Stepper motor

Stepper motors Stepper motor is a device that produce rotation though equal angles called as steps, for each digital pulse supplied to its input.

Stepper motors Variable reluctance motor Rotor is made of soft steel and is cylindrical with four poles, fewer poles than on the stator. When opposite pair of windings has current switched to them, a magnetic field is produced with line of force pass from stator to nearest poles of rotor. Rotor will until it is in minimum reluctance position. Step angle 7.5 deg to 15 deg.

Permanent magnet stepper Two phase four poles. Coils on opposite pairs of poles are in series. Current is supplied from dc source. Rotor is a permanent magnet. Rotor rotates in 45 deg steps. Step angles 1.8, 7.5, 15, 30, 34, or 90 deg available.

Hybrid stepper motor Combined features of both variable reluctance and permanent magnet motors. Permanent magnets are encased in iron caps which are cut to have teeth. It motor has n phase and m teeth on the rotor, the total number of steps per revolution will be nm 0.9 and 0.8 deg steps available. High accuracy positioning applications.

Specifications Phase Step angle Holding torque Number of independent windings on the stator, eg a three phase motor. Step angle Angle through which the rotor rotates from one switching change for the stator. Holding torque Maximum torque that can applied to a powered motor without moving it from its rest position and causing spindle rotation.

Pull – in torque Pull – out torque This is the maximum torque against which a motor will start for a given pulse rate and reach synchronism without losing a step. Pull – out torque Maximum torque against that can be applied to a motor, running at a given stepping rate, without loosing synchronism.

Pull – in rate Pull – out rate Slew range Maximum switching rate at which a loaded motor can start without loosing a step. Pull – out rate Switching rate at which a loaded motor will remain in synchronism as the switching rate is reduced. Slew range Range of switching rates between pull-in and pull- out within the motor runs in synchronism but cannot start up or reverse.

Bipolar stepper Unipolar stepper

H bridge

Stepper motor control Two phase motors are termed as bipolar motors when they have 4 connecting wires for signals. Solid state switches can be used to switch dc supply between the pair of stator windings.

Bipolar stepper

Merits and demerits Merits A high accuracy of motion is possible, even under open-loop control. Large savings in sensor (measurement system) and controller costs are possible when the open-loop mode is used. Because of the incremental nature of command and motion, stepper motors are easily adaptable to digital control applications. No serious stability problems exist, even under open-loop control. Torque capacity and power requirements can be optimized and the response can be controlled by electronic switching. Brushless construction has obvious advantages.

Demerits They have low torque capacity (typically less than 2,000 oz-in) compared to DC motors. They have limited speed (limited by torque capacity and by pulse- missing problems due to faulty switching systems and drive circuits). They have high vibration levels due to stepwise motion. Large errors and oscillations can result when a pulse is missed under open-loop control.