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Description  |
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This invention relates to the art of skid control systems for controlling
braking forces applied to the wheels on a vehicle having a braking system,
and, more particularly, to a skid control system for use with a vehicle
having a pair of spaced apart wheels.
When a vehicle operator actuates the vehicle's braking system, braking
forces are applied to the brake controlled wheels to slow the vehicle. The
vehicle is preferably decelerated to a desired lower speed or to a stop
condition, without a skid condition taking place. To prevent a skid
condition, the vehicle should be decelerated with decreasing wheel speed
which is slightly less than that of the vehicle. If, however, the wheel
speed decreases at a rate substantially faster than that of the vehicle
speed, then this is indicative of an impending wheel lock or skid
condition. To prevent the wheel lock, skid control systems serve to sense
this impending condition and release the brake forces. When the brake
forces are released, the wheels are permitted to spinup toward the vehicle
velocity. The braking forces are then restored to continue slowing the
vehicle.
As a vehicle is being decelerated, a condition may arise where the wheel
speed of one wheel being monitored by the skid control system is
substantially different than that of the other wheel. This is normally
indicative of a split coefficient of friction between the road surface and
each of the two wheels. The slower wheel may be approaching a skid
condition. Consequently, when the wheel speed difference becomes too great
it is desirable to release the brake forces to prevent wheel lock up of
the low speed wheel. However, the faster of the two wheels may still
provide effective braking of the vehicle and it is frequently desirable,
paticularly in a split coefficient of friction situation, that the braking
forces applied to the faster wheel be continued for an added time duration
even though the differential wheel speed exceeds the desired limits for
differential wheel speed.
If both wheels being monitored by the skid control system are decelerated
too fast, then this is indicative of an incipient wheel lock condition.
Since the faster wheel produces less braking than the slower wheel, it is
desirable to compare the deceleration of the faster wheel with a
deceleration reference. If the deceleration of the faster wheel is greater
than the reference the brake forces may be released.
Once an incipient skid condition has been sensed and the vehicle brake
forces have been released, the vehicle wheels will be permitted to
spin-up. It is known that the ideal wheel velocity for slowing a vehicle
is below the vehicle's speed. During an incipient skid condition, the
wheel velocity decreases substantially below this ideal velocity. Once the
brake forces are released by an anti-skid system, the wheels are permitted
to spin-up toward the ideal velocity and then the brake forces are
permitted to be reapplied. On a dry surface, the coefficient of friction
between the vehicle wheels and the surface is relatively high, and it has
been determined that a spin-up rate on the order of 5g will quickly bring
the wheels up to the ideal velocity and there is no need to keep the
brakes off, since the wheels will probably reach the ideal velocity even
if the brakes are applied. It is important that the wheel velocity does
not overshoot the ideal velocity since this would tend to increase the
vehicle stopping distance. Consequently, it is desirable to sense a high
spinup rate, such as a 5g rate, and then permit brake reapplication so
that the braking forces may take effect as the wheel speed approaches
ideal wheel speed.
On low coefficient of friction surfaces, such as ice, the wheel spin-up
rate will be between 0.5g and 5.0g which may allow a fixed bleed reference
to fall too low to prevent wheel lock up. To correct this condition, it is
desirable that the brakes be held off by the acceleration of the wheel,
with the limits being 0.5g to 5.0 g. On any surface the wheel acceleration
will drop to zero as the wheels reach vehicle speed and with an
acceleration logic, the brakes may be applied when the acceleration falls
below 0.5g. In addition to the foregoing, it is further desirable that a
skid control system be provided with monitoring circuits for monitoring
various operating conditions of the skid control system to determine
whether the operation is within prescribed limits. Thus, if the skid
control system employs a voltage regulating circuit to provide regulated
DC voltage to operate the skid detector circuits, erroneous indication of
either an incipient skid condition or no skid incipient condition may
result if the regulated voltage decreases in magnitude below a limit
level. Also, the typical skid control system employs a valve solenoid
which, when actuated, serves to act upon the vehicle's brake control
system to release braking forces. This valve solenoid should not be
energized when no incipient skid condition has been detected and it should
be energized when the incipient skid condition has been detected. In
addition, if a skid detector circuit provides a skid signal representative
of an incipient skid condition for an unduly long time period, this may be
indicative of a malfunction in the control system. If any such faulty
operating condition takes place, it is desirable that the skid control
system be, at least temporarily, deactivated.
