Accelerometer Suggestions

[6600gt]

Originally Posted by Donut

acceleration of 32 m/s^2, which is actually 3.3 gs!

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!!!

[Donut]

Originally Posted by 6600gt

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!!!

As I said, this was in perfect conditions. In reality, this isn't possible for a few reasons:

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.

[6600gt]

Originally Posted by Donut

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.

Thanks for the code. But first I need to "understand" accelerometer. Haven't really had the time to do that. I will start with the kit of parts accel and LabView. I have no idea if I can finish this in 6 weeks, but I will try none the less. Is your's based off of Kevin Watson code?

[Donut]

Originally Posted by 6600gt

Thanks for the code. But first I need to "understand" accelerometer. Haven't really had the time to do that. I will start with the kit of parts accel and LabView. I have no idea if I can finish this in 6 weeks, but I will try none the less. Is your's based off of Kevin Watson code?

Yes, my code is based off of Kevin Watson's gyro code. It requires his ADC code to run, and will look very familiar to his gyro stuff. I don't know what LabView will do for it, I have only used the accelerometer with this code I developed and don't have any past experience using accelerometers. Good luck!

[6600gt]

Originally Posted by Donut

Yes, my code is based off of Kevin Watson's gyro code. It requires his ADC code to run, and will look very familiar to his gyro stuff. I don't know what LabView will do for it, I have only used the accelerometer with this code I developed and don't have any past experience using accelerometers. Good luck!

He did have an accelerometer code made in 2005, I think. You might want to check it out(probably similar to yours); it's somewhat similar to the gyro.

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.

[Donut]

Originally Posted by 6600gt

He did have an accelerometer code made in 2005, I think. You might want to check it out(probably similar to yours); it's somewhat similar to the gyro.

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.

I've seen his '05 accelerometer code; it had the ADC built into it (rather than seperate like he did with the gyro in '06), so it wouldn't work with other sensors using analog ports at the same time. It does acceleration similar to mine, I just went a step further and tried to calculate velocity and position as well.

14 bits is alot better resolution than you'll get on the robot controller; keep that in mind, as there is a huge difference in what you can measure between these. A 14 bit A/D will make each number in the output practically 1 millig for the kit accelerometer, while the A/D on the FRC controller has about 16 milligs for each number on the output.

[6600gt]

Originally Posted by Donut

I've seen his '05 accelerometer code; it had the ADC built into it (rather than seperate like he did with the gyro in '06), so it wouldn't work with other sensors using analog ports at the same time. It does acceleration similar to mine, I just went a step further and tried to calculate velocity and position as well.

14 bits is alot better resolution than you'll get on the robot controller; keep that in mind, as there is a huge difference in what you can measure between these. A 14 bit A/D will make each number in the output practically 1 millig for the kit accelerometer, while the A/D on the FRC controller has about 16 milligs for each number on the output.

Well that s why I am working on trying to get an external PIC on board to do the extra crunching. I will certainly using the oversampling feature to attain the max res possible. External PIC might should be able to output 13bit easily and maybe 14bit with some optimizing.(It also has to do the gyro and the trig calculations)

You should be able to do 12 bit on the RC. In fact, that is the initial setup of the Kevin's ADC code.