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Topic: Measuring Force (Read 3304 times)previous topic - next topic

OzGrant

Feb 01, 2011, 07:20 am
G'day,
Attempting to make a clapper to strike a bell with variable forces, to obtain different volumes (at least three).
Am driving a rotary voice coil(from an old disk drive) with a DF-Robot motor shield and attached a A7269 tri axes accelerometer where the disk heads use to be.
As the A7269 path is an arc, it is just a simple task of adding both the X and Y values to obtain the g force produced.
As force = mass * acceleration, will add mass to the clapper to match the bell requirements (that I do not have at this stage)
So my first problem is measuring the actual G force when it hits the bell.
Trying to get the theory worked out before to much code is written.
Tks

retrolefty

#1
Feb 01, 2011, 07:43 am
Well my first thought is to use a strain gauge / load cell. It certainly is a direct measure of force. However they are usually used for reading static weight or at least slowly filling weight, so I'm not sure how accurate they would respond to short impulse force of the clapper. I'm sure you would get some kind of measurement response.

Second thought would be a piezo sensor. Cheaper but you would probably have to rectify/filter to get a average force impulse output.

http://cgi.ebay.com/5x-New-FT-20T-6A1-MINIATURE-UNCASED-PIEZO-TRANSDUCER-/250445825335?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item3a4fbc0537

Anyway, good luck, you sound like you have an interesting project and I like instrumentation/measurement applications. Keep us posted.

Lefty

OzGrant

#2
Feb 01, 2011, 10:02 am
Lefty,
Sort of want to stick with the A7260 to measure the acceleration, as it's comes pre calibrated, and the resultant Force measurement then determined by F=ma. In theory I can alter the PWM to the voice coil until the required acceleration is reached, wait for the increased coil current when the clapper hits the bell and then reverse the drive to the coil and retract the clapper from the bell.
Well it sounds ok in theory. But my first task is to accurately measure orbital acceleration along the circular path before impact.
Not even sure if I'm using the correct accelerometer, as I only want to measure dynamic acceleration and not the static component due to gravity.
If no solution is found in a few days will just suck it and see, but historically that approach normally takes a lot longer.

liudr

#3
Feb 01, 2011, 05:10 pm
All these physics terms make me want to see a sketch. I can't quite put your design in my head yet. So you have an accelerometer on a part that strikes a bell, once or many times? You want to use your measurement to decide if the bell is struck hard enough? Why don't you figure that out beforehand and strike with certainty? I can help with accelerometer code. I've got my code to measure how hard I knock on my music box with accelerometer so it's easy to translate into your problem but I don't see the need for an accelerometer unless you sketch it out to show me. Here's my post. Code is not cleaned up yet. Will post this weekend.

http://arduino.cc/forum/index.php/topic,49937.0.html

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