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Low-frequency vibrations in ServoExpert are assumed to be repeatable oscillations with a frequency proportional to the motor speed. They often cause a pronounced hum, especially as the motor turns faster. Sources of vibration can be mechanical, magnetic, or from feedback devices.
The key source of low-frequency vibrations are from errors in the motor feedback device. Resolver-based and low-cost encoders systems are particularly subject to this type of error, especially when the load inertia is very high or when servo gains are high. The position error from a resolver creates velocity error which, in turn, creates current ripple. Note that this problem occurs independently of whether the motor is in velocity mode or position mode (however, it should be eliminted if the system is put in torque mode with no external velocity or position loop). This current ripple creates torque ripple in the motor which often manifests itself as excessive vibration. This is the problem rated in this page.
There are other sources of vibration. These include motor cogging, where the magnetic construction of a motor creates torque ripple from the motor. This ripple is generally small, especially in motors designed for servo applications. For example, Kollmorgen's Goldline XT motor is one of the lowest cogging motors available for commercial applications. Another source of ripple is current sensor offsets in brushless motors. These offsets cause ripple, althought the amount is usually small. Transmission components, such as gear boxes and couplings, can also generate vibration because of dimensional tolerances such as eccentricity. Finally, six-step commutation, a low-cost method of controlling brushless motors, often generates torque harmonics of the electrical frequency of the motor. Note that these causes are unrelated to the motor sensor and will be present even when the system is in torque mode. However, the remainder of this page focuses on the vibration induced by position sensor error.
Low-frequency vibration that is induced by position sensor error can be verified by increasing the high-frequency servo gain. Higher gains should increase the amount of vibration. Alternatively, lower gains should decrease it. For PI or PDFF (PI+) velocity loops (used with or without a position loop), the high-frequency servo gain is the velocity loop proportional gain. For PID position loop, increase the P, I, and D gains simultaniously. If the velocity ripple goes down while the vibration goes up, the problem may be due to position sensor error. Check the frequency of the vibration. Resolvers typically cause ripple at the 2nd, 4th, 6th, and 8th harmonic of the resolver error. If the vibration frequency is not an even harmonic of the motor speed, the problem is probably unrealated to position-sensor error.
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