BLDC motor torque control using current measurement

Thread Starter


Joined Oct 5, 2016
I would like to control 18V BLDC motor of battery powered drill. What I want to do remake the drill to nut runner (torque wrench), so I need to sense a motor current which is proportional to motor torque. My controller will include microcontroller. Here are some questions:
  1. How battery voltage discharge can affect motor current measurement depending on the same load (for example I would like to wrench the same nut ant the same torque Nm)? Does it will affect my current reading and the current will increase?
  2. Could I sense the BLDC motor current directly on the battery high side or do I need to sense it on all three the motor phases?
  3. Is it enough to sense only current to create closed loop torque control, or do I need some additional parameter for torque control based on the different battery discharge levels?

What I plan to do is to create relationship table between current readings and torque (Nm) of a load, and save these values in a microcontroller memory. So when the torque wrencher reaches knowing current value, controller will know that the bolt or nut is wrenched with a fixed known torque and motor stops.

Thank you for your answers.


Joined Jun 19, 2012
The big problem with this idea is the energy stored in the rotating mass.

Don't you think every cordless drill would be an electronic torque wrench if this idea worked?

The rotating parts store enough energy to over tighten your nuts, the current rises after this energy has been dissipated and its too late.