Note: Descriptions are shown in the official language in which they were submitted.
SENSOR FOR BRAKING SYSTEMS
Technical Field and Background of the Invention
.
As is known to persons skilled in the art of
braking rotating members such as automotive vehicle
wheels, where brake modulators are used for varying the
braking effect exerted, it is necessary to sense the
'~ 5 rate of change of changing rotational speeds o~ a ro-
tatable element. Other examples of such needs are
known to persons skilled in the applicablc arts. A
variety of approaches to sensing rates of change of
changing rotational speeds and/or vehicle wheel slip
have been proposed heretofore, including certain prior
sensors disclosed by the inventor of the sensor de-
scribed hereinafter.
In connection with the development and use
of sensors of the type briefly described hereinabove,
one line of development has been directed to sensors
having a flyweight coupleable for rotation in response
to wheel rotation and selectively decoupleable in
response to the exertion on the flyweight of a torque
having a magnitude greater than a threshold magnitude
due to a change in rotation of a vehicle wheel. In
such sensors, it has now become known to provide a
- control means operativel~ connected with a flyweight
for exerting thereon torques resisting decoupled ro-
tation of the flyweight and a signalling means such
as an electrical switch, which may preferably be of a
.
-2~
magnetically actua-ted type such as a reed switch or a Hall
effect semiconductor switch, responsive to deco~pled rota-
tion of -the Elyweight for siynalling occurrences of an
excessive rate of change in changing rota-tional speeds of
the wheel. Those forms of sensors which have achieved
:~particular success in accommodating a wide range of
vehicle operating conditions have done so, at least in
part, by providing an improved control means capable of
applying a torque which is an average of a plurality of
:10 different torques individually applied in a rapidl~
fluctuating series. Such sensors, while successful, may
under cer-tain opera-ting circumstances and in certain
systems require insertion of special electrical circuits or
the use of other special components in order to accommodate
generation of signals as a train of rapidly fluckuating
pulses.
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Brief Summary of the Invention
With the above discussion in mind, it is an object
of the present invention to accomplish proper operation of a
sensor while simplifying certain characteristics of a system
incorporating that sensor. In realizing this object of the
present invention, a sensor of the general type described is
improved by causing a signal to be emitted continuously for
the interval of time during which the rate of change of
changing rotational speeds of a vehicle wheel is excessive.
By such continuous signalling, the necessity of providing
special modulators or circuitry is reduced and alleviated.
Yet another object of the present invention is to
accomplish more precise control over variations in the
operating characteristics of a sensor. In realizing this
object of the present invention, control means are provided
for controlling the level of torque transmitted by said
control means and operable in response to an occurrence of
a rate exceeding the threshold rate for changing the level
of torque from a first level to a second level and thereby
for causing said signalling means to signal continuously for
the interval of time during which the threshold rate ;`s
exceeded.
, Brief Description_of the Drawincls
Some of the objects having been stated, other
objects will appear as the description proceeds, when
taken in connection with the accompanying drawings in
which --
Figure 1 is a perspective view of one ~orm of
sensor responsive to the rate of chanye of changing
rotati~nal speeds of a rotatable element, in accordance
.~ with the present invention;
Figure 2 is an elevation view, partly in section,
of the sensor of Figure l;
Figure 3 is a schematic diagram of electrical
circuit elements as used with certain sensors in
: accordance with this invention;
Figure 4 is a diagram illustrating the course
of events during deceleration of the rotational speed of
a rotatable element and reflecting the operation of sensors
: such as that OL Figures 1 and 2;
Figure 5 is an enlargement of a portion of the
diagram of Figure 4;
Figure 6 is a schematic view, partly in elevation
and partly in section, of an arrangement by which the
: parameters of operation of the sensor of Fiyures 1 and 2
may be accommodated to vehicle operating conditions;
. Figure 7 is a view similar to Figure 1, showing
25 a second form of sensor in accordance with the present
invention; and
~igure 8 is a view similar to Figure 2, of the
sensor of Figure 7.
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Det,ail.ecl.Dc!script.ion of the Invention
_ _ . _ _ _
While the present inventiorl will be described
more fully hereinafter with reference -to the accompany-
ing drawi.nys, it is to be understood at the outsct of
the detailed descript.ion of this invention -that it is
contemplated that various modifications of the inven-
tion as clescri.~ed may be made by persons skilled
in the appropriate arts. For that reason, the cletailed
descri.pti.on is to be read broadly, and not as lirniting
on the scope o~ this invention.
Referring now more particularly to the accom-
panyiny drawincJs, Fiyure 1 shows a sensor for respond-
iny to the rate of chanye of chanyincJ rotational speeds
of a ro-tatiny mcmber suc}- as a vehicle wheel. The
sensor of Fiyurc 1 inc].udes a flyweiyht 10 coupleable
for rotation in response to wheel rotation by means of
a shaft l.l. The flyweight 10 and shaft 11 are couplea-
ble throuyh mealls includiny a plane-tary gearing
generally indicated at 12 and haviny an orbit gear 14,
planet year 15, and a sun gear 16. The sun year 16
is formed inteyrally with an interconnectiny shaft
18 to which the flyweiyllt 10 is fixed and which is
supported for rotation by a suitable beariny 19. The
flyweight 10 and the input shaft 11 are arranyed so as
to permit relativc rotation therebetween, in a manner
broadly similar to the prior sensors referred to briefly
hereinabove such as those of United States Patent
4,061,212.
--6--
More par~icularly, a stator 20 encircles the input
shaft 11 and is.mounted, by appropriate bearings 21, 22,
for movement through a restricted arc. A projection 24
extending from the stator mounts a pair of counteracting
permanent magnets, one of which is visible as a magnet 25,
which are positioned to either side of a reed switch 28.
Tlle position of the magnets re:Lative to the reed switch
28 controls the opening and closiny of the contacts of
the reed switch. The extent of arcuate movement of the
stator is controlled by a pair of limit screws 29, 30
arranyed to engage opposite sides of -the projecting magnet
carrier, while a setting spring 31 biases the stator
toward one particular rotative position (opposite the
driving direction of the input shaf-t 11 indicated by the
arrow 26 in Figure 1).
Mounted within the stator is an electromagnetic
coil or.winding 32 which serves to apply a magnetic field
across a right circular cylindrical groove or gap 34 in
the stator 20 and into which a cooperating coupling
rotor member 35 projects from the orbit gear 14 of the
planetary arrangement 12. As so arranged, the stator 20,
winding 32 and rotor 35 cooperate to function as a control
means for the leveI of torque transmitted from the input
shaft 11 through the planetary 12 to the flyweight 10.
The specific form of arrangement illustrated in Figures 1
and 2, and now described, exerts a selected plurality of
levels of torque, preferably two, through a non-contact
type coupling.
Certain of the sensors proposed heretofore
and briefly referred to above use yieldable coupling
--7--
means for achieving an operation in accordance with a
particular sequence of events by which a flyweigh-t is
` decoupled and the rate of deceleration of the decoupled
flyweight is controlled. It is recognized that the
. 5 word "coupling" has been used for mechanical devices
. operating in such a manner that relative movement is
: possib~e between different members so that connection
: and disconnection of -the membe:rs relative to each other
. is possible. The word "brake" is generally used for
devices giving a retarding eEfect upon other members.
Certain classes of devices, which have been used in
; certain of the prior sensors briefly referred to
above, function both as couplings and as brakes and
. the present invention (as described more fully herein-
: 15 after) contemplates the use of a wide range of various
devices. By way of example and not by way of limita-
tion, such dev.~ces may include contact devices such
. as helically wound wires or bands having one or more
turns and electxomagne-tically operated frictional
clutches. Such devices may also include non-contact
devices such as eddy current couplings, magnetic powder
couplings, magnetic hysteresis couplings, viscous
couplings of various types, and dynamoelectric devices.
