Brake control system

A brake system using a pair of brake apply and release force units, the units having electric motors with non-backdriveable mechanical output members to supply the needed brake apply force to limit of capability of the motors. Piezoelectric elements that generate high forces with low expansion during rapid rates of change of applied voltage are positioned in brake apply force-transmitting series with the motor output members. These elements are alternately energerized with applied voltage and deenergized, in opposite phase relation. The piezoelectric expansion effect of each energized element is mechanically captured in each energization cycle by the motor unit having the deenergized element so that the brake apply forces actually applied to actuate the brake are increased well beyond the maximum output of the motors. This is obtained by the alternating energization of the piezoelectric elements and the alternating follow-up actions of the motors, with the non-backdrivable arrangements acting to store the mechanical force increases so attained. Wheel lock control may be attained. When actuated, this feature sets the output force generated by the motors, deenergizes the piezoelectric elements, and then concurrently energizes and deenergizes them to obtain a brake pumping action while preventing excessive wheel slip.

FIELD OF THE INVENTION 
The invention relates to brakes and more particularly to an arrangement for 
applying and releasing brakes. A brake embodying the invention has at 
least two sets of primary and secondary actuating members. The primary and 
secondary actuating members of each set are arranged in brake apply force 
series relation. 
BACKGROUND OF THE INVENTION 
Brakes are commonly actuated by hydraulic, pneumatic, mechanical or 
electrical actuating members. An electric actuating member is disclosed in 
U.S. Pat. No. 3,420,580, entitled, "Skid Control Device" and issued Jan. 
7, 1969. Another example of an electric brake actuating member is 
disclosed in U.S. Pat. No. 4,653,815, entitled, "Actuating Mechanism in a 
Vehicle Wheel Brake and Anti-lock Brake Control System" and issued Mar. 
31, 1987. 
An electric motor and alternatively hydraulic motor are the primary 
actuating members disclosed in U.S. Pat. No. 4,602,702, entitled, "Brake 
Apparatus" and issued July 29, 1986. In the electric motor embodiment, a 
switch changes operation from a driver for the electric motor, stopping 
the electric motor, to a driver for a piezoceramic element positioned to 
act in parallel to and thereafter take over from the stopped electric 
motor when the load on the electric motor exceeds a predetermined value. 
In the hydraulic motor or piston embodiment, when the load on the piston 
exceeds a predetermined value, the hydraulic actuating pressure is trapped 
behind the piston by closing a valve, and the piston is effectively locked 
in place. The driver for the piezoceramic element is energized at the same 
time. In either instance the piezoceramic element acts in parallel to and 
takes over from the initially actuated motor or piston, and the initially 
actuated motor or piston causes no further brake applying movement of the 
friction braking member. 
A piezoceramic member electrically actuated brake, using the piezoceramic 
member as the only power brake actuator, is disclosed in U.S. Pat. No. 
4,623,044, entitled, "Brake Apparatus" and issued Nov. 18, 1986. 
In U.S. Pat. No. 4,765,140, entitled, "Piezoelectric Servomechanism 
Apparatus" and issued Aug. 23, 1988, a brake has a hydraulic piston 
actuator that has hydraulically trapped pressurized brake fluid in the 
wheel cylinder chamber in an apply position. It also has a floating plate 
in a chamber in the hydraulic piston that, with the head of the piston, 
defines another hydraulic pressure chamber. Within the piston and acting 
axially on the floating plate are two sets of piezoelectric elements. One 
set reacts on another part of the piston, and the other set reacts on 
another piston-like working member reciprocally mounted in the piston with 
one side exposed to the wheel cylinder chamber. When the piezoelectric 
elements are electrically energized at a low voltage, they contract to a 
reference position. The group of piezoelectric elements acting on the 
floating plate and reacting on the hydraulic piston then has a higher 
voltage applied to it, expanding that group. The other group is then 
shorter than the first group, and the working member on which the shorter 
other group reacts moves to decrease the pressure in the wheel cylinder 
chamber. High voltage is then applied to the other group of piezoelectric 
elements and low voltage is applied to the group that previously had the 
high voltage applied to it. The high voltage is then applied to both 
groups of elements concurrently, with the displacement amount of the 
working member being zero, followed by concurrently applying only the low 
voltage to both groups of elements concurrently. This switching of 
voltages to both groups of elements concurrently is then followed, and is 
said to displace the working member by twice the displacement obtainable 
when only one group of elements is activated with high voltage. 
