WO2007135673A1 - Angular position sensor with semicircular shielding plate - Google Patents

Angular position sensor with semicircular shielding plate Download PDF

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Publication number
WO2007135673A1
WO2007135673A1 PCT/IL2007/000610 IL2007000610W WO2007135673A1 WO 2007135673 A1 WO2007135673 A1 WO 2007135673A1 IL 2007000610 W IL2007000610 W IL 2007000610W WO 2007135673 A1 WO2007135673 A1 WO 2007135673A1
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WO
WIPO (PCT)
Prior art keywords
shielding plate
angular position
rotation
photodetector
shaft
Prior art date
Application number
PCT/IL2007/000610
Other languages
French (fr)
Inventor
Igor Friedland
Yaakov Gaber
Original Assignee
Elbit Systems Electro-Optics Elop Ltd.
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Publication date
Application filed by Elbit Systems Electro-Optics Elop Ltd. filed Critical Elbit Systems Electro-Optics Elop Ltd.
Publication of WO2007135673A1 publication Critical patent/WO2007135673A1/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
    • G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
    • G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
    • G01D5/342—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells the sensed object being the obturating part
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D3/00—Indicating or recording apparatus with provision for the special purposes referred to in the subgroups
    • G01D3/028—Indicating or recording apparatus with provision for the special purposes referred to in the subgroups mitigating undesired influences, e.g. temperature, pressure
    • G01D3/036—Indicating or recording apparatus with provision for the special purposes referred to in the subgroups mitigating undesired influences, e.g. temperature, pressure on measuring arrangements themselves
    • G01D3/0365—Indicating or recording apparatus with provision for the special purposes referred to in the subgroups mitigating undesired influences, e.g. temperature, pressure on measuring arrangements themselves the undesired influence being measured using a separate sensor, which produces an influence related signal
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
    • G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
    • G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
    • G01D5/347—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells using displacement encoding scales
    • G01D5/3473—Circular or rotary encoders