. While the range of devices described above
are useful in sensors in accordance with the present
. invention, those capable of giving a rapid and distinct
connection and disconnection when an electrical current is
applied and removed may be preferred for certain
configurations as described hereinafter. Devices
having those characteristics include, among others,
magnetic hysteresis and magnetic powder couplin~s which
have the additlonal advan-tage o~ exerting a desi~ecl
total torque without the necessity that portions of
the coupling move relative to one another. While at
least some oE the other above-mentioned types of devices
may appear simpler and less expensive, such other devices
frequently require -that at least a certain relative
turning movement occur between portions of the coupl-
ing in order for a torque to be exerted, giving rise
to discernible differences for the values a-t which
the sensors produce signals. Such differences will be
found acceptable for certain applications of sensors
in accordance with the present invention, and foun-l
unacceptable for others.
With sensors of the type to which the present
invention relates, signaling occurs in response to
certain g-value settings. The so-called g~value of a
sensor as related to its use in brake control systems
for vehicles is deEined with reference to the vehicle
speed change which causes an alteration of the rota-
tional speed of a flyweight. Broadly stated, therelationship is v=r.~, wherein v is the vehicle speed,
~ is the wheel radius and ~ is the angular speed for
the vehicle wheel. Additionally, the average figure
for the acceleration of ob~ects in the gravitational
field of the earth, namely 9.81 m/sec2, is used as a
reference. High retardation values thus give high
g-values, with low retardation values giving low g-values.
Normal g-value settings for sensoxs included in vehicle
brake control systems are between 0.7 and 1.5g. The
disclosure here of couplings of magnetic powder and
magnetic hysteresis types reflect the ability of such
_9_
couplings to operate within such normal ranges, but
; it is further contemplated that many o-ther coupl-
ings may operate within such ranges, including those
types briefly described hereirlabove.
For purposes of clari-ty and in order to assure
full understanding of the non-contact couplings particu-
larly disclosed, it is appropr:iate to briefly describe
the functional characteristics and features of magnetic
powder and magnetic hysteresis couplings. As indicated
briefly hereinabove, such devices may have a stator
and a rotor. The stator may comprise a rotationally
symmetrical member of iron having an inner cylindrical
coupling surface. I'he rotor may also be formed, at
least in par-t, by a ro-tationally symme-trical iron
; 15 member rotatably mounted relative to the stator and
having an outer cylindrical coupling surface which
turns with a relatively small gap or spacing with
regard to the coupliny surface of the s~ator. A
solenoid or electrical winding is arranged so that the
stator and rotor provide opposite poles, such as where
the stator provides a south pole and the rotor a north
pole. The gap between the stator and the rotor may
be filled by magnetic particles or powder (if so desired)
in which case the device is of a magnetic powder type.
Under the influence of the magnetic field in the gap
between the rotor and stator, such magnetic powder
will coalesce and resist relative turning motion
between the rotor and the stator, with the magnitude
of the resistance being dependent upon the dimensions
and proportions of the coupling device and the quantity
of magnetic powder employed. For any certain coupling,
the magnitude of the resistance to turning movement is
.
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~ 3~
~ 10
directly proportional to field s-trength, that is to
the current applied to the electrical winding or
solenoid. The torque is essentially independen-t of
rotational speed. A magnetic powder coupling can pro-
vide large torques from a couplinc3 of relatively smallsize and wi-th low current consumption, but suffers
from a possible deficiency in that the coupling is
subject to wear and deterioration of the magnetic
powder.
In a hysteresis coupling, the rotor is generally
provided by a drum or flat disc of magne-tizable ~aterial.
Such couplings are broadly known and, in at least
certain forms, are available from manufacturers known
to persons skilled in the art. In a coupling of the
magnetic hysteresis drum type, the drum may have only
one end wall, to which a shaft is mounted concentrically
; with an outer surface of the drum. A stator may be
provided by an outer and an inner part, each concentric
with the rotor shaft and drum. A central opening through
the inner part of the stator may receive the shaft
of the drum and accommodate bearings for the shaft.
