I need to feed a very fine nylon thread in a process by controlling a spool motor. There is not enough pull on the thread from the process to have the thread activate a push-button, even with a 1"-long lever on the button. Anyways ideally there should be no tension on the thread, so I guess you can say I want "slack management".

The system is controlled with an MCU but it doesn't really matter, all I want is a signal when the thread begins to tension to activate the feed motor for a few seconds (it is a 1/3 RPM 120VAC synchonous motor controlled by a SSR relay). I am not very well versed in mecanical things, perhaps there is a mecanical solution to this. I checked sewing machine-type thread tension regulator principles but these rely on a strong pull from the needle mechanism to regulate via springs, as I stated earlier the process can't and shouldn't pull on the thread.

As far as electrical detection goes, I am too newbie to know many solutions. I was thinking perhaps when the thread is slack it could rest on a plane with two contacts, shunting the gate of a mosfet to ground in a (very) high impedance input circuit, kind of like a "touch button" circuit. I'm not sure the thread has enough conductivity for this, I'll go now and check that, but perhaps someone can already tell me that can or can't work.

Of course all solution suggestions are welcome. Thanks in advance, J.

  • \$\begingroup\$ Hard to visualise, but could do with a diagram and an idea of whether expensive and complicated solutions are ok \$\endgroup\$
    – pjc50
    Jan 29, 2022 at 20:08
  • \$\begingroup\$ What are you feeding the thread into? \$\endgroup\$
    – DKNguyen
    Jan 29, 2022 at 20:29
  • \$\begingroup\$ The thread is fed into a succion tube - there isn't enough succion to provide much pull. There is no receiving spool. \$\endgroup\$
    – Joe
    Jan 29, 2022 at 20:33
  • \$\begingroup\$ @Joe So all of the tension available on the string comes from a suction tube (and it's not much?) How do you manage tension control, exactly? What are the details of the closed loop? What do you modify, assuming you can detect slack in the tension? \$\endgroup\$
    – jonk
    Jan 29, 2022 at 21:06
  • 2
    \$\begingroup\$ this question should really be asked at engineering.stackexchange.com \$\endgroup\$
    – jsotola
    Jan 29, 2022 at 23:27

2 Answers 2


Strain gauge could be a solution for sensing very low forces: Strain gauge There will be a change of resistance when the wire puts any force on the stain gauge. Resistance is easy to measure. There should be a hole or loop at the tip of the straing gauge and the other end would be fixed. Strain gauges come in many sizes and formats.

  • \$\begingroup\$ Might also be able to replace the strain gauge with a fiber optic whisker. Won't measure force but will indicate disturbance. \$\endgroup\$
    – DKNguyen
    Jan 29, 2022 at 20:31
  • \$\begingroup\$ Yea, I think the second best option would be by some form of an optical measurement. Something as simple a "slot photointerrupter" such as: media.digikey.com/Photos/Omron%20Elect%20Photos/… \$\endgroup\$
    – Ralph
    Jan 29, 2022 at 20:45
  • 1
    \$\begingroup\$ A strain gauge seems a good solution - I'm reading on them as I'm not familiar with those. I believe the succion from the vacuum pump would be enough pull for a precision strain gauge. They are cheap enough and could be easily interfaced with the on-chip precision analog comparator. I might flag that as solution after investigating both strain gauges and slot photointerrupters. Thanks gents! \$\endgroup\$
    – Joe
    Jan 29, 2022 at 21:53

In industry this would be referred to as a dancer-controlled unwind system.

enter image description here

Figure 1. A label unwind stand with dancer-controlled motor. Image source: AZCO.

A longer dancer arm will give you more torque for a given thread tension. This one, for example, has a long lever and a maximum actuation force of 1.5 N (0.15 kg). If this is too much then consider an optical or inductive proximity switch to give contactless monitoring of the dancer position.

For engineering you need numbers. Get a digital kitchen scales or similar, put an object on it, tare the scales and pull the thread down both sides of the object with the maximum tension your want. Record the measurement in kg. Multiply by 10 (9.81 m/s/s) to convert to newtons (N). Then look at the switch specifications and see if it is sensitive enough.

  • \$\begingroup\$ Thanks for your reply! I learnt a lot today. The push-button with lever I mentioned in my original question is the same microswitch type as the one you linked, except it has a smaller lever. I could try to extend the lever and see if it is actuated when the pump pulls on the thread. \$\endgroup\$
    – Joe
    Jan 29, 2022 at 22:03
  • \$\begingroup\$ @Joe: See the update on tension measurement. \$\endgroup\$
    – Transistor
    Jan 29, 2022 at 22:13

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