Motor driven brake pressure modulator with motor position control

A multi-phased electrically commutated brushless DC motor driven brake pressure modulator includes a pressure actuator the position of which is controlled by the motor to establish the desired brake pressure. The movement of the actuator is determined by monitoring the motor rotation as represented by the output of the rotor position sensors used in commutation of the motor windings. The position of the actuator is represented by a counter value which is incremented or decremented in response to the output of the rotor position sensors and the direction of motor rotation. The counter value is related to the actuator position by presetting the counter value when the actuator in a known position such as a mechanical limit position. The known position is established by causing the motor to rotate in a direction to position the actuator toward the limit position until further outputs of the rotor position sensor are not sensed indicating the actuator is at its mechanical limit position.

BACKGROUND OF THE INVENTION 
This invention relates to an electric motor driven brake pressure modulator 
for a motor vehicle braking system. 
The use of motor driven pressure modulators in a vehicle braking system is 
known. For example, such use includes a motor driven braking pressure 
modulator in vehicle antilock braking systems. In these systems, the brake 
pressure is modulated by means of a DC torque motor driving a piston in a 
cylinder whose volume is modulated to control the hydraulic pressure at 
the wheel brake. In one such known system, the motor is controlled to 
position the piston in an initial, fully extended home position at which a 
check valve is unseated to couple the brake system master cylinder to the 
wheel brake to allow normal braking. When antilock brake pressure 
modulation is required, the motor retracts the piston (which allows a 
check valve to close to isolate the master cylinder from the wheel brake) 
to reduce brake pressure and thereafter modulates the piston position to 
provide pressure control for antilock braking. When antilock braking is no 
longer required, the motor returns the piston to its extended home 
position. Typically in these prior systems, the braking pressure is 
established based upon a relationship between a motor current, motor 
torque and the hydraulic pressure acting on the head of the piston. Motor 
current then becomes the controlled parameter to establish a desired 
braking condition via the brake pressure/motor current relationship. 
The control of braking pressure in such systems wherein a brake pressure 
actuator is controlled by a motor may be substantially simplified by 
controlling the pressure based upon the position of the actuator 
controlled by the electric motor as opposed to motor current. This is 
because the functional relationship between position and pressure is 
substantially simpler than the functional relationship between motor 
current and pressure due to the motor dynamics. However, this would 
necessitate the addition of a position sensor on the actuator in order to 
establish the positional information. 
SUMMARY OF THE INVENTION 
This invention relates to a motor driven brake pressure modulator in which 
the position of a pressure actuator is controlled by the motor to 
establish the desired brake pressure. In accord with this invention, the 
position of the actuator is determined by monitoring the incremental 
rotation of the motor rotor from a reference actuator position. 
In one aspect of the invention, the position of the actuator is represented 
by a counter value which is incremented and decremented based upon the 
direction and incremental rotation of the motor rotor. This invention 
provides for relating the counter value to the actuator position by 
presetting the counter value when the actuator is in a known position such 
as a mechanical limit position. The known position is established by 
causing the motor to rotate in a direction to position the actuator toward 
the limit position until further rotation of the motor rotor is not sensed 
indicating the actuator is at it's mechanical limit. 
In accord with another feature of the invention, the motor is a 
multi-phased electrically commutated brushless DC motor and the 
incremental movement of the rotor is monitored by observing the output of 
the conventional rotor position sensors used in commutation of the motor 
windings.

DETAILED DESCRIPTION OF THE DRAWINGS 
An electric motor driven antilock braking system incorporating the 
principles of this invention is generally depicted in FIG. 1. Referring to 
FIG. 1, the braking system comprises a hydraulic boost unit 10, master 
cylinder 11, a wheel brake 12 associated with one wheel of the vehicle, an 
electric motor driven hydraulic pressure modulator 14, and an electronic 
controller 16 for operating the modulator 14 with current from the vehicle 
storage battery 18. The master cylinder 11 develops hydraulic pressure in 
line 20 in relation to the force applied to an operator manipulated brake 
pedal, the line 20 being connected to the brake 12 via modulator 14 and 
brake line 22. The brake 12 is depicted as a disk brake caliper which 
develops braking force on the wheel rotor 26 in relation to the hydraulic 
pressure in the brake line 22. The wheel includes a wheel speed sensing 
assembly comprised of an exiter ring 28 rotating with the rotor 26 and 
therefore at the speed of the vehicle wheel and an electromagnetic sensor 
30 which monitors the rotation of the exiter ring and provides a signal 
having a frequency proportional to the speed of the vehicle wheel. The 
wheel speed signal from the sensor 30 is provided to the controller 16. 
