Limitless Potentiometers

[popo308]

We used encoders on are drive train for the wheel rotation. It worked fine and never broke. It was also very reliable. However we had a pot on the arm and that broke a lot we went through quite a few of those. Mainly because it was not protected very well though. When it was working it did not give very consistent readings though

[Leav]

Here are my tips regarding potentiometers:

• Use the best quality pots you can get - 0.25$ pots wont perform as well as the 3-4$ pots... it's a small amount that's worth it.
• mount the pots securely making sure they cannot move.
• pots should not be load bearing parts. use them in a place where they will not be damaged by the load applied to your robot.
• remeber that if you need accuracy it is best to use a small gearing system that should make the pot turn more, that way you are minimizing the effect of the looseness in the pot itself.

that's about it I guess.... good luck!

-Leav

[Bruce Newendorp]

We use Digi-Key part 6639S-1-103-ND for our arms and linkages. It is a Bourns Series 6639 potentiometer. It provides electrical output over 340 degrees of rotation but there are no mechanical stops in it. That means it is easy to install and calibrate since you do not need to worry about breaking the stops due to over travel. It is somewhat expensive at $11.

[PhilBot]

Hi

Team1629 uses this pot from Mouser.com (My favorite parts vendor)

http://www.mouser.com/search/refine....594-357-00-502

Datasheet here:

http://www.vishay.com/docs/57059/357.pdf

At $15 it's not cheap, but it gave us position feedback on our 2007 arm position all season and several off-season competions with no problems.

Phil.

[Qbranch]

Originally Posted by sumadin

Sorry to hijack the thread, but I'm wondering how you would tell what direction you moved. I don't doubt this can work very well for something that only goes under a revolution. However, let's say you moved from 100 degrees to 200. How do you know if you moved 100 forwards or 260 back?

Thanks.

Before I even start, here's the, well, you might call it the Nyquist rate for sampling these rotary analog encoders (aka Limitless Pots). Unless you use adaptive sampling rates (DO NOT if you are using a PID and don't want to have dynamic constants on the terms) you have to sample the voltage about four times per revolution, or else you won't know which way you went (there may be a more efficient way but this is what I used last season). Therefore, I suggest you figure how many RPM the shaft you are coupled to could do in the worst possible situation, then add some to that (use your engineering sense). Obviously if you miss a revolution you'll be in trouble and have problems like you mentioned above.

I drew a diagram (using my graphic tablet I got on Black Friday... ) that explains how this system works. Essentialy you break the waveform that the analog encoder outputs into sections, and do a state-based decode. Since you're sampling at a rate four+ times that of the rate of revolution, you will be able to detect a high-contrast change in the values comming back from the analog encoder and be able to figure that you went from a high-to-low or low-to-high transition, thus indicating the direction in which you moved.

Aside from that, the rest of the system functions on deltas: you add or subtract how much the values have changed depending on which direction the analog encoder is moving.

Here's the code snippet that handles one of the analog encoders (there were two) on our robot. This function was called at a rate of 100Hz using a CCP module for timing (it also was running a high-performance pwm signal for the pid'ed traction motors on the robot):

void encoder_left_handler(void)
{
	if(ServoOn)	//if servos running, sample encoder
	{
		leftencoderpos = Get_Analog_Value(LEFTENCODER);		//get value from other encoder
		if (leftencoderpos <= 253)
		{
			leftencoderstate = 0;	//state a
		}
		else if (leftencoderpos <= 505)
		{
			leftencoderstate = 1;	//state b
		}
		else if (leftencoderpos <= 757)
		{
			leftencoderstate = 2;	//state c
		}
		else
		{
			leftencoderstate = 3;	//state d
		}
		if (leftencoderstate == 0 && oldleftencoderstate == 3)	//from top to bottom, rev
		{
			//going backwards
			leftencoder -= (int)((1009 - oldleftencoderpos) + (leftencoderpos-2));
		}
		else if (leftencoderstate == 3 && oldleftencoderstate == 0)	//from bottom to top, fwd
		{
			//going forward
			leftencoder += (int)((1009 - leftencoderpos) + (oldleftencoderpos-2));
		}
		else	//intra-state position calculation
		{
			if (leftencoderpos < oldleftencoderpos)
			{
				//goingfwd, intra state
				leftencoder += (int)(oldleftencoderpos - leftencoderpos);
			}
			else
			{
				//goingbkwd, intra state
				leftencoder -= (int)(leftencoderpos - oldleftencoderpos);
			}
		}
	
	
	
	
	

		oldleftencoderstate = leftencoderstate;	//save encoder state
		oldleftencoderpos = leftencoderpos;	//save old encoderposition
	}	//end if servo on
//...

