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What is Stepper Motor | Types , Application , Advantages

What is Stepper Motor

A stepper motor is a type of DC motor that works in discrete steps. It is a synchronous brushless motor where a full rotation is divided into a number of steps. The two main components of a stepper motor are the rotor and the stator. The rotor is the rotating shaft and the stator consists of electromagnets that form the stationary part of the motor. When a discrete DC voltage is applied, the stepper motor rotates in a particular angle called the step angle; thus a stepper motor is manufactured with steps per revolution of 12, 24, 72, 144, 180 and 200, with a corresponding step angle of 30,15, 5, 2.5, 2 and 1.8. It can be operated with or without a feedback control.

A stepper motor is a particular type of DC motor that does not rotate continuously. Instead, a full rotation is divided into a number of equal steps. A stepper motor consists of phases, which are multiple coils that are organized into groups. By applying the energy from the input voltage to each phase in a sequence, the stepper motor rotates by taking one step at a time. Thus a stepper motor converts electrical energy or an input digital pulse into mechanical shaft rotation.

A stepper motor works under the principle of electromagnetism. A permanent magnet or soft iron is used as the rotor and is surrounded by electromagnetic stators. The poles of the rotor and stator may be teethed. When voltage is applied at the terminals, the rotor aligns itself with the stator or moves to have a minimum gap with the stator due to the magnetic effect. The stators are energized in a sequence and the rotor moves accordingly, giving a full rotation that is divided into a discrete number of steps with a particular step angle.

Stepper Motor Construction :

A stepper motor consists of a permanent magnet sandwiched between the two rotor halves (causing axial polarity), which make up the spinning part of the motor, placed into a stator housing where the stator coils of wire make up the different motor phases. With a 2 phase stepper motor, each phase has four coils. The phase is magnetized where the A and A- phases (or B and B-)are magnetized at the same time, so that both A phases are magnetized as one pole, and both A- phases are magnetized as the opposite pole because the winding direction of the A phase is opposite the winding direction of the A- phase.

The rotor is connected to the motor shaft, which gives the output rotation and torque of the motor when voltage and current pulses are applied to the motor windings. The bearings on either side of the rotor allow for smooth rotation with little friction and wearing. The bearings are placed into their designated spaces in the front and rear endcaps, which allow for concentricity of the rotor inside the stator. Perfect alignment of the rotor and the stator is very important because the airgap between them, where the motor torque is generated, must be equal on all sides and is only a few nanometers wide – thinner than a strand of hair.

Types of Stepper Motor:
There are three main types of stepper motors, they are:

Permanent magnet stepper
Hybrid synchronous stepper
Variable reluctance stepper

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.

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.

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.

Stepper motor system

A stepper motor system consists of three basic elements, often combined with some type of user interface (host computer, PLC or dumb terminal):

Indexers: The indexer (or controller) is a microprocessor capable of generating step pulses and direction signals for the driver. In addition, the indexer is typically required to perform many other sophisticated command functions.

Drivers: The driver (or amplifier) converts the indexer command signals into the power necessary to energize the motor windings. There are numerous types of drivers, with different voltage and current ratings and construction technology. Not all drivers are suitable to run all motors, so when designing a motion control system the driver selection process is critical.

Stepper motors: The stepper motor is an electromagnetic device that converts digital pulses into mechanical shaft rotation. Advantages of step motors are low cost, high reliability, high torque at low speeds and a simple, rugged construction that operates in almost any environment. The main disadvantages in using a stepper motor is the resonance effect often exhibited at low speeds and decreasing torque with increasing speed.

Advantages Of Stepper Motors

Low cost for control achieved
High torque at startup and low speeds
Ruggedness
Simplicity of construction
Can operate in an open loop control system
Low maintenance
Less likely to stall or slip
Will work in any environment
Can be used in robotics in a wide scale.
High reliability
The rotation angle of the motor is proportional to the input pulse.
The motor has full torque at standstill (if the windings are energized)
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.
Excellent response to starting/stopping/reversing.
Very reliable since there are no contact brushes in the motor. Therefore, the life of the motor is simply dependent on the life of the bearing.
The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.
It is possible to achieve very low-speed synchronous rotation with a load that is directly coupled to the shaft.
A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses.

Applications Of Stepper Motors :

Industrial Machines – Stepper motors are used in automotive gauges and machine tooling automated production equipments.
Security – new surveillance products for the security industry.
Medical – Stepper motors are used inside medical scanners, samplers, and also found inside digital dental photography, fluid pumps, respirators and blood analysis machinery.
Consumer Electronics – Stepper motors in cameras for automatic digital camera focus and zoom functions.