Hi all,
Imagine that we have a closed loop control system that goes like this:
A/D -> DSP -> Switching Power Amplifier -> Fine Motion Actuator -> Feedback -> A/D (thus completing the loop)
I have a dilemma in understanding here. Assuming that this system was trying to achieve the most precise control of the actuator possible. Obviously, we'd want sensitive feedback, a high resolution A/D, and a well damped system. However, we'd also want lowest noise possible as I see it. However, this can only be achieved, best I know, by providing extensive filtering of unwanted bands (which is A LOT for a slow system like this, I'd wager) and selecting low noise components, low drift components. Not to mention that we'd need to limit bandwidth to avoid A/D aliasing.
But here's a problem that I can't seem to find anyone talking about. Anytime you try to filter (especially if there is a high gain analog input stage from the feedback to the A\D), you get wild open loop phase shifts in the analog circuitry...and often well above the critical 1dB cutoff point. SO...close that control loop and tell me what you think will happen?
Thus the issue. How can one filter out unwanted bands and get away with not having total system oscillations? I'd assume that digital techniques for the DSP would introduce phase shifts as well, so it seems like a serious limitation to any kind of precision control. I would LOVE to be told that I'm wrong about this by someone much wiser than I, or that another way exists.
-Logan Snow
Imagine that we have a closed loop control system that goes like this:
A/D -> DSP -> Switching Power Amplifier -> Fine Motion Actuator -> Feedback -> A/D (thus completing the loop)
I have a dilemma in understanding here. Assuming that this system was trying to achieve the most precise control of the actuator possible. Obviously, we'd want sensitive feedback, a high resolution A/D, and a well damped system. However, we'd also want lowest noise possible as I see it. However, this can only be achieved, best I know, by providing extensive filtering of unwanted bands (which is A LOT for a slow system like this, I'd wager) and selecting low noise components, low drift components. Not to mention that we'd need to limit bandwidth to avoid A/D aliasing.
But here's a problem that I can't seem to find anyone talking about. Anytime you try to filter (especially if there is a high gain analog input stage from the feedback to the A\D), you get wild open loop phase shifts in the analog circuitry...and often well above the critical 1dB cutoff point. SO...close that control loop and tell me what you think will happen?
Thus the issue. How can one filter out unwanted bands and get away with not having total system oscillations? I'd assume that digital techniques for the DSP would introduce phase shifts as well, so it seems like a serious limitation to any kind of precision control. I would LOVE to be told that I'm wrong about this by someone much wiser than I, or that another way exists.
-Logan Snow