Longitudinal grooves are formed in the inner surface of
the outer stator part and in the outer surface of the
inner stator part, with the stator parts being so
mounted that the grooves are displaced by half a pitch
relative to each other. The material between the grooves
thus provides a plurality of elongate pole pieces extending
generally in the direction of the mounting shaft.
The winding is so arran~ed that one of the stator parts
functions as one magnetic pole and the other functions
.
as the opposite magnetic pole, posltion:iny the rotor
between two magnetic f:ields in such a way that the
material of the rotor is maynctized in a particular
pattern and, during relative movement, the pattern of
magnetization must be displaced in the mass o~ the rotor.
Such continuous change in a magnetic field causes a
loss, referred to as "hysteresis loss", which results
in the exertion of a torque.
In the couplings described, the torque exerted
is independent of rotational speed and will have the
same magnitude at standstill or at any rotational speed.
Losses in addition to air resistance and bearing losses,
which are so small in couplings o~ the size with which
the present invention is concerned as to be neglectable,
essentially amount only to an unavoidable eddy current
loss. Suitable structural design for such a coupling
as a total Ullit and material choice ~or the components
may maintain any non-linear part of the total torque
exerted below one per cent of total torque. Generally
speaking, torque is proportional to stator magnetiza-
tion, i.e. directly proportional to current intensity.
In accordance with this invention, it is pre-
ferred tha* both magnetic powder and maynetic hysteresis
couplings be supplied with electrical current through a
temperature compensated cons-tant current circuit in
order that the torque exerted may be maintained at con-
trolled levels notwithstanding varying temperatures of
the winding, as the resistance of the windings may vary
in connection with such varying temperatures. A constant
current circuit may also be necessitated by the current
~12-
sup~ly, such a~ an automotive vehicle-electrical system,
not being able to maintain a constant voltage.
In operation of the sensor of Figures 1 and 2,
while the input shaft 11 and fl.yweight 10 are stationary,
the spring 31 acting against the projecting magnet carrier
24 will maintain the magnet carrier agains-t one set-
ting screw 29, with the permanent maynets being so
positioned relative to the reed switch 28 that the reed
switch is maintained open. Thus, a controllable cons-tant
current circ~1it (Figure 3) is caused to supply the wind-
ing 32 with electrical current. Any turning movement of
the input shaft 11 is then transmitted through the
: planetary gearing 12 to the interconnecting shaft 18 and
thence to the flyweiyht 10. In the event that the input
shaft 11 is subjected to a rate of retardation which is
greater than a threshold rate which, according to the
mechanics of the sensor, corresponds to the turning
. moment from the spring 31 and the common inertia of
. the flyweight 10, interconnecting shaft 18 and the
20 co-rotating parts of the bearings connected thereto,
: the tension of the spring 31 is overcome and the project-
ing magnet carrier 24 and stator 20 will turn to
engage the other setting screw 30. The permanent
magnets are thus moved to such a position that the reed
i 25 switch 28 is closed, in which event the current supplied
to the winding 32 is switched to a controlled substantial-
ly lower current resulting in a lower torque being
exerted by the coupling. ~t the same time, elec-trical
current .is supplied to a brake modulator.
'B~
An illustration of the function of a sensor
accordiny to the present invention is given in Figure
~ where, after braking is begun, vehicle wheel speed
dips while vehicle speed decreases at some lesser decel- -
eration rate. The average speed of a flyweight of asensor lies between vehicle speed and vehicle wheel
speed. The uppermost line ln Figure ~ illustrates the
transmit-tal of a signal frorn the sensor in accordance
with the present invention to a bra]ce modulator circuit,
showin~ that the signal to tlle modulator is on continu-
ously during intervals of time that vehicle wheel speed
is below the speed of the flyweigh-t.
The curves representing vehicle speed,
speed of a flyweigh-t, and vehicle wheel speed in Pigure
~ may also be used in order to clarify the meaning of
the term "g-value". More particularly, the slope
of the line indicating vehicle speed, or the negative
derivative thereof, is a measure of the retardation of
the vehicle expxessed in appropriate units such as
m/sec2. The alternating positive and negative slopes
or derivatives of the wheel speed line are indicative
of momentary retardations and accelerations of the wheel
expressed, ~or example, in radians/sec2. Similarly,
the somewhat jagged speed curve for the sensor flyweight
is indicative of momentary retardation and acceleration.
