Anti-lock brake system for vehicles

An anti-lock brake system for vehicles with an electronic system which regulates the braking pressure of a monitored wheel as a function of the wheel speed, so that locking is prevented. The system is designed so that each wheel has its own speed sensor, and both wheels of one axle are pressurized with pressure medium by means of a joint control valve controlled by the electronic system. This arrangement corresponds to that of a select-low regulation. To shorten the extended braking distance on roads with different traction on each side, which occurs with a select-low regulation, the control system is designed so that when a retard signal (-b) occurs, the electronic system does not switch as usual to "remove pressure", but, at least until the occurrence of a slip signal (.lambda.), the control valve is switched to the "maintain pressure" position.

FIELD OF THE INVENTION 
The present invention relates, in general, to vehicle brake systems and, 
more particularly, this invention relates to an anti-locking brake circuit 
for such vehicle brake system. 
BACKGROUND OF THE INVENTION 
Prior to the present invention, various control concepts are known and are 
in use for anti-locking brake systems which are arranged in series. 
Included in the prior art are various regulation concepts for anti-lock 
brake systems that are in mass production. 
For example, the single wheel or individual control and regulation (IC) 
anti-lock brake system envisions each wheel on the vehicle having a 
separate control circuit including its own sensor and setup unit or 
actuator. Such setup unit or actuator is a solenoid valve. This system 
provides optimum control or regulation of the brake system, but at a 
significant expense. 
A modified individual control and regulation (MIC) anti-lock brake system 
is taught in German Patent DE-OS 28 51 107. As taught therein, this (MIC) 
anti-lock brake system provides improved control of the steering effort on 
roadways having variable or different force requirements on a roadway in 
which traction conditions change and differ from one side to the other. 
This improved steering control is accomplished by reducing the side drift 
or yawing moment of the vehicle during normal driving. For the so-called 
"high wheel", which is the wheel operating on the good side of the road, 
it has, for example, a pressure-holding or maintenance phase which is 
ensured if and when the so-called "low wheel" is operating on a bad side 
of the road and this (MIC) anti-lock brake system receives a pressure drop 
indication as a consequence of a deceleration and/or a wheel skid or slip 
signal. 
In addition to individual control and regulation, it is also known in the 
prior art to utilize anti-lock brake regulation systems for an axle or a 
group of wheels. All such prior art systems can be used on a vehicle, 
regardless of whether the brake system is pneumatically operated or 
hydraulically operated. In the regulation system for an axle or group of 
wheels, one sensor is used for each wheel but, for cost consideration, a 
common setting or actuator unit is used for both wheels on an axle. For 
this reason, the brake cylinders of both wheels are connected in a 
parallel manner. 
In a so-called select low control or regulation anti-lock brake system 
(SLC), that is frequently used on vehicles, the common brake pressure on 
both wheels is reduced and, thereafter, further regulated and controlled 
as a function of the movement of a particular wheel having the lower 
traction or lower power application (low wheel). In any event, a pressure 
reduction takes place in each prior art device as soon as a delay signal 
(-b) and/or a slip signal (.lambda.) is generated by the sensor associated 
with the low wheel. In such SLC anti-lock brake systems, certain low 
control settings will lead to a relatively high excess commencing force. 
In addition, this can, on occasion, lead to a significant amount of 
under-braking of the high wheel operating on a good side of the road. This 
then is a contributing factor to substantially extended braking distances 
encountered at times as compared with the (IC) anti-lock brake system, or 
even when braking with locked wheels. Such extended braking distances 
occur most frequently on so-called .mu.-split roadways, as well as on 
roadways with changing values for the coefficient of friction (such as icy 
conditions, etc.) or on uneven surfaces (such as cobblestone pavement, 
potholes, or generally poor roads) and with wheel brakes having different 
ratings. 
In some cases, therefore, a so-called select high control or regulation 
(SHC) anti-lock brake system is used. Such SHC anti-lock brake system does 
not take into consideration the motional reaction in movement of the low 
wheel but, instead, reacts only to the retarding and/or slip signals 
generated from the sensor associated with the high wheel. Of course, the 
unavoidable result is a locking of the low wheel, even for rather long 
periods of time. Such a lockup of the low wheel (even if for an extended 
period of time) is, however, included with deliberate intent. 
Nevertheless, such (SHC) anti-lock brake system can lead to an undesirable 
flat tire condition at the low wheel, and to reduced conditions of vehicle 
stability when braking occurs on a curve on roadways having excellent 
traction conditions. 
