Unable to Move Motor Performing Torque Control With AKD Drives, Servo Motor and LabVIEW

Updated Sep 25, 2020

Reported In

Hardware

  • AKD Series Servo Drive

Software

  • LabVIEW
  • LabVIEW SoftMotion Module

Issue Details

I am using the Pos Vel Torque Setpoints.lvproj example provided in LabVIEW 2018 to control my AKM 32e Servo Motor through the AKD00606 Drive. The position and velocity controls are working fine. However, the torque control is not working as expected. For example, from my motor datasheet I can see that the static friction is 0.18 lb-in, but when I introduce a value of 0.20 lb-in in the torque setpoint my motor does not move at all. I need a value of 7 lb-in to start the movement, why?

Solution

The torque control is made by sending current commands instead of torque commands. The electrical current creates magnetic flux and that flux creates the torque. The current is measured and regulated, but not the torque. Torque Setpoint input of the block diagram shown below indicates (on the Help menu) that the value, for AKD EtherCAT Drives, should be given as thousandths of the rated torque, although it should be read as thousandths of the continuous current of the Drive. 



For example, the continuos current for the AKD00606 is 6 A. The AKM 32e Servo Motor has a torque constant of 6.5 lb-in/Arms and static friction of 0.18 lb-in. If you want to know which value should introduce in LabVIEW in order to beat the static friction, here is the formula to get the theoretical value1:

Setpoint = Desired Torque · 1000 / Continous Drive Current · Torque Constant. 

For this example, Desired Torque is equal to the static friction. Any Setpoint higher than this value theoretically should move the motor.

Additional Information

1. It is very important to know that this is just a theorical value. There are large tolerance values for torque which can produce differences between the real and the theoretical value. One of the main reasons is the manufacturing process that could modify the properties from one motor to another. It is recommendable to use the position or velocity control instead of torque control.