Non-Dissipative Control Saves Power And Cost In Stepper Motor Applications

by Enrico Poli and Jean-Jacques Meneu, STMicroelectronics, Agrate, Italy, How2Power Today, Aug 30 2012

Focus:
A stepper motor needs to control the current in the coils in order to control the torque and speed of the motor. To obtain the desired current, the traditional method senses the current flowing through the motor and provides feedback to the controller chip, which then decides whether to increase or decrease the current in the coil. In general, this method has required the use of expensive, large and highly dissipative shunt resistors. This article presents two new control techniques that avoid using these shunts resistors, and hence, decrease cost, complexity and power dissipation. One approach uses current-mode control while the other uses voltage-mode control. These solutions do not increase the cost of the application nor require extra room on the PCB as the power level increases. With current-mode control, current sensing is integrated on silicon: an H-bridge integrates a current mirror that generates a small current proportional to the current flowing in each MOSFET. Motor driver ICs using this method include STMicroelectronics’ easySPin L6474 and dSpin L6472. With voltage-mode control, current to the motor is controlled indirectly by applying a voltage. The applied voltage is adjusted to compensate for the effects of the motor’s back EMF, which increases with the speed of the motor. This achieves a flat current amplitude (and therefore constant torque) whatever the speed of the motor.

What you’ll learn:

• How to decrease the power dissipation associated with current-sensing in stepper-motor applications
• How to understand the operation and benefirs of nondissative current-mode control and voltage-mode control in stepper-motor applications

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