Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FRIC~ION CONTROLLERS
The invention relates to friction controllers whereby
frictional forces in brakes, clutches and the like may be varied
independently of the normal operating mechanism.
The invention can advantagecusly be applied to vehiale
braking systems, especially in motor-cycles, as an anti-lock
device, and as a slipping clutch in mechanisms transmitting
power or control movements, for example winches on cranes etc,
but is not limited to these functions.
The invention provides a means of reducing frictional Eorces
in controlled manner by utilising the known effect of the
application of ultrasonic vibration.
In wheeled vehicles, the application of brakes tends to
wheel locking especially in wet or otherwise slippery conditions
leading to skidding which can cause flats on the wheels cf rail
vehicles and loss oE directional control in other cases coupled
with reduction in braking effort. Wheel locking whilst always
undesirable is particularly serious in the case oE motor-cycles.
A recent survey has shown that motor-cycles (including scooters
and mopeds~ account for only abou-t 2% of the total road vehicle
mileage in ~ritain but produce 14~o of casualties in accidents,
many of which occur because braking is inadequate or incorrectly
applied in prevailing conditions.
Anti-lock braking systems are well known and usually comprise
a sensor for detecting excessive deceleration in a wheel and
which is connected tG release the applied brake pressure until
the wheel recovers a prede-termined rate of rotation~ when braking
is reapplied; the sequence being repeated until the wheal comes
to rest or the brakes are released normally.
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Such systems are in practlce usually limited to brakes
applied hydraulically or pneumatically since they are difficult
to adapt to mechanically applied brakes.
A friction controller assembly according to the invention
includes a movable member a friction member arranged to engage
the movable member and a ~ connected to apply ultrasonic
vibration to at least one of the members.
In one embodiment~ the movable member is a disc connected
to a wheel, the friction member is a brake pad and the sensor is
arranged to monitor the angular velocity of the wheel on brake
application and to apply ultrasonic vibrations -to the brake pad
when the angular velocity falls below a predetermined value.
In another embodiment, the movable and friction members are
coac-ting clutch elements.
Various embodiments of -the invention will now be described,
by way of example only, with reference to the accompanying
diagrammatic drawing which shows part of the braking system of
a vehicle.
The braking system includes a metal disc 1 connected to a
wheel (not shown) so as to rotate with it. A pair of brake pads
2 (one only shown), of a material having a high coefficien-t of
friction, on either side of the disc, are connected by a caliper
mechanism (not shown) whereby the pads may be urged into contact
with the disc by a suitable actuating arrangement so as to apply
a braking force. The actuating arrangement may be of any suitable
conventional kind including the use of hydraulic, pneumatic or
~echanical pressure.
A sensor 3 is connected to an ultrasonic genera-tor 4 which is
connected in turn by wires 5~ 6 to the coil 7 of a mag~netostrictive
transducer 8 which is in contact with one of the brake pads 2.
Alternatively there may be two magnetostrictive transducers, one
in contact with each brake pad.
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The sensor 3 is arranged to monitor the a~gular velocity
of the vehicle wheel either directly or through the medium of
the disc 1 during brake application. Thereafter if the wheel
decelera-tes at a rate greater than a predetermined setting
(usually 1 g) the sensor applies a signal to the ultrasonic
generator 4 which energises the coil 7 thus exciting the
magnetostrictive transducer(s) whereby an ultrasonic vibration
is applied to the brake pad or pads 2. This has the known effect
of reducing the frictional force between the pad or pads and -the
disc 1 whereby the wheel recovers velocity at which points the
sensor 3 ceases to apply the signal to the ultrasonic generator 4
and normal brake force is reapplied. ~his sequence of operations
continues repetitively whenever there is a tendenoy to wheel
locking un-til the vehicle either stops or brakes are released.
By this means the braking force applied by the pad or pads
2 is maintained to the maximum degree possible. This can be
especially important during "emergency" situations when a road
surface is wet and slippery~ and optimum control, particularly of
a motor-cycle, can be maintained.
Since the system is independent of the normal brake actuating
means and has no moving parts, there is a minimum time lag in its
operation which is more rapid than that which might be reasonably
expected where the brake actuating means are intermittently applied
and released.