As far as such (SLC) or (SHC) anti-lock brake systems are used in 
commercial or utility vehicles, the pressure control and regulation in 
most cases is generally carried out by the use of a 3/2 way 
multi-directional solenoid valve. Such 3/2 way multi-directional solenoid 
valve has only a pressurization and evacuation phase, and not a pressure 
maintenance or holding phase. 
SUMMARY OF THE INVENTION 
The present invention provides an anti-lock brake system for a vehicle. 
Such anti-lock brake system includes an electronic control system which, 
as a function of the wheel speed, forms internal signals for the wheel 
slip (.lambda.), the wheel acceleration (+b), and/or the wheel retardation 
(-b). The wheel rotational speed is determined by a speed sensor 
associated with two wheels on one axle or on one side of the vehicle. The 
internal signals generated in the electronic control system are 
transmitted to a common control valve for controlling the brake pressure 
in selected brake cylinders. Such signals transmitted to such control 
valve are one of reduce, maintain constant, or increase brake pressure as 
necessary. Such electronic control system with the receipt of signal from 
the speed sensor associated with a low-wheel and determined by such 
electronic control system to be a retarding signal (-b) will switch the 
control valve to a maintain pressure mode. The electronic control system 
with the receipt of a signal from the speed sensor associated with such 
low wheel and determined by such electronic control system to be a slip 
signal (.lambda.) will switch the control signal valve to a 
reduce-pressure mode, and when the electronic control system determines 
when the slip signal (.lambda.) disappears then it will transmit a control 
signal which switches the control valve to an increase-pressure mode. 
OBJECTS OF THE INVENTION 
It is, therefore one of the primary objects of the present invention is to 
provide a vehicle anti-lock braking system in which brake cylinder control 
is arranged in pairs, and in which a braking action is improved over prior 
art (SLC) anti-lock brake systems. 
Another object of the present invention is to provide a vehicle anti-lock 
braking system in which wheel lock-up, which occurs in prior art (SHC) 
anti-lock brake systems, is minimized or eliminated completely as far as 
possible. 
In addition to the above objects and advantages of the anti-lock brake 
system of the present invention, various other objects and advantages of 
such invention will become more readily apparent to those persons skilled 
in the vehicle braking art from the following more detailed description of 
the invention, when such description is taken in conjunction with the 
attached drawings and with the appended claims.

BRIEF DESCRIPTION OF THE INVENTION 
Prior to proceeding to a more detailed description of the invention, it 
should be noted that identical components have been identified with 
identical reference numerals throughout the drawing Figures. 
Now refer more particularly to FIG. 1 in which an axle 14 of a vehicle is 
illustrated. The axle 14 is illustrated as a driven axle, but it could 
also be an axle of a trailer, for example. Connected to each end of the 
axle 14 is a wheel designated 2 and 3. Each wheel 2 and 3 on this axle 14 
has its own speed sensor designated 5 and 6, respectively. Such speed 
sensors 5 and 6 can be revolution counters. Further, these speed sensors 5 
and 6 generate signal values that are representative of the rotation 
and/or rotational behavior of the associated wheel. The signal values from 
the speed sensors 5 and 6 are conducted as an input signal to an 
electronic control system 1. The processing of these input signals in the 
electronic system 1 will be explained in more detail hereinafter. 
Each of the two wheels 2 and 3 are equipped with a corresponding brake 
cylinder 9 and 10, respectively. The brake cylinders 9 and 10 are 
pressurized by a common control valve 8 and depressurized by evacuation of 
such common control valve 8. The control valve 8 is a solenoid-operated 
valve and is assembled as a three position valve. In addition to an 
increase-pressure position and a decrease-pressure position, the control 
valve 8 includes a pressure-holding or maintenance position. The 
activation or control coil of the control valve 8 is connected to an 
output terminal of the electronic control system 1 and is controlled into 
one of these three positions based on the output control signal from such 
electronic control system 1. 
Although the anti-lock brake system illustrated in FIG. 1 is a pneumatic 
brake system supplied by a compressed air system 13 it could also be a 
hydraulic brake system. The compressed air system 13 is shown as an air 
reservoir which would be supplied by a source of compressed air (not 
shown). The desired brake pressure is set by the driver, of the vehicle, 
activating the brake valve 12. Such brake valve 12 is illustrated as a 
foot-pedal-operated brake valve, but it could be an electrically 
controlled brake valve as well. 
The anti-lock brake system illustrated in FIG. 1 is generally similar in 
design to the (SLC) anti-lock brake system of the prior art or, depending 
on the control applied, it may be generally similar to a (SHC) anti-lock 
brake system. 
