Patent Application
20090310215
The present invention relates to a sample positioning stage, such as a stage used to position a sample for inspection by an optical inspection apparatus, for instance a microscope or spectroscope.
Sample positioning stages can be used, for example, to position samples relative to microscopes and spectroscopes, and can comprise a plate onto which a sample to be inspected can be placed. A generally planar plate is typically mounted on at least one carriage so that the plate can move relative to the carriage in at least one substantially horizontal degree of freedom. The plate and carriage typically have cooperating bearing members so that the plate and the carriage can be moved relative to each other in at least one degree of freedom under the control of a motorized drive mechanism. The plate or the carriage can be coupled to a further carriage via bearings which facilitates movement of the plate in another degree of freedom. The accuracy and repeatability of positioning of the plate is important in order to enable accuracy and repeatable position of samples on the plate relative to an inspection device, for instance.
It is known to use motors for driving the plate and carriage of a sample positioning stage relative to each other in response to signals received from an electronic input device. The use of a motor for driving a stage can be advantageous for many reasons. For example, they enable the automation of stage movement (e.g. a sample to be inspected or can be automatically be placed in a plurality of different positions such that a montage of the sample can be obtained). The use of motors can also provide for more accurate positioning of the stage. Motors can also facilitate stage movement when it is not possible for an operator to be close to the stage.
It can be important to ensure that there is little or no play between the bearings of the plate and the carriage; that is little or no freedom of movement between the bearings of the plate and carriage in dimensions other than that in which the plate and carriage are intended to move. This is particularly the case when the plate and carriage are held together only by way of the interaction of the cooperating bearing members. It can also be important to ensure that the force between the cooperating bearing members does not become too large so as to make it difficult to move the plate and carriage relative to each other; this could result in too much load being put on the drive mechanism or even cause the bearing members to jam.
These undesirable situations can arise due to manufacturing inaccuracies. It is known to provide adjustable bearings, the position of which can be set prior to operation of the stage for example through the use of adjustment screws, so as to avoid the above mentioned situations. It is also known to provide spring loaded bearings between the plate and carriage which compensate for any non-uniformity in the stage and thereby avoid the need for adjustable bearings.
However, it has been found that stages with spring loaded bearings can be less accurate and provide less repeatable movement than those in which all of the bearings are rigid.
The present invention relates to improvements in sample positioning stages, and in particular provides a sample positioning stage having rigid and compliant bearings extending and biased between the moveable bodies of the stage, in which a motorized drive system is configured to act on the bodies near to the rigid bearing, and in which a position measurement device for providing a measure of the relative position of the bodies is positioned closer to the rigid bearing than the resiliently compliant bearing.
Accordingly, in a first aspect, the invention provides a sample positioning stage for positioning a sample to be inspected relative to an optical inspection device, the stage comprising: a first generally planar body on which a sample to be inspected can be carried; a second body directly coupled to the first body via at least one rigid bearing and at least one resiliently compliant bearing provided between the first and second bodies, the at least one rigid bearing and the at least one resiliently compliant bearing being arranged generally opposite each other, and configured such that the first and second bodies are preloaded against each other in a dimension substantially parallel to the plane of the first generally planar body via the bearings, and such that movement of the first body relative to the second body is constrained to a first plane that is substantially parallel to the plane of the first body via the bearings; a motorized drive system operable to drive the first and second bodies relative to each other in the first plane toward a demanded relative position received from a position input device, in which the motorized drive system imparts its driving force on the stage closer to the at least one rigid bearing than the at least one resiliently compliant bearing; and a position sensing device on at least one of the first and second bodies closer to the at least one rigid bearing than the at least one resiliently compliant bearing, for providing a measure of the relative position of the first and second bodies.
It is an advantage that configuring the motorized drive system such that it acts on the bodies closer to the rigid bearings than the compliant bearings, and positioning the position sensing device closer to the rigid bearing than the compliant bearings improves the accuracy and repeatability of movement of the stage.
The relative movement of the first and second bodies could be constrained via the bearings to a first degree of freedom which lies substantially parallel to the plane of the first body. The first degree of freedom can be a linear degree of freedom. Optionally, the degree of freedom can be a rotational degree of freedom.
Suitable bearings for use with the present invention (for use with either or both the rigid bearing or the resiliently compliant bearing) include sliding bearings and roller bearings such as cross-roller bearings. For example, at least one of the rigid and resiliently compliant bearings can comprise roller bearings, such as ball bearings, being located between tracks on the first and second bodies.
Preferably, each of the at least one rigid bearing and at least one resiliently compliant bearing comprise a first bearing formation on the first body and a first bearing formation on the second body which cooperate to form the bearing. The bearing formation can comprise a bearing part which facilitates relative movement between the bodies in the first plane. The bearing formation can also comprise a mount via which the bearing part is mounted to one of the first and second bodies. For example, the first and second bearing formations can comprise a track on one of the first and second bodies and a runner on the other for cooperating with the track. As will be understood, the track can be any suitable feature on one of the bodies which defines a path which the runner on the other of the bodies is configured to follow. The track can be profiled, for example toothed. In this case the runner could be cog-shaped such that the track and runner engage like a rack and pinion mechanism. Preferably the track is smooth. In this case the runner could be configured to engage the track such that it can slide along the track. Optionally, the runner could be a roller configured to roll along the track. For instance, the runner could be a wheel.
As will be understood, different combinations of the types of bearings could be used for the at least one rigid bearing and at least one resiliently compressible bearing. For instance, the at least one rigid bearing could be a sliding bearing and the at least one resiliently compressible bearing could be a roller bearing. Preferably, the same type of bearing is used for each of the at least one rigid bearing and the at least one resiliently compressible bearing. Preferably, the at least one rigid bearing and the at least one resiliently compressible bearing each comprise a track and a roller for rolling along the track. Preferably, the track for the at least one rigid bearing and the track for the at least one resiliently compliant bearing are provided on the same body, for example the first body. Accordingly, preferably, the runner for the at least one rigid bearing and the runner for the at least one resiliently compliant bearing are provided on the same body, for example the second body. In embodiments in which the first degree of freedom is rotary, the runner of the at least one rigid bearing could engage the same track as the runner of the at least one resiliently compliant bearing.
A track can be provided by at least one recess in the first or second body. A track can be provided by at least one projection on the first or second body. The profile of a track taken perpendicular to the length of the track can be curved, for example at least part elliptical, preferably at least part circular, for example semi-circular. A track can be provided as an integral part of one of the first and second bodies. For example, a track and the body on which it is provided can be formed as one piece. A track can be provided as a separate piece which is mounted on one of the first and second bodies. For example, one of the first and second bodies can comprise a mounting for a track. Preferably the mounting supports a track substantially along its length.
