Multiple threshold wheel slip control apparatus and method

Anti-wheel spin control systems are useful, for example, in industrial off-road vehicles. It is desirable that such systems operate automatically and not be unduly influenced by false slip signals caused by steering of the vehicle. It is further desirable that the operability of the control be readily determined by the operator. The instant anti-spin control includes an apparatus for producing a slip signal having a value responsive to the difference in rotational velocity between the vehicle wheels, and for producing a test signal. A processor receives the slip signal and test signal, compares the slip signal value with a first predetermined reference value in response to receiving the slip signal in the absence of the test signal, and compares the slip signal value with a second predetermined reference value in response to receiving both the slip signal and the test signal. A brake control signal is produced in response to the slip signal value exceeding the compared reference value, and the braking mechanism corresponding to the slipping wheel is activated. In a second embodiment of the invention, a steer signal is also produced and applied to the processor. The steer signal is utilized to negate the effect of steering the vehicle on the produced slip signal. Thus, the disclosed anti-spin system accounts for false slip indications produced by steering the vehicle, and provides a ready means for the operator to test the operability of the control system.

TECHNICAL FIELD 
This invention relates generally to wheel slip control systems for vehicles 
having differentially driven wheels in which slip is controlled by 
application of a braking force to the slipping wheel, and, more 
particularly, to an apparatus and method having multiple wheel slip 
threshold levels. 
BACKGROUND ART 
It is well-known that vehicles having spaced apart drive wheels or 
wheel-sets powered by a single engine through a differential mechanism, 
are problematic when one of the differentially driven wheels or wheel-sets 
loses traction. Conditions which give rise to a loss of traction commonly 
exist in construction sites and other off-road locations. A vehicle having 
one of two differentially driven wheels or wheel-sets on a slippery 
surface and the other on a surface providing good traction is often unable 
to move, owing to the fact that the differential directs full engine power 
to the wheel having no traction. The result is a slip condition in which 
the wheel having no traction rotates at higher than normal speed and the 
wheel having traction remains stationary. 
To alleviate such problems, various mechanical anti-spin devices have been 
developed and placed in commercial use. Such mechanical devices have been 
proven to have various problems, especially during cornering of a vehicle. 
Some devices fail to accommodate the normal wheel speed differential which 
arises during a turn, causing excessive tire wear owing to the dragging of 
the radially outer wheel or wheel-set. 
Other devices drive only the slower wheel in a turn, making the vehicle 
hard to steer and applying excessive torque to the wheel being driven, 
often causing failure of the final drive. 
An alternative approach involves the provision of separately actuatable 
drive wheel brakes. An operator selectively applies a braking force to the 
spinning or slipping wheel, and effects a balancing of power through the 
differential mechanism. The application of the braking force to the 
slipping wheel simulates increased traction and results in a more even 
distribution of power between the differentially driven wheels. This 
approach is commonly used on farm vehicles. 
A more sophisticated approach to the just described system, utilizes 
electronics to supply the braking force to the slipping or spinning wheel. 
An effective example of this approach is described in U.S. Pat. No. 
4,344,139, issued to Miller et al. on Aug. 10, 1982, and assigned to the 
assignee of this invention. Miller discloses an apparatus for applying a 
proportionally varying braking force to the wheel which loses traction, 
during a slip control time period. A slip signal is produced corresponding 
to any difference between the rotational velocity of the differentially 
driven wheels, and the slip signal is compared with a predetermined 
reference signal. In response to the slip signal exceeding the reference 
signal, the system selectively applies a braking force to the faster 
turning wheel. The braking force is modulated proportionally according to 
the degree of slip represented by the slip signal. 
One problem with the various prior control systems involves the normal 
difference in rotational velocities of the inner and outer vehicle wheels 
encountered while cornering. To avoid this problem, a fully automatic 
system, such as that described by Miller, must establish the reference 
signal at a level higher than the maximum slip signal that is produced 
solely in response to cornering the vehicle. Thus, the automatic control 
is inhibited for small values of wheel slip. Other prior systems have 
eliminated the problem completely by providing only manual operation. 
