Abstract:
An optical torque sensor comprises a radiation source emitting radiation of at least one wavelength. At least one sensor senses the emitted radiation and generates at least one intensity signal indicative of the intensity of the emitted radiation. At least one signal conditioner is receptive of the emitted radiation and is positioned on a shaft between the radiation source and the at least one sensor thereby conditioning the emitted radiation. A circuit is receptive of the at least one intensity signal and determines the torque acting upon the shaft and compensates for the attenuation of the emitted radiation. A method of compensating for signal attenuation in the sensor comprises generating radiation of at least one wavelength, conditioning the radiation and sensing the radiation. Responsive to the sensed radiation, at least one intensity signal indicative of the intensity of the radiation is generated. The intensity of the radiation due to a combination of the torque acting upon the shaft and the contamination of the sensor is determined. The intensity of the radiation due to the contamination of the sensor only is determined and the difference between the intensity of the radiation due to a combination of the torque acting upon the shaft and the contamination of the sensor and the intensity of the radiation due to the contamination of the sensor only is calculated to generate a compensated signal indicative only of the torque acting upon the shaft.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to torque sensors and to a method and apparatus for correcting for attenuation in an optical torque sensor.  
         BACKGROUND OF THE INVENTION  
         [0002]    Electric power steering (EPS) has long been the subject of development by automobile manufacturers because of its fuel economy and ease-of-control advantages as compared to the traditional hydraulic power steering (HPS). However, commercialization of EPS systems has been slow in coming and is presently limited to smaller automobiles due to the cost and performance challenges associated with larger automobiles. Among the challenging technical issues addressed is the use of a sensor positioned on a steering shaft coupled to a steering device and the effect of dirt build-up on the sensor. Dirt build-up typically distorts sensed information associated with the type of high performance sensors needed to meet the steering requirements.  
           [0003]    At least two methods for the sensing the rotary positioning of a steering shaft exist. One method utilizes semiconductor-based magnetic sensors (magnetoresistors, or MRs). A second method utilizes the sensing of a signal based on optical detection and magnetic field variation. In the second method, the functionality of optical detectors and encoders is dependent upon environmental and operating conditions. Such conditions typically include dirt build-up and temperature variations, which increase the probability of the occurrence of distortion in the sensed signal.  
         BRIEF SUMMARY OF THE INVENTION  
         [0004]    A method and apparatus for correcting for the effects of dirt and oil build-up on the polarizers of an optical torque sensor are disclosed. An optical torque sensor comprises a radiation source emitting radiation of at least one wavelength. At least one sensor senses the emitted radiation and generates at least one intensity signal indicative of the intensity of the emitted radiation. At least one signal conditioner is receptive of the emitted radiation and is positioned on a shaft between the radiation source and the at least one sensor thereby conditioning the emitted radiation. A circuit is receptive of the at least one intensity signal and determines the torque acting upon the shaft and compensates for the attenuation of the emitted radiation.  
