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#1
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Accelerometer Suggestions
Over the past week I've gotten the kit of parts accelerometer from last year working on the Vex for getting position and velocity (or it's mostly working).
Does anyone have suggestions for an accelerometer to use for inertial navigation on an FRC bot? The current one works on Vex, but I'm afraid it won't be able to handle the faster accelerations and speeds of an FRC bot. I like the MMA7260Q from Freescale, largely because I can choose the sensitivity that works best on it, but it's a 3.3V part and I'm not sure our team wants to deal with making a power converter (we have no skilled electrical students; we have 2 mentors but I'm not sure they'll have the time to do it). |
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#2
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Re: Accelerometer Suggestions
Look at Analog Devices for ratiometric 5 V parts.
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#3
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Re: Accelerometer Suggestions
If you don't want to wait to get something shipped SparkFun has some good accelerometers.
http://www.sparkfun.com/commerce/cat...hp?cPath=23_80 Also can you post some code? If you really have that working I want to see it. ![]() |
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#5
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Re: Accelerometer Suggestions
I was looking at Sparkfun about Accelerometers, as they seem to be one of the best carriers for usable ones (boo BGA soldering, we bought a gyro directly from Analog accidently 2 years ago, and thankfully we knew someone who could solder it for us).
Chris, if you use that soon please tell me how it works, as the ADXL203 seems to be the best option available if we don't want to build a power converter for voltage. The only worry about that is if a 1.7g range will be enough. Attached are the .c, .h, and readme files for the code. It's designed to work with Kevin's ADC code and is based largely off of his gyro code (so if it looks familiar, that's why). So far it's accurate to about 3 inches for 1-1.5 seconds, and like 6-8 inches for 2 or 3 seconds. It's not as accurate in reverse as it is going forward. Based on those errors, I definitely need a more accurate sensor for a large bot. |
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#6
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Re: Accelerometer Suggestions
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However i have never used an accelerometer so maybe that number means something else |
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#7
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Re: Accelerometer Suggestions
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Of course I hope on having a fast robot, so it might not be an issue since that will have a slower acceleration. I'm just gathering suggestions. |
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#8
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Re: Accelerometer Suggestions
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#9
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Re: Accelerometer Suggestions
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This would theoretically mean that, if it could go that fast, it could reach a speed of 54ft/s from 0ft/s in just one second. I thought that such G's might only be "possible" during a full speed collision. |
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#10
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Re: Accelerometer Suggestions
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Reducing our RPM from the motors to 344, increases our stall torque to 148 N*m (this is ignoring all gear train inefficiencies). Dividing this by the radius of the wheel (.0762 meters) gives us 1942 N of force at stall. F = ma, so we divide the force by the robot's mass of 60.5 kg, and we get an acceleration of 32 m/s^2, which is actually 3.3 gs! I had originally gotten 1.6 gs, but that's because I used the wheel's diameter in place of its radius. Of course, this acceleration is actually impossible because the wheel traction would probably never allow for any force greater than 1000 N. Collisions are much worse; I registered a 1 g acceleration on the accelerometer when it was only 5 degrees from vertical, which meant the impact generated was more like a 12 g acceleration. Driving around with it being used for position I've gotten .5 gs from the Vex robot, which is no where near the power of an FRC bot. |
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#11
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Re: Accelerometer Suggestions
Thats close to that of a Space Shuttle! Even F-1 cars can pull that much during acceleration. There is no way that a FIRST robot can pull more Gs than a F-1 car(800Hp vs 2) during anything, let alone a Space Shuttle.
The reason I am going after this is because I want to use the accelerometer for same purpose(get position form it). And if this is true then everything I believed is completely false! I think you might be forgetting the INERTIA of the robot... It can't physically accelerate that fast!!! Last edited by 6600gt : 04-01-2007 at 16:15. |
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#12
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Re: Accelerometer Suggestions
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1. The wheel traction won't allow for it. Going through the math, an odd coincidence is that the coefficient of friction between your wheels and the ground is the same as the maximum amount of gs your robot can accelerate at before the wheels slip (assuming your robot has no dead weight and has the same friction on all wheels). This means FIRST robots using skyway wheels can't get above .8gs of acceleration, and the roughtop/wedgetop tread are limited to 1.3 gs. 2. This is an instantaneous acceleration where your robot has not moved yet and your motors are instantly outputing stall torque. Your motors can't output that much torque without ramping up to it first, and applying torque through the wheels will start your robot to move, which means you won't be stalling anymore. Under very unique (and basically impossible) conditions, I believe these motors could create that much acceleration; however, as I said, I believe that is impossible. Don't be surprised if you get readings like these from collisions though; collisions are capable of creating accelerations of several 1000s of gs, meaning collisions will still render position sensing invalid much like they do for encoders. I didn't discover some of these numbers until trying to prove everything again; seeing the new data about what gs are possible with the wheel traction we have, I'm no longer worried about choosing an accelerometer. Feel free to try the code I developed for position sensing. It took me over a week to develop it working on no other code, and that's alot of valuable time come build season. It does work, better accelerometers are needed for better accuracy is all. |
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#13
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Re: Accelerometer Suggestions
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#14
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Re: Accelerometer Suggestions
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#15
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Re: Accelerometer Suggestions
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Our team was lucky enough to receive a LabView data acquisition module and software($2700) to play around with. It has a 14bit A/D converters to acquire data that can be shown graphs. I want to try and use this to speed up our development. |
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