ENCODER READHEAD

• US 20110316527A1
• Filed: 03/01/2010
• Published: 12/29/2011
• Est. Priority Date: 03/02/2009
• Status: Active Grant

• Current Assignee: Renishaw PLC, RLS merilna tehnika d.o.o. (Renishaw PLC)
• Original Assignee: Renishaw PLC, RLS merilna tehnika d.o.o. (Renishaw PLC)
• Inventors: Dolsak, Gregor, Ogrin, Aljaz, Janezic, Matjaz
• Granted Patent: US 9,797,751 B2
• Time in Patent Office: Days
• US Class Current: 324/207.11
• CPC Class Codes: G01D 5/2492 Pulse stream

Abstract

A readhead is provided for reading an absolute scale, optionally a passive magnetic scale, that encodes a series of data bits. The readhead includes a plurality of sensors, such as an array of Hall sensors, for producing a plurality of sensor signals. A plurality of signal combiners are also provided to receive at least two of the sensor signals and produce therefrom a combined sensor signal. A plurality of data bit extractors are arranged to receive at least two combined sensor signals and to determine the value of a data bit encoded in an associated absolute scale. The readhead also comprises an incremental signal generator for generating at least one incremental signal from the combined sensor signals produced by a plurality of the signal combiners. In this manner, both absolute and incremental position is measured.

Claims

1. A readhead for reading an absolute scale encoding a series of data bits, the readhead comprising;

a plurality of sensors for producing a plurality of sensor signals;

a plurality of signal combiners, each signal combiner arranged to receive at least two of the sensor signals and produce therefrom a combined sensor signal; and

a plurality of data bit extractors, wherein each data bit extractor is arranged to receive at least two combined sensor signals and to determine the value of a data bit encoded in an associated absolute scale,

wherein the readhead comprises an incremental signal generator for generating at least one incremental signal from the combined sensor signals produced by a plurality of the signal combiners.

2. A readhead according to claim 1 suitable for reading an absolute magnetic scale, wherein the plurality of sensors comprises a plurality of magnetic sensors.

3. A readhead according to claim 1, wherein the at least one incremental signal comprises a pair of quadrature phase signals.

4. A readhead according to claim 1, wherein the incremental signal generator applies a weighting to the plurality of combined sensor signals when generating the at least one incremental signal.

5. A readhead according to claim 1, comprising a first interpolator for interpolating the at least one incremental signal to provide a first interpolated signal.

6. A readhead according to claim 5, wherein each data bit extractor determines a data bit value by analysis of a selected one of the at least two combined sensor signals received thereby, wherein each data bit extractor receives the first interpolated signal and selects which combined sensor signal to analyse based on the first interpolated signal.

7. A readhead according to claim 1, comprising

a second interpolator for interpolating the at least one incremental signal to provide a second interpolated signal;

and

an absolute position calculation unit for combining incremental position data from the second interpolated signal with absolute position data from the plurality of data bit extractors,

wherein the absolute position calculation unit generates absolute position data having a finer resolution than the bit length of an associated absolute scale read by the readhead.

8. A readhead according to claim 1, wherein the plurality of sensors are substantially evenly spaced apart from one another.

9. A readhead according to claim 1 for reading an absolute scale that comprises a series of data bits of bit length L, wherein the plurality of sensors of the readhead are spaced apart so as to provide M sensors per bit length L, wherein M is an integer of two or more.

10. A readhead according to claim 9 for reading an absolute scale that comprises a series of codewords formed from N data bits, N being an integer value of two or more, wherein the readhead comprises J sensors, J being greater than or equal to the product of N and M.

11. A readhead according to claim 1, wherein the combined sensor signal generated by each signal combiner is derived from the difference between a pair of sensor signals produced by a selected pair of the plurality of sensors, wherein each such selected pair of sensors is spaced apart by substantially half the bit length L of the series of data bits of the associated absolute scale.

12. A readhead according to claim 1, wherein each signal combiner generates its combined sensor signal by combining the sensor signals from a plurality of adjacent sensors.

13. A readhead according to claim 12, wherein the plurality of signal combiners generate a plurality of combined sensor signals (Si) from the plurality of sensor signals (Hi) produced by the plurality of sensors in accordance with the summation scheme:

S_i=H_i−

H_i+1−

H_i+2+H_i+3;

i=1 . . . (4·

N)

wherein N is the number of data bits in each codeword encoded by the associated absolute scale, wherein the readhead comprises 4N+3 sensors, wherein the sensors are spaced apart so as to provide four sensors per bit length L of the associated absolute scale.

14. A readhead according to claim 1, wherein the plurality of sensors comprises a plurality of magnetic sensors and the readhead further comprises a magnet.

15. An encoder device comprising, a readhead according to claim 1 and an absolute scale, wherein the absolute scale comprises a series of data bits of bit length L and the readhead comprises an integer number of sensors per bit length L.

16. An encoder device according to claim 15, including a passive absolute magnetic scale comprising a scale member and a plurality of scale markings for encoding a series of data bits, wherein the scale markings have a different magnetic permeability than the scale member.

17. An encoder device according to claim 16, wherein the passive absolute magnetic scale comprises scale markings in the form of a plurality of grooves, the plurality of grooves comprising grooves of at least a first groove type and a second groove type, the magnetic permeability of the passive absolute magnetic scale in the locality of grooves of the first groove type being different than the magnetic permeability of the passive absolute magnetic scale in the locality of grooves of the second groove type, wherein a groove of the first groove type indicates a data bit having a first value and a groove of the second groove type indicates a data bit having a second value.

18. An encoder device according to claim 15, wherein the absolute scale comprises a rod.

19. A readhead for reading an absolute magnetic scale encoding a series of data bits, the readhead comprising;

a plurality of magnetic sensors for producing a plurality of sensor signals;

a plurality of signal combiners, each signal combiner arranged to receive at least two of the sensor signals and produce therefrom a combined sensor signal; and

a plurality of data bit extractors, wherein each data bit extractor is arranged to receive at least two combined sensor signals and to determine the value of a data bit encoded in an associated absolute scale.