Abstract:
A vehicle wheel alignment sensor unit is configured with an absolute rotational position sensor to provide an absolute rotational position of a mounting shaft relative to the vehicle wheel alignment sensor unit enabling resumption of operation and recall of stored runout data subsequent to an operational disruption such as a system power loss. An internal power source maintains the integrity of the sensor memory for a definite span of time during momentary system power losses such as battery changes or during overnight shutdowns, permitting the mounting shaft runout compensation values to be maintained and utilized upon the restoration of system power.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is a continuation-in-part of U.S. patent application Ser. No. 10/378,305 filed on Mar. 3, 2003 now U.S. Pat. No. 6,796,036, herein incorporated by reference, and from which priority is claimed. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
   Not Applicable. 
   BACKGROUND OF THE INVENTION 
   The present invention relates generally to vehicle wheel alignment sensors of the type which are pendulously secured to a vehicle wheel by a mounting shaft during a vehicle wheel alignment procedure, and in particular, to an apparatus and method for identifying and maintaining tracking of an absolute mounting shaft rotational position of the vehicle wheel alignment sensor after mounting to a vehicle wheel. 
   Computer controlled vehicle wheel alignment systems, such as those shown in U.S. Reissue Pat. No. 33,144 to Hunter et al., U.S. Pat. No. 4,381,548 to Grossman et al., and U.S. Pat. No. 5,598,357 to Colarelli et al., utilize a number of wheel-mounted alignment sensors, such as those described in U.S. Pat. No. 4,879,670 to Colarelli, to obtain measurements related to vehicle wheel alignment parameters. The majority of vehicle wheel alignment sensors currently utilized in the market are “cordless”, relying on internal rechargeable batteries to power associated circuitry, and communicating to a console wheel alignment computer using conventional wireless communications technology. One example of a conventional computer controlled vehicle wheel alignment system is the Series 811 console, which utilizes the DSP-500 series cordless vehicle wheel alignment sensors, and is sold by Hunter Engineering Company of Bridgeton, Mo. 
   It is known in the industry that vehicle wheel alignment sensors which are pendulously secured to individual vehicle wheels must be compensated for any runout present between a plane in which the vehicle wheel alignment sensor hangs, and a plane perpendicular to the rotational axis of the wheel. The preferred procedures for obtaining runout compensation generally involve mounting a vehicle wheel alignment sensor to a vehicle wheel using a wheel clamp, rotating the wheel and mounting shaft to three distinct rotational positions relative to the sensor housing, and obtaining sensor readings for each position. Using the three sensor readings, a sinusoidal pattern representative of the amount of runout present between the vehicle wheel alignment sensor and the vehicle wheel may be calculated for any rotational position of the vehicle wheel and/or sensor. This runout compensation procedure for a vehicle wheel alignment sensor is described in detail in U.S. Pat. No. 5,052,111 to Carter et al. 
   Once the runout compensation procedure has been successfully completed, the vehicle wheel alignment sensor establishes a relative base rotational position of the mounting shaft. Utilizing an inexpensive relative rotational position sensor, the vehicle wheel alignment sensor tracks the rotation of the mounting shaft relative to the base rotational position. By tracking the change in the rotational position of the vehicle wheel alignment sensor from the base position, a runout compensation value for the current rotational position of the vehicle wheel alignment sensor is calculated from the previously obtained sinusoidal pattern. 
   One drawback to using inexpensive relative rotational position sensors is an inability of the sensor to identify an absolute rotational position of the vehicle wheel alignment sensor if the established base rotational position is lost. The established base rotational position in a conventional vehicle wheel alignment sensor can become lost for a number of reasons. For example, if the rechargeable batteries supplying power to maintain the wheel alignment sensor memory fail, or require replacement or recharging, data stored in the memory such as the established base rotational position and sinusoidal pattern will be lost, requiring an operator to repeat the time consuming compensation procedure before vehicle wheel alignment can be resumed. Similarly, in rare cases, battery supplied power can be lost momentarily due to poor or unclean battery contacts. 
