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

[EStokely]

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

[Gabe]

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

I love GlobeMotors!!!

[MrForbes]

Originally Posted by EStokely

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.

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!

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

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.

[Biff]

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

[MrForbes]

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

[Ian_Xodus]

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.

[Dan_Karol]

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.

[Rickertsen2]

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.

[Justin M.]

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.

[Andrew Blair]

Originally Posted by Justin M.

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.

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

[Justin M.]

Originally Posted by Andrew Blair

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

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

[Alan Anderson]

Originally Posted by Justin M.

Looks like we'll have to do it in programming then.

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.

[Justin M.]

Originally Posted by Alan Anderson

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.

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.

[Alan Anderson]

Originally Posted by Andrew Blair

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

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.

[petek]

Originally Posted by Alan Anderson

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.

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:

<R92> CIM motors and Fisher-Price motors must be connected to speed controllers. They can not be connected to relay modules.

Again, no mention of limit on number of motors per speed controller. From the 2007 Guidelines_Tips_Good Practices.pdf:

Only one large motor (CIM, Fisher-Price, Globe or similar motor) should be controlled per speed controller

As far as I know that is only a guideline, not a rule. It goes on to say:

In some cases, more than one low current small motor (window/Mabuchi) or actuator may be optionally connected to a single speed controller.

The closest I come to this being a rule is:

<R85> ... For examples of acceptable circuit designs, please see the 2007 Robot Power Distribution Diagram.

The Diagram shows just one motor per speed controller.