Re: PID How to hold an arm in a fixed position without a locking gear box
 

This might help you,

This is our team's robot from 2005. On the first stage of the arm we used two of the 6v FP motors (they messed up that year and gave a bunch of teams the wrong motor). On the second stage we used a single van door motor. We used cable and bicycle wheels to gear down the main arm about 18 to 1* and we geared the small arm down to about 9 to 1*. It had no problem holding a single 9 lb tetra 13' out. Here is a link to a picture of the robot.
http://www.chiefdelphi.com/media/photos/21524

As for programming we used a potentiometer on each arm and compared the value they gave us to a target position for the arm. Based on that information we passed a value to the motor to move.

If you have any questions please feel free to ask.

-Dan Karol


*numbers are a guess and may not be what we actually used.

Re: PID How to hold an arm in a fixed position without a locking gear box
 

While some of the motors may "take the heat", its a good design practice to attempt to minimise the load on the motor when the assembly is static. Counterweights help greatly with this. Another option is gas shocks like the ones used to hold up car trunks and hatchbacks. They not only put out alot of force but provide some dampening. Worm gears are definately a good idea as well. I would highly recomend them. I should note that the gearset in the van door motor is NOT a worm gear set. It is a healical gearset which can be backdriven.

Also, if you plan on using PID, you must make every effort to minimize the amount of backlash in your system or else put the sensor directly on the motor. Chains are especially bad about backlash unless active tensioners are used.

Re: PID How to hold an arm in a fixed position without a locking gear box
 

Keep in mind that when you need high-torque motors (like in a programmed non-back drive mode), it helps to use two of them, if possible. If you wire the two motors together (+ to + - to -, then the combined + and - to the victor controlling them), now you are taking two circuits and combining them into one, distributing the power evenly. Not only will the two motors operate in perfect harmony, it will also multiply your torque and stall rating by two.

Re: PID How to hold an arm in a fixed position without a locking gear box
 

Err, this is illegal as far as CIM's and FP's go. Each must be controlled individually by an independent Victor.

Re: PID How to hold an arm in a fixed position without a locking gear box
 

Hmm...I had no clue this way illegal. Looks like we'll have to do it in programming then.

Re: PID How to hold an arm in a fixed position without a locking gear box
 

I thought so too, and was about to respond saying so, but I couldn't locate the relevant rule to quote. Can you help me find what I'm certain I'm overlooking? So far, I haven't seen anything in the manual that says each Victor can connect to only one motor. The closest is <R98> which limits Spike relay modules to a single motor. <R91> restricts things in the other direction: "Each motor, actuator, and compressor must be connected to one, and only one, speed controller or relay module." But it doesn't say you can't connect multiple motors to the same speed controller.

Re: PID How to hold an arm in a fixed position without a locking gear box
 

You should have received two Y-connector pwm cables in the Kit of Parts. You can use them to control two Victor speed controllers from the same pwm output on the Robot Controller.

Re: PID How to hold an arm in a fixed position without a locking gear box
 

That will work as well...but I'm starting to think that two motors on one victor is legal. We did this last year with two CIMs in the '06 gearbox...or at least I did - somebody could of changed it.

Re: PID How to hold an arm in a fixed position without a locking gear box
 

In answer to some of your questions, The gearboxes that are on the Globe and the Keyang window motors pretty much match at output speeds. The part that I posted, one end goes on the globe shaft and the other picks up the "gear" on the window motor. The globe was monted to a plate and the plate was mouted to the frame and the window motor. The #25 chain teeth drive chain that is attached to a sprocket that was attached to the last arm. Thus the motors were mounted "nose to nose". The globe gave us the oomph and the window motor helped, and with power off, Held the arm in place. I'm attaching a picture of the last stage you can kindof get an idea of how we did it. You will not need as large a final sprocket as this year your not trying to move a 12 lb tetera. (the vision one). A multi turn pot with a #25 chain sprocket mounted to the frame read the position of the arm. The nice thing about pots with sprockets is you can change the number of teeth on the pot sprocket to get full range out of the pot. Just remember smaller sprockets get more truns on the pot for the same arm angle change. It seems kind of backward but if you work it out you will see.
(Also BTW each motor had it's own victor, although one victor is plenty to drive this setup if this years rules allow it, Also triple check wiring so the motors work together, I tended to label with arrow which way things turned with a marker on the part of the motor showing so I knew if red went to pos or it it was the other way)
Biff

Re: PID How to hold an arm in a fixed position without a locking gear box
 

Generally, it is best to design your mechanical system so that continuous motor power is not required to have the arm hold its position. Doing this pretty much boils down to using a worm gear setup to prevent back drive, whether it be one of the motors in the KOP that has a worm gear built into it, or by building your own worm gear setup that is driven by one of the other gear boxes available in the kit such as the banebots unit. We use either of these options, depending on how much power and torque we need to support with the mechanism.

The control software then uses a small dead band window. Once the motor gets the arm into the window the power can be completely shut off in order to prevent overheating. The FP and the motors on the small bane bots gear boxes will overheat quickly if run at stall.

Have fun,
Eugene

Re: PID How to hold an arm in a fixed position without a locking gear box
 

This is looking like a good solution....thanks! McMaster Carr offers a nice selection of "gas springs".

Re: PID How to hold an arm in a fixed position without a locking gear box
 

Well, that makes at least two of us scratching our heads saying "I could have sworn that was a rule". Here's what I found:
Again, no mention of limit on number of motors per speed controller. From the 2007 Guidelines_Tips_Good Practices.pdf: As far as I know that is only a guideline, not a rule. It goes on to say: The closest I come to this being a rule is: The Diagram shows just one motor per speed controller.

Re: PID How to hold an arm in a fixed position without a locking gear box
 

In the past it has been a rule that only one motor may be attached to a Victor 884, however the rules seem to leave that open to interpretation this year. All I can say is that it's a very bad idea, as at stall torque the current draw of two CIMs well exceeds the rating of the Victor 884 and you'd be likely to see magic smoke at least once before the end of a regional...

Ask yourself this: Would you rather buy one extra victor and plan in the few ounces extra, or continually replace burned out victors because they overload and release magic smoke every few matches?

//Dillon Compton

Re: PID How to hold an arm in a fixed position without a locking gear box
 

In the Good Practices manual, section G.6.3.1 V884 Speed Controllers it states "Only one large motor (CIM, Fisher-Price, Globe or similar motor should be controlled per speed controller". Didn't look, but there is probably a similar statement for the Spikes as well.
In any circumstance, the motors should be individually fused.

Re: PID How to hold an arm in a fixed position without a locking gear box
 

With the motors we have used, we have never done special programming. Usually, they won't back drive with power applied (IE the robot is on). If, however, you turn off the motor, they will backdrive...such as our arm did in 2005. This year however, we made specific checks to make sure our arm will not come crashing down on a robot on our ramp at the end of the match. We also build a custom gearbox to use two motors.