Potentiometer [Archive]

2611.Shooter

07-02-2011, 15:18

just wire it with the correct gauge wire for the breaker, and connect it correctly to the cRIO, not a motor controller. Oh and make sure the parts are under the $ limit. other than that I think you will be fine.

Teched3

07-02-2011, 15:39

What are you planning to use the potentiometer for? :)

jtdowney

07-02-2011, 15:42

just wire it with the correct gauge wire for the breaker

Typically potentiometers are going to be wired into a port on the analog breakout which provides power and is already downstream of a breaker.

2611.Shooter

07-02-2011, 15:46

that works too, just another way of saying meet the wiring resrictions! (hey, mebbe its a ginormous one and needs 10 amps or something :ahh: )

Matt Krass

07-02-2011, 17:55

that works too, just another way of saying meet the wiring resrictions! (hey, mebbe its a ginormous one and needs 10 amps or something :ahh: )

Actually, it would be a fairly tiny one by Ohm's Law, assuming we're talking 12V since its through a breaker

V = IR
12 = 10R
R = 1.2 ohms

Our pots are nearly 5000 times 'bigger'! Of course, I'm just being silly now.
Back on topic, why do you say not to connect it to a speed controller? The position control mode on Jaguars can use the potentiometers (though I don't personally find it actually works that well).

EDIT:
Actually, since you'd be dissipating about 120W, you would need a fairly large physical pot to absorb that kind of heat!
Ok, really I'm gonna put on my serious face now :)

Matt

Ted Weisse

07-02-2011, 20:42

If you are asking if you can use a potentioimeter the answer is yes. If you are asking how to connect it check the following FIRST document:
http://usfirst.org/uploadedFiles/Robotics_Programs/FRC/Game_and_Season__Info/2011_Assets/Kit_of_Parts/Analog_Breakout%281%29.pdf page 5 for this:

How do I connect a potentiometer?
 Connect the wiper to the input pin – Top row.
 Connect the other two pins to +5 and ground – Middle and bottom rows.

As for the Power required if you use a 1kΩ you would only dispate 144 mW at 12 volts and with a 10kΩ of course 14.4 mW. 1/4 potentiometer would be fine.

A good 10 turn pot works wonderful for positioning feedback.

wilsonmw04

07-02-2011, 20:59

A good 10 turn pot works wonderful for positioning feedback.

We found an old pot in our junk drawer that we are using to get feedback for our arm. Why would you want a sensor that can turn 10 rotations when your arm may only move 180 degrees. Aren't you using a small range of the possible output voltage?

Ted Weisse

07-02-2011, 21:01

So true... A 300 degree one should be fine for most apps but I was thinking of a tower type of application that would have multiple motor rotations. One train of thought problem...

wilsonmw04

07-02-2011, 21:10

And I didn't think of using it on a idler or a sprocket. Can't wait to try that out on the next bot :-)

One more thing. I was expecting the output voltage to be linear (constant angle/voltage) but we found that not be remotely true.the first 40 degrees give us roughly a change of .3 volts but the next 45 degrees gives us a change of 1.7 volts. Is this normal?

Ted Weisse

07-02-2011, 21:20

One analog input vs many digital inputs... I'd go analog...

Alan Anderson

07-02-2011, 21:48

One more thing. I was expecting the output voltage to be linear (constant angle/voltage) but we found that not be remotely true.the first 40 degrees give us roughly a change of .3 volts but the next 45 degrees gives us a change of 1.7 volts. Is this normal?

It sounds like you picked up a "log" or "audio taper" pot. They're used as volume controls. We accidentally mounted one on our 2004 robot's wrist joint, and it was ridiculously difficult to program for.

If I were you, I wouldn't use it. Get yourself a "linear taper" one. You'll be a lot happier.

Retired Starman

07-02-2011, 21:52

Not all pots are linear. Some are called "audio taper and have a logarithmic function to adjust sound volume so turning the pot approximates what your ear perceives as linear. I've also run into some very strange special purpose pots made on the surplus market. These have a very odd response. Pots aren't that expensive. Go buy one that's linear, and of good quality if your arm's position is going to depend on it. Remember, if you are using a feedback circuit to help drive your arm, then the quality of the pot becomes a safety issue!

PAR_WIG1350

07-02-2011, 22:13

...

Ignore the troll. (location listed as /b/, ridiculous suggestion, rookie year 1919, etc.)

wilsonmw04

07-02-2011, 22:15

Ignore the troll. (location listed as /b/, ridiculous suggestion, etc.)

before that it was "4 chan /b/ board" but agreed. The first rule of internet usage: Never feed the trolls.

Team#2057-Vegas

07-02-2011, 22:44

before that it was "4 chan /b/ board" but agreed. The first rule of internet usage: Never feed the trolls.

were too busy feeding the watchdog and the watch puppies :ahh:

Matt Krass

07-02-2011, 22:46

were too busy feeding the watchdog and the watch puppies :ahh:

Can we train the watchdog to hunt trolls? Should get rid of those pesky watchdog not fed errors and clean up the forums. Win-win.

kamocat

08-02-2011, 00:26

RyanN

08-02-2011, 08:06

If you're looking for good potentiometers, I would suggest Spectrol (http://www.apke.com/spectrolcd/home.htm) or Bourns (http://www.bourns.com/ProductFamily.aspx?name=potentiometers).
I have a 10-turn Spectrol pot myself. Its linearity is rated within 0.25%.
(typical pots don't HAVE a rating for linearity)

Of course, feel free to try a cheap potentiometer, and see where it gets you.