   Even if the data values are stored in a persistent memory, such as one receiving power from a capacitor, which will maintain the data values for a limited period of time until the restoration of the normal power supply, any relative rotational movement between the vehicle wheel alignment sensor, mounting shaft, or vehicle wheel will not be recorded by the relative rotational position sensor, resulting in a discrepancy between the rotational position in which the sensor was compensated, and the current rotational position as identified by the relative rotational position sensor upon restoration of power. Finally, if an operator desires to suspend work on a vehicle in the middle of a vehicle wheel alignment procedure, and shuts down the alignment system (such as overnight), the stored data may be lost, and any rotational movement of the mounting shaft relative to the vehicle wheel alignment sensor will not be tracked, requiring the runout compensation procedures to be repeated upon the subsequent system startup. 
   It is known that an absolute rotational position sensor which relies upon unique identification markings associated with the mounting shaft to identify the current absolute rotational position of a fixed point on the mounting shaft relative to the vehicle wheel alignment sensor may be utilized in place of the relative rotational position sensor in a cordless vehicle wheel alignment sensor. However, to align modern vehicles, a very high degree of precision is required in the sensor rotational position measurements. When utilizing an absolute rotational position sensor in such a high precision environment, the conventional absolute rotational position sensor must be capable of identifying rotational positions to the same degree of accuracy, and therefore requires a number of unique markings proportional to the required degree of accuracy. Absolute rotational position sensors with unique markings on the mounting shaft which are capable of measuring rotational positions to the required accuracy levels for vehicle wheel alignment are delicate and costly items, and are generally unsuited for use in a vehicle service environment. 
   Accordingly, there is a need in the industry for an alternative device and method for maintaining cordless vehicle wheel alignment sensor runout compensation values and rotational positions following momentary or extended losses of power, which do not rely upon the use of delicate and costly absolute rotational position sensors with associated markings on the mounting shaft of the vehicle wheel alignment sensor. 
   BRIEF SUMMARY OF THE INVENTION 
   Briefly stated, an apparatus of the present invention incorporated into a conventional cordless vehicle wheel alignment sensor consists of a two-axis Hall-effect sensor disposed to provide non-contact sensing of an absolute rotational position of the mounting shaft relative to the vehicle wheel alignment sensor. The two-axis Hall-effect sensor is secured coaxially adjacent an axial end of the mounting shaft of the vehicle wheel alignment sensor. A permanent magnet is disposed on the axial end of the mounting shaft. Signals from the two-axis Hall-effect sensor generated as the poles of the permanent magnet rotate about the axis of the mounting shaft are conveyed to a sensor processor and utilized to store, in a sensor memory area, one or more absolute mounting shaft rotational positions. An internal power source, such as a capacitor maintains the integrity of the sensor memory for a definite span of time during momentary power losses such as battery changes or during overnight shutdowns, permitting the mounting shaft runout compensation values to be maintained and reutilized upon the restoration of system power, without the need to repeat the runout compensation procedures. 
   In an alternate embodiment, an apparatus of the present invention incorporated into a conventional cordless vehicle wheel alignment sensor consists of a pair of two-axis Hall-effect sensors each disposed to provide non-contact sensing of an absolute rotational position of the mounting shaft relative to the vehicle wheel alignment sensor. A first two-axis Hall-effect sensor is secured coaxially adjacent an axial end of the mounting shaft of the vehicle wheel alignment sensor. A permanent magnet is disposed on the axial end of the mounting shaft. Signals from the two-axis Hall-effect sensor generated as the poles of the permanent magnet rotate about the axis of the mounting shaft are conveyed to a sensor processor and utilized to store, in a sensor memory area, one or more coarse absolute mounting shaft rotational positions. The second two-axis Hall-effect sensor is secured adjacent an annular arrangement of permanent magnets secured about the mounting shaft. Signals from the second two-axis Hall-effect sensor generated as the poles of the annular arrangement of permanent magnets rotate about the axis of the mounting shaft are conveyed to a sensor processor and utilized to store, in a sensor memory area, one or more fine absolute mounting shaft rotational positions. An internal power source, such as a capacitor maintains the integrity of the sensor memory for a definite span of time during momentary power losses such as battery changes or during overnight shutdowns, permitting the mounting shaft runout compensation values to be maintained and reutilized upon the restoration of system power, without the need to repeat the runout compensation procedures. 
