GB2219404A - Differential inductance transducer circuit - Google Patents

Description

2219404 Connection of a Differential Inductancd Transducer for

Non-Electric Quantities with Dicital Output The invention re:ors to the field of measurement of non-electric, especially of mechanical quantities a7nd solves the connection of a di:forontial inductance transducer for non-d lectric quantities with digital output, especially intended for computer-cquippod facilities.

In measurements of non-eloctric, especially of mechanical quantities, differential inductance transducers are frequently usod, consisting of a pair of coils with common moving core. Chan-ro i.n the 0 0 Li quantity being measured is converted into the change of the tr-nsducer- core position, thus causing a phango of inductance ir, both transducer branches. Connections of a differential inductance transduc er with digital output known hitherto are ut;Llizing connection of both transducer branches conitituting an.a.c,bridge, led by a sine oscillator. The output voltage of the bridge following its=.plification using an a.c, amplifier is rectified by means of a pha3e-sonsitive rectifier. Upon necesshry filtering., the d.c. voltage thus obtained, which is.appro%11- 711tely proportional-to the quantity measured, isconverted into digital form by means of an analog-digital ccn-.ro,-tCr. A limitation of these known connections is their considorab1c complexity resulting from extreme requirements on all connecting clements. Specific demands are made upon smalL dis tort ion, amplitude and phaso related ocillator stability, amplitude-phaso transfer character-.44.stic of the a.c. amplifier, linearity of the phase-sensitivo rectifior as well as on accuracy of the analog-digital converter applied. The demands referrod-to are still intensified by the requirement, 'on their time- and temporature-rolatod stability. From the ccz:ploxity of these connections also high material costs result, the production and debugging is highly labour-oxacting, which again rciults from the necessity7 of accurate setting of all connecting circuits. The.)_21'. resulting properties of those kno.-.n connections are princ-,..6 limited partly by the origin ot harmonic distortion of the brid.o.

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output volta., given by tho properties of the t U ransducor-corb,partly by the mathematical model employed in which it is assumed that a rolativo chanff,,o in the inductance difference of both transducer branches is directly proportional to the transducer-coil displacement and, finally, by..known non-linear characteristics of the unbalanced bridn connection. A considerable susceptibility or these known connections to disturbing signals is also not to be neglected, having their origin cy U partly in the intrinsic connection (electronic component noise), partly to disturbing signals ponotratinff into the connection rron CO U 0 external environment overfooding cables and through eloctromnenotic:p ractice means a reduction of the dynanical range induction, which in of the measured quantity to max.50 decibels.

The afore-mentioned limitations of connections known hitherto are considerably re moved by the connottion of a differential induct- accord ance transducer for non-electric quantities with digital output ing to the invention, its rinciple consistin5 in that the co=on outlet of the diúferontial inductance transducer is attached to the first outlet of the oscillator, the first outlet of the differential inductance transd4or being connected tothe first change-ovor switch input, the second outlet of the differential inductance transducer being connected to the second change-ovor.switch input. The output of the chango-over switch is connected to thesecond oscillator outlet, which through its output is connected to the input of the control evaluation and indication block, its output being.connected to the control input of the change-over switch. .

The connection accordikg to the invention. has tho úolloivin,-,, advantages: it is very simple, does not require any special Attention in. production and during setting, no precise c.r expensive analo; elements are employed - th3 connection is especially suitable for attachment to a cortputer - direct conversion of the quantity measured to interval longth of the oscillator oscillations causes min=ium sensitivity to disturbing eúZocts and-enables to process a dynamical rango-oú the qu antity me"ured up to 100 decibels 3 shows oxcolleni.1ineirity of the transf or characteristic rosu Itin.. from:suitablo selection of its mathematical model, with potential further improvement through suitable approximation shows excellent thermal and long-term stability of the transfer connection characteristic onablos'easy roalization of multichannel measurements An oxamplo of tho connection of the difXcrcntial inductanco transducor' for non-olactric quantities with digital output according to Lhe nvention is illustrated on the attached drawing, showing thc wiring diagram.

Th.c corinion outlet 5 ol'tho di:l'orential inductance transducer 1 is connected to the first outlet 12 ofthe oscillator 3. Tho first outlet 6 of the differential inductance transducer I is cor.%ccted to Le fir-st input 8 of the change-over switch 2'and similarly, thc SOC0111 outlet 7 of the differential inductance transducer 1 is conncdtcd to t!he second input 9 of the change-over switch 2. The output 11 -of t'hjs c hange-over switch 2 is connected to the second outlet 13 of the oscil 1,ator 3. The output 14 of this oscillator 3 is connected to input 15 o.f the control, -evaluation and indication block 4, whose output 16 i interconnected with control input 10 of the c hange-over suitch.0 T,he control, evaluation and indication block 4 can be realized by means of microprocessor circuitry., e.g. using a pair of packaps with L; ISOS circuits and one LSTTL package.

