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Closed-Loop Pot Control

The Steervo holds the steering at the commanded angle with a closed-loop position controller: it reads the true steering angle from a potentiometer, compares it to the setpoint from the Teensy, and drives the motor to close the error. The control core (firmware/steervo/lib/steervo/steer_controller.*) is pure C++ under unit test; the ESP32 sketch wires it to the ADC, PWM, and NVS.

The feedback potentiometer

  • Wiper → ESP32 GPIO36 (SENSOR_VP, input-only, ADC1 channel 0).
  • Powered from 3.3 V — not 5 V. The ESP32's ADC pins are not 5 V tolerant; feeding the pot from 5 V would damage the pin at one end of travel.
  • Mechanically coupled through the 50:1 gearbox, so it tracks the real output steering angle and encodes total motor travel. It is therefore also the end-stop guard — never let the motor drive it off either end.
  • Read at 12-bit resolution, 16× oversampled (with a throwaway first sample) to tame the ESP32 SAR ADC's noise.

Calibration

The pot's raw ADC value has no inherent meaning until it is calibrated to physical angles. Calibration is guided over CAN (the STEER_CAL message) and stored in the ESP32's NVS flash, so it survives power cycles and is a one-time step per mechanical setup:

flowchart LR
    enter["ENTER<br/>→ CALIBRATING"] --> center["center by hand<br/>→ MARK_CENTER"]
    center --> left["full left lock<br/>→ MARK_LEFT"]
    left --> right["full right lock<br/>→ MARK_RIGHT"]
    right --> save["SAVE_EXIT<br/>→ persist to NVS, READY"]

SAVE_EXIT assigns the captured left/right stops to ±30.00° (±kSteerRangeCdeg) and commits; ABORT discards. The motor never runs while CALIBRATING. From the Teensy this is driven with STEER CAL ENTER / CENTER / LEFT / RIGHT / SAVE / ABORT (the kart-core must be in SAFE to send them).

The controller: pure proportional

After a long tuning effort, the shipped controller is pure proportional:

Gain Value Note
Kp 0.002 output fraction per centi-degree of error
Ki 0 —
Kd 0 deliberately zero
output limit 0.4 (40 %) conservative clamp on Talon command

This is stable, accurate (~0.15° steady-state), and oscillation-free even at full output — the 50:1 gearbox's own friction supplies the damping a derivative term would normally provide.

Leave Kd at 0

The pot jitters ~±12 cdeg at rest. Differentiating that over a 10 ms tick injects ~±1000–2000 cdeg/s of noise velocity, so any real Kd saturates the output and drives a rail-to-rail limit cycle — that was the entire "oscillation" saga. The PID (pid.h) uses derivative-on-measurement plus a low-pass filter, so it can accept a Kd if you ever add a cleaner velocity source, but with the raw pot, don't.

The setpoint is clamped to the calibrated range minus a soft-limit margin, so the controller never targets the mechanical stop itself.

Live tuning

Over CAN (STEER_CFG, driven from the Teensy as STEER CFG …) you can adjust KP, KI, KD, the output limit (LIM), and the soft-limit margin (MARGIN) while READY or CALIBRATING — never while ACTIVE. These are RAM-only on the Steervo (lost on reboot); good values are baked into the SteerConfig defaults in firmware/steervo/lib/steervo/steer_controller.h.

Wheel → motion direction

Which way a given input deflection turns the steering is a Teensy-side flag, kSteerInvertDirection in firmware/kart-core/src/config.h (currently true on this build — a left wheel turn drove the steering right without it). It is independent of the control-loop sign (motor lead polarity) and the pot calibration; flip only this if the command sense is backwards. If the motor runs away from the target instead, that is the loop sign — swap the Talon's motor leads, don't flip this flag.

Steering mechanicals not in the repo

The gearbox ratio (documented as 50:1), the CIM motor, and the pot's mounting and total mechanical travel are physical facts. Also note the pot pin: the firmware and current bring-up notes use GPIO36, while some older docs say GPIO32 — confirm the physical wiring. See Hardware & Open Items.