What are Potentiometers?

[Theory6RobotiX]

Hi,
I was doing a little research on 'pots' on CD and i came across posts that suggested using different potentiometers in different scenarios. I am a little confused over how potentiometers work and what the difference is between the different ones?
Thanks

[evolution]

This should answer your questions:

http://en.wikipedia.org/wiki/Potentiometer

[KenWittlief]

wow, that wiki page has a lot of errors! (weird for something as simple as a pot)

this page is a little more straight forward (without the archaic references to galvenometers and stuff): http://www.sound.westhost.com/pots.htm

[TimCraig]

Originally Posted by KenWittlief

wow, that wiki page has a lot of errors! (weird for something as simple as a pot)

We might be enlightened by what you think are errors. And as far as galvanometers being archaic, historical context is often helpful in understanding how a device has evolved.

[KenWittlief]

so many things - like "a volume control acts as a filter" - thats not correct

back before voltage and current were defined by agreed to names (voltage was named after Voltaire) a device for measuring voltage was referred to as a potentiometer (potential meter) but that terminology is now obsolete.

A potentiometer is a variable resistor, nothing more. Its not a voltage source, its not a current source, its not a measuring device.

If you dont know what a pot is, the Wiki entry is very misleading and confusing.

[Matt Adams]

I think that the two previous links explain technically how a pot works, but I think that often times people ask this sort of question because they want to know specifically how they're used in FIRST robots. I'll try to do this to add more to this discussion.

To summarize the what's been said, a pot is a variable resistor, and it's purpose typically in FIRST is for some sort of feedback, i.e. knowing the location of a device on a FIRST robot.

On a typical pot you have three wires, a +5 volts input into the pot (out from the controller), a ground wire, and an analog wire that carries a signal 0-5 volts back into controller. At one extreme on a pot, the voltage will be 0 volts, and on the other extreme the voltage will be +5 volts, depending on the shaft location. The robot controller takes this analog voltage signal and runs it through a analog to digital converter (ADC), and expresses this voltage as an integer somewhere between 0 and 1024.

A typical use is having a pot connected to a shaft on a robot arm. As the arm moves back and forth, the whipper on the pot will change the resistance, and hence the output voltage.

Going back to FIRST applications, you can use a potentiometer to have a push buttons on your operator interface move your arm to a known position. So instead of having your operator manually control the arm to move to 50 degrees, with a single button you could have the motor that powers the arm rotate until the controller sees the arm is in position, based on the voltage feedback you get from the potentiometer.

As a disclaimer, there's a whole discipline of engineering with regard to "having the motor that powers the arm rotate until it sees the arm is in position." You can look up some posts on PID control loops in these forums for more information.

I hope this answers your question!

ho ho ho!

Matt

[skimoose]

On a typical pot you have three wires, a +5 volts input into the pot (out from the controller), a ground wire, and an analog wire that carries a signal 0-5 volts back into controller. At one extreme on a pot, the voltage will be 0 volts, and on the other extreme the voltage will be +5 volts, depending on the shaft location. The robot controller takes this analog voltage signal and runs it through a analog to digital converter (ADC), and expresses this voltage as an integer somewhere between 0 and 1024.

Matt's right on the mark. The other application in FRC is using pots to build custom operator interfaces. If you haven't opened a joystick before, it has one or two (depending on whether its a one or two axis stick) pots connected to the joystick's gimbal. As the joystick is moved it changes the analog signal. After its converted, the value is used to set the PWM value (0-255) being sent to the programmed Victor speed controller or being used to match the value to the feedback pot on the robot arm. Teams have built small arms on their OI which mimic the location of the arm on their robot. The pots are used at the joints to tell the bot's arm what position to go to.