Now refer more particularly to FIG. 2. The anti-lock brake system 
illustrated in FIG. 2 is essentially identical to that of FIG. 1. In this 
case, however, the wheel brake control is not by axle, but instead, the 
wheel brake control is by vehicle sides or by lateral control of the 
wheels 2 and 4. The wheel 4 of the second axle, therefore, is equipped 
with a speed sensor 7 and a brake cylinder 11. The two brake cylinders 9 
and 11 for the wheel 2 and 4, respectively, are switched in parallel by 
the common solenoid control valve 8. Otherwise, the schematic illustration 
of the anti-lock brake system in FIG. 2 is the same as in FIG. 1. For a 
better understanding of the present invention, reference is now made to 
FIG. 3. FIG. 3 graphically illustrates the wheel velocities, the 
acceleration, the deceleration, and the slip signals which are generated 
in the electronic control system 1. These signals are shown together with 
a plot of the brake pressure system curve according to the state of the 
art, and a plot of the brake pressure system curve according to this 
invention with respect to time. 
As the basis of the schematic illustration in FIG. 3, the operational 
function of the anti-lock brake system, according to this invention, as 
used in a vehicle application, will now be explained in greater detail. 
For the purposes of the following discussion, it is assumed that the 
vehicle is in a controlled braking condition. In this case, the low wheel 
which is operating on the bad side of the roadway, shows the typical 
velocity wave shape speed curve v.sub.RLow. On the other hand, the high 
wheel which is operating on the good road side, operates without need for 
control, i.e., its velocity v.sub.RHigh corresponds by and large to the 
velocity of the vehicle. 
As a consequence of changes in the velocity of the low wheel, the 
electronic control system 1 will produce appropriate control signals for 
deceleration (-b), acceleration (+b), and for slippage (.lambda.) of the 
controlled wheel. The method by which such control signals are produced in 
the electronic control system 1 is known in the art. The slippage signal 
(.lambda.) generally means, that the velocity of the controlled wheel has 
fallen below the reference velocity of the vehicle by a certain 
predetermined percentile. 
The customary and standard pressure flow for a control circuit according to 
the state of the art (brake pressure P.sub.B), as evidenced by a 
conventional pressure curve, starts at a point in time t.sub.1 with a 
decrease in pressure for the duration of the occurence of the deceleration 
(-b) signal in the system. The pressure is then maintained or held steady 
for the duration of the slippage signal (.lambda.) to the termination of 
the acceleration signal (+b). Subsequently, a repressurizing will occur, 
until a new deceleration signal (-b) will appear. Such repressurizing may 
be pulsed. 
According to the present invention, however, no pressure drop will occur 
with the appearance of the deceleration signal (-b) at time t.sub.1, 
except for a pressure holding. A pressure decrease at time t.sub.2 will 
occur only when the slippage signal (.lambda.) is given. 
Alternatively, a brake pressure decrease can also occur when the second 
wheel (high wheel) generates a deceleration signal (-b) and/or a slip 
signal (not shown). 
Once such a brake pressure reduction does occur, the drop will last up to 
the renewed drop of the deceleration signal (-b). At a point in time (time 
t.sub.3), possible also in pulsated form, the brake pressure, at a holding 
phase at low pressure, will relatively soon, i.e., after disappearance of 
the slip signal (.lambda.), intensify or increase in pulsation at a point 
in time (time t.sub.5), as long as the high wheel has not surpassed its 
own predetermined slip signal (.lambda.) threshold, or has surpassed it 
only very briefly. Otherwise, the braking pressure will increase only in 
the customary way, as is shown in the prior art, after an increased 
acceleration (+b-decrease) of both wheels. 
It is important to note that with the invention generated anti-lock brake 
system control concept, a deliberately higher slip at the low wheel is 
tolerated on roadways with heavy differential power relationships on the 
left/right (.mu.-split) or in spots with very low power application (ice 
spots, sandy or snowy spots), in favor of maintaining a relatively high 
braking force for the high wheel. This applies to the axle specific 
anti-lock brake system control, according to FIG. 1, as well as to the 
lateral anti-lock brake system control, according to FIG. 2. 
As can be seen in FIG. 3, the tolerated level of the braking pressure is at 
times higher for the invention concept, than for the state of the art. 
With the invention, the traditional concept of anti-locking brake systems 
will be deliberately disregarded, whereby the controlled wheel is not 
permitted to exceed a maximum (and thus optimal) slip of approximately 
20%. 
As long as the high wheel itself does not cause the electronic control 
system 1 to generate control signals with respect to such high wheel, it 
will brake with the excess of lateral guide force incurred during the 
pressure holding or maintenance phase from time t.sub.1 to t.sub.2. 
