# Can I estimate the force on a mass by measuring the current?

Discussion in 'General Electronics Chat' started by Paul24, Nov 1, 2012.

1. ### Paul24 Thread Starter New Member

Nov 1, 2012
7
0
At the moment I am designing a device, for which I need to know the force exerted on a mass. I thought of this approach:
- Do some experiments to find out the relationship between the current drawn by the motor and the force on the mass.
- If I know the relationship, I can just measure the current in order to know the force on the mass, and I dont need a force sensor anymore

Is it possible to do it with this approach? Does a clear relationship between current and motor output exist?

I hope someone can help me with this (easy?) question.

See attached picture for clarification.

Ciao

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2. ### manulal Member

Dec 6, 2010
17
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The torque Vs current relationship need not be linear. I think you will have to calibrate it.

Make the motor lift various weights vertically and calibrate the set up for torque Vs current!

Note:- Its not force but torque as the load will vary with the dia of pulley you use.

3. ### Paul24 Thread Starter New Member

Nov 1, 2012
7
0
Thank you for your reply. Is it convenient to do it this way?
Maybe there are some difficulties with calibrating.
I was thinking of: Does the relationship change over time? When the motor is running for some seconds the relationship becomes different? And maybe the current drawn is history dependent? I mean; does the current drawn for a certain torque is different when the power output was high just before the certain torque in comparision with having just before a low power output?
Is this method fast enough for control purposes? Is there a big timedelay between current drawn and motor output?

Would be nice if anyone has an answer on this

4. ### crutschow Expert

Mar 14, 2008
20,029
5,634
The torque is proportional to both current and speed I believe since for a given torque the output power (and thus the input current) is proportional to speed. That means you would have to measure both motor speed and current to determine torque.

A common way to measure motor torque is to mount the motor on a platform with a pivot hinge and use a scale on the side opposite the pivot to measure the force. From that and the dimensions of the platform (lever arm) you can calculate the torque.

5. ### Sensacell Moderator

Jun 19, 2012
1,977
613
If you actually have a true linear moving coil motor, you could calibrate it by rigging it up to press on a platform scale.

The current to force relationship should be relatively linear, until you reach saturation.

Perhaps some variation related to coil position also, so test for this too.

6. ### Paul24 Thread Starter New Member

Nov 1, 2012
7
0
Actually I will use a motor with a leadscrew attached to it, so as Sensacell mentioned I will take the position into account, and measure the velocity as well to get the torque.
But history dependency and time delay is not a thing to consider?