FRC971 Spartan Robotics 2016 Release Video

[AustinSchuh]

Originally Posted by mman1506

Would you mind ELI5, are you saying that you are able to zero the joints without the use of limit switch (hall effect switch, etc), a potentiometer or a hardstop?

I had to go look ELI5 up

We use a potentiometer and index pulse to zero each joint. We do not have any limit switches, and we've been known to not put in hard stops. In 2014, one of the hard stops was the cRIO...

Let me try to write out an example with a pretend elevator.

The encoder moves 0.1 meters per revolution. Someone went and calibrated the elevator and told you that at 0.0971 meters, they found an index pulse.

This means that as you lift and lower the elevator, you will see index pulses at 0.0971 meters, 0.1971 meters, 0.2971 meters, 0.3971 meters, 0.4971 meters (I think you see the pattern).

They also calibrate the potentiometer so that it reads out the approximate height. Also, pretend that it has like 0.02 meters of noise in the reading. So, if you are at 0.1 meters, you might see readings of 0.09, 0.1, 0.11, 0.12, 0.08. Welcome to real life. It sucks at times.

So, we initiate a homing procedure by telling the elevator to move 0.2 meters towards the center of the range of travel. The procedure needs to be designed to not break your robot, but move at least 0.1 meters to find an index pulse. As we are moving, we see a pulse. We then go immediately look at the pot, and it reads 0.3100 meters. The closes index pulse is 0.2971 meters, so we now know that whatever the encoder value was at the index pulse, it really should have read 0.2971 meters. So, compute that offset, and you are homed!

DMA is a really cool feature on the FPGA of the roboRIO where you can set up a trigger and cause sensors to be captured. We have configured it to trigger when an index pulse rises, and save the encoders, digital inputs and analog inputs. The FPGA does this within 25 nanoseconds. This lets us record the encoder and pot value at the index pulse.


The fun part comes when the noise on your potentiometer is ~0.05 meters. We see this on our subsystems. If you get unlucky, you might pick the wrong index pulse, and be off by 0.1 meters (!). We can fix this by filtering. The encoder should read what the pot reads, with an offset depending on where the system booted. You can take (pot - encoder) as the "offset" quantity and average that over a long period (2 seconds is what we use). Add that filtered value back to the current encoder value, and, assuming Gaussian noise and all that jazz, you will have removed enough noise to make everything work again.

More concretely, say we are sitting at 0.05 meters. We get the following encoder, pot readings.

Encoder, pot
0.0, 0.0
0.0, 0.1,
0.0, 0.06,
0.0, 0.02,
0.0, 0.08

(I'm a horrible random number generator, FYI)

From averaging it all together, it looks like the pot started out at a bit above 0.05, and the encoder is at 0. Then, we get the following measurement in: encoder, 0.05, pot, 0.16. Before, we would say that this is closer to the 0.1971 value, so we would round there. Now, we would say that since the encoder moved by 0.05, and we think the pot was around 0.05, we are likely at about 0.1. The nearest pulse is 0.0971, so that's the zero we actually saw.