The modulator 14 comprises a piston 32 axially displaceable in a modulator 
bore 34, a check ball 36 resiliently seated on a ball seat 38 disposed 
between the brake lines 20 and 22, and a bi-directional electric motor 40 
coupled to to the piston 32 via a reduction gear-set 42 and a ball screw 
actuator 44 to control the axial displacement of piston 32. 
Energization of the motor 40 is controlled by the electronic controller 16. 
When the controller 16 energizes the motor 40 for rotation in a forward 
direction, the ball screw actuator 44 extends the piston 32 into the bore 
34. When extended soqs to engage the .RTM.nd of the bore 34, the piston 32 
functions to unseat the check ball 36. This opens the communication 
between brake lines 20 and 22, and represents the normal or quiescent 
state of the antilock brake system. When the controller 16 energizes the 
motor 40 for rotation in the opposite, or reverse, direction, the ball 
screw actuator 44 retracts piston 32 within the bore 34, permitting spring 
46 to seat the check ball 36 on the ball seat 38, thereby isolating the 
brake line 22 from the brake line 20 and therefore the brake 12 from the 
master cylinder 11. In this condition, the brake fluid in line 22 back 
fills the modulator bore 34 as the piston 32 is retracted, relieving the 
fluid pressure developed at the brake 12. By controlling the motor 40, the 
pressure at the wheel brake 12 can therefore be modulated to controlled 
values less than the hydraulic pressure in brake line 20 until such time 
that the piston 32 is again extended to unseat the check ball 36 at which 
time the brake pressure output of the master cylinder is again 
communicated to the brake 12. 
The antilock control system of FIG. 1 is operative at all times while the 
vehicle is in operation. It is necessary for normal vehicle braking via 
the master cylinder 11 for the modulator 14 to be in the home position 
illustrated in FIG. 1 with the check ball 36 being unseated from the seat 
38. When so positioned, when the vehicle operator applies the vehicle 
brakes, the modulator 14 is in the passive or quiescent operating mode so 
that the hydraulic fluid passes through the brake line 20 and the check 
valve to the brake 12 thereby providing normal wheel braking. 
Referring to FIG. 2, the DC motor 40 takes the form of a brushless 
electrically commutated DC motor comprising a four pole permanent magnet 
rotor 48 and three stator windings 50-54. The stator windings are 
connected in a Y configuration in which the windings are connected at one 
end to a common terminal and individually connected at their other ends to 
the output of a power switch 56 in the controller 16. The power switch 56 
is comprised of a conventional full wave bridge cross which the voltage 
VBAT of the battery 18 is applied. Current through the power switch 56 and 
therefore the stator windings 50-54 of the motor 40 is sensed by a current 
sense resistor 58 the voltage across which represents the actual motor 
winding current IACT. 
The motor 40 further includes conventional position sensors 60-64 in the 
form of Hall-effect sensors situated 60 electrical degrees apart in the 
magnetic field of the rotor 48 and which are utilized by a standard 
commutation and current control circuit 66 in the controller 16 to control 
the switches in the full wave bridge of the power switch 56 for 
commutating the phase windings 50-54. 
The commutation and current control circuit 66 may take any known form such 
as a Unitrode UC1625 brushless motor controller chip. This circuit 
responds to an enable signal EN, a direction signal DIR and the position 
signals from the position sensors 60-64 for controlling the rotation and 
direction of the rotor 48. Further, the circuit 66 controls the current 
IACT in the stator windings 50-54, and therefore motor torque, to an input 
current command value ICOM. While any form of current control may be 
utilized, the preferred mode provides for a fixed frequency, pulse width 
modulation of the power switch 56 to regulate the sensed motor current 
IACT to the commanded motor current value ICOM. In one embodiment, an 
error voltage may be generated based upon the difference between the 
commanded current value ICOM and the actual current value IACT. The error 
signal is compared by a comparator to the level of a triangular wave 
signal to provide the duty cycle signal for controlling the power switch 
56 to establish the commanded current ICOM. In another embodiment, in 
addition to this proportional control of the motor current, integral 
control may be provided to eliminate the error associated with solely 
proportional control. 