SUMMARY OF THE INVENTION 
The primary actuating members of brake actuating member sets each have a 
non-backdriveable driveline. The primary actuating member of each set is 
capable of being power driven either in the brake apply direction or the 
brake release direction. The primary actuating members are energized for 
power in the brake apply direction when the brake is to be actuated. When 
the brake apply force demand is greater than the maximum brake apply 
forces that the primary actuating members can generate, or greater than 
the actual brake apply forces that the primary actuating members are 
energized to generate for the particular brake application, the secondary 
actuating members of each set are, or may be, alternately energized and 
deenergized. Such alternate energization and deenergization takes place in 
opposite phase relation. 
When there are two sets of actuating members, the energized secondary 
actuating member generates additional brake apply force added to the brake 
apply force generated by its associated primary actuating member, further 
applying the brake. This further application of the brake decreases the 
resistance to brake apply force generated by the primary actuating member 
associated with the deenergized secondary actuating member, and that brake 
apply force moves the output of the set of actuating members with the 
deenergized secondary member in the brake applying direction until the 
resistance to the brake apply force generated by the primary actuating 
member is again equal to that brake apply force. This is effectively a 
follow-up action that catches up the set of actuating members having the 
deenergized secondary actuating member with the other set of actuating 
members. 
The energization and deenergization of the two secondary actuating members 
are cyclicly reversed, meaning that the two secondary actuating members 
alternate in opposite phase relation. When one secondary actuating member 
in one set of actuating members is energized, the other secondary member 
in the other set of actuating members is deenergized. 
In one embodiment of the invention the above-described mechanism is 
modified to add a wheel lock control function. When incipient wheel lock 
occurs during service braking of a vehicle, both primary actuator members 
are set or held so as to prevent further energization of them in the brake 
apply direction while the incipient wheel lock condition continues. This 
is easily accomplished by simply deenergizing both primary actuating 
members and using the non-backdriveable characteristics on the motor units 
to hold this position. Concurrently, the energized secondary actuating 
member is deenergized but the other one is not then energized in the 
alternate cyclic manner. Therefore both secondary actuating members are 
deenergized. This somewhat decreases the total brake apply force, 
resulting in a partial release of the brake to a lesser brake apply force 
value and a decrease in the amount of wheel slip. The secondary actuating 
members are then concurrently energized and deenergized to effectively 
pump the vehicle brake and keep the total brake apply force at or near the 
point where excessive wheel slip denoting incipient wheel lock would 
occur, but not attaining such excessive wheel slip. This keeps the brake 
at or near its maximum braking effort for the road conditions being 
traversed without causing the vehicle wheel being so braked to slip 
excessively on the road surface. It is well known that when the wheel does 
slip excessively it will quickly degenerate into a wheel lock condition 
which results in less effective braking and steering difficulties. When 
needed, the primary actuating members are controlled to increase or 
further decrease the brake apply forces exerted by them so as to maintain 
a total braking effort at which the wheel does not slip excessively 
irrespective of brake apply force demand placed on the brake by the 
vehicle operator. This occurs when different road surface characteristics 
resulting in different road surface coefficients of friction are 
encountered during the braking operation.

DETAILED DESCRIPTION 
The brake assembly 10 of FIG. 1 is schematically illustrated as a floating 
caliper disc brake. It is appreciated, however, that it may be any of 
several other types of brakes, such as a fixed caliper-sliding rotor disc 
brake; an opposed actuator, fixed caliper, disc brake with an axially 
fixed rotor; and various arrangements of drum brakes. 
The assembly 10 includes a rotatable rotor 12 to be braked, a caliper 
housing 14, and an outboard brake shoe assembly 16 mounted on the caliper 
housing outer leg 18 and in friction braking relation with one side of 
rotor 12 when the brake assembly is energized. It also includes an inboard 
brake shoe assembly 20 mounted for friction braking engagement with the 
other side of rotor 12 when the brake assembly is energized. 