Just one other bit on pots. FRC usually uses rotary pots, but there are also linear pots (think of sound mixing board slider pots). Rotaries also come in different resistance values (10k, 100k, 500k) and single turn or multi-turn versions. Single turn pots give you full range 0-5 volts in about 270 degrees of motion. This is good for arm feedback where the arm doesn't exceed the 270 degrees of motion. Multi-turn pots require several complete revolutions (usually 10 turns) to complete the full feedback range. These might be better on gearboxes or elevator feedback where the drive system is making several full rotations to complete its range of motion.

[TimCraig]

Originally Posted by KenWittlief

(voltage was named after Voltaire)

I think you mean the Italian physicist, Alessandro Volta, rather than the French writer and philosopher Francois-Marie Arouet de Voltaire.

http://inventors.about.com/library/i...ndro_Volta.htm
http://www.ideafinder.com/history/inventors/volta.htm

[KenWittlief]

yep, you are right. Amps and watts are also named after people, and I believe the unit of charge (coulomb) as well.

maybe someday they will name a unit of something after me (like stupidity maybe? :^)

[billbo911]

Originally Posted by KenWittlief

...maybe someday they will name a unit of something after me (like stupidity maybe? :^)

Ken, are you giving us license to begin using expressions like, "Boy, that was a full Wittlief." or maybe, "Just remember class, Stupidity is measured in Wittlief's"?
The question then becomes, "Is it a single Wittlief, broken down by percentage, or is it a small unit that requires multiples to describe the actual level of stupidity?"

I think I'll stop here, the door is already too far open.
Thanks for allowing me to run with this for a short distance, I'll hand the baton off to the next runner now.

[ChrisH]

Originally Posted by KenWittlief

yep, you are right. Amps and watts are also named after people, and I believe the unit of charge (coulomb) as well.

maybe someday they will name a unit of something after me (like stupidity maybe? :^)

When I was in college a group of us created a unit named after a professor who seemed to be somewhat behind the times.

1 Maradudin = 1 carat furlong/(fortnight^2)

The Maradudin is a unit of force, you can use existing quantities and just change the units to create a new unit. The hard part is getting it generally accepted.

[KenWittlief]

keeping with engineering tradition, the unit would be similar to my name (volts: Volta...)

like maybe "Wittless" ?

Im not sure what the units would be? how do you measure stupidity? number of dollars wasted? or dollars wasted per second?

number of seconds wasted doing something the wrong way? number of seconds it takes to find/debug/correct a mistake?

time would be more of an intenational standard than say dollars per second.

We found a 3.6k Wittless in the code (stupid mistake that took one hour to find and correct)....

[Al Skierkiewicz]

I like measuring length in femto parsecs. (1 x 10 **-15). Somehow a wittlief doesn't seem fitting.

[Richard Wallace]

Sometimes more that one person's name is used as a unit for the same physical quantity. Gauss is a common unit of magnetic flux density; the earth's magnetic field induces a flux density of 0.3 to 0.6 Gauss depending on where you measure it (e.g., higher in northern Canada, lower in central Africa). Another common unit for magnetic flux density is the Tesla, equal to 10,000 Gauss. The strongest rare-earth permanent magnets have a residual induction of about 1.2 Tesla, or 25,000 times the flux density due to earth's magnetic field.

So should we infer anything about Tesla and Gauss based on the relative size of the flux density units that bear their names? Tesla was arguably the most important inventor of the twentieth century (I think it would be hard to top polyphase AC electrical distribution and its associated electric machinery for impact on society) while Gauss was arguably the most important mathematician of the 19th century.

Anyway, if Ken is to have a unit of stupidity named for him, then I suggest we will need a much larger unit named for someone else.

[billbo911]

Originally Posted by Richard

Another common unit for magnetic flux density is the Tesla, equal to 10,000 Gauss....
Anyway, if Ken is to have a unit of stupidity named for him, then I suggest we will need a much larger unit named for someone else.

I like the 10,000:1 ratio. If we stick with that, I would like to volunteer the "Kendalla". It would be a unit of measure 10,000 times more stupid than a "Wittless"
All in favor, raise both feet and say "Hail the Kendalla!"