It is therefore a primary object of the present invention to provide a skid
control system which satisfies the above enumerated needs.
It is a specific object of the present invention to provide a skid control
system which, upon noting that the wheel speeds of the two wheels being
monitored by the skid control system exceeds a predetermined magnitude,
provides a time delayed skid signal so that a slight delay is provided in
releasing the brake forces, permitting additional braking forces to be
applied to the faster wheel.
It is an additional object of the present invention to provide a skid
control system to provide brake release when the average wheel speed of
the two wheels being monitored by the skid control system decreases below
a declining reference signal representative of a desired rate of decline
in the speed of the faster wheel.
It is still further object of the present invention to provide a skid
control system which serves to release braking forces applied to a pair of
wheels being monitored by the skid control system are decelerating and,
more particularly, when the faster of the two wheels decelerates at a rate
greater than a reference deceleration.
A still further object of the present invention is to provide monitoring
circuits for a skid control system to monitor the operation thereof and
deactivate the anti-skid system when the operating characteristics are not
within prescribed limits.
The present invention contemplates that the skid control system be used
with a vehicle having at least a pair of spaced apart independently
rotatable wheels and in which a braking system is provided for applying
braking forces to the wheels. It is also contemplated that a brake control
means be provided which responds to an applied skid signal for controlling
the braking system to release the braking forces on the wheels. Still
further, it is contemplated that sensor means provide first and second
wheel speed signals having magnitudes respectively representative of the
wheel speeds of the first and second spaced apart wheels.
In accordance with one aspect of the present invention, circuitry is
provided for generating from the first and second wheel speed signals an
average wheel speed signal having a magnitude respectively representative
of the average speed of the first and second wheels. Also, circuitry is
provided for comparing the average wheel speed signal with a reference
signal to provide a skid signal whenever the magnitude of the average
signal is less than that of the reference signal. The reference signal is
obtained from a reference signal generating means which serves to provide
a reference signal which decreases in magnitude when speed of the faster
wheel decreases. The reference signal decreases from a magnitude having an
initial value representative of the speed of the faster wheel and at a
decay rate initially representative of a first deceleration rate. The
reference signal generating means also includes circuitry for responding
to a skid signal to vary the decay rate of the reference signal from the
first rate to a second rate, representative of a slower rate of
deceleration so that when the braking forces are released the wheel speed
must increase to a higher velocity than that when the first decay rate is
effective before the brakes may be reapplied.
In accordance with a further aspect of the present invention, logic
circuitry serves to provide a skid signal when the wheel speed of one of
the wheels differs from that of the other by a predetermined amount. The
application of the skid signal to the brake control circuitry is delayed
so that additional braking forces may be applied for a limited time to the
faster rotating wheel.
In accordance with a still further aspect of the present invention,
circuitry is provided for releasing the brake forces so that the wheels
may spin-up to the vehicle velocity.
Still further in accordance with the present invention, monitoring
circuitry is provided for monitoring at least one operational
characteristic of the skid control system and inhibiting operation of the
brake control circuitry so that the brakes will not be released if the
monitored operating characteristics is not within desired operating limits
.
BRIEF DESCRIPTION OF THE INVENTION
The foregoing objects and advantages of the invention will become more
readily understood from the following description of the preferred
embodiment of the invention taken in conjunction with the accompanying
drawings which are a part hereof and wherein:
FIG. 1 is a block diagram illustration of the skid control system
constructed in accordance with the present invention:
FIG. 2 is a schematic circuit diagram illustrating a portion of the
circuitry of FIG. 1:
FIG. 3 is a schematic circuit diagram illustrating other portions of the
circuitry of FIG. 1; and,
FIG. 4 is a schematic illustration showing still further portions of the
circuitry of FIG. 1.
Referring now to the drawings wherein the showings are for purposes of
illustrating a preferred embodiment of the invention only and not for
purposes of limiting same, FIG. 1 is a block diagram illustration of the
skid control system constructed in accordance with the present invention.
