EP-4742603-A1 - METHOD FOR DEMODULATING AN AMPLITUDE MODULATED SIGNAL

Abstract

A method of demodulating an amplitude-modulated signal, which is the result of a baseband signal that has been amplitude-modulated onto a carrier signal having a carrier frequency, the method comprising: a) sampling the amplitude-modulated signal so as to obtain a series of sampled points comprising: i) a plurality of temporally-distinct sampled points each of which is obtained at a first phase with respect to the period of an unmodulated version of the carrier signal; and ii) a plurality of temporally-distinct sampled points each of which is obtained at a second phase with respect to the period of an unmodulated version of the carrier signal; b) using the sampled points of i) and ii) to form a reconstructed baseband signal.

Inventors

• The designation of the inventor has not yet been filed

Assignees

• Renishaw plc

Dates

Publication Date

20260513

Application Date

20241106

Claims (17)

1. A method of demodulating an amplitude-modulated signal, which is the result of a baseband signal that has been amplitude-modulated onto a carrier signal having a carrier frequency, the method comprising: a) sampling the amplitude-modulated signal so as to obtain a series of sampled points comprising: i) a plurality of temporally-distinct sampled points each of which is obtained at a first phase with respect to the period of an unmodulated version of the carrier signal; and ii) a plurality of temporally-distinct sampled points each of which is obtained at a second phase with respect to the period of an unmodulated version of the carrier signal, wherein the second phase is different to the first phase; b) using the sampled points of i) and ii) to form a reconstructed baseband signal.
2. A method as claimed in claim 1, in which the phase difference, with respect to the period of an unmodulated version of the carrier signal, between the first and second phases is 180°.
3. A method as claimed in claim 2, configured such that the sampled points of i) and ii) coincide with carrier peaks of the amplitude-modulated signal.
4. A method as claimed in any preceding claim, in which: the plurality of sampled points of i) are obtained at regular time-spaced intervals, such that a sample point is obtained at the first predetermined phase every N th cycle of an unmodulated version of the carrier signal; and/or the plurality of sampled points of ii) are obtained at regular time-spaced intervals, such that one sample point is obtained at the second predetermined phase every M th cycle of an unmodulated version of the carrier signal, where N and M are positive integers.
5. A method as claimed in claim 4, where N = M.
6. A method as claimed in any preceding claim, in which step b) comprises processing the sampled points to derive a reconstructed baseband signal which has reduced errors caused by non-linearities in the transfer function of the system for sensing and/or demodulating the amplitude-modulated signal.
7. A method as claimed in claim 6, in which the processing comprises averaging multiple sampled points.
8. A method as claimed in any preceding claim, in which the modulation depth of the amplitude-modulated signal is greater than 100%.
9. A method as claimed in any preceding claim, in which the amplitude-modulated signal is a signal output by a receiver coil of an inductive encoder apparatus.
10. A method as claimed in claim 9, in which the inductive encoder apparatus comprises: • a first member comprising a scale track; and • a second member comprising an excitation coil and the receiver coil; wherein: • the first and second members are relatively moveable along a measurement direction; and • the signal output by the receiver coil is induced in the receiver coil by an alternating carrier magnetic field generated by an alternating current passing through the excitation coil, the amplitude of the alternating carrier magnetic field being modulated by the scale track dependent on the relative position of the first and second members along the measurement direction.
11. A method as claimed in claim 10 in which the inductive encoder apparatus is a rotary encoder apparatus, wherein the first and second members are relatively rotatable about an axis of rotation, and wherein the measurement direction is a rotational measurement direction.
12. A method as claimed in claim 10 or 11, in which the excitation and/or receiver coil comprises a single-turn coil.
13. An inductive encoder apparatus comprising first and second members relatively moveable along a measurement direction, wherein: • the first member comprises at least a first scale track; • the second member comprises an excitation coil and at least a first receiver coil; • the inductive encoder apparatus is configured to: ∘ pass an alternating current through the excitation coil at a predetermined frequency so as to generate a carrier alternating magnetic field having a carrier frequency, the amplitude of which is modulated by the first scale track dependent on the relative position of the first and second members along the measurement direction, whereby an amplitude-modulated signal is induced in the first receiver coil by the alternating magnetic field as modulated by the first scale track; ∘ sample the amplitude-modulated signal induced in the first receiver coil so as to obtain: ▪ i) a plurality of temporally-distinct sampled points each of which is obtained at a first phase with respect to the period of the carrier alternating magnetic field; and ▪ ii) a plurality of temporally-distinct sampled points each of which is obtained at a second phase with respect to the period of the carrier alternating magnetic field; ∘ use the sampled points of i) and ii) to form a signal representative of the envelope amplitude of the first receiver coil's amplitude-modulated signal, and to use that signal in determining the relative position of the first and second members.
14. An apparatus as claimed in claim 13 in which the inductive encoder apparatus is a rotary encoder apparatus, wherein the first and second members are relatively rotatable about an axis of rotation, and wherein the measurement direction is a rotational measurement direction.
15. An apparatus as claimed in claim 13 or 14, in which the excitation comprises a single-turn coil and/or the receiver coil comprises a single-turn coil.
16. An apparatus as claimed in any of claims 13 to 15, in which the receiver coil comprises a differential coil.
17. A method of demodulating an amplitude-modulated signal, which is the result of a baseband signal that has been amplitude-modulated onto a carrier signal having a carrier frequency, the method comprising: a) sampling the amplitude-modulated signal so as to obtain sampled points which comprise: i) a plurality of temporally-distinct sampled points each of which is obtained at a first phase with respect to the period of an unmodulated version of the carrier signal; and ii) a plurality of temporally-distinct sampled points each of which is obtained at a second phase with respect to the period of an unmodulated version of the carrier signal; b) using the sampled points of i) and ii) to form a reconstructed version of the baseband signal that has reduced symmetrical error caused by non-linearities in the processing electronics for sensing and/or demodulating the amplitude-modulated signal.

