Stepper motors

Stepping motors can be viewed as electric motors without commutators. Stepper motors consist of a permanent magnet rotating shaft, called the rotor, and electromagnets on the stationary portion that surrounds the motor, called the stator. All of the commutation must be handled externally by the motor controller. The motors and controllers are designed so that the motor may be held in any fixed position as well as being rotated one way or the other. To move the rotor the electric magnets on the motor are activated in the right order. Every change in this process moves the motor one step. The order in which those electromagnets are activated determines the rotation direction. The stepper’s resolution is based on the steps (typically 1.8 deg or 3.6 deg per step). In the stepper system, the driver advances one step, and the stepper motor follows. For example a 1.8 deg. stepper will turn a full circle in 200 steps. No matter how you gear it, a stepper motor still moves in discrete steps. Each step covers a specific range of ‘swing’. In a nutshell, a stepper (with or without gear-train) is a set of ‘preset’ positions you can move to. Any position that’s not part of the ‘presets’ is unattainable by that motor or motor-and-gear-train combination, and can only be reached as an approximation.

Stepping motors come in two varieties, permanent magnet and variable reluctance. Lacking a label on the motor, you can generally tell the two apart by feel when no power is applied. Permanent magnet motors tend to ‘cog’ as you twist the rotor with your fingers, while variable reluctance motors almost spin freely. You can also distinguish between the two varieties with an ohmmeter. Variable reluctance motors usually have three (sometimes four) windings, with a common return, while permanent magnet motors usually have two independent windings, with or without center taps. Center-tapped windings are used in unipolar permanent magnet motors. For both permanent magnet and variable reluctance stepping motors, if just one winding of the motor is energised, the rotor will snap to a fixed angle and then hold that angle until the torque exceeds the holding torque of the motor, at which point, the rotor will turn, trying to hold at each successive equilibrium point.

For applications where precise measuring of a motors’ rotor position is critical, a stepper motor is usually the best choice. Stepper motors operate differently from other motors; stepper motors turn on a series of electrical pulses to the motor’s windings. Each pulse rotates the rotor by an exact degree. These pulses are called ‘steps’, hence the name ‘stepper motor’. Stepper motors are traditionally used in various motion control applications. Stepper motors are quite easy to wire and control. Stepper systems are economical. Stepper motors are video used in robotics control and in computer accessories (disk drives, printers, scanners etc.).

Stepper motors produce motion in discrete steps. Similar DC motors, steppers usually have permanent magnets on the rotor and coils on the stator with field movement provided by commutation from the power supply. Stepper motors have a specified number of steps per revolution (typically around 200 steps, or 1.8 degrees per step).

Stepper motors are usually controlled by digital signals from the controller to power drive, with one pulse corresponding to one step. Thus, the frequency of the digital signals controls the speed of the motor.

Stepper motors have limitations. They are available in limited power and their rotation speed is limited (usually maximum speed limit is about 2000 rpm). The energy of stepper motors is low and stepper motor systems have tendency to have resonances which needs to be avoided. Stepper motors have characteristic holding torque and pullout torque. Other torques can be difficult to achieve. Therefore, precise torque control is difficult with steppers. Because of open-loop nature of stepper motor controlling, they are not very good to be used with varying loads. It is possible for a stepper motor to loose steps if is loaded too much. Steppers are not recommended for high-speed or high-power applications, or for applications requiring precise torque control. The stepper motors typically have a rated voltage at what they can work without overheating. Operating the motor at this voltage limits the maximum speed and torque at high speed. The current limiting can be done by using power resistors. Stepping motors can be used in simple open-loop control systems; these are generally adequate for systems that operate at low accelerations with static loads, but closed loop control may be essential for high accelerations, particularly if they involve variable loads. If a stepper in an open-loop control system is overtorqued, all knowledge of rotor position is lost and the system must be reinitialized; servomotors are not subject to this problem.