Precision Torque Control

What is Precision Torque Control?

Precision Torque Control means control of motors for the purpose of measuring, or applying, a precise torque or force. Precision torque control is used in a wide array of industries and machines including bottle cap applicators, torque wrenches & screwdrivers, hardness measuring equipment, and more.

Design Considerations for 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.

 

precision-torque-control-motion-application-image-rev1-resized

 

Vital Precision Torque Control Techniques

 

  • DC Brush or Brushless DC motors
  • Leg current sensing
  • PI current control loop
  • Sinusoidal encoder-based commutation
  • Slotless BLDC motors
  • High precision ADC
  • High resolution PWM (Pulse Width Modulation)
  • Digital switching amplifier

 

 

Another important consideration in choice of motor is torque output linearity. In other words, as the motor rotates (or moves linearly) how constant is the torque generated? Slotless motor designs are preferred since they minimize detent torque, also called cogging torque. Similarly, encoder-based sinusoidal commutation is preferred over traditional six-step Hall-based commutation because the elimination of abrupt changes in commutation angle can reduce torque variability from 10 % to as low as 1 or 2 %.

DC Brush motors, which commutate themselves, can also be used in the precision torque control application. They are lower cost however the contacting brushes not only add friction, thereby distorting the direct motor torque output function, their mechanical characteristics may change over time as the brushes wear down. Nevertheless, DC Brush motors are a viable choice in more cost sensitive precision torque control applications.

Finally, linear BLDC motors can be used in the torque control application but this is uncommon because of their cost. Alternate linear actuators that are more cost effective are linear DC motors and voice coil actuators. Both of these actuator types are sometimes used for delivering small, electronically controllable levels of torque.


Precision Torque Control Architecture & Approach

The diagram below shows the control flow for a typical high performance digital switching amplifier with leg current sensing. Such an amplifier architecture, coupled with a PI-based current loop controls and a calibration procedure for sensing resistors and A/Ds can provide high accuracy current control and therefore highly precise torque control of rotary and linear BLDC motors.

precision-torque-control-motion-application-diagram

High Performance Digital Switching Amplifier with Leg Current Sensing for Torque Control


Precision Torque Control Solutions from PMD

Since 1992 Performance Motion Devices motor control ICs and drives have been used in precision torque control applications due to their unique combination of high performance and and low cost. PMD’s MC53113 Brushless Motor Control IC, which provides high performance current control, is ideally suited for precision torque control applications as is PMD’s MC73112 BLDC Torque Control IC. PMD’s N-Series ION Digital Drive represents a ready-to-go solution in a PCB-mountable module format which excels in precision torque control applications. 

 


Machines That Frequently Use Precision Torque Control

  • Bottle capping equipment
  • Hardness measuring machines
  • Torque wrenches
  • Torque screwdrivers
  • Automatic fastening equipment
  • Liquid dispensing equipment
  • Robotic grippers
  • Haptic control systems
  • Web tensioning equipment
  • Textile machinery
  • Coil winding equipment
  • Pick and place machines
  • Robotic end effectors
  • Mobile robotics

Need advice from an expert?