This system works very very well, we used it to run a 100Hz update rate PID (as I stated before, we were feeding the Victors a 16-bit PWM signal at 100Hz as well as opposed to the standard 8-bit 39.37Hz) which performed with silky-smooth motion control during our post-season autonomous work. I definitely reccomment analog encoders. As long as you show 'em a little love with shielded cables, smart wiring (keep the wires short!), and a little software know how, you get fantastic positional accuracy without the need to handle extreme high frequency (in the KHz) interrupts at high speed.

Questions? Post!

-q

[eugenebrooks]

Give Bournes 6639S-1-203 a try.

http://www.mouser.com/catalog/632/568.pdf

We have used it sucessfully when the mechanical
stops of a 270 degree pot posed a problem.

[Pavan Dave]

We have used 2 continuous potentiometers in the past. What we did was couple them together so we could account for 360 degrees. From there, all you have to do is work with the code a little bit to work between the two pots.

[popo308]

Originally Posted by Leav

Here are my tips regarding potentiometers:

• Use the best quality pots you can get - 0.25$ pots wont perform as well as the 3-4$ pots... it's a small amount that's worth it.
• mount the pots securely making sure they cannot move.
• pots should not be load bearing parts. use them in a place where they will not be damaged by the load applied to your robot.
• remeber that if you need accuracy it is best to use a small gearing system that should make the pot turn more, that way you are minimizing the effect of the looseness in the pot itself.

I would have to agree! Get really good pots and be sure they are not bearing load!!!

Also a good pot mount with a protector always makes things nicer!

[Qbranch]

Uhm, can a moderator merge this thread into the other identically named Limitless Potentiometers thread?

-q

[BanksKid]

i must say that there cant possible be anything at all wrong with endless pot. ........(but seriously my big problem with them in the past is defining a definite start and end....i dont like them too much...)

[MrForbes]

Originally Posted by 987HighRoller

This is because we had something like 8 potentiometers break on us last year, mainly because they get locked inside due to load on the shaft ...........Any advise?
Thanks

Sure, I'll offer some advice. Don't put any mechanical load on a potentiometer shaft.

Here's how we got around the problem.

And our shaft encoders were coupled to the transmission shafts with a piece of surgical tubing.

[=Martin=Taylor=]

Originally Posted by 987HighRoller

This is because we had something like 8 potentiometers break on us last year, mainly because they get locked inside due to load on the shaft . We were also thinking of encoders but are afraid of using too many interrupts and messing with the interrupts already used by sensors like the camera and gyros. Any advise?
Thanks

Our team has used magnet-sensors many times in the past. These sensors work two ways. Either you can glue magnets on a rotating surface and read them as they go by (like a big LP record).

Or you can actually sense the teeth of a sprocket - without any sort of preparation - provided of course that the sprocket is made of steel.

These sensors are a lot more durable than potentiometers since they have no physical coupling with the robot's moving parts.

We have used them in 2005 and 2007. However I think we had to make custom circuit boards for them.

[popo308]

Originally Posted by squirrel

Sure, I'll offer some advice. Don't put any mechanical load on a potentiometer shaft.

Here's how we got around the problem.

And our shaft encoders were coupled to the transmission shafts with a piece of surgical tubing.

Very interesting... so how exactly did this work?? was the sergical tubing just attached to the output shaft the immediately attached to the sensor so there was very minimal drag??

I am very interested in how you did this?!?!

[MrForbes]

ok, I fibbed....we did the surgical tubing trick last year, this year we used a short piece of rubber automotive fuel line. The concept is the same.

The AM transmissions have a shaft with an end that is exposed, and turns with the output sprocket. We drilled and tapped the end of the shaft, then made a short piece of metal 1/4" diameter with a thread sticking out the end, which screws into the transmission shaft. Then just slipped the tubing over this short shaft and over the encoder shaft, and made a mounting bracket for the encoder. A lathe was used to drill the hole and make the adapter shaft.

The picture shows the mounting and protection for the encoder, and if you look close you can see a red tube clamp in there...the tube was not a very tight fit on the shafts.

[popo308]

Ahh.. interesting concept... seems like it would work well