Optimal LVDT shaft material? - Page 1
Forum section: electronics / general

author: rhb
author_rank: Super Contributor
timestamp: October 23, 2022, 01:45:58 am
content: I just received a pair of new Trans-Tek 0216-00000 linear LVDTs with calibration certificates. These have a core threaded at both ends.What's the best material to use for the actuator arms to get the best accuracy? I'd like to get millionth accuracy from these if that is possible. Calibration was at ~7 kHz. Will aluminum do or should I use a plastic shaft?Thanks,Reg

author: Zoli
author_rank: Frequent Contributor
timestamp: October 23, 2022, 02:25:08 am
content: I've worked with someOmegaLVDT's; all the actuators were stainless steel.

author: KT88
author_rank: Frequent Contributor
timestamp: October 23, 2022, 11:45:49 am
content: I worked with Invar for least dimensional variations. It poses no significant challenges when machining it...CheersAndreas

author: voltsandjolts
author_rank: Unknown
timestamp: October 23, 2022, 12:35:02 pm
content: Quote from: rhb on October 23, 2022, 01:45:58 am1" stroke LVDT giving one thousandth of a thou displacement _accuracy_?Seems like a stretch from any LVDT, let alone one with ±0.25% INL.Even a one thou _resolution_ would seem non-trivial to me.But I'm no LVDT expert, happy to be proven wrong.

author: rhb
author_rank: Super Contributor
timestamp: October 23, 2022, 01:25:50 pm
content: Read the specifications for this:https://tesatechnology.com/en-us/products/palpeurs-de-mesure/palpeurs-axiaux-standards/gt21-standard-probes,-%C2%B1-1-mm,-4,3-mm-bolt-travel,-axial-cable_p49282.htm#/collapseSpecificationsIn particular:"Repeatability" 0.01 um" (micron)That's 4 ten thousandths of a millionth of an inch!Edit: I was able to find the datasheet for the Trans-Tek line. They recommend non-magnetic stainless steel.Unfortunately, unlike Tesa they don't state the repeatability.0.0001" is a stock mechanical indicator. An LVDT is a transformer with 2 secondaries and measures the position of the core by comparing the voltages on the 2 windings. Microvolts and smaller are not all that difficult.

author: Stray Electron
author_rank: Super Contributor
timestamp: October 23, 2022, 01:34:28 pm
content: We used LVDTs where I used to work and all of the structure was stainless steel. I don't recall what grade but it was food safe. But we weren't looking for that kind of accuracy. I think that if you want to get that kind of resolution and accuracy then you're going to have use something like Invar to obtain the required dimensional stability and you'll probably have to enclose everything in a temperature controlled enclosure and will have to pay close attention to the design of the structure and how it flexes with temperature and with aging.The last place that I worked we made some large precision optics and we tried making the structures out of aluminum and other metals but even in a very controlled enclosure we couldn't get the required temperature stability so we went to Invar $$$$$. It caused several years of delays and eventually the program was cancelled so I don't know if they ever actually were able to met their design goals or not.

author: rhb
author_rank: Super Contributor
timestamp: October 23, 2022, 01:45:26 pm
content: Fortunately I don't need millionth accuracy. My current limit is 100 millionths, so if I get to 5-10 millionths I'll be happy. At least for a while.The expansion coefficient of most steels is 6.2e-6 per degree F. Temperature becomes a very serious issue even at the 0.0001" level.