It is contemplated that the skid control system as disclosed herein be
used for controlling the brake forces applied to a pair of spaced apart
wheels. Any number of axles may be controlled by applying a like plurality
of the control systems, one for each axle.
Generally, the skid control system serves to monitor the wheel speed of the
two wheels on the axle being controlled and develop a control signal,
referred to hereinafter as a skid signal, if one or more conditions
prevail indicative of an incipient or actual skid condition. The skid
signal is used to actuate a valve driver circuit which, in turn, energizes
a solenoid which acts on the braking system to relieve brake forces. The
vehicle's brakes may be air pressure operated or hydraulic operated, and
in either case it is contemplated that upon sensing an incipient skid
condition, the brake forces on the wheels be relieved to prevent wheel
lock-up. If, for example, differences in wheel speeds of the two wheels on
the axle being controlled exceeds a reference level a skid signal is
developed. A skid signal is also developed if the acceleration of the
faster wheel on the monitored axle exceeds a reference level; however, if
the acceleration continues and becomes greater than a second reference
level the skid signal is removed. Also, if the deceleration rate of the
faster wheel is greater than a reference level a skid signal is developed.
The reasons for and conditions causing a skid signal will be explained in
greater detail hereinafter, it being the purpose at this point to indicate
the general purpose of the skid control system.
Referring now to FIG. 1, the skid control system employs wheel sensors WS-1
and WS-2 for respectively sensing the wheel speeds of the two wheels on
the axle being controlled. Any suitable mechanism may be employed for
sensing wheel speed. Preferably, however, each wheel speed sensor includes
a tachometer generator for developing an alternating signal having a
frequency proportional to wheel speed. The frequency signals developed by
sensors WS-1 and WS-2 are respectively applied to signal conditioner
circuits SC-1 and SC-2. Each signal conditioner circuit includes a
frequency to voltage converter for developing a DC signal having a
magnitude proportional to the applied frequency signal and, hence, to the
wheel velocity. Preferably, although not necessarily, the DC signal is of
positive polarity. The wheel velocity signals V.sub.1 and V.sub.2 obtained
from the signal conditioner circuits SC-1 and SC-2, respectively, are
applied to both a summing amplifier SA as well as to a high wheel speed
selector HS. The summing amplifier SA serves to provide an output signal
which has a magnitude representative of the average wheel speed of the two
wheels being monitored, whereas the high wheel speed selector HS
determines which wheel exhibits the greater velocity and to provide an
output signal representative of the magnitude of the speed of the faster
wheel.
The high wheel speed selector HS is shown in greater detail in FIG. 2 and
includes common emitter connected NPN transistors 10 and 12 each having
their collectors connected to a B+ voltage supply source and their
emitters connected in common through a resistor 14 to ground. The DC
velocity signal V.sub.1 is applied to the base of transistor 10 whereas
the DC velocity signal V.sub.2 is applied to the base of transistor 12.
Consequently, the output signal V.sub.H is proportional to the speed of
the higher velocity wheel less a small drop.
The summing amplifier SA is also illustrated in greater detail in FIG. 2
and includes an operational amplifier 16 having a feedback network
including resistors 18 and 20. The wheel velocity signals V.sub.1 and
V.sub.2 are respectively applied through summing resistors 22 and 24 to
the noninverting input of amplifier 16. Consequently, the amplifier serves
to provide a positive DC output signal having a magnitude equal to the
average of the input signals V.sub.1 and V.sub.2 times the gain (1.5) of
the amplifier, as dictated by feed back resistors 18 and 20.
ACCELERATION -- DECELERATION CIRCUITRY
As shown in FIG. 1, the higher wheel speed signal V.sub.H obtained from the
high wheel speed selector H.sub.S is applied to an
acceleration-deceleration logic circuit AD as well as to a variable
deceleration reference circuit DR. Briefly, circuit AD serves to
differentiate the high speed signal V.sub.H to provide an output signal
representative of rate of velocity change. As will be brought out in
greater detail hereinafter, the rate of change output signal is in a
positive direction as vehicle wheel speed decreases, and is in a negative
direction as the wheel speed increases. The positive, or deceleration,
signal is applied to a deceleration comparator circuit DC which serves to
compare the deceleration signal with a variable deceleration reference
signal obtained from circuit DR. If the deceleration signal exceeds the
reference signal then the deceleration comparator circuit DC provides a
high or binary "1" output signal which is applied through OR gate OG to
actuate the valve driver circuit VD. The valve driver circuit, in turn,
actuates a valve solenoid VS to relieve the brake forces.
