For Immediate Release
In disc brakes, squeal can occur when the brake pads contact the rotor
while the vehicle is moving at low speeds, setting up a vibration that
manifests itself as an annoying high-pitched squeal. The noise doesn’t
affect brake operation, but the problem – which occurs in cars,
trucks and buses – leads to needless replacement of brake pads and
the addition of shims, damping materials and other parts designed to stop
“A squealing brake still works, and from an engineering perspective,
there is no safety problem when the brakes are squealing,” said
Cunefare, an acoustics researcher in Georgia Tech’s School
of Mechanical Engineering. “But it’s a perceived problem
with the quality of the vehicle. If you’ve bought a new luxury car,
you don’t want the brakes to squeal. So manufacturers must spend
money on warranty repairs that shouldn’t be necessary.”
Automotive engineers have learned many tricks for designing quiet braking
systems, but despite their best efforts, squeal still appears unpredictably.
Designers have proposed feedback control systems that would detect the
noise and then generate out-of-phase vibrations to counter the specific
frequency of the squeal. Because of the complexity and cost, such systems
haven’t been implemented.
By contrast, the Georgia Tech system would use a simple piezoceramic
actuator mounted inside the brake piston to apply bursts of a “dithering”
frequency to the backing plate of the inside brake pad, suppressing the
vibrations that cause squeal. This active control would work despite temperature
and humidity changes – and normal brake system wear – all
of which can change the squeal frequency.
The system would be connected to vehicle brake light switches, turned
on whenever the brakes were applied.
“Compared to feedback control, our dither system would be much
simpler,” Cunefare said. “It would be an open loop control
system in which we won’t need to detect the presence of squeal.
All we would need to know is that the brakes have been applied.”
Without the need for detectors or logic systems to determine the proper
control frequency, the Georgia Tech system could be much simpler, with
fewer components. The piezoceramic stacks that Cunefare is now testing
cost $130 each today, but he estimates high-volume production should reduce
that to around $30 each – and perhaps even to a few dollars each.
A single frequency generator and power electronics system could serve
a vehicle’s entire braking system, though an actuator would be required
for each brake piston.
In extensive laboratory testing using a dynamometer and acoustic measuring
equipment, the system has been able to control brake squeal under a variety
of different conditions. Next, Cunefare and his collaborators would like
to field-test the system under real vehicle operating conditions.
“In terms of understanding the design constraints, we are pretty
far along with this,” he said. “We know the temperature changes
we’ll have to survive, and we know the forces that we’ll have
Long-term reliability of the system and its potential effects on braking
efficiency are among the critical long-term questions that must be answered
by field testing. The brake system would still stop the vehicle if the
squeal control system malfunctioned because the actuator would be located
inside the piston. So far, Cunefare’s testing shows minimal –
or no – impact on brake performance.
“This is fundamentally a fail-safe technology,” he added.
“If an actuator were to break, there would still be another load
path to allow the piston to operate the brakes.”
While the system would probably be installed first on high-end automobiles,
it could potentially be retrofitted to existing vehicles. “Our goal
is to have a drop-in module that slips directly into the brake piston
and connects to the vehicle wiring harness,” Cunefare said.
The same principle could also be applied to drum brakes, which are used
in heavier vehicles such as trucks and buses, and on the rear wheels of
The piezoceramic stacks consist of several layers of piezoelectric materials
that stretch or contract when electrical current passes through them.
Such devices are already used in vehicles, reliably powering fuel injection
systems. However, when used to control brake squeal, the actuators would
add to demands on vehicle electrical systems.
The actuators would operate at frequencies of about 20 kilohertz (kHz),
well above where brake squeal occurs – and above the range of human
hearing. The system, Cunefare said, would not be affected by anti-lock
braking systems (ABS) now used on many vehicles.
The research has been supported by the National Science Foundation, using dynamometers and other equipment provided by automotive manufacturers and suppliers, including the Ford Motor Company and General Motors. The results of the research were presented in late April at the Acoustical Society of America meeting and have been the subject of papers in several professional journals, including the Journal of Sound and Vibration. In May, a paper (2003-01-1617) was presented to the 2003 SAE NVH Conference in Traverse City, MI.
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WRITER: John Toon