Which sensor?

[Neal Turett]

I need a sensor much like a potentiometer, but one that measures linear distance instead of rotation. For example, one that I could attach to a pneumatic cylinder to know where it is.

While browsing the Digikey catalog for something that would work, I came across a "Distance Measuring Sensor" from Sharp. Has anyone used these? Would this do what I need, and could I just run it to an analog input?

[ttedrow]

I took a quick look at one of the spec sheets. I think that one of these units would work for you application. Vcc is +5vdc and the output is ~ .4vdc to 2.5vdc giving you a range. This can be used in an analog input. The output is not liner so be careful with the programming.

Good luck

Tim Tedrow

[Al Skierkiewicz]

Why not just use two micro switches, actuated by the connection hardware to the cylinder? If you need infinite control over the position, the pot is the best way.

[Lloyd Burns]

Before going into a long search for measuring extension of a piston, see if you can stop the piston at partial extension, and then check that when it gets there, and you lock it, that there is enough force available to hold the position.

Stopping it where you want it may be easy if the piston is slow (forget flow restrictiers, with those SMC valves how could it be fast?), but locking in that position is hotly debated elsewhere. Many say it can be done, but I don't see many "On our robot , we did thus and so, to get the effect..."

The worst idea may be using those self-coiling springs mounted in a slot in a 10-turn pot's shaft, the far end attatched to the far end (of the piston).

[Jeff Waegelin]

Well, our idea doesn't exactly center around a pneumatic cylinder. It was just an example. I guess a closer application would be measuring the extension of a lead screw. We want a sensor that would measure a short linear distance. Like a pot, but measuring linear instead of rotational motion.

[Al Skierkiewicz]

Microswitches is still the best bet on a lead screw. If it is a short distance and is capable of a gear reduction down to 270 degrees of rotation you can use a single turn pot. There are also 3, 5, and 10 turn pots available that you can attach to the lead screw, but accuracy may suffer due to the A/D in the robot controller. You will have to figure out the quantization of 256 counts vs. rotation or distance traveled to determine if a multiturn pot would work OK for you.