Active control prevents squealing brakes

Acoustics researchers at the Georgia Institute of Technology in the States have developed an active control system for the suppression of disk brake squeal in vehicles. And, when you consider that automotive manufacturers spend several hundred million dollars a year in warranty repairs on squeaking brakes, the developments could represent quite a breakthrough for the industry. The system currently under experimentation is designed to simulate light braking conditions at low speeds and comprises a brake dynamometer with a 40hp constant speed electrical motor, gear reducer, automotive half shaft and automotive disk brake assembly. The motor is connected to the torque sensor via a flew coupling, the other shaft of the torque sensor also utilises a flex coupling to connect to the 21.4:1 speed reducer. The output of the speed reducer is fixed to the automotive half shaft via the spine joint. The rotor is vented and the dynamometer uses a ‘floating’ caliper. Brake pressure is applied using a servo motor. The dither signal is applied to the system using a piezoelectric stack located in the brake piston. The data acquisition system in place then has the ability to measure the braking pressure, brake pad temperature, the normal force on the brake pads, braking torque, in-plane velocity of brake pads and rotor and acoustic measures using a microphone. The objective of the research, which is still ongoing and funded by Ford, GM and the National Science Foundation, is to use these parameters to determine the effect of dither control on the effective braking torque and to better understand the system’s modal characteristics at the onset of brake squeal. Additional experimental work will address improvements in actuator control, placement, power supply and control signals. Squealing of brakes is an age-old problem for car manufacturers. In disk 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 loud noises do not affect brake operation in any way, but the problem leads to needless replacement of brake pads and extra shims, damping materials and other parts designed to prevent the noise occurring. Automotive engineers have learned many ‘tricks’ for designing quiet braking systems , but despite their best efforts, squeal still occurs unpredictably. Designers have proposed using feedback control systems that would detect the noise and then generate out-of-phase vibrations to counter the specific frequency of the squeal. But due to the complexity and cost, such systems have yet to find their way into road vehicles. Kenneth Cunefare, acoustics researcher at Georgia Institute’s School of Engineering, commented: “Our system would use a simple piezoceramic actuator mounted inside the brake piston to apply short bursts of a ‘dithering’ frequency to the backing plate of the inside brake pad, suppressing the vibrations that cause squeal. This active control system would work despite temperature and humidity changes - and normal brake system wear – all of which can change the squeal frequency.” He added that the system would be connected to vehicle brake light switches and would be turned on whenever the brakes were applied. “Compared to feedback control, our open loop control dither system would be simpler and we don’t need to detect the presence of squeal. All we would need to know is that the brakes have been applied.” The piezoceramic stacks which Cunefare is now testing cost $130 each, but he estimated that this cost would drop to around $30 each for high volume production. A single frequency generator and power electronics system could serve a vehicle’s entire braking system although an actuator on each brake piston would be required. The true test of the system will be field tests, yet to be undertaken. Long term reliability and its potential effects on braking efficiency are among the critical long term questions that still need answering. The brake system would still stop the vehicle if the squeal control system broke down because the actuator would be located inside the piston. So far, Cunefare’s testing shows minimal or no impact on brake performance. As he explained, “If an actuator were to break, there would still be another load path to allow the piston to operate the brakes.” Cunefare’s system could also be applied to drum brakes which are often used in heavier vehicles such as trucks, buses and the rear wheels of many motor cars. Cunefare also insisted that his system would not be affected by anti-lock braking (ABS) systems now used on many vehicles.