Operational characteristics such as those shown in
Figure 4 may be achieved by an appropriate selection
of the relative slopes involved.
In accordance with important characteristics
of the present invention, the torque exerted by the
coupling is, at the normal high current intensity through
the winding 32, substantially higher than the torque at
the lower controlled current intensity. That is, the
;L~
slope oE -the por~ions oE the l:ine rcpresenting the
speed of the flyweight in Figure ~ and havj.ng a neyative
slope indic~tes that the flyweight is, for those portions
of its operation, braked by the coupliny supplied with a
: 5 controllecl low intensi-ty current. Those portions of the
flyweight speed representing line having a positive ~lope
correspond to circumstances in which the flyweigh-t is
being ~ccelerated while the coupling is supplied with
the controlled higher intensity current. Xt is to be
10 noted that the moment exerted by the spring 31 is to
be higher than the moment exerted between the rotor
member 35 and the stator 20 when the coupling i~ bcing
subjec-ted to the controlled lower in-tensity curren-t.
Figure 5 is an enlarged illustration of a
portion of certain of the curves of Figure ~, further
illustrating a manner in which the sensor of the present
invention is accommodated to vehicle wheel friction
conditions influential on vehicle braking. In connection
with the following description of events there illustrated
20 and other points in the present description, the expres-
sion "slip" has been and will be used. "Slip" is under-
stood by persons working in the applicable arts as
representing a difference between vehicle speed and any
corresponding peripheral speed of a vehicle wheel,
divided by vehicle speed. Such a number is multiplied
generally by a factor of 100 and expressed in a percentage.
Extensive tests have shown that slip values or slip
percentages for maximum braking effect should be in
-15-
the range of 15 to 25 per cen-t, depending upon e~isting
road conditions.
Referring now to Figure 5, the velocities of
a wheel and vehicle will fre~uently appear, immediately
before braking is begun, substantially constant and in
direct relationship one to the other (that is, with 0
percentage slip). Such a condition is indicated by
single straigh-t, generally hor:izontal line to the left
of point a in Figure 5. Upon :initiation of braking
effort applied to a wheel, wheel speed dips (toward
progressively greater slip percentages) while vehicle
speed decreases at some lower deceleration rate. As
indicated in Figure 5, a divergence appears between
the speed of the decelerating vehicle as indicated along
the line c and the speed of a braked wheel indicated
by two curves, namely a broken line curve i for a good
road surface condit:ion such as dry asphalt or concrete
and a full line curve k representing a bad road condition
such as wet ice. As will be appreciated, acceleration
of the vehicle wheel during modulated braking over a
good road surface (the broken line curve i) is charac-
terized by relatively reasonable retardation and
acceleration at a relatively high rate. A braking
sequence over bad road conditions (the full line curve
k) is characterized by sudden retardation of the braked
wheel and poor re-acceleration.
Where the "normal" higher intensity current
applied to the sensor winding is selected to be very high,
--16~
; the coupling will accelerate the flyweiyht so rapidly
that it follows vehicle wheel speed as soon as vehicle
wheel speed reaches the speed of the ~lyweight. Thus,
under good roac1 surface conditions, the sensor begi.ns
signalling for brake modulation at A poin-t e from which
vehicle wheel speed again retards. Vnder bad road
surface conditions (on the full line curve k), sensor
signalling begins at a point f at which wheel slip differs
significantly from the wheel slip under comparable good
road surface conditions ~at point e). Thus, the vehicle
wheel will be braked with a higher slip under ba~ road
; surface conditions than with good road surface conditions,
which is undesirable. ~y controlling the higher level
of current supplied to the winding of the sensor, the
torque transmitted through the coupling is restricted
so that the flyweight of the sensor does not directly
follow the speed of the vehicle wheel, but will instead
be accelerated along a different line such as is
indicated by a straight line g in Figure 5. For
simplicity, the slope or g-value of the thus controlled
acceleration has been chosen in such a way that the
sensor will begin to signal at the same point f under
bad road conditions, while signalling under good road
conditions does not occur at the point e, as previously
was the case, but at a different point h. As a result,
the vehicle wheel is braked with a slip value which does
not differ greatly between good and bad road conditions.
