STEPPER MOTORS Name: Mr.R.Anandaraj Designation: Associate. Professor Department: Electrical and Electronics Engineering Subject code :EC 6252 Year: II ECE A & B Unit : IV

Stepper Motor Basics S N Stator : made out of coils of wire called “winding” Rotor: magnet rotates on bearings inside the stator Direct control of rotor position (no sensing needed) May oscillate around a desired orientation (resonance at low speeds) Low resolution printers computer drives S N Electromagnet stator rotor N S Current switch in winding ==>Magnetic force ==>hold the rotor in a position

Increased Resolution Half stepping S S N N angle torque

Increased Resolution More teeth on rotor or stator Half stepping S S N N

Increased Resolution More teeth on rotor or stator Half stepping S S N N

How to Control? Step Table Step Red Blue Yellow White Lead Wire Configuration Clockwise Facing Mounting End Increase the frequency of the steps => continuous motion Each step, like the second hand of a clock => tick, tick

Motoring along... direct control of position precise positioning (The amount of rotational movement per step depends on the construction of the motor) Easy to Control under-damping leads to oscillation at low speeds torque is lower at high speeds than the primary alternative…

DC motors -- exposed !

Position Sensors  Optical Encoders  Relative position  Absolute position  Other Sensors  Resolver  Potentiometer

Optical Encoders Relative position - direction - resolution grating light emitter light sensor decode circuitry

Ideal Optical Encoders Relative position mask/diffuser grating light emitter light sensor decode circuitry Real A diffuser tends to smooth these signals

Optical Encoders Relative position - direction - resolution grating light emitter light sensor decode circuitry

Optical Encoders Relative position - direction - resolution grating light emitter light sensor decode circuitry A B A B A lags B

Optical Encoders Relative position - direction - resolution grating light emitter light sensor decode circuitry A B A leads B Phase lag between A and B is 90 degree

Optical Encoders Detecting absolute position wires ?

Other Sensors Resolver = driving a stepper motor Potentiometer = varying resistance

Control What you want to control = what you can control For DC motors: speed voltage N S NS V  V e back emf R windings’ resistance e is a voltage generated by the rotor windings cutting the magnetic field emf: electromagnetic force Control: getting motors to do what you want them to

Controlling speed with voltage DC motor model V e R The back emf depends only on the motor speed. The motor’s torque depends only on the current, I. e = k e   = k  I

k  k e Controlling speed with voltage DC motor model V e R The back emf depends only on the motor speed. The motor’s torque depends only on the current, I. e = k e   = k  I Consider this circuit’s V: V = IR + e I stall = V/R current when motor is stalled speed = 0 torque = max How is V related to  V = + k e   R k  - or -  = -  + R  k e V Speed is proportional to voltage.

speed vs. torque torque  speed   k e V at a fixed voltage  R k  V max torque when stalled no torque at max speed

speed vs. torque torque  speed   k e V at a fixed voltage  R k  V stall torque no torque at max speed Linear mechanical power P m = F  v Rotational version of P m = 

speed vs. torque torque  speed   k e V at a fixed voltage  R k  V stall torque max speed Linear mechanical power P m = F  v Rotational version of P m =  power output speed vs. torque

torque  speed   k e V  R k  V power output speed vs. torque gasoline engine max speed stall torque

Back to control Basic input / output relationship: How to change the voltage? We want a particular motor speed . We can control the voltage applied V. V = + k e   R k  V is usually controlled via PWM -- “pulse width modulation”

PWM  PWM -- “pulse width modulation  Duty cycle:  The ratio of the “On time” and the “Off time” in one cycle  Determines the fractional amount of full power delivered to the motor