STEPPER MOTORS

WHY USE STEPPER MOTORS? Because it is more precise than DC Motors Measured rotation can be held at a particular

position Alignment is much better in differential

drives

WHAT IS A STEPPER MOTOR?  A Stepper motor is a marvel in simplicity. It

has no brushes, or contacts. Basically it's a synchronous motor with the magnetic field electronically switched to rotate the armature magnet around.

A stepper is typically commanded by two digital inputs: a digital pulse train and a direction bit. The stepping drive and motor is used primarily for position control. And unlike all other motor types, stepper motor is moved in "steps" (just one step per one command pulse) and will hold at its present position if no command pulses are received. The frequency of the pulse train controls the velocity of the motor, where the number of pulses determines the length of the move. The direction signal determines in which direction the motor will rotate. For each pulse from the controller, the drive will move the motor "one step" in the direction indicated by the direction command.

High Level Commands

Step

Pulses

Motor

Current

User Interface

Indexer

Driver

Motor

The Indexer (or Controller) is a microprocessor capable of generating step pulses and direction signals for the driver. In addition, the indexer is typically required to perform many other sophisticated command functions.

The Driver (or Amplifier) converts the indexer command signals into the power necessary to energize the motor windings. There are numerous types of drivers, with different current/amperage ratings and construction technology. Not all drivers are suitable to run all motors, so when designing a Motion Control System the driver selection process is critical.

TYPES OF STEPPER MOTOR Unipolar motor: the current only flows in one

direction in the windings of the coils. i.e. the stator poles can only be polarized one way.

BIPOLAR MOTOR The current flows in both direction in the

windings of the coils. i.e. the stator poles can be polarized both way.

COMPARISON

Unipolar•Current flows in one direction•Simple drive circuit•Less Torque•Smooth drive

Bipolar•Current flows in both directions•Complicated drive circuit•High Torque•Not Smooth

UNIPOLAR STEPPER MOTOR Stepper Motor contains a permanent magnet

which is attached to the shaft at centre. This kind of motor has four coils on the four

sides. When energized in the correct sequence, it

causes the permanent magnet attached to the shaft to rotate.

There are 4 basic step sequences. After step 4, the sequence is repeated from step 1 again.

Reversing the order of the steps in a sequence will reverse the direction of rotation.

Actually the stator contains 100 teeth to get a step angle of 360/(2*100)=1.8degrees. But here for the sake of simplicity, we are showing only one tooth of stator. So don’t get misled into believing that it’s step angle is 90 degrees

STEP MODES/MODES OF OPERATION Full step mode

Single Coil Excitation: Only one out of the 4 coils are excited at a given time

Double Coil Excitation: Two adjacent coils are excited at a given time

Half step mode – Hybrid of single and double coil excitation modes

SINGLE COIL EXCITATION MODE

SINGLE COIL EXCITATION MODE

SINGLE COIL EXCITATION MODE

1000

0010

00010100

DOUBLE COIL EXCITATION

DOUBLE COIL EXCITATION MODE

DOUBLE COIL EXCITATION MODE Codes

HYBRID OF SINGLE AND DOUBLE COIL EXCITATION MODES

0100 0011

0010

0110

01001100

1000

1001

COMPARISON

Single Coil•Low Torque•Consume less energy•Settling time is more

Double Coil• High Torque• Consume

double energy• Settling time is less

ADVANTAGE OF HYBRID MODE Step angle gets halved So we can control the rotation with double

precision

ACTUAL STEPPER MOTOR Multiple "toothed" electromagnets arranged around a

central gear-shaped piece of iron. The electromagnets are energized by an external control

circuit, such as a micro controller. To make the motor shaft turn, first one electromagnet is

given power, which makes the gear's teeth magnetically attracted to the electromagnet's teeth 

When the gear's teeth are thus aligned to the first electromagnet, they are slightly offset from the next electromagnet.

So when the next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next one, and from there the process is repeated.

Each of those slight rotations is called a "step," with an integer number of steps making a full rotation. In that way, the motor can be turned by a precise angle.