Movement of the first body relative to the second body is constrained to a first plane that is substantially parallel to the plane of the first body via the bearings. Accordingly, the first and second bodies are constrained from moving relative to each other in a dimension perpendicular to the plane of the first body via the bearings. The bearing formations on the first and second bodies which cooperate to form the at least one rigid bearing and at least one resiliently compliant bearing could comprise cooperating formations which prevent the first and second bodies from moving relative to each other in a dimension perpendicular to the plane of the first body. For instance, the bearing formations could have inter-engaging formations, such as a projection and groove arrangement, which prevents the first and second bodies from moving relative to each other in a dimension perpendicular to the plane of the first body. Optionally, the bearing formations on the first and second bodies which cooperate to form the at least one rigid bearing and at least one resiliently compliant bearing could be configured to frictionally engage under the preload so as to prevent the first and second bodies from moving relative to each other in a dimension perpendicular to the plane of the first body via the bearings.
As will be understood, the at least one rigid bearing could permit relative movement of the first and second bodies in the first degree of freedom but be sufficiently rigid so as to prevent relative movement in all other degrees of freedom. The bearing formations provided on the first and second bodies which provide the rigid bearing could be sufficiently rigid such that their position cannot move relative to the body on which they are provided. In embodiments in which the bearing formation comprises a bearing part and a mount, preferably the bearing part and the mount are sufficiently rigid such that their position cannot move relative to the body on which they are provided. For example, in embodiments in which the rigid bearing comprises a track on one of the bodies and runner on the other, preferably both the track and the runner are fixed such that their position cannot move relative to the body on which they are provided. Of course, as will be understood, in embodiments in which the runner is a roller, for instance a wheel, the roller will be able to rotate about its bearing relative to the body on which it is provided. In embodiments in which a part extends between the formations on the first and second bodies, for example a roller bearing extending between a track on each of the bodies, the part could also be sufficiently rigid, and shaped and sized so as to permit relative movement of the first and second bodies in the first first degree of freedom but prevent relative movement in all other degrees of freedom.
In contrast, the at least one resiliently compliant bearing could permit relative movement of the first and second bodies in the first degree of freedom, and also be resiliently compliant in at least one other degree of freedom. This enables the resiliently compliant bearing to compensate for any non-uniformity in the shape and size of the first and second bodies and/or the bearing formations of the resiliently compliant bearing, as they move relative to each other. Preferably the resiliently compliant bearing is not compliant in the first degree of freedom. In particular, preferably the resiliently compliant bearing is only compliant in a dimension that extends substantially perpendicular to the first degree of freedom at the bearing. Preferably, the resiliently compliant bearing is rigid in a dimension that extends substantially perpendicular to the plane of the first body. Preferably the resiliently compliant bearing is resiliently compliant in a dimension that extends along a plane which contains both the at least one resiliently compliant bearing and the at least one rigid bearing.
As will be understood, either or both of the bearing formations provided on the first and second bodies can provide the resilient compliance. For instance, in embodiments in which the bearing is provided by a track on one of the bodies and a runner on the other, the track side can be resiliently compliant. Preferably, the runner side is resiliently compliant. Optionally, both the track side and the runner side can be resiliently compliant. In any case, preferably, the resiliently compliant formation is resiliently compliant in a dimension which extends substantially perpendicular to the length of the track at the runner, so as to bias the runner and track together. Preferably, the resiliently compliant formation is rigid in all other degrees of freedom.
As will also be understood, in these cases, a bearing part itself can be resiliently compliant. For example, the track itself can be resiliently compliant. For example, in embodiments in which the bearing part is a runner, the runner itself can be resiliently compliant. Further still, in embodiments in which a bearing formation comprises a roller bearing, the roller bearing itself can be resiliently compliant. Optionally, in embodiments in which a bearing formation comprises a bearing part and a mount, the mount can be resiliently compliant. Preferably the bearing part itself is rigid. For instance, preferably the track is mounted to the body on which it is provided by a resiliently compliant mount. In embodiments in which a bearing formation comprises a runner, preferably the runner is mounted to the body on which it is provided by a resiliently compliant mount.
The resiliently compliant mount can be provided as a separate component to the body. For instance, the resiliently compliant mount can be a spring device, such as a coil spring, which acts between the bearing formation and the body on which the bearing formation is provided. In this case, the bearing formation can be coupled to the body by at least one arm so as to support the bearing formation. The arm can be hinged to the body so as to allow the arm and bearing formation to move relative to the body. Optionally, the arm could be deformable so as to allow the arm and bearing formation to move relative to the body.
Preferably, the resiliently compliant mount and the body are formed as one piece. More preferably, the resiliently compliant mount and the body are formed from the same material. This can reduce the complexity of the stage as it reduces the number of components needed. This can decrease the manufacturing cost, and also can improve the reliability of the stage.
The resiliently compliant mount can be configured to have at least one predetermined point of weakness which is configured to deform when the first and second bodies are assembled together. This enables the deformation of the resiliently compliant mount to be controlled in a predetermined way.
Preferably, the resiliently compliant mount comprises at least a first arm which extends between the body and the bearing formation so as to suspend the bearing member from the body. The first arm can be configured to deform at the point at which the first arm meets the body. Accordingly, the first arm will maintain its shape along substantially its entire length, but will bend at its joint with the body.
Preferably, the resiliently compliant mount further comprises at least a second arm which extends between the body and the bearing formation so as to suspend the bearing formation from the body. Especially preferably, the resiliently compliant mount further comprises at least a third arm which extends between the body and the bearing formation so as to suspend the bearing formation from the body. Most preferably, the resiliently compliant mount further comprises at least a fourth arm which extends between the body and the bearing formation so as to suspend the bearing formation from the body.
When there is only a single rigid bearing and a single resiliently compliant bearing, then preferably they are arranged directly opposite each other. Preferably the rigid bearing and resiliently compliant bearing are arranged such that a straight line connecting the bearings extends perpendicular to the first degree of freedom at the bearings. However, as will be understood, the at least one rigid bearing and at least one resiliently compliant bearing need not necessarily be arranged directly opposite each other. As will be understood, when there are a plurality of rigid bearings, and/or a plurality of resiliently compliant bearings, then there can be various preferred arrangement of the bearings between the first and second bodies.
Preferably, the at least one rigid bearing and at least one resiliently compliant bearing are arranged substantially opposite each other such that the direction of the net bias force which the at least one rigid bearing is under is substantially opposite to the direction of the net bias force which the at least one resiliently compliant bearing is under.
