Abstract:
A photo-interruptive sensor and an associated interrupter disk are configured to operate in conjunction with a relative rotational position sensor to provide an absolute rotational position of a mounting shaft relative to a vehicle wheel alignment sensor unit. The interrupter disk is secured relative to the mounting shaft of the vehicle wheel alignment sensor, and the photo-interruptive sensor is secured in operative relationship to the interrupter disk. The interrupter disk is configured with a raised peripheral lip having multiple teeth and gaps, each of a unique arcuate length. Signals from the photo-interruptive sensor, together with relative rotational position signals from the relative rotational position sensor in operative relationship to 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 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  
         [0001]    Not Applicable.  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
         [0002]    Not Applicable.  
         BACKGROUND OF THE INVENTION  
         [0003]    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 tracking the absolute mounting shaft rotational position of the vehicle wheel alignment sensor after it has been mounted to a vehicle wheel.  
           [0004]    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.  
           [0005]    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 Carteretal.  
           [0006]    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 is calculated from the previously obtained sinusoidal pattern.  
           [0007]    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.  
           [0008]    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.  
           [0009]    It is known that an absolute rotational position sensor may be utilized in place of the relative rotational position sensor in a cordless vehicle wheel alignment sensor. An absolute rotational position sensor 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. 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 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 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.  
           [0010]    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 in the vehicle wheel alignment sensor.  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    Briefly stated, an apparatus of the present invention incorporated into a conventional cordless vehicle wheel alignment sensor consists of a photo-interruptive sensor and an associated interrupter disk configured to operate in conjunction with a conventional relative rotational position sensor to provide an absolute rotational position of the mounting shaft relative to the vehicle wheel alignment sensor. The interrupter disk is secured to the mounting shaft of the vehicle wheel alignment sensor, and the photo-interruptive sensor is secured to the body of the vehicle wheel alignment sensor, in operative relationship to the interrupter disk. The interrupter disk is configured with a raised peripheral lip having multiple gaps and teeth, each having a unique size. Signals from the photo-interruptive sensor, together with relative rotational position signals from the relative rotational position sensor in operative relationship to 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.  
           [0012]    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 sensor is rotated about the mounting shaft through at least an arc sufficient to completely traverse at least one uniquely sized tooth or gap on an interrupter disk associated with the mounting shaft. Signals from a photo-interruptive sensor, together with a relative rotational position sensor signal, identify the unique size of the traversed tooth or gap on the interrupter disk. The identified unique size of the traversed tooth or gap is compared with permanently stored information 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.  
           [0013]    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  
       [0014]    In the accompanying drawings which form part of the specification:  
         [0015]    [0015]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;  
         [0016]    [0016]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;  
         [0017]    [0017]FIG. 3 is a perspective view of an absolute rotational position sensor apparatus of the present invention shown coupled to a vehicle wheel alignment sensor unit mounting shaft;  
         [0018]    [0018]FIG. 4 is a top plan view of an interrupter disk of the present invention;  
         [0019]    [0019]FIG. 5 is a perspective view of the interrupter disk of FIG. 4;  
         [0020]    [0020]FIG. 6 is a block diagram view of the components of a cordless vehicle wheel alignment sensor incorporating the present invention;  
         [0021]    [0021]FIG. 7 is a perspective view of an alternate embodiment of the interrupter disk of FIG. 4;  
         [0022]    [0022]FIG. 8 is a block diagram view of the components of a cordless vehicle wheel alignment sensor incorporating an alternate embodiment of the present invention; and  
         [0023]    [0023]FIG. 9 is a perspective view of an alternate embodiment of the absolute rotational position sensor apparatus of the present invention, shown coupled to a vehicle wheel alignment sensor unit mounting shaft; 
     
    
       [0024]    Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings.  
       DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    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.  
         [0026]    Referring to FIG. 1, there is shown a vehicle wheel  10  of an automotive vehicle, to which a conventional cordless 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.  
         [0027]    The sensor unit  12  also carries a relative rotational position sensor  22  disposed in operative relationship to the mounting shaft  15 , which develops an electric signal when the sensor unit  12  and vehicle wheel  10  rotate relative to each other. The signal, which may consist of a train of unidirectional pulses generated for each predetermined increment of rotation between the sensor unit  12  and the mounting shaft  15 , is used to correlate the measurements made by the angle sensors  18  and  20  with the rotational position of the mounting shaft  15  relative to the vehicle wheel  10  and/or sensor unit  12  at the time the measurements are made. Measurements are made both during runout compensation procedures and in subsequent wheel alignment measurements. The relative rotational position sensor  22  may be of conventional design, directly driven by the mounting shaft  15 , or may be indirectly driven through a one or more gears  24 A and  24 B to provide an increased rotational position resolution.  