   As a method, the present invention requires a vehicle wheel alignment sensor which has been previously mounted to a vehicle wheel and compensated for runout. To restore or identify an absolute rotational position of the mounting shaft relative to the vehicle wheel alignment sensor, the wheel alignment sensor is rotated about the mounting shaft through at least an arc sufficient to provide a measurable change in a magnetic field associated with the two-axis Hall-effect sensor, to identify the current absolute rotational position of the vehicle wheel alignment sensor mounting shaft. The current absolute rotational position is then utilized to determine the associated runout compensation value for the current sensor rotational position, using data stored in a persistent sensor memory during a runout compensation procedure, thereby permitting an operator to return the vehicle wheel alignment sensor to a previous rotational position or utilize stored runout compensation data following a general power-down or momentary power loss, such as battery contact failure or during battery replacement or recharging. 
   An alternate method, the present invention requires a vehicle wheel alignment sensor which has been previously mounted to a vehicle wheel and compensated for runout. To restore or identify an absolute rotational position of the mounting shaft relative to the vehicle wheel alignment sensor, the wheel alignment sensor is rotated about the mounting shaft through at least an arc sufficient to provide a measurable change in a magnetic field associated with the two-axis Hall-effect sensor disposed coaxial with the mounting shaft, to identify a course absolute rotational position of the mounting shaft. Simultaneously, a fine absolute rotational position of the vehicle wheel alignment sensor mounting shaft is acquired by a second two-axis Hall-effect sensor disposed adjacent the mounting shaft. The current coarse and fine absolute rotational positions are utilized to determine the associated runout compensation value for the current sensor mounting shaft rotational position, using data stored in a persistent sensor memory during a runout compensation procedure, thereby permitting an operator to return the vehicle wheel alignment sensor to a previous rotational position or utilize stored runout compensation data following a general power-down or momentary power loss, such as battery contact failure or during battery replacement or recharging. 
   The foregoing and other objects, features, and advantages of the invention as well as presently preferred embodiments thereof will become more apparent from the reading of the following description in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     In the accompanying drawings which form part of the specification: 
       FIG. 1  is a side view in schematic form of a vehicle wheel together with a prior art cordless vehicle wheel alignment sensor mounted pendulously to the wheel; 
       FIG. 2  is an exemplary illustration of the sinusoidal waveform of the runout of the vehicle wheel and alignment sensor shown in  FIG. 1  in the toe plane; 
       FIG. 3  is a perspective illustration of a prior art two-axis Hall-Effect sensor and associated rotating permanent magnet; 
       FIG. 4  is a graphical representation of voltages generated in the two-axis Hall-Effect sensor of  FIG. 3  by the rotation of the permanent mag 
       FIG. 5  is a block diagram of the components of the prior art two-axis Hall-Effect sensor of  FIG. 3 ; 
       FIG. 6  is a perspective illustration of a vehicle wheel alignment sensor of the present invention incorporating a two-axis Hall-Effect sensor for obtaining absolute rotational position measurement; 
       FIG. 7  is a block diagram representation of the logic circuits of a vehicle wheel alignment sensor of the present invention; 
       FIG. 8  is a perspective illustration of an alternate embodiment vehicle wheel alignment sensor of the present invention, incorporating a pair of two-axis Hall-Effect sensors for obtaining absolute rotational position measurements with a fine degree of precision; 
       FIG. 9  is a block diagram representation of the logic circuit of an alternate embodiment vehicle wheel alignment sensor of the present invention adapted to provide coarse and fine rotational position measurements; and 
       FIG. 10  is a graphical representation of an exemplary prior art output of a pair of two-axis Hall-Effect sensors for obtaining absolute rotational position measurements with a fine degree of precision. 
   

   Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings. 
   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The following detailed description illustrates the invention by way of example and not by way of limitation. The description clearly enables one skilled in the art to make and use the invention, describes several embodiments, adaptations, variations, alternatives, and uses of the invention, including what is presently believed to be the best mode of carrying out the invention. 
