EP-4735835-A1 - ENCODER APPARATUS

Abstract

A position encoder apparatus comprising a scale and a readhead comprising at least one scale-signal sensor configured to detect a scale signal from which at least one position signal indicative of the relative position of the scale and readhead along the scale's measuring direction is generated and output, in which the encoder apparatus is configured to compensate for a change in the relative pitch of the readhead and scale so as to reduce pitch-induced position-displacement error in the at least one position signal.

Inventors

• HENSHAW, JAMES, REYNOLDS
• GORDON-INGRAM, IAIN, ROBERT
• REEVE, Theodore, Richard
• CLUFF, JULIAN, ALEXANDER

Assignees

• Renishaw plc

Dates

Publication Date

20260506

Application Date

20240626

Claims (19)

1. CLAIMS: 1. A position encoder apparatus comprising a scale and a readhead comprising at least one scale-signal sensor configured to detect a scale signal from which at least one position signal indicative of the relative position of the scale and readhead along the scale’s measuring direction is generated and output, in which the encoder apparatus is configured to compensate for a change in the relative pitch of the readhead and scale so as to reduce pitch-induced position- displacement error in the at least one position signal.
2. 2. A position encoder apparatus as claimed in claim 1, in which the encoder apparatus is configured to monitor continuously, as the scale and readhead move relative to each other, for an indication of a change in the relative pitch of the readhead and scale, and wherein said compensation is based on the outcome of said monitoring.
3. 3. A position encoder apparatus as claimed in claim 2, in which compensating for a change in the relative pitch of the readhead and scale so as to reduce pitch-induced position-displacement error in the at least one position signal comprises, based on the outcome of said monitoring, advancing or retarding the relative position indicated by the at least one position signal.
4. 4. A position encoder apparatus as claimed in any preceding claim, in which said monitoring comprises analysing the deformation of a scale signal falling on a sensor.
5. 5. A position encoder apparatus as claimed in claim 4, in which said monitoring comprises analysing the deformation of a signal falling on the at least one scale-signal sensor which is configured to detect a scale signal from which at least one position signal indicative of the relative position of the scale and readhead is generated.
6. 6. A position encoder apparatus as claimed in claim 5, in which said monitoring comprises analysing the deformation of the scale signal from which the at least one position signal indicative of the relative position of the scale and readhead is generated.
7. 7. A position encoder apparatus as claimed in any preceding claim, which is further configured to determine a separation signal indicative of the separation between the readhead and scale.
8. 8. A position encoder apparatus as claimed in claim 7, wherein the separation signal indicative of the separation between the readhead and scale is factored into said compensating for a change in the relative pitch of the readhead and scale so as to reduce pitch-induced position-displacement error in the at least one position signal.
9. 9. A position encoder apparatus as claimed in any preceding claim, in which the position encoder apparatus is an absolute encoder apparatus.
10. 10. A position encoder apparatus as claimed in any preceding claim, in which the readhead is configured to perform said compensation.
11. 11. A position encoder apparatus comprising a scale and a readhead comprising at least one scale-signal sensor configured to detect a scale signal from which at least one position signal indicative of the relative position of the scale and readhead along the scale’s measuring direction can be generated, in which the position encoder apparatus is configured to generate at least one such position signal from a scale signal detected by the scale-signal sensor and also is configured to: i) determine a separation signal indicative of the separation between the readhead and scale based on a scale signal; and/or ii) determine a pitch signal indicative of the relative pitch of the readhead and scale based on a scale signal.
12. 12. A position encoder apparatus as claimed in claim 11, in which the encoder apparatus is configured to determine said separation signal and/or said pitch signal based on a scale signal sensed by said at least one scale-signal sensor.
13. 13. A position encoder apparatus as claimed in claim 11 or 12, in which the encoder apparatus is configured to determine: a) said separation signal and/or said pitch signal, and b) said at least one position signal, based on the same scale signal sensed by said at least one scale-signal sensor.
14. 14. A position encoder apparatus as claimed in any of claims 11 to 13, in which the encoder apparatus is configured to determine said separation signal and/or said pitch signal based on the size of a feature of said scale signal.
15. 15. A position encoder apparatus as claimed in any of claims 11 to 14, in which the encoder apparatus is configured to determine said separation signal and/or said pitch signal based on the period size of said scale signal.
16. 16. A position encoder apparatus as claimed in any of claims 11 to 15, in which the encoder apparatus is configured to determine said pitch signal based on the deformation of said scale signal.
17. 17. A position encoder apparatus as claimed in claim 16, in which the encoder apparatus is configured to determine said pitch signal based the deformation of said scale signal along the measuring dimension.
18. 18. A position encoder apparatus as claimed in any of claims 11 to 17, in which said scale signal has periodic content, and in which determining said separation signal and/or determining said pitch signal comprises analysing the periodic content of the signal falling on the sensor.
19. 19. A position encoder as claimed in 18 when dependent on claim 16 or 17, in which determining said pitch signal comprises analysing how the period of the signal varies across the sensor, in particular along the measuring dimension.