This makes it possible to do without the limited lateral control of the low 
wheel, which has an already poor power application on poor roadways having 
low traction, for example. An extended blocking of the low wheel, as it 
occurs during select high control, will thereby be avoided. For the 
braking force of the low wheel, because of the respective (.mu.-.lambda.) 
characteristics of roadways with low traction (snow, ice, sand), it is of 
little significance whether the low wheel brakes with a low or a high 
slippage. It is more important to avoid unnecessary reductions of the 
braking force. (e.g. as a consequence of ice patches, or wheel vibrations) 
at the stably braked high wheel which may be operating on a better road 
surface. 
The control concept of this invention will also take the .mu.-split into 
consideration. When the high wheel also causes the electronic control 
system 1 to generate control signals, especially when both wheels are 
subject to a continuous slip signal (.lambda.) at, for example 100 ms, the 
electronic control system 1 will switch to a select low control method. 
This will utilize the lateral guide capability at both wheels of the axle 
with about equally low power requirements. On the other hand, roadways 
with high traction forces lead to an avoidance of heavy wheel slippage, 
and thus high tire wear. 
In conjunction with relatively slow reacting solenoid control valves 8, it 
can be advantageous to use the control cycle of the low wheel as a base 
first control cycle and to use the subsequent control operations for the 
above-described concept according to the invention. 
It can also be advantageous for low vehicle velocities, such as for V.sub.F 
&lt;15 km/hr., to utilize select low control method criteria, in order to 
lower the bottom control limit or to reduce the subsequently higher 
slippage. The anti-lock brake system, according to the present invention, 
has been tested as a new trailer anti-lock brake system. In these tests, 
the theory behind the above described improvements over the (SLC) and 
(SHC) anti-lock brake systems has been demonstrated as accurate. 
Now refer more particularly to FIG. 4 in which the electronic control 
system 1 is illustrated schematically. Such electronic control system 1 is 
positioned on the vehicle in a convenient manner. The electronic control 
system 1 is electrically connected at the input terminals thereof to 
receive electrical signal values from the speed sensors 5 and 6 (FIG. 1) 
and/or 5 and 7 (FIG. 2). Additionally, at the output terminals thereof, 
such electronic control system 1 is electrically connected to the input 
terminal of the common electrically actuated and controllable solenoid 
control valve 8 (FIGS. 1 and 2) to enable feeding electrical control 
signals generated in such electronic control system 1 to such electrically 
actuated and controllable solenoid control valve 8. 
In the electronic control system 1, there is provided a means 16 for 
determining when the electrical signal value being fed to such input 
terminal of the electronic control system 1 is representative of a 
deceleration (-b) signal value of a low wheel. A means 18 is provided in 
such electronic control system 1 for generating a brake pressure hold 
electrical control system 1 in response to a determination in means 16 of 
such electrical signal value fed to the input terminals thereof being 
representative of such deceleration (-b) signal value of the low wheel. In 
addition, a means 20 is provided in the electronic control system 1 for 
feeding such brake pressure hold electrical control signal to the input 
terminal of the electrically actuated and controllable solenoid control 
valve 8. In response to such brake pressure hold electrical control signal 
being fed to the input terminal of the electrically actuated and 
controllable solenoid control valve 8 a first predetermined brake pressure 
on each of a respective brakeable wheel 2 and 3 (FIG. 1) and 2 and 4 (FIG. 
2) is maintained. Such first predetermined brake pressure being the 
initial brake pressure applied at the start of a brake application. 
In the presently preferred embodiment of the invention, the electronic 
control system 1 further includes a means 22 positioned therein for 
determining when the electrical signal value being fed to the input 
terminal thereof is an electrical signal value representative of a slip 
signal value (.lambda.) of such low wheel. In connection with the means 
22, there is a means 24 provided in such electronic control system 1 for 
generating a brake pressure decrease electrical control signal in response 
to a determination by means 22 of such electrical signal value being fed 
to the input terminal of such electronic control system 1 being 
representative of a slip signal (.lambda.) value of such low wheel. 
Further, the electronic control system 1 includes a means 26 positioned 
therein for feeding the electrical control signal generated by means 24 
from the output terminal of such electronic control system 1 to the input 
terminal of such electrically actuated and controllable solenoid control 
valve 8. Upon receipt of such brake pressure decrease electrical control 
signal at the input terminal thereof, such electrically actuated and 
controllable solenoid control valve 8 evacuates a predetermined portion of 
such first predetermined volume of the fluid pressure medium from the pair 
of brake cylinders associated therewith thereby decreasing the brake 
pressure being applied. 
