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
    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.

  2. manulal


    Dec 6, 2010
    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
    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


    Mar 14, 2008
    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


    Jun 19, 2012
    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
    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?