         [0028]    It is well known that any wobble of the wheel  10  or of the sensor unit  12  as during 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 modify the actual toe and camber measurements to correct for the wobble or runout at each rotational position. 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.  
         [0029]    Turning to FIG. 3, an absolute rotational position sensor assembly of the present invention is shown generally at  26 , coupled to mounting shaft  15 . Housing components which surround and support the mounting shaft  15 , and which comprise the body of the vehicle wheel alignment sensor unit  12  are shown in phantom in FIG. 3 for purposes of clarity. The absolute rotational position sensor assembly  26  preferably consists of an interrupter disk  28  secured in a fixed coaxial relationship with the mounting shaft  15 , and a photo-interrupter sensor  30  operatively disposed adjacent the interrupter disk  28 . The absolute rotational position sensor assembly  26  operates in cooperation with the conventional relative rotational position sensor  22  which is operatively coupled to the mounting shaft  15  through gears  24 A and  24 B. Preferably, gears  24 A and  24 B are selected such that gear  24 B, about which the relative rotational position sensor  22  is disposed, rotates through one complete revolution for each ¼ revolution of gear  24 A, which is in a fixed coaxial relationship with the mounting shaft  15 , thereby providing increased precision in measuring the rotation of the mounting shaft  15 .  
         [0030]    The interrupter disk  28 , shown in FIGS. 4 and 5, consists of a circular base  31  which is mounted in a fixed relationship coaxial with the mounting shaft  15 . The circular base  31  may be either secured to the face of gear  24 A, which is, in turn, mounted in a fixed relationship coaxial with the mounting shaft  15 , or directly about the mounting shaft  15 . A plurality of merlons or teeth  32  are disposed on a face  34  of the circular base  31 , adjacent the circumferential edge  36 . Starting from a predetermined position on the circumferential edge  36 , designated as the 0° base point, each sequential tooth  32  has a unique and increasing arcuate length. Correspondingly, a crenellation or gap  38  is disposed between each sequential tooth or merlon  32 , having a unique and decreasing arcuate length.  
         [0031]    Preferably, there are a total of fifteen unique teeth  32  disposed on the face  34 , and a total of fifteen interposed unique gaps  38 . As shown in the preferred embodiment of FIG. 4, the smallest tooth  32  has an arcuate length of  40 , and each counter-clockwise sequential tooth  32  increases in arcuate length by 1.250, to a maximum tooth  32  arcuate length of 21.50. Correspondingly, the largest gap  38 , disposed between the first and second teeth  32  has an arcuate length of 20°, with each counter-clockwise sequential gap  38  decreasing in arcuate length by 1.250 to a minimum gap  38  arcuate length of 2.50. Since each tooth  32  and gap  38  has a unique arcuate length, each transition point between a tooth  32  and a gap  38  defines a known absolute rotational position measured relative to a predetermined rotational position or origin point about the circumferential edge  36 . Those of ordinary skill in the art will recognize that the number, sizes, and placement of the unique teeth  32 , and correspondingly, the unique gaps  38  may be altered within the scope of this invention, so long as each remains uniquely sized, and the circumferential placement of each is known relative to the 0° base point.  
         [0032]    Correspondingly, those of ordinary skill in the art will further recognize that alternate embodiments of the present invention may be constructed by replacing the interrupter disk  28  and photo-interrupter sensor  30  with one or more unique markings disposed in a fixed relationship to the mounting shaft  15 , and a corresponding sensor configured to observe the unique marking. For example, an identification disk having alternating light and dark markings of unique sizes at known rotational positions may be operatively secured coaxial with the mounting shaft  15 , and observed by an optical sensor in the same manner as the teeth  32  and gaps  38  of the interrupter disk  28 .  
         [0033]    As shown in FIG. 3, the photo-interrupter sensor  30  of the preferred embodiment is disposed in operative relationship to the interrupter disk  28 , such that during rotation of the interrupter disk  28 , the alternative teeth  32  and gaps  38  pass through a detector region of the photo-interrupter sensor  30 . Photo-interrupt sensor  30  is of a conventional design, and includes a detector region in which light passes, such that a sensor detects the presence or absence of light. Preferably, when a gap  38  is rotated through the detector region of the photo-interrupter sensor  30 , a first signal is generated, representing an uninterrupted state. When a tooth  32  is rotated through the detector region of the photo-interrupter sensor  30 , a second signal is generated, representing an interrupted state.  