These controls rely on the operator to activate the anti-slip control when 
he senses the need to do so. Such manual systems necessarily eliminate 
much of the advantage of an anti-slip control. A further problem with the 
fully automatic systems is the inability of the operator to determine the 
operational status of the anti-spin control prior to encountering an 
actual slip condition. 
The present invention is directed to overcoming one or more of the problems 
as set forth above. 
DISCLOSURE OF THE INVENTION 
In one aspect of the present invention, an anti-spin apparatus for 
controllably equalizing the power delivered through a differential 
mechanism to at least two wheels of a vehicle is provided. The vehicle has 
a steering mechanism and each of the wheels has a respective braking 
mechanism. The apparatus produces a slip signal having a value responsive 
to the difference in rotational velocity between the wheels. A test signal 
is also controllably produced. A processor receives the slip signal and 
test signal, compares the slip signal value with a first reference value 
in response to receiving the slip signal in the absence of the test 
signal, and with a second reference value in response to receiving both 
the slip signal and the test signal. A brake control signal is produced in 
response to the slip signal value exceeding the compared reference value, 
and the braking mechanisms are controllably operated in response to the 
brake control signal. 
In a second aspect of the present invention, apparatus is provided for 
supplying a steer signal to the processor means. The steer signal value 
varies in response to the position of the steering mechanism. The 
processor controllably negates the effect on the anti-spin apparatus of 
steering the vehicle, in response to receiving the steer signal in the 
absence of the test signal. 
In another aspect of the present invention, a method for controllably 
equalizing the power delivered through a differential mechanism to at 
least two wheels of a vehicle is provided. The vehicle has a steering 
mechanism and each of the wheels has an associated braking mechanism. A 
slip signal responsive to a difference in the rotational velocity between 
the wheels is produced. A test signal responsive to the position of a test 
switch is also produced. A first reference value is produced in the 
absence of receiving the test signal and a second reference value is 
produced in response to receiving the test signal. The slip signal value 
is compared with the first reference value in response to receiving the 
slip signal in the absence of the test signal and to the second reference 
value in response to receiving both the slip signal and the test signal. A 
brake control signal is produced in response to the slip signal value 
exceeding the compared reference value, and the braking mechanisms are 
controllably operated in response to the brake control signal. 
The present invention advantageously compares the slip signal with multiple 
reference signals, thus avoiding problems associated with prior control 
systems.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring first to FIG. 1, an anti-slip apparatus embodying certain of the 
principles of the present invention is illustrated. It should be 
understood that the following detailed description relates to the best 
presently known embodiments of the apparatus. However, the apparatus can 
assume numerous other embodiments, as will become apparent to those 
skilled in the art, without departing from the appended claims. 
Wheels 100,101 are driven by an engine (not shown) through an input or 
drive shaft 104, a differential mechanism 106, and respective half-axles 
108 and 110. A steering mechanism 111 is coupled to steerable wheels (not 
shown) of the vehicle. The drive system is conventional and no further 
details need be disclosed for an understanding of the present invention. 
The wheels 100,101 are stopped by spring engaged parking brake pistons, or 
by hydraulically engaged service brake pistons, of brake mechanisms 
112,114. The brakes are spring biased in the engaged position and are 
maintained in the disengaged position by application of fluid pressure, as 
disclosed in U.S. Pat. No. 3,927,737, issued Dec. 23, 1975 to P. F. M. 
Prillinger and assigned to the assignee of this invention. The service 
brakes are normally actuated via a service brake line 137 connected to the 
service brake and retarder master cylinders (not shown). The service brake 
system is well-known and does not form a part of this invention. The 
brakes are also actuated through the parking brake lines 136,138 as 
described in detail below. 
