Photonics Examines Brake Noise

Daniel S. Burgess

Most automotive brake noise is little more than an annoyance. But as advances in technology and design have made vehicles quieter, customers have a lower tolerance for these squeals, groans and moans, and the automotive industry is pressed all the more to eliminate sources of brake noise in a new car design before it makes it to the show floor. To do so, engineers are increasingly using photonic techniques, specifically laser metrology tools, to study brake systems and to verify their computer models of the causes of noise.

Automotive engineers use laser metrology tools to study brake noise, vibration and harshness and to verify the accuracy of their computer models of the phenomena. Images courtesy of Frank Chen, Ford Motor Co.

Frank Chen, a technical specialist at Ford Motor Co. in Dearborn, Mich., explained that the phenomena -- collectively called brake noise, vibration and hardness -- are complex, nonlinear and dynamic processes. High-frequency squeals may be the result of friction in the brake, but the low-frequency shudders caused by variations in torque in the brake and wheel assembly, perhaps due to deformation or distortion in a rotor, may involve the excitation of vibrational modes throughout the power train, frame and body components.

Theoretical work and experimental analysis in academia and in the industry have yielded computer-aided engineering tools, such as complex mode analysis, with which researchers diagnose the causes of noise and offer guidelines to optimize the performance of a new design, Chen said. However, a computer model must be correlated with experimental results to ensure that it correctly represents a real brake system. The high sampling rates of laser metrology tools make them ideal for assessing the accuracy of these models.

For example, he said, a common correlation parameter is squeal operational deflection shape, or instantaneous squeal mode, which describes the high-frequency vibrational pattern in a squealing brake. High spatial resolution beyond the capabilities of traditional, transducer-based techniques is essential to rapidly obtain quality measurements of the shape of a moving disc rotor. Laser Doppler vibrometry -- also known as laser Doppler velocimetry -- is the tool of choice.

Laser Doppler vibrometry reveals the vibration pattern in a disc rotor, enabling engineers to identify the cause of brake noise and to suggest a solution.

A laser Doppler vibrometer essentially adopts the Twyman-Green interferometer principle. The laser beam enters a beamsplitter, and half is directed to a mirror, while the other illuminates the object under test. The reference and object beams then recombine and interfere with each other to produce a beat signal caused by Doppler shifting that is collected by a photodetector. In practice, the sound or the vibrational signal of a squeal event triggers the sampling run, and several scanning heads and sensors are combined to yield three-dimensional measurements of the object.

Beyond this "must have" tool, Chen said, engineers in the automotive brake community gradually are adopting techniques such as pulsed laser holographic interferometry and pulsed laser electronic speckle pattern interferometry. A variant of the latter, which he suggested may be the most promising for the study of brake noise, incorporates high-speed, continuous-recording camera systems to measure sequential instantaneous squeal modes in a brake system at rates of thousands of frames per second.