Does anyone know of a wire pullstrength sensor ? - Page 1
Forum section: electronics / projects

author: Kjelt
author_rank: Super Contributor
timestamp: May 15, 2019, 08:54:10 pm
content: Ok problem is as follows, I have a tubemotor unwinding a solarscreen or windowscreen and on the other side a tubemotor with two wires pulling this screen so it is tense/flat.Actually so it covers the ceiling and if the pullstrength is too low it will hang towards the floor.I am looking for a sensor i can put inbetween the Nylon or metal wire that gives a uC an indication about the pullstrength on the wire so it can control the second motor to sync to the first and it does not have to be very accurate.Has anyone seen something like this or knows alternatives?

author: KL27x
author_rank: Super Contributor
timestamp: May 15, 2019, 09:03:04 pm
content: Sounds like you want a strain gauge.I would consider trying to estimate the relative load, indirectly, by way of motor current consumption.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 15, 2019, 09:09:18 pm
content: just use a load cell (an aluminum block with a strain gauge glued to it) with a suitable amplifier module (both of which you can find on amazon for dirt cheap).Just use a suitable load cell which goes up to the force you require.

author: Benta
author_rank: Super Contributor
timestamp: May 15, 2019, 09:33:10 pm
content: Far too complicated.Just let the "pulling" tubemotor run and disengage the other (let it run free or lightly braked). When the screen is in place, brake the second tubemotor and by monitoring current in the pulling motor stop it at this point.Your screen is now taut.Works in the other direction as well.

author: soldar
author_rank: Super Contributor
timestamp: May 15, 2019, 09:57:03 pm
content: Quote from: Kjelt on May 15, 2019, 08:54:10 pmI do not know the particulars but it sounds like a bad design to begin with. In principle you do not need two motors at all as you can have one rolling up the screen and working against a constant tension provided by weight, spring or whatever. Having to control two motors to achieve the correct tension seems unnecessarily complex IMHO.

author: Kjelt
author_rank: Super Contributor
timestamp: May 16, 2019, 08:18:18 am
content: Quote from: KL27x on May 15, 2019, 09:03:04 pmThanks for the term also OM222O thanks.I will certainly look into that. The motor current could also be a good parameter, it is 230VAC but with a small current clamp around the phase or something like that it is easy to measureQuote from: Benta on May 15, 2019, 09:33:10 pmHave to investigate this because I think the Somfy motors brake the shaft when power is off and I never noticed that they can be disengaged.They are usually used for window blinds, you know those heavy aluminium bars, and you don't want to have the whole blinds tumble down if you stop halfway.Quote from: soldar on May 15, 2019, 09:57:03 pmAt this moment it works with a heavy spring, so heavy that the metal wires can be used to play pianoTwice already the past ten years the spring has disengaged, a lot of noise and even damage. So I would like to replace this since it is too heavy for the application. Since I want to go to a new wider screen I am looking for alternatives. But thanks for bringing it up, valid point.

author: soldar
author_rank: Super Contributor
timestamp: May 16, 2019, 10:11:33 am
content: Quote from: Kjelt on May 16, 2019, 08:18:18 amIf the tension is too high then that is a reason to make it lower, it is not a reason to redesign the whole thing from scratch and throw the baby out with the bathwater. Adjusting the spring tension should be fairly easy.The way I see it the motor would have limit switches/sensors at both ends and adequate tension provided by spring or weights or whatever. Keep it simple.If you want to get fancy you can design a system where the tension increases as the shade reaches maximum extension and that keeps it well extended but as soon as it starts to be retracted the tension diminishes and makes the motor work less.IMHO having two motors is the worst solution and is asking for countless problems.ETA: I am attaching some ideas for tension activated switches which I have used in the past.

author: Housedad
author_rank: Frequent Contributor
timestamp: May 18, 2019, 03:00:16 am
content: Why couldn't it be done with just current sensors and a microcontroller ?

author: Kjelt
author_rank: Super Contributor
timestamp: May 23, 2019, 08:46:56 pm
content: Quote from: brennafullen on May 23, 2019, 08:09:42 pmHad? Did you solve it?

author: KL27x
author_rank: Super Contributor
timestamp: May 23, 2019, 09:49:58 pm
content: QuoteThis was my suggestion. Measuring current consumption of the motors. A strain gauge may give you higher precision/resolution?Any way you slice it, you are probably better off with springs in addition to electronics. What I presume is a worm gear motor will hold a high tension, but you should want springs, too. They wouldn't be there to roll up the screen; springs would be there to give a bigger zone of relatively constant tension. Even if you stop a worm gear motor at exactly the right spot, you may end up stretching the screen/wires or whatnot. Then your electronics wind the motor up again, either right then or on the next go around. And you risk cumulative damage to the screen/cables or suboptimal tension. I am imagining relatively short and stiff springs.Limit switches plus spring tension could be just as good, really.