Stepper motors can be used in various areas of your microcontroller projects such as making robots, robotic arm, automatic door lock system etc. This tutorial will explain you construction of stepper motors (unipolar and bipolar stepper motors ), basic pricipal, different controlling types (Half step and Full step), Interfacing Techniques (using L293D or ULN2003) and programming your microcontroller in C and assembly to control stepper motor.

Types of Stepper Motors

There are basic two types of stepper motors available in market.

Unipolar stepper motor

The unipolar stepper motor has five or six wires and four coils (actually two coils divided by center connections on each coil). The center connections of the coils are tied together and used as the power connection. They are called unipolar steppers because power always comes in on this one pole.

Bipolar stepper motor

The bipolar stepper motor usually has four wires coming out of it. Unlike unipolar steppers, bipolar steppers have no common center connection. They have two independent sets of coils instead. You can distinguish them from unipolar steppers by measuring the resistance between the wires. You should find two pairs of wires with equal resistance. If you've got the leads of your meter connected to two wires that are not connected (i.e. not attached to the same coil), you should see infinite resistance (or no continuity).

As already said, we will talk mostly on "Unipolar stepper motors" which is most common type of stepper motor available in the market.A simple example of 6 lead step motor is given below and in 5 lead step motor wire 5 and 6 are joined together to make 1 wire as common.

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Stepper Motor Coils

Working of Stepper Motor

Now lets discuss the operation pricipal of a stepper motor. When we energize a coil of stepper motor, The shaft of stepper motor (which is actually a permanent magnet) align itself according to poles of energized coil. So when motor coils are energized in a particular sequence, motor shaft tend to align itself according to pole of coils and hence rotates. A small example of energizing operation is given below.

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Stepper Motor Coils

You can see in the example, when coil "A" is energized, A north-south polarity is generated at "A+A\" as shown in the figure above and magnetic shaft automatically align itself according to the poles generated. When the next coil is energized the shaft again align itself and take a step. Hence the working principal.

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Stepper Motor Coils When energized

We have seen that to make the stepper motor work, we need to energize coil in a sequence.

Step Sequence

Stepper motors can be driven in two different patterns or sequences. namely, Full Step Sequence

  • Bulleted list item
  • Half Step Sequence

we will go through these sequences one by one.

Full Step Sequence

In the full step sequence, two coils are energized at the same time and motor shaft rotates. The order in which coils has to be energized is given in the table below.

Full Mode Sequence
Step A B A\ B\
0 1 1 0 0
1 0 1 1 0
2 0 0 1 1
3 1 0 0 1

The working of the full mode sequence is given in the animated figure below.

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Full mode Sequence Animation

Half Step Sequence

In Half mode step sequence, motor step angle reduces to half the angle in full mode. So the angualar resolution is also increased i.e. it becomes double the angular resolution in full mode. Also in half mode sequence the number of steps gets doubled as that of full mode. Half mode is usually preffered over full mode. Table below shows the pattern of energizing the coils.

Half Mode Sequence
Step A B A\ B\
0 1 1 0 0
1 0 1 0 0
2 0 1 1 0
3 0 0 1 0
4 0 0 1 1
5 0 0 0 1
6 1 0 0 1
7 1 0 0 0

The working of the half mode sequence is given in the animated figure below.

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Half mode Sequence Animation

Step Angle

Step angle of the stepper motor is defined as the angle traversed by the motor in one step. To calculate step angle,simply divide 360 by number of steps a motor takes to complete one revolution. As we have seen that in half mode, the number of steps taken by the motor to complete one revolution gets doubled, so step angle reduces to half.

As in above examples, Stepper Motor rotating in full mode takes 4 steps to complete a revolution, So step angle can be calculated as...

Step Angle ø = 360° / 4 = 90°

and in case of half mode step angle gets half so 45°.

So this way we can calculate step angle for any stepper motor. Usually step angle is given in the spec sheet of the stepper motor you are using. Knowing stepper motor's step angle helps you calibrate the rotation of motor also to helps you move the motor to correct angular position.

Step Sequence for 2-wire control of Unipolar stepper motor

As seen in above explanation, In every step of the sequence, two wires are always set to opposite polarities. Because of this, it's possible to control steppers with only two wires instead of four, with a slightly more complex circuit. The stepping sequence is the same as it is for the two coils A and B, and the opposite polarity value is given to A\ and B\. The sequence is given in the table below:

2-wire Mode Sequence
Step A B
0 0 1
1 1 1
2 1 0
3 0 0

Step Sequence for Bipolar stepper motor

Bipolar motor has simpler construction. It has two windings with no center taps and a permanent magnet at the center just like unipolar stepepr motors. Being simpler in contruction, the stepping sequence is a little complex, as the power for both the coils has to be controlled in such a way that the polarity of the poles get reversed. This polarity sequence is shown in the table below.

Polarity Sequence
Step A A\ B B\
0 +ve -ve -ve -ve
1 -ve -ve +ve -ve
2 -ve +ve -ve -ve
3 -ve -ve -ve +ve

The above polarity sequence can be interpreted in terms of logic levels for microcontroller by activating one coil at a time as shown in the table below.

