Stepper motor and servo motor principle

Stepper motor

A stepper motor is a brushless motor that rotates in fixed angular steps, one step for each input pulse. A drive circuit energises the stator phases in sequence, and the rotor turns to the minimum-reluctance (or magnetic alignment) position of each energised phase.

• Phases A, B, C are energised in order. The rotor moves through the step angle at each switching:

$$\beta = \frac{360^\circ}{m\,N_r}$$

($m$ = number of phases, $N_r$ = number of rotor teeth; for $m=3$, $N_r=4$, $\beta = 30^\circ$).

• Total rotation $=$ number of pulses $\times\ \beta$; speed (rpm) $= \frac{\beta f_p}{6}$ for pulse frequency $f_p$ in Hz.
• Reversing the sequence reverses the direction. It works in open loop and holds its position while a phase is kept energised.

 Pulse train -> [Driver / sequencer] -> A,B,C phases
                                          |
                                      [Stepper] -> step by step

Use: printers, plotters, CNC, robots.

Servo motor

A servo motor is a motor used in a closed-loop system. A sensor measures the shaft position (or speed), which is compared with the reference; the difference (error) is amplified and fed to the control winding (AC servo: two-phase induction motor with a high-resistance rotor; DC servo: armature or field controlled).

• The torque is proportional to the control voltage. As the error reduces, the control voltage falls, and the motor stops when the position matches the reference.
• The direction depends on the polarity or phase of the control voltage.
• The rotor has a small diameter and a long length, giving low inertia and fast response.

 Ref -> (+) -> [Amplifier] -> [Servo motor] -> load
         ^ -                                   |
         +------- [Sensor / tacho] <-----------+