Preferably, there is provided a bias mechanism configured to bias the at least one rigid bearing and at least one compliant bearing into bearing engagement between the first and second bodies. As will be understood, because the at least one rigid bearing and at least one resiliently compliant bearing are arranged generally opposite each other, the bias mechanism will tend to bias the at least one rigid bearing and the at least one resiliently compliant bearing generally away from, or toward, each other. The bias mechanism could comprise a resiliently deformable device, for instance a spring. The resiliently deformable device could be provided by one or both of the first and second bodies. For instance, one or both of the first and second bodies could comprise at least first and second portions corresponding to the at least one rigid bearing and at least one resiliently compliant bearing and the bias mechanism could be configured to bias the portions so as to bias the bearings into bearing engagement.
Preferably, the bias force is provided by the at least one resiliently compliant bearing. Accordingly, preferably the at least one resiliently compliant bearing biases the at least one rigid bearing and at least one resiliently compliant bearing into bearing engagement. In this case the at least one resiliently compliant bearing can have a normal configuration which it is biased towards. Preferably, the stage apparatus is configured such that the at least one resiliently compliant bearing is forced away from its normal configuration against its bias. Preferably, the at least one resiliently compliant bearing is forced away from its normal configuration against its bias by way of the interaction between the first and second bodies.
As described above, the at least one resiliently compliant bearing can comprise a bearing part which facilitates relative movement between the bodies in the first degree of freedom, and the bearing part can be mounted to one of the first and second bodies via a mount. The bearing part of the at least one resiliently compliant bearing can be resiliently compliant. Optionally, both the bearing part and the mount of the at least one resiliently complaint bearing can be resiliently compliant. Preferably, the mount of the at least one resiliently compliant bearing is resiliently compliant. The stage apparatus can be configured such that the resiliently compliant mount is forced away from its normal configuration against its bias such that the resiliently complaint mount is in a biased state. Preferably, the resiliently compliant mount is forced away from its normal configuration against its bias by way of the interaction between the first and second bodies, and the bearings.
As will be understood, when the at least one rigid bearing and at least one resiliently compliant bearing are in bearing engagement they facilitate relative movement between the first and second bodies. For example, in embodiments in which the rigid bearing comprises a track and a runner as discussed in more detail below, the rigid bearing is in bearing engagement when the track and runner are biased into engagement with each other such that the runner can be guided along the track. Preferably, when the at least one rigid bearing and at least one resiliently compliant bearing are in engagement there is substantially no freedom of movement in the bearings other than in the first degree of freedom.
Preferably, the at least one rigid bearing and the at least one resiliently compliant bearing are arranged such that, in embodiments in which the at least one resiliently compliant bearing provides the bias force, each of the at least one resiliently compliant bearing is configured to bias at least one of the rigid bearings in bearing engagement. Preferably, the at least one rigid bearing and the at least one resiliently compliant bearing are arranged such that the net force provided by the at least one resiliently compliant bearing biases each of the rigid bearings into bearing engagement. Preferably, the at least one resiliently compliant bearing and the at least one rigid bearing are configured such that the force provided by the bias of the resiliently compliant bearings on each of the rigid bearings is substantially the same.
As will be understood, there can be provided at least two resiliently compliant bearings.
The motorized drive system can impart its driving force (so as to cause relative movement between the first and second bodies in the first degree of freedom) on any part of the stage close to the at least one rigid bearing. The motorized drive system could act on the at least one rigid bearing. The motorized drive system could act on the first body. The motorized drive system could act on the second body. In this case, the body on which the motorized drive system does not act could be held against movement in the first degree of freedom. Optionally, the motorized drive system could act on the first and the second bodies to cause relative movement. The motorized drive system could be fixed relative to one of the bodies in at least the first degree of freedom, and configured to act on the other of the bodies so as to cause relative movement in the first degree of freedom.
Preferably the ratio of the smallest straight line distance between i) the point at which the motorized drive system imparts its driving force on the first body or the second body and its closest at least one resiliently compliant bearing; and ii) the point at which the motorized drive system imparts its driving force on the first body or the second body and its closest at least one rigid bearing is more than 1:1, more preferably more than 2:1, especially preferably more than 5:1, for example most preferably more than 10:1.
In embodiments in which the first degree of freedom is linear, preferably the ratio of the distance between i) the point at which the motorized drive system imparts its driving force on the first body or the second body and its closest at least one resiliently compliant bearing; and ii) the point at which the motorized drive system imparts its driving force on the first body or the second body and its closest at least one rigid bearing, each taken in a dimension perpendicular to the first degree of freedom is more than 1:1, more preferably more than 2:1, especially preferably more than 5:1, for example most preferably more than 10:1.
In embodiments in which the first degree of freedom is a rotational degree of freedom, preferably the at least one rigid bearing and the at least one resiliently compliant bearing are arranged such that all of the at least one resiliently compliant bearings are on one side of a plane which wholly contains the axis of rotation and which extends perpendicularly to the net bias provided by the resiliently compliant bearings, and so that all of the at least one rigid bearings are on the other side of the plane. In this case, preferably the motorized drive system acts on the body on the side of the plane on which all of the at least one rigid bearings are located.
In embodiments in which the first degree of freedom is a linear degree of freedom, preferably all of the at least one resiliently compliant bearings are provided on one side of a plane extending parallel to the first degree of freedom, and all of the at least one rigid bearings are provided on the other side of the plane extending parallel to the first degree of freedom. In particular, preferably all of the at least one resiliently compliant bearings are positioned toward one side of the stage and all of the at least one rigid bearings are position toward an opposing side of the stage in a direction perpendicular to the first degree of freedom. In these cases, preferably the motorized drive system acts on the side of the stage on which all of the at least one rigid bearings are located.
The motorized drive system can comprise a controller for controlling a motorized drive unit. Accordingly, in this case, the motorized drive unit imparts the driving force on the first body or the second body, and the controller controls the operation of the motorized drive unit in response to demanded relative positions received from a position input device.
Suitable motorized drive units for use with the present invention include belt drives, lead screws, rack and pinion drives, linear drives and direct drives. In a particularly preferred embodiment the motorized drive unit comprises a motor body from which a drive shaft extends for engagement with a motor track, such that rotation of the drive shaft by the motor body causes relative movement between the drive shaft and the motor track along the motor track's extent. Preferably, the motorized drive unit is configured such that rotation of the drive shaft causes relative movement between the drive shaft and the motor track in a dimension perpendicular to the rotational axis of the drive shaft. Preferably, the drive shaft is configured to frictionally engage a friction rod.
