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Audible noise describes the problems where a servo system emits an unacceptable level of audible noise. Sometimes noise is objectionable because the presense of excessive noise reflects poorly on a product. Other times, the emitted noise may exceed the level called out in a formal specifications. Many types of noise indicate that high-frequency torque perturations are present on the motor output and degrade system accuracy.
Noise can come from at least four sources:
High-frequency resonance produces a pure pitch. High-frequency resonance usually comes on slowly as servo gains are increased. Often, it is a low-amplitude signal which becomes audible only when the machine comes to rest.
Typical scope shot of high-frequency resonance in a nearly noise-free system.
Low-frequency resonance produces a rough, grinding noise. It often comes on very quickly as servo gains are increased, producing loud irritating noises something like a fog horn or a garbage disposal. Often there is no single pitch that can be identified by ear, but you can usually see oscillatory behavior on a scope picture of the velocity feedback.
Typical scope plot of low-frequency resonance in a nearly noise-free system.
Random noise is noise that has no discernible pitch. Random noise can be caused by electrical problems which sometimes sounds like static on a radio or sometimes is described as "bacon frying." Random noise is also caused by mechnical problems such as gearboxes, (especially when the motor runs at high speeds) and brushes in brush-type servo motors.
Low-frequency vibrations are repeatable oscillations with a frequency proportional to the motor speed. They often cause a pronounced hum, especially as the motor turns faster. This may cause "waves" to form on the finish of a machined surface or they may cause uneven coating on a web-handling machine.
