We all have been accustomed to see a block diagram for a linear control system, where in the forward path, first sits the controller, then actuator, and then plant.
What if we flip the blocks of actuator and plant. I mean what if we place plant before actuator?
We know that, the closed-loop transfer function does not depend on the consecutive order of blocks in forward path, since: C(s)/R(s)=G(s)/(1+G(s)H(s)), where G(s) is the forward-path gain and H(s) is the feedback-path gain (sensor). The forward path gain could be decomposes as:
Therefore, since these transfer functions in forward-path are multiplied, then it does not differ what are their consecutive order.
Hence, let's flip the plant and actuator blocks in the forward path: G(s)=Gcontroller(s)*Gplant(s)*Gactuator(s).
What happens next? Nothing about the stability, but a great revamp of the required embedded system and controller design. Because instead of measuring plant out-put to send it into the feedback path, we only need to measure actuator out-put which is more convenient through the control engineering practice. In other words, we have mixed controller and plant blocks together, to establish a new controller which physically controls the actuator.
For stabilization, I think it works well. For example, assume an aircraft autopilot (controller). Through this revamped controller design, instead of measuring and regulating aircraft states as pitch-rate, we only measure and regulate actuator (elevator) deflection. If we can stabilize the elevator angular position to stay at zero, then we have stabilized the aircraft at a level-flight (stabilization), since zero elevator-deflection means level flight has been sustained.
For command following (not stabilization), we may set a mathematical trick as:
in this way, even for the case of command following (control) we still need to only measure actuator output.
- in the classical loop, we use actuator model as a software block in the controller simulation and design, while using plant as a hardware to be regulated by physical sensing of its output.
-in my revamped controller setup, we use plant model as a software block in the controller simulation and design, while using actuator as a hardware to be regulated by physical sensing of its output.
It seems physically sensing and regulating actuator dynamics, would be much convenient than physically sensing and regulating plant dynamics.
Please share with me, your comments and ideas about my plan.
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