Preferably, in embodiments in which the bearing comprises a track, the motorized drive system acts on the track to cause relative movement. This is advantageous because it enables the stage to be made more compact and lightweight. It also reduces the amount of torque exerted on the bodies by the motor about the at least one rigid bearing. For instance, in embodiments in which the motorized drive system comprises a motor having a drive shaft for engaging a friction rod, preferably the drive shaft frictionally engages the track. Accordingly, the track can be the friction rod.
Preferably, the force applied by the motorized drive system on the track is substantially in the same direction as the first degree of freedom of relative movement of the first and second bodies at the point the motorized drive system acts on the track. More preferably, the motorized drive system acts on the bearing surface of the track. As will be understood, the bearing surface can be the surface which a cooperating runner engages. Preferably there is at least one point of contact between the motorized drive system and the bearing surface of the track. For instance, there can be two points of contact between the motorized drive system and the bearing surface of the track. Preferably, the direction of the force applied by the motorized drive system on the track extends substantially parallel to a bearing contact line which extends substantially parallel to the extent of the track and contains the point of contact between the runner and the track. More preferably, the point at which the motorized drive system acts on the bearing surface of the track lies on the bearing contact line. Accordingly, it can be preferred that the direction of the force applied by the motorized drive system on the track substantially extends along the bearing contact line. As will be understood, in embodiments in which the track is straight, then preferably the direction of the force applied by the motorized drive system on the track and the bearing contact line are substantially co-axial. As will also be understood, there can be more than one bearing contact line, for instance, in embodiments in which there are at least two points of contact between a runner and the track. In these embodiments it can be preferred that the motorized drive system acts on the track so as to apply a force that extends along at least two of the at least two bearing contact lines.
There can be provided at least two rigid bearings provided between the first and second bodies. For instance, there can be provided at least two rigid runners on one of the first and second bodies for engaging a rigid track provided on the other. As will be understood, the rigid runners can be spaced apart along the extent of the track. In this case, preferably the motorized drive system acts on the track at a point between the at least two spaced apart rigid runners. For instance, in the embodiment in which the motorized drive system comprises a drive shaft for frictionally engaging a friction rod, preferably the drive shaft engages the track at a point between the rigid runners. Preferably, the motorized drive system acts on the track at a point approximately midway between adjacent rigid runners.
The motorized drive system can be rigid in that the part of the motorized drive system which acts on the stage to cause relative movement cannot move relative to the body on which motorized drive system is anchored in a first dimension perpendicular to the first degree of freedom at the point of contact. Preferably, the part of the motorized drive system which acts on the stage to cause relative movement is resiliently compliant. Preferably, the part of the motorized drive system which acts on the stage to cause relative movement is resiliently compliant in a dimension which is wholly contained in a compliance plane which also wholly contains the dimension in which the at least one resiliently compliant bearing is compliant. Preferably, the part which acts on the stage to cause relative movement is biased into the body on which it acts. For example, in embodiments in which the motorized drive system comprises a drive shaft which engages a track, preferably there is provided a bias mechanism which biases the drive shaft into frictional engagement with the track. Preferably the bias mechanism is resiliently compliant. The bias mechanism can act between the body which the motorized drive mechanism is fixed relative to in the first degree of freedom and the drive shaft.
Preferably the ratio of the smallest straight line distance between i) the point on the first or second body from which the position sensing device determines the relative position of the first and second body and its closest at least one resiliently compliant bearing; and ii) the point on the first or second body from which the position sensing device determines the relative position of the first and second body and its closest at least one rigid bearing is more than 1:1, more preferably more than 2:1, especially preferably more than 5:1, for example most preferably more than 10:1.
In embodiments in which the first degree of freedom is linear, preferably the ratio of the distance between i) the point on the first or second body from which the position sensing device determines the relative position of the first and second body and its closest at least one resiliently compliant bearing; and ii) the point on the first or second body from which the position sensing device determines the relative position of the first and second body and its closest at least one rigid bearing, each taken in a dimension perpendicular to the first degree of freedom is more than 1:1, more preferably more than 2:1, especially preferably more than 5:1, for example most preferably more than 10:1.
In embodiments in which the first degree of freedom is a rotational degree of freedom, preferably the at least one rigid bearing and the at least one resiliently compliant bearing are arranged such that all of the at least one resiliently compliant bearings are on one side of a plane which wholly contains the axis of rotation and which extends perpendicularly to the net bias provided by the resiliently compliant bearings, and so that all of the at least one rigid bearings are on the other side of the plane. In this case, preferably the position sensing device measures the relative position of the first and second bodies at a point on the side of the plane on which all of the at least one rigid bearings are located.
In embodiments in which the first degree of freedom is a linear degree of freedom, preferably all of the at least one resiliently compliant bearings are provided on one side of a plane extending parallel to the first degree of freedom, and all of the at least one rigid bearings are provided on the other side of the plane extending parallel to the first degree of freedom. In particular, preferably all of the at least one resiliently compliant bearings are positioned toward one side of the stage and all of the at least one rigid bearings are position toward an opposing side of the stage in a direction perpendicular to the first degree of freedom. In these cases, preferably the position sensing device measures the relative position of the first and second bodies at a point on the side of the stage on which all of the at least one rigid bearings are located.
The position sensing device can comprise a scale on one of the first and second bodies and a scale reader on the other of the first and second bodies. In this case, preferably, the scale and scale reader are position closer to the at least one rigid bearing than the at least one resiliently compliant bearing. In particular, preferably the scale reader reads the scale at a point closer to the at least one rigid bearing than the at least one resiliently compliant bearing.
Suitable scales include those having marks defining a pattern which can be read by a readhead in order to determine relative movement between them. For instance, the scale can be an incremental scale having scale marks defining a periodic pattern which generates a periodic signal at the readhead when relative movement between the scale and readhead take place. These periodic signals produce an up/down count which enables displacement between the scale and the readhead to be determined. For instance, such a suitable scale is described in European Patent Application no. 0207121, the entire content of which is incorporated into the specification of the present application by this reference. The scale can have reference marks which are detectable by the readhead so that it can determine the exact position of the readhead relative to the scale. For example, such a scale is disclosed in Published International Patent Application WO 2005/124282, the entire content of which is incorporated into the specification of the present application by this reference. Optionally, the scale can be an absolute scale which has scale markings which enable the readhead to determine an exact absolute position relative to the scale without the need to incrementally count from a predetermined position. Such scales typically have scale markings which define unique position data. The data can be in the form of, for instance, a pseudorandom sequence or discrete codewords. Such a scale is disclosed in International Patent Application no. PCT/GB2002/001629, the entire content of which is incorporated into the specification of the present application by this reference.