Caliper housing 14 has an inboard leg 22 in which the mechanism for 
applying braking forces to the brake shoe assemblies is located. The 
inboard brake shoe assembly is operatively mounted on inboard leg 22. As 
is commonly known, it may deliver its brake torque to the caliper housing 
14 or to a fixed member. Since this forms no part of the invention, such 
details are not illustrated. 
In the typical hydraulic pressure actuated brake, this mechanism is a 
piston in a pressure chamber, with hydraulic brake actuating pressure 
acting on the piston to move the brake shoe assembly 20 into braking 
engagement with the rotor 12 and to react on the caliper housing 14 to 
move the brake shoe assembly 16 into braking engagement with the rotor. In 
this embodiment, however, the brake is actuated by motor units, two such 
units 24 and 26 being schematically illustrated. If a hybrid system is 
preferred, the electric motors in the motor units 24 and 26 may be 
replaced by any of several other noted types of actuators, such as 
hydraulic pressure actuators with the use of a master cylinder. However, 
further detailed description will be directed to the use of electric 
motors as the primary actuating members in those motor units. 
Motor unit 24 includes a reversible motor 28, labeled motor #1, an output 
drive 30 having a non-backdriveable linkage or output 32 formed by the 
rotatable internal drive threads of the output drive 30 and cooperating 
external threads of the motor output shaft 34. Shaft 34 is driven axially, 
or at least urged in an axial direction, when motor 28 is energized and 
output drive 30 is driven, or at least urged, in one rotatable direction 
or the other by the motor. Motor 28 and output drive 30 define a first 
primary actuating unit. The output drive 30, which includes its rotatable 
internal drive threads and the cooperating external threads of the motor 
output shaft 34, is a power screw driveline. A piezoelectric crystal or 
element 36 is located axially intermediate drive 30 and the brake shoe 
assembly 20. Crystal 36 is shown as being in axial force transmitting 
relation with the output shaft 34 and the brake shoe assembly 20. Crystal 
36 defines a first secondary actuating unit and is also a part of motor 
unit 24. 
Motor unit 26 includes a reversible motor 38, labeled motor #2, an output 
drive 40 having a non-backdriveable linkage or output 42 formed by the 
rotatable internal drive threads of the output drive 40 and cooperating 
external threads of the motor output shaft 44. Shaft 44 is driven axially, 
or at least urged in an axial direction, when motor 38 is energized and 
output drive 30 is driven, or at least urged, in one rotatable direction 
or the other by the motor. Motor 38 and output drive 40 define a second 
primary actuating unit. The output drive 40, which includes its rotatable 
internal drive threads or screw leads and the cooperating external drive 
threads or screw leads of the motor output shaft 44, is a power screw 
driveline. A piezoelectric crystal or element 46 is located axially 
intermediate drive 40 and the brake shoe assembly 20. Crystal 46 is shown 
as being in axial force transmitting relation with the output drive 40 and 
the brake shoe assembly 20. Crystal 46 defines a second secondary 
actuating unit and is also a part of motor unit 26. 
Motors 28 and 38 are mounted in and secured to caliper housing leg 22 so 
that they react on the caliper housing 14 when they are exerting brake 
apply forces on brake shoe assembly 20, and the reaction forces are 
transmitted through the caliper housing to the brake shoe assembly 16. 
These reaction forces become the brake apply forces acting on brake shoe 
assembly 16 when the motors are energized in the brake apply direction. 
Motors 28 and 38 are controlled by motor control 48, which is in turn 
controlled by a braking force demand input member schematically shown as a 
brake pedal assembly 50. Motor control 48 has a suitable source of power 
52. 
An alternating electrical power voltage source and controller 54 controls 
and alternately supplies electrical power to the piezoelectric crystals 36 
and 46. When power is being supplied to crystal 36, its voltage is 
identified as V.sub.1. When power is being supplied to crystal 46, its 
voltage is identified as V.sub.2. 
FIG. 2 is a graphic illustration of the operation of the invention during a 
full brake apply from the brake fully released condition, and FIG. 3 is a 
graphic illustration of the operation of the invention during a full brake 
release following the full brake apply of FIG. 2. Power voltages V.sub.1 
and V.sub.2 are applied to and removed from the respective crystals 36 and 
46 in a 180.degree. phase relation. That is, while power voltage V.sub.1 
is on, power voltage V.sub.2 is off, and vice versa. Three such cycles are 
shown in the top and center graphs, with the (+) sign indicating that the 
power voltage is on, and the (o) sign indicating that the power voltage is 
off. 
