DT0002 Design tip Reducing torque variation in half step drive By Thomas Hopkins Main components L6506 Current Controller For Stepping Motors L6207, L6227 Dual DMOS Full Bridge Driver with PWM Current Controller Purpose and benefits This note describes a technique to reduce torque variation when driving two phase bipolar stepper motors in half step. Reducing the torque variation can reduce the chance of the motor stalling and give more reliable operation. Description The stepping sequence for a half step drive of a two phase bipolar stepper motor alternates between states driving current in both phases and driving current in only one phase. In many implementations, the current level is set to be the same whether one or both phases of the motor are being driven, as shown in Figure 1a. Figure 1. Current waveforms in half step operation For the motor, torque is proportional to the magnitude of the stator and rotor magnetic vectors times the sine of the angle between the stator vector and the rotor vector. When driving a motor with two coils, the magnitude of the resulting magnetic vector is the vector sum of the vectors produced by each coil. If the two coils are oriented 90 degrees apart April 2012 DT0002 Rev 1 1/3 www.st.com and are driven with equal currents, the magnitude of the resulting magnetic vector is √2, or approximately 1.4, times the magnitude of the individual vectors. Since torque is proportional to the amplitude of this vector, one can easily see that as the magnitude of the vector changes from 1.4, with both coils energized, to 1, with only one coil energized, the torque also is changing and has a variation of about 40%. This variation can be reduced by increasing the current in the coil during the half steps when only one coil is being driven. If the magnitude of the current is increased by √2, as shown in Figure 1b, the magnitude of the magnetic vector will also be increased by √2 and will be equal to the magnitude of the vector when both coils were driven by the original current. 5V Figure 2 shows an easy way to implement a L6506, L circuit to drive the stepper as shown in Figure Microcontroller or L6227 1b. In this example, the microcontroller Vref1 Open Drain provides the phase and enable information for Vref2 GPIO the stepper to a driver like the L6207 or L6227 or a current controller like the L6506 used with a driver IC like the L6201, L6202, L6203 or Enable 1 L298. The resistor values are calculated so Enable 2 GPIO or IN1 that the reference voltage is reduced by a ratio Timer PWM IN2 of 1.4:1 when the open drain output is turned IN3 IN4 on. For a typical implementation, the values may be set at 0.7V and 0.5V. When the microcontroller turns both coils on, it also turns Figure 2. Connection Diagram on the open drain device and reduces the reference voltage. When only one coil is energized, the open drain device is turned off and the reference voltage is raised to the higher value so that the current in the single coil is increased to 1.4 times the current when both coils are on. With this technique, the variation in torque caused by the one on - two on drive pattern can be reduced or eliminated. Support material Documentation Datasheet L6506, Current Controller For Stepping Motors Datasheet L6207, Dual DMOS Full Bridge Driver with PWM Current Controller Datasheet L6227, Dual DMOS Full Bridge Driver with PWM Current Controller Revision history Date 19-Apr-2012 April 2012 Version 1 Changes Initial release DT0002 Rev 1 2/3 www.st.com Please Read Carefully Information in this document is provided solely in connection with ST products. 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