High precision impedance with motorized variable impedances ? - Page 1
Forum section: electronics / projects

author: Zoumey
author_rank: Newbie
timestamp: July 14, 2023, 05:16:05 am
content: Hey !I'm designing an LCR meter in one of my classes and one of the issue I got was the lack of repeatable impedance I can use to test if the device I'm designing actually works. I could spend a few bucks on precision components but one of the (assuredly stupid) thought I had was "why not use a motorized potentiometer / variable inductor / variable capacitor ?"I mean, they're continuous devices, so at some point or another you're crossing the exact impedance you want right ? why can't I just use an Ohmmeter circuit with an MCU and a servomotor with some reduction gearing and get a cheap, repeatable (since closed loop) custom resistor ?? is this part of the hidden knowledge held tight by big ppms to sell more lab equipment ?I can't be the first one to think of this so why have I never heard of it ? so many questions.here's a mock up of what i mean. It looks stupid but other than the glaring issue of hiding everything behind a "multimeter thingy", I don't know why this wouldn't work.

author: mrburnzie
author_rank: Regular Contributor
timestamp: July 14, 2023, 09:52:39 am
content: You mentioned precision components would cost a couple of bucks... how many couple of bucks would servos, power supply for the servos, designing and modeling the gear ratios, custom fitting to the potentiometers be?

author: alm
author_rank: Super Contributor
timestamp: July 14, 2023, 10:28:26 am
content: The range would be quite limited (maybe two orders of magnitude), so you would need a bunch of motorized pots. And in many cases you only need a few values per decade so fixed resistors and relays are cheaper.There are resistance simulators likediscussed here. I have also seen General Radio decade resistors with motors driving rotary switches. Looked like a hack on top of a manual decade resistor, and probably costed thousands of USD back then. These days you would solve it with relays or electronic switches, or with simulation.There were motorized pots for volume control for use in audio equipment with a remote. I don't know if they are still available. They would be logarithmic, however.

author: Zoumey
author_rank: Newbie
timestamp: July 14, 2023, 01:29:22 pm
content: Quote from: mrburnzie on July 14, 2023, 09:52:39 amHonestly ? with 3D printing i wouldn't think it'd cost that much. if the entire range of the potentiometer is used, it would be exponentially cheaper than buying each resistor separately, and most of the stuff needed is already laying around my workbench.Quote from: alm on July 14, 2023, 10:28:26 amit's clear that using log is impossible cause you would need to be exponentially more precise to get smaller values. I'm also using those volume pots for another project of mine ! I got the bourns ones, but they come with a simple DC motor and two gangs, one linear and one log. (I think most do)

author: TimFox
author_rank: Super Contributor
timestamp: July 14, 2023, 02:01:39 pm
content: Can you use an ensemble of reed relays with conventional precision components?

author: Infraviolet
author_rank: Super Contributor
timestamp: July 14, 2023, 02:29:44 pm
content: The other argument than cost is size, a pot and servo setup with gearing will be pretty bulky compared to a normal single component. Also, to my understanding you're proposing a variable setup for something which will only ever need varying once, when the device is first calibrated?Also, gearing will have backlash, which will precision adjustment a little trickier then you'd hope.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 14, 2023, 05:29:51 pm
content: If size doesn't matter, search for the Leeds & Northrup K2 Potentiometer. The big drum contains a spiral wound piece of resistance wire. You could make something like that and drive it with a motor and encoder. Precision would be really good. For inductance search for the General Radio 107N variable inductor. It's basically two concentric frames with a coil wound on each one. Make something like that (quite easy) and drive it with whatever. For the capacitor just buy a big air variable and drive that.

author: TimFox
author_rank: Super Contributor
timestamp: July 14, 2023, 05:35:19 pm
content: General Radio also made precision variable capacitors in the same shape as their variable inductors.

