Question

Make a write up on how a stepper motor control works. The Do's and Dont's. The motor is driven by TTL logic used to implement the controller. Use a 555 timer circuit to operate the controller at 3Hz. Write up to be 4-5 pages.

Answer 1

Stepper motor can be defined as brushless DC motors. Stepper motors doent consist of any brushes and commutators. The operation of this motor can be controlled without any feedback mechanism and in the form of steps.

There are three main types of stepper motors, they are:

1. Permanent magnet stepper
2. Hybrid synchronous stepper
3. Variable reluctance steppper

1. Permanent Magnet Stepper Motor: Permanent magnet motors use a permanent magnet (PM) in the rotor and operate on the attraction or repulsion between the rotor PM and the stator electromagnets.

2. Variable Reluctance Stepper Motor: Variable reluctance (VR) motors have a plain iron rotor and operate based on the principle that minimum reluctance occurs with minimum gap, hence the rotor points are attracted toward the stator magnet poles.

3. Hybrid Synchronous Stepper Motor: Hybrid stepper motors are named because they use a combination of permanent magnet (PM) and variable reluctance (VR) techniques to achieve maximum power in a small package size.

Construction of Stepper Motor:

Stepper motor consists of a rotor and stator. Rotor is the rotating part of the machine and stator is the standstill part. The stator is constructed with multipole and multiphase windings.

The number of poles should be more in order to have a smaller step angle, the smaller the step angle larger the number of steps. Number of windings should be kept low. Stepper motors operates differnt from general DC motors .It operates on a programmed discrete control pulses that are applied to the stator windings via an electronic drive. The rotation occurs due to the magnetic interaction between poles of sequentially energized stator winding and poles of the rotor.

Stepper motor driving ciruit using 555Timer IC

C 555 is a very famous timer IC that is often used for time delays, pulse generation and many oscillator applications. IC 555 has three modes of operations namely Astable Multivibrator (Pulse Generator), Monostable Multivibrator (Time Delays) and Bistable Multivibrator (Flip – Flop).

The working of this Stepper Motor Driver circuit is very simple. By adjusting the potentiometer, the frequency of the input square wave can be controlled. The input square wave is given to the counter IC (Ex: CD4017) as clock input. For every positive transition of the clock signal i.e. a low to high transition, the counter output advances by one count.

If we take an example of four trnasistors then the input square wave is given to 4 transistors as shown below.

Assume the points A, B, C and D are the contacts of the coils connected to the transistors. The common wire in the stepper motor is given to 12V supply.

When the first clock signal is applied to the CD4017, Q0 becomes HIGH. This will turn ON the corresponding Transistor.

As a result, the supply from the common wire goes through point A to ground. This will energize the coil and acts as an electromagnet. The rotor will get attracted and turns to that position.

During the second clock pulse, output Q1 become HIGH and as a result, the transistor associated with it is turned ON. Now, the current flows from common wire to GND through point B.

Hence, this coil will be energized and turns in to an electromagnet. This will further rotate the rotor. This process continues and depending on the frequency of the clock signal, the speed of rotation of the stepper motor varies.

Advantages and Disadvantages of stepper motor:

1. The rotation angle of the motor is proportional to the input pulse.
2. The motor has full torque at standstill.
3. Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3 – 5% of a step and this error is non cumulative from one step to the next.
4. Excellent response to starting, stopping and reversing.
5. Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply dependant on the life of the bearing.
6. The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.
7. It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the shaft.
8. A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses.