Process for correcting the rotational speeds of vehicle wheels determined by wheel sensors

A method for correcting the rotational speeds of vehicle wheels determined by wheel sensors, a measurement of wheel speeds of the wheels of a vehicle being carried out, with specific driving conditions being that the vehicle is being neither accelerated nor braked and has no transverse acceleration. In a first part of the method, a matching of the measured wheel speeds is carried out under the specific driving conditions when the condition that the vehicle speed is below a first threshold value is satisfied. The matching of the measured wheel speeds is carried out by determining a matching factor for each of the two vehicle sides, so that, for example for the left vehicle side, matched wheel speeds are obtained from the measured wheel speeds. One of the matched wheel speeds of the left vehicle side is made equal to the associated measured wheel speed and the other of the two matched wheel speeds of the left vehicle side is obtained from the associated measured wheel by multiplying by the matchine factor of the left vehicle side, in such a way that the two matched wheel speeds of the left vehicle side assume the same value. In a second part of the method, the measured wheel speeds are balanced under the specific driving conditions when the condition that the vehicle speed is above a threshold value is satisfied. The balancing is carried out by determining a balancing factor for each wheel in relation to a reference wheel, in such a way that the balance wheel speed and the wheel speed of the reference wheel coincide with one another, and the measured wheel speeds are multiplied by the balancing factors determined in the second part of the method.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to a method for correcting the rotational 
speeds of vehicle wheels determined by wheel sensors, and includes the 
steps of measuring wheel speeds of the individual wheels of a vehicle, and 
determining balancing factors of the individual wheels under specific 
driving conditions, the specific driving conditions being that the vehicle 
is being neither accelerated nor braked and has no transverse 
acceleration. 
A process of the above type is already known from German Patent Document DE 
3,738,914 A1, and according to this process the wheel pair of which the 
wheel speeds differ the least is determined during slip-free driving. An 
average value is formed from these wheel speeds, and this average value, 
when related to the wheel speeds of the other wheels, provides a 
correction value by means of which the other wheel speeds are corrected. 
This process is carried out only when no slip occurs on the wheels and 
when no curve is being negotiated. The occurrence of slip can be detected 
from the appearance of signals of an anti-lock system (ABS) and/or a 
drive-slip control (ASR). It is stated in this Patent Document that the 
steering-angle signal can be evaluated for the purpose of detecting the 
negotiation of a curve. 
An object of the present invention is to provide a process for correcting 
the rotational speeds of vehicle wheels determined by wheel sensors, in 
such a way that the correction of the rotational speeds of vehicle wheels 
determined by wheel sensors is simplified in terms of the signal 
acquisition. 
This and other objects are achieved by the present invention which provides 
a method for correcting the rotational speeds of vehicle wheels determined 
by wheel sensors, and includes the steps of measuring wheel speeds of the 
individual wheels of a vehicle, and determining balancing factors of the 
individual wheels under specific driving conditions, the specific driving 
conditions being that the vehicle is being neither accelerated nor braked 
and has no transverse acceleration. In a first part of the method, the 
measured wheel speeds are matched under the specific driving conditions 
and when the vehicle speed is below a first threshold value, the vehicle 
speed and the specific driving conditions being determined from the 
measured wheel speeds. The matching of the measured wheel speeds includes 
the step of determining a matching factor for each of the two vehicle 
sides so that matched wheel speeds are obtained from the measured wheel 
speeds that are associated with the matched wheel speeds. One of the 
matched wheel speeds of the left vehicle side is made equal to its 
associated measured wheel speed, and the other of the two matched wheel 
speeds of the left vehicle side is obtained from its associated measured 
wheel speed by multiplying by the matching factor of the left vehicle 
side, in such a way that the two matched wheel speeds of the left vehicle 
side assume the same value. One of the matched wheel speeds of the right 
vehicle side is made equal to its associated measured wheel speed and the 
other of the two matched wheel speeds of the right vehicle side is 
obtained from its associated measured wheel speed by multiplying by the 
matching factor of the right vehicle side, in such a way that the two 
matched wheel speeds of the right vehicle side assume the same value. 