Means 115 produces a slip signal having a value responsive to the 
difference in rotational velocity between the wheels 100,101. The slip 
signal producing means 115 includes a left wheel speed pick up in the form 
of an electromagnetic transducer 116 which provides pulses in cooperation 
with a gear-like device 118. The device 118 is mounted on and rotates with 
the axle portion 108. Signals from the transducer 116 are applied to one 
input of an electronic controller 120, the details of which are described 
below. In a similar manner, right wheel speed signals are provided by a 
transducer 122 operated in conjunction with a gear-like device 124 which 
rotates with the axle portion 110. Signals from the transducer 122 are 
applied to another input of the electronic controller 120. Each of the 
transducers 116,122 produce respective signals having values responsive to 
the rotational speed or velocity of the wheels 100,102. Additionally, a 
drive shaft speed signal is produced by a transducer 126 in conjunction 
with a gear-like device 128 which rotates with the drive shaft 104. The 
drive shaft speed signals are applied to a third input of the electronic 
controller 120. Each of the transducers 116,122,126 is preferably an 
electromagnetic device which produces a pulse type, time variable output 
voltage. Such transducers are well-known in the art. However, other 
transducers, such as optical and Hall effect devices, may be employed as 
alternatives. Means 127 for producing a test signal is also connected to 
an input of the electronic controller 120. The test signal producing means 
127 is preferably a manual switch 129. 
The electronic controller 120 is part of a processor means 121 for 
receiving the slip signal and producing a brake control signal in response 
to the slip signal value exceeding a compared reference value. A means 131 
receives the brake control signal and controllably operates the braking 
mechanisms 112,114 in response to the received brake control signal. 
A second embodiment of the present invention includes means 113 for 
producing a steer signal in response to the position of the steering 
mechanism 111. The steer signal is delivered to the controller 120. Means 
113 preferably includes a potentiometer 115 controllably connected to the 
steering mechanism 111. The output of the potentiometer 115 is a signal 
having a value that varies over a negative to positive voltage range, in 
response to the position of the steering mechanism 111. Other suitable 
angular position transducers may be substituted for the potentiometer 115 
in the means 113, as is well-known in the art. 
In both embodiments, the controller 120 operates upon the signal inputs, 
determines the existence, magnitude, and location of wheel slip during a 
loss of traction situation, and distinguishes between true wheel slip and 
a transducer failure. In response to detecting a true slip condition, the 
power transfer between the two differentially driven wheels 100,102 is 
balanced by applying a proportional braking force to the wheel which loses 
traction. This is accomplished by means of locating selection valve 130 
and proportioning valve 132, both of which are included in the means 131. 
The valves 130,132 operate in combination with a supply 134 of oil or brake 
fluid under pressure, the fluid lines from supply 134 running both through 
and around the proportioning valve 132 to the solenoid operated selection 
valve 130 which directs full pressure to one of the brake mechanisms 
112,114, and modulated or proportionally controlled pressure to the other 
of the brakes 112,114. Note that, owing to the utilization of spring 
biased brakes 112,114, brake pressure is the inverse of fluid pressure and 
is applied by relieving the fluid pressure in one of the two brake lines 
136,138, and could be straightforwardly implemented in the opposite 
fashion, increasing brake pressure in direct rather than inverse ratio to 
the applied fluid pressure. 
FIG. 2 is a block diagram of the preferred implementation of the anti-spin 
apparatus described above. A solid state digital microprocessor 142 
performs system control functions. The microprocessor 142 is programmed to 
establish a plurality of slip value bands, each band having associated 
therewith an applied brake force value expressed in terms of fluid 
pressure. The microprocessor 142 is interconnected with a band value 
memory 144 and a timer 146. Transducers 116,122,126 are connected to the 
microprocessor 142 through an input conditioning circuit 140. The input 
circuit 140 provides appropriately digitized input signals to the 
microprocessor 142. Retard brake pressure and service brake pressure 
switches 148,150 are likewise connected through an input conditioning 
circuit 152 to the microprocessor 142. The test signal producing means 127 
is also connected to the microprocessor 142 through the input circuit 152. 
Finally, in the alternative embodiment, the steer signal producing means 
113 is connected to the microprocessor 142 through the input circuit 152. 
