stepper motor kt

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Stepper MotorsThe motor which moves in steps

Prepared by Khurram Tanvir

Applications of Stepper Motors

• Dot Matrix printers• Disk Drives, Floppy Drive• CD-Drives• Robotics• Precise Industrial Controls• Low speed High Torque Applications

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Introduction to Stepper Motor• A stepper motor is a special kind of motor that moves in individual steps

which are usually .9 degrees each. Each step is controlled by energizing one or more of the coils inside the motor which then interacts with the permanent magnets attached to the shaft. Turning these coils on and off in sequence will cause the motor to rotate forward or reverse.

• The time delay between each step determines the motor's speed. • Steppers can be moved to any desired position reliably by sending them the

proper number of step pulses. Unlike servo motors, steppers can be used "open-loop" without the need for expensive encoders to check their position.

• Stepper motors are much more cost-effective than servo systems due to their simplified control and drive circuitry. There are no brushes to replace in a stepper motor, eliminating the need for maintenance.

• Even though a stepper motor system can not achieve the speed of a servo motor system, their ease of use and relatively higher torque make them attractive to be used in computerized control systems.


Each pulse moves a motor to a discrete position

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Common Characteristics of Stepper Motor• Voltage

Stepper motors usually have a voltage rating. This is either printed directly on the unit, or is specified in the motor's datasheet. Exceeding the rated voltage is sometimes necessary to obtain the desired torque from a given motor, but doing so may produce excessive heat and/or shorten the life of the motor.

ResistanceResistance-per-winding is another characteristic of a stepper motor. This resistance will determine current draw of the motor, as well as affect the motor's torque curve and maximum operating speed.

Degrees per stepThis is often the most important factor in choosing a stepper motor for a given application. This factor specifies the number of degrees the shaft will rotate for each full step. Half step operation of the motor will double the number of steps/revolution, and cut the degrees-per-step in half. For unmarked motors, it is often possible to carefully count, by hand, the number of steps per revolution of the motor. The degrees per step can be calculated by dividing 360 by the number of steps in 1 complete revolution Common degree/step numbers include: 0.72, 1.8, 3.6, 7.5, 15, and even 90. Degrees per step is often referred to as the resolution of the motor. As in the case of an unmarked motor, if a motor has only the number of steps/revolution printed on it, dividing 360 by this number will yield the degree/step value.


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•The stepping sequence is generated by microcontroller device

•Power drivers give power necessary to drive stepper motor coils

5 v – 48 v

Can be computer microcontroller or any driver IC

General Stepper Motor Driver Architecture


Types of Stepper Motors

• Permanent Magnet Stepper Motors– Unipolar– Bipolar– Universal

• Variable Reluctance Stepper Motors

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Number of wires in stepper motors

– Bipolar -> with 4 leads– Unipolar -> with 5 leads (common centre tap)– Unipolar -> with 6 leads (separate centre tap)– Universal -> with 8 leads


Types of Stepper Motors

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Bifilar

Bifilar

or UnifilarBifilar

Check resistance of coils using voltmeter to find the coil numbers e.g1a, 2a, 1b, 2b etc


Stepper Motor Basics

P1.0P1.1

P1.2

P1.3

+V

+V

1001

1010

0110

0101

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Stepper motor driver

#include <reg51.h>unsigned char code pattern[]= {0x5,0x9,0xa,0x6};

void delay(int);

void step(void){static unsigned char i;P1= pattern[i=++i&3];delay(8);}void main(void){while(1){step();}}


•Low Torque Full Stepping Mode

•Only one coil active at one time

Full Stepping Sequence for Unipolar Stepper Motor(Lower Torque)

Full Step

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High Torque Full Stepping



9


0110




0101

10



1001


•High Torque Mode

•Two coils active at one time

•Provides 1.5 times more torque

•Needs double the current

Unipolar MotorsHalf Step

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Bipolar Stepper MotorWithout centre tapping

1. Provides more torque

2. Requires complex driver circuitry

3. H-bridge is used as driver

4. Driver ICs like LM297/298 and LMD18T245


Half Step - Effectively doubles the stepping resolution of the motor

Half-Step---+--++--+--++--+--++--+---+--+

00010011001001100100110010001001

Hi Torque Hi-Torque, Two-Phase--++-++-++--+--+

0011011011001001

Consumes the least power. Only one phase is energized at a time.

