by                                     
Ashutosh M Bhatt
a_m_bhatt@indiatimes.com
www.multyremotes.com

This project is actually an educational kit. One can study the full operation of unipolar type stepper motor using this board. As it is micro controller based it can be programmable also and one can learn micro controller interfacing with LEDs, key board and stepper motor. Thus single board serves the purpose of learning stepper motor control as well as learning micro controller programming.

General description and system block diagram:-

The complete board consists of transformer, control circuit, keypad and stepper motor as shown in snap. The given figure shows the block diagram of project.          

The circuit has inbuilt 5 V power supply so when it is connected with transformer it will give the supply to circuit and motor both. The 8 Key keypad is connected with circuit through which user can give the command to control stepper motor. The control circuit includes micro controller 89C51, indicating LEDs, and current driver chip ULN2003A. One can program the controller to control the operation of stepper motor. He can give different commands through keypad like, run clockwise, run anticlockwise, increase/decrease RPM, increase/decrease revolutions, stop motor, change the mode, etc. before we start with project it is must that we first understood the operation of unipolar stepper motor.        

Unipolar stepper motor:-

In the construction of unipolar stepper motor there are four coils. One end of each coil is tide together and it gives common terminal which is always connected with positive terminal of supply. The other ends of each coil are given for interface.  Specific color code may also be given. Like in my motor orange is first coil (L1), brown is second (L2), yellow is third (L3), black is fourth (L4) and red for common terminal.

By means of controlling a stepper motor operation we can

•    Increase or decrease the RPM (speed) of it

•    Increase or decrease number of revolutions of it

•    Change its direction means rotate it clockwise or anticlockwise

To vary the RPM of motor we have to vary the PRF (Pulse Repetition Frequency). Number of applied pulses will vary number of rotations and last to change direction we have to change pulse sequence.

So all these three things just depends on applied pulses. Now there are three different modes to rotate this motor

1.    Single coil excitation

2.    Double coil excitation

3.    Half step excitation

The table given below will give you the complete idea that how to give pulses in each mode

Note:- In half step excitation mode motor will rotate at half the specified given step resolution. Means if step resolution is 1.8 degree then in this mode it will be 0.9 degree. Step resolution means on receiving on 1 pulse motor will rotate that much degree. If step resolution is 1.8 degree then it will take 200 pulses for motor to compete 1 revolution (360 degree).

Now let me give you the specification of the stepper motor that I have used.

Max rated voltage: -    5 V

Max rated current per coil: - 0.5 Amp

Step resolution: -    1.8 degree / pulse  

Max RPM: -    20 in single/double coil excitation mode and 60 in half step mode

Torque: - 1.5 Kg/cm²

RPM calculation:-

One can calculate the exact RPM at which motor will run. We know that motor needs 200 pulses to complete 1 revolution. Means if 200 pulses applied in 1 second motor will complete 1 revolution in 1 second. Now 1 rev. in 1 sec means 60 rev. in 1 minute. That will give us 60 RPM. Now 200 pulses in 1 sec means the PRF is 200 Hz. And delay will be 5 millisecond (ms). Now lets see it reverse.

•  If delay is 10 ms then PRF will be 100 Hz.
•  So 100 pulses will be given in 1 sec
•  Motor will complete 1 revolution in 2 second
•  So the RPM will be 30.

In same manner as you change delay the PRF will be changed and it will change RPM

Stepper motor control board circuit:-

The circuit consists of very few components. The major components are 7805, 89C51 and ULN2003A.

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For queries on the above please contact  a_m_bhatt@indiatimes.com

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