Attaching Potentiometers to VEX

[billbo911]

I've looked and searched, but I haven't found these questions asked or answered, so here they are.

The Vex kits use square drive shafts and holes to drive gears and wheels. I need to attach pots for position feedback, but can't seem to find any simple way to do it. I figure either coupling adapters with round holes for the pot on one end and square holes on the other to accommodate the axle. Or, round on one end and a square post/peg on the other to attach directly to a gear.
What I would love to hear is that this has already been solved. If not, that at least someone has a suggestion as to how to solve this simple issue.

The next minor issue: All inputs and outputs on the Micro Controller require male pins in their plugs. I have seen several sites that I can purchase female pins and plug sets and wire in bulk. These have all been R/C Hobby sites. What I have not seen, although my searching has not been all that extensive, are male pin and plug sets, let alone ones with keys, like the plugs in the VEX kit.
Again, I'm open to suggestions.

[foobert]

try shaft couplers. much like the shaft collars that come with the kit, but a little longer with two set screws. someplace like mcmaster-carr would have these.

check jameco or digikey or a local electronics shop for connectors and crimp pins. you'll have to put them together yourself and they won't be keyed. right now i'm playing with resistors soldered to the legs of leds and just sticking them directly into the holes on the vex. but, you know... carefully.

on the vex, the keyed end of the connector is the signal, the midlle pin is 5v on the a/d and interrupt sections and whatever's coming out of your batteries at the moment on the motor section and the pin opposite the keyed end is ground. i checked it with a meter but the colors of the wires pretty much tell the story.

[billbo911]

Originally Posted by foobert

try shaft couplers. much like the shaft collars that come with the kit, but a little longer with two set screws. someplace like mcmaster-carr would have these.
check jameco or digikey or a local electronics shop for connectors and crimp pins. you'll have to put them together yourself and they won't be keyed.

I knew someone had to have an idea where to find couplers!!! Using them was my first thought, I just haven't been able to find them, yet. I'll start with your links! Thanks!!!!!!

By the way, for those who need to know, the collars that come with the Vex kit measure: .434" OD X .158" ID X .252" L

Finding the right pins was a big concern. Using the wrong ones could potentionally damage the connectors in the Micro permanently. I already own crimpers that are designed for them, just a benefit of sevicing X-Ray systems for 20+ years, although, soldering is the most reliable. The correct plug is not as big of a deal, the biggest concern is getting the pin-out correct. The key is a bonus, and in my opinion, a good idea considering these kits are targeted to an entry to mid level robotics group.

Originally Posted by foobert

right now i'm playing with resistors soldered to the legs of leds and just sticking them directly into the holes on the vex. but, you know... carefully.

May I ask, what are you using the LED's for? I could throw out some guesses, but the list of possibilities is way too long.

[sanddrag]

A very quick, easy, and reliable way to couple a pot to a shaft of similar size is to heat shrink them together to make a flexible coupling. On a square shaft, it would work superbly because it would get a good hold on it. And on the round pot shaft, it should have enough surface area not to slip. But, you may want to put a small amount fo some sort of adhesive on the pot shaft.

[billbo911]

Originally Posted by sanddrag

A very quick, easy, and reliable way to couple a pot to a shaft of similar size is to heat shrink them together to make a flexible coupling. On a square shaft, it would work superbly because it would get a good hold on it. And on the round pot shaft, it should have enough surface area not to slip. But, you may want to put a small amount fo some sort of adhesive on the pot shaft.

Now that is something I had never even considered. It makes good sense on a couple different levels. Not only is it simple, and I like simple , it's inexpensive, and allows for imprecise alignment of the pot and drive shaft without undue stress on the pot.
The only drawback is you will need to get the pot very close to it's calibrated position prior to heating the shrink tube. You "should" be able to trim out the pot by rotating it's housing within the mounting plate or bracket to do any final cal. Then, in worst case, use adhesive as you mentioned.

OK, now I have some experimenting to try this weekend. I'll let you know how it turns out. I'll just need to find a pot that is not too stiff

[foobert]

just playing with the hardware. turning them on and off in the a/d section and now using the hardware pwm on the first four motor connectors to vary the brightness.

i call this project the Mobile Remote Controlled Light Dimmer, or MoReCoLD. Cool, huh?

[billbo911]

Originally Posted by sanddrag

shaft of similar size is to heat shrink them together to make a flexible coupling. On a square shaft, it would work superbly because it would get a good hold on it. And on the round pot shaft, it should have enough surface area not to slip....

Sanddrag you Mad Genius!!! That worked out GREAT!!!

Check out the pictures. You can see that I was able to find a 10K linear pot with a shaft very close in size. The other cool thing about the pot is the pins on it align perfectly with a Servo extension cable plug, except, they are small enough that the make poor contact, so I had to solder to the pot. I also felt the round shaft might not grip the heat shrink well, or vise verse, so I added a layer of smaller diameter shrink to it first. Then using a diameter that fit both the round and square shafts, I shrank them together. WORKS GREAT!!!




This is an edit to give an update:
I spent some time working with, and programming, last night. I got the programming module to work!!! I now can use the Radio Control stick to control the position of the are instead of just the speed. This is a big step for me, and my team, because we have NEVER used anything beyond the basic code on our bots.

BTW: When you program the VEX, the basic code turns over control of the PWMs to the USER, not the MASTER, controller. Until you write code to create the PWM signals, you will need to set the PWM control to MASTER or nothing will work.

[sanddrag]

Thanks for the pics. I'm glad my ideas could be of service.

[foobert]

was thumbing through my jameco catalogue today and came across spider couplers with inside diameters of 1/8", 1/4" and 5mm. they can be mixed and matched to connect axles of different sizes or an axle of one size to a pot shaft of another. costlier, of course, than shrink tubing.

0.125" - $1.35 (pn 162288ch)
0.250" - $1.35 (pn 162270ch)
0.196" - $1.09 (pn 161998ch)
spider - $1.29 (pn 162000ch)

[billbo911]

Originally Posted by foobert

was thumbing through my jameco catalogue today and came across spider couplers with inside diameters of 1/8", 1/4" and 5mm. they can be mixed and matched to connect axles of different sizes or an axle of one size to a pot shaft of another. costlier, of course, than shrink tubing.

0.125" - $1.35 (pn 162288ch)
0.250" - $1.35 (pn 162270ch)
0.196" - $1.09 (pn 161998ch)
spider - $1.29 (pn 162000ch)

Funny you should mention this today of all days. I am scheduled to receive a shipment today from Jameco today with 5 different sets of these exact Spider Couplers and spiders. There are actually a couple additional sizes available on-line that aren't mentioned here, but the application is identical.

Ahhh, the saying is true, "Great minds think alike"

BTW, adding a small zip tye to the heat shrink on a round shaft makes the bond more secure!