In this operation, the acceleration of the decoupled
rotating flyweight is limited to a rate less than a
controlled, substantially constant rate.
: ~
-17-
It is to be no-ted that mos~ oE the brake Eorce
modulators currently employed are provided with devices
delaying the reapplication of braking ~orce after cessa-
tion o the signal from a sensor. Such a delay is a
parameter that can be used in combination with controlling
the current levels applied to a coupling device in
accordance with the present invention to obtain "feed-
back" of information to the sensor regarding existing
road conditions. It is anticipated that this particularly
simple and attractive form of "feedback" will be entirely
sufficient for road vehicles no-t expected to be used on
roads having greatly varying fric-tional conditions and
for vehicles running upon rails.
Frictional conditions existing between a wheel
and a road surface are one of two important varying
factors which, in accordance with the present invention,
may have influence on the g-value settings of sensors for
a particular vehicle. The second factor is vehicle
loading, and Figure 6 illustrates an arrangement in
which the two variables can affect the adjustment of
a sensor. More particularly, Figure 6 includes a
schematic representation of a source 50 of a pressurized
braking fluid, such as a master cylinder supplying
; pressurized hydraulic oil, and a brake force modulator
51 Eor controlling vehicle wheel braking. An
appropriate conduit 52 connects the pressure source
50 with the modulator 51 and a further conduit 54
supplies pressurized braking fluid to a vehicle wheel
brake 55. Pressure supplied from the master cylinder 50,
whether or not modulated or reduced by the effect of the
'`
'
,
modulator 51, is supplied through the conduit 52 to
a cylinder 56 enclosing a piston 58 which operates a
: piston rod 59 and is acted on by a return spriny 60.
The position of the rod 59 controls the posi-tion of
one end of the setting spring 31, and thus the y-value
. setting of the sensor. Additionally, -the position of
the rod 59 may act, through a pin 61, on an electrical
circuit element of the controllable constant current
apparatus which may, for examp:Le, be a potentiometer 62.
The cylinder 56 is arranged to slide axially along two
guides 64, 65. Further, the cylincler 56 is operatively
connected with a Bowden wire 66, the other end of which
is connected to a lever 68 mounted for movement about
` a pivot 69, fixed in the frame 70 of the vehicle, in
response to relative displacernent of a portion of the
: vehicle ~rame 70 and the vehicle suspension 71. As
vehicle load increases, the vehicle suspension moves
relative to the frame, causing movement to be trans-
mitted through the lever 68 and Bowden wire 66 to the
cylinder 56.
When the brakes are not actuated and no
pressure is transmitted from the master cylinder 50
through the modulator 51 to the bralce cylinder 55 of the
vehicle wheel and the control cylinder 56, the return
spring 60 urges the piston S8 to the right in Figure 6,
with the piston rod 59 thus keeping one end of the
setting spring 31 in a position for a low g-value setting
and the potentiometer 62 at a position at which low current
intensity is applied to the winding 32. When the master
cylinder 50 is actuated, braking fluid pressure rises
. ' .
19-
in the cylincler 56, the force tllus applied to the piston
58 overcomes the force of the return spring 60, and
the piston 58 and rod 59 are moved (to -the left of
Figure 6). As a consequence, the force exerted by the
setting spring 31 is increased and the pin 61 of the
po-tentiometer 62 is -turned in such a way as to raise the
g-value se-ttings of the sensor.
At an increased brake fluid pressure a point
may be reached at which the vehicle wheel tends to lock
and the sensor generates a signal to the modulator to
lower brake pressure. The brake pressure at which the
sensor starts to signal is a measure of the fric-tional
conditions existing between the wheel and the road
surface and the system, as here described, has thus
adjusted the sensor to a g-value set-ting suitable for
the existing conclitions.
