Measurement of brush wear of starter motors with optical methods - Part I.

Abstract

An important element of the durability test of starter motors is the measurement of brush wear, which must be performed inside the motor through a borehole in the housing. The accuracy of the current mechanical measurements is around 0.1mm, on which we would like to improve orders of magnitude. Out of the available optical methods, the confocal distance sensor and the laser triangulation methods can be suitable for this task. Since the probe can only remain on the motor for the duration of the measurement, its precision reinstallation must be solved for the successive measurements. The design and construction of a special probe holder to ensure this was the most delicate element of this program. In the first part of our paper we also show the results of a test performed with the confocal Omron ZW-S40 probe. During the test (with 50 thousand starts) the device worked flawlessly. Based on many (at least one hundred) post-reinstallation position measurements we state that the guaranteed reinstallation precision is 2-3 µm, and the error caused by temperature uncertainties can be kept below 5 µm (during measurements in the planned time frame).