In a second part of the method, the wheel speeds are measured again to 
provide newly measured wheel speeds. The newly measured wheel speeds are 
balanced under the specific driving conditions and when the condition that 
the vehicle speed is above a second threshold value is satisfied. The 
newly measured wheel speeds are used to determine, with the matching 
factors being taken into account, the vehicle speed and the specific 
driving conditions. The step of balancing includes fixing one of the 
individual vehicle wheels as a reference wheel and determining a balancing 
factor for each individual vehicle wheel in relation to this reference 
wheel, in such a way that the balanced wheel speed coincides with the 
wheel speed of the reference wheel. Correcting measurements of wheel 
speeds measured after the balancing step are made by multiplying the wheel 
speeds measured after the balancing step by the determined balancing 
factors. 
One of the advantages of the present invention in relation to the known 
state of the art is that there is no need to use a steering-angle sensor. 
Apart from the wheel speeds, according to the certain embodiments, only 
signals which can be detected simply and accurately, such as, for example, 
the appearance of ABS or ASR signals, are used. 
In the method according to the present invention, the general driving 
situation in terms of the factors of vehicle speed v.sub.veh and 
transverse acceleration a.sub.q is estimated from the order of magnitude 
and ratio of initially uncorrected wheel speeds .OMEGA..sub.u. 
Furthermore, an estimation of the drive slip or brake slip .rho. is 
carried out at the same time. In the range of low transverse accelerations 
a.sub.q, the vehicle wheels on the individual vehicle sides run 
approximately on identical path curves, so that the speed of the front 
wheels .OMEGA..sub.vl, .OMEGA..sub.vr, and rear wheels .OMEGA..sub.hl, 
.sub.106 .sub.hr on the respective vehicle sides correspond with close 
approximation to one another when the dynamic wheel radii R.sub.dyn are 
equal. A further precondition, here, is that a negligible drive and brake 
slip .rho. should occur. 
Since a deviation of the dynamic wheel radii of a plurality of wheels with 
an increasing vehicle speed v.sub.veh leads to a speed difference 
.delta..OMEGA. of increasing amount, in the range of low vehicle speeds it 
is more easily possible to infer transverse acceleration from the 
difference between the wheel speeds of wheels of one axle, since as a 
result of different dynamic wheel radii the speed differences assume a 
smaller amount in this speed range. If a low transverse acceleration is 
detected in the range of low vehicle speeds, the speeds of the vehicle 
wheels on the individual vehicle sides are matched respectively at the 
front and at the rear. This matching can be carried out by providing the 
wheel speeds of the wheels of the rear axle with a matching factor, in 
such a way that the speed of the right rear wheel corresponds to the speed 
of the right front wheel and the speed of the left rear wheel corresponds 
to the speed of the left front wheel. 
Subsequently, in the range of higher vehicle speeds a balancing of the 
wheel speeds is carried out by fixing the speeds of all the vehicle wheels 
in relation to a reference wheel during a driving of the vehicle at least 
virtually free of transverse acceleration. Driving free of transverse 
acceleration is detected when the difference between the wheel speeds 
averaged on the two axles becomes equal to 0. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
As is evident from FIG. 1a, in the method according to the invention, on a 
2-axle vehicle with in each case two wheels on each axle the wheel speeds 
of all four wheels are detected when the execution of the process has been 
initiated by the signal 1.1. In step 1.2, the wheel speeds .OMEGA..sub.vl, 
.OMEGA..sub.vr, .OMEGA..sub.hl and .OMEGA..sub.hr are measured. 
In step 1.3, the driving state of the vehicle is derived from the wheel 
speeds .OMEGA..sub.vl, .OMEGA..sub.vr, .OMEGA..sub.hl and .OMEGA..sub.hr. 
and It is first established, in step 1.3.1, whether the vehicle is being 
accelerated or braked. This can be carried out, according to step 1.3.1.1, 
by a time differentiation of the measured wheel speeds .OMEGA..sub.vl, 
.OMEGA..sub.vr, .OMEGA..sub.hl and .OMEGA..sub.hr. If the amount of the 
wheel accelerations d.OMEGA..sub.vl /dt, d.OMEGA..sub.vr /dt, 
d.OMEGA..sub.hl /dt and d.OMEGA..sub.hr /dt thus determined is below a 
predetermined threshold value d.OMEGA./dt.sub.threshold, it can be 
concluded that the vehicle is being neither accelerated nor braked. 
According to step 1.3.1.2, this check can also be conducted by forming the 
average value AVFA of the wheel speeds of the front axle according to the 
equation: 
EQU AVFA=(.OMEGA..sub.vl +.OMEGA..sub.vr)/2 
and the average value AVRA of the wheel speeds of the rear axle according 
to the equation: 
EQU AVRA=(.OMEGA..sub.hl +.OMEGA..sub.hr)/2. 
If the amount of the difference between the two average values AVFA and 
AVRA is below a threshold value .delta.AV.sub.threshold, it can likewise 
be determined from this that the vehicle is being neither accelerated nor 
braked. If it is detected in step 1.3.1 that the vehicle is being 
accelerated or braked, the method according to the present invention is 
suspended by causing a return to step 1.2. Otherwise, a transfer to step 
1.3.2 takes place. 
In step 1.3.2, the vehicle speed v.sub.veh is derived from at least one of 
the measured wheel speeds .OMEGA..sub.vl, .OMEGA..sub.vr, .OMEGA..sub.hl 
and .OMEGA..sub.hr. Advantageously, this is carried out by averaging a 
plurality of these wheel speeds. The wheel speeds of the wheels of the 
nondriven axle of the vehicle can thus be used especially advantageously. 
According to step 1.3.3, a check is then made as to whether the vehicle 
speed v.sub.veh is below a first threshold value v.sub.veh,threshold1. A 
possible order of magnitude for this threshold value v.sub.veh,threshold1 
is of the order of magnitude of approximately 20-40km/h. If the vehicle 
speed v.sub.veh is above the threshold value v.sub.veh,threshold1, the 
method according to the invention is suspended by causing a return to step 
1.2. Otherwise, a transfer to step 1.3.4 takes place. 
In step 1.3.4, a check is then made as to whether the transverse 
acceleration a.sub.q is below a predetermined threshold value 
a.sub.q,threshold1. According to step 1.3.4.1, this check can be 
conducted, for example, by comparing the wheel speeds of wheels of one 
axle on different vehicle sides with one another. If the amount of this 
difference exceeds a predetermined threshold value 
.delta..OMEGA..sub.threshold1, the method according to the invention is 
suspended and there is a return to step 1.2. Otherwise, the method 
continues with the execution of step 1.4. 
Another possibility for checking the transverse acceleration is, according 
to step 1.3.4.2, to compare the amount of the difference between the 
average values of the wheel speeds of the wheels of the axles AVRA and 
AVFA with a predetermined threshold value .delta.AV.sub.threshold2. With 
an increase in transverse acceleration a.sub.q, the result of an 
understeering or oversteering driving behavior is that the wheels on the 
front and rear axles on the same vehicle side run on different path curves 
and therefore have different wheel speeds. In other words, the method 
according to the invention is suspended when the amount of the difference 
between the average values of the wheel speeds of the wheels of the two 
axles AVRA and AVFA is above the predetermined threshold value 
.delta.AV.sub.threshold2. Otherwise, the method according to the invention 
continues with the execution of step 1.4. It is possible, of course, to 
combine step 1.3.4.2 with step 1.3.1.2, since the check is identical, 
although the threshold values .delta.AV.sub.threshold1 and 
.delta.AV.sub.threshold2 to be checked can differ from one another. 
