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Suppose I want wo measure how strong someone (or somthing) pulls on a rope, i.e. I want to measure the tension (=dragging) force on the rope. The pulling would come from only one direction. I assume a maximum force of let´s say about 1000N (could be more, but let´s forget about the exact range for now).

What would be a reasonably priced method? I searched a bit on the web and found lots of force sensors which should do the job, just one example out of many would be , e.g.

https://www.burster.de/de/produkte/p/detail/8417/

Now, as far as I can see, this part has "just" some resistance strain gauge(s?) with a wheatsone bridge incorporated. Nevertheless, It is priced at about 800 EUR per piece, something I cannot afford.

I thought about building an own setup, i.e. sticking discrete strain wire gauges to some kind of cylinder. Of course the question arises which material to use for the cyclinder.

Can someone share experiences in building "custom" strain gauges for such a scenario? Maybe resistance strain gauges are even a bad idea, maybe an LVDT (Linear Variable Differential Transformer) in combination with a spring would be better suited?

EDIT: The solution should be small enough to be integrated in some kind of handheld device (let´s assume a box of 20 x 5 cm) Environmental protection such as IP5X is not needed since ideally, the solution can get some kind of enclosure later on.

Thanks!

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  • \$\begingroup\$ Just connect a dynamometer in series... \$\endgroup\$ – Eugene Sh. Jun 22 '17 at 14:43
  • \$\begingroup\$ Well, I search for some solution that can be made relatively small so I can integrate the setup into some kind of handheld device. Therefore, I also don´t need some heavy cover like most of the load cells that are rated for IP5x or IP6x \$\endgroup\$ – Junius Jun 22 '17 at 14:52
  • \$\begingroup\$ Hm. I don't get how can you have such a handheld device. Measuring tension involves connecting some sensor in series with the measured force. I don't think there is another way around. \$\endgroup\$ – Eugene Sh. Jun 22 '17 at 14:54
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    \$\begingroup\$ How accurate do you need to be? 1000N is only about 225lbs, so something like this would work, just need to find a larger scale... They have digital versions, with remote sensors, depends on what you want to spend... \$\endgroup\$ – Ron Beyer Jun 22 '17 at 14:58
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    \$\begingroup\$ WHat qty? what budget? I would get the 60 pc strain gauge kit from Vishay $30 the bond adhesive $60 and flat spring steel for mounting. But if just making 1 for DIY then I would buy a mechanical big fish scale or modify a home digital scale for a fixed application. \$\endgroup\$ – Sunnyskyguy EE75 Jun 22 '17 at 17:21
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If you don't wish to homebrew your own transducer, look into "strain type load cells". These are typically found in S and Z type configurations. An example of an S type cell that is available for $130 USD can be found here.

enter image description here

You will have to add your own means of mechanically fastening the rope ends to the cell but most come with tapped holes for easy attachment of eye bolts or similar.

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You can get your preferred style of sensor from China for less than 10% of the price of the German unit. In other words, maybe you should try harder.

enter image description here

There are also (probably less convenient) sensors in digital luggage scales that are even cheaper, even if you buy one and throw away the electronics.

enter image description here

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You don't need something that goes in series with your rope. Google "cable tension meter" and you will see a lot of configurations that work on the same principle as this https://www.youtube.com/watch?v=aDb0SnNdSh8

tension meter

I don't see any great difficulty in home brewing this kind of tension meter. A spring loaded plunger and a way to measure how far the plunger moves. Calibrate it with weights on the rope hanging vertically, and a look-up table in an Arduino Nano or similar simple micro.

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Get a few of these from eBay, £50 for 10:-

gauge

Glue it on with some epoxy and solder a couple of wires on. This is the standard technique for ad-hoc force measurements. Wheatsone bridge + opamp /instrumentation amp and you should be there. There's a good guide here as to gluing them on.

Glue them onto anything metal that's rectilinear. I mean it's long and thin so that you get even strain across the cross section:-

rod

You could even bend a steel strip say 500mm long to form hooks either end. Stick a gauge each side of the strip. Some simple formulae will allow you to determine the overall strain and allow for any bending moment. The type of metal doesn't matter as long as you know what it is to get the Young's modulus. Just don't make it too thin as to neck it at the gauge location when you pull. 1000N is nothing. Get a 10 x 2mm strip from somewhere that'll allow you to identify the metal.

This is really one for engineering.SE

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    \$\begingroup\$ No. Don't do this. Buy an off the shelf load cell. Strain gauges are a pain to work with. They don't solder easily, are fragile, and getting the mounting right is critical. Trying to do if yourself if you don't know what you're doing is a recipe for enormous headaches. There is a reason that there are engineers that have dedicated their careers to load cell design. \$\endgroup\$ – Matt Young Jun 23 '17 at 2:40
  • \$\begingroup\$ The load cell he referenced probably has 4 strain gauges arranged in a radial pattern on spokes and wired to compensate for off-axis strain. \$\endgroup\$ – Spehro Pefhany Jun 24 '17 at 13:34

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