   Referring to  FIG. 1 , there is shown a vehicle wheel  10  of an automotive vehicle, to which a vehicle wheel alignment sensor unit  12  is mounted by means of a suitable conventional wheel clamp  14 . The wheel alignment sensor unit  12  is pendulously mounted to the wheel  10  through wheel clamp  14  on a mounting shaft  15  so as to swing freely about an axis which is approximately coaxial with the axis of rotation  16  of the wheel  10 . The sensor unit  12  carries a first angle sensor  18  which develops an electric signal representative of the angular position of the sensor unit  12  relative to the vertical plane. A second angle sensor  20 , also carried by the sensor unit  12 , develops an electric signal representative of the angular position of the sensor unit  12  relative to the horizontal plane. The angle sensors  18  and  20  are conventional in the wheel alignment art for making camber and toe measurements, and additional sensors (not shown) are commonly carried by the sensor unit  12  for making other angle measurements used in the alignment of the wheels of a vehicle. 
   It is well known that any wobble of the wheel  10  or of the sensor unit  12  during rotation about the axis of rotation affects the measurements made by the angle sensor  18  and  20 . As can be seen in  FIG. 2 , wobble or runout present may be represented as a sinusoidal waveform, where the amplitude of the waveform at a given rotational position of the wheel and/or sensor represents the amount of runout present at that rotational position. It is necessary, therefore, either to eliminate the wobble or runout, or to compensate for it. Since in many cases it is impractical to eliminate the wobble, the usual practice is to compensate the acquired toe angle and camber angle measurements to correct for the wobble or runout at a corresponding rotational position at which the measurement was acquired. A suitable method for calculating and utilizing runout present at each rotational position is described in U.S. Pat. No. 5,052,111 to Carter, et al. 
   Turning to  FIG. 3 , an absolute rotational position sensor assembly  22  is illustrated generally. The absolute rotational position sensor assembly  22  includes a two-axis Hall Effect sensor  24  disposed coaxially about an axis φ with a single pole pair magnet  26  coupled to a rotating shaft  28 . The magnet  26  is magnetized diametrically, so that by rotating the shaft  28 , the magnetic field generated by the magnet  26  also rotates. The rotation of the magnetic field through the two-axis Hall Effect sensor  24  results in two generated voltages, Vx and Vy, which represent the sine and cosine of the magnetic field direction. As shown in  FIG. 4 , calculating a ratio of Vx to Vy yields a representation of the rotational position of the rotating shaft  28  relative to the two-axis Hall Effect sensor  24  from an initial position, i.e. provides an absolute rotational position of the rotating shaft  28 . A suitable two-axis Hall Effect sensor is available from Sentron AG, of Zug, Switzerland, or GMW of San Carlos, Calif., and sold under the product identification 2SA-10. 
   As shown in  FIG. 5 , the two-axis Hall Effect sensor  24  preferably includes an X-axis Hall effect sensor  30 , a Y-axis Hall effect sensor  32 , a pair of offset cancellation circuits  34 A,  34 B, amplification circuits  36 . Programming circuits  38  are provided for enabling and setting a bias circuit  40  for the Hall effect sensors, offset parameters, and amplification parameters. Preferably, signals from the Hall effect sensors  30 ,  32  are routed through the offset cancellation circuits  34 A,  34 B and modulated in a modulator circuit  42  prior to amplification. After the modulated signal is amplified, a demodulator circuit  44  separated the corresponding X and Y axis signal, which are then routed through corresponding filters and buffering circuits  46 A,  46 B for output. 
   Input signals to the two-axis Hall Effect sensor  24  shown in  FIG. 5  include a supply voltage (VDD), a supply ground (GND), a clock signal (PC), a programming voltage signal (PV), and a programming data signal (PD). Output signals include a common output signal (CO_OUT), an X-channel analog output (X_OUT), and a Y-channel analog output (Y_OUT). 
