Re: Crab drive without a pot
 

There are a couple of things that teams can do to use a standard 270 degree pot without breaking it. You can easily use a couple of nylon gears (from Small Parts) to couple the pot to your steering mechanism. You can mechanically limit the travel of your crab so that you stop turning before the pot is broken or you can spend some money and buy a continuous pot.
But dpick, as you can see, no one is telling you to do crab without a pot. There is no way for the robot or the driver to know that the crab system has actually gone where the driver has told it to go or for the software to get it back to driving forward.

Re: Crab drive without a pot
 

Why not an absolute encoder? (Like this.)

Re: Crab drive without a pot
 

Is there any functional difference between a pot and an absolute encoder? (I realize that they are different internally.) Why would someone use one over the other (for low speed applications, eg arms, steering)? Is there a reason teams use encoders that give counts instead of absolute encoders on higher speed applications (wheels)?

Re: Crab drive without a pot
 

Yes, the most significant advantage of this encoder is its free rotating shaft.

Plus it costs about the same as a 10 turn precision pot without the potential for value shifting due to vibration or temperature.

One possible reason to use raw quadrature input is to "count" beyond 1 revolution.

The source of this encoder USDigital has several other devices which can emulate pots by actively providing an analog voltage "incremented or decremented" by a quadrature pulse train (edac2) or an analog voltage feedback of the rate of pulses per second(etach2).

Additionally these devices incorporate the ability to self calibrate via a precise index marker pulse.

By selecting your counts per revolution carefully you can tailor the precision of the output to the neccessary resolution.

Re: Crab drive without a pot
 

Don't be afraid of potentiometers. Do use the correct value (I think 2.5k is the best match for the RC - or is it 5k??). Linear taper, as eshteyn noted, is what you need.

Connect a pot to the RC - one end to red, the other end to black, and the center to white, hooked up to an analog input - and get a feel for what kind of information you can get from it. You will see values in the range of 0 to 1023, varying linearly (linear taper, remember?) with rotational position.

Pots are available with 360 degree rotation, but these often have a small dead zone, where no signal is available.

Absolute encoders are great, can get costly (Mouser 594-601-1045 is one example, $60, get the connector wire too), but feature true 360 rotation, no dead zone, reliability and precision.

Multi-turn post can be a good solution, but you can eventually run out of turns if you don't enforce some kind of discipline on which way the wheels turn.

Running without pots (or similar) will lead to astoundingly poor performance.

Don

Re: Crab drive without a pot
 

100k wired as shown in the RC manual.

Re: Crab drive without a pot
 

100K is for the OI. The RC manual shows 10K in use, but says anything between 250 ohm and 100K can be used.

Re: Crab drive without a pot
 

From the 2007 RC Reference Guide...
9. Analog Inputs
The analog inputs on the Robot Controller can be used to measure various conditions on the robot and
trigger automatic responses by the control program. There are 16 analog inputs available on the
ANALOG INPUTS connectors on the Full-Size Robot Controller.
Any sensor which outputs a variable 0-5V signal may be read with 10-bit resolution on these inputs.
Popular devices include potentiometers and gyro (yaw rate) sensors. Potentiometers should be wired per
the diagram below when connecting to the Analog Inputs. Always use 100kΩ potentiometers. For
wiring yaw rate sensors, refer to the sensor manufacturer’s data sheet.

Re: Crab drive without a pot
 

The 2007 RC Reference Guide (dated 6-13-2006, and I just downloaded it from IFI) I have open says:
And the diagram below it shows a 10K pot being used.

Are you looking at an old version?

Re: Crab drive without a pot
 

Yes you're right, the version I had downloaded earlier this month was dated in February sometime and that was what I quoted. I just downloaded the newest version and it contains the above quote. Old is deleted.

Re: Crab drive without a pot
 

I'm not sure if everyones missed the link that tristan posted, but, there's everything to "fear" about pots.

Even the best Linear pots exhibit some degree of non linearity especially near the outer portions of their range.

They suffer from thermal drift to some degree.

Mechanical forces on the shaft (not true to the rotational axis) affect the value at the wiper.

Shocks to the housing can shift values even when the pot isn't being rotated.

And last but not least exceeding the stop at the end of travel usually causes bad things to happen.

Why not revisit Tristans post and have another look at an "active" system with the potential to eliminate most of these problems.

I know that to a team, every Niagara First Team 1680, 1503 and 1114 would caution that if you can find a device for as close to the price of a precision pot without the drawbacks. Try it. 1503 and 1680 won't be going back to pots again if they can help it.

Re: Crab drive without a pot
 

We have used pots successfully for many years. Ofcourse we have to recalibrate them from time to time, but as long as you make sure you dont break(mechanically, and wire them correctly, and use the correct potentiometer) them, they work wonderfully(atleast for us).