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

I have read a little of the PID stuff and i sort of get it. I am wondering a few things.
1. How does your team power the rotation of their arm and get it to stay in a lifted position?
2. Do you use the window motors or some of the smaller motors to power its rotation and lifting?
3. is there not a risk of burning up a motor if just barley enough voltage is applied to keep an arm lifted?

This is my second year in FIRST... Im not sure that the robot my team produced lastyear should be considdered something FIRST worthy. I really need help or pictures.

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

1. You can write a very simple piece of code that will do a decent job of keeping your arm up. Store your target potentiometer position, and set your motor output to 127, plus the target, minus the current position. (Or the other way around, depending on which direction is up.)

2. A window motor can lift a small arm. For bigger arms, you'll want to look at the RS-540, the FP, or the CIMs. The Good Practices document will tell you what torque is available without additional reduction, and what your peak power is.

3. Some motors burn up, others don't. FPs have a reputation for letting out magic smoke at stall, whereas CIMs have a reputation for being essentially indestructible.

Feel free to post if you have more questions.

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

How about the globe motor? can it take the heat? :)

(we are planning on loading it at about 20% of it's stall torque spec)

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

I don't know of any cases of motors burning up without being at or close to stall. I do know that the Globe gearboxes start to break if you sideload them.

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

Thanks, that's reassuring! We're aware of the side loading issue with the Globe, and I think we can support the shaft properly without too much work, after thinking about it for a week.

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

Make sure to set the victor your motor is hooked up on to the "brake" setting; this will make it more difficult to back drive and will make the motor better keep your current arm position.

Also, the type of system you use to move your arm can automatically help keep your arm stable. Screw drives are basically impossible to backdrive, but you're limited to less than 180 degrees of rotation with them.

Using some of these things may make it unnecessary to keep a PID loop in programming monitoring the position of your arm.

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

We are working on different designs using either the globe, BB, or window motor(s). The globe looks to be the closest to what we need as far as torque capacity and rpm, but the others are there as backup if it doesn't work.

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

And to pile on with another idea: A common solution to preventing back drive is to interface your motor with a worm gear, if possible. You just need to make sure whatever motor you use can withstand axial thrust. I believe some of the KOP motor transmissions utilize worm gears (i.e. the window motors)...

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

The window motors do use worm gears, although they are a bit weak (torque) compared to the globe and BB motors. But two window motors can work together if needed.

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

Or using a "big" motor, CIM as an example to do the main drive of an arm and use the window motor for its anti back drive feature in conjunction, it won't add much to lift but locks position pretty well.

BTW globe motors back drive pretty easily with seemingly little forces on them

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

Globemotors are indestructable little things that can take the heat, even when stalled. (Don't do it just for fun, though)

I love GlobeMotors!!! :D

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

The plan is to have the arm motor mounted "up high", so weight is a big concern....less weight up there means less tendency to tip. Adding a remote drive system to it also adds some weight "up high", but not as much as putting a CIM up there.

But that is an interesting idea of combining two different types of motors to drive the arm, thanks!

We did an experiment with a Globe using a wrench and a scale....the Globe will back drive with about 1/5 the load we plan to apply to it, so this could be a serious problem.

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

We used the two motor solution a few years back with an extendable ladder like structure. The trick is to make the window motors holding direction so that you are driving the armature toward the back of the motor. I.e. hold the motor in your hand and if the output shaft is facing you and the motor part is to the right and up, the stopped load you would want turning the output shaft clockwise. We have had very bad luck with the holding load the other direction. The other thing is to match the free speeds of the two motors you are using. The published info is good. Hooking them up and counting revolutions is even better. In 2005 A Globe and the Keyang window motor was a good match. We had a coupler with 16 teeth for #25 chain machined the linked the two shafts together. It was the best working part of the arms on that robot. And it stayed where you left it with out driving power. This year you get two Globes and two Keyangs. I will attach the inventor file. We did have some issues with the machining and had to do some of it by hand.
Biff

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

Thanks for the tips! I'll get my son to open the inventor files....I'm not that computer literate yet :)

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

Thank you for all of your input but my question has still not really been answered.
do you guys build your own gearboxes for the globe motors?
Do you use any special programming to make the motor stay slightly engaged to keep the arm up?
Do you use globe motors, van door, or window?

Sorry... only my second year in first.

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.