   Those of ordinary skill in the art will recognize that the two-axis Hall Effect sensor  24  may be implemented as an integrated circuit component, or as two discrete Hall Effect sensors mounted in suitable configuration to provide sine and cosine representative values of the rotating magnetic fields. Optionally, a single Hall Effect sensor may be employed to provide 180° of rotational resolution, combined with a suitable mechanism to identify within which half-circle of a complete rotation of the mounting shaft the rotational position measurement has been acquired. 
   Turning to  FIG. 6 , a vehicle wheel alignment sensor unit  100  of the present invention is shown with an absolute rotational position sensor assembly  22  operatively associated with an alignment sensor mounting shaft  102 . Housing components which surround and support the mounting shaft  102 , and which comprise the body of the vehicle wheel alignment sensor unit  100  are shown in phantom in  FIG. 6  for purposes of clarity. The absolute rotational position sensor assembly  22  is disposed on a supporting structure  104  coaxial with, and adjacent an end of, the mounting shaft  102 . 
   A magnet  106  which is magnetized across a diameter is disposed on the end of the mounting shaft  102 , parallel to the absolute rotational position sensor assembly  22 , such that rotation of the mounting shaft  102  will result in a corresponding rotation of the magnet  106  and an associated magnetic field. 
   As shown in  FIG. 7 , output signals from the absolute rotational position sensor assembly  22  are routed to a micro-processor or logic circuit  110  in the vehicle wheel alignment sensor unit  100 . In addition to receiving signals from the absolute rotational position sensor assembly  22 , the micro-processor or logic circuit  110  is configured to communicate with the conventional components of the wheel alignment sensor unit  100 . These include the angle sensors  18  and  20 , a sensor memory  112 , a communications transceiver  114 , such as a radio-frequency or infra-red communications unit, and one or more conventional operator I/O devices  116  such as buttons or LEDs disposed on the wheel alignment sensor unit  100 . The sensor memory  112  is preferably linked to a short-term power supply  113 , such as an internal battery or a super-capacitor, capable of providing sufficient power to maintain stored data in the sensor memory  112  during interruption or shutdown of a normal power supply (not show). Alternatively, sensor memory  112  may be a form of re-writable persistent memory, such as MRAM, which does not require a continuous supply of power to maintain stored data values. 
   In addition to being configured to perform the conventional functions of a vehicle wheel alignment sensor, the micro-processor or logic circuit  110  is configured to utilize the signals received from the absolute rotational position sensor assembly  22  to identify an absolute rotational position of the mounting shaft  102  relative to the vehicle wheel alignment sensor unit  100 . The absolute rotational position sensor  22  provides two pieces of information to the micro-processor or logic circuit  110 , a rotational distance and a direction of rotation. Using a known or identified mounting shaft parameter correlated with one or more absolute rotational positions stored in a persistent sensor memory  118  such as an ROM, EPROM, or EEPROM, the micro-processor or logic circuit  110  determines an absolute rotational position of the mounting shaft  102  relative to the vehicle wheel alignment sensor unit  100  and the vehicle wheel  10 , or to a vertical (gravity) orientation. Subsequent rotation of the mounting shaft  102  relative to the vehicle wheel alignment sensor unit  100  is tracked in a conventional manner by the micro-processor or logic circuit  110  using signals received from the absolute rotational position sensor  22 , once the initial absolute rotational position has been identified. 
   During use, a vehicle wheel alignment sensor unit  100  incorporating the absolute rotational position sensor assembly  22  of the first embodiment is secured to a vehicle wheel, such as through the use of a wheel clamp  14 . Prior to the obtaining the first vehicle wheel alignment measurements, the vehicle wheel alignment sensor unit  100  must be compensated for any runout or wobble present in the mounting to the vehicle wheel  10 . A runout compensation procedure is completed, and data representative of, or sufficient to reconstruct, a sinusoidal pattern of runout present for a complete rotation about the mounting shaft  102  is obtained and stored in the sensor memory  112 . 