Description

ENCODER APPARATUS This invention relates to an encoder apparatus, in particular a position measurement encoder apparatus comprising a scale and a readhead which are moveable relative to each other. As is well known, a position measurement encoder apparatus typically comprises a scale having a series of features which the readhead can read to determine and measure relative position (and/or its derivatives such as velocity and/or acceleration). Encoders are typically categorised as being either incremental or absolute. A scale for an incremental encoder comprises a series of generally periodic features which the readhead detects to determine a relative position and movement of the scale and readhead. One or more reference marks can be provided on the scale to provide reference positions from which the relative position of the scale and readhead can be counted. A scale for an absolute encoder comprises features defining unique positions along the scale length, for instance a regular (e.g. continuous) series of unique absolute positions, and can enable a readhead to determine its absolute position on start-up without requiring any relative motion. A position encoder apparatus can be sensitive to changes in the relative pitch of the scale and readhead. For instance, a change in the relative pitch of the scale and readhead could cause a change/displacement in the signal detected by the readhead, which could be incorrectly interpreted and reported as a change in the relative position/displacement of the readhead and scale along the position encoder apparatus’ measurement dimension, thereby leading to a pitch-induced position-displacement error. As will be understood, the term “pitch” in this document adopts the normal meaning of “pitch” in the established right-hand rule for the “pitch”, “roll” and “yaw” rotations of an object movable along a direction of travel (in this case, along the readhead’s/position encoder’s measuring dimension). Accordingly, relative “pitch” of the scale and readhead is rotation about a transverse/lateral axis, i.e. an axis that extends perpendicular to the measuring dimension/direction of travel and also extends perpendicular to the readhead’s “vertical” or “rideheight” dimension (i.e. the dimension of separation of the readhead and scale, which as will be understood will be normal/perpendicular to the scale). Accordingly, when the readhead and scale are installed, a change in the relative pitch is a change in the relative angular orientation of the readhead and the scale, at least at the point of measurement on the scale, about an axis that lies parallel to the scale at the point of measurement and extends perpendicularly to the scale’s measuring direction. As will also be understood, a change in relative pitch of the scale and readhead could be caused by a rotation of the readhead and/or scale as a whole, or for instance due to a change in the gradient of the scale along the measurement direction, e.g. due to scale undulations. Indeed, once the position encoder apparatus has been installed, the most likely cause of a change in the relative pitch of the scale and readhead at the point of measurement is a change in gradient of the scale due to undulations in the scale. The readhead of a position encoder apparatus might have a “pitch-insensitive axis”, which is an axis, about which the relative pitch of the scale and readhead may be changed without causing a position-displacement error in the signal detected by the readhead, and hence without causing a position measurement/displacement error. For example, when installed, the pitch- insensitive axis will typically be parallel to the scale and extend perpendicularly to the scale’s measuring direction. As will be understood, for rotary encoders having a ring-scale, (i.e. scale formed on the circumferential edge of a ring) the orientation of the pitch-insensitive axis to the scale will be as measured at the current readhead reading location. As explained in the previous paragraph, a likely source of relative pitch is a local change in gradient of the scale. Accordingly, the pitch-insensitive axis could be referred to as a “scale-gradient- insensitive axis”. Some known encoders, such as the TONiCTM and ATOMTM encoders available from Renishaw plc have a design which means that when installed correctly, the “pitch-insensitive axis” lies on the surface of the scale. Accordingly, in theory, a change in the relative pitch of the scale and readhead, e.g. a variation in the gradient of the scale along its length, should not cause a pitch-induced position- displacement error. However, even if such an encoder apparatus is set up such that the accuracy of their position signals are not sensitive to changes in the relative pitch of the scale and readhead, changes in the relative configuration of the readhead and scale during use (e.g. changes to the readhead’s “ride-height”, which is the distance between the scale and readhead measured normal to the