The present invention relates to a method of making metrological scale for scale reading apparatus. In particular, the invention relates to a method of making scale by electrochemical machining.
A known form of scale reading apparatus for measuring relative displacement of two members comprises a scale on one of the members having scale marks defining a pattern and a readhead provided on the other member. An optical scale reading apparatus has means for illuminating the scale and detecting means in the readhead responsive to a resultant light pattern to produce a measure of relative displacement of the scale and readhead. A scale having its marks in a periodic pattern is known as an incremental scale and provides an output of up and down counts. A scale may be provided with reference marks which, when detected by the readhead, enable the exact position of the readhead to be determined. The scale may have absolute code marks which enable the absolute position of the readhead to be determined anywhere on the scale.
Scale and readhead systems are not limited to optical systems. Magnetic, capacitance and inductive reading systems are also known.
Metrological scales may for example be linear, rotary or two-dimensional. Rotary scales may have the scale markings provided radially on a face or axially on the circumference of a rotary part.
The scale may be an amplitude scale or a phase scale. In the amplitude scale the scale pattern is made from two different types of sections. A first type of section reflects incident light to the readhead and the second type of section does not. For example an incremental amplitude scale may comprise alternate reflecting and non-reflecting lines, such as chrome on glass scale.
A phase scale has a form that reflects light from different sections at different phases when detected at the readhead.
European Patent EP 0274492 discloses a method of making a scale by passing an elongate scale member between a pair of cylindrical rollers, one roller having a plane surface and the other roller having a profiled surface. As a scale member is passed through the rollers and pressure is applied, a surface of the scale member is deformed with the pattern of the profiled roller.
A known technique for forming a workpiece with a specified shape dimension and surface finish is electrochemical machining. This is the controlled anodic electrochemical dissolution process of a workpiece (anode) with a tool (cathode) in an electrolytic cell during an electrolysis process.
A first aspect of the invention provides a method for making a metrological scale by electrochemical machining of a scale substrate using a tool having at least one feature, the method comprising the following steps, in any suitable order:
Preferably the tool and scale substrate are moved relative to one another.
The one or more sensor may be provided for sensing scale markings generated by the tool, and the method includes the step of:
At least one of the one or more sensor senses the pitch, depth or width of the scale markings. The at least one of the one or more sensor may be used to determine whether any scale markings can be detected. The feedback may also used for relative placement of the tool and scale substrate for the creation of subsequent scale markings.
The reflectivity of the scale markings may be selected by adjusting the system parameters. The electrochemical dissolution may occur between scale markings to produce a polished surface. This may occur in a separate step.
In one embodiment, the method includes a tool refurbishment step. The tool refurbishment step may comprise an electrochemical machining method. The extent of protrusion of the at least one feature from the tool may adjustable. The at least one feature may be tapered.
The tool may be moveable relative to the scale substrate such that movement of the tool assists in flushing through the electrolyte.
The tool may be moved relative to the scale substrate in steps in order to produce a repeating pattern of scale markings. The tool is moved continuously relative to the scale substrate in order to produce a repeating pattern of scale markings.
The electrical connection between the scale substrate and tool may be pulsed. The pulse rate of the electrical connection may be synchronised with the relative movement of the tool and scale substrate. The method may include the step of controlling the pulse parameters in response to feedback from the one or more sensor.
The electrical connection between the scale substrate and the tool may be a continuous current.
A second aspect of the present invention provides apparatus for making a metrological scale by electrochemical machining of a scale substrate comprising;
The scale may comprise a phase scale or an amplitude scale.
The invention will now be described with reference to the accompanying drawings in which:
Direct current is passed between the scale substrate 12 (anode) and the tool 14 (cathode), the direct current may be either continuous or pulsed. At the surface of the anodic scale substrate 12, metal is dissolved into metallic ions by a de-plating reaction. The shape of the tool 14 is thus substantially copied onto the scale substrate 12.
The tool 14 is moved along the scale substrate 12 and at each position a current is applied between the anode and cathode so that trenches are selectively etched into the scale substrate 12. The shape of the metal elements of the tool is substantially copied into the scale, thus a desired shape may be chosen. The shape of the elements may be only loosely copied onto the scale substrate. The tool may be stepped relative to the scale substrate by servo positioning one of the tool and scale substrate. The servo positioning may be velocity controlled.
If both the trenches and upper surface of the scale are reflective, then a phase scale will be produced. However, if the scale substrate is treated or manufactured so that one of the trench and upper surface is reflective and the other is non reflective, then an amplitude grating will be produced.