Definitions

  • the present invention relates to the field of non-contact angular position sensors, especially for use in galvanometric scanning systems.
  • Angular position sensors especially for use in galvanometric scanning systems have been proposed in the prior art.
  • Angular detection of high accuracy has, for example, been achieved using variable-inductance angular sensors, variable-capacitance angular sensors or analog optical angular position sensors.
  • Variable inductance angular position sensors such as described in U.S.
  • Patent No. 5,561 ,375 tend to be expensive, difficult to manufacture and physically large because of the size of the inductor elements.
  • Examples of angular sensors of the variable capacitance type have been described in U.S. Patent Nos. 4,142,144, 4,268,889 and 5,099,386. These sensors provide good accuracy in angular position measurement, but they are relatively expensive because they include circuits for modulation and demodulation of a high frequency carrier.
  • An optical angular position sensors has been described in U.S. Patent No. 5,235,180, and is used in measuring the mirror angle in a laser scanner.
  • the light receiving photo detector is a low cost, compact, light-weight and inexpensive device.
  • the sensing surface of the photo detector is divided into either two or four photosensitive sectors called A and B or A, B, C and D.
  • a and B photosensitive sectors
  • A, B are voltages corresponding to the intensities of light detected at the A and B sectors.
  • Such an angular sensor has a number of disadvantages: 1.
  • An induced noise voltage, designated by ⁇ decreases the accuracy of the sensor because if the voltages A and B are replaced by values A+ ⁇ and B+ ⁇ , the value of ⁇ given by equation (1) becomes:
  • the present invention seeks to provide a new angular position sensor, especially for use in a galvanometric scanning system, and including novel use of a sectorized photodetector with an illumination shielding plate attached to the shaft whose rotation is to be sensed.
  • the photodetector has two preferably diametrically opposed sectors, disposed such that the incident illumination from a source is differentially modulated by rotation of the shielding plate, and at least a third sector which is always illuminated by the source, regardless of the rotational position of the shielding plate.
  • the outputs from the diametrically opposed sectors are used to determine the rotational angle of the shaft, while the output from the third sector is used to stabilize the illumination source, such that the outputs of the two opposed sectors of the detector are insensitive to changes in temperature.
  • This arrangement enables accurate temperature compensation of the angular position sensor to be achieved, the need for temperature compensation arising because of significant changes in the sensitivity of the photodetector with temperature.
  • an angular position sensor comprising: (i) an input shaft whose rotation conveys the angular position,
  • a shielding plate attached to the shaft with its plane generally perpendicular to the axis of rotation of the input shaft, the plate covering a part of the plane of rotation of the shaft, (iii) a sectorized photodetector disposed in proximity to the shielding plate, such that rotation of the shaft causes the shielding plate to rotate over the sectorized surface of the photodetector, and
  • an illumination source directing its illumination generally in the direction of the surface of the photodetector, such that the shielding plate modulates the illumination falling on the photodetector surface in accordance with the rotation of the shaft
  • the photodetector comprises a first and a second sector disposed such that incident illumination falling thereon is differentially modulated by rotation of the shielding plate, and at least a third sector disposed such that incident illumination falling thereon is unaffected by rotation of the shielding plate.
  • the at least third sector preferably provides an output signal utilized to control the luminous output of the illumination source.
  • the disposition of the at least third sector on the same photodetector substrate as the first and second sectors enables the control of the luminous output of the illumination source to compensate for changes with temperature of the optical power of the illumination source and of the sensitivity of the first and second sectors of the photodetector.
  • an angular position sensor as described above and wherein the first and second sectors provide a differential output signal proportional to the rotation of the input shaft.
  • the shielding plate preferably has a generally semicircular form. Additionally, the shielding plate may preferably have a profile edge which passes through the axis of rotation of said shaft.
  • the illumination source may preferably be either a LED, a laser diode, or an incandescent lamp.
  • Fig. 1 is a schematic illustration of a angular position sensor, constructed and operative according to a first preferred embodiment of the present invention
  • Fig. 2 is a schematic view from the direction of the axis of the shaft, showing the geometrical structure and positions of the quadrants of the photodetector and the shield plate of the embodiment of Fig. 1
  • Fig. 3 is a schematic electronic circuit diagram, illustrating how the photodetector sector output signals are used to determine the angular setting of the shaft of Fig. 1;
  • Fig .4 is a graph showing the effect of temperature changes on the output voltage of the output amplifier of the common sector of the detector shown in Fig. 3.
  • Fig. 1 is a schematic illustration of a angular position sensor, constructed and operative according to a first preferred embodiment of the present invention.
  • the shaft 10 of the angular position sensor has its angular rotation input from the galvanometer motion 12, not shown in Fig. 1.
  • a mirror 14, such as for use in an optical scanning system which the galvanometer is driving, is attached rigidly to the shaft, such that its reflective plane rotates with rotation of the shaft.
