STEPPER MOTOR

Construction:

Stepper Motor is a brushless DC electric motor that is a positioning device. It does not provide continuous rotation instead of rotates in steps that are, one complete rotation is divided into many steps. That’s why the name stepper motor. It is capable of rotating in steps with the applied train of input pulses and make it rotate through a fixed angle every time. Stepper motors are open-loop devices. They don’t require any feedback circuitry.

Stepper motor system:

1. Indexers: A microprocessor generating step pulses and directions for the driver.
2. Drivers:   Provides power to the motor to energize windings by converting indexer commands.
3. Stepper motors: converts digital pulses to mechanical shaft rotation.

Principle:

Stepper motors are provided with electromagnetic teeth like structure arranged on the central Gear shaped piece of iron. An external circuit or a microcontroller can be made use to activate these electromagnetic teeth. To start running off the motor, first and the electromagnet is energized which attracts the teeth of the gear and aligned in one position. again the first one is made de-energized at the same time activating the next electromagnet aligning the teeth to that position. This makes the motor rotate through an angle. The same process is repeated to make one complete rotation that is by energizing the electromagnet each time. This circular arrangement of magnets around the central gear is divided into phases. The angle by which the motor rotates every time is called a step angle. It is given by 360/number of steps.

Types of stepper motors:

Based on the construction of the rotor there are three types of stepper motors:

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

Permanent magnet stepper motor:

The rotor of this is made of a permanent magnet. The stator windings create opposite polarity compared to the poles of the rotor. This the rotation of the rotor is achieved.

• Variable reluctance stepper motor:
• here the rotor is a non-magnetized soft iron type having teeth that are offset from the stator.
• When the stator windings or activated in a particular order rotor moves accordingly.
• Hybrid synchronous stepper motor:
• It contains a toothed rotor and toothed stator.
• The rotor consists of two sections which are of opposite polarity.
• Generally, we use an eight-pole stator of two-pole types say A and B (that is 4A and 4B). So if we energizepoleA two of them are polarized north and two of them are South. The teeth on the road or aligned accordingly. In the next cycle, we energized poles B. The rotor teeth align accordingly.
• Does the rotor rotate in steps?

Speed torque characteristics:

 There are four main types of torques associated with the stepper motor.

1.  Detent torque:

When the windings or not energized the motor produces a certain amount of torque which is called detent torque.

• Holding torque:

When the windings or energized but still rotor is stationary the amount of torque required to move the motor through one step is called holding torque.

•  Pull-in torque:

The maximum torque that a stepper motor develops for a given shipping rate without losing synchronism.

• Pull out torque:

The maximum torque that is stepper motor develops for a given stepping rate in a slewing mode without losing synchronism.

The motor can start, synchronize and stop or reverse for the load torque ƮL. if the pulse rate is less than S1. The stepping rate can be increased for the same load as the rotor started the rotation and synchronized. Now, for load ƮL1 the stepping rate can be eventually raised or increased up to S2 without losing any sort of synchronism of the motor.

If the stepping rate is increased beyond S2, the motor will lose synchronism. Thus, the area between curves 1 and 2 represents the various torque values, the range of stepping rate, which the motors follow without losing the synchronism when it has already been started and synchronized. This is known as Slew Range. The motor will operate in slewing mode.

Driving modes and driving circuits:

Driving circuits: As we discussed that pulses are converted into mechanical rotatory movements, this is achieved with the help of driving circuits.

Driving modes:

• Wave drive/single-coil excitation:

Each time a single coil is activated. Thus the rotor rotates.

• Full step drive mode:
• High torque is provided.
• At every particular instant of time, there are always active coils.
• But it does not provide many resolutions.

• Half step drive mode:
• It is a mixture of the above two drive modes.
• If one coil is active for the given instant of time we will find two coils energized for the next instant of time and this process repeats.

• Micro stepping:
• This micro-stepping technique employs pulse width modulated voltage to control the current in windings.
• Achieves higher resolution at low speeds.

Advantages:

• Greater torque.
• Cheaper, available at fewer prices.
• Long life.
• Provide standstill torque.

Disadvantages:

• Less efficient
• As Torque is inversely proportional to speed, torque declines rapidly.
• No feedback system present to indicate positional steps.
• Noisy.

You can visit sections of 8051 and 8086 for more information.