Lineal Potentiometer

willy1067

Have anyone seen any of this devices?·· they are use in equalizers, volume control, etc.

I need them..
Lineal Potentiometers

http://forums.parallax.com/showthread.php?p=637017



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Fernando Gomez

revinc.us
gomez-rivera.com

Never compare yourself with anyone else, there will always be someone bigger·or·smaller·than you.

Post Edited (willy1067) : 3/28/2007 9:49:40 PM GMT

T Chap

Spelled linear, they are everywhere. Try digikey, allelectronics.com, google, all depends on what you want, plastic conductive is nice, you need to specify analog taper(log scaled for audio volume control use) or linear taper. The Linear pot will have 50% resistance in the center of travel. Take a log pot for audio use from Radio shack, and a pot that is linear, and meter from the wiper to one end, notice the difference between the two types. Without knowing the goal here, it is impossible to say what to use.

Post Edited (TChapman) : 3/25/2007 9:59:08 AM GMT

willy1067

Thanks TChapman, you are a God send.

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Fernando Gomez

revinc.us
gomez-rivera.com

Never compare yourself with anyone else, there will always be someone bigger·or·smaller·than you.

Post Edited (willy1067) : 3/28/2007 9:49:20 PM GMT

PJAllen

Hey, lookit -- the device pictured is NOT a linear potentiometer, it is a slide potentiometer.· Since·wily1067 is looking for just such an item·used in "...equalizers, volume control, etc." then it does NOT (should not) have a linear taper, it must have a logarithmic (or "log", or audio) taper.· There are slide potentiometers available in both types (linear and log.)

Also -- this should have been posted in The Sandbox.

T Chap

My assumption is that he is trying to figure a way to control his robotic hand with a pot, just a guess though, as he only "implied" where he has seen the pots used, then linked his robotic hand thread.

Not that PJ needs any study on this, but for clarification, in audio, a graphic EQ could actually use a linear taper pot, depending on the function i.e., freq select, bandwidth select, cut/boost. Also, in LR pan functions, "dual gang" circuit linears are used often instead of log/antilog pots.

Machine27

Shot me an email.· I've got some really sweet linear potentiometers.

willy1067

TChapman, your assumption is correct, I need 20 of this devices for my project (5 fingers X 4 joints) the devices would be conected to the strings that pull and push each joint giving me a readout of where each joint is at all time.

PJ Allen, Now the question is.... Is there a difference between a linear potentiometer and a slider potentiometer? I am sure there is, because on my web search, the result came with regular circular potentiometer when I typed (linear potentiometers)

Thanks guys for all the help, I am new at all this.

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Fernando Gomez

revinc.us
gomez-rivera.com

Never compare yourself with anyone else, there will always be someone bigger·or·smaller·than you.

Post Edited (willy1067) : 3/28/2007 9:49:01 PM GMT

Robert Kubichek

You can get slider pots, that are "linear"" logrithmic" or "audio" in taper.
Its how the resistance changes as you move the wiper on the pot.

How are you going tomove the slider?? with 2 cords per pot, or a spring loaded pot???


Is it possible to use the flexiforce sensors; [url=http:// http://www.parallax.com/detail.asp?product_id=30056 ] http://www.parallax.com/detail.asp?product_id=30056 [/url]

Bob

Post Edited (Robert Kubichek) : 3/26/2007 3:24:52 AM GMT

T Chap

The term "slider pot" is simply referring to the mechanical structure and motion of the device, which affects into how you use it in an application. A pot can actually be any shape you want to mold the resistive element into, there are more than just rotary and slider variations, but the most common are rotary pots(using a knob that turns, and sliders). The "slider pot" in your photo technically is a linear sliding potentiometer. It's form is linear motion, not to be confused with the electrical resistive component, which can be "linear" in it's electrical function, or logarithmic in it's electrical function. In other words, a linear pot will have a consistent increase (or decrease) of resistance the farther along the element the wiper goes. With a log pot, the resistance will change along it's path according to a logarithmic scale. You can google logarithmic pot and see many graphs that depict how it looks compared to linear. In audio, there is a more natural effect to raise or lower the signal when using a log pot(which can be either rotary or sdlider). In other applications, it is more desirable to have the resistance change on a linear scale. Depending on your application, you choose which serves the purpose. In the example of a multi turn trim pot, where you want to adjust some voltage level using a voltage divider circuit, you would prefer a linear pot, so that each turn of the screw driver produed a similar effect on the voltage. For what you are doing, it would likely be a slider with a linear resistive component.

Just so this makes sense again, a slider can be linear or log taper(resistance). A rotary can be either linear or log too. The name rotary or slider only depicts the shape and motion of how it is operated. Just as an example, you could design a resistive element in the shape of a horseshoe if you wanted to, and have a wiper that tracked the shape of the element around its travel.

