Automotive Brake Testing

Application Summary
Automotive braking systems are designed for high reliability and are constantly monitored with data from warranty returns. Because brakes are one of the most safety-critical items on a vehicle, the OEM component manufacturers take these warranty returns seriously. All returned units are thoroughly examined for signs of abnormal wear and manufacturing or material defects, but most often, the parts prove to be flawless: the alleged fault lies elsewhere. 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 returned units that are good is an expensive, time-consuming process. In spite of the high cost, the OEMs continue to examine these returns for such features as run-out and thickness variations, but at the same time, they are constantly upgrading the test gear to make measurements and analyses more accurate and faster.

Potential Solution
For a relatively long while, most tests were conducted on braking systems on a component-by-component basis, which amounted to testing on a relatively static, bench-top system. The typical measurement process involved dial indicators and calipers to check for run-out and thickness. A more expensive dynamic test conducted in the lab involved a precision spindle and a controller to rotate it at a constant speed while measuring run-out with linear variable differential transformers (LVDTs), signal conditioners, and a read-out device that allowed technicians to manually measure and record the roundness, flatness, and thickness of brake discs.

IOtech’s Solution
Recently, however, one OEM brake supplier, Bosch Braking Systems of Farmington, Mich., 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 test. Robert McNaughton, Manager, Metrology/Technical Services, at Bosch, contacted Mark Malburg of Digital Metrology Solutions, Columbus, Indiana, 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 the vehicle – without removing components. Malburg recommended the IOtech WaveBook^™ and a special capacitive sensor 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 or LVDTs 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 take the 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 run out, but also relative thickness, parallelism, and other parameters on a brake disc and caliper assembly while spinning wheel-brake assemblies on the vehicle. The WaveBook records units of measurement that are in the microns.

One problem discovered was tilted parts, which resulted in uneven disc wear that left two thin-to-high spots. These anomalies produce audible oscillations plus multiple harmonics, which are annoying and indicate the malfunction. Another problem came from unbalanced rotors that had to be replaced. Before the IOtech system was used, the brakes were removed from a vehicle, sent to a lab for tests, and the data recorded and analyzed. The process took about a week. With the new system in the field, a vehicle is put on a hoist, the wheel is spun, and the data is recorded and analyzed within about 5 minutes, a remarkable reduction in time, labor, and ultimate cost.

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. This produces an extremely shaky ride. “Now, researchers understand the relation between orientation and thickness variations,” says Malburg. “The biggest issue is the thickness variation, because the caliper can slide back and forth a little to accommodate a slightly warped (potato-chip shaped) 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 the vehicle battery, this portability let McNaughton 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 module. “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,” says Malburg.

“Another significant benefit was derived from the new data acquisition and measurement system at another level,” says McNaughton. “We installed similar systems all the way from the development lab through the production line and to the dealerships to provide us with much more uniform, in-depth data for diagnosis and analysis. Each department can now submit their data in a common format, which helps considerably in decision-making.”

Conclusion
Bosch Braking Systems in cooperation with Digital Metrology Solutions, designed a measurement system for evaluating brake components returned under warranty. The new system using an IOtech WaveBook and capacitive or LVDT displacement sensors moved the static test from the lab bench to a garage hoist or the highway where dynamic tests can be run on actual vehicles. This reduces the test time from weeks to minutes: It eliminates the time and huge expense needed to remove the components from the vehicle, run the bench test, and return the parts to the point of origin. The new test system produces more in-depth data in a uniform format, and is remarkably faster and much more accurate.