           [0005]    A method of compensating for signal attenuation in the sensor comprises generating radiation of at least one wavelength, conditioning the radiation and sensing the radiation. Responsive to the sensed radiation, at least one intensity signal indicative of the intensity of the radiation is generated. The intensity of the radiation due to a combination of the torque acting upon the shaft and the contamination of the sensor is determined. The intensity of the radiation due to the contamination of the sensor only is determined and the difference between the intensity of the radiation due to a combination of the torque acting upon the shaft and the contamination of the sensor and the intensity of the radiation due to the contamination of the sensor only is calculated to generate a compensated signal indicative only of the torque acting upon the shaft. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 illustrates a schematic diagram of an electric power steering system;  
         [0007]    [0007]FIG. 2A is a diagram of a torsion bar;  
         [0008]    [0008]FIG. 2B is a diagram of an upper and lower steering shaft of the electric power steering system of FIG. 1;  
         [0009]    [0009]FIG. 2C is a diagram of the upper and lower steering shafts of FIG. 2B engaged to form a single steering shaft;  
         [0010]    [0010]FIG. 2D is a cross sectional view of the upper and lower steering shafts of FIGS. 2B and 2C;  
         [0011]    [0011]FIG. 3 is a first schematic diagram of a torque sensor coupled to the steering shaft of FIG. 2C;  
         [0012]    [0012]FIG. 4 is a schematic diagram of an electro-optic circuit having a multicolored light source;  
         [0013]    [0013]FIG. 5 is a schematic diagram of the electronic configuration of a set of photodetectors;  
         [0014]    [0014]FIG. 6 is a schematic diagram of signal processing electronics for the set of photodetectors of FIG. 5;  
         [0015]    [0015]FIG. 7 shows the optical transmission characteristics of two thin film polarizers with their polarization axes at right angles with respect to one another;  
         [0016]    [0016]FIG. 8 is a graphical representation of the intensity of light (or cos 2  of the polarization angle ψ) detected at a photodetector of FIG. 5 as a function of the change in differential polarization angle ψ, and  
         [0017]    [0017]FIG. 9 is a schematic diagram of the torque sensor of FIG. 3 including integrated circuits, printed circuit boards and a protective housing or cover. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    Referring to FIG. 1, a motor vehicle electric power steering system is shown generally at  10 . The system  10  comprises an upper steering shaft  18  in mechanical communication with a lower steering shaft  20  through a universal joint  22 , a steering mechanism, shown generally at  12 , operably connected to the lower steering shaft  20 , and to steerable wheels  28  (only one of which is shown) rotatably and operably connected to the steering mechanism  12 . The steering mechanism  12  is a rack-and-pinion type system and includes a toothed rack (not shown) and a pinion gear (not shown) located in a gear housing  14 . As a steering wheel  16 , disposed on the upper steering shaft  18 , is rotated, the upper steering shaft  18  and the lower steering shaft  20  correspondingly rotate axially to in turn rotate the pinion gear. The axial rotation of the pinion gear moves the toothed rack that causes the lateral translation of a set of tie rods  24  (only one of which is shown) that move a set of steering knuckles  26  (only one of which is shown) to turn steerable wheels  28  (only one of which is shown).  
         [0019]    Electric power steering, or torque, assist is provided through the unit generally designated by reference numeral  30  which includes a controller  32  and an electric motor  34  responsive thereto. The controller  32  is in electronic communication with, and is powered by, a power supply  36 . The controller  32  also receives a vehicle velocity input signal  40 . The steering wheel angle is measured using a position sensor  42 , which may be an optical encoding type sensor, a variable resistance type sensor, or any other suitable type of position sensor. A signal  44  representative of the angle of the steering wheel  16  is then provided as input to the controller  32 .  
         [0020]    As the steering wheel  16  is turned, a torque sensor  46 , positioned between the upper steering shaft  18  and the lower steering shaft  20  senses the torque applied to the steering wheel  16  by an operator of the motor vehicle. The torque sensor  46 , provides as output a variable analog voltage signal  102  to the controller  32  in relation to the degree of twist of a torsion bar (not shown) connecting the upper and lower steering shafts  18 ,  20 .  
         [0021]    In response to the vehicle velocity input  40 , the input from position sensor  42 , and the input from torque sensor  46 , the controller  32  transmits a signal  48  in the form of a current command or a voltage command to the electric motor  34 . In response thereto, the electric motor  34  supplies torque assistance to the steering system  10  through a worm  50  and a worm gear  52  in such a way so as to provide a torque to the vehicle steering, thereby assisting the driving force exerted by the vehicle operator on steering wheel  16 .  
