angular accelerations through accelerometers

[pierluk]

I have to measure angular acceleration in a vehicle and since I haven't gyrobox, I have to use two accelerometer placed at known distance. At work I have only accelerometer with +/- 50g full scale. Are they good for the measurements I have to carry out? I'm doubtful, they have a low sensitivy for my need.

 

[IRstuff]

I was trying to figure out how you came to the conclusion that "the maximum error of angle acceleration should be 0,21°/s^2"

TTFN

FAQ731-376

 

[pierluk]

IRstuff, I was in error for the calculation of the max error of angle acceleration. But why have you used that formula? Now I have another doubt, if I need a maximum error of +/- 1°/s for angle rate, for you what should be the maximum error for angle acceleration? Does a way exist to estimate that error?

 

[IRstuff]

Well, how are you going to measure angular motion without using two sensors and a known distance between them? Given that, the measurement error is:
sqrt(2)*[σ]/sensor_spacing

TTFN

FAQ731-376

 

[pierluk]

I know the sensors spacing, but I haven't understood the formula you have written. Look at this link: formula 4.4.4 at page 184; so the max err
if the max error is 0,25g and the sensor spacing is 0,85 should the max error for angular acceleration be 0,25/(2*0.85)=0,14deg/s^2?
Whereas I'm interested to a frequency range 0-5Hz, is there a way to compare that angle acceleration error to the angle rate rate error of 1°/s? Only for understanding if the measure should be good enough.

 

[SomptingGuy]

You need to do some units conversion here. Convert your 0.25g to m/s^2 (0.25g may also be RMS). Then convert your angular acceleration from rad/s^2 to deg/s^2.


- Steve

 

[IRstuff]

The equation is the simple propagation of errors, i.e., the root-sum-square of the errors, ignoring the uncertainty of the distance. Therefore, the sigmas are multiplied by the square root of 2.

Then, you need to pay attention to the units. This is pretty sloppy work as an engineer.

TTFN

FAQ731-376

 

[pierluk]

ohh....I'm in fail...thanks for correction


I have another question, if for example I need to measure the angle rate with the maximum error of +/-1°/s, but if I measure the angle acceleration what should its maximum error be for having the same max error in calculating the angle rate as integrate of the angle acceleration ?

 

[IRstuff]

Well, since rate is the integral of acceleration, you can get by with a slightly sloppier accelerometer, since some of the error will be averaged away. Classically, the noise reduction is proportional to the square root of the number of samples taken.

TTFN

FAQ731-376