Compensation of g-value set-tings solely in
response to braking fluid pressures will be found, in
some circumstances, to be sufficient for vehicles such
as heavy passenger cars in which wheel pressure does
not vary greatly in response to vehicle loading.
However, with cargo trucks and small cars of relatively
low weight, wheel pressures may vary considerably in
response to vehicle load. It is with such vehicles
that the efficiency and versatility of a brake control
system is substantially enhanced by using both vehicle
load conditions and hydraulic braking fluid pressures as
control parameters for g-value set-tings~ In such
instances and in the arrangement illustrated in Figure 6,
it is correct to say that the load dependent g-value
adjustment is superposed on the fluid pressure dependent
~-value adjustment.
'
-20-
Brake force modulating systems have been pro-
posed for compressed alr brake systems in which ai.r
pressure is not modulated but a hydraulic system is
providcd to oppose or counteract the brake force
exerted by the "normal" compressed air brake system.
The present invention is contemplated as being adap-table
to such arrangements, with it being understood that in
such arrangements the cylinder 56, piston 58, piston rod
59, and return spring 60 may be replaced by a diferential
pressure cylinder arrangement in which the piston is
balanced between the to-tal air pressure applied to a
wlleel cylinder and the counteracting hydraulic fluid
pressure. In the event that the pressure ranges for the
air and hydraulic fluid are of differen-t maynitudes,
the cylinder arrangemen-t may employ two i.nterconnected
pistons of different diameters in order to accommodate
the balancing effect. Normally, the "counteractin~"
hydraulic fluid pressure would be contemplated as
being higher than the "normal" air brake pressure and,
for that reason, the piston responding to air brake
pressure may be the larger one.
~ In the sensor of the present invention as des-
- cribed to this point, the two levels of torque trans-
mitted through the coupling are determined by two levels
of current applied, inasmuch as the coupling employed
is of a non-contact type. It is contemplated, however,
that the coupling eMployed may at least include a
contact type element by which at least one of the
levels of torque transmitted is determined. Such an
arrangement is illustrated in Figures 7 and ~, where
.
.
.
-21-
components comparable -to those descr:ibed -to this point
are iclentified by similar referellce characters of a
100 series. Inasmuch as there is subs-tantial similarity
between the sensor of Figures 7 and 8 and that of
Figures 1 and 2, the present discussion will be directed
particularly to the distinc-tions and di.fferences there-
betwecn .
Mor,e parti.cularly, the coupling of Figures 7and ~ i~c]udes a contact type element in the form of a
helical coil or spring 172. In a manner similar to
certai.n arrangements di.sclosed more fully in ~nited
States Patent 4,061,212, one end 173 of the coi.l 172
is fi~ed in the stator 120, while the other end is free
relative to the rotor mernber 135 which it encircles.
Torques accelerating the flyweiyht and exceeding that
which can be transmitted throuyh the non-contact coupling
are transMitted through the freewheel arrangement thus
provided by the contact type coupliny provided by the
coil 172. It should be noted that a yreat number of
different freewheel devices such as roller and sprag
clutches are known and can be used. The coil is
preferred because of its e~treme simplicity and low
cost. In similarity to the operation described above with
particular reference to Figure 5, where the driving
torque of the helical coil 172 is selected to be very
high, the coupling will accelerate the flyweiyht so
rapidly that it will follow vehicle wheel speed as
soon as it reaches the speed of the flyweight. By
restricting the driving torque which can be transmitt~d
through the helical coil 172 (in a manner known to
.
-22
designers of such devices), it is possible to control
operation in a manner similar to that described above
with reference to the selection of current levels
applied. Thus, operation as described above may be
accomplished by a combination of non-contact and contact
type couplings.
In the drawings and specification, there has
been set forth a preferred embo~iment of the invention,
and although specific terms are employed, they are used
in a generic and descriptive sense only and not for
purposes o~ limitation.
:
