March 27, 2014

What's Happening With the Robot Arm?

Not too much, sadly.  Term started, 2.007 started (introductory post to follow soon), and I've gotten mixed up in other side projects.

I have made a little progress on characterizing the arm's dynamics for feed-forward control as discussed at the end of the last update.  To start out I took some measurements to characterize the friction in the belt reductions.  Somehow in the past month my data from the 72 tooth reduction disappeared, but I still have everything from the 60 tooth one, and I remember they were virtually identical:

The process for calculating the friction was pretty simple.  I ran the motor off a bench supply at a constant current.  I then had the mbed spit back the steady-state angular velocity to my computer over serial.  Since I know the torque constant of my motor, I can convert the current draw read off the bench supply into torque produced by the motor.  Plotting all this data gives me a nice linear curve representing the friction in the belt reduction as a function of angular velocity.

While doing all this testing, all of a sudden the readings from one of my encoders got stuck.  I probed the encoder outputs, and found that one channel was stuck high.  I cracked open the encoder to investigate further, and found this:

Further probing around the inside told me something was wrong with the 339N comparator (the chip on the right).  Fortunately MITERS had some of these same chips floating around.  I cut the leads off the original and soldered a new one in its place:

This seems to have fixed things.  I can't imagine why the chip would have died in the first place though.

In terms of actual mechanical progress, I machined a new linkage to replace the carbon fiber one.  It's basically an aluminum I-beam, with clamping mounts at each end: