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Low cost, simple accelerometer?

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2005-2006 » Archive through June 13, 2005 » Low cost, simple accelerometer? « Previous Next »

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Mikehibbett
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Username: Mikehibbett

Post Number: 60
Registered: 04-2005

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Posted on Tuesday, 07 June, 2005 - 10:33 am:   Edit Post Delete Post Print Post

Hi all,

I have a project idea that needs a simple, inaccurate accelerometer. I have a ball at the end of a wire that is rotating, and I would like to detect changes in rotation speed. Something very crude and non-linear will be fine; cheap, small and light are the main criteria.

Any thoughts? I had once seen some resistive strain gauges somewhere but cannot find any sources.

Mike.
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Sparks
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Post Number: 7
Registered: 06-2005

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Posted on Tuesday, 07 June, 2005 - 12:26 pm:   Edit Post Delete Post Print Post

You wanted crude, how about this? attach a weight to an arm sticking out from a pot spindle at 90 degrees (I hope that conveys what I'm trying to say) and then attach a spring to return it to a home position. When the device moves, the weight will move away from rest dependant on the acceleration. Please say if this isn't a clear description.
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Mikehibbett
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Post Number: 61
Registered: 04-2005

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Posted on Tuesday, 07 June, 2005 - 12:33 pm:   Edit Post Delete Post Print Post

Thanks sparky,

I was trying to avoid mechanical solutions if possible. I find sourcing mechanical parts difficult, and even harder to know what to order :o).
If all else fails I'll give that a go though.
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John_becker
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Post Number: 20
Registered: 05-2005

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Posted on Tuesday, 07 June, 2005 - 12:39 pm:   Edit Post Delete Post Print Post

Hi Mike

We did a Pocket G-Meter in July '00 which used an ADXL105 accelerometer from Analog Devices (www.analog.com). There are several devices in the ADXL range. I suspect that this may do what you want as the ball will be constantly in acceleration as it is changing course all the time. A PIC's ADC will sense the analogue output, just a matter of figuring out the software :-).

Are you thinking of another Fido pedometer in some way? Incidentally, in Dimbleby's look at the beauty of the UK on TV this last weekend, ancient pedometers were mentioned in that. A bit of string was attached to the walker's boot, ran up to a pulley wheel at his waist and somehow the rotation of the pulley was monitored as distance - would certainly have got round the varying stride length problem!

John
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Mikehibbett
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Post Number: 62
Registered: 04-2005

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Posted on Tuesday, 07 June, 2005 - 12:53 pm:   Edit Post Delete Post Print Post

Hi John,

Do you know who supplies these parts in 1-offs? Farnell dont seem to stock that part.

I'm thinking of putting it into a musical toy, so it needs to be cheap. Thats why I thought of resistive strain guages...

On your second point, what a cool idea having a wire from shoe to a box on the hip. Although possibly cumbersome, it would be much more accurate. If the wire had black/white marks on it one might be able to use a simple opto-coupler to count pulses, like on a shaft encoder. You could almost take a standard retracting steel tape measure and modify it... Man, that would look odd!

Mike.
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John_becker
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Post Number: 22
Registered: 05-2005

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Posted on Tuesday, 07 June, 2005 - 01:18 pm:   Edit Post Delete Post Print Post

Mike, we quoted Thame Components Ltd in Oxon in Shop Talk at the time, www.impact.uk.memec.com. But if go you into Analog's site, as I've just done, they quote GB distributors. They are SMDs by the way - no problem to us in this instance if you maybe have us in mind.

