Antilock brake system motor speed control

A motor speed controller for an antilock brake system motor driven brake pressure modulator controls the speed during the pressure ramping phase of an antilock brake pressure control cycle by commanding periods of dynamic braking of the motor while the motor current is being controlled to ramp the pressure.

BACKGROUND OF THE INVENTION 
This invention relates to an antilock control method for vehicle wheel 
brakes. 
When the brakes of a vehicle are applied, a braking force is generated 
between the wheel and the road surface that is dependent upon various 
parameters which include the road surface condition and the amount of slip 
between the wheel and the road surface. This braking force increases as 
slip increases until a critical value of slip is surpassed. Beyond the 
critical value of slip, the braking force decreases and the wheel rapidly 
approaches lockup. Therefore, to achieve stable braking, an antilock 
control system seeks to operate wheel slip at or near the critical slip 
value. An antilock control system achieves this objective by detecting an 
incipient lock condition. Upon detecting an incipient lock condition, the 
antilock control system releases pressure at the wheel brake to allow 
recovery from the incipient lock condition. Once the wheel recovers from 
the incipient lock condition, brake pressure is reapplied. Criteria used 
to indicate an incipient lock condition includes excessive wheel 
deceleration and/or excessive wheel slip. 
One known antilock control system uses a motor driven pressure modulator in 
which a DC torque motor drives a piston in a cylinder whose volume is 
modulated to control the hydraulic brake pressure at the wheel brake. In 
this system, because of the relationship between motor current, motor 
torque and motor load represented by the hydraulic brake pressure on the 
head of the piston, the value of motor current is used as a representation 
of brake pressure and is controlled to provide control of the brake 
pressure. In general, when an incipient wheel lock condition is sensed, 
the value of motor current at this time is stored as a representation of 
the brake pressure producing the maximum braking force coexisting with the 
critical slip between the wheel and the road surface and the motor current 
is controlled to quickly retract the piston to release brake pressure to 
allow recovery from the incipient wheel lock condition. When a recovery 
from the incipient wheel lock condition is sensed, the motor current is 
controlled to extend the piston to reapply brake pressure. In reapplying 
the brake pressure, the pressure is quickly established substantially at 
the brake pressure producing the maximum braking force by quickly 
establishing the motor current at a significant fraction of the motor 
current stored at the time an incipient wheel lock condition was sensed. 
Thereafter, brake pressure is ramped at a controlled rate that is a 
function of wheel slip and acceleration by ramping the motor current until 
an incipient wheel lock condition is again sensed after which the cycle is 
repeated. In general, the ramp rate is decreased with increasing wheel 
slip and wheel deceleration so that the ramp rate is smaller as the wheel 
approaches an incipient wheel lock condition. This lower ramp rate 
prevents an overshoot of the brake pressure resulting from system inertia 
when an incipient wheel lock condition is sensed. 
In the foregoing form of motor driven pressure modulator, the following 
dynamic relationships exist: (a) when the brake pressure load on the motor 
is equal to the motor torque, the motor does not rotate, the piston 
remains stationary, and motor current is a measure of the brake pressure 
and (b) when the brake pressure load on the motor is small compared to the 
motor torque, the motor accelerates and rotates at a high rate and the 
piston travels at a high speed. In this latter situation, the speed of the 
motor is unknown and the motor current is not a true indicator of brake 
pressure. If this condition exists when the wheel slip approaches the 
critical slip, the high motor/piston speed may cause the brake pressure to 
overshoot the pressure producing the critical slip and will result in 
storing a current when an incipient wheel lock condition is sensed that 
represents a brake pressure other than the pressure producing the maximum 
braking force. 
Excessive speed resulting in the brake pressure overshooting the pressure 
producing the critical slip may also occur when the ramp rate of the motor 
current is decreased in response to increasing slip and/or deceleration as 
the wheel slip approaches the critical slip value. At the time the current 
ramp rate is decreased, the motor speed is related to the prior higher 
ramp rate and the motor current is not a true measure of the actual 
pressure. If the critical wheel slip is reached soon after a decrease in 
the current ramp rate, the excessive motor speed may result in the brake 
pressure overshooting the pressure producing the critical slip and in the 
storing of a current representing a brake pressure other than the pressure 
producing the maximum braking force. 
Another characteristic of the foregoing form of motor driven pressure 
modulator is that its compliance varies with load on the piston. When the 
motor load is low (i.e., low pressure present on the piston head), the 
motor position change necessary to create a change in pressure is greater 
compared to the motor position change required to produce a change in 
pressure when the motor load is high (i.e., piston head pressure high). 
Thus, for a given motor current ramp rate during application of brake 
pressure, the real motor speed will actually be higher at lower pressures 
than it will be at higher pressures. When beginning a pressure reapply 
from a low pressure value, if motor torque is allowed to increase at a 
very high rate, the motor travels at an effective speed higher than is 
actually desired, which may cause the system to overshoot the desired 
pressure. 
Thus from the foregoing, it can be seen that when controlling motor current 
to control re-application of brake pressure following recovery from an 
incipient wheel lock condition, it is desirable to provide controlled 
movement of the motor such that the motor does not exhibit an overspeed 
condition. 
SUMMARY OF THE INVENTION 
In general, this invention provides for limiting the speed of the motor of 
a motor driven pressure modulator of an antilock braking system. This is 
accomplished by commanding periods of dynamic braking during the 
re-application of pressure to limit periods of motor acceleration and to 
prevent the motor speed from becoming excessive. 
In order to prevent excessive motor speeds when the ramp rate is shifted to 
a lower rate as the wheel approaches the critical slip, the amount of 
braking is increased with the decrease in the ramp rate to quickly slow 
the motor speed to the new slower ramp level and to thereafter inhibit 
excessive motor speeds. 
The dynamic braking is also provided to linearize the pressure ramp for a 
given ramp rate. This is accomplished by providing an amount of motor 
braking that is inverse to the magnitude of the motor current. This 
results in lower braking forces at high currents and higher braking forces 
at lower currents to compensate for the variation in the compliance of the 
motor driven pressure modulator. 
In a specific form of the invention, the motor braking is provided by 
applying a zero current command of the motor in the reverse direction. The 
amount of this dynamic braking is adjusted by varying the duration of the 
brake period.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A wheel lock control system for a wheel of a motor vehicle is illustrated 
in FIG. 1. In general, the wheel includes a brake unit 10 operated by 
hydraulic pressure provided by a master cylinder 12 and a hydraulic boost 
unit 14 operated by the vehicle operator. The hydraulic fluid under 
pressure from the master cylinder 12 is provided to the brake unit 10 via 
brake lines 16 and a pressure modulator 18. The brake unit 10 is 
illustrated as a disk brake system that includes a caliper 20 located at a 
rotor 22. The wheel includes a wheel speed sensing assembly comprised of 
an exciter ring 24 rotated with the wheel and an electromagnetic sensor 26 
which monitors the rotation of the exciter ring to provide a signal having 
a frequency proportional to the speed of the wheel. The wheel speed signal 
from the sensor 26 is provided to an electronic controller 28. 
The pressure modulator 18 is controlled by the electronic controller 28 to 
limit the brake pressure applied to the wheel brake assembly 10 to prevent 
wheel lockup. The modulator 18 is illustrated in an inactive position when 
it is transparent to the braking system. This is the modulator home 
position during normal vehicle braking. In general, when the controller 28 
senses a braking condition whereat the wheel is approaching an incipient 
wheel lock, the pressure modulator 18 is controlled to regulate the 
braking pressure to the wheel to maintain the braking of the wheel in a 
stable braking region. The pressure modulator 18 includes a DC torque 
motor 30 whose output shaft drives a gear train 32 which, in turn, rotates 
a linear ball screw actuator 34. The ball screw actuator contains a 
linearly stationary ball screw which, when rotated, linearly positions a 
nut 36. A nut 36 terminates in a piston 38 such that as the linear ball 
screw rotates, the piston 38 is either extended or retracted depending 
upon the direction of the rotation of the torque motor 30. The modulator 
20 includes a housing 40 in which a cylinder 42 is formed. The piston 38 
is reciprocally received within the cylinder 42. The cylinder 42 forms a 
portion of the fluid path between the master cylinder 12 and the wheel 
break unit 10. Included within this fluid path is a normally closed ball 
check valve 44 which, when closed, isolates the master cylinder 12 from 
the wheel brake unit 10. The ball check valve 44 is operated to an open 
position by the piston 38 when it is positioned in an extended position 
within the cylinder 42 as illustrated in FIG. 1. This position is the home 
position of the modulator 18. 
