Cheap 1-axis gyro?

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I have an AVR device inside of a rotating car wheel that needs to detect when it's in top position (biggest distance from the ground) at up to 400rpm (ideally up to 1200rpm). I can not use tacho, IR, or any other type of sensor that needs fixed or external parts out of my AVR device. So, it came to my mind that some 1-axis gyro chip could be used for this. Is this a good idea? Do you know some cheap one that could be easily connected to AVR? I googled around but found only some helicopter gyros for direct servo control, and that's not what I need.

Thanks

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I don't see how a gyro will sense the top position, gyros detect rotation and the output will have to be integrated, the gyro drift will quickly accumulate and the zero (or top) position will be lost.

An accelerometer may detect the top position by sensing when the gravitational force is at a minimum. One acceleration that will be present in the accelerometer will be the effect of rotation - it will be constant if the rotational velocity is constant. On top of this will be the gravitational field modified by the change in orientation of the accelerometer - the AC component. When the AC portion reaches a minimum value it is at the top (G is subtracted from the centripetal acceleration), when the AC portion reaches a maximum value it will be at the bottom (G is added to the centripetal acceleration)

These sensors are available from many sources and you may even be able to make one depending on your field of expertise. The classic piezoelectric sensor is capacitive and can be configured as a high pass filter thus detecting just the ac part.

This sensor will also detect rotation rate (frequency of AC output).

GK

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Quote:
I don't see how a gyro will sense the top position, gyros detect rotation and the output will have to be integrated, the gyro drift will quickly accumulate and the zero (or top) position will be lost.
I am confused on this, because in my understanding (possibly wrong) it turns out that helicopter with gyro based stability system could not make multiple rotations because it would quickly loose horizont position information? Wikipedia articles didn't help either because of my limited knowledge.

Could accelerometer detect top position even when driving for few hours (up to 1200 rpm)? Is it fast enough not to miss any top positions (I have found some with 1600Hz so they should be)? How can it differ top from bottom position? What force range should I choose (I found from 1.2g up to 18g)?

Anyway, what cheap 1-axis accelerometer can be recommended? At conrad.com I found ADXL320 and ADXL330. Are they OK for my application? Are there any suitable in DIP package? I found more at http://search.analog.com/search/default.aspx?query=accelerometer&local=en but it will be hard for me to get them from my country.

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you can't use an accelerometer either. centrifugal force is constant all around and this is what the accelerometer will measure. You probably can't do anything, that would sense the top position, at least not 1200rpm (you know, this is 20 rotations per second! that is Damn Fast!).
maybe you can use something like used on bicycles? the speedometer I mean. only a small unit (magnet) out of the box.

Rain

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What about 300-400rpm?

Can I detect any position, if top is not an option?

Design does not let me use anything outside of the unit, like the magnet you mentioned for getting one pulse per rotation.

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Where's the unit? If it's in the tire than maybe measre the deformation of the thing? But accelerometers and gyroscopes (unless placed on the inner part of the tire) aren't the way to go IMHO.

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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Inside of a wheel, not inside of a tire.

If it was in the tire then distance changes once per revolution... Nice idea but many problems with that approach since I need to have a visible part...

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Quote:
you can't use an accelerometer either. centrifugal force is constant all around and this is what the accelerometer will measure.

Yes the centripetal force is constant, and so is gravity but the sensor is turning! This means that it will sense the centripetal force as constant but it will sense the gravitational force as a sinusoidally varying (by + and - 1g). This is superimposed on the constant centripetal force (ac ripple). How easy it is to detect 1g on top of the constant centripetal force depends on the noise floor of the accelerometer and pre-amp.

The situation is improved if you can place the sensor near the axis of rotation where centripetal force is the least (and gravitational force is the same).

It may also be possible to update a gyro (synchronise it) with the accelerometer, it gets a zero signal, indicating top) whenever the ac component of the accelerometer has a negative peak, similar to a lock in amplifier. A very small signal can be extracted from noise if you use lock-in techniques.

for the small added cost the use of a 2 axis accelerometer may be worth considering to improve the signal to noise, one will be 90 degrees out of phase with the other.

Another useful input would be rotation rate (to set a filter for the ac signal recovery). The Gyro supplies this but has no positional integrity, or it may be available from elsewhere. Remember that any filter has a delay which must be known because it adds to the phase of the filtered signal.

Some terms to Google
Lock in amplifier or Lockin amplifier
synchronous detector

GK

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Quote:
Could accelerometer detect top position even when driving for few hours (up to 1200 rpm)? Is it fast enough not to miss any top positions (I have found some with 1600Hz so they should be)? How can it differ top from bottom position? What force range should I choose (I found from 1.2g up to 18g)?

I don't see why not, the problem is more likely the reporting of false signals related to bumps in the road but if you augment the accelerometer with a gyro this can be avoided, gyros are very good at ignoring lateral movement which accelerometers aren't. Again, the synchronous demodultion will help to ignore noise and maintain phase integrity.
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Anyway, what cheap 1-axis accelerometer can be recommended?

At 1200 RPM the centripetal acceleration is about 125m/s^2 or 12g for a 1m radius, at 0.3m radius you get only 3g.