Similarly, the negative or acceleration signal obtained from circuit AD is
applied to an acceleration comparator AC where the acceleration signal is
compared with a reference signal. If the acceleration is greater than the
reference signal comparator AC provides a skid signal in the form of a
binary 1 signal which is applied through OR gate OG to actuate valve
driver circuit VD. As will be brought out in greater detail hereinafter,
the acceleration comparison is a two-stage comparison in that a skid
signal is developed when the acceleration signal obtained from circuit AD
exceeds a level representative of 0.5g. If, however, the acceleration
increases sufficiently to attain a level representative of 5.0g then an
acceleration inhibitor circuit AI responds to this condition to inhibit
comparator AC from providing a skid signal. The circuitry to accomplish
the foregoing acceleration and deceleration comparison functions is
illustrated in greater detail in FIG. 3, to which reference is now made.
The high speed signal VH obtained from the high wheel speed selector HS is
applied to the base of an NPN transistor 30 in a low speed cutoff circuit
LSC-1. The emitter of transistor 30 is connected to ground through a
resistor 32. Transistor 30 serves as an emitter-follower with unity gain.
Since it does not conduct below the base-emitter threshold voltage, the
derivative circuits are inactive for speeds below this threshold voltage,
i.e., under 5 miles per hour. Above this speed, the output voltage from
the low speed cutoff circuit is applied to the inverting input of an
operational amplifier 36 through resistor 34 and a capacitor 38, in the
acceleration-deceleration logic circuit AD. A feedback resistor 40 is
connected between the output of the amplifier and its inverting input.
Capacitor 38 and resistor 40 provides a differentiation path and a
capacitor 42, connected in parallel with resistor 40, serves in
conjunction with resistor 34 to provide immunity to high frequency noise.
The output signal from amplifier 36 varies in proportion to the rate of
change of the velocity of the faster wheel. The signal varies in a
positive sense as wheel speed decreases and varies in a negative sense as
wheel speed increases.
The output signal from circuit AD is applied to the noninverting input of
an operational amplifier 50 in the deceleration comparator circuit DC. The
reference signal applied to the inverting input of operational amplifier
50 is comprised of a fixed level and a velocity variable level. The fixed
level is obtained from a voltage divider network made up of resistors 52,
54 and 56 connected together in series between a B+ voltage supply source
and ground. The velocity variable level is obtained from a second low
speed cutoff circuit LSC-2 which includes an NPN transistor 60 having its
base connected to receive the high speed signal V.sub.H and its collector
connected to a B+ voltage supply source. The emitter of transistor 60 is
connected through series connected resistors 62 and 64 to a B- voltage
supply source. Consequently, above a low speed level, such as 5 miles per
hour, the output signal obtained from the low speed cutout circuit LSC-2,
through resistor 66, is proportional to the speed of the faster wheel.
Resistors 66 and 56 serve as a voltage divider for this signal which is
then applied through resistor 54 to the inverting input of the operational
amplifier. When the deceleration signal applied to the noninverting input
of amplifier 50 becomes greater, in a positive sense, than the variable
reference signal applied to the inverting input, amplifier 50 will provide
an output skid signal in the form of a positive or binary 1 signal. This
positive skid signal is applied through a diode 70, poled as shown, to
actuate the valve driver VD. This will energize the solenoid valve VS to
relieve the brake forces on the wheels, permitting them to spin-up toward
the vehicle velocity.