A first output of the microprocessor 142 is connected through a pulse-width 
modulated servo valve driver 154 to the servo operated proportioning valve 
132. A second output from the microprocessor 142 is connected to the 
solenoid driver 133 associated with a left direction selection valve 130a, 
and a third output from the microprocessor 142 is connected through the 
solenoid driver 136 to a right direction shuttle valve 230b. The pulse 
width modulated servo valve driver 154 proportionally controls the servo 
valve 132, in a manner well-known in the art. Therefore, adverting 
momentarily to FIG. 1, fluid pressure is modulated via the proportioning 
valve 132 to the brake line 136,138 selected by the position of the valve 
130. 
Referring now to FIG. 3, a functional flowchart defining the internal 
programming for the microprocessor 142 is shown. From this flowchart, a 
programmer of ordinary skill can develop a specific set of program 
instructions for a general purpose microprocessor that defines the 
necessary slip signal value bands, timing cycles, and brake fluid pressure 
values necessary for implementation of the instant invention. It will be 
appreciated that, while the best mode of the invention is considered to 
comprise a properly programmed microprocessor 142, the result of which is 
the creation of novel hardware associations within the microprocessor 142 
and its associated devices, it is possible to implement the instant 
invention utilizing traditional hard wired circuits. 
INDUSTRIAL APPLICABILITY 
The following description refers to FIGS. 1, 2, and 4, and to the flowchart 
depicted in FIG. 3. The operation of the anti-spin apparatus is first 
described according to the embodiment in which the steer signal producing 
means 113 is absent from the apparatus. The initial discussion further 
assumes that the test signal producing means 127 is in the "non-test" or 
"normal" position. 
Data delivered by the transducers 116,122 is sampled in block 164 of FIG. 
3. In response to one of the wheel velocity signals being equal to zero, 
control progresses along the right side of the flow diagram to block 166. 
The input shaft speed is determined in block 166, and is compared in block 
168 with the non-zero wheel velocity. If the ratio of the non-zero wheel 
velocity to drive shaft speed is equal to or less than a constant, for 
example, 1.5, it is assumed that the wheel indicating zero speed is 
actually turning and that the zero indication is the result of a failed 
transducer. In this case, the program returns to the original starting 
point and, if desired, an indication of the apparent transducer failure 
can be presented to the operator. The value of the constant selected is 
determined according to the gear ratio of the differential mechanism 106. 
As a matter of convenience for purposes of this discussion, a differential 
ratio of 1:1 is assumed throughout. 
If the rotating wheel velocity is greater than 1.5 times the drive shaft 
speed, the program progresses to blocks 170a,b,c. If one of the wheel 
velocities is not zero, the program progresses to blocks 172a,b, which is 
an event counter requiring several successive cycles of the above 
conditions prior to activation of the anti-spin apparatus. This delay 
filters out short term wheel speed aberations and has been found to be 
advantageous for efficient system operation. If the event counter has not 
incremented or counted a sufficient number of cycles, the program 
progresses to blocks 174a,b which increment the counter. The program then 
returns to the original starting point. 
Once the event counter 172a,b has incremented a sufficient number of 
cycles, the program progresses to block 176. In block 176 a determination 
is made of the input switch conditions controlling the service brake, the 
retarder, and/or the vehicle speed. It is additionally advisable to check 
the condition of brake fluid pressure at this point. 
If both wheel velocities are zero, block 170c advances the program to 
blocks 178,180,182,184, releasing the parking brake 112,114 and resetting 
the timer 146. The same result occurs if automatic control is 
contra-indicated in block 176. 
If block 176 is satisfied, the program progresses to block 186 to determine 
whether the slip control system is activated. If so, the effect is the 
same as entering band 5, the most severe slip condition band, and control 
passes to block 194. Block 194 increases the wheel brake force by 
decreasing the wheel brake hold off pressure by an increment X, for each 
timed interval established by the microprocessor timer 146. If the slip 
control system is not operative in block 186, the program progresses to 
block 188 which activates the system, and then through blocks 190 and 192 
to begin the timing cycle and reduce the brake hold off pressure to a 
value just sufficient to maintain the brakes in the released position. On 
the next pass through the above sequence, a positive result will be 
obtained at block 186, and in block 194 the brake hold off pressure will 
be decreased by X psi. The control repeats this sequence at timed 
intervals until either the zero speed wheel begins to turn or the brake 
hold off pressure is decremented to nearly zero psi. In the latter case, 
the brakes are then released completely. In the former case, the control 
follows the path from block 164 to block 196, and continues as described 
in the following paragraphs. 