Various modifications consistent with the state of the art
may be applied to the arrangement described. ~or ins-tance the
sensor 3 migh-t include an accelerometer to measure the rate of
retardation of a vehicle, the brake pads may be applied directly
to a wheel eg at the rim, or conventional drum brakes may be used,
while the magnetostrictive transducer may be replaced by one of
the piezo-electric type which is an alternative device for
converting electrical oscillation to mechanical. Ultrasonic
vibrations may also be applied to the disc 1.
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The displacement caused by the magnetostrictive or piezo-
elec-tric transducer may be applied to the appropria-te friction
member in any desired direction. In the foregoing embodiment,
this is applied in -the plane of rotation and along the line of
instan-taneous direction of rotation which is believed to be most
suitable for that particular application~ but any other direction
either in the plane of rotation or out of it might be more
advantageous according to the prevailing circumstances.
It is known in ultrasonic transducers, particularly of
the piezo electric type, to apply a dc bias to the osoillatory
input signal in order to provide more definite response.
In a modification to the foregoing construction~ a short
duration dc pulse of greater magnitude than the normal dc bias
is applied at the onset of the input signal.
Referring again to the drawing, a function generator 9
(shown dotted) is interposed between the sensor 3 and the ultra-
sonic generator 4 and has an additional output which passes by
way of a dc pulser 10 (dotted~ to the output connection 6 from
the ultrasonio generator to the transducer 8. The function
generator comprises a starter and dc pulse timer whereby the onset
of a signal from the sensor causes a dc pulse of duration equal
to one or two cycles at an operating frequency of ~ 20 kHz to
be fed to the transducer 8 effectively simultaneously with the
output from the ultrasonic genera-tor 4. This produces a surge
which will rapidly break any friction bond likely to exist between
the friction member and the movable member without the need for
build up of normal oscillations. This will give more rapid
reduction of the frictional force than woulcl normally be possible
otherwise and thus a greater degree of control. An incidental
advantage is that less power will be needed overall since the
amplitude of the ultrasonic vibration can be less than if it were
required to break the frictional bond unaided. The effect of the
dc pulse is probably more pronounced in the case of a pie~o-
electric transducer where it will act to compress the crystal
very rapidly but similar results have been achieved using a
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magnetostriotive -transducer. It has also been found advantageous
in this con-text for the displacement caused by the transducer
to be applied radially to the friction member with respect to a
rotating movable member.
In a further modification to the basic construction wherein
hydraulic means is employed -to actuate the friction member, a
transducer is applied directly to the closed end of the ac-tuating
cylinder and the ultrasonic vibrations emanating from the transduoer
are transmitted through the operating fluid to the piston which
may conveniently carry the friction member. ~he end wall of -the
cylinder is of a suitable matching material eg stainless steel
which will facilitate transmission of ul-trasonic energy and the
pis-ton is appropriately formed according to its own oharacteristics
and those of -the operating fluid so as to minimise any attenua-tion
f ultrasonic energy at the piston/fluid interface.
In another embodiment, the invention is applied to friction ;
clutches, which rnay operate in either angular or linear senses,
and the sensor arranged to detect other undesirable conditions
such as overloading eg by strain gauge, or overspeed whereby the
application of ultrasonic vibrations to one of the clutch elements
allows the clutch to slip during such time as the adverse conditions
are maintained.
Very precise control of a clutch is obtainable by the use of
a function generator interposed between a sensor and ultrasonic
generator whereby the input to the ultrasonic generator may be
varied to give gradual engagement thus:
a. partial engagement -to provide slip;
b. partial engagement prior to full engagement to lessen
shock loading of driving unit and/or transtnission;
c. movement towards full engagement in a pre-programmed
manner, ie to engage in accordance with a linear or non-
linear function maintaining the speed of a driven load
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within specified limits might best be achieved using a ~ `
conven-tional closed loop control system but i-t can be
advantageous to allow the clutch to slip whilst varying
the speed of the driving means.
The sensor can be arranged to detect any appropriate signals,
analogue or digital, representing torque, temperature or
other measurable predictable parameter.
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