According to step 1.4, a matching of the wheel speeds is carried out by 
providing each of the wheel speeds of the wheels of the rear axle with a 
matching factor, in such a way that the speed of the right rear wheel 
corresponds to the speed of the right front wheel and the speed of the 
left rear wheel corresponds to the speed of the left front wheel. It is 
also possible, of course, to provide each of the wheels of the front axle 
with a matching factor, so that the speed of the right front wheel 
corresponds to the speed of the right rear wheel and the speed of the left 
front wheel corresponds to the speed of the left rear wheel. As a result 
of the matching factors, matched wheel speeds .OMEGA..sub.vlmatch, 
.OMEGA..sub.vrmatch, .OMEGA..sub.hlmatch and .OMEGA..sub.hrmatch are thus 
obtained from measured wheel speeds .OMEGA..sub.vl, .OMEGA..sub.vr, 
.OMEGA..sub.hl and .OMEGA..sub.hr. 
As an example, the matching factor for the right side of the car can be 
determined as a quotient of the measured speed of the right front side of 
the measured speed of the right rear side wheel. This wheel quotient is 
the matching factor of the right rear wheel. The matched value of the 
right rear wheel, .OMEGA..sub.hrmatch, is found by multiplication of the 
measured speed of the right rear wheel with the quotient. The matched 
value of the right front wheel will then be identical to the measured 
value of the right front wheel. The matched value for the left side is 
determined in a similar manner. Alternatively, the matching factor can be 
the quotient of the measured speed of the right rear side wheel to the 
right front side wheel. 
In the second part of the method according to the invention, shown in FIG. 
1b, the wheel speeds .OMEGA..sub.vl, .OMEGA..sub.vr, .OMEGA..sub.hl and 
.OMEGA..sub.hr are measured once again according to step 1.5. In step 1.6, 
in accordance with the matching factors determined in step 1.4, matched 
wheel speeds .OMEGA..sub.vlmatch, .OMEGA..sub.vrmatch, .OMEGA..sub.hlmatch 
and .OMEGA..sub.hrmatch are derived from these measured wheel speeds 
.OMEGA..sub.vl, .OMEGA..sub.vr, .OMEGA..sub.hl and .OMEGA..sub.hr. 
In step 1.7, the driving state of the vehicle is derived from the matched 
wheel speeds .OMEGA..sub.vlmatch, .OMEGA..sub.vrmatch, .OMEGA..sub.hlmatch 
and .OMEGA..sub.hrmatch that were derived earlier. It is first 
established, in step 1.7.1, whether the vehicle is being accelerated or 
braked. This can be carried out, according to step 1.7.1.1, by a time 
differentiation of the matched wheel speeds .OMEGA..sub.vlmatch, 
.OMEGA..sub.vrmatch, .OMEGA..sub.hlmatch and .OMEGA..sub.hrmatch. If the 
amount of the wheel accelerations d.OMEGA..sub.vlmatch /dt, 
d.OMEGA..sub.vrmatch /dt, d.OMEGA..sub.hlmatch /dt and 
d.OMEGA..sub.hrmatch /dt thus determined is below a predetermined 
threshold value d.OMEGA./dt.sub.threshold, it can be concluded that the 
vehicle is being neither accelerated nor braked. The subscript "match" has 
been placed higher in the drawing merely for representative reasons. 
According to step 1.7.1.2, this check can also be carried out by forming 
the average value AVFA.sub.match of the wheel speeds of the front axle 
according to the equation: 
EQU AVFA.sub.match =(.OMEGA..sub.vlmatch +.OMEGA..sub.vrmatch)/2 
and the average value AVRA.sub.match of the wheel speeds of the rear axle 
according to the equation: 
EQU AVRA.sub.match =(.OMEGA..sub.hlmatch +.OMEGA..sub.hrmatch)/2. 
If the amount of the difference between the two average values 
AVFA.sub.match and AVRA.sub.match is below a threshold value 
.delta.AV.sub.thresholdlmatch, it can likewise be derived from this that 
the vehicle is being neither accelerated nor braked. If, in step 1.7.1, it 
is detected that the vehicle is being accelerated or braked, the second 
part of the method according to the invention is suspended by causing a 
return to step 1.5. Otherwise, a transfer to step 1.7.2 takes place. 
