What is Precision Torque Control?

Motors, particularly DC Brush and Brushless DC motors, are increasingly used to deliver or measure precise amounts of torque or force. The reason for this is that modern electronics allow the amount of current flowing through the motor to be commanded, and controlled, very accurately. Therefore, the amount of torque generated by the motor is similarly controlled. This method of torque generation and control is called Direct Motor Torque Output. Both rotary and linear motors can be used in the torque control application although rotary motors are far more common.

Precision torque measurement using direct motor torque output uses the same principle but in reverse. When the torque delivered by the motor equals the torque returned from the object being measured the motor output torque equals the torque being measured. This is the basis of hardness measurement devices. These devices work either by applying a specific torque or force via the motor and recording the distance of deflection in to the object being measured, or the motor torque is increased until a specific deflection occurs and the torque output is then recorded.

It should be noted that some torque control systems (particularly those delivering very high amounts of torque) use an external device such as a strain gauge to explicitly measure and control torque output. In this motion application however we will focus just on direct motor torque output.

What type of controls are needed for precision torque control? For greatest current accuracy a calibrated current loop control scheme is used. For three-phase brushless motors a leg-current sense resistor scheme is typically used. Leg current sensing improves over other schemes because it references the measured current to electrical ground, reducing noise and simplifying downstream signal processing.

These signals are then passed to an ADC (Analog to Digital Converter) with at least 12 bits of resolution, and these reading are then input to the torque controller’s current loop which combines them with the desired current (torque) command to develop a PWM (Pulse Width Modulation) amplifier voltage command for each coil. The accuracy of the resultant current controller is further enhanced by calibrating the resistors and ADCs, allowing current control accuracies of 1.0% or better.

Are all motors equally suited for precision torque control? No. When motor current is used to generate a commanded torque there is a premium on having a minimum of extraneous torque altering elements. This means the motor should not use a gearhead, should have bearings that are smooth with low friction, and any attached mechanical couplings should have as little stiction or backlash as possible.