Step Sequence
Step A A\ B B\
0 1 0 0 0
1 0 0 1 0
2 0 1 0 0
3 0 0 0 1

Stepper Motor Connections

Connecting Unipolar Stepper Motor

There are actually many ways you can interface a stepper motor to your controller, out of them the most used interfaces are:

  1. Interface using L293D - H-Bridge Motor Driver
  2. Interface using ULN2003/2004 - Darlington Arrays

We will discuss both connection techniques one by one. The above mentioned methods need 4 controller pins for interface.

Connecting Unipolar stepper using L293D

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Unipolar Stepper with L293D

As you see in the circuit above the four pins "Controller pin 1",2,3 and 4 will control the motion and direction of the stepper motor according to the step sequence programmed in the controller.

Connecting Unipolar stepper using ULN2003/2004

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Unipolar Stepper with ULN2003/ULN2004

As already discussed in case of L293D, Here in this circuit too the four pins "Controller pin 1", 2, 3 and 4 will control the motion and direction of the stepper motor according to the step sequence sent by the controller.

2-wire connection for Unipolar Stepper Motor

We have seen the generally used 4-wire connection method for interfacing unipolar stepper motor, but we can simplify the design to make controller use less pins with the help of 2-wire connection method. The circuit for 2-wire connection is shown below.

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Unipolar Stepper Motor connection

Connecting Bipolar Stepper Motor

As we have studied that, Bi-polar stepper motors has 2 different coils. The step sequence for Bipolar stepper motor is same as that of unipolar stepper motors. The driving circuit for this require an H-Bridge as it allows the polarity of the power applied to be controlled independently. This can be done as shown in the figure below:

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Bipolar Stepper Motor connection

Now we have seen the methods for connecting stepper motors with your microcontroller. So keeping these circuits in mind,we will now look at the programming of microcontroller to control stepper motors.

Stepper Motor Programming

Programming Full step Sequence

C Programming

I am assuming that stepper motor is connected at Port 1.0 to Port 1.3. Adjusting the delay will increase or decrease the speed of the motor. Here just for demonstration i have taken some delay, you can change it as you want.

[Tip: Do testing... ]

#include <REG2051.H>
#define stepper P1

void delay()
{
	unsigned char i,j,k;
	for(i=0;i<6;i++) {
		for(j=0;j<255;j++)
			for(k=0;k<255;k++);
	}
}

void main()
{
	while (1) {
		stepper = 0x0C;
		delay();
		stepper = 0x06;
		delay();
		stepper = 0x03;
		delay();
		stepper = 0x09;
		delay();
	}
}

Assembly Programming

	org 0H

	stepper equ P1

main:
	mov stepper, #0CH
	acall delay
	mov stepper, #06H
	acall delay
	mov stepper, #03H
	acall delay
	mov stepper, #09H
	acall delay
	sjmp main

delay:
	mov r7,#4
wait2:
	mov r6,#0FFH
wait1:
	mov r5,#0FFH
wait:
	djnz r5,wait
	djnz r6,wait1
	djnz r7,wait2
	ret
	end

The working of the above code can be seen in the demo animation below.

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Full Step Sequence Simulation

Programming Half step Sequence

C Programming

Just the main routine changes rest everything remains same, i mean same delay routine.

void main()
{
	while (1) {
		stepper = 0x08;
		delay();
		stepper = 0x0C;
		delay();
		stepper = 0x04;
		delay();
		stepper = 0x06;
		delay();
		stepper = 0x02;
		delay();
		stepper = 0x03;
		delay();
		stepper = 0x01;
		delay();
		stepper = 0x09;
		delay();
	}
}

Assembly Programming

Here also the main routine changes rest everything remains same.

main:
	mov stepper, #08H
	acall delay
	mov stepper, #0CH
	acall delay
	mov stepper, #04H
	acall delay
	mov stepper, #06H
	acall delay
	mov stepper, #02H
	acall delay
	mov stepper, #03H
	acall delay
	mov stepper, #01H
	acall delay
	mov stepper, #09H
	acall delay
	sjmp main

The working of the above code can be seen in the demo animation below.

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Half Step Sequence Simulation

Programming for 2-wire connection of Unipolar Stepper Motor

C Programming

void main()
{
	while (1) {
		stepper = 0x03;
		delay();
		stepper = 0x01;
		delay();
		stepper = 0x00;
		delay();
		stepper = 0x02;
		delay();
	}
}

Assembly Programming

main:
	mov stepper, #03H
	acall delay
	mov stepper, #01H
	acall delay
	mov stepper, #00H
	acall delay
	mov stepper, #02H
	acall delay
	sjmp main

The working of the above code can be seen in the demo animation below.

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2-wire connection of Unipolar Stepper Motor simulation

Programming for Bipolar Stepper Motor

C Programming

void main()
{
	while (1) {
		stepper = 0x08;
		delay();
		stepper = 0x02;
		delay();
		stepper = 0x04;
		delay();
		stepper = 0x01;
		delay();
	}
}

Assembly Programming

main:
	mov stepper, #08H
	acall delay
	mov stepper, #02H
	acall delay
	mov stepper, #04H
	acall delay
	mov stepper, #01H
	acall delay
	sjmp main

Now you're ready to use stepper motors. If you have any doubts, please post in the fourm.

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