  • a shield plate 16 is rigidly attached to the shaft, preferably at its end, either by direct affixation to the end, as shown in Fig. 1 or by means of an attachment strip which attaches the plate along the length of the shaft, or in any other suitable manner.
  • a photodetector 18 is disposed beyond the end of the shaft, such that when the shaft rotates, the shield plate 16 rotates in front of the fixed photo-detector.
  • a light source 20, such as a light emitting diode, LED, or a laser diode, is positioned such that its illumination is projected past the shield plate and onto the photodetector.
  • the light source 20 can be disposed either coaxial with the shaft or adjacent to the shaft, so long as it directs its illumination reasonably axially through the plane of the shield plate and onto the photodetector.
  • Fig. 2 is a schematic view from the direction of the axis of the shaft, showing the geometrical structure of the sensitive surface of the photodetector 18, and the mutual positional relationship between the shield plate 16, which rotates with the galvanometer shaft, and the fixed photodetector 18.
  • the profile of the shield plate 16 is such that it shields the light from reaching the detector surface over part of the angular revolution of the shaft, and preferably has a semicircular shape. Any other shape is acceptable, however, on condition that the top edge 17 clearly delineates those regions of the detector surface receiving radiation, and preferably passes through the center of rotation of the plate.
  • the photodetector 18 is preferably of the common quadrant divided type, having 4 quadrant sectors 22, 24, 26 and 28.
  • the detector is aligned such that when the shaft is in its null position, the shield plate 16 is positioned to provide approximately equal exposure of diametrically opposite sectors 22 and 24 to the incident illumination, which arrives from the general direction of the axis of the shaft 10. Rotation of the shield plate is limited such that the central sector 26 of the photodetector is always fully exposed to the illumination. Sector 28, on the other hand, is always shielded, and therefore essentially unused. A photodetector having only three sectors could thus be used in the present invention, but such types are not generally available. Rotation of the shaft 10, for instance anticlockwise, thus results in increase of the illumination on one of the opposing sectors 24, and a corresponding decrease in the other 22.
  • the position of the light source 20, if not coaxial with the shaft should be symmetrical to the diametrically opposite sectors 22 and 24, to ensure linearity, and should thus be positioned on the symmetry line between these two sectors.
  • Fig. 3 shows a schematic electronic circuit diagram, illustrating how the photodetector sector output signals are used to determine the angular setting of the shaft.
  • the output voltages from sectors 22 and 24 are equal, such that the amplified output V from differential amplifier 30 is zero.
  • the differential change in illumination of sectors 22 and 24 is proportional to the fractional change in the sector surfaces exposed to the illumination, namely 2 ⁇ /360.
  • the output voltage V of amplifier 30 is thus proportional to ⁇ , and is shown in Fig. 3 as k ⁇ . This relationship assumes uniformity of the photosensitive surface of the photo detector 18 and of the illumination falling on it.
  • Fig. 4 is a graph showing the effect of changes of temperature on the output voltage of amplifier 30.
  • the curve labeled ti is that obtained at a predefined reference temperature T1 , and as expected, since the sensor assembly is calibrated at this temperature, the curve passes through the origin and has a certain slope.
  • T1 a predefined reference temperature
  • the characteristic curve becomes that labeled ⁇ 2 in Fig. 4.
  • the slope changes. This effect has to be compensated for to provide the sensor with accuracy over a range of temperatures.
  • the change in detector sensitivity is compensated for by means of a signal taken from sector 26 of the photodiode.
  • This sector is always fully exposed to the incident illumination regardless of the angular position of the shaft, and therefore provides a monitor signal whose magnitude is a function of the source output and the detector sensitivity. Since all of the sectors of the detector are on one substrate and in the same package, it is assumed that the environmental characteristics of the opposing angular sensor signal sectors closely follow that of the monitor signal.
  • the output signal from sector 26 is amplified 32 and its level is compared in operational amplifier 34 with a reference voltage V re f, which defines a predetermined calibration output taking into account the source illumination level and the detector sensitivity.
  • a corresponding correction voltage V cor r appears at the output of amplifier 34.
  • This correction voltage is amplified by current driver 36, which supplies the drive current to the light source 20, thus closing the feedback loop and maintaining a constant overall output characteristic of the source/detector combination.
  • the working point of the overall illumination/detection control system is defined by a reference offset voltage 38, against which the correction voltage V co rr is compared in current driver amplifier 36.
  • the gain of this amplifier is sufficiently high to maintain good control of the illumination/detection feedback loop, and for example, a 30% change in lamp intensity, which would result in a 30% change in the angular sensor's scale factor, can be compensated for to result in only a 0.08% change in the sensor's scale factor
  • this control circuit is operable to maintain the output of sector 26, and hence of amplifier 30, largely insensitive to changes both in illumination output level, and in detector conversion sensitivity.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Mechanical Optical Scanning Systems (AREA)