         [0034]    As shown in FIG. 6, the first and second signals from the photo-interrupter sensor  30  are routed to a micro-processor or logic circuit  100  in the vehicle wheel alignment sensor unit  12 , and provide sufficient information to identify edge transition points on the interrupter disk  28  between each tooth  32  and an adjacent gap  38 , as the edge transition passes through the detector region of the photo-interrupter sensor  30 . In addition to receiving signals from the photo-interrupter sensor  30 , the micro-processor or logic circuit  100  is configured to communicate with the conventional components of the wheel alignment sensor unit  12 . These include the angle sensors  18 ,  20 , the relative rotational position sensor  22 , a sensor memory  102 , a communications transceiver  104 , such as a radio-frequency or infra-red communications unit, and one or more conventional operator I/O devices  106  such as buttons or LEDs disposed on the wheel alignment sensor unit  12 . The sensor memory  102  is preferably linked to a short-term power supply  103 , such as an internal battery or a super-capacitor, capable of providing sufficient power to maintain stored data in the sensor memory  102  during interruption or shutdown of a normal power supply (not show). Alternatively, sensor memory  102  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.  
         [0035]    In addition to being configured to perform the conventional functions of a vehicle wheel alignment sensor, the micro-processor or logic circuit  100  is configured to utilize the signals received from the photo-interrupter sensor  30  together with signals received from the relative rotational position sensor  22  to identify an absolute rotational position of the mounting shaft  15  relative to the vehicle wheel alignment sensor unit  12 . The relative rotational position sensor  22  provides two pieces of information to the micro-processor or logic circuit  100 , a rotational distance and a direction of rotation. The rotational distance identifies the length of an arc about the mounting shaft  15  in which the vehicle wheel alignment sensor unit  12  has rotated, while the direction of rotation identifies if the rotation is clockwise or counter-clockwise. Combined with a pair of sequential signals from the photo-interruptive sensor  30  identifying a first and second edge transitions between a tooth  32  and a gap  38 , the micro-processor or logic circuit  100  can identify the unique arc length of each tooth  32  or gap  38  passing through the detector region of the photo-interruptive sensor  30 .  
         [0036]    Using a look-up table or known relationships correlating tooth and gap arc length with absolute rotational positions stored in a persistent sensor memory  108  such as an ROM, EPROM, or EEPROM, the micro-processor or logic circuit  100  determines, from the identified unique arc length of a tooth  32  or gap  38  passing through the detector region of the photo-interruptive sensor  30 , an absolute rotational position of the mounting shaft  15  relative to the vehicle wheel alignment sensor unit  12  and vehicle wheel  10 . The absolute rotational position from the 0° base point is identified at the second edge transition between a tooth  32  and gap  38  as it passes through the detector region of the photo-interruptive sensor  30 . Subsequent rotation of the mounting shaft  15  relative to the vehicle wheel alignment sensor unit  12  is tracked in a conventional manner by the micro-processor or logic circuit  100  using signals received from the relative rotational position sensor, once the absolute rotational position has been identified.  
         [0037]    In an alternate embodiment of the present invention, shown in FIG. 7, a circumferential flange  39  is incorporated into the base  31  of the interrupter disk  28  to facilitate identification of one or more predetermined rotational positions of the mounting shaft  15  relative to the vehicle wheel alignment sensor unit  12  without requiring rotational movement thereof. The circumferential flange  39  includes a pair of diametrically opposed slots  41 A and  41 B. Slot  41 A is radially aligned with a gap  38 , while slot  41  B is radially aligned with a tooth  32 , thereby providing for unique identification of each slot  41 A and  41 B. During assembly, one of the slots  41 A or  41 B is aligned with an indicator marking on the mounting shaft  15 , thereby providing a fixed reference rotational position for the mounting shaft  15  relative to the vehicle wheel alignment sensor unit  12 .  
         [0038]    An adapter photo-interrupter sensor  40  of the alternate embodiment is disposed in operative relationship to the circumferential flange  39 , such that during rotation of the interrupter disk  28 , the slots  41 A and  41 B pass through a detector region of the photo-interrupter sensor  40 . Preferably, the adapter photo-interrupter sensor  40  and the photo-interrupter sensor  30  are in radial alignment about the mounting shaft  15 , as shown in FIG. 9, however, those of ordinary skill in the art will recognize that the adapter photo-interrupt sensor  40  and the photo-interrupt sensor  30  may be radially displaced from each other in a known relationship. Adapter photo-interrupt sensor  40  is of a conventional design, and includes a detector region in which light passes, such that a sensor detects the presence or absence of light. Preferably, adapter photo-interrupt sensor  40  is configured to detect a reflection of light from a reflective surface disposed behind the circumferential flange  39  when slots  41 A and  41 B are within the detector region.  
