Method and apparatus for providing runout compensation

A wheel alignment system includes a gauge head which is adapted to be pendulously mounted to a wheel to be aligned for free rotation about an axis which is coaxial with the axis of rotation of the wheel. The gauge head carries a plurality of level sensors, which are coupled to a microprocessor, and an encoder which is mounted between a shaft, that is mounted to the wheel for rotation therewith, and the gauge head to transmit to said microprocessor signals representative of the angular position of said wheel.

The present invention relates in general to methods and apparatus for use 
in aligning the wheels of automotive vehicles, and it relates more 
particularly to a new and improved method and apparatus for compensating 
for the runout of a wheel alignment instrument relative to the axis of 
rotation of a wheel to which it is mounted. 
BACKGROUND OF THE INVENTION 
In order to measure the camber and toe of the wheels of an automotive 
vehicle the common practice is to mount inclinometers to the wheels of the 
vehicle and to use these instruments to measure the deviation of the 
planes of the wheels from the horizontal and vertical planes. If the 
instruments are not precisely aligned with the axis of rotation of the 
respective wheels the camber and toe measurements will be inaccurate and 
thus result in misalignment of the wheels of the vehicle. 
The inclinometers are generally mounted on a gauge head which is adapted to 
hang freely like a pendulum from a stub shaft carried by a wheel clamp 
mounted to the rim of the associated wheel. For various reasons the stub 
shaft will not be exactly coaxial with the axis of rotation of the 
associated wheel, a condition known as runout. Many ways of correcting for 
such misalignment or runout of the instruments are known and described in 
the literature. 
For example, in U.S. Pat. No. 4,138,825 there is described a method of 
providing runout compensation by measuring and recording the runout at 
three different angular positions of the associated wheel while the wheel 
is elevated and then locking the instrument to the wheel to prevent 
relative movement between the instrument and the wheel and movement of the 
wheel from the final runout position before letting that wheel back down. 
A similar procedure is described in U.S. Pat. No. 3,892,042 where the 
runout measurements are made at two angular positions one-hundred eighty 
degrees apart before the instrument is locked to the wheel. In U.S. Pat. 
No. 4,192,074 still another method of compensating for the runout of the 
wheel alignment instrument is described. In this latter method the wheel 
is initially rotated through at least one complete revolution to measure 
and record the runout before the instrument is locked to the wheel and the 
various wheel alignment measurements are made. 
It will be apparent that in all of these prior art systems it is necessary 
to lock the instruments to the wheels and prevent rotation of the wheel 
and gauge head combination after runout measurements have been made. 
Moreover, it is important that the relationships between the wheels and 
the associated instruments be maintained throughout the wheel alignment 
procedure or faulty measurements and a consequent misalignment of the 
vehicle will result. Because of the location of some of the devices which 
must be adjusted and in some cases because of carelessness, it is not 
unusual for the technician to accidentally bump one of the instruments and 
possibly cause it to move relative to the wheel to which it is mounted. 
When this occurs it is necessary to again jack up the vehicle and repeat 
the runout measurement procedure to ascertain if the instrument did indeed 
move relative to the wheel, whether it did in fact move. 
SUMMARY OF THE INVENTION 
Briefly, in accordance with the present invention there is provided a new 
and improved method of compensating for the runout of a wheel alignment 
instrument and a new and improved wheel alignment instrument which may be 
used to easily measure runout and to indicate if the instrument has been 
moved relative to the wheel after runout has been measured and recorded. 
The instrument embodying the invention utilizes a conventional wheel clamp 
including a generally horizontal shaft on which a gauge head is rotatably 
mounted. The gauge head carries at least one inclinometer. In addition, 
however, an encoder is mounted between the shaft and the gauge head to 
provide information which directly relates the angle measured by the 
inclinometer to the angular position of rotation of the wheel. 