Assuming that, in block 164, neither wheel velocity is found to equal zero, 
the program branches to block 196, representing a calculation sub-routine. 
Essentially, block 196 determines the location of the actual slip signal 
value within one of five slip bands represented by Table 1. The slip bands 
are contiguous, the upper limit of one band being the lower limit of the 
next, with the highest slip value being the first predetermined reference 
value referred to above. The first predetermined reference value must be 
exceeded by the slip control signal value before anti-slip control begins. 
The processor means 121 makes this determination by comparing the slip 
signal value with the first predetermined reference value, and with the 
other reference values shown in Table 1. 
Since normal cornering of the vehicle produces an apparent slip condition, 
with the radially outer wheels of the vehicle rotating faster than the 
radially inner wheels, to prevent actuation of the anti-slip control in 
response to maneuvering the vehicle while operating in this mode, the 
first reference value must be established higher than the maximum slip 
signal value produced by steering the vehicle in a hard cornering 
maneuver. This value varies according to the design of the vehicle. As 
shown in Table 1, for purposes of this discussion, a value of 1.7 is 
considered to be greater than the maximum slip signal value produced by 
steering the vehicle. 
Continuing through the program of FIG. 3, program blocks 198,200,202 
represent the determination of which of the wheels is rotating at a higher 
velocity, and the disposition of the valve 130 to direct modulated brake 
fluid pressure to the highest velocity wheel and unmodulated brake fluid 
pressure to the lower velocity wheel (remembering the inverse relationship 
between brake pressure and brake fluid pressure). The program then 
advances to block 204 and determines whether a sufficient slip time period 
has elapsed to begin operation of the control system. If insufficient time 
has elapsed, program block 206 increments a counter and the program 
advances to the starting point. If sufficient time has elapsed, the 
program advances to block 208 in which the various signal conditions which 
might contra-indicate operation of the slip control system are considered. 
If the conditions are satisfied, the program advances to the appropriate 
one of the blocks 210,212,214,216,218, as determined in block 196. 
Initially, the vehicle can enter the slip control mode only via band 5. 
Therefore, the slip value must exceed the first predetermined reference 
value, for example, 1.7. This prevents unintentional activation of the 
control system while steering the vehicle. Once the control mode has been 
entered through band 5, control is exited by sequencing through the lower 
bands 4-1, producing a smooth transition back to the uncontrolled mode. 
Assuming that band 5 is entered, and that a positive indication is reached 
in block 186, the brake force is periodically incremented in block 194 by 
reducing the brake pressure by the increment X for every timing cycle, 
until the slip signal value causes entry into another slip band. This is 
best seen in FIG. 4, where the first step from the hold off pressure of 
400 psi represents an abrupt reduction in pressure to 200 psi, followed by 
three additional incremental reductions of approximately 33 psi each, 
increasing the parking brake force through spring action with each 
incremental step. 
Next, the slip condition in the example represented in FIG. 4 enters band 
4, in which the slip has been reduced to the point where the slip signal 
value is within the range between 1.5 and 1.7. Blocks 220,222 cause the 
brake force to be increased by a smaller increment Y for every timing 
cycle. Accordingly, the system approaches the full brake force condition 
in a gradual curve, with the brake force increments becoming smaller 
toward the full brake force condition. 
As is also represented in FIG. 4, reaching a lower slip band value and 
qualifying for successive entry into slip bands 3,2,1, causes gradually 
increasing incremental reductions in brake force until the system is back 
to the unbraked condition represented by application of the full brake 
fluid pressure of 400 psi. Preferably, band 3 is the mirror image of band 
4, and causes incremental reductions in braking force through incremental 
increases in brake fluid pressure. Band 2 is the mirror image of band 5, 
and causes large incremental reductions in brake force. Band 1 has no 
counterpart, and causes still larger reductions in brake force as 
represented by the increment W in the diagram of FIG. 4. 
From the foregoing it is apparent that, with the test signal producing 
means 127 in the "normal" mode, the anti-spin apparatus operates to detect 
a slipping wheel, to apply braking force to the slipping wheel, and to 
periodically and incrementally modulate the brake force either positively 
or negatively in accordance with the degree of slip which is detected by 
the system. Conditions giving rise to a slip signal value of 1.7 or less 
do not cause entry into the slip control mode, because the first 
predetermined reference value is not exceeded. This prevents unintentional 
operation of the anti-spin control owing to steering of the vehicle. 
However, actual slip that does not exceed the first predetermined 
reference value is likewise not corrected for by the control system. 
The utility of the test signal producing means 127 is now described. When 
the test switch 129 is moved from the "normal" to the "test" position, the 
first predetermined reference value is replaced by a second predetermined 
reference value. For example, as is shown in Table 1, the reference value 
required to enter slip band 5 is reduced from the first value of 1.7 to 
the second or modified value of 1.25. Band values 4-1 are likewise 
reduced. These are the values stored in the band value memory 144 and 
utilized in the calculation of block 196. Therefore, in response to 
selecting the "test" mode, the processor means 121 receives the slip 
signal and the test signal and compares the slip signal with the second 
predetermined reference value. Anti-slip control is thus initiated in 
response to a slip signal value less than the slip signal produced by 
steering the vehicle. This produces two important results. 
First, the operator can readily determine whether or not the anti-spin 
apparatus is functioning properly by merely steering the vehicle into a 
hard turn. If the slip control is operating properly, the operator can 
observe the application of the vehicle brake 112,114 to the radially outer 
of the differentially driven wheels 100,101, in response to the slip 
signal value produced by steering the vehicle exceeding the modified 
threshold value. Thus, it is relatively easy for the operator to determine 
the operability of the control system prior to encountering a low traction 
situation. 
Second, when the operator encounters slippery working conditions in which 
it is desirable that the anti-spin control system respond in a rapid 
fashion, he can select the "test" position with the test switch 129. This 
causes the control system to enter the slip control mode with the lower 
slip signal value, making it more sensitive to low traction conditions, 
and enhancing controllability of the vehicle. Since this mode of operation 
is selected only under a particular set of extreme circumstances, the 
interference with normal steering functions is minimal and acceptable. 
The alternative embodiment of the anti-spin apparatus, in which the steer 
signal producing means 113 is included, as shown within the dotted area of 
FIG. 3, is a further enhancement of the control system. In one 
implementation of this embodiment, the processor means 121 receives the 
steer signal and controllably modifies the slip signal value in response 
to the value of the steer signal, in the absence of the test signal. 
Assuming that the test switch 129 is in the "normal" position, and that 
neither wheel velocity is found to be zero in block 164, program control 
progresses to block 250, in which a slip value is calculated as S.sub.c. 
The sign of S.sub.c is determined in the blocks 252,254,256. If the right 
wheel is the faster of the two wheels, the quantity S.sub.c is positive, 
and if the right wheel is not the faster of the two wheels, the quantity 
S.sub.c is negative. S.sub.c is next delivered to block 262. The inverted 
steer signal value S.sub.s is delivered from the block 260, and also 
supplied to the block 262. The signals S.sub.c and S.sub.s are summed and 
delivered to the block 196, in which the appropriate slip band is 
calculated and control progresses as described above. As shown in Table 1, 
the slip band reference or threshold values utilized by the control system 
are much lower as applied to the instant embodiment of the invention, than 
the values utilized in the embodiment having no steer input signal. 
For example, assume that the steer signal producing means 113 supplies a 
positive signal to block 260 in response to steering the vehicle in a left 
turn, and a negative signal in response to steering the vehicle in a right 
turn. In response to the vehicle executing a right turn, the left or 
radially outer wheels 100,101 rotate at a higher velocity than do the 
right or radially inner wheels. Assuming a "no-slip" or good traction 
condition, S.sub.c is calculated in block 250 and negative S.sub.c is 
delivered from block 254 to block 262. The steer signal polarity is 
inverted in block 260 and the signal is delivered as a positive signal to 
block 262, effectively canceling the false slip signal caused by turning 
the vehicle, thus preventing entry into the control mode. 