In step 1.7.2, the vehicle speed V.sub.veh is derived from at least one of 
the matched wheel speeds .OMEGA..sub.vlmatch, .OMEGA..sub.vrmatch, 
.OMEGA..sub.hlmatch and .OMEGA..sub.hrmatch that were derived earlier. 
Advantageously, this is carried out by averaging a plurality of these 
derived matched wheel speeds. The wheels speeds of the wheels of the 
non-driven axle of the vehicle can thus be used especially advantageously. 
According to step 1.7.3, a check is then made as to whether the vehicle 
speed v.sub.veh is above a threshold value v.sub.veh,threshold2. A 
possible order of magnitude for this threshold value v.sub.veh,threshold2 
is of the order of magnitude of approximately 60-80 km/h. If the vehicle 
speed v.sub.veh is below the threshold value v.sub.veh,threshold2, the 
second part of the process according to the invention is suspended by 
causing a return to step 1.5. Otherwise, a transfer to step 1.7.4 takes 
place. 
In step 1.7.4, a check is made as to whether the transverse acceleration 
a.sub.q is below a predetermined threshold value a.sub.q,threshold1match. 
According to step 1.7.4.1, this check can be carried out, for example, by 
comparing the wheel speeds of wheels of one axle on different vehicle 
sides with one another. If the amount of this different exceeds a 
predetermined threshold value .delta..OMEGA..sub.threshold1match, the 
second part of the process according to the invention is suspended and 
there is a return to step 1.5. Otherwise, the process continues with the 
execution of step 1.8. 
Another possibility for checking the transverse acceleration is, according 
to step 1.7.4.2, to compare the amount of the difference between the 
average values of the wheel speeds of the wheels of the two axles 
AVRA.sub.match and AVFA.sub.match with a predetermined threshold value 
.delta.AV.sub.threshold2match. With an increasing transverse acceleration 
a.sub.1, the result of an understeering or oversteering driving behavior 
is that the wheels of the front and rear axles on the same vehicle side 
run on different path curves and therefore have different wheel speeds. In 
other words, the second part of the method according to the present 
invention is suspended when the amount of the difference between the 
average values of the wheel speeds of the wheels of the two axles 
AVRA.sub.match and AVFA.sub.match is above the predetermined threshold 
value .delta.AV.sub.threshold2match. Otherwise, the method according to 
the invention continues with the execution of step 1.8. It is possible, of 
course, to combine step 1.7.4.2 with step 1.7.1.2, since the check is 
identical, although the threshold values .delta.AV.sub.threshold1match and 
.delta.AV.sub.threshold2match to be investigated can differ from one 
another. 
In step 1.8, a balancing of the wheel speeds measured in step 1.5 then 
takes place, in that the speeds of all vehicle wheels are balanced with a 
reference wheel by determining the particular associated balancing factor 
f.sub.vl, f.sub.vr, f.sub.hl and f.sub.hr. The exemplary embodiment of 
FIG. 1b shows that the right front wheel, the left rear wheel and the 
right rear wheel are balanced with the left front wheel as a reference 
wheel. This then means, therefore, that the balanced wheel speeds 
.OMEGA..sub.vlbal, .OMEGA..sub.vrbal, .OMEGA..sub.hlbal and 
.OMEGA..sub.hrbal coincide with the measured wheel speed .OMEGA..sub.vl of 
the reference wheel. It is also possible, however, to select one of the 
other wheels as a reference wheel and to balance the other wheels with 
this reference wheel accordingly. The balanced wheel speeds 
.OMEGA..sub.vlbal, .OMEGA..sub.vrbal, .OMEGA..sub.hlbal and 
.OMEGA..sub.hrbal are thus obtained from the measured wheel speeds 
.OMEGA..sub.vl, .OMEGA..sub.vr, .OMEGA..sub.hl and .OMEGA..sub.hr. 
An arrangement of sensors 2.1, 2.2, 2.3 and 2.4 with an associated 
conventional data-processing unit 2.5 for the carrying out of the method 
according to the invention can be taken from the representation of FIG. 2. 
