Adapted from an article that appeared in Automotive Engineering, July 2005

The major components comprising the test hardware include a custom-designed signal-conditioning module (upper portion) and an IOtech WaveBook (lower module), a data acquisition system that feeds a notebook computer. A thermocouple and a pair of capacitive probes surrounding the brake rotor drive the custom module, which in turn, supplies analog temperature and displacement signals to the WaveBook for storage and analysis.

A common warranty issue is a customer complaint of a pulsing sensation felt during a brake stop, called shutter and judder. Uneven level changes in the rotor thickness (measured in microns) during the rotor’s application life can produce such pulses. However, sometimes a “no trouble found” report comes out of a warranty investigation or an engineering forensic study. In these cases, either the warranty concern could not be replicated in the warranty lab, or the limited data or information from the dealership makes the engineering forensics and traceability difficult. Engineering forensics concerning a brake issue requires data and information obtained during the earliest stages of the investigation. For example, more significant results can be uncovered when suppliers receive complete and accurate brake-plate runout and thickness-variation data at the start.

Brake suppliers have invested enormous sums of money on test equipment to validate brake reliability, durability, and life for ensuring the utmost safety, so analyzing and testing non-defective returned units is an expensive, time-consuming process. In spite of the high cost, OEMs continue to examine these returns, but at the same time, they are constantly upgrading the test gear to make measurements and analyses more accurate and faster.

The example shows only two lobes on a defective rotor, but the rotor assembly produces some extremely high frequencies as well as multiple harmonics. The software package allows this image to rotate on the computer screen so investigators can examine it from several different angles.

Recently, however, Robert Bosch began looking for a better way to measure brake geometry and to lower inspection costs by increasing brake component throughput without compromising the accuracy and thoroughness of the inspection. Robert McNaughton, Manager, Metrology/Technical Services, Bosch, contacted Mark Malburg of Digital Metrology Solutions, a metrology consultant dealing with surface and form measurement and analysis, to assist in the design and development of a new testing and analysis system. The initial system was intended to test a single wheel on a vehicle—without removing components. Malburg recommended the IOtech WaveBook and non-contact displacement capacitive sensors purchased by Bosch along with a custom signal processor package to replace the existing test and measurement system. Signal conditioners scale and filter the output of the capacitive sensors to provide the WaveBook with noise-free voltage signals. Malburg used the IOtech software drivers but wrote the math-intensive instrumentation software in Visual C++ that was necessary to make measurements, analyze the data, and control the system.

The new system eliminates the need for speed control, and it can measure not only roundness, flatness, and runout, but also thickness variation, parallelism, and other characteristics of the brake motor and caliper assembly while rotating and driving on the road, in a chassis dynamometer, or in a standard dynamometer.

The system block diagram for dynamic brake testing of warranty returned parts is relatively simple and perhaps belies the importance and significance of the test. However, the IOtech WaveBook-based measurement system equipped with Digital Metrology’s custom analysis software can detect displacements in microns and sort out the few truly defective parts from the greater number of misdiagnosed parts.

Before the new system was in place, one to six weeks (or more) was required to perform an in-depth analysis of rotor runout and thickness variation in the OEM or supplier inspection lab. Once the wheels and rotors are removed from the vehicle, the OEM or supplier misses a critical data point. The best-case scenario is to measure the rotor runout and thickness variation in the “as received from the customer state.” In some cases, removed and shipped rotors were received damaged or had rust build-up, so the true condition could not be measured with a high degree of certainty. But with the new system, all the data in the field, assembly plant, test track, or dealership can be collected, recorded, and analyzed within about five minutes per wheel corner. The data then can be sent via the Internet or saved on a memory stick. Only one-third the time is now required to collect, distribute, and analyze the data to satisfy the complaint.

Further studies include analyzing different frequencies and harmonics generated by the shape of the brake. Investigators can detect discs that have two or three lobes just from the frequency it generates. As a result, several important facts were learned. For example, when a caliper is tilted, high spots wear in on both sides of the rotor, which eventually produces two thin spots as well. This combination accounts for an extremely shaky ride. “Now researchers understand the relationship between caliper orientation and thickness variations,” said Malburg. “The biggest issue is the thickness variations because the caliper can slide back and forth a little to accommodate a slightly warped disc. But eventually it wears to the point where the disc has thick and thin spots that produce the shaky ride.”

The test rig graduated from the hoist and garage setting to being used on a vehicle running over the road, a bigger challenge. But because the WaveBook can operate from a car battery, this portability allowed McNaughton to install the data-acquisition system relatively easily. Moreover, the advantage of eliminating speed from the equation let researchers measure actual on-road variables on all wheels simultaneously with the WaveBook expansion options. “Because all wheels don’t turn at the same speed while cornering, for example, the challenge was finding the relationship between the wheel position and the displacements. Picking up the speed of an individual wheel allows me to interpolate backward to determine the values of the displacements at the actual positions on the rotor,” said Malburg.