Now, given a similar hypothetical situation but under poor traction 
conditions, assume that the left wheel loses traction. The calculated slip 
signal S.sub.c becomes larger, as does the absolute value of the negative 
signal delivered to block 262. Since the steer signal delivered to block 
262 modifies but no longer cancels the slip signal, the true slip 
condition is detected and is delivered to block 196. In response to this 
signal exceeding the first predetermined reference signal, shown in Table 
1, band 5 of the control mode is entered and slip control commences. 
Finally, under the same hypothetical conditions, assume that the left wheel 
maintains traction and the right wheel loses traction. The calculated slip 
signal S.sub.c becomes smaller or passes through zero and becomes larger, 
as the relative wheel velocities change. Correspondingly, the negative 
signal delivered to block 262 becomes less negative or goes positive and 
is modified but no longer canceled by the steer signal, and the control 
mode is entered once the first predetermined reference value is exceeded. 
The consequences of other combinations of wheel slip and steering angle can 
be readily determined by applying the above described logic. 
In a second implementation of this embodiment, the processor means 121 
accomplishes the same result as that just described, by continuously 
modifying the band threshold values in response to receiving the steer 
signal in the absence of the test signal. The first predetermined 
reference value is continuously maintained a predetermined amount greater 
than the actual slip signal value produced in response to steering the 
vehicle. 
Adding the steer signal producing means 113 to the anti-spin control system 
accounts for differences in wheel velocity caused solely by steering the 
vehicle, and prevents such wheel velocity differences from appearing to 
the control as an actual slip condition. By negating the effect of 
steering on operation of the control, the anti-spin system is made 
extremely sensitive to small slip conditions that prior systems are unable 
to automatically contend with. Keeping the above principles in mind, and 
having the flexibility of a programmable microprocessor, one skilled in 
the art can produce other related methods of accomplishing this same 
result. 
In the steer signal embodiment, switching the test signal producing means 
127 to the "test" mode inhibits the action of the steer signal producing 
means 113 and causes the control to operate on the second predetermined 
reference value, as fully described above. Thus, in each embodiment, the 
anti-spin control system can be tested by steering the vehicle in a hard 
turn while the test signal producing means 127 is in the "test" mode. Of 
course, if the test mode is not provided in the second embodiment, the 
control mode will never be entered solely in response to steering the 
vehicle. 
It will be appreciated by those skilled in the art that it is not essential 
to incorporate all of the steps represented in the flowchart of FIG. 3 in 
a given system, nor is it necessary to incorporate the steps of FIG. 3 
utilizing a microprocessor. 
However, such an implementation is deemed to be the best mode of practicing 
the invention owing to the broad and widespread availability of suitable 
microprocessor circuits, the widespread understanding of programming 
techniques for such microprocessors, the cost reduction in such circuitry 
which has been realized in recent years, and the flexibility which a 
programmable device affords. 
Other aspects, objects, advantages and uses of this invention can be 
obtained from a study of the drawings, the disclosure, and the appended 
claims. 
TABLE 1 
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BAND THRESHOLD REFERENCE VALUES 
S = SLIP SIGNAL VALUE 
"NORMAL" "TEST" STEER SIGNAL 
BAND MODE MODE MODE 
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1 1.0 &lt; S &lt; = 1.2 
1.0 &lt; S &lt; = 1.1 
1.0 &lt; S &lt; = 1.1 
2 1.2 &lt; S &lt; = 1.3 
1.1 &lt; S &lt; = 1.15 
1.1 &lt; S &lt; = 1.15 
3 1.3 &lt; S &lt; = 1.4 
1.15 &lt; S &lt; = 1.2 
1.15 &lt; S &lt; = 1.2 
4 1.4 &lt; S &lt; = 1.7 
1.2 &lt; S &lt; = 1.25 
1.2 &lt; S &lt; = 1.25 
5 1.7 &lt; S 1.25 &lt; S 1.25 &lt; S 
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