FIG. 2 shows an arrangement of sensors, in which each wheel 2.6, 2.7, 2.8 
and 2.9 is assigned a sensor 2.1, 2.2, 2.3 and 2.4 for measuring the wheel 
speed. It is also possible to conduct an appropriate balancing of the 
measured wheel speeds .OMEGA..sub.vl, .OMEGA..sub.vr, .OMEGA..sub.hl and 
.OMEGA..sub.hr, for example if the two sensors 2.3 and 2.4 of the wheels 
2.8 and 2.9 of the rear axle 2.10 are combined to form one sensor, by 
means of which, for example, the speed of the rear-axle drive shaft is 
then measured. 
The method according to the present invention is carried out in the 
data-processing unit 2.5 when the signal 1.1 appears. Different dynamic 
wheel radii occur not only when there are tire defects, but also as a 
result of a loss of air pressure in the tire and the wear of the tires. 
This signal can be generated, for example, by a control unit (not 
illustrated) after a particular mileage covered by the vehicle or at 
specific time intervals, the time intervals and the mileage appropriately 
being selected so that, under general conditions, it can be reckoned that 
there is a loss of air pressure expressed by a change of the dynamic wheel 
radii. 
The output signals 2.11, 2.12, 2.13 and 2.14 coming from the 
data-processing unit 2.6 represent the balancing factors f.sub.vl, 
f.sub.vr, f.sub.hl and f.sub.hr determined in step 1.8 of the method flow 
according to FIG. 1b and are then used to balance wheel speeds 
.OMEGA..sub.vl, .OMEGA..sub.vr, .OMEGA..sub.hl and .OMEGA..sub.hr measured 
thereafter by multiplying the corresponding measured wheel speeds 
.OMEGA..sub.vl, .OMEGA..sub.vr, .OMEGA..sub.hl and .OMEGA..sub.hr, by the 
balancing factors f.sub.vl, f.sub.vr, f.sub.hl, f.sub.hr determined, so 
that the balanced wheel speeds .OMEGA..sub.vlbal, .OMEGA..sub.vrbal, 
.OMEGA..sub.hlbal and .OMEGA..sub.hrbal are obtained from them. Different 
dynamic wheel radii R.sub.dyn of the individual wheels can thus be 
equalized. The wheel speeds .OMEGA..sub.vlbal, .OMEGA..sub.vrbal, 
.OMEGA..sub.hlbal and .OMEGA..sub.hrbal balanced in this way can then 
advantageously be used as input 10 signals for controls or regulations of 
chassis systems, such as, for example, anti-lock systems (ABS), drive-slip 
controls (ASR) or other systems. In the exemplary embodiment of FIGS. 1a 
and 1b, in step 1.8 all the vehicle wheels have been balanced with the 
left front wheel as a reference wheel. In this case, therefore, the 
balancing factors f.sub.vl, f.sub.vr, f.sub.vl and f.sub.hr are thus 
obtained according to the equations: 
EQU f.sub.vl =.OMEGA..sub.v1 /.OMEGA..sub.vl, f.sub.vr =.OMEGA..sub.vr 
/.OMEGA..sub.vl, f.sub.hl =.OMEGA..sub.hl / .OMEGA..sub.vl and f.sub.hr 
=.OMEGA..sub.hr /.OMEGA..sub.vl, 
that is to say, in particular, that in the exemplary embodiment of FIG. 1 
the balancing factor f.sub.vl is equal to 1. 
The actual programming of a conventional data-processing unit to perform 
the individual calculations of the average wheel speeds, the matching 
factors and the balancing factors, as well as the comparisons to threshold 
values, and to follow the method of the invention described above, are 
well within the level of one of ordinary skill in the art using the 
disclosure of the present invention. 
Although the invention has been described and illustrated in detail, it is 
to be clearly understood that the same is by way of illustration and 
example, and is not to be taken by way of limitation. The spirit and scope 
of the present invention are to be limited only by the terms of the 
appended claims.