Patent Application
20050221721
The present invention relates to grinding and polishing ophthalmic surfaces, such as plastic and glass ophthalmic lenses and glass molds.
Conventional devices for grinding and polishing ophthalmic surfaces first apply a hard master shape to an affected surface so as to remove any form error prior to polishing the surface to optical clarity. In the context of an optical laboratory, where prescription lenses are produced, each prescription value requires its own hard master shape. Hence, an optical laboratory typically has to store and maintain a large inventory of hard laps, which may be cumbersome and expensive.
Furthermore, conventional methods are limited to the production of spherical and toric surfaces. Certain specialized Computerized Numerical Control machines may be employed to generate non-conventional surface forms such as aspheric, atoric, and progressive geometries. However, these kind of asymmetric shapes are difficult to grind or polish when employing the above mentioned conventional methods. The equipment currently used for grinding and polishing non-conventional surface forms is from the field of high precision optics. The quality, tolerances, and control in this field are very rigorous in comparison to the field of ophthalmic optics (generally more than one order of magnitude). For this reason, the cost of such high precision equipment is very high, as well as requiring specialized operator training, which essentially renders it unavailable.
Hence, there is a need for a system that allows for automatic grinding and polishing of ophthalmic surfaces, either glass or plastic, which provides a greater surface conformity with respect to the original surface contour, i.e., substantially without any surface deformation, and which obviates the need for an optical laboratory to maintain a large stock of hard master shapes.
There is also a need for providing an apparatus and method which automatically grinds and polishes an ophthalmic surface to geometries that do not provide a symmetry of revolution.
The present invention provides a reduced cost apparatus and method as compared with equipment presently available from the field of high precision optics. The present invention also provides an apparatus and method for grinding and polishing ophthalmic surfaces which matches or surpasses mechanical specifications required in the area of ophthalmic optics.
In accordance with the present invention, an apparatus for grinding and polishing ophthalmic surfaces is provided. In one embodiment, the resultant surface is provided with a symmetry of revolution. The resultant surface is either concave or convex depending on the desired lens properties. In another embodiment, the apparatus includes a rotating flexible shaping pad, which is substantially smaller than the affected surface. The relatively small size of the flexible pad allows it to deform and contour onto any desirable area on the surface.
The shaping pad is preferably maintained in contact with, and is moved across, the affected surface so as to produce a removal profile which is greatest at the center of pad movement and smallest at the periphery of pad movement.
The apparatus traces a substantially equally-spaced spiraling path across the affected surface at a substantially constant contouring speed. The rotating speed of the pad and its contact pressure are preferably constant as well. The pad support is allowed to pivot in all directions about its rotating axis so as to maintain the plane of said pad orthogonal to the normal of the surface contact point. Preferably, by controlling the movement rate along the contouring path as well as controlling the pad rotation, the material removal rate is controlled.
In one embodiment, the present invention provides an apparatus for grinding or polishing an ophthalmic surfaces. The apparatus includes a surface shaping pad and a rotational pad driver coupled to the pad for maintaining the pad in constant contact pressure with an affected surface and for rotating the pad about a first rotational axis. A position drive means of the apparatus is coupled to the pad driver to controllably move the pad relative to the surface being ground or polished along a substantially spiraling contour path. The contour path is centered at the center of the surface and has parallel spiral arcs, which are spaced at a constant distance along any given radius of the spiraling contour path. The contour path further extends between the contour perimeter and the contour center such that a removal profile is produced along the contour path, which has circular symmetry with peak removal at the center of pad movement and minimal removal at the extremes of pad movement.
In operation, the pad 20 is controllably moved by the pad drive mechanism 21 in a substantially equally-spaced spiraling path across the surface of the specimen, at substantially constant contouring speed. The pad drive mechanism 21 is advantageously controlled to maintain constant pad pressure against the surface of the specimen 28. The pad drive mechanism 21 rotates the shaping pad 20 about a center axis (R).
In one embodiment, a four-axis positioning arrangement (“X”, “Y”, “Z”, “W”) guides the pad 20 along the desired spiraling path. In this embodiment, the positioning arrangement maintains the pad drive shaft 22 properly aligned with the normal of the surface 28. The positioning arrangement preferably includes the pad drive mechanism 21 and the surface support 34.