Abstract

An angular position sensor, especially for use in a galvanometric scanning system, and including a sectorized photodetector with an illumination shielding plate attached to a shaft whose rotation is to be sensed. The photodetector has two opposed sectors, preferably disposed diametrically opposite each other, such that incident illumination from a source is differentially modulated by rotation of the shielding plate. In addition, at least a third sector is provided which is always illuminated by the source regardless of the rotational position of the shielding plate. The output from the diametrically opposed sectors is used to determine the rotational angle of the shaft, while the output from the third sector is used to stabilize the illumination source, such that the output of the two opposed sectors of the detector is insensitive to changes in temperature.

Description

ANGULAR POSITION SENSOR WITH SEMICIRCULAR SHIELDING PLATE
FIELD OF THE INVENTION
The present invention relates to the field of non-contact angular position sensors, especially for use in galvanometric scanning systems.
BACKGROUND OF THE INVENTION
A number of angular position sensors especially for use in galvanometric scanning systems have been proposed in the prior art. Angular detection of high accuracy has, for example, been achieved using variable-inductance angular sensors, variable-capacitance angular sensors or analog optical angular position sensors. Variable inductance angular position sensors, such as described in U.S.
Patent No. 5,561 ,375 tend to be expensive, difficult to manufacture and physically large because of the size of the inductor elements. Examples of angular sensors of the variable capacitance type have been described in U.S. Patent Nos. 4,142,144, 4,268,889 and 5,099,386. These sensors provide good accuracy in angular position measurement, but they are relatively expensive because they include circuits for modulation and demodulation of a high frequency carrier. An optical angular position sensors has been described in U.S. Patent No. 5,235,180, and is used in measuring the mirror angle in a laser scanner. It has one or two fixed LED light sources, a shield plate rotatable in unison with the rotating mirror, and the light receiving photo detector is a low cost, compact, light-weight and inexpensive device. The sensing surface of the photo detector is divided into either two or four photosensitive sectors called A and B or A, B, C and D. For the two sector case, the angular position function of such a sensor is given by: Θ = (A - B)/(A + B) (1) where Θ is the galvanometer mirror angle, and A, B are voltages corresponding to the intensities of light detected at the A and B sectors.
Such an angular sensor has a number of disadvantages: 1. An induced noise voltage, designated by Δ, decreases the accuracy of the sensor because if the voltages A and B are replaced by values A+Δ and B+Δ, the value of Θ given by equation (1) becomes:
Θ = [ (A+ΔMB+Δ)] / [(A+Δ+B+Δ)] = (A-B) / (A+B+2Δ) (1a) and this results in a spurious signal being added to the output .
2. No means are provided to compensate for zero drift. However, the output is relatively insensitive to change of temperature since both the numerator and the denominator of equation (1) behave similarly with change in temperature.
3. The circuit execution of the division operation in equation (1) is relatively expensive.
There therefore exists a need for a system for determining the angular position of a rotating galvanometer mirror, without the disadvantages associated with the above described optical angular position sensor.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.
SUMMARY OF THE INVENTION
The present invention seeks to provide a new angular position sensor, especially for use in a galvanometric scanning system, and including novel use of a sectorized photodetector with an illumination shielding plate attached to the shaft whose rotation is to be sensed. The photodetector has two preferably diametrically opposed sectors, disposed such that the incident illumination from a source is differentially modulated by rotation of the shielding plate, and at least a third sector which is always illuminated by the source, regardless of the rotational position of the shielding plate. The outputs from the diametrically opposed sectors are used to determine the rotational angle of the shaft, while the output from the third sector is used to stabilize the illumination source, such that the outputs of the two opposed sectors of the detector are insensitive to changes in temperature. This arrangement enables accurate temperature compensation of the angular position sensor to be achieved, the need for temperature compensation arising because of significant changes in the sensitivity of the photodetector with temperature.
There is thus provided in accordance with a preferred embodiment of the present invention, an angular position sensor comprising: (i) an input shaft whose rotation conveys the angular position,
(ii) a shielding plate attached to the shaft with its plane generally perpendicular to the axis of rotation of the input shaft, the plate covering a part of the plane of rotation of the shaft, (iii) a sectorized photodetector disposed in proximity to the shielding plate, such that rotation of the shaft causes the shielding plate to rotate over the sectorized surface of the photodetector, and
(iv) an illumination source directing its illumination generally in the direction of the surface of the photodetector, such that the shielding plate modulates the illumination falling on the photodetector surface in accordance with the rotation of the shaft,
(v) wherein the photodetector comprises a first and a second sector disposed such that incident illumination falling thereon is differentially modulated by rotation of the shielding plate, and at least a third sector disposed such that incident illumination falling thereon is unaffected by rotation of the shielding plate. In the above described position sensor, the at least third sector preferably provides an output signal utilized to control the luminous output of the illumination source. Furthermore, the disposition of the at least third sector on the same photodetector substrate as the first and second sectors enables the control of the luminous output of the illumination source to compensate for changes with temperature of the optical power of the illumination source and of the sensitivity of the first and second sectors of the photodetector.
There is further provided in accordance with yet another preferred embodiment of the present invention, an angular position sensor as described above and wherein the first and second sectors provide a differential output signal proportional to the rotation of the input shaft.
In any of the above described angular position sensors, the shielding plate preferably has a generally semicircular form. Additionally, the shielding plate may preferably have a profile edge which passes through the axis of rotation of said shaft.
The illumination source may preferably be either a LED, a laser diode, or an incandescent lamp.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: Fig. 1 is a schematic illustration of a angular position sensor, constructed and operative according to a first preferred embodiment of the present invention; Fig. 2 is a schematic view from the direction of the axis of the shaft, showing the geometrical structure and positions of the quadrants of the photodetector and the shield plate of the embodiment of Fig. 1 ; Fig. 3 is a schematic electronic circuit diagram, illustrating how the photodetector sector output signals are used to determine the angular setting of the shaft of Fig. 1; and
