Linear actuator, 12 V

A 12 V electric linear actuator: a motor and worm gear push a rod out and pull it back, and a limit switch stops it at each end.

Aliases: linear actuator, electric actuator, push rod actuator, PA-14. Two wires and no electronics: the polarity is the direction, so it needs an H bridge such as an L298N or an IBT-2, or two relays.

Pins

Pin	What it does
M+	Positive on M+ extends the rod (the red wire).
M-	Positive on M- retracts it.

The limit switches

At each end of the travel a switch in series with the motor opens, and a diode across it lets current through the other way only. So at full extension, "out" does nothing at all (no current flows and the rod stays put), but "in" runs at once, through the diode, until the rod leaves the switch. The same at full retraction.

That is why a sketch can simply drive it out for longer than the stroke takes and let it stop itself, and why the current drops to zero at the end, which a current sensor such as an ACS712 can see.

const int IN1 = 8, IN2 = 9;    // an L298N's inputs, ENA jumpered

void setup() {
  pinMode(IN1, OUTPUT);
  pinMode(IN2, OUTPUT);
}

void loop() {
  digitalWrite(IN1, HIGH); digitalWrite(IN2, LOW);   // out
  delay(10000);                                     // longer than the stroke takes
  digitalWrite(IN1, LOW); digitalWrite(IN2, HIGH);   // in
  delay(10000);
}

Speed and current

The defaults follow Progressive Automations' PA-14 in its 150 lb (668 N) version: 12.7 mm/s (0.5 in/s) with no load at 12 V, a 100 mm (4 in) stroke and limit switches inside. The motor is the dcmotor model, so the speed follows the voltage: 6 V moves it at well under half speed. The winding (2.4 Ω) and the back EMF (0.9) are assumptions that give 5 A stalled and about 0.5 A running free at 12 V, inside the PA-14's 5 A at full load.

Properties

Property	What it sets
voltage	The rated voltage, 12 V.
stroke	The travel in millimeters, 100.
speed	The rod's speed at full motor speed, mm/s.
resistance, back_emf, inertia	The motor, as on a dcmotor.
position	Where the rod starts, in mm.

The probe is the rod's extension in millimeters.

What is not simulated

A load on the rod (a real one slows down and draws more as it pushes), the duty cycle and heat, the motor's inductive kick, and position feedback: this part has no potentiometer or Hall sensor output.