Description

The present invention relates to a method of demodulating an amplitude modulated signal, in particular relates to a method of demodulating an inductive encoder signal, and relates to an apparatus (e.g. an inductive rotary encoder apparatus) configured to demodulate an amplitude-modulated (encoder) signal. Inductive encoders are known and typically comprise: i) a first member (e.g. what is commonly referred to as a "rotor", in the case of a rotary encoder) for mounting to a first part of a machine (e.g. such as a shaft that is rotatable relative to a static part of the machine about an axis); and ii) a second member (e.g. what is commonly referred to as a "stator", in the case of a rotary encoder) for mounting to a second (e.g. static) part of a machine. Typically, one of the members (e.g. the second member/"stator") is an active/powered component and comprises transmit (or "excitation") and receiver coils (and associated electronics) for creating and sensing an alternating magnetic/electromagnetic field. The terms "magnetic field" and "electromagnetic field" are used interchangeably herein because the magnetic fields referred to herein are created by an electric current and therefore can also be referred to as an electromagnetic field. Also, as will be understood, references herein to a magnetic field created by an excitation or transmit coil are references to an "alternating" magnetic field created thereby, and is often referred to herein simply as a "magnetic field" for brevity. Typically, the other member (e.g. the first member/"rotor") is a passive/unpowered component comprising scale features which manipulate the electromagnetic field sensed by the second member's (e.g. the stator's) sensor coils such that the output(s) of the second member's (e.g. the stator's) receiver coil(s) are dependent on the relative position (e.g. relative rotational orientation about an axis, in the case of a rotary encoder) of the first and second members about the axis. Accordingly, the relative (e.g. rotational) position (and/or derivatives thereof) of the second member/stator and first member/rotor (and hence of the different parts of the machine) can be measured from the output(s) of the second member's/stator's receiver coil(s). In a typical setup, the second member's (e.g. stator's) excitation coil generates an alternating "carrier" electromagnetic field (i.e. a "carrier signal") which alternates at a predetermined carrier frequency, and the first member (e.g. rotor) comprises scale features which manipulate the amplitude of the alternating carrier electromagnetic field (as sensed and output by the receiver coil(s)) dependent on the relative (e.g. rotational) position of the second member (e.g. stator) and first member (e.g. rotor) along the measurement direction (e.g. about the axis of rotation). Therefore the scale features modulate baseband information (i.e. a "baseband signal") on the alternating carrier electromagnetic field. Accordingly, the second member's (e.g. stator's) receiver coil(s) sense an amplitude-modulated alternating electromagnetic field and output an amplitude-modulated alternating voltage. The envelope of the amplitude modulated alternating voltage output by the receiver coil(s) is dependent on the relative (e.g. rotational) position of the second member (e.g. stator) and first member (e.g. rotor) along the measurement direction (e.g. about the axis of rotation), and the frequency of the amplitude modulated alternating voltage will be the same as the predetermined carrier frequency of the alternating carrier electromagnetic field. Accordingly, to determine and output a reconstructed baseband signal which can be used for position determination, the amplitude modulated alternating voltage output by the receiver coil(s) is demodulated. As mentioned earlier in this paragraph, the envelope amplitude of the amplitude-modulated alternating voltage output by the receiver coil(s), and therefore the demodulated/reconstructed baseband signal, is dependent on and hence indicative of the relative (e.g. rotational) position of the stator and rotor along a measurement direction (e.g. about the axis of rotation). Accordingly, the quality of the demodulated/reconstructed baseband signal can directly impact the positional accuracy of the output of the inductive (e.g. rotary) encoder. The present invention relates to an improved demodulation process. According to a first aspect of the invention there is provided a method of demodulating an amplitude-modulated signal, which is the result of a baseband signal that has been amplitude-modulated onto a carrier signal having a carrier frequency, the method comprising: a) sampling the amplitude-modulated signal so as to obtain a series of sampled points. The sampled points can comprise: i) a plurality of temporally-distinct sampled points each of which is obtained at a first phase (position/angle) with respect to the (e.g. the, or a notional) period of an unmodulated versi