         [0039]    The reflective surface may optionally be disposed on the face of the gear  24 A or other structure disposed behind the circumferential flange  39  when the interrupter disk  28  is secured to the mounting shaft  15  as previously described. Preferably, the circumferential flange  39  is composed of, or coated with, a non-reflective material, thereby enhancing the contrast with the reflective material. However, those of ordinary skill in the art will recognize that the non-reflective and reflective materials may be varied or swapped, provided that the adapter photo-detector  40  or other suitable sensor can optically detect the presence of a slot  41 A or  41 B within the detector region.  
         [0040]    As shown in the alternate embodiment circuit of FIG. 8, the signals from the adapter photo-interrupter sensor  40  are routed to a micro-processor or logic circuit  100  in the vehicle wheel alignment sensor unit  12 , together with the signal from the photo-interrupter sensor  30 . The signals from the photo-interrupter sensor  30  provide sufficient information to identify the presence of a tooth  32  or gap  38  within the detector region of the photo-interrupter sensor  30 . Correspondingly, signals from the adapter photo-interrupter sensor  40  provide sufficient information to identify the presence or absence of a slot  41 A or  41 B within the associated detector region.  
         [0041]    In addition to being configured to perform the conventional functions of a vehicle wheel alignment sensor as described above, the micro-processor or logic circuit  100  is configured to utilize the detection of a slot  41 A or  41 B, together with the signal from the photo-interrupter sensor  30  identifying the presence of either a tooth  32  or gap  38 , to uniquely identify the detection of slot  41 A or  41 B, and the corresponding rotational position of the mounting shaft  15  relative to the vehicle wheel alignment sensor unit  12 , i.e. relative to the vehicle wheel  10 . This unique identification of slot  41 A or  41 B, or the detection of the absence of slot  41 A or  41 B from the detector region of adapter photo-interrupter  40  occurs without the need to rotate the mounting shaft  15  relative to the vehicle wheel alignment sensor unit  12 .  
         [0042]    Using a look-up table correlating slots  41 A and  41 B with absolute rotational positions, stored in a persistent sensor memory  108  such as an ROM, EPROM, or EEPROM, the micro-processor or logic circuit  100  determines, from the detected presence of a slot  41 A or  41 B, together with the detection of a tooth  32  or gap  38  in the detector region of the photo-interruptive sensor  30 , an absolute rotational position of the mounting shaft  15  relative to the vehicle wheel alignment sensor unit  12  and the vehicle wheel  10 . In addition, the micro-processor or logic circuit  100  determines a relative rotational position between a predetermined marking on the mounting shaft  15  and the vehicle wheel alignment sensor unit  12 .  
         [0043]    During use, a vehicle wheel alignment sensor unit  12  incorporating the absolute rotational position sensor assembly  26  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  12  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  15  is obtained and stored in the sensor memory  102 .  
         [0044]    As previously described, to compensate a vehicle wheel alignment measurement for runout between the vehicle wheel alignment sensor unit  12  and the vehicle wheel  10 , it is necessary to know the rotational position of one relative to the other about the mounting shaft  15 , as well as the corresponding runout value for that rotational position. Upon completion of the runout compensation procedure, the micro-processor or logic circuit  100  continuously tracks all subsequent rotational movements of the mounting shaft  15  relative to the vehicle wheel alignment sensor unit  12  through signals obtained from the relative rotational position sensor  22 . In addition, upon completion of the runout compensation procedure, the absolute rotational position sensor assembly  26  of the present invention is utilized by the micro-processor or logic circuit  100  to identify an absolute rotational position RC 1  of the vehicle wheel alignment sensor unit  12  associated with at least one point on the runout compensation sinusoidal waveform. Position RC 1  is stored in the sensor memory  102 , together with sufficient information to reconstruct the runout sinusoidal waveform for each rotational position of the vehicle wheel alignment sensor unit  12 .  
         [0045]    Upon restoration of power following an interruption in power supplied to the vehicle wheel alignment sensor unit  12 , 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  15  relative to the wheel alignment sensor unit  12 , the micro-processor or logic circuit  100  is configured to utilize the data stored in the sensor memory  102 , together with a new absolute rotational position measurement, to resume normal sensor operation without the need to repeat the runout compensation procedures.  