author: CurtisSeizert
author_rank: Regular Contributor
timestamp: November 06, 2022, 08:31:54 pm
content: I would try to avoid having any aluminum or plastic be within the measurement loop for microinch accuracy as a general rule. They tend not to be stiff enough for most serious dimensional metrology purposes, especially given the thin section of the shaft that holds the core to the probe tip.Ideally the measurement setup and test piece will be made from material with identical thermal expansion coefficients. That way, the absolute temperature of the measurement is not particularly important as long as thermal variations within the measurement loop are negligible. Invar tends to be a poor choice unless your test piece is also invar or you're in a strictly temperature controlled environment. Even with steel, it is best to keep temperature variations to a minimum because thermal expansion coefficient can vary quite a bit between samples. The goal is not to be able to cancel out the effect of absolute temperature, because that is not realistic. The goal is to minimize such effects. Generally most of your measurement loop will be steel or cast iron, so shooting for 10 ppm/K is a good guideline.Another thing to consider with austenitic stainless steel (i.e. all the non-magnetic types) is that austenite is not a thermodynamically stable phase with respect to martensite at ambient temperatures due to the large amounts of nickel generally added to these alloys. As such, these alloys are generally susceptible to strain- or work-hardening, where sheer strain essentially provides the activation energy necessary to promote the phase transition to martensite. The martensite formed has greater than unity magnetic permeability. Therefore, my choice would be 316LMo stainless because annealing at 1050 C after machining the shafts will allow you to reduce the magnetic permeability back down to unity.In my experience, the most common issues that affect repeatability at the microinch level are stick-slip motion, surface roughness effects, and lack of temperature homogeneity. If you are comparing different materials, you also need to be aware of the force on the tip and its radius to account Hertzian contact mechanics. For designing a probe with microinch repeatability, stick-slip will be the thing to think about. A flexure mount or air bearing is probably the most robust way to achieve that. Colorado Precision Products makes an air bearing LVDT with a Schaevitz MHR010 (I think), and that can repeat around 0.1 microinch. Any mechanical bearing (non-flexure) will have uncertain hysteresis in the single digit microinch range. Good luck.

author: mendip_discovery
author_rank: Super Contributor
timestamp: November 06, 2022, 09:14:16 pm
content: There are plenty of calculators out there for the Hertzian contact issues I think NIST has one. Temperature coefficients can also be taken into account just a search for Kaye & Laby (https://web.archive.org/web/20190506031327/http://www.kayelaby.npl.co.uk/)I would use stainless, possibly with a ruby at the tip depending on what you are trying to measure. Then again I have the hq of renshaw just up the road.

author: rhb
author_rank: Super Contributor
timestamp: November 06, 2022, 11:17:04 pm
content: Curtis,Excellent points. On the positioning system for his grinder Robin Rennitz (robrenz on youtube) backs up and then moves forward do that the slide is in a hydrodynamic state when he advances it.Primary application of the LVDTs I envision is as a high resolution test indicators.I've been playing with my lathe, but I'll try to cobble together a test setup to evaluate the LVDTs.At the low frequencies used with LVDTs an HP 34401A will give microvolt resolution of a 1 volt AC differential. One rapidly gets into all manner of heat effects. The voltage references are heat sensitive as are the transducers.I agree that 10 ppm/K is the best you can expect in absolute terms. The voltage reference is worse than that in most cases.Have Fun!Reg

author: CurtisSeizert
author_rank: Regular Contributor
timestamp: November 08, 2022, 02:02:33 am
content: Robin's way around stick slip on his CNC grinder is definitely clever, but it doesn't directly translate to things you might do with a microinch resolution lvdt. If you are, for example, sweeping a parallel, you will be uncertain if a given area that appears flat is actually just not out of flat enough to get the bearing to slip. I have been tossing around the idea of designing a flexure mount for a handful of Schaevitz MHR-010s I picked up. I think the easiest thing would be something similar (but much less stiff) than the fine adjust flexure mounts used on some indicator holder stands. I expect an air bearing would probably give better results, but that would be a project in itself.It is possible to use calculators for Hertzian contact deformation to get close, but things like surface roughness and the uncertainty of a material's elastic modulus can be limiting. Robin Renzetti uses a G5 silicon nitride bearing ball silver soldered to a shaft with the required #4-40 thread so that the contact radius is larger than a standard tip. I have generally used the largest radius ruby tip I could find on McMaster or ebay. Coefficient of thermal expansion is similarly uncertain. I cannot remember the source, but I remember seeing raw data for such measurements and the standard deviation was fairly large, maybe in the range of 10%. I think the most robust strategy is to minimize the potential influence of such things and try to correct for the residual error.Quote from: rhb on November 06, 2022, 11:17:04 pmWhen I say 10 ppm/K, I mean 10 microinches per inch per Kelvin.Reading this post gives me some flashbacks, because I think that solving the problems that you will find limit the utility of such a setup is what put me on the road of learning electronics when the maker's space that I had used to turn money into chips closed. I don't know exactly what level of experience you are coming in with, so please forgive me if this insultingly basic. These are the things I wished I had known then.The transfer function is not VRMS_primary = constant * displacement * VRMS_secondary. This is true even on one side of null. For a well-made transducer, it approximates this, but it isn't quite. The displacement is linearly proportional to (the integral of the secondary voltage taken while the primary is positive) - (the integral of the secondary voltage taken while the primary is negative). This was traditionally measured with a lock-in amplifier (see Linear Technology Application Note 3, p. 9). The one in my Lion Precision LVDT conditioner uses AD630s to do pretty much the same thing. The more modern approach to taking this measurement is to just digitize the waveform and do the calculation in software. In my opinion, it's less fun, but I believe it tends to give better linearity. This is the approach taken by the PGA970 SoC made by TI. I bought their EVM, but it's not really practical to use due to the complete absence of input protection and its apparent propensity to succumb to ESD. Also, the sensitivity is related to excitation frequency, so it helps to have a sine wave source with good stability and low distortion to drive the primary.I have been tempted at times to roll a board that uses the PGA970, but it always seems too boring a solution to be able to get me through the project without losing interest. I would probably try to build up the Jim Williams circuit and measure some gage blocks to get sensitivity and linearity. You could send the DC out to your 34401. If you are going to use this in the shop, you will want some sort of input protection for the LT1043.Good luck!Curtis