The control, evaluation and indication block 4 connects by mvzns of the change-over switch 2 individual branches of the di:fcrontial inductance transducer I to oscillator 3. Inductanco L of the branch of the differential inductance transducer 1 actually connected by r.canz of the change-over switch 2 is constituting a member of a'tunod circujit of the oscillator 3 and for the oscillatica cycle duration T of osci,',Llator 3, the following known relation applies:

T = 2 9 -1 VL_C (1) - 4 hore C capacity. constituting with inductance L of one branch.oú transducer 1 a tuned eiriuit. It. i!ith the connected first brancl; of transducer 1 for the oscillaton cycle. duration Tl of oscillator 3, the relationship Tl r_ 217r IL1C applies, where L,= inductance of the first branch of tranoducor r. then, also with cnnectod second branch of transducer 1 for oscillation cycle duration T2 of oscillator 3, the relationship T2 = 271-VL2C (3) applies, where L.2 = inductance of the second branch or tr--ns,-lucer 1.

For the relationship between the measured quantity y and inductances L, and L2 of the branches of transducer 1, the following relationship can be written:

y (4) As a suitable approximation of runctiofi g( L1,1.), funct..on -Irr 11, (5) g( Li;L2). - canbe used.

Ir the change-over switch 2 is switching individual branchos of transducer 1 relative to required dynamical properties sufficiently quickly then, substituting oquatiozz(2) and (3) in equation (5), we can wr te:

-. T, T - 1 (G) T 1 + T2 being evident that the value of this function will not be depending 1 W on variations of oscillation cycle duration T1, T. or oscillator caused by thermal or 17ong-t'or= Instability or oscillator 3, that JS to say that all,instabilitios with the exception of instabilities of transducer 1 had been compensated.

(4) uj3jn& eqqation (6) can thdn be written a.

The expression CD R T2 - T,. (7) T T2 It can be verified that even a linoor approximation of th4s tunction leads to a better connection transfer characteristic than in case of connection of the same transducor in known a.c. bridgo connections, being possible to further improve the linoarity through approximation of function (7), for example using a polynomial.

Control-o 2 the chango-over sivitch 2, measuremont of oscillat-.Lon cyclo durations T, and T. of oscillator 3, calculation of the measured quantity y and its display and/or further processing are provided by the control, evaluation and indication blocj':6 4 To incrnaso the resolution at simultaneously smallor demands upon the mcasurcriont rate, it is possiblo, the oscillation cycle.duration Tl, T, of osc4-llator 3 to be interchanged with durations T'O, 'T" of a convenient 2 number of successive oscillations of oscillator 3 Connection of the difiorential inductance 'transducer for-non-eloctric quantities with difti7tal output according to the invention can bo I utilized in metrology, control and automation engineering.

List of reference symbols 1 Differential Inductance transducer 2 Change-over switch 3 LC oscillator 4 Control, avaluaion and indication block Co=on outlet of the differential inductance transducer 6 First outlet of the differential inductance transducor 7 Second outlet of the differential inductance transducer a First chango-over switch input 9 Second change-over switch input Control change-over switch input 11 Change-over switch output 12 Firs oscillator outlet 13 Second oscillator outlet 14 Oscillator output Input of the control,- evaluation and indication block 16 Output of the control,;valuation and indication block 1 4 1 C L A 1 9 S 1. Connoction.of a diú1.orontial inductance tranhducer úor non-oloctric quantities with digital output, characterized in that tho co=on outlot (5) of the differential intructance transducer (1) is connocted to the first outlet (12) of the oscillator (3), th6- rirst outlet(6) of tliodirfoiential inductance transducer (1) beint.,r connoctcd to the first input (8) or the change-over switch (2) the 0 & f the differential inductance transducor (1) second outlet (7) o being conticted to the second input (9) or the change- over snitch (2), whose output (11) is connected_ to the second outlct (13) or the oscillaior (3), which through its output (14). is connected to C3 input (15) of tho cc ntrol, evaluation and indication block (4), whose output (16) Is connected with the control input (10) of the change-over switch (2).

Published 1989atThe PatentOffice, State House, 66/71 High Holborn, LondonWCIR 4TP.Purther copies maybe obtainedfrom. The Patent Office. Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con. 1/87