Re: Yaw Sensor
 

theres no way your bot can move with a 40Hz response rate - to make a mass of 130 pounds move with unique yaw rates at 25mS intervals would take hundreds of HP

we only have about 2HP at our disposal

I wish I had the time to instrument the step response of our bot - switch the steering from zero to a full power spin on its axis and measure how fast the robot torques up to speed

trust me on this, you will not see any drastic changes in yaw rate a 25mS intervals with these machines - there is no need to sample the yaw rate any faster than that - the bot has too much inertia to be twittering around at 40Hz or more like a hummingbird :c)

Re: Yaw Sensor
 

It can easily see fundamental or harmonic frequencies of 40Hz if it runs into something hard :D.

-Kevin

Re: Yaw Sensor
 

I agree with that - there is no yaw sensor that can measure what happens when you get smacked by another bot - if you get bashed your yaw rate sensor will either max out, or glitch - so even then, it dosnt matter how fast you sample it- the output you get will be wrong for a sample or two.

max yaw rate bandwidth
 

Ken,

I don't think zero to full power spin is the limiting case. The worst case impulse in yaw rate is probably during a collision. That's when I expect the integrated gyro to get wrong - no longer represent my absolute turn angle. My intuition may be wrong here, and I'd love to see an explanation, but my swag is that in a full speed collision we could see a change of 30 degrees or so in a tenth of a second or so. If that happened, the sensor is saturated without even starting to consider bandwidth, since that guess yields a 300 degrees/second change. What's the frequency content of such a change? I dunno, too late tonight for me to want to think that hard. Seems like more than 20 Hz though, which is all the bandwidth you can get sampling at 25ms. Moreover, don't you have to use pretty good low pass filters if you're sampling near the Nyquist limit? Which probably none of us are doing. In order to get by with lousy or no filters, you have to sample quite a bit faster than Nyquist's limit. OK, am I all screwed up about this? I'll admit it's been a long time since I've done much with signal processing.

Bill

Re: max yaw rate bandwidth
 

If you want to sample at the Nyquist frequency, you can. However, given that analog sensors are prone to noise, I think it makes more sense to sample at the lowest frequency that will yield usable information in order to reduce the risk of sampling brief electromagnetic transients. I do not anticipate that our robot is likely to be able to navigate if it experiences yaw rates anywhere near the ADXRS150EB's maximum during autonomous mode; whether this would be due to mechanical failure or lack of usable sensor data remains to be seen, but I would prefer that the system be less useful in extreme conditions to having the robot wander all over the place due to EMI from fluorescent lighting or other sources.

Re: max yaw rate bandwidth
 

i dont think this is entirly accurate. though you may not be getting usable information in real time, it is still best to sample at fastest practical frequeency possable. take a metaphor for a moment: say you are driving. is it better that you blink 40 times a minute for 1/10 of a second or 10 times a minute for 3 seconds?

stability of zero reading with ADXRS150 or ADXRS300
 

With all this sidetrack on sampling rate, one of my original questions got lost. I'm wondering how much variation people are seeing on the no-motion output of the ADXRS150 or 300. I am using the 300, since it happened to be in stock as a free sample at the time. In my setup, it has about +/- 10% variation, and I'm wondering if that's typical or should I try to figure out why?

Thanks
Bill

Re: stability of zero reading with ADXRS150 or ADXRS300
 

Bill,

Gyro drift is very common. The simple way around it is to just periodically sample the output when you know you aren't moving and using that value as a baseline. Another cool thing to look into is the use of a Kalman filter to reduce the amount of noise in your measurements.

-Kevin

Re: stability of zero reading with ADXRS150 or ADXRS300
 

With our 150, at startup our neutral varies between 519 and 521. We are ignoring the first 5 to 10 samples @ 4ms sample rate, and averaging the next 25 to 250 samples (rounding the average).

[edit]We aren't getting no-motion drift if we refrain from trying to increase the accuracy.

[edit][edit] Ran a Kalman filter on it and didn't significantly improve drift results while moving, but that may be due to the individual robot nature of the noise. Didn't try it with no-motion. I need to revisit the errors in drift. Maybe play with more filter variations.

Re: Yaw Sensor
 

these Analog Device parts do have a temp output, and you can correct the output for temp, which would reduce the drift over time

I think the spec sheet called out something like 300° accumulated drift over one hour if you compensate for temp

+/- 10% seems too high if the bot is sitting perfectly still - are you sure you are not hitting the analog to digital converter too fast? I dont know its max sample rate

I would look at the output on a scope if you are getting that much drift - seems way to high to me.

nother thought - where are your signal wires running? ours are very short, and there are no power wires or pwm wires in the area- if you have the gyro cable bundled up with other wires you could be getting a lot of cross talk from them.