The bottom graphs in FIGS. 2 and 3 schematically show the movements of the 
brake shoe assembly 20 and the forces and movements of the output drives 
30 and 40 and the crystals 36 and 46 that cause the movements of the brake 
shoe assembly. FIG. 2 shows the brake assembly 10 being energized from a 
full released condition to a full apply condition. FIG. 3 shows the brake 
assembly 10 being released from the full apply condition to the full 
released condition. 
The labels used in FIGS. 2 and 3 and/or in the following discussion have 
the following meanings. Some of the below-listed labels are presented for 
root meanings of the labels so used. 
TF=Total Force of brake apply or release at any moment in time. It is 
referred to as TF.sub.a if apply force and TF.sub.r if brake release 
force. 
M=Motor apply force (Obtained when a motor is energized in the brake apply 
direction.) 
M.sub.x =M that a motor can deliver. 
M.sub.1 =M exerted by motor #1. 
M.sub.2 =M exerted by motor #2. 
R Motor release force. It is negative in relation to M and G. (Obtained 
when the motor is energized in the brake release direction.) 
G=Movement and force attributed to axial crystal Growth caused by voltage 
applied to a crystal. The amount of growth attained may be controlled, 
within limits, by the rate application of voltage increase. 
G.sub.1 =G of crystal 36, connected with motor #1 
G.sub.2 =G of crystal 46, connected with motor #2. 
C=Movement and force attributed to crystal Contraction caused when voltage 
is removed from a crystal. 
C.sub.1 =C of crystal 36, connected with motor #1. 
C.sub.2 =C of crystal 46, connected with motor #2. 
F=the Follow-up action occurring to the contracted (not-energized) crystal 
and output drive, driven by the motor associated with that crystal and 
output drive, when the expanded (energized) crystal moves the brake shoe 
assembly 20 away from the contracted crystal and output drive during brake 
application. During brake release, it occurs when the expanded (energized) 
crystal holds the brake shoe assembly 20 in a position that the contracted 
crystal and output drive is not so loaded, and the reversing action of the 
output drive occurs. 
F.sub.1 =Follow-up action of crystal 36 and output drive 30, driven by 
motor #1. 
F.sub.2 =Follow-up action of crystal 46 and output drive 40, driven by 
motor #2. 
The following relationships apply: 
EQU TF=(M-R)+G (general 
formula) 
During brake apply: 
M=a force value acting toward rotor 12. 
R=0 
Therefore, 
EQU TF=M+G (apply formula) 
When the crystal is deenergized, it contracts by the axial distance C, 
which is equal and opposite to G. Thus, when C occurs, G becomes zero and 
TF.sub.a =M 
During brake release: 
M=0 
R=a force value acting away from rotor 12. It is mathematically negative in 
relation to M. 
Therefore, 
EQU TF.sub.r =G-R (release 
formula) 
When the crystal is deenergized, it contracts by the axial distance 0, 
which is equal and opposite to G. Thus, when C occurs, G becomes zero and 
TF.sub.r =-R 
In FIG. 2, it is assumed that the alternating voltage source and controller 
54 begins to send its alternate voltages to crystals 36 and 46 as soon as 
the pedal 50 is actuated, and that the motor control 48 is also activated 
at that time. It is further assumed that the first alternate voltage cycle 
begins with voltage V.sub.1 energizing crystal 36, and crystal 46 is not 
energized. 
Motors 28 and 38 are energized in the brake apply direction when the motor 
control 48 is activated, rotating both screw output drives 30 and 40 and 
driving shafts 34 and 44, crystals 36 and 46 and shoe assembly 20 toward 
rotor 12. At the same time, crystal 36 is energized, and has an axial 
growth G.sub.1 that has the effect of axially lengthening shaft 34 
relative to the length of shaft 44. Shoe assembly 20 frictionally engages 
rotor 12 and the brake apply force is increased until motors 28 and 38 
either reach their maximum force outputs or their controlled maximum force 
outputs M.sub.1 and M.sub.2. However, because of the crystal growth 
G.sub.1 of energized crystal 36, but no crystal growth of deenergized 
crystal 38, the motor shaft 34 does not move axially quite as far (by the 
amount G.sub.1) as does the motor shaft 44. Yet the total movement of the 
outer faces of the crystals 36 and 46 where they engage the brake shoe 
assembly 20 is the same for each motor, being M.sub.1 +G.sub.1 for unit 24 
and M.sub.2 for unit 26. At this point, each motor is exerting the same 
force on the brake shoe assembly, as graphically shown by line 60 in FIG. 