author: rhb
author_rank: Super Contributor
timestamp: November 08, 2022, 02:18:20 pm
content: What's the title of the App note? Searching for "AN-3 " doesn't produce anything relevant on analog.com.Since you've fooled around in this rabbit hole, what are some good references on LVDTs? I've ordered a copy of David S. Nyce's "Position Sensors", 2016.BTW Did you watch Robin's "repeat-o-meter" video?

author: CurtisSeizert
author_rank: Regular Contributor
timestamp: November 08, 2022, 06:10:34 pm
content: The app note is "Applications for a Switched-Capacitor Instrumentation Building Block" by Jim Williams (https://www.analog.com/media/en/technical-documentation/application-notes/an03f.pdf). I have watched every video Robin has put out, most of them several times. He does solid work. I followed him down the precision spindle rabbit hole and built my own using a pair of 71910 bearings rather than integrating the races into the shaft and the housing. The LVDT was definitely a critical part of that effort.What I know about LVDTs is mostly cobbled together from a variety of sources. The Schaevitz Handbook of Measurement and Control is a decent source. Portions of it may be out of date, but most of the movement in the past few decades that I am aware of has been towards digital signal conditioning. Some of the things LVDTs used to be used for are now done with other measurement tools, like capacitive displacement sensors. There is some vague information about LVDT signal conditioning in the development reports by Lawrence Livermore of an LVDT signal conditioner designed there and commercialized by Lion Precision (https://www.osti.gov/servlets/purl/10189724). That collaboration also involved Colorado Precision, who made the air bearing probes, and the system was tested to have a resolution around 50 nanoinches. The AD630 datasheet has a very brief blurb about LVDT signal conditioning, and there is presumably some amount of useful information in the AD598 and AD698 datasheets, but I haven't looked at them in some time. If you are looking for the PGA970 datasheet, go to the Mouser website. TI keeps it behind a registration wall.By random chance, I scored the Lion Precision conditioner on eBay for $200. I have never seen it show a digit I would trust below a microinch, but some of that may have been the insufficient isolation from vibration and the Starrett probe I was using. I generally saw a noise floor of around 0.5 microinches and a drift of about 0.5 microinches per hour. Repeatability was conservatively around 4 microinches. The thing about microinches is that to be confident at the single digit level, a lot of things need to be kept tightly controlled, and every reading is suspect. For example, a 7.5" diameter reference flat that I lapped to about 2 microinch total deviation (which required me to wear a winter coat and mittens inside for thermal isolation) would become concave by about 5 microinches when the heat kicked on in the winter because of internal temperature differences of less than 1 C. In my experience, even fine grinds that have been stoned are subject to a fair degree of uncertainty in repeatability (though this is probably helped by having a larger radius tip), and everything I have needed ("needed") to repeat below 10 microinches has been lapped. The feeling I have (and a lot of times feelings are the best you can do at microinch level without shelling out some serious $$) is that this is a product of probe tip force and Hertzian deformation dependence on surface roughness.I am not trying to warn you off from trying to get microinch repeatability, but it won't happen by accident. And if and when it does, you will almost certainly be left doubting the results you just saw. It's fun if you're into that sort of thing. I have been thinking about building up a prototype signal conditioner, so if more than zero other people are interested, maybe that will spur me on. I am certainly game to collaborate.

