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  • Users: jblc
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  1. jblc

    Feedforward: why does feedBACK reduce itself?

    "Feedback is negative!" Yeah definitely :D Not sure what this is referencing, but that's certainly clear. It should be obvious that the output will be lower because we are not adding Kp*tar)vel or Kd*tar_acc" Are you responding to my question on why your I-PD is more poor tracking? LQI has...
  2. jblc

    Feedforward: why does feedBACK reduce itself?

    Thanks PNachtwey :) The closed loop gains can interfere with the feed forwards if the closed loop gain is only used in the feed back path ( act only on changes in the actual position instead of the error ). The difference between PID and the PI-D is that the D term is in the feed back path...
  3. jblc

    Feedforward: why does feedBACK reduce itself?

    Thanks for the reply PNachtwey. What you mentioned is already clear, but I think my issue is more related to the full state feedback (per my last post), which acts differently than PID. "The closed loop gains should not affect the feed forwards nor should the feed forwards affect the closed...
  4. jblc

    Feedforward: why does feedBACK reduce itself?

    Ah -- a clue finally occurred to me. There is actually a difference between PID and LQI. They both have equivalent-type correction terms (proportional, integral, derivative), but LQI does NOT use error as inputs to P and D: LQI uses state feedback (which could be from sensors, or estimators). It...
  5. jblc

    Feedforward: why does feedBACK reduce itself?

    1) "If FF is working correctly there should be no divergence between the setpoint and the output for FB to do anything with." Thanks 3DDave, and agreed. This is disturbance FF though, ie no setpoint change. FF is the cmd input to the plant. In this case (as in the image) the disturbance Gd acts...
  6. jblc

    Feedforward: why does feedBACK reduce itself?

    Thanks xnuke :), and I agree with that all as well -- typically I think of it as "FB is only needed because FF isn't perfect". But, that's not what I'm seeing in this instance: Concrete example, with sims to support: - FF sensor acts instantly to disturbance, and the plant is commanded to...
  7. jblc

    Feedforward: why does feedBACK reduce itself?

    FF alone is not a perfect correction (hence having both FF and FB), and FB alone works better than FF (as expected). My expectation, though, is that FF can fill the gaps left by FB, but that doesn't seem to be the case, since FB throttles back when FF is active.
  8. jblc

    Feedforward: why does feedBACK reduce itself?

    Thanks. Agreed that any deriv term could be countering contributions. In this case, FB is tuned correctly though, since it's a model-based controller that tries to keep the system behavior at the input specs (a higher-level spec like response time constant), and I can verify that the response to...
  9. jblc

    Feedforward: why does feedBACK reduce itself?

    "How does feed forward "work fine by itself"? Feed forward does not close the control loop (thus open loop)." Feedforward is still a control strategy, it's just not closing a loop -- it's open-loop as you said. When i say it works fine, i mean that it is effective (just not as effective as FB in...
  10. jblc

    Feedforward: why does feedBACK reduce itself?

    This questions related to feedforward (FF) and feedback (FB) interactions. Say there's a system who's goal is to keep a car centered in a lane. Say it includes a FF sensor (like detecting sideways wind hitting the car, or an inclinometer to detect the sideways slope of the road), and a FB sensor...
  11. jblc

    Switching between several controllers, depending on SP error.

    I'm curious about your preferred way to analyze and develop a hybrid feedback control system, where the controller is actually made up of a few unconnected controllers that you switch between. The loop is a standard feedback loop: SP --> SPerror --> controller --> plant --> PV, and PV is fed...
  12. jblc

    Coupled Setpoints ... how controllable?

    Thanks for the reply. Yes, as you surmised this was an example. The main points I was trying to raise through the example -- vs the practicalities of sensing, or force control, or control bandwidth and such -- is the general concept of a setpoint being changed directly at each time step by what...
  13. jblc

    Coupled Setpoints ... how controllable?

    Thank you, I appreciate your being willing to lend help. Here are some further details. First I'm trying to find out what this specific general class of coupled problems is called, where a reference setpoint is directly determined by one of the states of the system. Second, I'd like to...
  14. jblc

    Coupled Setpoints ... how controllable?

    If the reference tracking setpoint is directly coupled to the plant's state, is the system always nonlinear? What is such a coupled system called? 1) Say you have a fast dynamic system, and the actuator's reference tracking target is given by its position. This is directly coupled. Eg at 0...
  15. jblc

    Discretized transfer function feedback implementation

    Yep, exactly on the time. I tested various sample periods, from 1e-4 to 1e-7. The results are almost the same, so I stuck with 1e-4 sec for the numbers given in the post. I've checked the bode plots and they look fine, when overlaid on top of the continuous bode. You're right, good call. At...
  16. jblc

    Discretized transfer function feedback implementation

    Thanks. (That is a very nice and clearly written document, by the way!). Yes, this is a simple spring-damper. I understand in-depth PID (and more sophisticated controllers). I recognize that two gains are needed for two complex poles given the oscillatory and damped nature of the system, but my...
  17. jblc

    Discretized transfer function feedback implementation

    Thanks for the reply. Yes, that's right, K is the feedback gain. I don't believe K is too large: the settling value changes as K varies, but the waveform never actually changes form; it always oscillates. Agreed, small enough K should cause the closed-loop system response to be massively...
  18. jblc

    Discretized transfer function feedback implementation

    I'm applying feedback to a discretized time-series transfer function in software, to simulate how a physical system moves (there are other ways, I'm doing it this way for a reason). The step response of the derived time-series system IS what i'd expect when I match it with a matlab "step"...
  19. jblc

    DC model motor and viscous damping: behavior inconsistent?

    Thank you for the response, that clarifies that question! (Together with xnuke's reply).
  20. jblc

    DC model motor and viscous damping: behavior inconsistent?

    Here's a simpler follow-up question instead, related to the poles of a transfer function: Between 1/(s+2) --> (1/2) / [(1/2)s + 1] and 1/(s+4) --> (1/4) / [(1/4)s + 1], the 2nd function has the faster pole, since T = 1/-p. Here p is the pole, and T the time constant. For the 2nd function, T =...

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