2 in the schematic first half of the schematic first cycle. Depending upon 
the length of time required for each motor to reach its maximum force 
output, and the frequency of the alternating voltage cycles, this may 
actually take several such cycles to attain this condition, but it is 
easier to graph and understand when presented as if it occurs in the first 
half of the first cycle. This simplified graphing and explanation will be 
used throughout, but it is to be appreciated that it is only for graphing 
and simpler descriptive purposes. 
The second half of the cycle then occurs, and the voltage V.sub.1 to 
crystal 36 becomes zero while the voltage V.sub.2 is positive. When 
crystal 36 is deenergized, it contracts axially by the distance C.sub.1. 
This lessens the shoe assembly's resistance to force M.sub.1, so the motor 
28 immediately moves shaft 34 and crystal 36 axially until the maximum 
motor force M.sub.1 is again applied to the shoe assembly 20. 
Concurrently, energization of crystal 46 causes that crystal to expand 
axially by the value G.sub.2. The force value of G.sub.2 is added to the 
force value M.sub.2. This occurs because the motor 38 is non-backdriveable 
and the attained distance M.sub.2 is not decreased. The total force 
TF.sub.a applied to the shoe assembly by unit 26 is therefore greater than 
the maximum apply force generated by motor 38 alone, and this moves shoe 
assembly 20 more tightly into engagement with rotor 12. This is 
schematically indicated by line 62 in FIG. 2. At the same time, the 
crystal 36 and shaft 34 are moved in a follow-up manner by the amount 
F.sub.1 so that the maximum brake apply force M.sub.1 continues to be 
exerted on the brake shoe assembly 20 by motor 28. 
As graphically shown in FIG. 2, this continues through the second and third 
cycles to force lines 64 and 66, and force lines 68 and 70, when, for 
graphical reasons, it is presumed that the demanded brake force has been 
reached when force line 70 is attained. The alternating voltages are then 
discontinued, and with that occurrence, G.sub.2 in the last half of the 
third cycle becomes zero, and motor 38 moves M.sub.2 up to line 70 as 
well. The non-backdriveable features of drive outputs 30 and 40 prevent 
any backing up of the shafts 34 and 44 or motors 28 and 38, so this brake 
apply force is maintained until either a greater or lesser apply force is 
demanded. Both motors 28 and 38 may be deenergized if this is to be held 
for some period of time. The brake parking brake mode may be invoked in 
this manner if desired. When operating in the service braking mode, the 
brake apply force demand is usually changing so that it is not necessary 
to so deenergize the motors. 
A slightly modified method of control and operation of the brake assembly 
1? during service brake apply may be that of delaying energization of 
either of the piezoelectric crystals 36 and 46 until the maximum capacity 
of the motors 24 and 26 to apply braking force is exceeded by the demand 
for more braking force. Thus moderate braking demands that can be met by 
the motors without any energization of the crystals are in fact met by 
energization of the motors only. This can be accomplished by sensing the 
fact that the motor output torques are at a maximum and that concurrently 
there is greater braking force demand than that attained by the motors 
alone. 
Such sensing may use the values of the electric current delivered to the 
motors, which will be at a maximum when the motors are at maximum output 
torque. The crystals are then alternately energized as above described and 
the cycle graphically illustrated in FIG. 2 is followed once a selected 
motor current value is attained. 
Another manner of sensing the attainment of maximum motor output torque is 
based on the fact that the motor speed becomes zero when its torque is at 
its maximum. Motor brush electrical noise, generated each time the motor 
rotor passes a brush, may be detected as an indicator of motor speed. This 
noise will cease when the motor speed is zero, indicating that the motor 
is at its maximum output torque value. Thus, when the electrical brush 
noise ceases while a greater brake force demand still exists, the crystals 
are then alternately energized as above described, and the cycle 
graphically illustrated in FIG. 2 is followed. 
