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#1




Using an encoder and a gyro to find absolute displacement
I am working on a project that requires the robot's displacement from the starting position to be known. The only useful sensors that I have are two encoders, one on each side of the robot, and a gyro. My general idea so far is that the encoders can be used to measure how far the wheels have spun. This can only be used to find displacement when driving in a straight line. I think the gyro could be used to correct for the turning of the wheels. I can also take multiple measurements along the path if needed.
Thanks! 
#2




Re: Using an encoder and a gyro to find absolute displacement
What exactly is being asked here?

#3




Re: Using an encoder and a gyro to find absolute displacement
Assuming you mean the robot's distance from the starting point, not the distance the robot has traveled, this would be my suggestion:
If you imagine the robot on a coordinate plane starting at (0,0) and facing 0 degrees, each tick of the code you could update the position using the distance traveled and the angle. For example, if you track that you've moved 2 inches forward in the last tick while facing 0 degrees you could update your coordinates to (2,0) and update the angle. The next tick if you moved 3 inches forward but you were facing 15 degrees you'd take your current X and add 3*cos(15) and take your current Y and add 3*sin(15) and that'd be your new position in the coordinate plane. Then to take the distance traveled from the origin it'd simply be the Pythagorean theorem. 
#4




Re: Using an encoder and a gyro to find absolute displacement
This is a process that is used by many teams and is commonly referred to as "odometry" or "state estimation". You can basically calculate the distance traveled by the robot at a fixed interval by averaging your left and right encoders to get d. If your dt is small enough, you can assume the robot goes in straight lines and turns. Your gyro angle would then be your theta. Use vector addition from the initial pose to calculate your position on the field.
<d cos theta, d sin theta> + previous pose = current pose. Some teams like 254 use differential geometry to represent the robot's movement as movement along an arc of constant curvature at each dt interval. More info can be found here: https://www.chiefdelphi.com/forums/s...6&postcount=15 Be mindful that this method of finding your position on the field is usually only accurate for the first 15 seconds or so. Wheel slip, encoder slip, and other factors will cause this approximation to deviate significantly after a while. 
#5




Re: Using an encoder and a gyro to find absolute displacement
Like Prateek said, to get the distance you just do
Code:
(leftEncoder + rightEncoder) / 2 Code:
speed = 1 // 100% turnAmount = (DESIRED_ANGLE  gyro) / 10 // in degrees // the PID part with P leftSpeed = speed * (1  (turnAmount < 0 ? turnAmount : 0)); rightSpeed = speed * (1  (turnAmount > 0 ? turnAmount : 0)); // OR you can just do this: leftSpeed = speed * (1 + min(turnAmount, 0)); rightSpeed = speed * (1  max(turnAmount, 0)); 
#6




Re: Using an encoder and a gyro to find absolute displacement
Quote:
I don't know of any team that builds control strategies around the assumption that they will know absolute position at any given time. This isn't to say you shouldn't pursue how to get this answer  simply to make sure you levelset your expectations. 
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