I am trying to see if I could accurately apply a static pull to a cord to create tension. I should not be measuring the tension directly, only be sure that the pull I apply is the force in Newtons that I want. The range is from a few tens of Grams to 7 kG. Either in one range or in several sub-ranges (say, 80G - 500G, 500G - 2kG, 2kG - 7kG) or such. With say, accuracy of 1% of the value.

I do not specialize in drives so I do not know how accurate a, say, BLDC motor torque could be when applying current, and what range I would get.

Any ideas? Any that have been tried?

  • 1
    \$\begingroup\$ Make a strain gauge into a pull force sensor and use feedback to control the motor. \$\endgroup\$
    – Andy aka
    Commented Mar 15, 2021 at 15:48
  • \$\begingroup\$ You might try using the arm of a hard disk drive as the actuator. To achieve tension accuracy of 1% might still require a sensing and feedback circuit. \$\endgroup\$
    – glen_geek
    Commented Mar 15, 2021 at 15:59
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    \$\begingroup\$ My first thought is to use a servo motor and a spring (or elastic band). Probably won't have the range you want though. The spring translates position into force. \$\endgroup\$ Commented Mar 15, 2021 at 16:09
  • \$\begingroup\$ This can be done -- maybe. What sort of precision do you need? What range of mechanical motion (if any)? Will the thing you're pulling on be moving, and how fast? Do you need high- as well as low-speed accuracy? Note that grams and kg are measures of mass, force is measured in Newtons. It's good to keep it straight. \$\endgroup\$
    – TimWescott
    Commented Mar 15, 2021 at 16:58
  • \$\begingroup\$ What range of distance do you want this force to be applied over? This is rather like the voltage compliance specification of a current source. \$\endgroup\$
    – Neil_UK
    Commented Mar 15, 2021 at 17:55

1 Answer 1


This is a fairly common thing to do with electric motors. Essentially you need to control the motor's current. The current is proportional to torque using a constant called the torque constant \$K_t\$ with units \$(Nm/A)\$, which is often given in a motors datasheet. If you can control the current flowing into the motor, say using a PI(D) controller then you can fairly accurately know the torque by the following equation \$\tau = iK_t\$.

The accuracy will depend on how you measure the current feedback and how you design your controller. Typically current measurements are fairly noisy so you will need low pass filtering to reject noise before being fed back into your controller.

It is important to note that the torque constant will not always be provided to you on a motor datasheet, and instead you will be presented with another constant known as the back emf constant \$K_e\$. If you convert the back emf constant into units of \$Vs/rad\$ then it will be the same value and the same units as the torque constant.


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