As the tool is moved along the scale, it is important that the tool is at the correct position when the current is applied. This may be achieved by providing an encoder 28 to measure relative position of the scale substrate and tool. Alternatively, a readhead system may be connected to the tool, so that as the system is passed over the scale markings that have been produced, the output of the readhead is used as feedback to position the tool in order that the next scale markings are produced in the correct relationship with the previous scale markings. Other sensors can be used to read the scale markings, for example a camera or surface profiler.
The sensors may also be used to determine whether the scale markings are of sufficiently good quality to be detected. This quality of the scale markings may be effected by debris, such as metal hydroxide produced by the de-plating reaction and not adequately removed by the flow of electrolyte.
If the metal elements of the tool are spaced at the pitch of the scale, then the tool is moved so that a section of the scale is made at a time. However if the pitch of the metal elements is a multiple of the pitch of the scale, then the tool must be moved in small steps so that a scale is produced with the desired pitch. Alternatively, an averaging technique may be used so that the trenches are built up on the scale substrate with successive pulses from metal elements as the tool is moved along the scale substrate.
Several system parameters effect the dimensions and quality of the scale markings produced by this method. For example, it is advantageous to have a small gap between the scale substrate and tool, typically in the range of 50-300 μm. The smaller the gap, the more defined the features on the scale substrate will be. However, a small gap may result in the electrolyte boiling and producing hydrogen gas at the anode, thus inhibiting the de-plating reaction.
This problem is overcome by monitoring the scale markings and using this information to adjust system parameters, thus maintaining optimum conditions for the production of well defined features on the scale substrate.
Parameters such as the working voltage and current between the anode and cathode, the flow rate and temperature of the electrolyte, the interval between the tool and scale substrate and the interval between electrical pulses can be closed loop controlled by feedback from the sensor reading the scale markings so that a consistent depth of material is removed from the scale substrate and so that the scale markings have the desired dimensions (e.g. width, depth and shape of feature).
The system parameters are measured and most are measured by conventional means, whether directly or indirectly.
These measurements of the gap may be verified by touching the tool against the scale substrate and thus providing a zero value for the gap. It can be determined when the tool is touching the scale substrate by measuring the current flowing through the system which will increase when the tool and scale substrate are in contact. Alternatively, as illustrated in
In a second embodiment of the invention, the tool has the same profile as that described in
In this method, each trench on the scale is machined by multiple tool elements. However, it is possible to use a single tool finger to produce the scale markings.
The elements are not necessarily isolated from the rest of the tool.
The polishing may take place in a separate step, e.g. the scale may be polished (e.g. by electrochemical machining) in a first step and the scale marks created in a second step (or vice versa). By adjusting the system parameters, different levels of reflectivity can be achieved. For example a slower removal rate provides a more reflective finish. Thus the reflectivity can be adjusted making this technique suitable for both amplitude and phase scales.
A third embodiment of the invention is described with reference to
Current between the cathode (rotating tool wheel 34) and anode (scale substrate 32) is pulsed as the rotating tool wheel 34 is continuously rolled over the scale substrate 32. A reference encoder is provided to synchronise the current pulse rate with the movement of the scale substrate 32 and rotating tool wheel 34. The reference encoder could comprise for example a rotary encoder 46 provided on the rotating tool wheel 34 which is linked to a linear encoder 48 on the moving stage 33.
Alternatively as illustrated in
In the embodiments illustrated in
It may be desirable to be able to adjust the pitch of the scale markings.
Any of the previous embodiments are suitable for making both continuous scale and short lengths of scale.
Although the embodiments describe moving the tool relative to the scale, it is also possible to move the scale on a stage and fix the tool.
The design of the tool will now be described in further detail.
The tool may contain channels through which the electrolyte is delivered to the scale substrate.
Electro chemical machining is also suitable for forming two dimensional scales. For small scales the tool may be provided with elements in the desired scale pattern, and the scale is formed in a single step.
For larger scales, a step and repeat method is suitable.
Correct positioning between the tool and scale substrate may be achieved using an encoder, with respective parts placed on the tool and scale substrate.
Electrochemical machining is also suitable for manufacturing rotary scales. Rotary scales may either have axial scale markings on an edge or radial markings on a face.
As the tool wheel 86 and scale disc 90 are rotated, scale markings will be formed on the edge of the scale disc where it is adjacent to the elements 88 on the tool wheel.