In accord with this invention, to provide for antilock controlled braking, 
the controller 16 includes a computer 67 which executes an operating 
program permanently stored in memory to regulate the brake pressure 
applied to the wheel brake 12 in response to an incipient wheel lock 
condition by controlling the position of the piston 32 in the modulator 
14. To monitor the position of the piston 32, the controller utilizes a 
rotor position encoder and relative position counter circuit 68 which 
tracks the movement of the piston 32 by means of a counter that counts in 
one direction (such as down) the state changes in the position sensors 
60-64 when the rotor 48 is rotating in a direction retracting the piston 
32 away from its home position and counts in the opposite direction (up in 
this example) when the rotor 48 is rotated in the direction extending the 
piston 32 towards its home position. In order to establish a known 
relationship between the count in the counter and the position of the 
armature 32, the counter is preset to a predetermined count when the 
piston is in a known position. This known position is the fully extended 
home position at which the piston 32 is seated at the limit position 
within the bore 34 at which it contacts the end of the bore 34. At this 
position, the check ball 36 is unseated by the piston 32 from the seat 38. 
Thereafter, by incrementing and decrementing this count based upon the 
state changes of the sensors 60-64 and the direction of rotation of the 
rotor 48, the counter tracks the position of the piston 32 relative to its 
home position. 
According to the foregoing, the computer 67 provides for initializing the 
counter in the circuit 68 by commanding the commutation and current 
control circuit 66 to cause the motor 40 to rotate in direction to extend 
the piston 32 to the home limit position. This is accomplished by issuing 
the enable EN and direction DIR commands along with a current command Icmd 
to a digital-to-analog converter 70. The current command output ICOM of 
the converter 70 is then provided to the commutation and current control 
circuit 66. When the computer determines the piston 32 has reached its 
home position, the computer 67 presets the position counter in rotor 
position encoder 68 and relative position counter to a calibration home 
position value HPOS. Thereafter, the actual position of the piston 32 is 
tracked by the circuit 68 and provided upon command to the computer 67. 
In accord with the operating program stored in memory, the computer 67 
monitors the condition of the wheel via the wheel speed signal provided by 
the wheel speed sensor 30. When an incipient wheel lock condition is 
detected, the computer thereafter modulates the position of the piston 32 
to regulate braking pressure to prevent a wheel lockup condition by 
determining a current command Icmd based upon the difference between a 
desired position and the actual position as provided by the circuit 68. 
This current command is provided to the digital-to-analog converter 69 
which supplies the analog current command signal ICOM to the commutation 
and current control circuit 66 along with the direction DIR and enable EN 
signals to control the motor 40 to establish the desired position. 
Referring now to FIG. 3, there is illustrated the rotor position encoder 
and relative position counter 68. The count representing the position of 
the armature 32 is contained in an up/down counter 70. This counter is 
preset to a desired value by the computer 67 by applying the desired 
preset count such as the home position count HPOS to the inputs of the 
counter and then setting the chip select CS line and the read/write R/W 
inputs to a NOR gate 71 to the proper logic levels. Thereafter, the 
counter 70 is either incremented or decremented based upon the output of 
the position sensors 60-64 and direction of rotation of the rotor 48 of 
the motor 40. 
A change in the state of the position sensors 60-64 is sensed by a latch 
circuit 72 and a logic circuit comprised of EXCLUSIVE OR gates 74-80 whose 
outputs are coupled to an OR gate 82. In general, each of the EXCLUSIVE OR 
gates 74-80 compares the last latched state of one of the position sensors 
with the present state of the position sensor and if a difference is 
detected, the output of the respective EXCLUSIVE OR gate is a logic 1 
which is coupled to the clock input of the counter 70 via the OR gate 82 
and the OR gate 73. 
Simultaneously with clocking the counter 70, the output of the OR gate 73 
latches the new state of the position sensors 60-64 into the latch circuit 
72 via an AND gate 84 as long as the computer 67 is not in the process of 
loading the counter 70 such that the output of an inverter 86 to the AND 
gate 84 is a logic 1. If the computer is in the process of presetting the 
counter 70, the logic 1 output of the NOR gate 72 applied to the inverter 
86 functions to disable the AND gate 84 to inhibit latching of the output 
of the position sensors 60-64. 
