[archiver]

Posted by Paul Copioli at 04/26/2001 8:37 AM EST


Engineer on team #217, Team Macomb - Royal Fusion, from Utica Schools, Fraser Schools, Warren Cons. School and Ford Motor Company.


In Reply to: questions from a speed/torque guy...
Posted by Joe Johnson on 04/25/2001 7:04 PM EST:



Joe,

BEWARE! This is a long response due to the nature of your questions and the fact that I can't draw any pictures!!!

I will try to answer your 3 questions, but first I would like to address the 3 problems you mentioned.

: 1) Inability to easily implement a PID control
: algorithm on the current STAMP2 CPU.

--- Yes. This is a big problem, but pretty solvable. Actually, this problem amplifies the need to have a favorable load-to-rotor(LR) ratio, because the control algorithm is limited. Up until about 3 years ago, any machine using a FANUC motor (all of our robots) was required to keep the LR ratio below 3:1 or we would have control problems. As the control system (and encoder resolution) improved, our LR ratios can be as high as 10:1. One last thing about the algorithm: when we use pots for feedback, we add a "velocity gain loop" in our system. The velocity loop is pretty simple: it takes how large your position difference is and reduces power as you close in on the command position. It works great.

: 2) The dead band in the Victors (a PWM output value of 127 +/- 10% yeilds 0 Volts output -- very annoying).

--- Again, Yes. Very annoying. We have learned to deal with it (although, ever since we went to this velocity control loop, the problem is less severe).

: 3) My poor design skills preventing me from properly
: counter-balancing the arm so that the arm is more or
: less neutrally balanced over a large range of motion.

--- Yeah right, poor design skills. What, are you a comedian? Anyway, I have read all the white papers, etc. that you have put out there regarding balancing. I am curious, do you always use the gas shocks? If you do, then it is very difficult to get a perfect balance. There are little gems in the kit called springs --- work great. With a spring you can configure the geometry such that you get a perfect balance for an individual arm. With that said, If you have an inner arm and an outer arm like you guys did this year and we did last year, then you can not perfectly balance the inner arm, because of the position variance on the outer arm. What we do (at FANUC and on Royal Fusion) is take the entire mass of the outer arm (plus payload) and assume it is at the end of the inner arm. If you plot Torque vs. Inner arm angle you can see that the gravity load is sinusoidal. A spring with a constant stiffness can be configured to perfectly match gravity. One last comment on balancing: The arm will not be perfectly balanced because of the outer arm location, but if you point the outer arm straight down; then you will see your arm perfectly balanced. This year we only had one arm, so it was perfectly balanced (we used the 2 extension springs in the kit in parallel & latex tubing to fine tune it). Can you believe we used a globe motor with a 1:1 gear ratio to move our 3' arm? Our arm only had a few positions to stay at so the LR ratio did not bite us. It did amplify how important LR ratio is, however; because our arm would resonate at other positions due to a 10:1 LR ratio.


: I why do we want a 1 to 1 ratio?

--- Think of a spring-mass system with 1 mass at each end and 3 springs in series connecting the masses. The 1st mass is your rotor inertia, the second mass is your load inertia. The 3 springs are you coupling, gearbox, and arm. More important than LR ratio is stiffness matching-find the least stiff member (usually the arm) and make it stiff!!!. Remember that you also have your controller "gain", which you want to make as high as possible. As you make it higher, the mechanical natural frequency has to be higher as well. Here is where the LR ratio comes in. We try to get it down to 1:1, but 2:1 is O.K.


: What good things does it do for us if we can get a 1 to 1 ratio?

Easy way to help control performance. The alternative is to use a low pass filter (I have an excellent 2 page article on it, but I will have to fax it to you). Actually, it is not necessary for FIRST robots, but is an outstanding way to eliminate vibration.

: Finally, does it make sense to add mass to the armature of the motor in order to improve the reflected inertia ratio? If I can get good things for the extra work, I may end up doing it.

Absolutely!! It is even easier than that. You just have to add mass which is rigidly connected to your motor. For example, if you attached a pinion to the motor shaft of the van door motor using a trantorque, then the inertia of the pinion and the trantorque could be considered part of the rotor inertia. The main drawback of adding mass to the rotor is the acceleration performance decreases due to the added inertia. On all of the robot arms I have engineered, the gear ratio needed to obtain a favorable LR ratio was always higher than the gear ratio needed for acceleration performance. Just something to chew on.

I apologize for being so winded, but you asked some detailed questions.

I hope you find this information useful,

-Paul