         [0022]    Referring to FIGS. 2A, 2B,  2 C and  2 D in conjunction with one another, a torsion bar  78 , the upper steering shaft  18  and the lower steering shaft  20  are shown. In particular, FIG. 2C shows the assembly of the torsion bar  78 , the upper steering shaft  18  and the lower steering shaft  20 . In FIG. 2B, one end  82  of the upper steering shaft  18  has a stoptooth or blade  82   a  that is positioned across the end  82  of the upper steering shaft  18  and extends from the surface thereof. The end  82  of the upper steering shaft  18  also has a center hole  72  (FIG. 2D) equal to the diameter of the torsion bar&#39;s largest diameter milled to a depth of slightly less than one half of the length of the torsion bar  78 . One end  60  of the lower steering shaft  20  has a notch  60   a  that is positioned across the end  60  of the lower steering shaft  20  and extends from the surface thereof inward to a depth of slightly greater than the length of the stoptooth  82   a . The end  60  of the lower steering shaft  20  also has a center hole  56  (FIG. 2D) equal to the diameter of the torsion bar&#39;s largest diameter milled to a depth of slightly less than half of the length of the torsion bar  78 .  
         [0023]    The torsion bar  78 , with a known torsional spring rate, k, (typically about 1.5 Nm per degree) is pressed into the center holes  56 ,  72  of the upper  82  and lower  60  shaft ends with the stoptooth  82   a  centered within the notch  60   a . Holes  78   a  are drilled through the upper and lower shafts  18 ,  20  and the torsion bar  78  as indicated in FIG. 2C. Oversized pins  78   b  are pressed into the holes  78   a  to lock the torsion bar  78  and the upper and lower steering shafts  18 ,  20  together. At this point the only mechanical connection between the upper  18  and lower  20  steering shafts is through the torsion bar  78  and the assembly comprises essentially a single steering shaft  94 . The stoptooth  82   a  and notch  60   a  are designed to allow about ±10 degrees of twist of the torsion bar  78  before the stoptooth  82   a  and notch  60   a  engage to carry any further torsional load. Since the spring rate, k, of the torsion bar  78  is known, if a measurement of the differential angle, θ, between the upper  18  and lower  20  steering shafts is made, then the torque, T, acting upon the steering shaft  94  can be calculated as follows:  
           T=k×θ.    (1)  
         [0024]    Automotive requirements make the measurement of the differential steering angle, θ, between the upper steering shaft  18  and the lower steering shaft  20  difficult. The automotive environment with dust, dirt, various fluids and extreme temperature variations make long term and high-resolution measurements very difficult.  
         [0025]    The resolution required for the measurement of torque, T, is about 0.02 Nm, which translates into about 0.013 degrees. For a sensor with a sensing diameter of about 65 mm or 2.56 inches, 0.013 degrees is a mechanical resolution of about 7.4 μm or about 0.0003 inches. Since the upper  18  and lower  20  steering shafts are only connected by the relatively smaller diameter torsion bar  78 , motions, other than rotation, are possible between the shafts  18 ,  20 . These extraneous motions introduce mechanical noise, which can be as much as 40 times greater than the measurement resolution (about 300 μm or about 0.012 inches). Sensor manufactures have used various coupling joints, (e.g., Oldham coupler) to make sensors immune to unwanted radial and axial motions only to introduce hysteresis and noise. Any contacting method, such a variable resistor, has a problem with long term rotary contact wear. The automotive industry typically requires parts to have life times in excess of 10 years; this means that almost all sensors that have been contacting will change to a non-contacting sensors.  
         [0026]    An optical polarization method of measuring the differential steering angle, θ, has many advantages over other measurement techniques. This method, being non-contacting and insensitive to unwanted mechanical motions, is the basis of a differential angle sensor. Polarization of light is insensitive to the radial or axial motions of polarizers or of the source. A change in the intensity, I, (e.g. attenuation) of light after passing through two polarizers occurs if the angle, ψ, between the polarization axes of the polarizers changes.  
         [0027]    The transfer function of light intensity, I, passing through two polarizers varies as the square of the cosine of the angle, ψ, such that at an angle of ψ=0°, or when the polarization axes of the two polarizers are parallel with one another, the transmitted light intensity will be at a maximum or 1, and when ψ=90°, or when the polarization axes of the two polarizers are perpendicular to each other, the transmitted light intensity will be at a minimum or 0. At an angle of ψ=45°, the transmitted light intensity will be 0.5 or 50% (i.e., cos 2  of 45°=0.5). Since the stoptooth  82   a  on the upper steering shaft  18  captured within the notch  60   a  of the lower steering shaft  20  limits the angle ψ to about ±10° the place for the sensor to operate is at ψ=45°±10°. At ψ=45°±10° the intensity of the light is about 67% at 35° and about 33% at 55° for a total intensity range of 34%, or 50% ±17%. Even with a cos 2  transfer function, within the range of ψ=45°±10°, the intensity change is linear to about 0.1%. It will be recalled that the steering wheel  16  can be turned about ±2.5 turns from the straight-ahead position and it is only the differential steering angle, θ, between the upper and lower steering shafts  18 ,  20 , or of the torsion bar  78 , that is being measured. Since the measurement of the differential steering angle, θ, is directly related to the measurement of changes in light intensity, or the electrical signal representing the change in light intensity, any other factors that could affect the change in light intensity or electrical signals must be controlled.  
         [0028]    Referring to FIG. 3, the upper  18  and lower  20  steering shafts are shown assembled as in FIG. 2C without showing the notch  60   a  and the stoptooth  82   a . A torque sensor, for sensing the torque applied to the steering shaft  94  is shown generally at  46 . A longitudinal axis  70  extends through the upper and lower steering shafts  18 ,  20 , and the torsion bar  78 . The torque sensor  46  includes a first signal conditioner, such as a polarizer  64 , disposed on the outer surface of and at the end  60  of the lower steering shaft  20 . The first polarizer  64  is generally disk-shaped and includes a first surface  66  and an opposing second surface  68  and has an axial center that corresponds with the longitudinal axis  70 .  
         [0029]    The torque sensor  46  includes a second signal conditioner, such as a polarizer  86  disposed on the outer surface of and at the end  82  of the upper steering shaft  18 . The second polarizer  86  is generally disk-shaped and includes a first surface  88  and an opposing second surface  90  and has an axial center that corresponds with the longitudinal axis  70 . The positioning of the first and second polarizers  64 ,  86  is such that any rotation of the upper steering shaft  18  with respect to the lower steering shaft  20  results in a corresponding rotation of the torsion bar  78  and of the second polarizer  86  with respect to the first polarizer  64 . Thus, Δθ=Δψ.  
         [0030]    Continuing in FIG. 3, light  104   a  emitted from a source  104 , such as a light emitting diode (LED), passes through the first polarizer  64  where it is polarized and falls upon a first sensor or photodetector  106 . The light  104   b  also passes through the second polarizer  86  and falls upon a second sensor or photodetector  108 . As best understood from FIG. 3, the reference numerals  104   a  and  104   b  designate the same beam of light, i.e., a single light beam of a specific wavelength, and are differentiated only to indicate different geometrical paths of the different segments  104   a ,  104   b  of the beam. The intensity of the light falling upon the second photodetector  108  follows the cosine squared of the differential angle, ψ, between the polarization axes of the two polarizers  64 ,  86 . However, one must consider the stability of the light source. The intensity variations measured by the second photodetector  108  could come from a change in the angle ψ or from a change in the light intensity, ΔI, of the source  104 . Placing a photodetector near the light source  104  and making a feedback loop to keep the light output constant could correct this effect. However, most light sources are at least weakly polarized and, even with a constant light output from the source  104 , the light level after the first polarizer  64  could vary a slight amount depending upon the amount of polarization of the source  104  and the angle, Φ, between the polarization of the source  104  and the axis of polarization of the first polarizer  64 . This second effect can be corrected by simply moving the photodetector from near the source  104  to a position after the first polarizer  64 . The placement of the photodetector  106  after the first polarizer  64  and connected in a feedback control loop with the LED source  104  assures a constant light level going into the second polarizer  86 .  
         [0031]    The only effect that can now cause a major error that is unaccounted for at this point is contamination on the second polarizer  86 . Any contamination on the second polarizer  86  will decrease the light intensity at the second photodetector  108  and may be misinterpreted as a change of differential polarization angle, Δψ. A novel method for correcting for the effects of contamination on the second polarizer  86  is presented.  
         [0032]    The first photodetector  106  is positioned such that the first polarizer  64  is positioned between the light source  104  and the first photodetector  106 . The second photodetector  108  is positioned such that both the first polarizer  64  and the second polarizer  86  are positioned between the light source  104  and the second photodetector  108 . The second polarizer  86  is positioned between the first and second photodetectors  106 ,  108 .  
         [0033]    The polarizers  64 ,  86  are configured to rotate about the centerline  70  relative to one another while the light source  104  and the first and second photodetectors  106 ,  108  remain stationary relative to the polarizers  64 ,  86 . Light generated by the light source  104  passes through the relative moving polarizers  64 ,  86 . A portion of the light  104   a  generated by the light source  104  that passes through first polarizer  64  is received by the first photodetector  106 . The portion of the light  104   b  that is not received by the first photodetector  106  passes through the second polarizer  86  and is received by the second photodetector  108 .  
         [0034]    Referring now to FIG. 4, the operation of the light source  104  and its relationship with the first photodetector  106  and the second photodetector  108  is described. The light source  104  comprises a multicolored light emitting device, such as one or more light emitting diodes (LED) operative to generate light at a plurality of wavelengths. The multicolored light source  104  generates a first beam of light at a first wavelength which is polarizable, and a second beam of light at a second wavelength that is not polarizable. The polarizable light is typically visible light having a characteristic wavelength range of about 565 nm to about 630 nm, and the non-polarizable light is typically infrared light having a characteristic wavelength range of about 900 nm to about 950 nm. The beam of light  104   a  passes through the first polarizer  64  and is detected by the first photodetector  106 . A quantity of the beam of light  104   b  also passes through the second polarizer  86  where it is detected by the second photodetector  108 .  
         [0035]    The above multicolored light is generated by the configuration of elements shown in FIG. 4. The multicolored light source  104  includes an input lead  112  and a first LED  116  and a second LED  118 . The two LEDs  116 ,  118  are electrically configured whereby when a bi-polar input signal  120  is applied to input lead  112 , the two LEDs  116 ,  118  are alternately activated to generate light. For example, LED  116  may be a diode that emits green light at a wavelength that is within a range subject to transmission reductions by the polarizers  64 ,  86 . LED  118  may be a diode that emits infrared light at a wavelength that is not within a range subject to transmission reductions by the polarizers  64 ,  86 .  
         [0036]    Certain polarizers do not polarize infrared light. For instance, the dyes used in the manufacture of long chain polymers which are subsequently used to construct thin film polarizers, have little or no effect upon light having wavelengths in the infrared. The change in the intensity of light in the visible wavelengths, when the axes of the polarizers  64 ,  86  go from crossed (ψ=90°) to uncrossed (ψ=0°), can range between about 30-40 decibel (dB). When the wavelength is greater than about 900 nanometers (nm) little or no change in intensity is realized. However, a certain amount of fixed attenuation does occur due to the polarizers  64 ,  86 . FIG. 7 illustrates the relationship between the transmission of light through a pair of crossed thin film polarizers as a function of wavelength. As shown in FIG. 7, when the wavelength is greater than about 900 nm, e.g., infrared (IR), the effect of the polarizers upon the transmission of light through the polarizers is negligible, and nearly all of the light is transmitted therethrough. Contrariwise, when the wavelength is less than about 800 nm, e.g. in the visible, the effect of the polarizers upon the transmission of light is significant and little light is transmitted therethrough.  
         [0037]    Referring to FIG. 5 in conjunction with FIG. 6, the first photodetector  106  for receiving the light beam  104   a  passing through the first polarizer  64  is shown connected to the first part of an electric circuit  100   a ,  100   b  for determining the torque, T, applied to the shaft  94  and for compensating for attenuation in the torque sensor  46 . The first photodetector  106  comprises a photo-diode that receives or captures light  104   a  from the first polarizer  64  and transforms such light  104   a  into an electrical signal  130  that is transmitted to a first amplifier  132 . An amplified output signal  134  from the first amplifier  132  is provided as input to a first solid state switch  136 . The first solid state switch  136  is controlled by a timing signal  138  that causes the first solid state switch  136  to select or alternate between a pair of output leads  140 ,  142 . The timing signal  138  will preferably be in synchronization (in phase) with the bi-polar input signal  120  of FIG. 4 wherein the polarizable portion and the non-polarizable portion of the output of the multi-colored LED device are measured in synchronization. A first output signal at  140 , from the first solid state switch  136 , is amplified, rectified and filtered by a second amplifier  144  which produces a DC voltage output signal  170  indicative of the intensity of the polarized visible light captured by the first photodetector  106 . A second output signal at  142 , from the first solid state switch  136 , is amplified, rectified and filtered by a third amplifier  146  which produces a DC voltage output signal  172  indicative of the intensity of the non-polarized infrared light captured by the first photodetector  106 .  
         [0038]    Still referring to FIG. 5 in conjunction with FIG. 6, the second photodetector  108  for receiving the portion of light  104   b  passing through both polarizers  64 ,  86  is shown connected to the first part of the electric circuit  100   a ,  100   b . The second photodetector  108  comprises a photo-diode that receives or captures light  104   b  from the second polarizer  86  and transforms such light to an electrical signal  154  that is transmitted to a fourth amplifier  156 . An output signal  158  from the fourth amplifier  156  is provided as input to a second solid state switch  160 . The second solid state switch  160  is controlled by the bi-polar signal  138  that causes the second solid state switch  160  to toggle between a pair of output leads  162 ,  164 . As above, timing signal  138  will preferably be in synchronization (in phase) with bi-polar input signal  120  of FIG. 4 wherein the polarizable portion and the non-polarizable portion of the output of the multi-colored LED device are measured in synchronization. A third output signal at  162 , from the second solid state switch  160 , is amplified, rectified and filtered by a fifth amplifier  166  which produces a DC voltage output signal  174  indicative of the intensity of the polarized visible light captured by the second photodetector  108 . A fourth output signal  164  from the second solid state switch  160  is amplified, rectified and filtered by a sixth amplifier  168  which produces a DC voltage output signal  176  indicative of the intensity of the non-polarized infrared light captured by the second photodetector  108 . The values of signals  170 ,  172 ,  174  and  176  are retained for the appropriate number of cycles.  
         [0039]    Amplifiers  144  and  146  rectify and filter the time switched signals  140  and  142  to produce the DC voltage output signals  170  and  172 . These signals  170 ,  172  are initially adjusted to equal a reference voltage, V D , at  180  in FIG. 6. As such, the error signals  180   a  and  182   a  are initially equal to zero. During the operation of the torque sensor  46 , the output signals  170  and  172  are each compared to the reference voltage, V D ,  178  at  180  and  182  (FIG. 8). If signal  172  is not equal to the reference voltage, V D ,  178 , a first nonzero error signal  180   a  indicative of the difference between the output of amplifier  146  and the reference voltage, V D ,  178  is generated. Likewise, if signal  170  is not equal to the reference voltage, V D ,  178 , a second nonzero error signal  182   a  indicative of the difference between the output of amplifier  144  and the reference voltage, V D ,  178  is generated. The error signals  180   a ,  182   a  are provided as input to an LED drive circuit  184  where they are processed. The LED drive circuit  184  provides as output a drive signal  120 . The drive signal  120  is delivered to the light source  104  and is operative thereby to adjust the intensities of the outputs  104   a ,  104   b  of the visible and infrared LEDs  116 ,  118  of the light source  104  so as to cause signals  170  and  172  to again equal the reference voltage, V D ,  178 . The reference voltage, V D ,  178  is the output voltage of a bandgap diode having a temperature stability of about 10 parts per million per degree centigrade and a similar stability per year. Thus the light intensity after the first polarizer  64 , as measured by the first photodetector  106 , will remain substantially constant over time, temperature, source polarization and contamination.  
         [0040]    The wavelength being emitted from the light source  104  is dependent upon the polarity of the bi-polar signal  120  (FIG. 4) at  112 . If the bi-polar signal  120  is a square wave that toggles or alternates between a positive and a negative value (e.g., +/−V 0 ) with respect to a zero reference voltage (V R ), then the output of the light source  104  will toggle or alternate between a first wavelength, λ 1 , and a second wavelength, λ 2 . Therefore, by selecting one wavelength in the visible range and a second wavelength in the infrared range, the attenuation of the light output  104   a ,  104   b  of the source  104  due to changes in ψ, as well as the attenuation due to contamination of the second polarizer  86  can be measured. The visible and infrared intensities measured by the first photodetector  106  are used only for keeping the light intensities constant going into the second polarizer  86 . The first photodetector  106  is not used in the determination of the change in the polarization angle nor determination of the contamination of the second polarizer  86 . For example, in FIG. 5, when the multi-colored LED  104  is switched to a visible wavelength, the output  134 ,  158  of the photodetectors  106 ,  108  is concurrently switched, by the solid state switches  136 ,  160 , to the circuitry  100   a ,  100   b  for measuring the attenuation due to changes in the angle ψ, and the attenuation due to the contamination of the second polarizer  86 . Switching the LED  104  to an infrared wavelength, concurrently switches the output  134 ,  158  of the photodetectors  106 ,  108  to the circuitry  100   a ,  100   b  that measures the attenuation due only to the contamination of the second polarizer  86 . The measured infrared attenuation (M 1 ), which is due only to the contamination of polarizer  86 , is used as a correction factor, (A c ), and is applied to the intensity indicative of the attenuation due to changes in the angle ψ combined with attenuation due to contamination. In automotive steering applications, the rate of switching would generally be in the range of about 20-50 kHz. The infrared intensity signal  176 , from the second photodetector  108 , is originally adjusted to match the reference voltage, V D , from the reference voltage source  178  at calibration. Thus, any increase in the optical path attenuation through polarizer  86  will cause the infrared intensity signal  176  to have a lower value and would be represented as a voltage change that is compared to the reference voltage level, V D . Furthermore, the use of infrared light, which is not polarized by the polarizers  64 ,  86 , allows for the measurement of the attenuation due to the second polarizer  86 . When all of the optical intensities are accounted for, it is possible for an optical torque sensor, based upon a change in the angle, ψ, between the polarization axes of the polarizers, to be accurate, stable, and substantially immune to contaminates.  
         [0041]    When the angle ψ, between the axes of polarization of the polarizers  64 ,  86 , changes, the total path attenuation between the light source  104  and the second photodetector  108  changes. The total path attenuation can also be changed by contamination from dirt and oils on the polarizers  64 ,  86 . If the path attenuation can be measured without the attenuation due to Δψ but with the attenuation due only to contamination (M 1 , in the infrared wavelengths), and then measured with the attenuation due to Δψ combined with the attenuation due contamination (M 2 , in the visible wavelengths), then the difference between the two measurements, M 2 −M 1 , equals the attenuation due only to a change in ψ, which is a measure of the torque applied to the shaft  18 ,  20 , thus compensating for contamination of the second polarizer  86 . This calculation is exemplified in Eq. 2:  
           M   2   M   1 =attenuation due only to Δψ 
         { A   p   +A   c   }−A   c   =A   p    (2)  
         [0042]    where A p  is the attenuation due to a change in ψ and A c  is the attenuation due to contamination of the second polarizer  86 . Equation 2 is performed at the summing device  190  in FIG. 6.  
