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EETE JAN 2014

ANALOG & MIXED SIGNAL DESIGN Innovative back EMF based stall detection simplifies stepper-motor drive designs By Don Jacques Steper motors provide significantly more flexibility and control than traditional continuously rotating electric motors, and are widely used in a variety of industrial, consumer, and automotive applications. Many applications require reliable detection of when a mechanical end point is reached without using an external sensor, which can be achieved using motor stall detection. This article describes an innovative method of stepper-motor stall detection and its application. A general definition for a stepper motor is an electromechanical machine that moves a rotor shaft in small precise increments without feedback to govern motor speed. Stepper motors offer several advantages. They exhibit good speed stability when the load fluctuates, because the stepper can maintain constant torque. They have good startup characteristics, with maximum torque at zero speed. Stepper motors also have wide dynamic ranges and can accelerate faster than servo motors. Owing to the narrowness of the step angle, stepper motors have small mechanical transient responses, which makes Fig.1 : Typical behaviour of bipolar stepper drive at a stall condition; (left) waveforms in normal phase; (right) waveforms adjusted for comparison. Fig. 2: Typical hard stall behaviour; (left) motor continuing to attempt to rotate in same direction; (right) motor reversing direction. position and speed control possible without a complicated control loop. As a result, the cost of drive solutions for stepper motors is very affordable. Certain disadvantages occur with steppers, although advancements in electronics have helped to minimise their effects. For example, open-loop operation fails to provide information about absolute position, or whether the motor is responding to input commands. In addition, resonance can cause vibration if motor speed or winding current is not controlled properly, and motors can lose step if the speed is too high. Requirement for stall detection In practical systems, a means of stall detection is required to detect when the rotor is stationary: something that can be caused by any of several conditions. Rotation of the electrical field generated by the driver may lose synchronicity with the mechanical rotation of the stator, or the mechanical load may exceed the design limits of the motor. Any obstruction of the load path, including a fixed mechanical stop, can also cause the motor to stop rotating, but in these instances, without information on absolute position, the motor will attempt to drive through the obstruction in order to ensure that the load reaches the end point. This can cause wear, audible noise, heating, and mechanical failures. In addition, driving a stepper into a fixed stop by design inherently reduces the efficiency of the system, which is critical in battery-operated applications. To moderate these negative effects, an electronic integrated stall detection function can be used to ensure the load has reached its desired position, or to notify the user if the load is obstructed. When absolute positioning is not required, electronic stall detection can often replace a costly slip-clutch or optical encoder to provide stall detection. How stall detection works Electronic stall detection works by measuring the effect of back EMF on the number of PWM cycles. When a motor is stopped or is moving slowly there is little back EMF to impede the current in the phase windings. This allows the current to rise quickly to the limit and causes the PWM current control to activate. However, when a motor is rotating at normal operating speeds, the back Don Jacques is Strategic Marketing Manager at Allegro MicroSystems, LLC - www.allegromicro.com – He can be reached at djacques@allegromicro.com Fig.3 : Stepper motors come in a variety of sizes and power levels, with many options for precision of stepping performance. 26 Electronic Engineering Times Europe January 2014 www.electronics-eetimes.com


EETE JAN 2014
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