[HN Gopher] Linear Variable Differential Transformer (LVDT) Basics
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Linear Variable Differential Transformer (LVDT) Basics
Author : userbinator
Score : 19 points
Date : 2024-10-09 04:32 UTC (2 days ago)
(HTM) web link (www.te.com)
(TXT) w3m dump (www.te.com)
| K0balt wrote:
| lol. Interesting stuff but here I was thinking this was about the
| new transformer architecture.
|
| Differential Transformer https://arxiv.org/abs/2410.05258
| topspin wrote:
| Likewise. I was pleased to find this instead.
| mikewarot wrote:
| I've always been intrigued by LVDTs since I learned that they can
| be used to measure millionths of an inch displacements. With the
| advent of cheap computing with good A/D, perhaps it's time to add
| a DIY LVDT to my project list.
| HeyLaughingBoy wrote:
| You can find pretty cheap resolvers on eBay (I have a couple of
| Singer units that I paid $30 for). I'm sure you can also find
| LVDTs. Resolvers & LVDTs only differ by motion type: resolvers
| are rotary and LVDTs are translational.
|
| At least that way you don't have to do the "annoying" part of
| the project, which is likely to be the mechanical aspects of
| winding and placing the coils.
|
| I built a product that basically simulates the process: it
| takes an analog or digital input and outputs sine & cosine
| signals that look like position information for a motion
| controller that expects an LVDT or resolver.
| kragen wrote:
| I've been thinking about using the magnetic and mechanical design
| of an LVDT in a different application: a high-reliability
| keyboard with a four-dimensional scan matrix to reduce the number
| of electrical lines required.
|
| For a conventional keyswitch-matrix multiplexed keyboard with 81
| keys, you need 18 GPIO lines, 9 row lines and 9 column lines.
| Even with Charlieplexing, I believe you need 13 GPIO lines to get
| to 81 keys. (1/2(14*13) = 91.) Keyswitch matrices are also
| mechanically and chemically delicate; a spill of solvent, battery
| acid, or sometimes even saltwater can damage the keyswitches, and
| they do not work underwater unless the keyboard is hermetically
| sealed. Such seals have to be flexible and are regularly flexed
| during usage, so they usually fail after only a few years. Some
| keyswitch contacts were often made of metal, which suffers
| oxidation over time resulting in keyswitch failure; many current
| keyswitches instead use contacts made of graphite-filled rubber,
| which doesn't form a solid oxide surface layer. (Keyswitches also
| generally require debouncing, though I suspect this is less of a
| problem with the graphite-filled rubber contacts.)
|
| Capacitive keys avoid contact bounce and oxidation, but tend to
| suffer even worse from submersion because of the high electrical
| permittivity of water. They are also more sensitive to electrical
| noise.
|
| By contrast, a differential-transformer key mechanism would
| permit an 81-key keyboard with only 12 GPIOs, high EMI immunity,
| and extreme mechanical robustness.
|
| Each key contains a differential transformer, similar to an LVDT
| but without any attention given to linearity. When the key is not
| depressed, the core in the differential transformer is at its
| nulled position, where a pulse of current through the primary
| will produce exactly canceling voltages across the two opposing
| secondaries. But when the key is depressed, the core is
| substantially displaced, so that the net voltage pulse induced
| across the two opposing secondaries is significant.
|
| Submersion poses no problem for the mechanism, because the
| magnetic permeability of water is basically the same as air or
| vacuum, so water filling the tube around the core is not a
| problem. As the TE page explains, the same is true of things like
| high-pressure hydraulic oil and even low-temperature molten
| metals. The mechanism would not work if you submerged it in a
| ferrofluid, or if you heated the core past its Curie point, but
| that is not much of a problem in most practical environments.
|
| The four-dimensional multiplexing works as follows. There is a
| 3x3 primary-winding matrix and a 3x3 differential secondary-
| winding matrix. Each of the 9 primary-winding-matrix cells has
| the primary windings of 9 different keys in it, each of which
| belongs to a different cell of the secondary-winding matrix.
| These 9 primary windings in a single primary-winding-matrix cell
| are preferably in parallel. By pulling one of the three row lines
| of the primary-winding matrix high and pulsing one of its three
| column lines low, while maintaining the other 4 row and column
| lines tristated, you send a pulse of current through those 9
| primary windings.
|
| Similarly, each of the 9 secondary-winding matrix cells contains
| the 9 opposing-series-wound secondary-winding coils, in parallel,
| one for each of the 9 primary-winding-matrix cells. So each of
| the 81 keys represents a unique combination of a primary-matrix
| cell and a secondary-matrix cell.
|
| I'm not yet entirely clear on how to scan the secondary-coil
| matrix for a given primary cell. It would be fairly
| straightforward if you had an electromechanical relay for each
| row, a diode on the anti-series secondaries of each key, and a
| sense resistor to ground on each column: the voltage induced on a
| depressed key connected to an open-circuited row would not be
| able to draw any current from its open-circuited row, so it would
| drive no current through its column's sense resistor, which would
| therefore remain at ground, and the open-circuited row line would
| be driven below ground. But if you close the relay to select its
| row, connecting the row to ground, then the voltage induced
| across the anti-series secondaries would drive current from the
| row to the column and through the column's pulldown resistor,
| raising the column voltage up to an easily detectable level.
|
| The part I'm not entirely clear about is how to do this with
| regular CMOS GPIOs, which have clamping diodes to prevent them
| from going above Vcc or below ground. So an induced secondary
| voltage that attempts to drive a tristated CMOS GPIO below ground
| will only drive it to a diode drop below ground, at which point
| it will start to source current enthusiastically to protect the
| chip, looking very much like a GPIO being driven low. I suspect
| there's a simple solution to this problem, but I'm just a
| beginner in the art of electronics; undoubtedly it would be
| obvious to one skilled in the art.
|
| However, for keyboards of ordinary sizes, the four-dimensional
| keyboard matrix is an aspect of only minor, marginal benefit
| compared to the mechanical robustness and reliability of the
| keyboard mechanism. A three-dimensional matrix or a conventional
| two-dimensional matrix is easily realizable.
|
| One way to do a conventional two-dimensional matrix is to connect
| all the secondary-coil pairs in parallel, each with a series
| diode, and scan the primary matrix as above to get a pulse on the
| single output line shared across all secondaries only when you
| happened to scan across a key being depressed. This would require
| 18 tristate output lines and one input line. This variant of the
| system permits independent analog measurement of each core's
| position.
|
| Another way would be to connect each primary between one of 9
| primary row lines and Vcc, driving one of them low at a time
| while tristating the others, and to connect each secondary
| between one of 9 secondary column lines and Vcc. This would
| require 9 tristate output lines and 9 input lines with pullup
| resistors; it's closely analogous to a conventional keyswitch
| matrix. (Of course you can interchange the polarities as
| desired.)
|
| A three-dimensional matrix could be realized by, for example,
| combining the two above setups: dividing up to 100 keys among a
| 5x5 matrix of primary cells, with up to 4 keys in each cell, each
| with its secondary connected to a different secondary column
| line. Activating one of the 25 primary cells by pulling its row
| line high and its column line low would induce currents through
| secondaries that drive low some subset of the 4 column lines.
| This requires 10 tristate output lines and 4 input lines with
| pullup resistors. Again, polarities can be interchanged as
| desired.
|
| I suspect you can play various charlieplexing-like tricks with
| diodes to reduce the number of required lines further.
|
| Back to the issue of mechanical and corrosion robustness. If the
| cores are encapsulated in glass, porcelain, or teflon, and
| similarly for the tubes they slide within, the keyboard should be
| able to survive even fairly aggressive environments such as
| extended immersion in boiling sulfuric acid, unless the
| insulation on the coils is degraded by the high temperature.
| Exposed key return springs that are subject to mechanical fatigue
| and chemical attack might be able to be made of long ceramic
| flexure blades, or if high-temperature resistance is not
| required, they could be replaced with repulsion between small
| rare-earth magnets which are themselves encapsulated in
| corrosion-resistant housings.
| HeyLaughingBoy wrote:
| In any practical application, the number of GPIOs simply
| wouldn't be an issue. Transistors are as cheap as sand.
|
| What would you foresee as the application for a keyboard like
| this? It sounds like Hall-Effect switches would work just as
| well and cost significantly less.
| kragen wrote:
| You may be right.
| peter_d_sherman wrote:
| >"Infinite Resolution
|
| Since an LVDT operates on electromagnetic coupling principles in
| a friction-free structure, _it can measure infinitesimally small
| changes_ in core position. This infinite resolution capability is
| limited only by the noise in an LVDT signal conditioner and the
| output display 's resolution. These same factors also give an
| LVDT its outstanding repeatability."
|
| Related:
| https://en.wikipedia.org/wiki/Linear_variable_differential_t...
|
| >"A counterpart to this device that is used for measuring rotary
| displacement is called a rotary variable differential transformer
| (RVDT)."
| (https://en.wikipedia.org/wiki/Rotary_variable_differential_t...)
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