   As previously described, to compensate a vehicle wheel alignment measurement for runout between the vehicle wheel alignment sensor unit  100  and the vehicle wheel  10 , it is necessary to know the rotational position of one relative to the other about the mounting shaft  102 , as well as the corresponding runout value for that rotational position. Upon completion of the runout compensation procedure, the micro-processor or logic circuit  110  continuously tracks all subsequent rotational movements of the mounting shaft  102  relative to the vehicle wheel alignment sensor unit  100  through signals obtained from the absolute rotational position sensor  22 . In addition, upon completion of the runout compensation procedure, the absolute rotational position sensor assembly  22  of the present invention is utilized by the micro-processor or logic circuit  110  to identify an absolute rotational position RC 1  of the vehicle wheel alignment sensor unit  100  associated with at least one point on the runout compensation sinusoidal waveform. Position RC 1  is stored in the sensor memory  112 , together with sufficient information to reconstruct the runout sinusoidal waveform for each rotational position of the vehicle wheel alignment sensor unit  100 . 
   Upon restoration of power following an interruption in power supplied to the vehicle wheel alignment sensor unit  100 , such as may be caused by a battery discharge, poor electrical contact with the battery leads, or an intentional operator shutdown while in use, which results in a discontinuity in the tracking of the rotational movements or position of the mounting shaft  102  relative to the wheel alignment sensor unit  100 , the micro-processor or logic circuit  110  is configured to utilize the data stored in the sensor memory  112 , together with a new absolute rotational position measurement, to resume normal sensor operation without the need to repeat the runout compensation procedures. 
   Assuming that the vehicle wheel alignment sensor unit  100  has not been dismounted from the vehicle wheel  10  during the interruption in power or shutdown, the runout compensation values previously obtained and stored in the sensor memory  112  remain valid for all rotational positions of the vehicle wheel alignment sensor unit  100 . What is unknown immediately after restoration of the power or restart of the system is, the current rotational position of the mounting shaft  102  relative to the vehicle wheel alignment sensor unit  100 . For example, it is possible that the mounting shaft  102  was rotated relative to the vehicle wheel alignment sensor unit  100  during the time the power was interrupted, or the vehicle wheel  10  was rolled forward or backwards. 
   To re-synchronize the current rotational position of the vehicle wheel alignment sensor unit  100  and the stored runout compensation values, the micro-processor or logic circuit  110  is configured to utilize the absolute rotational position sensor assembly  22  of the present invention to obtain a current absolute rotational position RC 2  for the vehicle wheel alignment sensor unit  100 . Once the current absolute rotational position RC 2  of the mounting shaft  102  relative to the vehicle wheel alignment sensor unit  100  is obtained by the micro-processor or logic circuit  110 , the current absolute rotational position RC 2  is utilized together with the stored data representative of the sinusoidal runout pattern and previous absolute rotational position RC 1  to re-synchronize the rotation of the mounting shaft  102  relative to the vehicle wheel alignment sensor unit  100  with the previously determined runout compensation sinusoidal waveform. Subsequent rotation of the mounting shaft  102  relative to the vehicle wheel alignment sensor unit  100  is tracked by the absolute rotation position sensor  22 , and an associated runout compensation value obtained by the micro-processor or logic circuit  110  using the stored runout sinusoidal waveform data. 
   Using the absolute rotational position sensor assembly  22  of the present invention further permits the micro-processor or logic circuit  110  to identify a specific or predetermined absolute rotational position of the mounting shaft  102 , such as a “zero” position, “gravity referenced” position, or other operator identified rotational position, and to guide an operator to return the vehicle wheel alignment sensor unit  100  to the identified absolute rotational position at any point during a vehicle wheel alignment procedure, including subsequent to a loss of power to the vehicle wheel alignment sensor unit  100  or system shut down. 
   Turning to  FIG. 8 , an alternate embodiment vehicle wheel alignment sensor unit  200  of the present invention is shown with a pair of absolute rotational position sensor assemblies  22 A and  22 B for providing increased absolute rotational position measurements. The first absolute rotational position sensor assembly  22 A is operatively disposed adjacent to, and coaxial with, and end of the mounting shaft  202  of the vehicle wheel alignment sensor unit  200 . The second absolute rotational position sensor assembly  22 B is operatively disposed parallel to, and adjacent, the mounting shaft  202 . Housing components which surround and support the mounting shaft  202 , and which comprise the body of the vehicle wheel alignment sensor unit  200  are shown in phantom in  FIG. 8  for purposes of clarity. 