The surface support 34 preferably includes a surface holding block 30, a chuck 31, and a holding block drive mechanism 32. The surface holding block 30 is mounted onto the chuck 31 which rotates about a first axis (Z). The pad drive mechanism 21 traverses along a second axis, (X), such that the synchronized motion of the pad 20 along the first axis and the second axis provides a relatively spiraling trajectory with respect to the surface 28. The surface support 34 is adapted to move along a third axis (Y), which is parallel to the first axis (Z). This movement is used to adjust the position of the surface 28 and reciprocate any change in surface height. A fourth axis (W), coupled to the pad drive mechanism 21, allows for pivoting the pad drive mechanism so as to maintain the pad axis of rotation (R) substantially perpendicular to the surface 28.
The pad support 23 allows for the pad 20 to pivot in all directions about the pad rotation axis (R) so as to maintain the geometric plane of the pad oriented orthogonal to the surface contact point.
Tables A illustrates parameters used to facilitate the mathematical control procedure used with the control module of the apparatus. The control module comprises a dedicated personal computer (PC), an input-output digital interface board, an operating system for the PC with real-time extensions which allow for a deterministic interrupt handling response better than 10 microseconds, and a control program which executes low level and high level code.
Both levels of code are executed in a concurrent manner. The low level (real-time) code requires to be serviced by a fast responsive interrupt service routine, so as to be able to read all of the status conditions of the machine, and further being able to send control signals and commands to the different actuators and motors of the machine. High level (user) code provides an operator interface and advantageously uses the resources of the operating system that do not require a real-time response (math co-processor, graphics processor, communications, libraries, etc.).
Said user code is responsible to bring about a valid geometrical description of a tool path for processing an ophthalmic surface. Generating the tool path comprises three steps: a complete and precise description of the ophthalmic surface, a definition of the parameters of the spiraling path to be used across said surface and a description of the kinetic limits of the machine.
The descriptive parameters of the surface include: type of geometry (spherical, toric, aspherical, atoric, progressive, etc.), back and front curvatures of the blank specimen, diameter and thickness of the blank specimen, etc. Non-conventional surface geometries are preferably represented by NURB (non uniform rational B-splines) curves.
The machining parameters of the tool path include: tangential velocity, distance between parallel spiral arcs, rotating speed of polishing pad, etc. The kinetic parameters of each positioning axis include acceleration and velocity limits.
Table B illustrates high level user code which facilitates control of the pad positioning means.
Considering the above mentioned parameters, the high level user code generates incremental positioning motions to be executed by the low level real-time code. The motion increments for each positioning axis are simultaneously executed, by said low level code, within a corresponding time increment. The time increments used are preferably constant.
The control module employs coordinated motion between the four-axis positioning mechanism of
Motion control along the four axes X, Y, Z and W is facilitated by an open loop manner, preferably stepping motors, so as to obtain a reduced cost apparatus. However, the present invention contemplates the use of feedback position sensing devices on any axis to provide a higher accuracy closed loop control. The configuration and operation of such feedback sensing device would be apparent to those of ordinary skill in the art. In the illustrated embodiment, the pad drive mechanism 21 is preferably provided with a closed loop speed control, which maintains constant speed pad rotation without substantial fluctuations.
Speed information is preferably provided to the speed control means so as to maintain a constant rotational speed with minimal fluctuations. In one embodiment the system obtains a series of pulses from a Hall Effect sensor. The frequency of these pulses is proportional to the speed of rotation and is used to determined a rotation speed. However, as may be appreciated there are many other methods to obtain a tachometric signal.
The pad support 23 has a spherical surface 52, whereon the flexible pad 20 rests, with a base-curvature value. The present invention contemplates a set of various pad supports, each with a different curvature value, ranging from piano to 10 diopters (concave or convex) in constant increments of 2 diopters. Each individual pad support 23, in combination with a flexible pad 20, will cover a continuous subrange of curvatures to be shaped. The pad support curvature to be applied will depend on the desired lens properties.
The present invention further contemplates the use of pad supports with curvature values greater than 10 diopters (both, concave and convex). However, the limit value of 10 diopters, used at present, allows for the processing of practical prescription values.
Although the present invention was discussed in terms of certain preferred embodiments, the invention is not limited to such embodiments. A person of ordinary skill in the art will appreciate that numerous variations and combinations of the features set forth above can be utilized without departing from the present invention as set forth in the claims. Thus, the scope of the invention should not be limited by the preceding description but should be ascertained by reference to claims that follow.
