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Lesson 27

Card 3.5: Feedback Control and PID

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Unit: Control & Actuation Format: Socratic seminar (or self-study path below) Time: 30 to 40 minutes Prerequisites: Card 3.1 (PWM), Card 3.7 helps but is not required

Core Question

Our float needs to stop AT 2.5 meters, not blow past it and bounce. A human can do this by feel. How does a machine do it with only a pressure sensor and a piston?

Resource (read/watch before seminar, ~10 minutes)

  1. Read TUNING.md from the team float repository (github.com/slvusd/float). Focus on what kp, ki, and kd each do.
  2. Watch: any short “PID explained” video of your choice (search “PID controller explained simply”, pick one under 8 minutes). Note one thing that made sense and one thing that did not.

Prep Notes (bring to seminar, this is your artifact)

Answer in writing, a few sentences each:

  1. In your own words: what is “error” in a control system? What is the error if the float is at 1.9 m and the target is 2.5 m?
  2. The P term pushes harder when error is bigger. Why is P alone not enough? What goes wrong?
  3. Our float’s actual starting gains are kp = 40.0, ki = 8.0, kd = 15.0. Pick ONE of these and predict: what would happen at the pool if we doubled it?
  4. Write one honest question you still have. (Real questions score; fake questions are obvious.)

Seminar Questions (teacher facilitates, students carry it)

  • Why does the float overshoot at all? Where does the momentum come from?
  • What does the D term “know” that the P term does not?
  • Why does the code cap the integral term (imax = 0.5)? What disaster is that preventing?
  • Is there a version of this problem in the ROV, not just the float? Where?
  • Cars, thermostats, drones: where else is PID hiding in your life?

Clearing This Card

Choose one:

  • Oral check (90 seconds, during build time): Explain to the teacher what happens to the float if kd = 0, and why. Then: your team doubles kp and the float oscillates violently. What do you adjust and why?
  • Teach-back (5 minutes at sprint standup): Teach your team the three terms using the float as the example. Your team asks one question; you field it.

If You Miss This Class

Do the resource, write the prep notes, then clear via oral check or teach-back as above. Same standard, same artifact. Nothing extra, nothing less.

Why This Matters for Competition

The float’s depth hold is worth real points, and judges will ask WHY your control approach works, not just what it does. A student who owns this card can defend the PID choice at the engineering presentation. Pool tuning of these exact gains is a sprint task, so this lesson feeds directly into build work.

CC BY-NC-SA 4.0.