As the wheels spin-up, circuit AD provides a negative going acceleration
signal representative of the spin-up rate. This spin-up rate is monitored
by acceleration comparator AC to provide a skid signal for brake release
for acceleration rates between 0.5g and 5.0g. The acceleration produced
signal is therefore ignored when the wheels spin-up at a rate greater than
5.0g. This is desirable to offset electrical and pneumatic system delays
so that the wheel does not overshoot the ideal velocity for braking. But,
the spin-up should not cause the wheel velocity to overshoot that ideal
level. It has been determined that a spin-up rate of over 5.0g is
indicative of a fast wheel speed recovery and, unless the acceleration
produced skid signal is removed, the wheel speed may overshoot the desired
level. For this reason, the acceleration comparator is programmed to
remove its generated skid signal at the 5g spin-up rate. But, on a low
coefficient of friction surface, such as ice, the wheel spin-up rate may
never exceed 5.0g and as the wheels approach vehicle speed the spin-up
rate will fall off to zero, the brakes being applied when the rate is
0.5g. Therefore, below this acceleration rate the acceleration comparator
does not provide a skid signal.
The output signal from circuit AD is applied to the inverting input of an
operational amplifier 80 in the acceleration comparator circuit AC. Here
the negative going acceleration signal is compared with a fixed reference
taken from a voltage divider consisting of resistors 82 and 84 connected
between ground and a B-voltage supply source. The junction of resistors 82
and 84 is connected to the non-inverting input of amplifier 80.
Consequently, a fixed reference is defined and the values of the resistors
are chosen such that the fixed reference is representative of an
acceleration level of 1.5g. When the acceleration signal attains a level
such that it is more negative than the reference signal, then amplifier 80
will provide a skid signal in the form of a positive or binary 1 signal
and this is applied through a diode 86, poled as shown, to actuate the
valve driver VD to cause the brake forces to be released.
The positive skid signal provided by acceleration comparator AC will be
inhibited if the acceleration signal increases beyond a higher reference
level, preferably on the order of 5.0g. The acceleration inhibit circuit
AI serves, when this condition is sensed, to remove the positive skid
signal to permit brake reapplication. The acceleration inhibit circuitry
is illustrated in FIG. 3, to which reference is now made.
The acceleration signal obtained from circuit AD is applied to the
noninverting input of an operational amplifier 90 in the acceleration
inhibit AI. The acceleration signal is compared against a reference signal
applied to the inverting input of operational amplifier 90. The reference
signal is obtained from the voltage divider comprised of resistors 62 and
64 and is made speed dependent by its inter-connection with the low speed
cutoff circuit LSC-2. When the acceleration signal exceeds the reference
signal, i.e., it becomes more negative than the reference signal, the
output of amplifier 90 will change from a high level to a low level so as
to forward bias a diode 92, poled as shown, connected in its output
circuit. This diode forces the input to the noninverting input of
amplifier 80 in the acceleration comparator circuit AC to become more
negative and thereby prevent the output circuit of amplifier 80 from
carrying a positive skid signal. Capacitor 94 connected between ground and
the junction of diode 92 and the noninverting input of amplifier 80
serves, in conjunction with resistor 84, to provide an RC time delay to
return the acceleration reference signal to its normal level once the
output of amplifier 90 has returned to its normal high level.
Consequently, so long as the acceleration signal is greater than the fixed
reference for the acceleration comparator circuit AC, but less than the
reference signal for the acceleration inhibitor circuit AI, a positive
skid signal is provided and this signal is applied through diode 86 in OR
gate OG to actuate the valve driver VD.
DIFFERENTIAL WHEEL SPEED-FIXED BLEED CIRCUITRY
The average wheel speed signal VA obtained from the summing amplifier SA is
applied to a differential wheel speed fixed bleed comparator DFC. This
comparator has two different modes of operation. In one mode it compares
the two wheel speeds on the axle being controlled, and if the speed of one
wheel exceeds that of the other by a fixed amount a positive skid signal
is transmitted to the valve driver VD through a time delay network DFD and
OR gate OG. In the second mode of operation, comparator DFC serves to
compare the average wheel speed (times a gain of 1.5 as dictated by the
gain of amplifier SA) with a reference signal having a two stage decay
rate. As will be developed in greater detail hereinafter, the decay rate
is dependent on the state of the logic circuitry and initially presents a
high decay rate followed by a slow decay rate, respectively representative
of high and low decelerations.