When the ball check valve 44 is opened, fluid communication is provided 
between the master cylinder 12 and the wheel brake unit 10. This position 
is the normal inactive position of the pressure modulator 18 so that 
normal braking of the wheel of the vehicle is provided upon actuation of 
the brakes by the vehicle operator. However, when the torque motor 30 is 
operated by the electronic controller 28 to modulate the braking pressure 
in the wheel brake unit 10, the piston 38 is retracted, allowing the ball 
check valve to seek and isolate the master cylinder 12 from the wheel 
brake unit 10 as long as the pressure in the cylinder 42 is less than the 
pressure from the master cylinder 12. Further retraction of the piston 38 
functions to increase the volume in the cylinder 42 thereby decreasing the 
pressure applied to the wheel brake unit 10. By controlling the DC torque 
motor 30, a pressure at the wheel brake can therefore be modulated to 
controlled values less than the master cylinder 12 pressure outlook until 
such time that the piston 38 again unseats the ball check valve 44 or 
until the pressure generated by the pressure modulator at the wheel brake 
unit 10 exceeds the fluid pressure output of the master cylinder 12. When 
this latter condition exists, the ball check valve 44 is opened by the 
differential fluid pressure which limits the pressure of the wheel brake 
unit 10 at the master cylinder 12 pressure. In this manner, the wheel 
cylinder pressure can never exceed the operator established pressure. 
Referring to FIG. 2, the electronic controller 28 of FIG. 1 is illustrated 
and generally takes the form of a digital computer based controller. The 
controller includes a microprocessor 46 that is standard in form and 
includes the standard elements such as a central processing unit which 
executes an operating program permanently stored in a read-only memory and 
which stores tables and constants utilized in controlling the modulator 
18, an analog-to-digital converter, a random access memory and 
input/output circuitry utilized to provide motor control signals to a 
motor driver interface circuit 48. The input/output circuit further 
includes input ports for receiving the wheel speed signal from the output 
of an interface and squaring circuit 53 having an input from the wheel 
speed sensor 26. 
The motor driver interface circuit 48 receives an enable signal, a motor 
current command signal I.sub.c and a forward/reverse direction signal from 
the microprocessor 46 and controls an H-switch driver 50 to establish the 
commanded motor current I.sub.c in the required forward or reverse 
direction. The current to the torque motor 30 is controlled to the 
commanded value via a closed loop that responds to the actual motor 
current represented by the voltage across a sense resistor 52. In response 
to the direction and motor current command, the motor driver interface 48 
energizes the upper and lower forward gates via the upper gate signal UGF 
and lower gate signal LGF to control the DC torque motor 30 in the forward 
direction to apply brake pressure and energizes the upper and lower 
reverse gates via the signals UGR and LGR to control the DC torque motor 
30 in the reverse direction to retract the piston 38 to reduce pressure at 
the wheel brake. The microprocessor 46 may take the form of a Motorola 
single chip microcomputer MC-68HC11. The motor driver interface 48 and 
H-switch 50 may take the form of the driver illustrated in the U.S. Pat. 
No. 4,835,695 issued May 30, 1989. 
As previously described, when the speed of the DC torque motor 30 is low as 
current is controlled in the forward direction to apply pressure to the 
brakes 20, the motor current is a measure of the torque and therefore the 
brake pressure. However, when the motor 30 is rotating, the motor current 
sensed by the resistor 52 is not a true indicator of brake pressure due to 
the back EMF of the motor 30. 
During a typical antilock brake control cycle established by the system of 
FIGS. 1 and 2, when an incipient wheel lock condition is sensed, the motor 
current is controlled to quickly retract the piston 38 to release brake 
pressure to allow recovery from the incipient wheel lock condition. This 
reversal is accomplished by commanding a reverse motor direction and 
setting the command current I.sub.c at a reverse current value I.sub.r. 