Consequently, rotation of the wheel and wheel clamp shaft will cause data 
from the encoder and the camber inclinometer to be entered into the 
microprocessor for computation of the runout compensation necessary for 
the camber and toe readings. Furthermore, it will not be necessary to 
prevent rotation of the wheel after the runout information has been 
gathered and recorded. The proper amount of runout compensation will be 
continually updated at each gauge head until the next step in the 
alignment process. This will not occur until all wheels have been runout 
compensated, the vehicle lowered back down and the gauge heads locked to 
the wheelclamps. 
During the other wheel alignment procedures where relative movement between 
the instrument and the wheel cannot be tolerated the encoder provides an 
indication if any such movement occurs. An alarm is actuated by a signal 
from the encoder to warn the technician that such movement has occurred. 
At that time it is only necessary for the technician to readjust the gauge 
head to its original position while being assisted by the output of the 
encoder.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
Referring particularly to FIGS. 1 and 2, a vehicle wheel 10 has a wheel 
clamp 12 mounted to the wheel rim 14 by any suitable means such, for 
example, as the three mounting pins 15 in the wheel adapter assembly shown 
in U.S. Pat. No. 3,892,042. The wheel clamp 12 includes a pair of parallel 
rods 16 and 17 to which a central mounting plate 18 is mounted for 
adjustable movement relative to the wheel rim so as to align a centrally 
located blind reference and mounting hole 20 with the axis of rotation of 
the wheel. The hole 20 is provided with an internal thread to which a 
shaft 22 is threadedly connected. A gauge head 24 is pendulously mounted 
for free rotation on the shaft 22 and a housing 26 of an encoder 27 is 
mounted to the gauge head 24 for unitary motion therewith. 
It may be seen from FIG. 2 that the gauge head 24 includes an elongated 
tubular section 28 which carries a pair of spaced bearings 30 and 32 which 
rotatably support the gauge head on the shaft 22. A plurality of machine 
screws 34 extend through holes in the inner wall 36 of the encoder housing 
26 and are threaded into threaded blind holes provided in the distal end 
of the tubular section 28 of the gauge head 24 to affix the encoder 
housing to the gauge head. 
The encoder housing 26 may be seen to consist of a hollow body section 38 
and an outer circular plate-like wall 40 having a central opening 42. The 
inner wall 36 is also provided with a central opening 44 through which the 
shaft extends into the space between the walls 36 and 40. A pulse wheel 46 
is fixedly mounted to the shaft 22, and as best shown in FIG. 3, is 
provided with a plurality of equally spaced radial slits 48. The angular 
width of each slit and of the space between adjacent slits are the same. A 
pair of position sensors 50 and 52 are mounted to the encoder housing 26 
and respectively include a light source and a light detector positioned on 
opposite sides of the wheel 46. Position sensors of this type are well 
known in the art and generate an electric signal each time one of the 
slits 48 is positioned between one of the light sources and its associated 
sensor. The sensors 50 and 52 are positioned relative to one another and 
to the pulse wheel such that at any single angular position of the pulse 
wheel only one or the other of the sensors detects a slit and produces an 
electric signal. Consequently, as the wheel 46 is rotated, a square wave 
signal is developed by each of the sensors and the two signals are 90 
degrees out of phase. As is well known in the art, the use of two position 
sensors quadruples the accuracy of the system to detect and measure the 
angular position of the wheel. 
In addition to the wheel angle sensors 50 and 52 a plurality of 
inclinometers 54 and 56 are carried by the gauge head 24 for measuring the 
angle at which the axis of rotation of the wheel meets the vertical plane 
for use in measuring the camber and steering axis inclination of the 
associated wheel to which the gauge head is mounted. Other sensors may be 
mounted to the gauge head for measuring other wheel alignment factors such 
as toe angle and the like. Typically, the gauge heads which are mounted to 
the rear wheels of the vehicle have two sensors, one for measuring toe and 
the other for measuring camber while those which are mounted to the front 
wheels have four sensors, one for measuring front toe, one for measuring 
camber, one for measuring toe angle relative to its respective rear wheel, 
and one for measuring steering axis inclination. 
