View Full Version : [Question(s)] Position Sensing Possibility

08-06-2013, 12:58 AM
A quick google search did not really show many related products, so I'll ask before spending more time/money digging deeper. Anyone ever tried using extruded carbon fiber tube as a position sensor for a linear actuator (essentially just a big linear potentiometer)?

I had previously thought of using aluminium tube as structural members for a mech while also using them as the body cylinder of a pneumatic actuator. The thought involved using a magnet inside the tube to link the air movement to a magnet in a sliding carriage on the exterior which be attached to a prismatic-spherical-spherical linkage like a certain high speed movement camera platform (http://spectrum.ieee.org/automaton/robotics/industrial-robots/superfast-robotic-camera-mimics-human-eye). Hopefully, this would make it mechanically simpler and cheaper than a standard pneumatic actuator and prevent any need for puncturing the tube or having any sliding seals for a piston sticking out the end of the cylinder. It should also add a bit of safety should the joint experience too large a load, since the magnetic linkage should disengage the actuator from the joint without anything breaking.

I was curious if a change to carbon fiber tube with a sliding contact on the carriage would permit an easy means of sensing the position of the carriage without needing to add any additional sensors. Another possibility is just embedding some graphite into a groove in an aluminium tube should the resistance of the carbon fiber tube be too small for useful position sensing.

08-06-2013, 05:32 AM
There are two types of CF tube in common use, extruded and wrapped. Inside diameter is going to be most consistent in wrapped tubing as it is built on a mandrel of consistent size. Even then I would hesitate to use it for the bore of a pneumatic cylinder as it's very abrasive material, and would quickly tear up any seal. I also doubt that a consistent reading is going to work all that well as far as resistance. You have a matrix of resin and fiber, and the density of that is not always consistent. Extruded tube would be somewhat better in that regard.
Magnets are used for position sensing in commercial pneumatic cylinders, many are available with magnets in the piston and reed or hall sensors outside of the cylinder. They are generally of the type that sense end limit of the cylinder only, as most pneumatics make use of simple valving which is on or off. Valving of the type you would be looking for (position control) is going to be a LOT more expensive and heavy. Lot of cylinders come up on ebay and the like as many people buy custom runs and then don't need them. Surplus places too.

08-06-2013, 02:41 PM
It was just an idea that might have made positon sensing easier/cheaper, but kind of doubted it would work. Thinking about it a bit more, the idea might have started as polycarbonate tube instead of aluminium... did not write down all the details at the time and my brain keeps faulting out. I think it started as a 1DOF leg segment for a quad with the slide/carriage linked to a small lever on the next leg segment, sort of like an excavator but moving the piston completely within the structure of the segment and replacing it with a solid linkage. No possibility of leakage of the fluid within the cylinder to outside of system except at tube fittings, and hopefully little to no forces/torques on the piston/magnet that might cause uneven/excessive wear on the seals. Just that the magnetic link between piston and slide/carriage would be pretty damn weak with the gap needed for wall thickness required to maintain structural integrity (unless both sides of the cylinder are well pressurized like a soda can/bottle). I'm babbling again...