Unipolar Stepper Motor

Introduction

The stepper motor is an electromechanical device that converts electrical pulses into angular displacement. It is CapAd forward a series of steps (degrees) depending on the value of their tickets.
There are two types unipolar and bipolar; in our case study first.
The microcontroller system we use is the arduino, an easy way to control these motors.

Experimental Material

 - Arduino Uno

- Driver unipolar motors

- Unipolar motor

Structure

The stepper motor consists of a rotor and a stator. In the rotor are certain numbers of magnets, while the stator coils are located.

Figure 1. Structure of unipolar motor.

Wiring

Usually has 5 output cables, which are: four for the coils and one for the common (in certain engines can be 2 wires and not be attached). To use an encapsulated diode is needed, in our example ULN2803 (8 diode array) is used.

Coils normally come determined by A, B, C and D.

Figure 2. Connecting the unipolar motor with ULN2803.

Performance

For the engine to run, it must be properly polarize the coils and in the right order, so that the magnet is moved to one side or the other. If the sequence used is not correct, the engine may have awkward movements skipping any steps (degree) turn in the desired direction, or is not moving and just vibrate.

If the sequence used is correct, the motor will run perfectly, the only problem you may encounter is the frequency of rotation, because if we use too fast the engine will be unable to move or skip some steps.

There are three ways to polarize the coil:

• Step by step polarizing two coils: This is the most used, since two coils activating the motor torque is large. In this case the motor will move from step to step, so you will need 4 steps to turn around.

Figure 3. Sequence for operation step by step activating two coils.

In our case, the internal engine needs to perform 8 times this sequence, so you need 4x8 = 32 steps for a spin.

It also has a reducer 64 so to give back to the outer shaft should be given 32x64 = 2048 steps.

• Step Medium: in this case, two coils are applied first, then one and so on, so that steps twice that in the case above is performed. This sequence is used when we need more accuracy in the rotation, because to make a turn employ 8/2 steps instead of 4 steps.

  At times the torque is lower than in the previous case, but the accuracy is compensated.

Figure 4. Sequence for operation at half steps.

As in the previous case, our engine must perform this sequence 8 times to perform a rotation of the internal engine, so that 8x8 = 64 steps perform media.

The outer engine must perform 64 times this sequence because too reductive, so the outer shaft should give 64x64 = 4096/2 steps for a spin.

• Footsteps activating a coil: this sequence is the least used because the torque is small to activate only one coil, and the number of steps is equal to the first example, so you do not gain in precision and lose torque.

Because of this we have not tested this sequence.

Comments

To make a change in the engine rotational just have to make the sequence in reverse.

The speed is increased step by step making a half steps. This can be seen in the videos.

If you do not have the motor data and know no coils, can be identified as follows:

If you do not have the motor data, and we know that each coil wire is for then is explained a way to identify each coil and common.

Select a cable and connect to ground. That will be called Cable A.

A cable holding grounded, test which of the remaining three wires cause a step counter being also connected to ground sense.

 That will be the cable B.

A cable holding grounded, test which of the two remaining wires clockwise causes a step to be grounded sense. That will be the cable D.

The last cable should be the cable C. To check, simply connect to ground, which should not cause any movement because it is the opposite coil to the A.

The name of each coil has been set arbitrarily, and may be designated in any manner.