It is well known that any wobble of the wheel as it rotates on the 
associated axle affects these measurements, so if the wheels are to be 
properly aligned it is necessary that either the wobble be eliminated or 
its effects on the said measurements be compensated for. Since the runout 
of the axle causes the wobble, and it cannot be entirely eliminated, the 
usual practice is to modify the actual toe and camber measurements to 
compensate for such runout. In the past this was done by initially 
rotating the wheel to a plurality of specified angular positions and 
measuring and recording the runout at each of these plurality of positions 
while the wheel is locked against rotation. Thereafter the wheel must be 
prevented from rotating throughout the wheel alignment procedure or 
erroneous measurements of toe and camber will result. This is particularly 
difficult while checking the runout of the other three wheels where the 
wheels are coupled together by a differential. For example, if both wheels 
are off the ground, which is customary in wheel alignment during the 
runout measurement procedure, when one wheel is rotated in one direction 
the other wheel rotates in the opposite direction. With limited-slip open 
differentials the amount of torque required to keep the first runout 
compensated wheel from rotating when the other wheel is being checked for 
runout can be as great as 100 foot pounds. Spurious rotation of a runout 
compensated wheel may also occur after all of the wheels have been 
compensated for runout by the accidental bumping of one or more of the 
gauge heads. In any case, rotation of a wheel after the runout 
measurements have been made and the runout compensating factor has been 
calculated results in inaccurate wheel alignment readings. 
In FIG. 4 there is shown in graphic form the effect of wheel axle runout on 
the camber measurement. It may be seen that depending on the angular 
position of the wheel the effect of runout varies. In FIG. 4 the 
horizontal axis is the rotational angle of the wheel clamp in relation to 
the stationary gauge head, and the vertical axis is the correction factor 
required to correct the camber measurement for runout at the particular 
angle of rotation of the wheel. It will be seen that the correction factor 
curve is a sine wave. 
In the runout compensation systems known in the prior art runout is 
measured at either two or four points along this curve and no subsequent 
angular movement either of the gauge head or of the wheel can be 
tolerated. 
OPERATION 
At the beginning of the wheel alignment procedure and after the wheel clamp 
and associated gauge head 24 have been mounted to the wheel to be aligned, 
the technician actuates the microprocessor 58 shown in FIG. 5 to measure 
the runout of that wheel. The wheel is then rotated through 360 degrees or 
more and the signals generated by each of the encoder 27 and the 
inclinometer 56 are transmitted to the microprocessor 58 where they are 
computed and recorded. This process is then repeated for each additional 
wheel. When the toe and camber measurements are subsequently made, the 
runout of the associated wheel axle is factored into the measurement and 
the true toe and camber angles are displayed on a display at the base unit 
60 driven by the microprocessor 58 and the main computer in the base unit 
60, the toe compensation factor being 90 degrees out of phase from camber 
on the correction factor curve. 
If the operator should accidentally bump a gauge head during the wheel 
alignment procedure, a signal will be generated by the associated position 
sensors 50 and 52 and the microprocessor 58 will transmit to the base unit 
the amount of angular movement which resulted so that the operator can 
quickly reposition the gauge head to its original position. In the prior 
art systems, when a gauge was bumped it was necessary for the technician 
to completely remeasure the runout by again rotating the associated wheel 
to the two or four angular positions at which the system measures runout. 
It may be seen that with the provision of the encoder 27 in the gauge head 
24 the conventional wheel alignment procedure is greatly improved in that 
more accurate measurements can be made while the overall time required to 
measure the runout of each wheel is reduced and such measurement need be 
made only once. Moreover, if the gauge head has been moved unbeknownst to 
the technician, the movement is detected by the encoder and such fact is 
displayed on the display at the base unit 60 and an alarm is given. 
While the present invention has been described in connection with a 
particular embodiment thereof, it will be understood by those skilled in 
the art that many changes may be made without departing from the true 
spirit and scope of the present invention. Therefore, it is intended by 
the appended claims to cover all such changes and modifications which come 
within the true spirit and scope of this invention.