FRC | PartQuest™ Explore

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Designer

Description

This example shows a motor speed control loop that uses a PID control strategy (Proportional, Integral and Derivative), with continuous function blocks representing the controller up to the ideal voltage motor drive.

The parameter values of the generic DC motor model were calibrated to represent an FRC - First Robotics Competition CIM Motor, as described in the article “<a href="https://www.systemvision.com/blog/first-robotics-frc-motor-modeling-may-6-2016">First Robotics (FRC) Motor Modeling</a>”. The voltage drive, the motor and the mechanical fan load were “assembled” using a conservation-based modeling approach. This approach to system modeling is much like assembling a hardware prototype, and does not require the user to develop an analytical model of the “plant” being controlled. Both the static and dynamic system characteristics “emerge” naturally from the model, simply because they are connected on the schematic. Additional electrical circuit components, mechanical loads and other “physical” elements can easily be added by simply placing them on the schematic and “wiring” them together. In fact, a more detailed implementation of the motor drive is shown in the companion example, “<a href="https://www.systemvision.com/groups/mikedonnellys-workspace/designs/pid-speed-control-loop-switching">PID Speed Control Loop – Switching</a>”. In that example, a design for the logic and power electronics needed to implement a PWM-based, switched MOSFET H-bridge drive is included.

Because this version simulates very quickly compared to the switching version, it is well suited for early validation of the control strategy and PID tuning, as well as loop stability and frequency response analysis. Note that the P, I and D "gains" for this nominal design were computed using the <a href="https://en.wikipedia.org/wiki/Ziegler%E2%80%93Nichols_method">Ziegler-Nichols</a> classic PID tuning method.

Another version of this design, which simulates quickly but includes the important and practical aspect of a limited drive voltage range, is show in “<a href="https://www.systemvision.com/groups/mikedonnellys-workspace/designs/pid-speed-control-loop-continuous-and-limited">PID Speed Control Loop – Continuous and Limited</a>”. It is instructive to compare the speed step response between that design and this one.

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PID Speed Control Loop - Switching compact

Designer

Description

This example shows a more detailed circuit- and logic-level implementation of the PID Control Loop shown in the companion example, “PID Speed Control Loop – Continuous”. The ideal motor drive block of the “Continuous” version is expanded here, to include both a H-bridge motor drive, and also the digital logic necessary for converting the continuous PID controller output into the desired PWM signals that are distributed to drive the gates of the power MOSFET switches. The MOSFET model was calibrated to represent an IRF3710, using only information published on the manufacturer’s datasheet.The rest of the system, including the PID block-diagram controller, the mechanical fan load and the DC Motor characterized to represent an FRC CIM Motor, are the same as in the Continuous version. While the simulation time for this switching version is significantly longer, more detailed information about practical circuit performance and component sizing is available. For example, the fan speed step response is somewhat different from the conceptual design, because of the losses in the MOSFETs under high current conditions, as well as voltage drop in the battery. Also, information regarding component stress levels within the “datasheet specified” MOSFETs and Diodes is provided.

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