Stepper MotorsStepper Motors

Stepper MotorStepper Motor

A A stepper motor is a motor that moves in stepsstepper motor is a motor that moves in steps ..

A A stepper motor is 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…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 operationStepper motor operation

Step angle Step angle ββangle through which motor shaft rotates for each angle through which motor shaft rotates for each

command pulsecommand pulse ββ = (Ns~Nr)*360 / (Ns*Nr) = (Ns~Nr)*360 / (Ns*Nr)

or or ββ = 360 / (m*Nr) = 360 / (m*Nr)where Ns=No. of stator poles (teeth)where Ns=No. of stator poles (teeth)

Nr=No. of rotor poles (teeth)Nr=No. of rotor poles (teeth)m=No. of phasesm=No. of phases

Displacement = Displacement = ββ * No of control pulses * No of control pulses

(Rotation) (Rotation) Speed =(60*f) / SRSpeed =(60*f) / SR rpm rpm

step resolutionstep resolution SR=360 / SR=360 / ββ step/rev step/rev

Comparison to Servo Motors

Servo MotorServo Motor Stepper MotorStepper Motor

Position ControlPosition Control require some form of require some form of analog feedbackanalog feedback not subject to this not subject to this problemproblem

Position ControlPosition Control often open loopoften open loop

at high accelerations at high accelerations with variable loads, all with variable loads, all rotor information is lost, rotor information is lost, and closed loop is and closed loop is required for accurate required for accurate controlcontrol

Types of Stepper MotorsTypes of Stepper Motors

Permanent MagnetPermanent Magnet

Variable ReluctanceVariable Reluctance

HybridHybrid

Permanent Magnet Stepper motorsPermanent Magnet Stepper motors

Rotor is made of permanent magnetic Material

No teeth on rotor No freewheeling 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 Permanent Magnet Stepper motors…Stepper motors…

Full SteppingFull Stepping

A-CA-C 0 0˚̊B-DB-D 90˚ 90˚

Half steppingHalf stepping

A-CA-C 0 0˚̊A-C,B-D 45˚A-C,B-D 45˚B-DB-D 90˚ 90˚

Variable Reluctance Variable Reluctance Stepper motors Stepper motors

No permanent magnetNo permanent magnetFree wheeling Free wheeling

possiblepossible

A - 0A - 0˚̊

C - 15˚C - 15˚

B - 30˚B - 30˚

Variable Reluctance Variable Reluctance Stepper motors….Stepper motors….

less torqueless torque

Hybrid Stepper motors 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 motorsHybrid Stepper motors

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 ModeFull step Mode

Full step ModeFull step Mode

Animation shows how coils are energized for full steps

Half Step mode

Half Step modeHalf Step mode

the second phase is turned on before the first the second phase is turned on before the first phase is turned off. Thus, sometimes both phase is turned off. Thus, sometimes both phases are energized at the same time. phases are energized at the same time.

During the half-steps the rotor is held in between During the half-steps the rotor is held in between the two full-step positions.the two full-step positions.

A half-step motor has twice the resolution of a A half-step motor has twice the resolution of a full step motor. full step motor.

Fast responseFast response Absence of detent torqueAbsence of detent torque

Half Step modeHalf Step mode

Micro Stepping ModeMicro Stepping Mode

Smooth and low speed operation with high Smooth and low speed operation with high resolutionresolution

Printing, photo type settingPrinting, photo type setting

A – constant B –increased by very small A – constant B –increased by very small

incrementsincrements till 1 or max.till 1 or max.

A -decreased by very A -decreased by very B – constant B – constant

small stepssmall steps till 0 or mintill 0 or min

AdvantagesAdvantages

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

ApplicationsApplications

Precise positioning and speed controlPrecise positioning and speed controlOpen loop position controlOpen loop position controlTorque : 1Torque : 1µNm to 40 NmµNm to 40 NmPower : 1w to 2500wPower : 1w to 2500w

ApplicationsApplications

Film DriveFilm DriveOptical ScannerOptical ScannerPrintersPrintersATM MachinesATM MachinesComputer peripheralsComputer peripherals IC fabricationIC fabrication

ApplicationsApplications

robotics

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