Laser Vibrometry: DC Motor Applications

Noise measurement in micro electromechanical drive systems

Drive systems that have damaged or deformed components, such as gears, will generate undesirable noises during operation. Therefore, it is important to detect them during production so they can be dealt with accordingly. Laser vibrometry has proved to be an optimum measurement technique for this task, enabling non-contact, automatic and reliable quality inspection.

The demands for highest quality and reliability require ever more time-intensive procedures for checking quality. It is often essential to subject each individual product to an additional final check before delivery in order to ensure an error rate of zero as far as possible. Non-contact optical measurement procedures distinguished by useful features such as absence of interaction, accuracy and, especially, their advantages with regard to simple adaptation and stability are being used
increasingly for this purpose.

Studies were carried out first of all on the feasibility of automated quality monitoring regarding unwanted operational noise for a customized drive unit from FAULHABER which includes two motors, each with two drive units that have three gears. Even small deviations of the gear geometries produce typical noise characteristics that can be subjectively described as “rattling”, “whimpering”, “clicking” or “chirping” and that it should be possible to automatically detect in a test stand. The aim was to automatically and reliably analyze the noise characteristics described above at a throughput of 10,000 to 16,000 parts per day with a measurement duration of less than 50 s per test piece in a cost-effective manner. The manual noise test stands constructed in an initial development phase were to be designed in such a way that integration in the automatic production line would be possible. In a trial run, the laser vibration meter proved to be the best measurement technique compared with airborne sound measurement and the acceleration transducer.

Construction of the noise test stand

The test stand allows vibration-decoupled mounting of the test piece, where the vibration meter measurement head and the test piece holder are mounted on a common base plate. The vibration measurement is performed at suitable, empirically determined location of the drive at the optimum working distance of the laser. The evaluation software performs the recording and evaluation of the vibration meter data as well as the motor current for the calculation of the system rpm. From the two signals, the energy bands are analyzed using frequency analysis in the vicinity of the 1st and 2nd order of rotation. The classification of the test pieces is then carried out by comparison with a threshold value.

The system reliably identifies all noise characteristics; a test time of 17 seconds, including handling, was achieved. It was also possible to realize the necessary throughput of up to 16,000 drives per day with 8 test positions. After the positive experiences obtained with the manual noise test stand, testing within an automatic production line is now in preparation with the aim of subsequent integration in an existing assembly line. Thus, the micro drive systems can be identified/sorted automatically and reliably in accordance with acoustic standards a further contribution towards ensuring the recognized high quality of FAULHABER products.

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