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So, this might be a somewhat strange need, as I don't seem to see many resources on this. I'm trying to create a device where I can set an amount of force for it to generate, and when it would behave as such:

  1. If there is no or only a small opposing force, it would drive forward in the direction of the force.
  2. If there is a constant force being applied that is equal and opposite to the force being generated, it would stall indefinitely without damage.
  3. If there is an opposing force larger than the generated force, it would be pushed back, while still providing the constant force as a resistance, therefore reducing the effective backward force; and it should be able to handle all this without damage.
  4. All this only need to occur in the range of no more than 5 cm, and the simpler and smaller the system is, the better.

I was wondering if a linear motor, considering that they are also called a force motor, can handle such a demand, and if so, what type? There's also the concern that such a small linear motor can't be found anywhere (or might be prohibitively expensive).

Another thought that I had was to use a torque motor (with some gears to translate torque into linear force, of course), but I couldn't find a definitive source saying that those can indeed provide a constant torque even when being pushed back by the load, I just see that they provide high torque at low speed. Also, on the topic of torque motors, do DC torque motors exist, or are they AC only?

Of course, if you have a better idea for achieving this need of a constant force no matter what's actually happening in terms of movement and outside load, please let me know!

Finally, please do tell me if you think a different community would be a better place for this question.

Thank you all for your time!

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  • $\begingroup$ Just thinking out loud: For DC motors, torque is always linearly proportional to the current that flows through the motor. I believe you can achieve constant torque generation with a constant current source. You can easily convert torque to a linear force. $\endgroup$
    – Sina Atalay
    Commented Sep 4, 2023 at 11:57
  • $\begingroup$ Need for a constant force isn't uncommon. Sounds like you just need to load a constant force spring (possibly with a motor to wind it a set amount). Force wouldn't be adjustable (you'd have to add more springs or replace with a wider spring). A common constant force spring use is retracting measuring tape. $\endgroup$
    – Abel
    Commented Sep 4, 2023 at 12:20
  • $\begingroup$ Could you make a drawing of the situation, indicating what force you mean, and what velocity? $\endgroup$
    – Chris_abc
    Commented Sep 4, 2023 at 13:59
  • $\begingroup$ @SinaAtalay That's my thoughts as, well, but I wasn't sure about it considering that I don't see many sources about whether a regular motor would be damaged when forced to stall or even go backwards indefinitely. $\endgroup$
    – Jason Long
    Commented Sep 4, 2023 at 15:30
  • $\begingroup$ DC motors won't care about the rotation direction or stalling (there is no need for explicit research on this; this is the basics). You can only damage them by burning them with resistive heating caused by excessive amounts of current. This won't happen in your case since you will have a constant current, which will be carefully adjusted according to the motor. You might need a cooling fan though, if you plan to run this device for a long period. $\endgroup$
    – Sina Atalay
    Commented Sep 4, 2023 at 17:22

1 Answer 1

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If the changes are slow enough, a simple pulley with counterweight might do the job. Constant force hangers are used very commonly for piping, where you can find interesting designs using combinations of springs and lever arms. One of the simplest designs is here. enter image description here

Similar effect is also achieved in compound bows, that are optimised to resist the middle part of the draw with the maximum force in order to store maximum energy (the force also drops at the end of the draw, so you can hold the fully drawn bow with a lots of energy with smaller effort than traditional bow).

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  • $\begingroup$ Interesting idea on the CES, since I can just change the compression of the spring to achieve adjustable force, but I'm not sure I quite understand how it achieves that constant force (considering the size that I'm looking for I'll probably have to make my own). The force exerted by a spring is linearly related to deformation distance, so the force from the spring should be different between the two extremes, I believe? Or is the design of the pivot arm key to this as it changes the effective leverage along the travel distance? $\endgroup$
    – Jason Long
    Commented Sep 6, 2023 at 3:31
  • $\begingroup$ @JasonLong It it possible the force is not exactly constant, but close enough for practical purposes. Here is a good diagram from a similar design, where a moment equilibrium is shown at 3 different stages and I guess you might be able to calculate some key dimensions from that. $\endgroup$
    – Tomáš Létal
    Commented Sep 6, 2023 at 15:50
  • $\begingroup$ I see now. Either way, checking the site that you provided does indicate quite a hefty system. I wonder if I can replace the spring with a spiral spring and attach that to a motor... Either way, I'll wait a few more days to see if any other ideas pop up, especially about the motor part. If not I guess I'll accept this answer $\endgroup$
    – Jason Long
    Commented Sep 7, 2023 at 2:53

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