current measurement DC Motor and torque control

Good day,
I am using a 24V brushed DC Motor. It is driven by a TB9051FTG from pololu. I am measuring the current using a ina219. The ina219 is between the powersupply and the motor driver.
With the measured current I implemented a torque controller on an arduino.
So far so good. It worked perfect. Now after using this setup for around 2-3 months, for around 30minutes a day, somehow the setpoint for the current is not sufficient enough to reach the desired torque. So I had to adapt the setpoint.
I am curious what causes this?
My thinking is either:

  • Friction has changed
  • motorconstant has changed
  • something is affecting the ina219 reading

Thank you very much
Greetings

You failed to mention how you are powering all this?

Paul

Given all the conditions, I would think:

  • Are these 30min a day close to noon? Is this system exposed to the weather or close to any heat source? Maybe light or temperature could be affecting the sensor, drive or the arduino itself.

and this are the most common:

  • Check the motor brushes, they wear out due to constant friction and this is a first signal that maybe it's time for a replace

  • Check the lub in the load attached to the motor or any presence of dust which can be interfering in the torque demand.

Hope I have helped

Sorg

Friction has changed - very likely if system components are subject to wear/changes
motorconstant has changed- very unlikely.
something is affecting the ina219 reading - easy to verify with an ammeter in series with your motor.

thanks for the replies:

Paul_KD7HB:
You failed to mention how you are powering all this?

Paul

I am using a 24V power supply from meanwell (RSP 500).

sorg:
Given all the conditions, I would think:

  • Are these 30min a day close to noon? Is this system exposed to the weather or close to any heat source? Maybe light or temperature could be affecting the sensor, drive or the arduino itself.

and this are the most common:

  • Check the motor brushes, they wear out due to constant friction and this is a first signal that maybe it's time for a replace

  • Check the lub in the load attached to the motor or any presence of dust which can be interfering in the torque demand.

Hope I have helped

Sorg

It is beeing used inside and through out the day, so no special condition changes. For the brushes: this will lead to resistance change, right? And a different resistance will lead to different current if the voltage stays the same right? But the current controller will make sure the right current is reached. And as the motor constant stays the same, the torque should remain.

Looking at existing torque control for motors, would you say that this is a common problem that at some point the torque will not be the same (if you use currentmeasurement and dont measure torque directly), or is it just my special case ?

Thanks

jacko91:
I am curious what causes this?
My thinking is either:

  • Friction has changed
  • motorconstant has changed
  • something is affecting the ina219 reading

Thank you very much
Greetings

The motor constant depends only on geometry and the permanent magnets (I presume permanent
magnet motor - you never posted details of the motor though). There may be a small dependence on
temperature for the magnet, but it shouldn't drift with time.

Probably friction, friction is very hard to tame, you might have bearing wear, dried out bearings,
dirt getting into the bearings, grime build up on the commutator, or simply bedding in of the
commutator might have some effect. Temperature will also affect the friction/drag in the bearings if
they are pre-greased, as grease viscosity varies with temperature. Bearings with rubber seals can
have drag due to the seal rubbing (metal seals don't actually seal, so don't rub, but do let dust in
eventually)