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  #16   Spotlight this post!  
Unread 30-11-2009, 15:53
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Re: Sensors!

So is the drift great enough to impact the readings over the 2 minutes? And has anyone done any work with a compass?
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Unread 30-11-2009, 20:37
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Re: Sensors!

HiTechnic sells one that uses I2C. I've used it a bit, but within LEGO projects only. It needs pretty careful mounting away from iron and high current circuits or it will give erratic values. Otherwise works pretty well.

http://www.hitechnic.com/cgi-bin/com...on&key=NMC1034

As for gyro drift, 120 seconds is a long time to integrate the signal, and you will find that your gyroCompass will probably have a pretty measurable error term at the end of the match. Another issue which occurs much more quickly is when the rate gyro exceeds its max rate. In other words, the gyro will only report X degrees of turn in a time period, and you have a powered turn or a bump that exceeds X. The reported value will be X, and the excess will become an error in your compass reading.

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Unread 09-12-2009, 13:23
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Re: Sensors!

Hi all!

Gyros are tricky beasties - that is certainly for sure, but since the little accelerometers and gyros are becoming ubiquitous, I've notice a trend towards the digital serial-enabled gyros - here's one on Sparkfun
http://www.sparkfun.com/commerce/pro...oducts_id=8372
It is worth noting that though 300 degrees/sec seems like a lot, a second is actually quite long, and 300 degrees is less than one rotation - if you're doing 1 rev/s, you'll max out the range on most low-cost gyros.
As for drift, the previous posts are quite accurate - the amount of drift depends heavily on the sensor, the environment, and the algorithm. Many sensors you'll find today are temperature-compensated, which will help with environmental issues, but hard shocks and sudden rotation will confuse them fairly easily - they're more used to smooth motion, which FIRST robots aren't used to performing. That being said, if you feel like some clever programming, there are filters out there that can get ridiculously accurate - the Kalman filter, for example, is a state-space recursive estimation algorithm that, if properly tuned, can give very accurate readings from a gyro. It takes some fairly high-level math, though, so unless you've got time or someone who has done one, keep looking.

As for compasses (or magnetometers), I've had some luck with the Honeywell sensors - the particular model I've used isn't sold anymore - but they are designed to operate on extremely weak magnetic fields, and placing it anywhere close to a motor or ferrous metal will make it nearly inoperable. In short, it can be done, it just requires either clever programming or isolation.

Good luck!

Sparks
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Unread 09-12-2009, 14:41
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Re: Sensors!

I've experimented a little with capacitive and inductive sensors. These fall under the radar of "proximity" sensors, if you look in the mouser electronics catalog. There is a gargantuan list of sensors that can be used with the CRio. Again, look in the mouser catalog. I've also used the PIR sensor from parallax that you can get for $10 from RadioShack. RadioShack also sells an ultrasonic rangefinder, for about $40, also made by parallax.

Also, I recently went to home depot and picked up a set of Garage Door sensors. These are those things that go behind the door, and send a beam of IR between them. When something is blocking that, the door will not close. The weird thing is, both the transmitter and receiver have two terminals. This would make sense for the transmitter, as it would have one for +V and the other for GND. I would expect, however, that the receiver has a +V, GND, and Signal. but it doesn't. I'll be playing around with this, and when I find what's what, i'll let you know.
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