Servos & Steppers: Position Without Guesswork
Two opposite answers to 'go to this angle': the servo closes a loop and fights for its position; the stepper counts perfect steps — until it silently loses count.
Builds on: 1.1 DC Motors: Fast, Weak and Hungry
Same idea, other course:Spark: driving them from firmware
The servo: a control loop in a box
A hobby servo is a whole robotics course in a plastic case: DC motor, gearbox, a potentiometer watching the output shaft, and a tiny controller comparing that measured angle with the one you asked for. Command 90° and it fights its way there — push it off with a finger and it pushes back. That is sense–think–act, purchased whole.
The classic limits: about 180° of travel, and no report back — you command, you trust. Its grown-up sibling, the smart bus servo, fixes exactly that: servos daisy-chained on one cable, each with an address, each answering with its true angle, speed and current. Ask a modern low-cost robot arm what it is, and the honest answer is six smart servos and some bolts.
The stepper: position by arithmetic
A stepper motor takes the opposite bet. Its rotor clicks between magnetic detents — typically 200 per revolution, 1.8° each. Pulse it 50 times and it has turned exactly 90°. No sensor, no loop: position is just counting.
The fine print is brutal in a specific way: overload a stepper — too much torque, too fast a start — and it skips steps silently. The electronics keep counting pulses that the rotor never made. Nothing errors; the robot’s idea of where it is simply becomes fiction. This is why 3D printers re-home against limit switches: to reconcile the count with reality.
Choosing between them
- Servo: knows where it is (internally), fights disturbances, limited travel. Joints, grippers, steering.
- Stepper: unlimited travel, exact and repeatable while unloaded within limits, but trusts arithmetic over truth. Printers, plotters, camera sliders.
- Smart servo: the feedback of a servo plus the honesty of a report. Robot arms live here now.
The stepper is the course’s first pure open-loop actor: it acts without checking. The servo is closed-loop: it measures and corrects. Keep the pair in mind as characters — Unit 3 is the theory of why the closed one wins whenever the world pushes back.
⚗️ Lab — The Position Duel
A servo and a stepper, side by side, both commanded to the same angles — with a disturbance lever that shoves both shafts.
- Send both to 120°. Identical arrival — arithmetic and feedback agree in a quiet world.
- Now shove: the servo snaps back; the stepper stays pushed and its display quietly lies.
- Raise the load until the stepper skips, then command 0° — the servo returns true, the stepper returns to a fiction. Re-home to reconcile.
Work it by hand — 0 / 2
No multiple choice here: compute the value and type it. Suffixes like 2.5k or 20m are understood; answers within ±2% count.