         [0043]    This is accomplished in this method by using two different wavelengths of light from the source  104  and polarizers  64 ,  86  that polarize light at a first wavelength and not at a second wavelength (FIG. 6). The output from the photodetectors  106 ,  108  will first measure the intensity of the output of the first LED (in the visible) followed in time with the measurement of the intensity of the second LED (in the infrared). The intensity signals  170 ,  172  from the first photodetector  106 , are compared to the reference voltage, V D ,  178  at  180 ,  182 . Error signals will result if the intensities of signals  170  and  172  do not match the voltage reference, V D , at  178 . The visible error signal  182   a  and the infrared error signal  180   a  are provided as input to the LED drive circuit  184  which adjusts the drive current for each polarity of the LED drive signal  120  to keep the measured visible and infrared intensities constant at photodetector  106 . The time-separated visible  174  and infrared  176  output signals from the second photodetector  108  are amplified, rectified and filtered and their difference, ΔI t , is calculated at  190  to remove the component of attenuation due to contamination. The difference  190   a , is indicative of the attenuation due to a change in ψ.  
         [0044]    The difference  190   a  is then sent to the scaling electronics  188  where the difference  190   a  is scaled as needed to meet customer requirements. When the torque sensor  46  is initially calibrated the gain of amplifiers  144 ,  146 ,  166  and  168  is adjusted so that the intensity levels of  170 ,  172 ,  174  and  176  are referenced to the reference voltage reference, V D ,  178 . Thus, if the infrared total path attenuation increases, then signal  176  decreases and the difference signal  190   a  (indicative of Δψ) can be multiplied by a gain factor, equal to the ratio of the value of signal  178   a  to the value of signal  176 , at  188  to return the difference signal  190   a  to the correct level. For example, if the value of signal  176  decreases by a factor of two, the gain factor is equal to two. This adjustment is done in the scaling  188  portion of the electronics.  
         [0045]    At calibration, when the polarizers are set at 45°, zero torque is acting upon the shaft  18 ,  20 . The differential range of ψ from the starting point of 45° is limited to ±10° and, as seen in FIG. 8, there is a linear change in the intensity of light (or cos 2  of the polarization angle ψ) captured at the second photodetector  108  with the change in differential polarization angle ψ. In the scaling  188 , zero torque (or 45° polarization angle) is set at 2.5V DC at  178   a  and, for example, at +10° is adjusted to 4.5V DC which is a 2 volt span for a 10° differential angle change. This is 0.2V DC per degree or 0.005° per mV. The steering controller  32  uses this known linear transfer function to calculate torque by Eq. 1 as T=k×θ or  
           T=k ×({sensor output in  mV− 2500  mV}× 0.005°)   (3)  
         [0046]    It will be appreciated that the circuit  100   a ,  100   b  may be located either within the controller  32  remote from the LED  104 , the photodetectors  106 ,  108  and the polarizers  64  or may be located in close proximity thereto as an integral part thereof remote from the controller  32 . It will also be appreciated that, as seen in FIGS. 3 and 9, the circuit  100   a ,  100   b  may comprise integrated circuits mounted on printed circuit boards  96 ,  98 ,  100 ,  102 . Still further, the radiation source  104 , first and second photodetectors  106 ,  108  and first and second polarizers  64 ,  86  are encased within a protective housing or cover  200 .  
         [0047]    While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration only, and such illustrations and embodiments as have been disclosed herein are not to be construed as limiting the claims.