The motor driver interface 48 responds to these commands by energizing the 
upper and lower reverse H-switch gate switches to drive the motor 30 in 
reverse direction at the commanded current level. When recovery from the 
incipient wheel lock condition is sensed, brake pressure is reapplied and 
ramped by commanding a forward motor direction and setting the command 
current I.sub.c at a forward apply current value I.sub.a. The motor driver 
interface responds to these commands by energizing the upper and lower 
H-switch gate switches to drive the motor in a forward direction at the 
commanded level. Brake pressure is ramped by ramping the value of the 
apply current value I.sub.a. This ramp function is continued until an 
incipient wheel lock condition is again sensed after which the cycle is 
repeated. In general, the ramp rate is decreased with increasing wheel 
slip and wheel deceleration so that the ramp rate is smaller as the wheel 
approaches an incipient wheel lockup condition. 
In accord with the principles of this invention, as the current is being 
ramped to establish a controlled ramp of the pressure applied to the wheel 
brake, the DC torque motor 30 is repetitively dynamically braked at 
constant intervals for a specified time period. This braking is provided 
by enabling the H-switch 50 in a motor reverse direction and commanding a 
zero current to the DC torque motor 30. This establishes a dynamic braking 
of the DC motor tending to retard motion of the motor output shaft. The 
repetitive dynamic braking of the motor 30 as the motor current is ramped 
to ramp the brake pressure is illustrated in FIG. 5. By this controlled 
braking, the acceleration periods of the DC torque motor 30 is limited and 
the motor speed is controlled to relatively low values. This periodic 
braking assures that the brake pressure does not overshoot the brake 
pressure producing the maximum braking force when the incipient wheel 
lockup condition is sensed and assures that the motor current value stored 
when an incipient wheel lockup condition is sensed substantially 
represents the brake pressure applied to the wheel brake 20 producing the 
critical slip. Further, the dynamic braking of the motor 30 is provided so 
as to substantially linearize the pressure ramp provided by the motor 30 
as the current is ramped to ramp the pressure from a low to high value. 
The operation of the electronic controller 28 in controlling the DC torque 
motor 30 in accord with this invention is illustrated in FIGS. 3 and 4. 
The read-only memory of the microprocessor 46 contains the instructions 
necessary to implement the algorithm as diagrammed in those figures. 
Referring first to FIG. 3, when power is first applied to the system from a 
vehicle battery 54 (FIG. 1) such as when a conventional vehicle ignition 
switch (not illustrated) is rotated to its "on" position, the computer 
program is initiated at a point 56 and then provides for system 
initialization at step 58 which entails clearing registers, initializing 
various RAM variables to calibrated values and other functions. When the 
initialization routine is completed, the program then proceeds to perform 
antilock brake control functions as required. These antilock control 
functions are performed by executing a control cycle in response to each 
of repeated control cycle interrupts which are generated at predetermined 
fixed time intervals such as 5 milliseconds. Upon the occurrence of a 
control cycle interrupt, the digital computer begins executing the 
functions embodied in the control cycle. First, at step 60, wheel speed 
sensor information is read and wheel speed is computed for each of the 
vehicle wheels. Thereafter, the routine determines the individual wheel 
accelerations at step 62 and the individual wheel slip values at step 64. 
From the computed values of wheel acceleration and wheel slip, the program 
determines at step 66 whether or not those parameters represent the need 
for antilock brake pressure modulation for any wheel. 
If antilock control of wheel brake pressure is not required, the program 
proceeds to perform background tasks at step 68. These tasks may include 
diagnostic functions as well as other functions. However, if step 66 
determines that a need for antilock brake pressure modulation for any 
wheel is required, the program proceeds to a step 70 where antilock brake 
control functions are executed. Once those functions are executed, the 
program proceeds to the step 68 previously described. 
The foregoing steps 60 thru 70 are repeated once for each control cycle. 
Thus, when a control cycle interrupt occurs, a new cycle begins at step 60 
and the functions represented by steps 60 thru 70 are again repeated as 
previously described. 
Repeated executions of step 70 when antilock brake control is required 
establishes the following brake cycle. When the wheel slip and 
acceleration conditions represent an incipient wheel lockup condition, a 
pressure release mode is indicated and brake pressure is quickly released 
to allow the wheel to recover from the incipient wheel lockup condition. 