author: mikerj
author_rank: Super Contributor
timestamp: July 14, 2023, 05:45:29 pm
content: You are effectively creating your own short term transfer standard. A typical potentiometer used as a resistor will have poor long term stability, disastrous temperature stability and it will only ever be as accurate as the device you use to set it. If none of these things matter to you then sure you can do it, but I don't see much advantage in motorising it for occasional use.Variable capacitors tend to be limited to a few hundreds of pF so this may not be very useful, particularly as you'll need a decent LCR meter to set it in the first place. You can use regular capacitors as your short term transfer standard, provided you have some way of measuring them to an accuracy you are happy with, and bear in mind that many regular caps also have poor temperature stability. Don't use ceramic caps with X or Y types dielectrics.

author: Neepa
author_rank: Regular Contributor
timestamp: July 14, 2023, 06:06:43 pm
content: QuoteWhich is why you can see quite a lot of chain or belt/string drive transmissions in older pre 70ies equipment, which depending on tensioning have no appreciateable backlash. Precision can be achieved by using large knobs one can set more accurately with a large dial and a two to three step down reduction to turn 10° of rotation into less than a tenth of that.Provided the OP really wants to go through with this I would strongly suggest going the belt drive route today. Fiber or rubber belts need no lubrication and can be more easily utilized with tensioning rollers than gears where gear meshing becomes an issue with axle center distance. Belts are also more easily sourced these days with the ubiquity of robotics as a hobby.An idea worth mentioning would be to custom make variable capacitors to linearize an impedance curve. Provided one got enough space the plates in the capacitor could have all sorts of crazy shapes to wildly vary the capacitance depending on the angle to some arbitrary null line. Alignment would be tricky then to make sure these custom components track accurately relative to each other but that can be achieved with jigs one can accurately 3D print these days with a resin printer.Sure the contruction becomes a much larger mechanical task than electronic but it can be done. Although beware it might look like this then:Siemens B83600

author: ajb
author_rank: Super Contributor
timestamp: July 14, 2023, 07:00:53 pm
content: Seems to me that this method would fundamentally be more amenable to inductors and capacitors than potentiometers. With a potentiometer, I think you'd be much more likely to run into problems with monotonicity when making very small adjustments. Unless it's a very high quality potentiometer to begin with, contact resistance may change in either direction due to surface roughness and contact wiping effects, and minute variations in the resistance material's thickness and composition may not result in the same magnitude--or even sign--of change in resistance for a given change in position. That uncertainty is liable to set a floor on the minimum increment you can realistically achieve. You may still have sliding contacts in a variable cap or inductor, depending on what type you use, but at least those can be sliding metal-on-metal contacts, and changes to contact resistance aren't adirecterror term in the parameter you're trying to tune with them.

author: donlisms
author_rank: Frequent Contributor
timestamp: July 15, 2023, 09:22:03 am
content: Yup. I've been playing with various precision resistances, and my conclusion is that a workable, typical resolution for a pot is 100 parts per turn. I can do better under the right circumstances, but expecting to set a pot much more precisely than that generally leads to frustration. There are so many physical factors at work. So to get 0.1% resolution, reliably, I'd want to use a 10 turn pot. And if I'm lucky, and it's a good pot, maybe I can do a little better that that. I don't know if I'd call it precise. I don't think putting a motor on it would change things much.There's a really good, really old paper by one of the makers on all the pot specs and varieties and construction techniques; many great details. Sorry I can't remember who. Bournes or Spectrol or some other well known company. I could dig it up.

author: TimFox
author_rank: Super Contributor
timestamp: July 15, 2023, 01:43:45 pm
content: There used to be (large) 40-turn pots.For 10-turn pots, 0.1% linearity is a premium part, most good ones are 0.2%.Of course, the specifications for multi-turn potentiometers are based on their use as voltage dividers, not variable resistors.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 15, 2023, 04:21:33 pm
content: A lot of the precision comes down to track length. That's why the old L&N K2 did so well- it used multiple turns on a very large drum. A big 10-turn pot uses the same idea. GR did well with a single turn, but they had an adjustable ramp and lead/lag follower so you could linearize the pot. It was also large and with dials to match. There were also some ancient teaching pots with a single element a foot or two long and an accurately marked slider. Dial diameter and the knob are important for resolution and leverage. The GR 1304 beat frequency oscillator did 20-20000 Hz with a large dial and big air variable capacitor. Resolution was good and accuracy excellent because you could bend the sectioned fins to tweak the linearity. Their big air variables used a worm gear to drive the mechanism, resolving 1-2 pF out of 1100 pF and, with a correction table, could be accurate to about the same.

