# Accurate Static Pulling (with electric motor?)

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?

• Make a strain gauge into a pull force sensor and use feedback to control the motor. Commented Mar 15, 2021 at 15:48
• 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. Commented Mar 15, 2021 at 15:59
• 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. Commented Mar 15, 2021 at 16:09
• 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. Commented Mar 15, 2021 at 16:58
• 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. Commented Mar 15, 2021 at 17:55

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 $$\\tau = iK_t\$$.
It is important to note that the torque constant will not always be provided to you on a motor datasheet, and instead you will be presented with another constant known as the back emf constant $$\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.