Usually, in the service braking mode, within a short time after the brake 
is so applied, the brake apply force demand is decreased, or becomes zero 
as the brake is to be fully released. When that occurs, the mechanism is 
again operated, this time as graphically illustrated in FIG. 3. The motors 
28 and 38 are energized to turn their output drives 30 and 40 in their 
opposite directions from the apply mode, generating release forces R.sub.1 
and R.sub.2. The crystals 36 and 46 are again alternately energized and 
deenergized, with follow-ups in the release direction occurring as first 
one, then the other, of the crystals grows and takes the load from the 
brake shoe assembly 20. The total force TF is therefore decreased 
incrementally from line 70 as shown incrementally by lines 72, 74, 76, 78, 
80, 82 and 84. Line 84 represents zero brake apply or release force. When 
it is reached, the zero brake force demand has been attained, and motor 
control 48 and alternating power source and control 54 deenergize both 
motors 28 and 38 and crystals 36 and 46. 
In some applications, the crystals 36 and 46 need not be energized during 
part or even all of the brake release operation. They may be energized as 
above described for a few (e.g., one or two) cycles to obtain breakaway 
torque, and then no longer be energized, letting the motors alone complete 
the brake release operation. If initial releasing energization of the 
motors will obtain the necessary breakaway torque, the crystals will not 
have to be energized at all during brake release. In some such 
applications, the crystals need not be energized during brake release once 
the instantaneous actual, but decreasing, brake apply force becomes less 
than the maximum capacity of the motors, because the motors alone will act 
to continue to back off irrespective of the expansion and contraction of 
the crystals. 
Since piezoelectric crystals also generate electricity in response to 
compression loading, and during any brake apply force conditions the 
crystals that transmit the brake apply forces from the motors 26 and 28 to 
the brake shoe assembly 20 are under compression, the crystals 36 and 46 
may be used as actual brake apply force sensors while they are not 
energized. Where alternate energization of those crystals does not permit 
the employment of them as actual brake apply force sensors, another set of 
piezoelectric crystals, positioned in physical force-transmitting series 
with the main set of crystals 36 and 46, may be used for such sensing. In 
either event the electricity, generated when such crystals are under 
compression and the brake apply force sensed, causes a signal indicating 
that the actual brake apply forces have increased until the fully 
energized motor maximum apply force capacities have been attained. This 
signal will then activate the crystal controller 54, and crystals 36 and 
46 will begin their alternating expansion-contraction cycles. 
During brake release, this signal ceases and the crystal controller 54 will 
be deactivated. The action of the motors 28 and 38 in brake release will 
then continue without further expansion and contraction of the crystals. 
The modified brake assembly 110 schematically illustrated in FIG. 4 has 
many elements common with brake assembly 10 of FIG. 1, and such common 
elements have the same reference characters. Modified elements and 
subassemblies in FIG. 4 that are similar but not identical with elements 
and subassemblies in FIG. 1 are assigned related reference numerals in the 
100 series, and added elements and subassemblies are assigned reference 
numerals in the 200 series. 
Modified elements include assembly 110, motor units 124 and 126, 
piezoelectric crystals 136 and 146, motor control 148, and alternating 
voltage source and control 154. Added subassemblies or elements are the 
wheel slip sensor and signal generator 202, the control 204 for wheel 
anti-lock, and the concurrent voltage control 206 for wheel lock control 
only. 