author: Zoumey
author_rank: Newbie
timestamp: July 15, 2023, 07:16:14 pm
content: Quote from: Infraviolet on July 14, 2023, 02:29:44 pmI was more or less thinking about a variable load for low power application. Something that you can use in a pinch instead of waiting for shipping when you're missing a specific value of component.Quote from: Neepa on July 14, 2023, 06:06:43 pmI don't ! At least I don't think I do ? This seem like a dumb idea but it'd be fun to mess around with it, since I first joined uni to do mechatronics and ended up staying for EE, and also because I never care about precision in my projects. (except that damned LCR meter i'm forced to design. At least i can keep it !)Quote from: ajb on July 14, 2023, 07:00:53 pmYes, I thought about it ! tbh i think it'd be a "simple" fix since you know which way the motor is turning and so whether the impedance is supposed to get bigger or not.Quote from: TimFox on July 15, 2023, 01:43:45 pmHold on, I knew about parasitics but are you implying that disconnecting one of the end of a potentiometer and measuring between one of the fixed end and the variable pin wouldn't be exactly like a variable resistor ? If not, why would that 0.2% precision change ?Quote from: Conrad Hoffman on July 15, 2023, 04:21:33 pmYeah , i guess making a 2m long strip of resistive film is the only way to do it

author: ajb
author_rank: Super Contributor
timestamp: July 15, 2023, 07:39:01 pm
content: Quote from: Zoumey on July 15, 2023, 07:16:14 pmSure, but the question is what you do when the value should go down but doesn't. At a minimum, this complicates your control algorithm somewhat--more akin to focus seeking in cameras than a thermostat. The repeatability of the system is another question: if you scan through a range to find the position that's closest to the target, you may not get quite the same value when you go back to that minimum error position. More time moving the wiper around seeking the right position also means more wear on the resistance track.The same sorts of problems could crop up in variable inductors or capacitors as well--minute errors in the profiles of moving plates in a cap, for example, will have similar effects on the linearity and monotonicity of the cap at small enough increments. None of these are necessarily dealbreakers, but they will limit the realistic resolution you can acheive.QuoteFor one thing, the contact resistance of the wiper appears in series with the wiper terminal. That means that it does not affect the voltage read in the potentiometer configuration, but DOES add to the resistance of the rheostat configuration. Other error terms, such as local variations in the bulk resistivity or thickness of the resistance material, will have similar effects.

author: TimFox
author_rank: Super Contributor
timestamp: July 15, 2023, 10:30:22 pm
content: Also, the 0.2% linearity spec is just that: the overall resistance probably is 5% tolerance or so.Remember what "potentiometer" means literally.A variable resistor is a "rheostat".

author: alm
author_rank: Super Contributor
timestamp: July 15, 2023, 10:55:11 pm
content: Quote from: Zoumey on July 15, 2023, 07:16:14 pmKeep in mind that the rated power of a potentiometer or rheostat is when using the entire length of the track at the maximum setting. If you have it set to 1% of its maximum setting, then the maximum power would be roughly 1% of the rated maximum power.

author: Tizian
author_rank: Newbie
timestamp: July 16, 2023, 11:57:47 am
content: Buongiorno, usare resistenze variabili come carico non è pratico, per quello esistono i carichi elettronici. E poi quanti Ohm? 0,1, 1 ,10?Anche il K2 di Conrad Hoffman (ho il K2) ha meno di 10 ohm e funziona solo come divisore di tensione. Avrebbe senso motorizzare LCR solo se l'operatore è lontano o si lavora in ambienti tossici, oppure in controlli automatici. Alla fine basta inserire l'impedenza giusta con i relè. A casa mia la vera motorizzazione sono le mie mani.Con due cassette decennali controllo la resistenza da 1 ohm a 1 Mohm con salti di 1 mohm e da 100 pF a 10 microfarad con salti di 1 pF.E' il mio primo post e scusate il disturbo.Buona vita.