When wheel acceleration and slip conditions represent a recovered 
condition, an apply mode is indicated and wheel pressure is reapplied, 
such as to a significant fraction of the wheel pressure at the time 
pressure was released, and thereafter ramped until another incipient wheel 
lockup condition is sensed at which time the release mode is indicated and 
the cycle is repeated. In the form of control to which this invention 
relates, the control of the brake pressure is established via control of 
the current through the DC torque motor 30. Accordingly, brake pressure is 
released in response to a detected incipient wheel lock condition by 
controlling current through the DC motor 30 in a reverse direction and 
pressure is applied by controlling the current through the motor 30 in a 
forward direction. During brake pressure application, the current is 
ramped at a controlled rate with intermittent dynamic braking periods to 
limit the motor speed and to provide the desired linear relationship of 
the ramping of the brake pressure. 
Referring to FIG. 4, there is illustrated the antilock brake control 
functions executed once for each braking channel where each channel 
includes a modulator 18. Where the four wheels of the vehicle are 
controlled independently, this requires the routine of FIG. 4 to be 
executed four times, once for each wheel with its related parameters. In 
another system, the rear brakes may be controlled by a single modulator 
such that the routine of FIG. 4 then is executed once for each front wheel 
and once for the combined rear wheels. 
The antilock brake control routine begins with a step 72 that determines 
the required apply or release brake mode. In general, the apply or release 
brake mode is determined based upon the conditions of wheel acceleration 
and wheel slip. In this embodiment, the brake mode is determined by a 
lookup table stored in ROM establishing the threshold between pressure 
apply and release modes as a function of wheel acceleration and wheel 
slip. When the combination of wheel acceleration and wheel slip represents 
an incipient wheel lockup condition, the lookup table indicates a brake 
release mode whereas if the combination of wheel acceleration and wheel 
slip represents a recovered condition, the lookup table indicates a brake 
apply mode. 
Step 74 then determines whether the brake mode determined at step 72 is an 
apply mode. If not, indicating a release mode in response to an incipient 
lockup condition, the program proceeds to a step 76 where the duration of 
a dynamic motor brake period interval is reset. In this embodiment, the 
motor brake period interval is a constant interval representing the 
frequency at which the DC torque motor 30 is dynamically braked. From step 
76, the program executes a brake release mode at step 78. In the preferred 
embodiment, when an incipient wheel lockup condition is first detected, 
step 78 stores the commanded motor current as representative of the motor 
current at the time the incipient wheel lockup condition is detected. This 
stored current value represents a measure of the brake pressure producing 
the maximum brake effort that corresponds to the wheel critical slip. 
Thereafter with repeated executions of step 78 for the respective wheel, 
step 78 releases the brake pressure by commanding a release current 
I.sub.r in a reverse direction. In one embodiment, steps 72 and 78 can 
also provide for a hold mode wherein the brake pressure is held at a 
constant value when the wheel slip and acceleration represent the wheel 
beginning to recover from the incipient wheel lockup condition. 
Release of brake pressure in response to repeated execution of the steps 72 
through 78 results in the wheel recovering from the incipient lock 
condition. This recovery condition is detected at step 72 when the lookup 
table indicates a pressure apply mode for the wheel acceleration and wheel 
slip conditions. When step 74 determines that step 72 has determined a 
pressure apply mode, the program then proceeds to a step 80 where the 
apply current I.sub.a for reapplying brake pressure is determined. In the 
preferred mode, the apply current is first set to a significant fraction 
of the current stored in step 78 when the incipient lockup condition was 
first detected. Thereafter, upon repeated executions of the step 80, the 
apply current I.sub.a is ramped at a controlled rate to increase the brake 
pressure at the wheel brake 20 until an incipient wheel lock condition is 
again sensed at step 72 wherein a brake release mode is then again 
indicated at which time the cycle is repeated. In this embodiment, the 
ramp rate of the motor apply current I.sub.a for increasing the brake 
pressure during the brake apply mode is a function of wheel slip and wheel 
acceleration. In general, this ramp rate is decreased with increasing 
wheel slip and wheel deceleration results in a lower ramp rate as the 
wheel approaches an incipient wheel lock condition. 
