Stepper Motors

Stepper Motors

Stepper Motor A stepper motor is a motor that moves in steps.

A stepper motor is one whose shaft moves in precise angular increments for each electrical input. Typical step sizes are 2°, 2.5° ,3°, 7.5° , 15°, 30°.

Stepper Motor…  salient poles on both stator and rotor. The stator poles are wound with coils which carry relatively long pulses of current. The rotor poles are made from either magnetically soft or hard iron.  Ns always different from Nr.

Stepper motor operation  Step angle β angle through which motor shaft rotates for each command pulse β = (Ns~Nr)*360 / (Ns*Nr) or β = 360 / (m*Nr) where Ns=No. of stator poles (teeth) Nr=No. of rotor poles (teeth) m=No. of phases  Displacement = β * No of control pulses (Rotation) Speed =(60*f) / SR rpm step resolution SR=360 / β step/rev

Comparison to Servo Motors Servo Motor Position Control  require some form of analog feedback  not subject to this problem

Stepper Motor Position Control often open loop at high accelerations with variable loads, all rotor information is lost, and closed loop is required for accurate control

Types of Stepper Motors  Permanent Magnet  Variable Reluctance Hybrid

Permanent Magnet Stepper motors Rotor is made of permanent magnetic Material No teeth on rotor No freewheeling more torque during rotation Less acceleration Difficult to make small step pm rotors with a large number of poles. i.e. step sizes are limited to 30° - 90°.

Permanent Magnet Stepper motors…  Full Stepping A-C B-D

0˚ 90˚

 Half stepping A-C 0˚ A-C,B-D 45˚ B-D 90˚

Variable Reluctance Stepper motors  No permanent magnet  Free wheeling possible 

A - 0˚ C - 15˚ B - 30˚

Variable Reluctance Stepper motors…. less torque

Hybrid Stepper motors  A hybrid stepping motor has characteristics of both PM and VR motors the rotor is a permanent magnet but has blades like VR motors Rotor is magnetised axially to create the poles motor has very high torque and very small, precise step increments

Hybrid Stepper motors

Hybrid Stepper motors…. 200 rotor teeth and rotate at 1.80 step angles. Also available in 0.9ºand 3.6º step angle configurations. used in a wide variety of industrial applications.

Full step Mode • One phase is energized at a time

Full step Mode

Full step Mode

Animation shows how coils are energized for full steps

Half Step mode

Half Step mode the second phase is turned on before the first phase is turned off. Thus, sometimes both phases are energized at the same time. During the half-steps the rotor is held in between the two full-step positions. A half-step motor has twice the resolution of a full step motor. Fast response Absence of detent torque

Half Step mode

Micro Stepping Mode Smooth and low speed operation with high resolution Printing, photo type setting A – constant A -decreased by very small steps till 0 or min

B –increased by very small increments till 1 or max. B – constant

Advantages  They produce the highest torque at low speeds  holding torque (not present in DC motors)  Rotor has no winding, commutator or brushes – quite, robust and reliable operation

Applications  Precise positioning and speed control  Open loop position control  Torque : 1µNm to 40 Nm  Power : 1w to 2500w

Applications  Film Drive  Optical Scanner  Printers  ATM Machines  Computer peripherals  IC fabrication

Applications  robotics      

digital control systems tape drivers disk drives tool positioning process control X-Y recorders