Force Gauges and PureData?
March 4, 2010 8:56 AM   Subscribe

I want to hook up a bunch of force gauges (or accelerometers?) to PureData--what's the cheapest way to do this?

This is intended to be a learning project for me to take my first steps into the realm of PureData. The idea was simply to hook up five or six digital force gauges to a kid's trampoline, and send the data to PD. My thought was then to average them in order to determine to location of the force on the trampoline, and use that data to send MIDI notes.

You know, to have a digital timpani (or steel drum) set that I could play with club hammers.

Looking online, digital force gauges seem to cost hundreds of dollars. I'm sure there's a cheaper solution out there, but I'm completely at sea. I imagine that I could use accelerometers for this as well, right? But I'm equally unable to find those for cheap.

I warn you, though, if you answer this question you are setting yourself up for a hundred more as I try to learn this stuff (I have a feeling that the word "Arduino" will be an important one for me). And then the endless Mefi Music links of synth-timpani (sympani?).
posted by Squid Voltaire to Computers & Internet (6 answers total) 1 user marked this as a favorite
 
I think you want strain gauges, not necessarily force gauges. You'd need some sort of I/O board, though, and I'm not sure how cheap that would be.
posted by Comrade_robot at 9:42 AM on March 4, 2010


You probably do want strain gauges -- here's a 10 pack from Omega for about $50 -- but a major problem is going to be hooking that up. The way the strain gauges work is that the resistance of gauge varies depending on the strain it's subjected to, so you need to be able to figure out what the resistance is at any given point in time and somehow translate that data into what you need for PureData.

This page seems to be really useful for seeing how they work. In most engineering/scientific applications, you'd probably be using some sort of data acquisition (DAQ) hardware, which can translate raw electrical signals into useful data depending on the sensor settings you give it. DAQs, and the software you use them with, are really, really expensive because they're made for precision applications and have built-in circuits to handle a number of different measurements.

I suppose you could try to use the analog voltage i/o ports on an Arduino board to try to measure the resistance of the gauges and pass that information on to your computer for further processing, but there are lots of potential issues in having to deal with tiny resistance changes. The page I linked to details what the problems are (such as: a strain gauge with a nominal resistance of 120 ohms would see a change of about ~0.1%) and how they're usually mitigated.

I've definitely thought of doing stuff like this with my Arduino, but it seems like it will maybe be more trouble than it's worth. And if you really want to do this with something like a trampoline, you'll probably have other problems as well -- the strain gauges won't work very well on super elastic materials and you'll have problems with the scale of it all.


Just realized as I was typing this out, though: if I wanted accelerometers that were big and durable (and that I could use with other stuff), I'd go ahead and buy a couple of WiiMotes and sync them to a computer using bluetooth. You could try to hook them up to the trampoline in a couple of places and use their data output (I think they'll do velocity and acceleration, in three dimensions) for whatever you wanted to do. If you wanted to do multiple hits in quick succession, though, you might have an issue with detecting the different hits since the trampoline will be moving a lot.

Good luck!
posted by malthas at 10:26 AM on March 4, 2010


Best answer: What a cool project.

"the strain gauges won't work very well on super elastic materials"

IAAME - strain gauges will not work at all on elastic materials like a trampoline. Strain is typically expressed in microinches per inch, and the stretching of the trampoline material itself is on the order of inches at a time. For all intents and purposes, consider strain gauges rigid, they are backed with thin metal (does not stretch compared to the elastic in a trampoline). They are typically bonded to the material you are investigating, if you attempt to install them on a fabric like a trampoline, they will just pop off when you jump/smash your mallet near by.

You would probably have better luck with accelerometers, or even a linear potentiometer. You could bond each end of the potentiometer in place so they all point towards the middle of the trampoline, and they would act like macro-scale strain gauges.

You could also install force gauges between the trampoline springs and the trampoline frame, so that at any time you could measure the force going through each spring. This would provide similar data. Along the same lines, you could just install a solid metal block between the spring and the frame, and put a strain gauge on that, measuring the strain on the block in line with the springs. As a first try, you could just install strain gauges on the trampoline frame itself, at each spring location, and measure the stresses on the frame! *lightbulb goes on*
posted by kenbennedy at 10:54 AM on March 4, 2010


(kenbennedy, I'm a mechanical engineer too, but I do thermal/fluids stuff and haven't worked with strain gauges since undergrad)
posted by malthas at 11:16 AM on March 4, 2010


Response by poster: kenbennedy : you could bond each end of the potentiometer in place so they all point towards the middle of the trampoline, and they would act like macro-scale strain gauges.

That was my first thought, and it looks like the keyword that I need to know was "linear potentiometer". Google shopping has them ranging from $1.5 to $400, which looks like a great place to start!
posted by Squid Voltaire at 11:30 AM on March 4, 2010


My 0.02;

1) Yes, get yourself an Arduino. I think they've got around 5-6 analog ins, if I remember....

2) Look into some cheap infrared distance sensors (Allellectronics, sparkfun, or digikey might be good places to start.) Construct a little sensor ring under your trampoline, with the sensors pointing up. They'll tell you how close the fabric of the trampoline is, and you can use that information to figure out where the jumping's going down. As a bonus, they won't get totally destroyed from being bonded to the trampoline / jumped on.
posted by Horizontally a Champion at 2:29 PM on March 4, 2010


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