So who's going to do the B&W thingy - you or me?
Oh, alright then, you do it --- :-)

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Mikehibbett
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Post Number: 63
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Posted on Tuesday, 07 June, 2005 - 01:50 pm:   Edit Post Delete Post Print Post

lol

I'm going to be busy with the mark 2 camerawatch for a few months so you can count me out :o)

I'm just doing a bit of background research for the next project. I'm finding now that locating distributors willing to sell in one-offs is the biggest hurdle. Although I'm sure homebase will be willing to sell tape measures, so that project is covered!!
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Mike_b
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Username: Mike_b

Post Number: 16
Registered: 05-2005

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Posted on Tuesday, 07 June, 2005 - 02:30 pm:   Edit Post Delete Post Print Post

Radiospares do the ADXL in either the analogue or pcm output - 331-4504,4510

rgds

mb
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Mikehibbett
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Post Number: 64
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Posted on Tuesday, 07 June, 2005 - 02:49 pm:   Edit Post Delete Post Print Post

I think I'll go with the strain gauge, which at £4 is probably going to be the cheapest option.

Thanks gentlemen
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Boris
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Username: Boris

Post Number: 25
Registered: 05-2005


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Posted on Tuesday, 07 June, 2005 - 07:52 pm:   Edit Post Delete Post Print Post

"..I have a ball at the end of a wire that is rotating, and I would like to detect changes in rotation speed.."

Not sure I understand this.


Just the other day, I was thinking of how to make a simple accelerometer. I had some start-point ideas, but before I got very far, I got stuck on a fundamental problem: How can an accelerometer tell the difference between acceleration and tilting? Anyone got an answer?


Anyway, one of my ideas:

A small spring, taken from a retracting biro, fixed at one end, and with a weight and an LED at the other end. An LDR is fixed in position such that, at rest, the LED points at the LDR. Acceleration causes resistance changes in the LDR.

If the LDR and LED are swapped, the (fixed) LED could have some filtering or shading to provide a more linear resistance change.

(Message edited by boris on 07 June, 2005)
www.pyxia.com - BASIC was never less basic.
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Mike_b
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Post Number: 18
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Posted on Tuesday, 07 June, 2005 - 10:41 pm:   Edit Post Delete Post Print Post

Hi Boris,

I have just had to solve this very probelm. You do this with a dual axis accltmr. One axis points in the horizontal direction you wish measure. The other measures the vertical axis value of 'g'. Taking the arcsin of g allows the tilt to be measured and hence deducted from the horizontal components to give the 'real' value unaffected by tilt.

rgds

mb
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Mike_b
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Post Number: 19
Registered: 05-2005

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Posted on Tuesday, 07 June, 2005 - 10:44 pm:   Edit Post Delete Post Print Post

Hi Mike H,

How about some of those ballbearing tilt switches. If several were arranged at different angles, then you would have a very rough measure of accln. as each one came on

rgds

mb
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Boris
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Username: Boris

Post Number: 28
Registered: 05-2005


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Posted on Tuesday, 07 June, 2005 - 10:49 pm:   Edit Post Delete Post Print Post

"..The other measures the vertical axis value of 'g'.."

Just might work!
www.pyxia.com - BASIC was never less basic.
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Boris
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Username: Boris

Post Number: 29
Registered: 05-2005


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Posted on Tuesday, 07 June, 2005 - 11:25 pm:   Edit Post Delete Post Print Post

Just a little picture of what I was trying to describe.


Accelerometer
www.pyxia.com - BASIC was never less basic.
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Boris
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Username: Boris

Post Number: 31
Registered: 05-2005


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Posted on Tuesday, 07 June, 2005 - 11:58 pm:   Edit Post Delete Post Print Post

Alternative version of same idea.

Accelerometer
www.pyxia.com - BASIC was never less basic.
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John_becker
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Post Number: 23
Registered: 05-2005

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Posted on Wednesday, 08 June, 2005 - 12:09 am:   Edit Post Delete Post Print Post

What you seem to have arrived at Boris is a variant of the technique I used with my seismograph, only I used a suspended magnet and a nag field detector - which are maybe a bit more responsive.

John
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Grab
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Username: Grab

Post Number: 7
Registered: 05-2005

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Posted on Friday, 10 June, 2005 - 11:00 am:   Edit Post Delete Post Print Post

If Farnell don't stock it, you can ring them up and get them to order it for you direct from the manufacturer. They only do this for "selected" manufacturers, but this covers basically all electronics companies. Analog Devices are certainly on the list.

Graham.

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