Quote:
Originally Posted by Trevor_Decker
So if I wanted to use an accelerometer to tell how fast a robot is moving (constantly adding/ subtracting the acceleration from the speed) , I would want a 2g accelerometer since the field is 54 feet in length, which divided by 32.174(earths gravity) = 1.678 which rounds up to 2.
would that be correct?
How accurate is a 2g accelerometer?
Thanks again for the help!
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As a side note, if you want to know how far your robot has travelled, you're much better off using encoders. With that being said, I have 10 years of engineering experience in control systems using intertial sensors (inertial sensors = accelerometers and gyros), so I enjoy talking about this stuff. So, here goes the answer to your question...
Is your robot slip limited? In other words, if you put your robot on the carpet facing against a wall and you apply full power, do the wheel slip or do the motors stall?
If the robot is slip limited, the highest acceleration that the robot can experience under its own power is the coefficient of friction between the wheels and the carpet (if you don't believe that, go through the math/physics, if you wish). Therefore, if your coefficient of friction is 1.5, you can accelerate up to 1.5g. Once again, you'd want to pad this to account for vibration and noise, so you might want to select a 3g accelerometer. Why pad for vibration and noise? See the "VERY IMPORTANT" section below.
If the robot is torque limited (wheels don't ever slip), then you need to calculate the force applied to the carpet by all of the wheels. You can do this by either going through all of the gear-train calculations, or just put a scale against the wall as you drive into it. Then use Newton's handy-dandy 2nd law (Force = mass*acceleration) to solve for the maximum acceleration.
VERY IMPORTANT: If you EVER saturate the accelerometer, your velocity and distance caculations will be forever wrong after that point. For example, let's say you are using a 3 g accelerometer and your robot bumps into something and experiences 4 g's for 0.1 seconds. Your accelerometer can only measure 3 g's, so for 0.1 sec you think you're accelerating 3 g's when in fact you're actually accelerating 4 g's. Your velocity calculation will then be off by 3.2 ft/s and your position calculation error will grow to infinity (at a rate of 3.2 feet per second - see the connection?). Not a good situation. There is a lot of engineering work that goes into solving these types of problems on antilock brake systems and traction control systems - a lot more time than you have in the 6 week period, which is why I would suggest encoders.