How to get Resistive Traces on PCB

[bitogre]

I am looking at building a micro servo board for some personal RC projects.  The Servo mechanics I plan on using require that there is a resistive pad as part of a variable resistor on the PCB that is used in the feedback to measure the servo position (see pictures below).  Does anyone know how to get PCBs with these resistive pads (somewhere in the 2K Ohm to 20K Ohm per inch range) or to add them myself after I receive the PCB?

[NiHaoMike]

What about make a digital encoder with many closely spaced traces?

[CatalinaWOW]

You could also salvage the resistive track from a linear pot.  Those are at least designed to stand the wear of a slider.  I don't know how the resistive carbon printed on a pwb would hold up.

Depending on the resolution you need you could just lay out the pcb with copper fingers and use chip resistors between them.  The servo would hunt back and forth across between fingers closest to the commanded point.  Could probably get more than 20 elements without too much trouble, but 8 bit resolution would be a real tour de force.

[LaserSteve]

Don't know if you can get it easily except from China, but Google "Resistive Card" . Used in older microwave gear.

[bitogre]

What about make a digital encoder with many closely spaced traces?

Interesting idea, but, unfortunately, I don't think I can get the resolution I need doing that.  I figure I need at least 9 bits of resolution and that would be 512 different encodings in 0.5" of travel.

[bitogre]

Many PCB fabs offer printed carbon trace process, which is to print a resistor on the PCB. Absolute resistance accuracy is horrible, but the linearity is usually pretty good, so for ratiometric applications like a pot, this should work well.

I have been emailing several PCB placing I can find on a Google search and I the only ones I found do low resistance traces (about 20 Ohms).  That is too low of a resistance.  I figure I want something between 2K Ohm to 20K Ohm per inch as I will be putting a 1 cell LiPo battery (4.2 V max voltage) across the resistive pad.  Can you recommend a specific PCB Fab (preferably in the US) that can do this?

[bitogre]

You could also salvage the resistive track from a linear pot.  Those are at least designed to stand the wear of a slider.  I don't know how the resistive carbon printed on a pwb would hold up.

Depending on the resolution you need you could just lay out the pcb with copper fingers and use chip resistors between them.  The servo would hunt back and forth across between fingers closest to the commanded point.  Could probably get more than 20 elements without too much trouble, but 8 bit resolution would be a real tour de force.

Salvaging a resistive track sounds like too much work.  I am confident some one does it as there are existing mass produced product that do this.  In fact, I am planning on using the replacement mechanics for mine.  The difference between the the existing product and what I am doing is the supported RF protocol (the existing is DSM2/DSMX).

Given the size (about 0.5" of travel), I think 8bit of resolution is impractical and I figure I need at least 9 bits of resolution.

[T3sl4co1l]

I took apart a cordless drill speed control, which uses the traces-and-resistors method.  And, yeah, it's noticeably lumpy (with less than 8 divisions in it, I think it was?).

The traces are cut on a bias (about a 20 degree slant), while the sliding contact is a bunch of spring contacts in a row.  So a few (1-3?) traces will be shorted together at any given time, which is both redundant, and can potentially ~double the number of steps (because of parallel combinations of steps acting as interpolation), if the width of the contact is just right.

You could do better with fewer steps but more rows; a Gray code perhaps might give okay results, but would have to be decoded, and now you have a whole mess of things to deal with (reliable contact and debouncing on each bit position, ensuring the contacts track exactly parallel, an MDAC or digital pot for output, etc.). There may be a decoding, using a minimal number of discrete resistors and contact pads, which yields maximal number of steps, but it would be a lot of thinking to come up with it (or, likely it's been done already -- but you have to find it; it may well be equivalent to a long-since solved math-hobby sort of problem, but who knows? ).

So yeah, it's not likely to be a viable method, for any more than coarse control.

Tim

[CatalinaWOW]

You could also salvage the resistive track from a linear pot.  Those are at least designed to stand the wear of a slider.  I don't know how the resistive carbon printed on a pwb would hold up.

Depending on the resolution you need you could just lay out the pcb with copper fingers and use chip resistors between them.  The servo would hunt back and forth across between fingers closest to the commanded point.  Could probably get more than 20 elements without too much trouble, but 8 bit resolution would be a real tour de force.

Salvaging a resistive track sounds like too much work.  I am confident some one does it as there are existing mass produced product that do this.  In fact, I am planning on using the replacement mechanics for mine.  The difference between the the existing product and what I am doing is the supported RF protocol (the existing is DSM2/DSMX).

Given the size (about 0.5" of travel), I think 8bit of resolution is impractical and I figure I need at least 9 bits of resolution.

Lots of things are available when purchase in 10,000 or 100,000 unit quantities that cost an arm and a leg when purchased in small quantities.  Great if you can find someone who will do a low cost low quantity build, otherwise work will be needed.

These dual servo boards are used in all sorts of consumer helicopters and planes - perhaps you can find crashed ones and salvage the boards (with appropriate resistive trace).  In my area craigslist has people advertising for broken stuff, and apparently they get it.

You might also try painting your own with Aquadag or similar.  Aquadag is relatively low resistivity ~800 ohms-sq air dried, but that ought to work for your geometry needs.  Claimed to be very erosion resistant.  If other resistivities are required there are things like https://www.coating-suisse.com/en/products/electrically-conductive-heating-paint/

I would also explore your real needs on resolution.  RC planes flew for years on essentially two bit control (left-center-right), and eight bit or less resolution was all anyone had until sometime in this century.  Unless you are doing a helicopter or precision aerobatics you may not need much more than four or five bits.  Lower resolution will mean that you need to do flight trimming mechanically instead of electronically.