Active Control of Automotive Disc Brake Squeal

Sponsor: Integrated Acoustics Laboratory, General Motors, Trelleborg

Students: Jeff Badertscher, Michael Michaux

Visiting Scholars: Ricardo Borjas, Alberto Ortecho

The objective of this project is to investigate the use of dither control for the suppression of automotive disk brake squeal. A fundamental investigation into the mechanisms for dither suppression of brake squeal and significant theoretical and modeling activities are being used to develop an improved comprehension of the dynamics involved. The use of dither control for the suppression of automotive disc brake squeal suppresssion is experimentally investigated using a brake dynamometer.

Brake Dynamometer

The brake dynamometer consists of a 40 hp constant speed motor, torque sensor, gear reducer, automotive half shaft and automotive disc brake assembly. The motor is connected to the torque sensor via a flew coupling, the other shaft of the torque sensor also utilizes 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. These components are designed to simulate light braking conditions at low vehicles speeds.

Dither Control

Dither control is the use of a high frequency disturbance signal to suppress brake squeal. It is believed that the dither signal stabilizes friction-induced self-oscillations. A dither control signal is introduced into the system using the piezo-electric stack shown to the right. The stack is housed in a steel cage which ensures that the dither signal is always applied in the normal direction. The cage also allows for a load cell to be placed in line with the stack so that the longitudinal dither force can be recorded. The cage is placed in the brake piston. This allows the dither signal to be applied directly to the inboard brake pad (as shown in the picture on the left).

Controls and Data Acquisition

The motor speed is controlled using the white motor control unit shown to the right, it is a first order controller and is not interfaced with any PC. The PC shown to the right controls the brake pressure and can be used to acquire data such as brake pressure, brake pad temperature and braking torque. A Labview VI (virtual instrument) takes these measurements from a NI-4351 and NI-6713 data acquisition card and can store the data in a several data formats. The program utilizes a PID (proportional, integral, derivative) controller to send an output signal to the servo motor to control brake pressure. A second PC utilizes a much faster data acquisition card and is used to record the dither signal voltage, dither force and sound pressure measurements. A scanning head laser vibrometer with a dedicated PC can be used to measure surface velocity with the added capability of one extra data acquisition channel.

Modal Characterization of Squeal

In order to better the understanding of squeal, a working knowledge of the modal charateristics of the braking components needs to be known. Using the piezo-electric stack as an excitation device, a signal of any frequency content can be used to excite the system to determine what modes of the rotor/pads could be causing the audible response. Using this technique can also reveal which modes are contributing to the audible squeal response. Shown here is an example of a mode shape of a squealing brake. Future research in this area is focused on producing the frequency response function of a squealing brake as it comes into and goes out of a squeal condidtion. This information can be used to better isolate the vibration event responsible for squeal.

Effect of Dither Control on Brake Effectiveness

The use of an external dither signal has been shown to suppress automotive disc brake squeal in experiments with a brake dynamometer, but the effect of this control on the system's braking torque has yet to be determined. The dither control signal is applied using a piezo-electric stack located in the brake piston applying a normal vibration directly to the brake pads. There are many studies that lead to the conclusion of a lower effective braking torque due to the high frequency dither control signal. Under the assumption of Hertzian contact stiffness it has been speculated that the loss in braking torque is due to a lowering of the average normal force. There has also been work done that proves that the application of a dither signal in the normal direction eliminates the 'stick-slip' oscillation that causes brake squeal by an effective decrease in the friction force. It is not apparent whether any of these models accurately portrays the interaction of the brake pads and brake rotor. The current research will examine the effect of normal dither control on the effective braking torque using a brake dynamometer.

Control Effort Mapping/Burst Investigation

Many papers have addressed the successful use of dither control in the suppression of automotive disc brake squeal, but the exact nature of the suppression is not well known. A host of control signals have been utilized varying frequency, amplitude, waveform and burst modulation parameters. One explanation of the effect of dither control is the stabilization of friction-induced self-oscillations (or limit cycles). When dither is applied in the normal direction it serves to stabilize the non-linear limit cycles by reducing the friction coefficient. Past research has confrimed the success of continuous signals of 20 kHz and burst signals of the same frequency. The current research will document the effectiveness of various dither control signals for the suppression of automotive disc brake squeal and look more closely at the root cause of the dither signals effectiveness for burst control signals.

Alternative Actuator Designs

The current piezo-electric stack has proven to be an effective device to introduce dither into an automotive brake system. Unfrotunately it requires an external signal source (function generator), amplifier and impedance matching network. Consequently this makes this actuator design hard to implement into automotive braking systems. One of the focuses of this research program is to develop an actuator design that will be easier to implement into automotive systems.

"Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF)."