   The first absolute rotational position sensor assembly  22 A is disposed on a supporting structure  204  coaxial with, and adjacent an end of, the mounting shaft  202 . A magnet  206  which is magnetized across a diameter is disposed on the end of the mounting shaft  202 , parallel to the absolute rotational position sensor assembly  22 A, such that rotation of the mounting shaft  202  will result in a corresponding rotation of the magnet  206  and an associated magnetic field. 
   The second absolute rotational position sensor assembly  22 B is disposed on a second supporting structure  208  oriented adjacent to, and perpendicular with, the axis  16  of the mounting shaft  202 . An annular magnet  210  is fixed about the mounting shaft  202 , coplanar with the second absolute rotational position sensor assembly  22 B on the second supporting structure  208 . The annular or ring magnet  210  includes four or more equally spaced pole pairs  210 N,  210 S. Rotation of the mounting shaft  202  will result in a corresponding rotation of the ring magnet  210  about the axis  16  and the oscillation of an associated magnetic field at the location of the second absolute rotational position sensor assembly  22 B. The oscillations of the magnetic field associated with the annular magnet  210  results in “n” electrical cycles of sine and cosine voltage signals from the second absolute rotational position sensor assembly  22 B, where “n” is the number of pole pairs  210 N,  210 S in the annular magnet  210 . 
   As shown in  FIG. 9 , output signals from the first and second absolute rotational position sensor assemblies  22 A and  22 B are routed to a micro-processor or logic circuit  211  in the vehicle wheel alignment sensor unit  200 . Output signals from the first absolute rotational position sensor assembly  22 A provide one cycle of sine and cosine voltage signals per rotation of the mounting shaft  202 , identical to the operation of absolute rotational position sensor assembly  22  as previously described in connection with embodiment  100 . In contrast to the output signals from the second absolute rotational position sensor assembly  22 B, the output signals from the first sensor assembly  22 A are considered “coarse” rotational position measurements. The “coarse” rotational position measurement is utilized by the logic circuit or micro-processor  211  to identify which pole pair  210 N,  210 S of the ring magnet  210  is currently disposed adjacent to the second absolute rotational position sensor assembly  22 B. 
   Since each pole pair  210 N,  210 S of the annular or ring magnet  210  is equally sized and spaced, i.e. occupies an equal arc about the circumference of the annular or ring magnet  210 , identification of a single pole pair  210 N,  210 S identifies a arcuate range within which the rotational position of the mounting shaft  202  is currently disposed. Output signals from the second absolute rotational position sensor assembly  22 B may then be used to identify a highly accurate or “fine” rotational position of the mounting shaft  202  within the “coarse” arcuate range (360°/n) identified by the output signals from the first absolute rotational position sensor assembly  22 A. The degree of accuracy within the “fine” range is limited to the measurement precision of the second absolute rotational position sensor assembly  22 B. Exemplary output signals from the pair of absolute rotational position sensor assemblies  22 A and  22 B are shown in  FIG. 10 . 
   In addition to receiving signals from each absolute rotational position sensor assembly  22 A,  22 B, the micro-processor or logic circuit  211  is configured to communicate with the conventional components of the wheel alignment sensor unit  200 . These include the angle sensors  18  and  20 , a sensor memory  212 , a communications transceiver  214 , such as a radio-frequency or infra-red communications unit, and one or more conventional operator I/O devices  216  such as buttons or LEDs disposed on the wheel alignment sensor unit  200 . The sensor memory  212  is preferably linked to a short-term power supply  213 , such as an internal battery or a super-capacitor, capable of providing sufficient power to maintain stored data in the sensor memory  212  during interruption or shutdown of a normal power supply (not show). Alternatively, sensor memory  212  may be a form of re-writable persistent memory, such as MRAM, which does not require a continuous supply of power to maintain stored data values. 