Fig .4 is a graph showing the effect of temperature changes on the output voltage of the output amplifier of the common sector of the detector shown in Fig. 3.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Reference is now made to Fig. 1 , which is a schematic illustration of a angular position sensor, constructed and operative according to a first preferred embodiment of the present invention. Though the angular position sensor of this embodiment is described it terms of its use in a rotating mirror galvanometer, it is to be understood that the present invention is not limited to use in a rotating mirror galvanometer, but can be used whenever small angular rotations of a shaft are to be measured. The shaft 10 of the angular position sensor has its angular rotation input from the galvanometer motion 12, not shown in Fig. 1. A mirror 14, such as for use in an optical scanning system which the galvanometer is driving, is attached rigidly to the shaft, such that its reflective plane rotates with rotation of the shaft. A shield plate 16 is rigidly attached to the shaft, preferably at its end, either by direct affixation to the end, as shown in Fig. 1 or by means of an attachment strip which attaches the plate along the length of the shaft, or in any other suitable manner. A photodetector 18 is disposed beyond the end of the shaft, such that when the shaft rotates, the shield plate 16 rotates in front of the fixed photo-detector. A light source 20, such as a light emitting diode, LED, or a laser diode, is positioned such that its illumination is projected past the shield plate and onto the photodetector. The light source 20 can be disposed either coaxial with the shaft or adjacent to the shaft, so long as it directs its illumination reasonably axially through the plane of the shield plate and onto the photodetector.
Reference is now made to Fig. 2, which is a schematic view from the direction of the axis of the shaft, showing the geometrical structure of the sensitive surface of the photodetector 18, and the mutual positional relationship between the shield plate 16, which rotates with the galvanometer shaft, and the fixed photodetector 18. The profile of the shield plate 16 is such that it shields the light from reaching the detector surface over part of the angular revolution of the shaft, and preferably has a semicircular shape. Any other shape is acceptable, however, on condition that the top edge 17 clearly delineates those regions of the detector surface receiving radiation, and preferably passes through the center of rotation of the plate. The photodetector 18 is preferably of the common quadrant divided type, having 4 quadrant sectors 22, 24, 26 and 28. The detector is aligned such that when the shaft is in its null position, the shield plate 16 is positioned to provide approximately equal exposure of diametrically opposite sectors 22 and 24 to the incident illumination, which arrives from the general direction of the axis of the shaft 10. Rotation of the shield plate is limited such that the central sector 26 of the photodetector is always fully exposed to the illumination. Sector 28, on the other hand, is always shielded, and therefore essentially unused. A photodetector having only three sectors could thus be used in the present invention, but such types are not generally available. Rotation of the shaft 10, for instance anticlockwise, thus results in increase of the illumination on one of the opposing sectors 24, and a corresponding decrease in the other 22. The position of the light source 20, if not coaxial with the shaft, should be symmetrical to the diametrically opposite sectors 22 and 24, to ensure linearity, and should thus be positioned on the symmetry line between these two sectors.
Reference is now made to Fig. 3, which shows a schematic electronic circuit diagram, illustrating how the photodetector sector output signals are used to determine the angular setting of the shaft. When the shield plate 16 is in its null position, the output voltages from sectors 22 and 24 are equal, such that the amplified output V from differential amplifier 30 is zero. When the shaft is rotated by an angle α° from its null position, the differential change in illumination of sectors 22 and 24 is proportional to the fractional change in the sector surfaces exposed to the illumination, namely 2α/360. The output voltage V of amplifier 30 is thus proportional to α, and is shown in Fig. 3 as kα. This relationship assumes uniformity of the photosensitive surface of the photo detector 18 and of the illumination falling on it. However, the simple differential arrangement described above, is satisfactory only if the environmental conditions are constant, since it is known that the sensitivity of the photodetector 18 varies with temperature. Reference is now made to Fig. 4, which is a graph showing the effect of changes of temperature on the output voltage of amplifier 30. The curve labeled ti is that obtained at a predefined reference temperature T1 , and as expected, since the sensor assembly is calibrated at this temperature, the curve passes through the origin and has a certain slope. When the temperature changes to T2, the characteristic curve becomes that labeled \2 in Fig. 4. As is observed, because of the change in overall temperature sensitivity of the detector, the slope changes. This effect has to be compensated for to provide the sensor with accuracy over a range of temperatures.
Referring back again to Fig. 3, the change in detector sensitivity is compensated for by means of a signal taken from sector 26 of the photodiode. This sector is always fully exposed to the incident illumination regardless of the angular position of the shaft, and therefore provides a monitor signal whose magnitude is a function of the source output and the detector sensitivity. Since all of the sectors of the detector are on one substrate and in the same package, it is assumed that the environmental characteristics of the opposing angular sensor signal sectors closely follow that of the monitor signal. The output signal from sector 26 is amplified 32 and its level is compared in operational amplifier 34 with a reference voltage Vref, which defines a predetermined calibration output taking into account the source illumination level and the detector sensitivity. If a change occurs either in the radiant intensity of the light source 20, or in the output sensitivity of the photodetector, for instance, because of a change in ambient temperature, a corresponding correction voltage Vcorr appears at the output of amplifier 34. This correction voltage is amplified by current driver 36, which supplies the drive current to the light source 20, thus closing the feedback loop and maintaining a constant overall output characteristic of the source/detector combination. The working point of the overall illumination/detection control system is defined by a reference offset voltage 38, against which the correction voltage Vcorr is compared in current driver amplifier 36. The gain of this amplifier is sufficiently high to maintain good control of the illumination/detection feedback loop, and for example, a 30% change in lamp intensity, which would result in a 30% change in the angular sensor's scale factor, can be compensated for to result in only a 0.08% change in the sensor's scale factor
As a result of this correction technique, the slope of the sensitivity characteristic t2 in Fig. 3 is rotated, such that it becomes parallel to and essentially co-linear to that of the reference characteristic curve ti, and the temperature sensitivity of the detection system is thus compensated for. Compensation over a temperature range of -30 to +700C can be readily obtained. It should be noted that this control circuit is operable to maintain the output of sector 26, and hence of amplifier 30, largely insensitive to changes both in illumination output level, and in detector conversion sensitivity.
It is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art.

