Stepper speed control with a TMC2209 and a knob on an Arduino Uno

// Stepper motor speed control with a knob, on a TMC2209: the middle of the
// knob is stop, either side turns the NEMA 17 that way, faster further out.
//
//   A0         the potentiometer's wiper (its ends on 5 V and GND)
//   TMC2209    STEP -> pin 2, DIR -> pin 3, EN -> pin 4
//              VIO -> 5V, GND -> GND, VM -> 12 V
//              M1A, M1B -> one coil, M2A, M2B -> the other
//
// The TMC2209 is the quiet driver in most 3D printers. Wired like this, with
// MS1, MS2 and SPREAD left open, it runs StealthChop at 1/8 step: 200 full
// steps a turn times 8 is 1600 pulses a turn, and it interpolates each one to
// 1/256 inside. AccelStepper only has to pulse STEP and set DIR.
//
// EN is active low, so setPinsInverted(false, false, true) tells AccelStepper
// to pull it low to enable. With the knob in the middle for two seconds the
// sketch disables the outputs, so the motor and driver stop drawing holding
// current and run cool; the shaft turns freely then.
//
// A stepper cannot jump from stopped to full speed: past a few hundred steps a
// second it misses steps and stalls with a buzz. So the speed follows the knob
// at ACCEL steps a second every second, and a reversal slows through zero.
//
// Press Run, then turn the knob either side of the middle.

#include <AccelStepper.h>

const int KNOB = A0;
const int STEP_PIN = 2;
const int DIR_PIN = 3;
const int EN_PIN = 4;

const float MAX_SPEED = 3200;  // steps a second: 2 turns a second at 1/8 step
const float ACCEL = 4000;      // steps a second, per second
const int DEAD = 40;           // counts either side of 512

AccelStepper stepper(AccelStepper::DRIVER, STEP_PIN, DIR_PIN);

float speed = 0;
unsigned long lastRead = 0;
unsigned long stoppedAt = 0;
bool enabled = true;
int printed = 9999;

float target() {
  int offset = analogRead(KNOB) - 512;
  if (abs(offset) <= DEAD) return 0;
  float s = (abs(offset) - DEAD) * MAX_SPEED / (511 - DEAD);
  if (s > MAX_SPEED) s = MAX_SPEED;
  return offset > 0 ? s : -s;
}

void setup() {
  Serial.begin(9600);
  stepper.setEnablePin(EN_PIN);
  stepper.setPinsInverted(false, false, true);  // EN is active low
  stepper.setMaxSpeed(MAX_SPEED);
  stepper.enableOutputs();
  Serial.println("TMC2209 speed knob: middle is stop, turn either way");
}

void loop() {
  // Step as often as possible; read the knob 20 times a second.
  stepper.runSpeed();
  unsigned long now = millis();
  if (now - lastRead < 50) return;
  lastRead = now;

  float want = target();
  float most = ACCEL * 0.05;  // the change allowed in 50 ms
  if (want > speed) speed = min(speed + most, want);
  if (want < speed) speed = max(speed - most, want);
  stepper.setSpeed(speed);

  if (speed == 0 && want == 0) {
    if (enabled && now - stoppedAt > 2000) {
      stepper.disableOutputs();
      enabled = false;
      Serial.println("stopped: outputs off");
    }
  } else {
    stoppedAt = now;
    if (!enabled) {
      stepper.enableOutputs();
      enabled = true;
    }
  }

  int rpm = (int)(speed * 60 / 1600);
  if (rpm != printed && enabled) {
    printed = rpm;
    Serial.print(rpm);
    Serial.println(" rpm");
  }
}