A means 28 is provided in such electronic control system 1 for determining 
when the electrical signal value fed to the input terminal thereof by such 
speed sensing means 5 and 6 or 5 and 7 is an electrical signal value that 
is representative of a decrease in the slip signal (.lambda.) value of 
such low wheel determined in means 22. Within the electronic control 
system 1 there is a means 30 provided for generating a brake pressure 
increase electrical control signal in response to a determination in means 
28 of such electrical signal value being fed to the input terminal of such 
electronic control system 1 being representative of such decrease in the 
slip signal (.lambda.) value of the low wheel. This brake pressure 
increase electrical control signal is fed from the output terminal of the 
electronic control system 1 to the input terminal of such electrically 
actuated and controllable solenoid control value 8 by a means 32 provided 
in such electronic control system 1. Upon receipt of this brake pressure 
increase electrical control signal at the input terminal thereof, such 
electrically actuated and controllable solenoid control valve 8 
communicates a second predetermined volume of such fluid pressure medium 
from an outlet port thereof to each of such predetermined pair of brake 
cylinders thereby applying a second predetermined brake pressure on each 
of the respective brakeable wheels. 
As presently preferred, the electronic control system 1 includes a means 34 
positioned therein for determining when an electrical signal value being 
fed to the input terminal thereof is an electrical signal value 
representative of at least one of a deceleration signal (-b) value and a 
slip signal (.lambda.) value of a high wheel. In response to a 
determination in means 34 of such electrical signal value being fed to the 
input terminal of the electronic control system 1 is representative of 
either a deceleration signal (-b) value on a slip signal (.lambda.) value 
of such high wheel a brake pressure decrease electrical control signal is 
generated in a means 36 disposed within such electronic control system 1. 
A means 38 for feeding the brake pressure decrease electrical control 
signal generated in means 36 from the output terminal of such electronic 
control system 1 to the input terminal of such electrically actuated and 
controllable solenoid control valve 8 is also disposed within such 
electronic control system 1. When such brake pressure decrease electrical 
control signal is fed by means 38 to the input terminal of such 
electrically actuated and controllable solenoid control valve 8, then such 
electrically actuated and controllable solenoid control valve 8 will 
evacuate from the predetermined pair of brake cylinders associated with 
such high wheel a predetermined portion of such first predetermined 
volumes of the fluid pressure medium thereby decreasing such first 
predetermined brake pressure being applied thereto. 
In a presently preferred embodiment of the invention, when it is determined 
in the electronic control system 1 by the means 34 that the electrical 
signal value being fed to the input terminal of such electronic control 
system 1 is representative of the slip signal (.lambda.) value of the high 
wheel, then a select low control mode electrical control signal is 
generated by a means 40 which is also contained within such electronic 
control system 1. The select low control mode electrical control signal 
generated by means 40 is fed from the output terminal to such electronic 
control system 1 to the input terminal of the electrically actuated and 
controllable solenoid control valve 8 by a means 42 disposed within such 
electronic control system 
1. This results in the anti-lock brake system switching to such select low 
control mode. 
A timing means 44 is provided in the electronic control system 1. Such 
timing means 44 determines a time duration of the electrical signal value 
which is representative of such slip signal (.lambda.) value of each of a 
high wheel and a low wheel being fed to the input terminal of such 
electronic control system 1. In response to a determination by the timing 
means 44 of such slip signal (.lambda.) value being present for a time 
duration of about 100 ms, a select low control mode electrical control 
signal is generated in a means 46 disposed in the electronic control 
system 1. This select low control mode electrical control signal generated 
by means 46 is fed from the output terminal of such electronic control 
system 1 to the input terminal of the electrically actuated and 
controllable solenoid control valve 8 by a means 48 disposed in such 
electronic control system 1. In this manner, the anti-lock brake system is 
switched to such select low control mode. 
According to the present invention, a means 50 is positioned in the 
electronic control system 1 for switching the electronic control system 1 
into a select low control mode during the first control cycle of the 
anti-lock brake system. 
In addition, there is a means 52 positioned within such electronic control 
system 1 for switching such anti-lock brake system into such select low 
control mode at a vehicle speed of less then about 15 km/hr. 
The basic concept consists of the fact that as compared with the known 
individual or select low controls, as taught in the German Reference DE-OS 
27 40 419, the deceleration signal (-b) and possibly the slip signal 
(.lambda.) of the low wheel, reacts not simultaneously with brake pressure 
reduction, but only when the brake pressure is being held constant. 
Although a number of embodiments of the anti-lock brake system, according 
to the present invention, have been described in detail above, it should 
be apparent to those persons skilled in the vehicle braking art that 
various other modifications and adaptations of this invention may be made 
without departing from the spirit and scope of the attached claims.