   In addition to being configured to perform the conventional functions of a vehicle wheel alignment sensor, the micro-processor or logic circuit  211  is configured to utilize the signals received from the absolute rotational position sensor assemblies  22 A and  22 B to identify a high precision absolute rotational position of the mounting shaft  202  relative to the vehicle wheel alignment sensor unit  200 . Using a known or identified mounting shaft parameter correlated with one or more absolute rotational positions stored in a persistent sensor memory  218  such as an ROM, EPROM, or EEPROM, the micro-processor or logic circuit  211  determines an absolute rotational position of the mounting shaft  202  relative to, the vehicle wheel alignment sensor unit  200  and the vehicle wheel  10 , or to a vertical (gravity) orientation, to a high degree of precision. Subsequent rotation of the mounting shaft  202  relative to the vehicle wheel alignment sensor unit  200  is tracked in a conventional manner by the micro-processor or logic circuit  211  using signals received from the absolute rotational position sensors  22 A and  22 B, once the initial absolute rotational position has been identified. Use of the vehicle wheel alignment sensor unit  200  is substantially identical to that embodiment  100  described above, but with a greater degree of precision in the absolute rotational position measurements. 
   In an alternate method of use, a vehicle wheel alignment sensor  100  or  200  of the present invention may be mounted to a conventional “no-compensation” type wheel adapter. A no-compensation wheel adapter, such as shown in U.S. Pat. No. 6,427,346 B1 to Stieff et al, herein incorporated by reference, is designed to facilitate attachment of a wheel alignment sensor unit  100 ,  200  to a vehicle wheel  10  without the need for any runout compensation. This type of wheel adapter operates on the assumption that the runout of the vehicle wheel is negligible, and that the manufacturing process of the wheel adapter itself does not induce any additional runout in the system, hence there is no need to rotate the vehicle wheel  10  or the wheel alignment sensor unit  100 ,  200  to different positions to compensate for runout within the system. These no-compensation wheel adapters are configured to minimize orientation errors. By configuring the wheel adapter to contact a vehicle wheel  10  (or other suspension component) in a reliable and repeatable manner, and by choosing points on the vehicle wheel  10  (or other suspension component) that provide a reference which closely represents that plane of rotation of the vehicle wheel  10 , mounting errors incurred by the wheel adapter can be minimized. Careful fabrication of the wheel adapter itself to minimal tolerances minimizes any position and orientation errors between the mounting shaft  102 ,  202  and the wheel adapter, and the wheel adapter contact points on the vehicle wheel  10  (or other suspension component). 
   During mounting of the vehicle wheel alignment sensor unit  100 ,  200  to a no-compensation type wheel adapter, a technician is required to determine when the wheel alignment sensor unit  100 ,  200  is aligned with the scribed mark on the mounting shaft  102 ,  202  at the top-dead-center position, thereby mounting the wheel alignment sensor unit  100 ,  200  to the no-compensation adapter in a repeatable manner. By predetermination of an absolute rotational position of the mounting shaft  102 ,  202  relative to the vehicle wheel alignment sensor unit  100 ,  200  at the scribed mark, the logic circuit or micro-processor  110 ,  211  of the present invention may be configured to guide an operator with electronic guidance to correctly mount the wheel alignment sensor unit  100 ,  200  on a no-compensation type wheel adapter. Signals from the absolute rotational position sensor  22  identify to the micro-controller or logic circuit  110 ,  211  when the wheel alignment sensor unit  100  is rotational aligned to the desired position. 
   The micro-processor or logic circuit  110 ,  211  may be configured to provide LED illumination or a directional indication identifying the rotational position or direction to which the operator should rotate the wheel alignment sensor unit  100 ,  200  for mounting on the no-compensation type adapter at the top-dead-center or desired position. 
   The present invention can be embodied in the form of computer-implemented processes and apparatuses for practicing those processes. The present invention can also be embodied in the form of computer program code containing instructions embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, or an other computer readable storage medium, wherein, when the computer program code is loaded into, and executed by, an electronic device such as a computer, micro-processor or logic circuit, the device becomes an apparatus for practicing the invention. 
   The present invention can also be embodied in the form of computer program code, for example, whether stored in a storage medium, loaded into and/or executed by a computer, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. When implemented in a general-purpose microprocessor, the computer program code segments configure the microprocessor to create specific logic circuits. 
   In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results are obtained. As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.