In accord with the foregoing, each time a change in the state of the 
position hall-effect sensors 60-64 changes states, the counter 70 is 
clocked by the output of the OR gate 82 via the OR gate 73. In order that 
the counter is clocked in direction according to rotation of the rotor 48, 
a quadrature detector comprised of AND gates 88-92 and OR gate 94 senses 
the direction of rotation of the rotor 48. Accordingly, when the rotor 48 
is rotated in a direction retracting the armature 32 from the home 
position, the output of the quadrature detector circuit via OR gate 94 
sets the counter 70 in a countdown mode such that each clock pulse from 
the OR gate 82 functions to decrement the count in the counter 70. 
Conversely, when the rotor is rotated in direction extending the piston 32 
toward its home position, the output of the quadrature detector via the OR 
gate 94 sets the counter 70 in a countup mode such that each pulse output 
of the OR gate 82 in response to a change in the state of the position 
sensors 60-64 functions to increment the count in the counter 70. In this 
manner, once preset by the computer 67 as described above, the count in 
the counter 70 provides an indication of the position of the piston 32 
relative to its fully extended position. 
At any time the computer desires to read the actual position APOS of the 
piston 32, the CS and R/W lines are controlled to open a tristate buffer 
96 via an AND gate 98 and inverter 100. 
The computer 67 may take the form of a Motorola single chip microcomputer 
MC68HC11. This computer executes an operating program stored in a read 
only memory that contains the instructions necessary to implement the 
algorithm as set forth in FIGS. 4-6. Referring first to FIG. 4, when power 
is first applied to the system from the vehicle battery 18 such as when a 
conventional vehicle ignition switch is rotated to its on position, the 
computer program is initiated at point 102 and then proceeds to execute an 
initialization routine at step 104 which, in addition to the normal 
initialization steps of clearing registers, initializing various random 
access memory variables to calibrated values and other functions, a rehome 
routine is executed to establish the known relationship between the count 
in the counter 70 of FIG. 3 and the position of the piston 32 so that the 
system may thereafter track the position of the piston relative to its 
fully extended home position. Without this known relationship, it would 
not be possible for the computer 67 to know the position of the piston 32 
in the bore 34. Without this information, intelligent control of the 
pressure for antilock brake pressure regulation based on position would 
not be possible. 
Referring now to FIG. 5, the rehome routine to establish the known 
relationship between the piston 32 position and counter 70 count is 
illustrated. This routine is entered at point 106 and then proceeds to 
sample the state of a rehome flag at step 108. This flag is initially in a 
set condition indicating that the routine has not yet completed the rehome 
routine. Accordingly, assuming that the flag is set, the program proceeds 
to sample the state of a first time flag at 110. This flag indicates that 
the rehome routine is being executed for the first time since the power up 
of the system. If it is the first time, it is desirable to preset both the 
actual position count APOS in the counter 70 and a commanded position 
count CPOS both to a minimum count value. This is accomplished via step 
112 where the counter 70 is preset to the minimum count value in the 
manner previously described in reference to FIG. 3. The commanded position 
CPOS is also set to this minimum value at step 112. The first time flag is 
reset at step 114 causing step 112 to be bypassed during subsequent 
executions of the rehome routine. 
Following step 114, the commanded position count CPOS is incremented at 
step 116. The routine then determines a current command value Icmd based 
upon the difference between the commanded position CPOS of the piston 32 
and the actual position APOS resident in the counter 70. In one 
embodiment, the current command ICOM determined at step 118 is an amount 
having a proportional relationship to the position error. In another 
embodiment, an integral term based upon the error may also be summed with 
this proportional term to establish the final current command Icmd 
provided to the digital-to-analog circuit 70 of FIG. 2. As previously 
described in relation to FIG. 2, the resulting current command ICOM output 
of the digital-to-analog converter 70 is provided to the commutation and 
current control circuit 66 which establishes a current value in the motor 
windings according to the commanded value. 
Following step 118, the program exits the rehome routine at step 120. The 
routine of FIG. 5 is thereafter re-entered on a time interrupt basis and 
repeated until the rehome function has been completed as will be indicated 
by a reset condition of the rehome flag. 
