Hmmm. Based on those definitions I'm not sure what's most important.
I'm testing a group of people's ability to slide the device in time with an external object. Several people will be using a device at the same time. If the devices aren't accurate, then it might make the testing irrelevant. I might be able to, though, "zero" the inputs and just look at the rate of change of each device.
I'll think about this a bit more. I may be able to favour precision over accuracy.
So, it sounds like you are investigating a small population's ability to "track" a moving target using a slider. Perhaps a "bouncing ball" target on a video monitor? That could be an interesting experiment to measure the "frequency and phase response" of a human being in a closed-loop positioning system. For the slider itself, I think an
LVDT would be most appropriate because they offer no-wear and very small inertia and friction. Many
commercial versions exist at prices in the $500 to $1000 range for the length you require.
A
linear potentiometer is a far-second choice, but the price is in the $200 to $300 range. The friction can be annoying for rapid movements by hand, and the resistive element will eventually wear out requiring replacement of the entire potentiometer.
Another approach might be a so-called "string potentiometer" which is a rotary potentiometer shaft connected to a pulley with a stainless steel "string" wound around the pulley. The string is internally spring-loaded to provide a retraction tension, and this could be a problem. However, these string pots are also available with quadrature-output incremental encoders instead of potentiometers. One example is the WS31/WS42 series from
ASM Sensor. The problem with string pots is the spring tension required to keep the cable taut, and the possibility that rapid retraction could cause the cable to go slack, introducing an inaccurate reading. I have no idea what these cost, but the optical encoder type should be the least expensive. Visit the link and download the datasheet for the WS31/WS42 series if this sounds interesting.
Rotary optical shaft encoders are very inexpensive and are easily interfaced to digital microprocessors like the Arduino. All you have to do is figure out how to attach the slider to a rack-and-pinion mechanism to drive the shaft without backlash. Not a trivial problem, but potentially very inexpensive. There are plenty of
ready-made anti-backlash rack-and-pinion gear trains available on line. Mount the rack to the bottom of the slider and mate it with the pinion attached to the incremental encoder. Add some electronics.
@(*steve*) suggested using a digital optical encoder ribbon and this may be the easiest DIY approach, if you can find a junk printer to salvage for a prototype. The optical position sensors (two in quadrature) will be the only part of the print head you will need. The optical ribbon must be carefully removed without damaging it and re-purposed as part of your slider mechanism. The quadrature outputs are derived by precise positioning of the two optical sensors with respect to the ribbon. Try not to disturb this positioning by carefully removing the optical sensors as a mated pair if you want to use the original optical ribbon.
In lieu of a transparent optical ribbon, I would investigate etching a pattern of parallel lines on a ten-inch long circuit board and use a pair of reflective IR LED and photo-transistor sensors to detect the line transitions. You could have the boards commercially manufactured at low cost, but you might have to experiment with the optical sensors (and perhaps some lenses) to obtain the resolution you need. One of the sensors needs to be adjustable in position with respect to the other one to obtain the quadrature output pulses needed for up/down counting. You could mount the PCB on the bottom of the slider though, eliminating the need for flexible wiring to the sensors. Or, in lieu of etched lines on a PCB, you could even produce a printed paper target of alternating black and white lines to attach to the bottom of the slider. Please send 0.1% of any profits you earn from this idea to me.
You can photographically produce your own optical encoder ribbon at whatever position resolution you require. The slider would be mechanically attached to the sensor optics with ultra-flexible ribbon cable, perhaps salvaged from the print head. You would have to design and build the optical signal conditioning electronics: a quadrature up/down counter and a means to read it, perhaps converting counts to an analog signal with a digital-to-analog converter. Discrete-circuit quadrature up/down counters are tricky to design because you must allow for count hysteresis at the quadrature encoder step transitions to avoid a cumulative count error. A microprocessor might be able to handle this problem better in software. A "reset" or "home" position sensor (magnetic reed switch?) attached to the slider would be needed to initialize the up/down counter to a known initial slider position. Despite all that, I like the optical encoder approach because of it has the potential to meet your "under $15" goal, especially if mass produced by an Asian vendor.
My idea of using a variable-density strip is probably not within the capabilities of the home DIYer. It may not even be practical. It requires a dark-room setup and a means to variably expose a moving strip of film to create the variable-density strip of the length (250 mm) your require. There are also problems with read-out stability to solve since this is an analog approach. The only "advantage" I see is a continuously variable (as compared to a discrete incremental digital) output. On further thought, I do not recommend this approach.
Please keep us posted on your project. It sounds like fun!
73
de AC8NS
Hop