...But if you do buy a cheap potentiometer, or actually, most potentiometers (but cheap ones mostly). Even though they rotate at 270 degrees, don't expect to get all 270 degrees of data from it. Cheap ones get probably 200 degrees of reliable data. Decent 270 degree ones maybe a little more.

My point: don't get a potentiometer that will just have enough rotation to work. It won't.

Matt Krass

08-02-2011, 12:22

...But if you do buy a cheap potentiometer, or actually, most potentiometers (but cheap ones mostly). Even though they rotate at 270 degrees, don't expect to get all 270 degrees of data from it. Cheap ones get probably 200 degrees of reliable data. Decent 270 degree ones maybe a little more.

My point: don't get a potentiometer that will just have enough rotation to work. It won't.

Agreed, plus you want to be able to sense your soft limits before you're right on top of them, which requires some extra room at either end of the range.

We've used 300 degree pots for about 250 degrees maximum range, with some breathing room.

Matt

rsisk

08-02-2011, 15:35

We use these for counting the revolutions of our motor. We mount them to the output shaft of the gearbox with a piece of surgical tubing. One of our mentors manufactured some spiffy plastic mounts that strap onto the side of the motor to hold the potentiometer. Works pretty nice.

http://www.alliedelec.com/search/productdetail.aspx?SKU=9700063

kamocat

08-02-2011, 21:13

dmitch

08-02-2011, 21:55

One of our mentors manufactured some spiffy plastic mounts that strap onto the side of the motor to hold the potentiometer. Works pretty nice.

Could we get a picture of these?

Joe Ross

09-02-2011, 00:32

Do you know what the deadband on that is? (How much space is there between when it hits the bottom and when it starts again at the top?)

It's in the specs on that page (electrical rotation 340 degrees +/- 3)

rsisk

09-02-2011, 02:54

Could we get a picture of these?

http://www.rsisk.com/potentiometerholder.jpg
http://www.rsisk.com/potentiometerholder1.jpg

koo_04

11-02-2011, 15:55

Hey everyone. Here is detailed shots, as well as a shot giving dimentions, of the POT holder for the 2.5" CIM motor. Sorry I can't find my CAD drawing. Might of went with my computer when I had it wiped.

http://doug.devicariis.org/images/robotics/

I am sure one of you guys could recreate it in no time. I have been busy, or else I would. Have fun!!

Ross3098

07-12-2011, 10:02

Quick question and I dont want to start a thread about it. Do potentiometers have to be reset at all some way? Ive been tinkering with code and simply using the AnalogChannel->GetVoltage() and printing it to the console gives a voltage reading somewhere around ~30,000,000 which I assume means it isnt hooked up. No one has touched it since competition season and the potentiometer was fine since the last competition.

It is plugged into port 2 on an analog module.

:confused: :confused: :confused: :confused:

Alan Anderson

07-12-2011, 10:21

...simply using the AnalogChannel->GetVoltage() and printing it to the console gives a voltage reading somewhere around ~30,000,000...

What format string are you using when you print the value? The GetVoltage() function returns a float. You might have accidentally tried to print it as an int instead.

Ross3098

07-12-2011, 10:30

That might be the problem... Im retrieving it to a double value.

And Im printing to int. Honestly I dont know how to print anything but int. Just getting to know this stuff. :/

Michael Blake

07-12-2011, 11:35

Here's the VEX potentiometer we used for our arm position:

http://www.vexrobotics.com/products/accessories/sensors/276-2216.html

It worked well once we had a completed gripper on the end of the arm that _worked_... ;-)

Ross3098

16-12-2011, 09:14

Sorry but our potentiometer is still not working. Potentiometer is hooked up to Analog Channel #2. Power is going to the analog module.

Code for printing is as follows:

(Im typing it because its on a different computer.)

AnalogChannel *m_Pot;
///////////////////////////
m_Pot = new AnalogChannel(2);
///////////////////////////
if (m_gamepad1->GetButton07() == 1){

m_LCD->Printf(DriverStationLCD::kUser_Line3,1,"v: %4f",m_Pot->GetVoltage());
m_LCD->UpdateLCD();

Driver Station is reading a constantly changing value ~0.003 that is sometimes negative.

:confused: :confused: :confused: :confused: :confused:

Alan Anderson

16-12-2011, 09:27

Potentiometer is hooked up to Analog Channel #2.

How exactly is it connected? There should be three wires. Where does each of them go?

When you measure the voltage with a meter, does it do what you expect?

Ross3098

16-12-2011, 09:59

Not in the room atm but I'm sure that all three wires are connected to a pwm which then feeds into the analog module. It hasnt been changed (or at least not that I know of because OCCRA happened and fuses/pwms were taken off of the robot.) since last competition.

Al Skierkiewicz

16-12-2011, 11:03

Colin,
Assuming that the code is functional, the reading sounds like what I would expect with a shorted wire. Use a voltmeter and check at both the pot and the wiring entry to the analog module.

Ross3098

16-12-2011, 16:49

*Facepalm* once again on our team the fault is not programming it is electrical. The solder holding the positive wire to the potentiometer broke off.

Only noticed after trying to stick a volt meter down there.....:rolleyes:

Thanks for the help nonetheless as it did help solve our problem. :D