         [0046]    Assuming that the vehicle wheel alignment sensor unit  12  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  102  remain valid for all rotational positions of the vehicle wheel alignment sensor unit  12 . What is unknown immediately after restoration of the power or restart of the system is, the current rotational position of the mounting shaft  15  relative to the vehicle wheel alignment sensor unit  12 . For example, it is possible that the mounting shaft  15  was rotated relative to the vehicle wheel alignment sensor unit  12  during the time the power was interrupted, or the vehicle wheel  10  was rolled forward or backwards. Hence, the tracking of the rotational position by the micro-processor or logic circuit  100  using the relative rotational position sensor  22  is no longer in-sync with the stored runout compensation values.  
         [0047]    To re-synchronize the current rotational position of the vehicle wheel alignment sensor unit  12  and the stored runout compensation values, the micro-processor or logic circuit  100  is configured to utilize the absolute rotational position sensor assembly  26  of the present invention to obtain a current absolute rotational position RC 2  for the vehicle wheel alignment sensor unit  12 . To obtain the current absolute rotational position RC 2 , an operator is required to rotate the mounting shaft  15  in either direction relative to the vehicle wheel alignment sensor unit, either by rotating the vehicle wheel alignment sensor unit  12  or by rolling the vehicle wheel  10 , through a rotational arc sufficient to pass at least one complete unique tooth  32  or gap  38  through the detector region of the photo-interruptive sensor  30 . The arc length of the rotation is tracked by the micro-processor or logic circuit  100  using signals from the relative rotational position sensor  22 , as previously described.  
         [0048]    Once the current absolute rotational position RC 2  of the mounting shaft  15  relative to the vehicle wheel alignment sensor unit  12  is obtained by the micro-processor or logic circuit  100 , 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  15  relative to the vehicle wheel alignment sensor unit  12  with the previously determined runout compensation sinusoidal waveform. Subsequent rotation of the mounting shaft  15 ,relative to the vehicle wheel alignment sensor unit  12  is tracked by the relative rotation position sensor  22 , and an associated runout compensation value obtained by the micro-processor or logic circuit  100  using the stored runout sinusoidal waveform data.  
         [0049]    Using the absolute rotational position sensor assembly  26  of the present invention together with signals from the relative rotational position sensor  22  further permits the micro-processor or logic circuit  100  to identify an absolute rotational position on the mounting shaft  15 , such as a “zero” position, or other operator identified rotational position, and to guide an operator to return the vehicle wheel alignment sensor unit  12  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  12  or system shut down.  
         [0050]    The optional embodiment of the present invention shown in FIGS. 7 and 8 is intended for use when the vehicle wheel alignment sensor unit  12  is 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  12  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  12  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  15  and the wheel adapter, and the wheel adapter contact points on the vehicle wheel  10  (or other suspension component).  
         [0051]    During mounting of the vehicle wheel alignment sensor unit  12  to a no-compensation type wheel adapter, a technician is required to determine when the wheel alignment sensor unit  12  is aligned with the scribed mark on the mounting shaft  15  at the top-dead-center position, thereby mounting the wheel alignment sensor unit  12  to the no-compensation adapter in a repeatable manner. By providing a known slot  41 A or  41 B in a fixed rotational position corresponding to the top-dead-center of the mounting shaft  15 , the alternate embodiment of the present invention shown in FIG. 7 through FIG. 9 provides an operator with electronic guidance to correctly mount the wheel alignment sensor unit  12  on a no-compensation type wheel adapter. Signals from the adapter photo-detector  40  identify to the micro-controller or logic circuit  100  when the wheel alignment sensor unit  12  is rotational aligned with either slot  41 A or slot  41 B. Additional signals from the photo-detector  30 , indicating the presence of either a tooth  32  or gap  38  at the same rotational position provide unique identifying information, permitting the micro-processor or logic circuit  100  to positively identify which slot  41 A or  41 B has been detected.  
         [0052]    Predetermined information identifying either slot  41 A or slot  41 B as being aligned with the top-dead-center position of the mounting shaft  15  provides the micro-processor or logic circuit  100  with sufficient information to signal a correct mounting position on the no-compensation adapter for the wheel alignment sensor unit  12 , or to provide an the operator with sufficient guidance to achieve the desired mounting position. The micro-processor or logic circuit  100  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  12  for mounting on the no-compensation type adapter at the top-dead-center position.  
         [0053]    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.  
         [0054]    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.  
         [0055]    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.