Position data from the position sensing device could be used be used in order to determine the relative position of the first and second bodies. This could be used for instance to provide a relative position readout to a user via a user interface display. Optionally, the position data could be used in a control algorithm to detect when the first and second bodies are moving relative to each other. Furthermore, position information from the position sensing device could be used by the motorized drive system, for instance by the motorized drive system's controller, as part of a servo loop in order to accurately drive the first and second bodies relative to each other to a demanded relative position.
Preferably, the first and second bodies are generally planar. Preferably, one of the first and second bodies extends between the other. Preferably, the bearings are provided towards opposing sides of the bodies. Preferably, one of the bodies is held within another body by the bearings. In particular, preferably one of the bodies has first and second side walls depending from opposing sides of the body, between which the other body is held against relative movement in a dimension perpendicular to the plane of the bodies by the bearings. Preferably, the first and second side walls are linear. Preferably, the first and second side walls are parallel to each other. Preferably, each of the first and second walls provides a track. Accordingly, preferably the mating sides of the other body provide at least one runner for engaging the corresponding track.
Preferably, at least a portion of each of the first and second bodies (and third body if present) permits the transmission of light. As will be understood, light can be visible, infrared or ultraviolet light. This is advantageous as it enables the illumination of a sample placed on the first body. The portion can be a transparent or translucent part of the body. Optionally, the portion can be an opening in the body. Preferably, in embodiments in which the first degree of freedom is linear, preferably the portion of at least one of the first and second bodies is elongate. The portion can be elongate in a dimension perpendicular to the linear degree of freedom in which the first and second bodies can move relative to each other. In embodiments in which the second body is directly coupled to a third body, preferably the second body's portion is elongate in a dimension parallel to the linear degree of freedom in which the second and third bodies can move relative to each other.
The stage apparatus can further comprise: a third body directly coupled to the second body via at least one rigid bearing and at least one resiliently compliant bearing between the second and third bodies, the at least one rigid bearing and the at least one resiliently compliant bearing being arranged generally opposite each other and configured to preload the second and third bodies against each other so as to constrain movement of the first body relative to the second body to a second degree of freedom which lies in a plane that is substantially parallel to the plane of the first body and which extends substantially perpendicularly to the first degree of freedom. A second motorized drive system can be provided and configured to drive the first and second bodies relative to each other along the second degree of freedom toward a demanded relative position received from a position input device. Preferably, the motorized drive system is configured to impart its driving force on the stage to cause the relative movement in the second degree of freedom closer to the at least one rigid bearing than the at least one resiliently compliant bearing.
The stage can further comprise a second position sensing device on at least one of the second and third bodies closer to the at least one rigid bearing than the at least one resiliently compliant bearing, for providing a measure of the relative position of the second and third bodies.
The stage can be for use in an inspection apparatus, such as a microscope or a spectroscope. Preferably, the stage is for use in high resolution systems. For example, the stage can be for use in a high resolution system in which the resolution of the positioning of the first body to the second body is at least four orders of magnitude higher than the range though which the first and second bodies can move relative to each other, more preferably at least five orders of magnitude, especially preferably at least six orders of magnitude.
Preferably the position input device is an electronic position input device. Suitable electronic position input devices include a joystick, trackball or other device which a user can manipulate to input a demanded relative position. The electronic position input device can be a memory device which contains pre-stored demanded relative positions. Optionally, position input device is a processor unit, for instance a general purpose computer which can provide demanded relative positions from a computer program. Accordingly, user can program a sequence of positions which the program runs through to control the position of the stage apparatus.
According to a second aspect of the invention, there is provided an optical inspection apparatus comprising: an optical inspection device; and a sample positioning stage as claimed in any preceding claim for positioning a sample to be inspected relative to the optical inspection device.
According to a third aspect of the invention, there is provided a sample positioning stage for a microscope apparatus, comprising: a first generally planar body having first and second linear tracks substantially opposing and parallel to each other, at least the first linear track being rigidly fixed relative to the first body; a second body directly coupled to the first body so that it can move relative to the first body in a degree of freedom, the second body having at least first and second rigidly mounted runners in bearing engagement with the first linear track at points spaced along the first linear track so as to define the degree of freedom, the second body also having at least one resiliently compliant runner in bearing engagement with the second linear track, in which the runners and tracks constrain movement of the first body relative to the second body to a first plane that is substantially parallel to the plane of the first body; a motorized drive system fixed relative to the second body in the degree of freedom and configured to act on the first linear track at a point between which the first and second rigidly mounted runners engage the first linear track so as to drive the first and second bodies relative to each other along the degree of freedom toward a demanded relative position received from a position input device; and a position sensing device on at least one of the first and second bodies provided on the side of the bodies that is closer to the rigidly mounted runners than the resiliently compliant runner, for providing a measure of the relative position of the first and second bodies.
An embodiment of the invention will now be described with reference to the accompanying drawings in which:
a and 5b are plan and perspective views of the carriage shown in
a is a perspective underside view of the sample positioning stage shown in
b is a perspective topside view of the second carriage in isolation;
a and 8b are perspective views of the drive mechanism shown in
a is schematic underside view of the second guide rod, drive shaft and bearing wheels of the first and second carriages shown in
b is a schematic cross-sectional view of the drive shaft and guide rod shown in
c is a schematic cross-sectional view of a bearing wheel and guide rod shown in
Referring now to
The microscope 6 comprises an objective lens 10, first 12 and second 14 eye piece lenses, and an arm 16 which supports the sample positioning stage 4.
As will be understood, the optical inspection device need not necessarily be a microscope, and can be any device suitable for examining a sample 8 placed on the stage 4. For instance, the examination device could be a spectroscope. Furthermore, it will be understood that there need not be an optical inspection device at all. For example, the stage 4 could be used to support a sample 8 which is to be examined by the naked eye.
Referring in particular to
The second carriage 22 is fixed relative to the microscope 6, and in particular is fixed relative to the objective lens 10, in the X and Y dimensions.
The plate 18 has an upper face 24 which is substantially planar and substantially rectangular in shape. In the embodiment described, a formation in the form of a recessed area 26 is provided for receiving a sample 8 to be examined. Also provided is an aperture 27 through so that a light source (not shown) located below the plate 18 can illuminate a sample 8 located in the recessed area 26. First 28 and second 30 skirts depend from opposing sides of the plate 18.
