I would like to control 14V 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:
- 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?
- 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?
- 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?
Okay, for now this is more clear. Please correct me if I am wrong: I do not need to sense voltage of the battery for any calculations or compensation. The only thing I have to sense is motor current, because the current is proportional to the torque. The battery discharge will not have any impact for the current measurement, depending on the same load torque. So shortly, I will get the same current readings when the battery is fully charged and the same current readings when the battery is discharged let say to 60% of full voltage.
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.
The things with current and voltage sensing seems ok. But I forgot another thing - temperature of motor. What impact to the torque (current) will have temperature changes in motor? What is the relationship between temperaute and current of BLDC motor?
Thank you for your answers.