To limit the speed of the torque motor 30 and to provide a linear 
relationship between the pressure ramp rate and the current ramp rate, the 
subject invention provides for periodic dynamic braking of the DC torque 
motor as the current is ramped to provide for ramping of the pressure. By 
preventing excessive motor speed, the overshoot of the pressure at the 
time an incipient wheel lockup condition is sensed is minimized and the 
motor current value stored in step 78 when the incipient wheel lockup 
condition was first sensed will be representative of the actual brake 
pressure existing at that time which corresponds to the pressure producing 
the critical slip between the wheel and the road surface. 
The periodic braking of the DC motor is provided at constant intervals. The 
program determines whether or not it is time to initiate dynamic braking 
at step 82. In one embodiment, this step indicates time to initiate a 
brake period at constant intervals of 55 milliseconds. Assuming it is time 
to initiate dynamic braking of the motor 30, the program proceeds to a 
step 86 where the duration of the dynamic braking is established. In 
general, the duration is inversely related to the ramp rate of the applied 
current and inversely related to the magnitude of the motor current. As 
previously described in regard to step 80, the controlled rate of increase 
of the apply current Ia is established as a function of wheel acceleration 
and wheel slip. By establishing the dynamic brake period as an inverse 
relationship to the ramp rate, a larger brake period is provided when step 
80 establishes a transition to a slower ramp rate. This increased brake 
period quickly reduces the speed of the motor associated with the higher 
ramp rate to the speed established by the lower ramp rate to thereby 
prevent the potential of an overshoot condition if the wheel parameters 
are in proximity to an incipient wheel lockup condition. The inverse 
relationship to the motor current provides a larger dynamic brake period 
at low current levels with a decrease in the brake period as the current 
level is ramped. This provides a linearization of the pressure ramp rate 
and limits the motor speed at the lower pressures at which the pressure 
ramp rate for a given current ramp rate is higher than at the higher 
pressures. The dynamic brake period is determined in the preferred 
embodiment via a lookup table in ROM addressed as a function of the ramp 
rate of the apply current determined at step 80 and the magnitude of the 
commanded apply current I.sub.a. The lookup table stores the various 
dynamic brake periods associated with the particular combinations of 
current ramp rate and current level. 
Following the determination of the brake period, the program proceeds to a 
step 88 which provides for dynamic braking of the torque motor 30. This is 
accomplished by commanding a reverse direction of the motor to the motor 
driver interface and commanding a brake current value I.sub.b. In the 
preferred embodiment, the command current I.sub.b is zero so that the 
motor is set in the reverse direction with a zero command current. The 
effect of the foregoing is to establish dynamic braking in the motor to 
slow the speed of the torque motor 30. 
Returning to step 82, if it is determined that it is not time to initiate a 
brake period, the program proceeds to a step 90 where it determines 
whether or not a previously initiated brake period has expired. If a brake 
period had previously been initiated but the duration established at step 
86 has not expired, the program proceeds to step 88 where braking of the 
motor is continued. However, when the brake period expires, the program 
proceeds from step 90 to a step 92 where the normal apply current I.sub.a 
in the forward direction established via step 80 is commanded to the motor 
driver interface. When antilock braking is provided for all the brake 
channels as required, the program exits the antilock brake control 
function routine 70 and proceeds to perform the background task 68 
illustrated in FIG. 3. 
The effect of the foregoing is to recurrently interrupt the ramping of the 
motor current ramp to apply the brake current in the opposite direction 
for the determined brake period. 
By providing for the periodic braking of the motor via the routine of FIG. 
4 as described, the motor speed is limited so as to prevent an overshoot 
of the pressure required to establish the maximum braking condition 
occurring at the critical slip between the wheel and the road surface. 
Further, in the antilock brake control system wherein the motor current is 
stored upon the sensing of an incipient wheel lockup condition to 
establish a reference for re-application of brake pressure, the value 
stored is representative of the actual brake pressure occurring at the 
critical slip between the wheel and the road surface. 
The foregoing description of a preferred embodiment of the invention for 
the purpose of illustrating the invention is not to be considered as 
limiting or restricting the invention since many modifications may be made 
by the exercise of skill in the art without departing from the scope of 
the invention.