A handle 40, which is substantially cylindrical in shape, depends from one corner of the plate 18. The handle 40 is placed on the side of the plate 18 that is distal to the arm 16. This aids accessibility of the handle 40. A recess 42 is provided in the upper face 24 of the plate 18 for receiving a thumb of a user. The handle 40 and recess 42 facilitate gripping of the plate 18 by a user.
A first position measurement device is provided in the form of a first scale 31 (a part of which is shown in
Referring to
The body 21 of the first carriage 20 has an elongate aperture 64, and a plurality of portions of reduced depth such as those indicated by reference numeral 65. The aperture 64 allows the passage of light through the body 21 as described in more detail below, and the reduced depth portions 65 reduce the weight of the body 21.
A second position measurement device is provided in the form of a second scale 35 provided on the underside of the first carriage 20, which can be read by a second readhead 37 mounted on the second carriage 22. The second readhead 37 is electrically connected to the control system 200 via a line (not shown) in cable 146, and can output a signal which can be used by the control system 200 to determine the position of the first carriage 20 relative to the second carriage 22. A suitable scale is the scale sold under product number RGS40 available from Renishaw plc. A suitable readhead for is the readhead sold under product number RGH34 readhead available from Renishaw plc
Third 66 and fourth 68 skirts depend from the body 21, and extend between the first and second sides on which the first 44, second 46 and third 48 wheels are mounted. The third skirt 66 has a third elongate recess 70 extending along its length for receiving a third guide rod 72. The fourth skirt 68 has a fourth elongate recess 74 extending along its length for receiving a fourth guide rod 76. The third 74 and fourth 78 guide rods are held within the third 70 and fourth 74 elongate recesses.
First and second rigid bearings, generally indicated by 43 and 45, are provided between the plate 18 and the first carriage 20 one side of the stage 4. A first resiliently compliant bearing generally indicated by 47, is provided between the plate 18 and the first carriage 20 on the opposite side of the stage 4. The bearings 43, 45 and 47 facilitate the relative movement of the plate 18 and first carriage 20 in the X-dimension.
The bearings 43, 45 and 47 are provided by cooperating bearing formations provided on the plate 18 and carriage 20, as described in more detail below.
The first skirt 28 has a first elongate recess 32 extending along its length for receiving a first guide rod 34. The first guide rod 34 provides the bearing part for the bearing formation on the plate 18 for the first 43 and second 45 rigid bearings. The second skirt 30 has a second elongate recess 36 extending along its length for receiving a second guide rod 38. The second guide rod 38 provides the bearing part for the bearing formation on the plate 18 for the first resiliently compliant bearing 47.
The first carriage 20 has first 44 and second 46 rigid wheels positioned spaced apart on a first side of the first carriage 20. The first 44 and second 46 rigid wheels provide the bearing parts for the bearing formations on the first carriage 20 for the first 43 and second 45 rigid bearings. The first carriage 20 also has a rigid third wheel 48 positioned on a second side of the first carriage 20, opposite to the first side. The rigid third wheel 48 is positioned along the length of the second side so that it lies midway between the first 44 and second 48 rigid wheels on the first side. The third rigid wheel 48 provides the bearing part for the bearing formation on the first carriage 20 for the first resiliently compliant bearing 47.
The first 44 and second 46 rigid wheels are mounted within first 58 and second 60 circular recesses in the body 21 so that a portion of them extends beyond the boundary of the body 21. This is so that when the sample positioning stage is assembled as shown in
The plate 18 and the first carriage 20 are configured so that when they are assembled together, the force on the wheels on the first carriage 20 and the respective guide rods on the examination plate is sufficiently low that the first carriage 20 is free to move within the plate 18 along the guide rods, but is sufficiently high that there is no play between the plate 18 and the first carriage 20 in all other dimensions.
The first 44, second 46 and third 48 rigid wheels each have circular apertures and the wheels are mounted by the apertures forming an interference fit with first 52, second 54 and third 56 square pegs within the first 58, second 60 and third 62 circular recesses. The use of circular apertures with square pegs enables an accurate and secure mount between the wheels and the pegs. The first 44, second 46 and third 48 wheels contain bearings which enable them to rotate relative to the pegs. The circumferential edge of the first 44, second 46 and third 48 rigid wheels is grooved so that the wheels can partially wrap around their respective guide rod. This prevents the wheels, and hence the carriage moving relative to the plate in the Z dimension, i.e. perpendicular to the plane of the plate 18.
The first 52 and second 54 pegs are mounted on the first carriage 20 so that they cannot be moved relative to the body 21. In contrast, the third peg 56 is mounted on the first carriage 20 so that it can resiliently move relative to the body 21 in a direction perpendicular to the length of the second guide rod 38 of the plate 18 when the sample positioning stage 4 is assembled.
In particular, the third peg 56 is mounted on a planar base 82 which is connected to the body 21 by first 84, second 86, third 88 and fourth 90 arms which define first 92, second 94 and third 96 apertures. The first 84, second 86, third 88 and fourth 90 arms are resiliently deformable and allow the planar base 82, and so the peg 56 and the third wheel 48 mounted on it, to move into the body 21 in the direction indicated by arrow A, on the application of a force on the planar based 82 in the direction indicated by arrow A. The first 84, second 86, third 88 and fourth 90 arms are configured to deform along their length as illustrated in
The stage 4 is configured such that when the plate 18 and first carriage 20 are assembled together the first 84, second 86, third 88 and fourth 90 arms are deformed. Accordingly, due to their resilience, the first 84, second 86, third 88 and fourth 90 arms will bias the third rigid wheel 48 into the second guide rod 38. This in tutn will cause the first 44 and second 46 rigid wheels to be biased into the first guide rod 34.
In the described embodiment, the stage 4 is configured so that the force on the planar base 82 (caused by the force on the third wheel 48 by its engagement with the second guide rod 38) is greater than the yield stress of the first 84, second 86, third 88 and fourth 90 arms. Accordingly, the first 84, second 86, third 88 and fourth 90 arms are plastically deformed on assembly of the plate 18 and the first carriage 20.
Referring to
Third and fourth rigid bearings, generally indicated by 81 and 83, are provided between the first carriage 20 and the second carriage 22 on one side of the stage 4. A second resiliently compliant bearing generally indicated by 85, is provided between the first carriage 20 and the second carriage 22 on the side of the stage 4 opposite to that on which the third 81 and fourth 83 rigid bearings are provided. The bearings 81, 83 and 85 facilitate the relative movement of the first carriage 20 and second carriage 22 in the Y-dimension.