Returning now to step 108, during the second and subsequent executions of 
the rehome routine, the program proceeds from step 108 and step 110 to a 
step 122 where the actual position APOS represented by the count output of 
the counter 70 is sampled and compared to the last actual position LPOS 
determined during the prior execution of the routine. If the piston is 
moving in response to the current command indicating that the piston has 
not yet reached the physical limit position where it engages the end of 
the bore 34, the step 122 will detect a change in position represented by 
the inequality of APOS and LAPOS. Assuming this condition, a rehome timer 
is preset to a predetermined value at step 124. In general, the routine 
requires the piston to be stationary for a predetermined time represented 
by the preset value before it is assumed that the armature has been fully 
moved to its extended position. Thereafter, the commanded position CPOS is 
again incremented and a current command Icmd is determined at step 118 
based upon the resulting error in the actual and commanded position. 
The foregoing steps 108, 110, 122, 124, 116 and 118 are repeatedly executed 
at the time interrupt interval until such time that step 122 indicates 
that the present and last positions of the piston 32 (represented by the 
last and present counts from the counter 70) are equal. When this 
condition is sensed, the rehome timer is sampled at step 126. If not zero, 
the timer is decremented at step 128 after which the commanded position is 
again incremented at step 116 and a new current command value established 
based upon resulting error at step 118. Whenever step 126 determines that 
the rehome timer has been decremented to zero indicating that the piston 
32 has been stationary for the required period of time indicating it has 
been moved to its fully extended position, the routine proceeds to a step 
130 where the count in the counter 70 is preset to the predetermined home 
position count HPOS representing the position of the piston 32 in the 
fully extended limit position. When preset to this value, the count in the 
counter 70 thereafter has a predetermined known relationship to the actual 
position of the piston 32 as it is moved in the bore 34 via operation of 
the motor 40. With this knowledge, intelligent control of the pressure 
applied to the wheel brakes 12 for antilock brake control may be 
established based on a direct correlation between brake pressure and 
piston 32 position. 
When the preset step 130 has been executed, the rehome flag is then reset 
at step 132 so that during subsequent executions of the initialization 
routine, the rehome routine is bypassed via step 108. 
When the initialization routine 104, which may include various other 
routines, is completed, the program 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 a predetermined fixed time 
interval such as 5 milliseconds. Upon the occurrence of a control cycle 
interrupt, the digital computer 67 begins executing the functions embodied 
in the control cycle. First, at step 134, wheel speed sensor information 
is read and wheel speed is computed for each of the vehicle wheels. In 
this respect, it is understood that while the system illustrated in FIG. 1 
shows a single channel for antilock brake control, multiple channels such 
as 2, 3 or 4 channels may be provided as required for the particular 
application. For example, a separate modulator 14 may be provided for each 
front wheel and a single modulator may be provided for the combined rear 
wheels for antilock brake control. The computer 67 will have associated 
with it a rotor position encoder and relative position counter 68, a 
commutation and current control circuit 66, a power switch 58 and a 
converter 69 for each of the wheel brake channels. 
Thereafter, individual wheel accelerations are determined at step 136 and 
individual wheel slip values are determined at step 138. The routine next 
executes once for each braking channel (where each channel includes a 
modulator 14) a step 140 to determine whether the parameters for the 
selected channel indicate an incipient wheel lockup condition requiring 
entry into antilock brake pressure regulation and, if such a need is 
indicated, an antilock brake control function routine 142. For a four 
channel system, this requires the steps 140 and 142 to be executed four 
times, once for each channel with its related wheel parameters as 
determined via steps 134-138. 
Step 140 determines from a lookup table stored in read only memory whether 
or not antilock controlled braking is required based upon a predetermined 
schedule that is a function of wheel acceleration and wheel slip. The 
table establishes a boundary condition such that when the combination of 
acceleration and wheel slip indicate an incipient wheel lockup condition, 
a need for antilock controlled braking is indicated. If step 140 does not 
indicate such an incipient wheel lockup condition, the routine then 
continues for the next channel. However, if step 140 determines via the 
lookup table that an incipient wheel lockup condition exists, the program 
then proceeds to execute an antilock brake control routine 142 as 
illustrated more specifically in FIG. 6. 
Referring now to FIG. 6, the antilock brake control routine is entered at 
step 146 and proceeds to determine the braking mode at step 148. In 
general, the selection is made from (A) a number of release modes, such as 
3, each having associated with it a specific pressure release and pressure 
hold periods, (B) a number of apply modes, such as 3, each having a 
related rate of increase in brake pressure, and (C) a pressure hold mode. 
