Experimental Investigation of Dither Control on Effective Braking Torque

Automotive brake squeal is a problem that has plagued the automotive industry for years. Many noise cancellation techniques have been published. One such technique is the use of an external dither signal, that 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. By imposing a high frequency disturbance normally into the brake pad, squeal is suppressed. 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. Yet another theory indicates that the effective contact area is reduced causing a lowering of the mean coefficient of friction. It is not apparent whether any of these models accurately portrays the interaction of the brake pads and brake rotor. This paper will examine the effect of normal dither control on the effective braking torque using a brake dynamometer. A dither control signal of 23 kilohertz with several force amplitudes are examined. A statistically significant reduction in the braking torque has been found that is directly related to the amplitude of the dither signal applied. These tests were repeated at varied brake pressure to prove that the results were not dependent on the specific braking conditions used.