Re: Acceleration

Gareth wrote:
Gareth wrote:
Hi

I am having trouble ( a lot of trouble ) trying to get meaningful and
timely acceleration data.

In essence, I have a wheel speed sensor that picks up around 40 to 100
pulses per wheel revolution (adjustable). This is captured by a 16bit
Micro/DSP.

All I want to assert, with reasonable accuracy is the wheel
acceleration.
My acceleration data at the moment is all over place.

How can I filter the data in real time so that I can assert wheel
acceleration? eg. DO I need to implement a Kalman filter or is there a
faster way of doing this?

In other words, as a test, I have fitted this setup to a car and
steady
acceleration does not show steady acceleration data - it's all over
the
place. I can see the acceleration component, but is is saturated with
acceleration spikes.



Gareth,
The methood you describe is very sensitive to any errors in pulse-timing

caused by road vibration and wheel eccentricity. Also, if you are taking the readings of a driven wheel you will have the influence of the pulsating torque of the engine as well.

I suggest the following:
1. Measure the time between pulses, as you are doing, and compute the differences. This gives you dt/d(theta) for 40 to 100 instants per revolution.
2. Compute the difference between these values to give you a set of
d^2t/d(theta)^2 values. The reciprocal of these gives you a set of acceleration values.
3. Average the acceleration values for one complete revolution of the wheel at a time to minimise errors due to wheel eccentricity.
4. If you can afford a slower response, put the acceleration values through a FIR filter, the longer the better.

Regards,
John


Hi John

Thank you for your reply. Most helpful. Please may I ask you to help me
through your formulas so that I can implement them.

You have given: A.) dt/d(theta) and B.) d^2t/d(theta)^2

I take it that in A, dt is the difference in time, what is d(theta)?

Sorry if I am coming accross as dumb, I just need some clarity please.
Do you have any simple working examples that I can play about with?

..Gareth


Gareth,
First a correction. As you already have the time between pulses,
step 1 is not needed.

On reflection, what I would suggest is:

Compute (1/t2 - 1/t1)* 2 /(t1 + t2)
Where t1 and t2 are successive measurements of the time between pulses.
This will give you a measure of the acceleration in degrees/s^2.

Filter or average the stream of acceleration values.

Multiply by (360 / pulses per rev.) to scale the results to degrees per second per second.

Unfortunatley I don't have any useful examples to send you.

Regards,
John





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