author: rhb
author_rank: Super Contributor
timestamp: November 08, 2022, 09:15:56 pm
content: I'm currently turning a precision test bar on a worn out Clausing 4902. Goal is a tenth or two, but that is proving difficult for reasons I don't understand yet. If I measure the infeed with a tenth indicator on the QCTP I get 4x the infeed reduction in diameter. I've been taking cuts < 0.005" so I'm assuming something is moving without my knowledge. I've locked the compound by tightening a gib screw.The test bar is simply for the sake of doing it. I've bought precision ground bars for all my machine spindles. The bar was my first project on the Clausing and I decided to revisit it and see if I can succeed this time.I'm also rebuilding a new Grizzly G0937 to instrument maker specs. Major effort right now is making laps to lap the spindle 1-3 tenths and probably similar adjustments to the housing bores so I can fit ABEC 7 angular contact bearings.And it that's not crazy enough, I'm scraping a small block of ductile iron bar into a precision prism using only tools available 150 years ago.Temperature is a *big* deal. 10 ppm/K accuracy demands fraction of a degree temperature control. Fortunately, for comparator use things are not so difficult. They should be fairly linear for 0.0001" around center and 3 gauge blocks will do a basic cal. I've got B blocks, so I have a 50 microinch tolerance. At least until I assemble an interferometer from CD/DVD drive parts.Making a flat is on my bucket list. I am awaiting a pair of 80 mm Soviet era flats from Ukraine. These will be my first flats. So time to assemble the light source.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 08, 2022, 09:32:48 pm
content: I've found precision cuts require razor sharp tools and rake angles that give neutral forces in or out. Having built a fair number of interferometers, beware of anything bolted together. Almost any two materials, sometimes even the same material, will form a bimetal strip and deflect way more than you'd expect for a given temperature change. At this level a good rule is, "everything's made of rubber."

author: rhb
author_rank: Super Contributor
timestamp: November 09, 2022, 12:20:47 am
content: Ah, the voice of experience.My interest in an interferometer was rekindled by the realization I could mill one out of aluminum plate with a piece of 16 gauge for a cover using the small bits that are used in optical disks.A one inch range would provide a cal check on any LVDT I'll ever own.

author: CurtisSeizert
author_rank: Regular Contributor
timestamp: November 09, 2022, 02:44:28 am
content: I did not have a grinder when I was doing the spindle I made, so every accurate cylindrical part was hard turned and lapped. I would second the neutral cutting forces theory. Beware taking too small a chip because it can deflect the part quite a bit. Stefan Gotteswinter has a video about how to address this. Generally, you would want to take between 0.01 to 0.02" infeed (on diameter) on non-hardened parts for the best accuracy, so the way to go is to shoot for about 0.04" oversize, wait for the part to cool, then mic it. Take your first 0.02" then mic it again. If that 0.02" cut only took 0.019" off, set the infeed for the next one to 0.021" to compensate. In my experience, that method is good enough to get you within a few tenths.Don't forget the CMD #3 for your spindle rebuilds!

author: rhb
author_rank: Super Contributor
timestamp: November 09, 2022, 01:15:00 pm
content: Quote from: CurtisSeizert on November 09, 2022, 02:44:28 amPlease explain.

author: CurtisSeizert
author_rank: Regular Contributor
timestamp: November 13, 2022, 07:45:38 pm
content: Quote from: rhb on November 09, 2022, 01:15:00 pmIt's an extreme pressure lubricant to prevent fretting corrosion.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 14, 2022, 11:39:28 am
content: When going to highest precision (going to the ppm range in the dimensions) it is a good idea to avoid anything magnetic: that magentization and magneto-striction adds another degree of uncertainty.There is also an analog to the dielectric absorbtion - a small port of the "elastic" deformation can be delayed. The effect can be quite a bit (1% range) in things like cast iron and magensium alloys.

author: JOFlaherty
author_rank: Contributor
timestamp: November 18, 2022, 12:13:49 pm
content: Per Google search bar, 0.01 micron ~= 0.4 microinches, not 0.0004 microinches.