Although this will start to get very expensive, you could have encolders on the motors that are set to a known value on boot up, and would allow you to monitor the position of the motor. I assume you are considering using an ADC for each slider pot to produce a position in the form of a voltage. Pretty soon you are going to need to switch to the Propeller for this project.

Post Edited (TChapman) : 3/26/2007 5:31:38 AM GMT

willy1067

TChatman, First thanks for your insightful explanation of linear pots, you are always teaching me a thing or two. (kudos to you my man)
second, I can't use encoders because my project only uses one motor, and the position will never be the same, not even at boot-up.

Robert, The slider head will be attached to each joint cord (one) as it moves up or down. my travel distance will be less than 1/2"; so the slider doesn't have to be big.

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Fernando Gomez

revinc.us
gomez-rivera.com

Never compare yourself with anyone else, there will always be someone bigger·or·smaller·than you.

Post Edited (willy1067) : 3/28/2007 9:48:43 PM GMT

T Chap

I think you need a brushless DC motor with encoder. On boot, home each joint to a hard stop and store the position in a Joint1Var, Joint2Var etc. Each time you want to move a joint, you call up the stored value and that value is affected by the encoder count, You could get a chip that reads the quad encoder A/B pulses, outputs a Count and Dir pulse, meaning 2 pins on the Stamp or external "counting engine" (SX, Another Stamp etc, counter IC).

Sounds like a lot of code, but encoder counting/tracking code is far less cumbersome than the hardware and code to read 20 slider pots(ADC or however you intend to do it)!

willy1067

Lets Say the arm is turn off, how will the program know where each joint is? that is why adding the sliders will facilitate the program not to go to a home position, but continue doing, what ever it needs doing.

I can then add the counter engine to keep track of position, without reading the pots. that will reduce timing and then from time to time, read the pots to double check accuracy.

What do you think of that?

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Fernando Gomez

revinc.us
gomez-rivera.com

Never compare yourself with anyone else, there will always be someone bigger·or·smaller·than you.

Post Edited (willy1067) : 3/28/2007 9:48:30 PM GMT

T Chap

What holds everything in place when no holding torque is applied by a motor? You have added brakes to every string that is not currently active?

Are you intending to use 20 ADC's to track the sliders, or are you going to multiplex access to the DAC with some switching?

From the pictures you have provided,it is hard to determine what is the best way without more info. I rerally think this is too much task for the Stamp, and that a Propeller is a more suitable platform.

allanlane5

Yes, you definitely need some position feedback.

Note that Potentiometers (variable resistors), aka "pots", do suffer some mechanical wear over time and use. That's why an optical solution was suggested, as these have zero wear.

And if your arm has been turned off, who knows what's happened from that time to when it's turned on?

Now, a typical pot is a knob, with a wiper, that moves with the knob over a coil. If the coil is wrapped 'linearly', you have a 'linear taper potentiometer' -- move the knob to the 10% position, you get 1/10 the full resistance, move to 90% you get 90%.

Now, human hearing (for a volume control, say) is logarithmic -- the volume goes as the log of the voltage. So you have "logarithmic" tapers, too, where the coil is wound wider and wider as you move toward the 100% position.

Now you're asking for a 'slider' potentiometer. Instead of the "knob" rotating a 'wiper', these 'slide' the wiper up and down the coil. And you expect to move the slider with strings? Well, the human hand works with tendons, why not?

But the more typical solution to this problem is to add a disk with an optical sensor to each joint. Then the processor monitors the sensor to know the position of the joint. As the joint rotates, it changes the position of the disk. This reduces the amount of mechanical 'strings' and 'pulleys' that have to be managed.

In any event -- if you go with your current design, you probably need a "slider" potentiometer, with a "linear" taper. Probably the 100% resistance should be 10,000 ohms.

willy1067

TChapman, I am just in the testing phace, posibilities are that by the time I finish the mechanical part of the hand and move into the electronics will realize that the Stamp is not the way to go. But at this time it's the easiest way to start and troubleshooting all the in's and out's of this project. (must do with what you have. lol)

Allan, no strings are used in my design, no stoppers either; it's all gears. just like a car transmission. each string pushes and pulls at the same time.

I am also reconsidering the Linear potentiometers, as you say "the wear and tear" Thinking optical now, but not a disk.· I will post a picture tonigh and see what you guys think about it.

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Fernando Gomez

revinc.us
gomez-rivera.com

Never compare yourself with anyone else, there will always be someone bigger·or·smaller·than you.

Post Edited (willy1067) : 3/28/2007 9:48:13 PM GMT

T Chap

Since you are suggesting that the fingers will stay braked where they last were, with encoders you could write the variables to an external EEPROM or flash/SD drive, including the last active joint, load it on boot and you are back where you started from.