Wave Drive, One-Phase---+--+--+--+---

0001001001001000

DescriptionNamePolaritySequence

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Stepper Motor Driver ICs


1. One H-bridge can drive single coil

2. Two H-bridges or two driver ICs are needed to driver bipolar stepper motor.

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MOSFET H-Bridge


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Variable Reluctance Stepper Motors• Variable reluctance stepper motors are the simplest to control over other

types of stepper motors.• Their drive sequence is simply to energize each of the windings in order,

one after the other.• This type of stepper motor will often have only one lead, which is the

common lead for all the other leads. • This type of motor feels like a DC motor when the shaft is spun by hand; it

turns freely and you cannot feel the steps.• This type of stepper motor is not permanently magnetized like its unipolar

and bipolar counterparts.


Holding Torque Concept in Stepper Motors• Stepper motors also have another characteristic, holding torque, which

is not present in DC motors. Holding torque allows a stepper motor to hold its position firmly when not turning. This can be useful for applications where the motor may be starting and stopping, while the force acting against the motor remains present. This eliminates the need for a mechanical brake mechanism.

• Steppers don't simply respond to a clock signal, they have several windings which need to be energized in the correct sequence before the motor's shaft will rotate. Reversing the order of the sequence will cause the motor to rotate the other way.

• Stepper motors produce high torque at low speeds as compared to the the DC Motors.

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Stepper Motor vs DC Motor• Steppers don’t require encoders and have simpler controller hardware

so are less expensive.• Steppers are susceptible to missing steps: the controller won’t know if

external torque stalls the motor. A higher torque margin is therefore required. Steppers should be avoided where external conditions may cause large torques

• to be applied.• Steppers are easy to drive—you can easily write software• allowing your microcontroller to control a stepper motor directly with

just a driver chip. Servos are more complex to drive and most applications rely on third-party servo controller boards.


Good Things about Bipolar Stepper Motors

• You can get more torque in a smaller package with bipolar because you are pulling from both ends at once.

• Bipolar steppers require fewer windings, so the motors can be smaller.

• For the same motor power dissipation, bipolar drive provides 40% more torque than unipolar.

• There are fewer electromagnet circuits to deal with.

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Speed Issues of Stepper Motor

• If you step faster than the motor can respond, the driver and motor fall out of synchronization, and you can't be sure exactly where the motor is.

• Please note that the maximum stepping rate of a motor depends on the load that it is turning. If your motor is turning a heavy mass, it will take longer to spin into the new position.


Acceleration Issues of Stepper Motor

• The spinning motor has mass in motion, i.e. inertia. When you hook up the motor shaft to turn something, you end up with more mass andmore inertia. Going from a full stop to the maximum step rate means all that mass wants to stay at rest. You have to use plenty of torque to get it moving. (Remember Newton's laws of motion.)

• But if the motor is already spinning at 90% of its maximum rate, less torque is required to speed it up a little more.

• Of course, the same problem exists when you are slowing down and/or reversing.

• All this means is that stepper motors can only be accelerated at a given rate. Specifications for this should be available from the manufacturer. Your driving circuitry must take this acceleration into account, in order to keep the motor locked to the driver.

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Microstepping

• The half-step is a specific form of microstepping. There are other variations, because you are not limited to turning the electromagnets fully on or off. You could feed 1/4 power to coil A and 3/4 power to coil B, and get a position that is nearer to coil B than coil A.


Good Things about Stepper Motors

• You can speed up or slow down the motor rotation by adjusting the step rate.

• You can reverse the direction of spin by energizing the coils in reverse sequence.

• Relative positioning is very precise: For a single step, you know exactly how much the shaft will turn.

• This precise relative positioning can be accomplished "open-loop". You don't need feedback to tell you where the position is right now.

• If you stop stepping and still apply power, the motor holds its position well.

• They are very good for precision movements. • No brushes in stepper motors. • It is possible to achieve very low speeds.

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A word of caution1. When making connections to either a PC parallel port, or I/O pins of a

microcontroller, be sure to isolate the motor well. High voltage spikes of several hundred volts are possible as back EMF from stepper motor coils.

2. Always use clamping diodes to short these spikes back to the motor's power bus.

3. The use of optical isolation devices (optoisolators) will add yet another layer or protection between the delicate control logic and the high-voltage potentials which may be present in the power output stage.

4. Whenever possible, use separate power supplies for the motor and the translator / microcontroller. This further reduces the chance of destructive voltages reaching the controller, and reduces or eliminates power supply noise that may be introduced by the motor.

5. If you're using a computer that has a parallel port as part of its onboard I/O, you may want to consider purchasing a parallel port card to use instead. Not only does this reduce the risk of permanently damaging or destroying your motherboard but it will also allow you to experiment without the need for swapping cables or flipping a switchbox when you want to use your parallel printer, since your experiments won't be sharing its port. It is much cheaper to throw out some money on parallel port card than it is to replace your motherboard!

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