GK

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At 1200 RPM the centripetal acceleration is about 125m/s^2 or 12g for a 1m radius, at 0.3m radius you get only 3g.

Oops, that should be w^2, the centripetal acceleration is more like 1600g per meter or 483g at 0.3m

I wonder if the electronics will survive that force.

GK

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Hang on, acceleration is v^2/r... in a car at ~60mph ~=30m/s (at the outside of the tyre) that gives 900/0.3 = 2700m/s^2 at the outside of the tyre; about 300g.

A little closer to the hub, say 0.2m, it's going to be 400/0.2 = 2000m/s ~= 210g

Sanity check - I think you have answers in m/s^2, not in g, Klave. Or I'm insane :)

Neil

(Edit - another sanity check... car tyre is usually about 60-65cm diameter, circumference therefore about 2 metres, so 1200 rpm = 2400m/min = 40m/s - so I should be in the right sort of range. But I did it all on my fingers, so don't quote me on it :) )

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Is your rotational velocity going to be constant?

"I only speak one language .... 101001 ..." - Mr. Rat from The Core (r)

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This is possibly the worst aproach, but if your tire's a little excentric (not in the exact center of the axis of the driving shaft) it will vibrate. Measure the frequency of the vibrations and it migth work. (OK stupid idea, just poopped into my mind).

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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How about using an electronic level http://www.freepatentsonline.com/5031329.html? It will be level twice per revolution.

Cheers,

Tom

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Quote:
Hang on, acceleration is v^2/r...

That's quite correct, at 1200RPM, linear velocity is 15800m/s for a 1m radius etc - maybe only wagon wheels are that big but it does allow me to work "per meter" (I use the equation for angular velocity a=w^2.r where w=2.pi.(revs/sec) )

For a 0.3m radius the acceleration is 4737 m/s^2 or 483g but this is 83mph. It doesn't seem like much more than 60mph but remember the v^2 will multiply by almost 2.

Now your 60mph car (27m/s) with a 60cm diameter tyre will be rotating at 850rpm giving 2377m/s^2 or 243g.

Mounting the sensor nearer the hub, the rotation rate is the same but the velocity at the same radius is now 17m/s and the centripetal acceleration is now 1585m/s^2 or 162g.

Ideally the sensor is at the centre, there will be no centripetal acceleration, just the sensor spinning around sensing gravity and outputting a sine wave plus the potholes.

Quote:
How about using an electronic level

I think that this will work in theory but it will suffer the same effects from the centripetal acceleration, the rotation will be a small variation around zero but the problem is the inertia of the liquid which will cause a phase error depending on frequency. In reality it is an accelerometer.

GK

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Of course, Klave. I failed to note the 'per meter'. Doh.

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danderson wrote:
Is your rotational velocity going to be constant?

Cars accelerate/decelerate so the answer is no.

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Klave wrote:
Yes the centripetal force is constant, and so is gravity but the sensor is turning! This means that it will sense the centripetal force as constant but it will sense the gravitational force as a sinusoidally varying (by + and - 1g)...

This really gives me hope :P :D :P

Thanks to everyone!

Btw. Does anyone have an idea how to power such a device? I would like to avoid changing batteries if possible, and I have no elegant idea how to reach car's wires from a rotating wheel. Could vibration be used somehow? Or anything else? Ideas?

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Somehow RF IDs 'rectify' the incoming RF, and charge up a cap. When its charged up enough, it transmits back the ID or whatever. So you need a coil and tuned circuit and a diode and cap in the tire, and a coil driven by a big amp... how ever many watts or mw it takes to jump across the air and thru the tire. Frequency to use? Beats me. Dont RF IDs use 125KHz? Hint: dont use the local police frequency.

Imagecraft compiler user

Last Edited: Mon. Aug 6, 2007 - 04:19 PM
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Why not power it using a freely spinning weight attached to a motor/generator?

Math is cool.
jevinskie.com

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When the wheel hits a hole, the bounce up 'tosses' the pendulum up, and it starts spinning around the hub instead up hanging down and generating. The thing that hangs down needs to weigh enough to offset the torque of the generator.. lets say several ounces. Now all of a sudden the wheel has this weight spinning around causing everthing to bounce.

Imagecraft compiler user

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couldn't you use a single axis accelerometer and orient the sensing axis perpendicular to gravity?
That way at top dead center and bottom dead center your accelerometer would report 0 Gs, the max readings would be at 90degrees and -90degrees.
You'd need a little stateful information to know the difference between top and bottom, but that could be easy to deal with.
In this configuration the centrifugal force should be pulling perpendicular to the accelerometer's sensing axis so should not read. Mounting would have to be precise though.
There are look-up tables and formulas at Freescale.com

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What about mounting a piezoelectric element from an electronic lighter, it generates a voltage when it is subject to deformation.

Have not tried it, but I suspect it to work, when it is fixed in one end and have a weight balance attached to the other end, and mounted near the rotational centre.

AC coupled with large gain and then a zero crossing detection could maybe do the job.

The element must be cheap considering the cost of a whole gas lighter.

ekh