The first mode of operation takes place whenever the high speed signal VH,
representative of the speed of the faster wheel on the axle being
controlled, is constant, or is decreasing at a rate less than the first
stage delay rate referred to above. With reference to the circuitry
illustrated in FIG. 2, the comparator DFC includes an operational
amplifier 100 having its inverting input connected to summing amplifier SA
to receive the average wheel speed signal VA. The noninverting input of
operational amplifier 100 is connected to the output side of a computed
speed reference circuit CR which has its input circuit connected to
receive the high velocity signal VH from the high wheel speed selector HS.
The computed speed reference circuit CR serves to provide a reference
signal essentially equal to high velocity signal VH less a diode drop.
This is accomplished by applying the high speed signal VH through a diode
102 and a resistor 104 to charge capacitor 106. The capacitor 106 is
thereby charged to a level representative of the speed of the faster
wheel, less the voltage drop through the charging circuit. Reference
circuit CR also includes a diode 110 in the discharge circuit of capacitor
106 together with a diode 112 connected across the series connected
circuitry comprised of diode 102, resistor 104 and diode 110.
Consequently, at constant speed or during acceleration the voltage at the
junction of diode 110 and diode 112, serving as the output of reference
circuit CR, is equal to or greater than the voltage stored by capacitor
106. The reference signal applied to the noninverting input of operational
amplifier 100 is essentially equal to that of the faster wheel speed VH
less the voltage drop across diode 112.
If one of the wheels is rotating substantially faster than the other wheel
by a sufficient amount, then the average speed signal VA will be less than
the reference signal and, hence, amplifier 100 will serve to provide a
positive skid signal. This skid signal is delayed in time by delay circuit
DFD before being applied through OR gate OG to actuate the valve driver VD
to release the brake forces. This mode of operation ensues so long as the
faster wheel speed VH is constant or increasing or is decreasing at a rate
less than a predetermined rate.
The second mode of operation of comparator DFC comes into play once the
circuitry senses that the faster wheel speed VH is decreasing at a rate
greater than a predetermined rate. That is, once the circuitry senses that
the faster of the two wheels is decelerating, capacitor 106 will discharge
through diode 110 with the discharge rate being controlled by a two stage,
fixed bleed circuit FB. The discharge rate of capacitor 106 is controlled
in two states so that it initially decays at a rate representative of a
percentage of the speed of the faster wheel decreasing at a rate of 1.0g.
This is accomplished by limiting the discharge current with a pair of
constant drain NPN transistors 120 and 122 in the fixed bleed circuit FB.
NPN transistors 120 and 122 have their collectors connected together in
common and thence to the junction of diodes 110 and 112. The emitters of
the two transistors are connected through resistors 124 and 126
terminating in a common connection and then through a Zener diode 128,
poled as shown, to ground, as well as through a resistor 130 to a
B-voltage supply source. The base of transistor 120 is connected to ground
whereas the base of transistor 122 is connected to ground through a
resistor 132. Consequently, both transistors are normally forward biased
to drain discharge current from capacitor 106 through the parallel current
drain paths provided by the two transistors at a controlled decay rate of
1g. Hence, the computed speed reference signal applied to the noninverting
input of amplifier 100 will decay from an initial level, representative of
the speed of the faster wheel before that wheel decelerated, at a fast
decay rate, on the order of 1g, representative of a maximum brake
controlled deceleration rate to be permitted before the brake forces are
released.
If the average wheel speed signal VA decreases sufficiently fast then its
magnitude will become less than the decaying reference signal and
operational amplifier 100 will provide a positive skid signal. The signal,
however, is delayed by delay circuit DFD before application through OR
gate OG to actuate valve driver VD to relieve the brake forces. This time
delay permits the faster wheel to be braked for a longer duration before
the brake forces are released in response to the skid signal.
The delayed skid signal or any skid signal is used to actuate the fixed
bleed circuit FB to its second stage so that the discharge rate of
capacitor 106 iis decreased from a 1g decay rate to 0.5g decay rate. This
is accomplished by a feedback network wherein the positive skid signal is
applied to the inverting input of an operational amplifier 140. The
noninverting input for amplifier 140 is held at a fixed reference level
from a voltage divider including resistors 142 and 144 connected between
ground and a B+ voltage supply source. The positive skid signal is
sufficiently positive relative to the reference level to cause the output
of amplifier 140 to be switched to a low level. This level ch | | |
