Rotation detection apparatus

A rotation detection apparatus comprising:

• a beam source for emitting a beam of polarised light;
• a detector for outputting a signal in dependence on the intensity of light which is incident thereupon, the detector being arranged so that the beam of polarised light is incident upon it;
• a polariser located in the path of the beam of polarised light such that rotation of the polariser relative to the beam of polarised light results in a variation of the intensity of light incident upon the detector from the beam source and a corresponding variation in the signal output from the detector;
• and
• a processor for detecting the relative rotation based on the variation of the signal output from the detector;
• wherein the beam of polarised light comprises first and second beam portions having first and second different respective polarisation states;
• wherein the apparatus comprises an arrangement for providing, for each of the first and second beam portions, first and second different relative orientations between a polarisation axis of the polariser and a polarisation direction of the polarised light of that beam portion, thereby correspondingly providing first to fourth different output signals from the detector relating respectively to:
• (1) the first relative orientation for the first beam portion;
• (2) the first relative orientation for the second beam portion;
• (3) the second relative orientation for the first beam portion;
• and (4) the second relative orientation for the second beam portion;
• wherein the processor is operable to detect the relative rotation based on the first to fourth output signals from the detector;
• and
• wherein the processor is operable to derive first and second values from the first to fourth output signals, and to detect the relative rotation based on a difference between the first and second values, wherein the first and second values are based respectively on (i) a difference between the first and second output signals and a difference between the third and fourth output signals;
• or (ii) a difference between the first and third output signals and a difference between the second and fourth output signals.

A rotation detection apparatus comprising:

• a beam source for emitting a beam of polarised light;
• a detector for outputting a signal in dependence on the intensity of light which is incident thereupon, the detector being arranged so that the beam of polarised light is incident upon it;
• a polariser located in the path of the beam of polarised light such that rotation of the polariser relative to the beam of polarised light results in a variation of the intensity of light incident upon the detector from the beam source and a corresponding variation in the signal output from the detector;
• and
• a processor for detecting the relative rotation based on the variation of the signal output from the detector;
• wherein the beam of polarised light comprises first and second beam portions having first and second different respective polarisation states, thereby correspondingly providing first and second different output signals from the detector, and wherein the processor is operable to detect the relative rotation based on a difference between the first and second output signals from the detector;
• and
• wherein the apparatus comprises a semi-transparent detector arrangement located in the path of the beam of polarised light before the polariser, such that the first and second beam portions are incident upon the arrangement and at least part of the incident light passes through the arrangement, the arrangement being adapted and arranged to influence the detection of relative rotation by the processor as a substantially linear function of the difference between the intensities of the first and second beam portions received at the arrangement, wherein the arrangement is adapted to measure an offset term of the function and to adjust the influence provided by the arrangement to counteract the effect of the offset term, and/or wherein the offset term does not drift significantly with environmental conditions and/or the wavelength of the first and second beam portions.