The bearings 81, 83 and 85 are provided by cooperating bearing formations provided on the first carriage 20 and second carriage 22, as described in more detail below.
The fourth guide rod 78 provided on the first carriage 20 provides the bearing part for the bearing formation on the first carriage for the third 81 and fourth 83 rigid bearings. The third guide rod 72 provided on the first carriage 20 provides the bearing part for the bearing formation on the first carriage 20 for the second resiliently compliant bearing 85.
As with the first carriage 20, the second carriage 22 has first 77 and second 79 rigid wheels which are positioned spaced apart on a first side of the second carriage 22. The first 77 and second 79 rigid wheels provide the bearing parts for the bearing formations on the second carriage 22 for the third 81 and second 83 rigid bearings. The second carriage 22 also has a third rigid wheel 80 positioned on a second side of the second carriage 22, opposite to the first side. The rigid third wheel 80 is positioned along the length of the second side so that it lies midway between the first 77 and second 79 rigid wheels on the first side. The rigid third wheel 80 provides the bearing part for the bearing formation on the second carriage 22 for the third resiliently compliant bearing 85.
The first carriage 20 and the second carriage 22 are configured so that when they are assembled together, the force on the wheels on the second carriage 22 and the respective guide rods on the first carriage 20 is sufficiently low that the second carriage 22 is free to move within the first carriage 20 along the guide rods, but is sufficiently high that there is no play between the first carriage 20 and the second carriage 22 in the other dimensions.
The first 77 and second 79 rigid wheels are mounted on the second carriage 22 so that they cannot be moved relative to the body 23. In contrast, the rigid third wheel 80 is mounted on the second carriage 22 so that it can move relative to the body 23 in a direction perpendicular to the length of the fourth guide rod 72 of the plate 18 when the sample positioning stage 4 is assembled.
In particular, the rigid third wheel 80 is mounted via a peg 87 which in turn is mounted on a planar base 89 which is connected to the body 23 by first 91, second 93, third 95 and fourth 97 arms which define first 99, second 101 and third 103 apertures. The first 91, second 93, third 95 and fourth 97 arms are resiliently deformable along their length and allow the planar base 89, and so the peg 87 and the third wheel 80 mounted on it, to move into the body 23 in the direction indicated by arrow B, on the application of a force on the planar base 89 in the direction indicated by arrow B.
The stage 4 is configured such that when the first carriage 20 and second carriage 22 are assembled together the first 91, second 93, third 95 and fourth 97 arms are deformed. Accordingly, due to their resilience, the first 91, second 93, third 95 and fourth 97 arms will bias the third rigid wheel 80 into the third guide rod 72. This in turn will cause the first 77 and second 79 rigid wheels to be biased into the fourth guide rod 78.
In the described embodiment, the stage 4 is configured so that the force on the planar base 89 (caused by the force on the rigid third wheel 80 by its engagement with the third guide rod 72) is greater than the yield stress of the first 91, second 93, third 95 and fourth 97 arms. Accordingly, the first 91, second 93, third 95 and fourth 97 arms are plastically deformed on assembly of the first carriage 20 and second carriage 22.
Referring to
It has been found that providing the drive unit and the position measurement devices on the side on which the rigid bearings are located improves the accuracy and repeatability of the stage positioning.
Referring to
The drive shaft 104 has first 106 and second (not shown) conical portions which each converge to a reduced diameter portion toward the centre of the length of the drive shaft. When the sample positioning stage 4 is assembled, both of the first 106 and second conical portions contact the fourth guide rod 78. Accordingly, the drive shaft 104 has two points of contact with the guide rod 78. As illustrated in
The second drive unit 100 comprises a mounting arm 108. The motor body 102 is secured to the mounting arm so that they cannot move relative to each other. The mounting arm 108 has a base portion 110 to which the motor body 102 is secured by first 112 and second 114 screws, and an arm portion 116 which extends away from the motor body 102 along the length of the fourth guide rod 78. The end of the arm portion 116 distal to the motor body 102 has an extension 118 which defines an aperture 120.
The mounting arm 108 is mounted on the second carriage 22 via a spring 122. The spring 122 is secured to the second carriage 22 via a screw 124, and to the mounting arm 108 by the end of the spring being hooked through the aperture 120. Accordingly, in this way the mounting arm 108 is mounted on the second carriage so that the mounting arm 108, and hence the motor body 102 and drive shaft 104, is free to move relative to the second carriage in all dimensions other than in the dimension defined by the rotational axis B of the drive shaft 104. Rotation of the mounting arm 108, and hence the motor body 102 and drive shaft 104, is restricted in a first direction by the spring 122 and in a second direction opposite to the first direction by the abutment of the extension 118 against the body 23 of the second carriage 22.
The second drive unit 100 further comprises a bias mechanism 126 for biasing the drive shaft 104 onto the fourth guide rod 78. The bias mechanism 126 comprises a body 128 which has a head 130 and an arm 132, and a spring 144 which, when the sample positioning stage 4 is assembled, acts between the head 130 and the body 23 of the second carriage 22.
The head 130 has first 134, second 136, third 138 and fourth 140 bearings. When the sample positioning stage 4 is assembled, the bearings engage the drive shaft 104 and facilitate rotation of the drive shaft 104 relative to the head 130.
A first end of the arm 132 is secured to the head 130, and the arm 132 has an aperture 141 toward its second end which is distal to the head 130. The bias mechanism 126 is coupled to the body 23 via a bolt 142 which extends through the aperture 140 in the arm 132 and which engages a threaded bore in the body 23. The arm 132 is flexible so that the head 130 can be biased onto the second guide rod 78 by the spring 144.
To assemble the sample positioning stage 4, the first carriage 20 is slid into the plate 18 so that the first 44 and second 46 wheels engage the first guide rod 34, and so that the third wheel 48, and the drive shaft 104 of the first drive unit 98 (which is mounted on the first carriage 20 in the manner described above) engage the second guide rod 38.
The dimensions of the plate 18 and first carriage 20 are such that in order for the first carriage 20 to be received within the plate 18, the first 84, second 86, third 88 and fourth 90 arms are deformed so that the third wheel 48 is compressed into the body 21 of the first carriage 20. Furthermore, in the embodiment described the first 84, second 86, third 88 and fourth 90 arms are deformed to such an extent so as to plastically deform the first 84, second 86, third 88 and fourth 90 arms. Accordingly, when the first carriage 20 is received in the plate 18, the first 84, second 86, third 88 and fourth 90 arms bias the third wheel 48 onto the second guide rod 38.