Step 148 further provides for an initial release mode providing the most 
aggressive release of brake pressure. Accordingly, when a release mode is 
indicated at step 148 in response to an incipient wheel lockup condition, 
the routine proceeds to a step 150 where the commanded position CPOS of 
the piston 32 of the associated wheel brake modulator 14 is set to a full 
release position for a predetermined period that is a function of the 
particular release mode. For the initial release mode, step 150 may 
provide for the most aggressive release by establishing the longest period 
of release represented by a predetermined number of control cycle 
interrupt intervals. Release periods corresponding to the release mode are 
otherwise provided. The step 152 then determines whether or not the period 
that the commanded position is established at the fully retracted position 
of the piston 32 has expired. If not, the program proceeds directly to a 
step 154 where a proportional current control term IP is determined as a 
predetermined function of the position error represented by the difference 
between the commanded position CPOS and the actual position APOS obtained 
from the counter 70 of FIG. 3. Also at step 154 a derivative current 
command term IP is determined as a predetermined function of the rate of 
change in position error. The final current command value Icmd is 
determined at step 156 as the sum of the proportional and derivative 
current control terms. This value is then provided to the corresponding 
digital-to-analog converter 70 for controlling the modulator 14 associated 
with the selected wheel. 
Returning to step 152, when the routine determines that the period of 
release for the selected wheel has expired, the mode is set to a hold mode 
at step 158 with a hold period identified based upon, for example, the 
particular release mode determined at step 148. Thereafter, at step 148 
for selected wheel, a hold mode is executed for a predetermined number of 
interrupt cycles by freezing the commanded position CPOS at the actual 
position of the armature 32 represented by the output of the counter 70. 
When the step 148 determines an apply mode such as when wheel slip and 
acceleration indicate the wheel has recovered from the incipient wheel 
lock condition, a step 162 determines if this is the first cycle of 
antilock brake control since step 140 initiated antilock controlled 
braking. If so, the commanded position CPOS is set equal to a value that 
is a predetermined function of the time of pressure release after which 
the commanded position CPOS is ramped at rate that is a function of the 
particular apply mode determined by step 148. In the preferred embodiment, 
the ramp rate of brake pressure is controlled by varying the period of 
ramp rather than the size of the change in the commanded position CPOS. 
This maximizes the position resolution by allowing the step size of the 
commanded position to always be a small value. 
If step 162 determines that this is not the first antilock brake pressure 
control cycle, a step 166 is executed to establish the commanded position 
for apply. At step 166, the commanded position of the piston 32 is 
initialized to a predetermined substantial fraction of the maximum 
position of the piston during the previous cycle. The maximum position 
represents maximum brake pressure corresponding in time to the step 148 
first indicating a release mode. This piston position (brake pressure) 
represents the pressure substantially corresponding to the maximum braking 
force for the vehicle wheel. By setting the commanded position CPOS at a 
predetermined fraction of the maximum position during the prior cycle, the 
brake pressure is quickly established at a pressure substantially at the 
pressure producing a maximum braking force between the tire and the road 
surface. In another embodiment, the initial commanded position established 
via step 166 may be based upon the release distance and the difference 
between the actual and commanded positions of the piston at the time of 
release. Thereafter, via repeated executions of the step 166 for the 
selected wheel, the commanded position CPOS is incremented in the same 
manner as previously described in relation to step 164 to establish a ramp 
rate that is a function of the apply mode determined at step 148. The 
resulting current command is established as previously described via steps 
154 and 156. Through repeated executions of the routine of FIG. 6, the 
position of the armature 36 is continually modulated to modulate the wheel 
brake pressure for antilock controlled braking to prevent wheel lockup. 
The routine of FIG. 6 is repeated for each braking channel as previously 
described. Following execution of the steps 140 and 142 (if applicable) 
for each braking channel, the routine then proceeds to step 144 where 
background tasks are performed until the receipt of the next control cycle 
interrupt at which time the steps 134 through 142 are repeated as 
described. 
The foregoing antilock brake control system provides for control of brake 
pressure based upon the position of a pressure actuator, such as the 
piston 32, wherein the position is tracked based on the output of position 
sensors used in the commutation of a brushless motor controlling the 
actuator position. Further, the counter utilizing the output of those 
position sensors for tracking the position of the pressure actuator is 
preset to a predetermined count to establish a known relationship between 
the count and the position of the pressure actuator when the actuator is 
driven to a known position. 
The foregoing description of a preferred embodiment of the invention for 
purposes of illustrating the invention is not to be considered as limiting 
the invention since many modifications may be made by the exercise of 
skill in the art without departing from the cope of the invention.