What's relationship motion control Loop and Encoder sampling

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What's relationship motion control Loop and Encoder sampling

Postby mars_bruce_lee » Fri Jul 10, 2015 12:22 am

What's relationship motion control Loop and Encoder sampling rate For a good hydraulic motioncontrol?
For example
Control loop is 10ms,The Encoder sampling rate only is less than 10ms or Smaller。
Someone thinks at least 5 times,In other words Encoder sampling rate is less than 2ms, Is it Right? Why?
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Re: What's relationship motion control Loop and Encoder samp

Postby Jacob Paso » Fri Jul 10, 2015 11:07 am

The sampling rate of digital encoders only needs to equal the motion control loop. However, for good motion control with the RMC (which uses a deterministic loop time), it is very important that the sampling jitter is very low, otherwise when calculating the velocity from the encoder position, there will be a lot of inaccuracy in the velocity. This is a particular issue for magnetostrictive transducers, since the measurement time can be quite long. If the measurement is not done at the same time in the control loop, the jitter can cause large velocity errors, which can be a major problem. It is important that the magnetostrictive transducer always measures at the same point in time relative to the motion controller's loop time. With Start/Stop and PWM signals, the measurement inherently is done at the correct time. With SSI signals on a magnetostrictive transducer, it is important to get the synchronous versions of the transducer so it guarantees low jitter.

For analog sensors, the sampling rate only needs to be as fast as the control needs to be. Usually, for high-performance control, Delta prefers a transducer internal bandwidth of 1 kHz or more, but if the system only needs to move slowly, then it can be less.

For optical encoders, jitter is normally not a problem because the measurement time is so fast. Internal to the encoder, the most recent value is sent when the motion controller requests the data.

This is a different question than how fast the sampling has to be to accurately measure a waveform without any aliasing. The Nyquist Theorem addresses this.

This is also a different question than how high a frequency of a sine wave the controller can achieve. Typically, about sampling 10 points per period will give reasonable control, so at 1 milliseconds, the controller can do a 100 Hz sine wave (not many hydraulic systems can go 100 Hz).
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