Claims

We claim: 1. An angular position sensor comprising: an input shaft whose rotation conveys said angular position; a shielding plate attached to said shaft with its plane generally perpendicular to the axis of rotation of said input shaft, said plate covering a part of the plane of rotation of said shaft; a sectorized photodetector disposed in proximity to said shielding plate, such that rotation of said shaft causes said shielding plate to rotate over the sectorized surface of said photodetector; and an illumination source directing its illumination generally in the direction of said surface of said photodetector, such that said shielding plate modulates the illumination falling on said photodetector surface in accordance with the rotation of said shaft; wherein said photodetector comprises a first and a second sector disposed such that incident illumination falling thereon is differentially modulated by rotation of said shielding plate, and at least a third sector disposed such that incident illumination falling thereon is unaffected by rotation of said shielding plate.
2. An angular position sensor according to claim 1 and wherein said at least third sector provides an output signal utilized to control the luminous output of said illumination source.
3. An angular position sensor according to claim 2 and wherein the disposition of said at least third sector on the same photodetector substrate as said first and second sectors enables said control of said luminous output of said illumination source to compensate for changes with temperature of the optical power of said illumination source and of the sensitivity of said first and second sectors of said photodetector.
4. An angular position sensor according to any of the previous claims and wherein said first and second sectors provide a differential output signal proportional to the rotation of said input shaft.
5. An angular position sensor according to any of the previous claims and wherein said shielding plate has a generally semicircular form.
6. An angular position sensor according to any of the previous claims and wherein said shielding plate has a profile edge which passes through the axis of rotation of said shaft.
7. An angular position sensor according to any of the previous claims and wherein said illumination source is any one of a LED, a laser diode and an incandescent lamp.
PCT/IL2007/000610 2006-05-22 2007-05-21 Angular position sensor with semicircular shielding plate WO2007135673A1 (en)

Applications Claiming Priority (2)

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IL175831A IL175831A0 (en) 2006-05-22 2006-05-22 Angular position sensor
IL175831 2006-05-22

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WO2007135673A1 true WO2007135673A1 (en) 2007-11-29

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Cited By (1)

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CN104180828A (en) * 2013-05-24 2014-12-03 北京金海创科技发展有限公司 Detection device used for determining angular position

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EP0169657A2 (en) * 1984-06-25 1986-01-29 Energy Innovations Inc. Non-contact shaft angle detector
DE3635107A1 (en) * 1985-10-16 1987-04-16 Fuji Electric Co Ltd ROTATION DETECTOR
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EP0169657A2 (en) * 1984-06-25 1986-01-29 Energy Innovations Inc. Non-contact shaft angle detector
DE3635107A1 (en) * 1985-10-16 1987-04-16 Fuji Electric Co Ltd ROTATION DETECTOR
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Publication number Priority date Publication date Assignee Title
CN104180828A (en) * 2013-05-24 2014-12-03 北京金海创科技发展有限公司 Detection device used for determining angular position
CN104180828B (en) * 2013-05-24 2016-11-23 北京金海创科技发展有限公司 For determining the detection device of angle position

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