I have a question regarding the design of anti-aliasing filter in feedback control system.
The application is a mechanical control system that the controller compute torque command with position and torque sensor data. The ADC is neccessary for torque data acquisition.
Anti-aliasing filter must be necessary for the ADC. My question is regarding the design of the filter. Here are some specifications regarding the signal.
- disturbance torque 1: < 0.5 Hz
- disturbance torque 2: < 2 Hz
- disturbance torque 3: < 180 Hz
- motor torque: < 1000 Hz (control loop frequency)
The torque sensor basically sense the motor torque + disturbance torque. The frequency of the control loop is 1000 Hz, which means the torque from the motor have 0~1000Hz information.
Clearly the data rate of the ADC should be 1000 Hz, since the controller loop time is 1000 Hz. According the Nyquist Theorem, all the signal above 500 Hz would cause aliasing and degrade the signal quality, right? This means in order to prevent the aliasing, the stop band of anti-aliasing filter should start from 500 Hz.
Now, considering the practical filter system, setting the stop band -40 dB. By employing 6th order filter one could get -3dB pass band around 220Hz.
Is it natural to design anti-aliasing filter like this, only utilizing data about 1/5th of controller band width, or is there any misconceptions? Please enlighten me.