It is important to control the relative speeds of the tool wheel and scale disc, although these speeds are not necessarily the same.
It is advantageous if the number of elements 88 on the tool wheel is not a factor of the scale pitch. Thus as the scale markings are built up over several rotations, a different tool element will act on each scale marking during each rotation. Thus any errors due to the spacing of the scale marking are averaged out.
The tool may be arranged to have elements which are not spaced at every scale pitch (e.g. they may be spaced every 20th or 50th scale marking), so that the intervening scale markings are filled in during subsequent rotations. This has the advantage that errors caused by indexing (i.e. relative positions of rotation of the scale disc and tool wheel) are averaged out.
The scale disc may have a fundamental error trace, for example as illustrated in
Both the scale disc and the tool wheel are provided with encoding means so that their relative speeds can be determined.
A readhead may be provided to read the scale markings produced on the rotary disk, as illustrated in
Scale markings may also be produced on a face of a scale disc.
On rotation of the scale disc and tool wheel, scale. markings will be produced. These will be parallel lines with tapered gaps.
In an alternative embodiment suitable for both linear and rotary scales, the tool has a single element mounted on a piezo electric stage. As the scale substrate moves relative to the tool, the piezo electric stage will move in a saw tooth profile so that the element keeps stationary with respect to the scale marking being produced. The element then returns to its original position and repeats this process for the next scale marking. The stage may include a feeder to feed the element towards the scale substrate should the element degrade.
Although the embodiments above describe an incremental scale pattern, these methods are also suitable for providing other patterns, such as reference marks or absolute scale patterns. In this case the scale elements may be arranged in the desired pattern, so that this pattern is imprinted onto the scale.
A reference mark may be provided in the scale by several methods. In a first method, the tool may not receive an electric pulse when positioned over a region of the scale substrate, so that no scale markings are formed in that region.
A chirp scale (of varying pitch) may be formed using the embodiment illustrated in FIGS. 2,3 and 9 by varying the speed of the tool wheel relative to the scale substrate. This chirp scale may be used as a reference mark.
Absolute scale patterns can be formed on the scale substrate by a variety of techniques. A simple technique is to provide the tool with elements positioned in the pattern which is to be copied onto the scale substrate.
An absolute pattern can be formed on the scale substrate using the embodiments illustrated in FIGS. 2,3 and 9 by continuously rotating the tool wheel and pulsing the tool only when scale markings are desired. By rotating the tool wheel at twice the rate of rotation of the scale disc, scale markings can be placed side by side on the scale disc, thus producing a region of continuous scale markings.
For the formation of an iron based scale substrate, a suitable electrolyte is a chloride solution in water, for example a 10% NaCl solution. The tool material may typically comprise copper, brass or steel. An advantage of this method is that there is very little tool wear. The gap between the tool features and scale substrate is typically 50-3000 μm.
The system typically works when a voltage range of 5-300V and a current range of 50-40,000 A.
The electrolyte is typically at a temperature of 20-50° C., with a flow rate of typically 1 L/Min/100 A, a velocity of 1500-3000M/Min, inlet pressure of 0.15-3 Mpa and outlet pressure of 0.1-0.3 Mpa.
As the tools in the embodiments described have small elements, it is advantageous to provide a tool regeneration process for keeping the tool elements in optimum condition.
A rotating tool wheel of the type described with reference to FIGS. 2,3 and 9 may include a tool refurbishment station so that the elements may be refurbished during each rotation.
This embodiment has the further advantage that by adjusting the gap between the scale substrate and tool, the amount of de-plating at the scale substrate can be controlled, thus controlling the shape of the scale substrate. The tool can thus be servoed in Z as the scale substrate moves relative to the tool, to create the desired scale profile.
Thus large sinusoidal relative movement of the tool provides flushing of the electrolyte whilst small movement enables the gap between the scale substrate and tool to be adjusted. This relative movement of the tool and scale substrate up and down may be controlled by closed loop feedback or open loop motion.
Electrochemical machining forms a good surface finish, unlike processes such as chemi-etching which tends to leave a pitted granular surface. Electrochemical machining is a good method for polishing surfaces. It can be used to provide a highly reflective surface on both the crests and troughs of the scale, thus being suitable for producing phase scales.
Number | Date | Country | Kind |
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0416952.0 | Jul 2004 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/GB05/03008 | 8/1/2005 | WO | 00 | 1/30/2007 |