The brake assembly 110 includes the features of brake assembly 10, with the 
addition of a wheel lock control arrangement that will control the brake 
assembly to prevent or minimize wheel skidding during braking due to 
lock-up of the braked wheel or wheels of the vehicle actuated and released 
by action of motor units 124 and 126. The piezoelectric crystals 136 and 
146, which are normally used in service braking in the same manner as are 
crystals 36 and 46, are also used in cycling a wheel lock control system 
when needed. The service brake mode of brake operation is attained through 
operation of the brake pedal assembly 50 in the manner described with 
regard to FIG. 1. When the sensor and signal generator 202 senses a 
percentage wheel slip that indicates incipient wheel lock due to the 
amount of service braking force being applied under the particular 
road-to-tire adhesion conditions being encountered (such percentage wheel 
slip being in the range of about 15% to 25%, and commonly about 20%), the 
actually-attained brake apply forces generated by the primary actuating 
motors 128 and 138 at the time the wheel slip signal is generated is set 
as the desired maximum attainable braking force during wheel lock control 
braking. This is done by deenergizing both of those motors. Crystals 136 
and 146 are both deenergized, and then concurrently cyclicly energized and 
deenergized to prevent the percentage wheel slip from increasing to and 
beyond the incipient wheel lock stage and to obtain the desired pumping 
action of the brake. 
Since the values of the output forces G.sub.1 and G.sub.2 generated by the 
expansion of the crystals 136 and 146 (and crystals 36 and 46 as well) are 
proportional to the rate of change in the input voltages to those 
crystals, the range of brake actuating force changes occurring during the 
pumping action of the brake to prevent excessive wheel slip can be 
controlled by varying the rate of change in the input voltages to the 
crystals 136 and 146. A larger range of brake actuating forces is obtained 
by a higher rate of input voltage application, and a smaller range of 
brake actuating forces is obtained by a lower rate of input voltage 
application. 
If excessive wheel slip incipiently occurs even at the motor settings for 
that actually-attained braking force, the wheel slip signal to the motor 
controller causes the controller to reverse the motors until a lesser 
motor setting is attained that does not cause the wheel slip to be 
incipiently excessive, and a new, lower motor setting is made. 
The modifications of the brake assembly 10 to that of the brake assembly 
110 shown in FIG. 4 provide for these procedures. With the rotor 12 being 
braked by normal service braking actuation of brake assembly 110, and with 
incipient excessive wheel slip occurring, the wheel slip sensor and signal 
generator 202 (which has been sensing wheel slip whenever the brakes are 
applied so as to cause any wheel slip) generates a signal that such 
incipient excessive wheel slip has been reached, and the signal is sent to 
the control 204 for wheel anti-lock or wheel lock control. Control 204 
controls the alternating voltage and source control 154 so that control 
154 sends a signal to the motor control 148. Control 148 then sets the 
braking force attained by motor actuation at the time of the occurrence of 
incipient excessive wheel slip as the maximum braking force now attainable 
by motor actuation. The motor control also generates a signal sent to the 
concurrent voltage control 206. That control in turn causes control 154 to 
concurrently deenergize and then energize the piezoelectric crystals 136 
and 146. The rate of concurrent voltage removal from and application to 
the crystals 136 and 146 is preferably varied as the wheel lock sequence 
continues, such variation being controlled by motor control 148 and 
concurrent voltage control 206, acting through control 154 and using 
signal information being provided to the motor control and hence to the 
concurrent voltage control based on the signals from the sensor and 
generator 202 and the control 204. The initial rate of voltage changes is 
preferably at a relatively high frequency so that a greater initial 
crystal contraction occurs to quickly decrease the actual brake 
application force and prevent or minimize the opportunity for the wheel 
slip to increase beyond the incipiently excessive wheel slip value first 
sensed. As the wheel lock control mode of operation continues, the rate of 
voltage changes may be decreased, resulting in less growth and contraction 
of the crystals in each cycle, and reducing the difference between the 
maximum braking force being applied and the lesser braking force being 
applied during each pumping cycle while keeping the sensed wheel slip 
below the incipiently excessive amount of wheel slip. 
If, instead of a constant value of applied braking force resulting in a 
constant amount of wheel slip, due to changes in the friction 
characteristics of the road surface and therefore changes in the 
tire-to-road surface adhesion, a higher or lower value of total applied 
braking force is desirable, the signals generated by sensor and signal 
generator 202 will cause the control 204 to call for changes in the 
applied braking force through the voltage source and control 154 to the 
motor control 148 as well as to the crystals 136 and 146. This will 
typically cause the motors 36 and 38 to be actuated to either increase or 
decrease the motor-caused brake apply forces as needed. It may also modify 
the voltage change rates to the crystals as well, via controls 206 and 
154. This will set a new, lower or higher (as appropriate) motor setting 
of maximum brake apply force attained due to the motors.