Furthermore, the spring 144 is compressed by the interaction between the drive shaft 104 of the first drive unit 98 with the second guide rod 38 of the plate 18, so that the drive shaft 104 is biased onto the second guide rod 38.
The second carriage 22 is then slid into the first carriage 20 in a manner similar to that described above in relation to the first carriage 20 and the plate 18.
The second carriage 22 is then fixed to the microscope 6 so that it cannot move relative to the objective lens 10 in the X and Y dimensions. Accordingly, the position of the plate 18 can be moved relative to the objective lens 10, in the X dimension by operation of the first drive system 98, and in the Y dimension by operation of the second drive system.
A light source (not shown) can be positioned below the sample positioning stage 4. Light from the light source can pass through the aperture 67 in the second carriage 22, the aperture 65 in the first carriage and the aperture 27 in the plate 18 so as to illuminate a sample 8 located on the plate 18.
As shown in
The basic operation of the control system 200 will now be described in connection with
The demanded position variable can be changed by a user inputting a new demanded position via the input device 210. In particular, the user can input a demanded X position and a demanded Y position to the control system via the input device 210. In the described embodiment, the user inputs an absolute demanded position (i.e. move the plate 18 to a certain X/Y position). However, it will be understood that the demanded position input to the control system 200 can be a relative position (i.e. move the plate 18 in the X/Y dimension by a certain amount).
The position maintenance module continually monitors the output from the first 33 and second 37 readheads and updates the current position variable on detection of a change of position.
At step 304, the position maintenance module 206 continually checks to see if the demanded position and the current position are the same. If not, then at step 306, the position maintenance module applies a DC output voltage (“V”) across either, or both, of the motor bodies 102 of the first 98 and second 100 drive systems as required so as to move the plate 18 toward the demanded position. The output voltage V can be increased up to a maximum output voltage so as to progress the plate 18 towards the demanded position. As the plate 18 is moved, the current position variable is continually updated so as to reflect its current actual position. This process continues until the current position variable is the same as the demanded position variable.
The user can manually drag the plate 18 by applying a force to the plate 18 in the X and/or Y dimension. The user can apply such a force to the plate 18 by manipulating the handle 40.
The collision/drag detection module 208 runs in parallel to the position maintenance module 206, and is used to determine if the plate 18 is being manually dragged by the user, or if the plate 18 has collided with an object. If the collision/drag detection module 208 does detect such a situation, then it deactivates the motor bodies 102 of one of, or both of, the first 98 and second 100 drive systems so as to stop them driving against the external force. The operation of the collision/drag detection module 208 is explained in connection with
The collision/drag detection module 208 begins at step 500 when the control system 200 is turned on. At step 502, the output voltage V applied by the position maintenance module 206 at step 306, is continually monitored to see if it is at its maximum level.
As can be seen from
If the output voltage V is at is maximum voltage, then the process proceeds to step 504. At step 504, the position error is recorded in a variable PositionError0 and a timer (“t”) is started. The position error is the difference between the demanded position and the current position.
Control proceeds to step 506 in which the timer is continually incremented until it has reached half its maximum value (“T”). In the described embodiment, “T” is 25 mS. Accordingly, after approximately 12.5 mS, it is determined if the output voltage V is still at is maximum level. If the output voltage V is not still at its maximum level then this means that the external force on the plate 18 has been removed and so the process restarts.
If the output V is still at its maximum level, then control proceeds to step 508 at which the current position error is recorded in a variable PositionError1.
At 510, the timer t is continually incremented until it has reached its maximum value T. Accordingly, after approximately 25 mS from the start of the timer t, it is determined if the output voltage V is still at is maximum level. If the output voltage V is not still at its maximum level then this means that the external force on the plate 18 has been removed and so the process restarts.
If the output voltage V is still at its maximum level, then control proceeds to step 512 at which the current position error is recorded in a variable PositionError2.
At 514, it is determined if a force external to the plate 18 is being applied to it. If the sample positioning stage 4 has collided with an object and stalled, then the position of the plate 18 between the timer t being started and the timer t reaching the maximum number of increments T will not have changed significantly. Accordingly, if the difference between the position in PositionError0 and the position in PositionError2 is smaller than a predetermined maximum movement threshold X, then the method proceeds to step 518. The predetermined maximum movement threshold X enables a collision still to be detected even if the plate 18 moves a small distance between the recordal of PositionError0 and PositionError2. In the described embodiment, the predetermined threshold X is 100 μm.
If at step 514 the method does not determine that the plate 18 has collided with an object, then the method proceeds to step 516 at which it is determined if the plate 18 is being dragged manually.
At step 514, it is determined if either of the following are true:
0>PositionError0>PositionError1>PositionError2
0<PositionError0<PositionError1<PositionError2
These conditions are true whenever the plate 18 is being dragged away from its demanded position. If either of these conditions are true, then the process proceeds to step 518. If neither of these conditions are true, then the process returns to step 502.
At step 518 the motor body 102 is turned off. This is done by the collision/drag detection module 208 interrupting the position maintenance module 206 and reducing the DC output voltage V applied across the drive unit to zero. Accordingly, the motor body 102 will no longer try to drive the plate 18 to the demanded position and the drive unit 100 can be easily backdriven by the user.
Control then proceeds to step 520 in which a timer Voff is started. Timer Voff has a maximum value of one second. At step 522, it is determined if the plate 18 is stationary. If the examination plate is not stationary, then control returns to step 520, and the Voff timer is reset to zero.
If it is determined that the plate 18 is stationary, then control proceeds to step 526 at which it is determined if Voff has reached its maximum value (i.e. one second). If not, then control returns back to step 522 and the process is configured so that step 524 executes once every 200 μS after the timer Voff is started. If Voff has reached its maximum value, (i.e. one second) then control proceeds to step 528, at which point the demanded position variable is set as being the current position, and the position maintenance module 206 is restarted (from step 304).
The process described in relation to
As will be understood, other mechanisms other than the above described method can be used to determine when to the turn the motors 100 and 104 off so as to not to drive the plate 18 to a demanded position against an external force. For instance, a switch could be provided on the stage 4 for enabling the user to switch the mode of operation between one mode in which it drive tries to drive the plate 18 to a demanded position and another mode in which it freely allows a user to backdrive the drive units 98 and 100 so as to manually position the plate 18. Such a switch could be provided, for example by a button on the stage 4. For example, the button could be placed in the location of the recess 42 on the plate 18.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0700506.9 | Jan 2007 | GB | national |
| 0700507.7 | Jan 2007 | GB | national |
| 0700519.2 | Jan 2007 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2008/000096 | 1/11/2008 | WO | 00 | 6/30/2009 |
