CA1039971A - Direct reading temperature measuring bridge circuit - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An a.c.-fed temperature measuring bridge in the form of a balanced Wheatstone bridge containing a temperature sensor R(x) which, depending upon. the measured tempera-ture x°C,generates an output voltage which is balanced by a variable voltage y provided by ohmic ratio arms in such a way that if the sensor R(x) has a non-linear characteristic, there will still be a linear relation-ship such that x = const.?y, the inventive feature being that the fixed ratio arms of the balanced Wheatstone bridge are formed by an inductive potential divider and that for correcting the non-linearity of the sensor the voltage across the variable ohmic ratio arms is a pre-determined ratio

Description

, ~0399~1 B~CKGROUND OF THE INVENTION
The invention relates to a direct reading temperature measuring bridge circuit containing a platinum resistance type sensor for the highly precise measurement of temperature variations, particularly of very small temperature variations. ``
In the case of temperature measuring bridges diverse circuit arrange-ments have been proposed which are usually based on the principle of the Wheatstone bridge. The temperature sensor, i.e. transducer, used for accurate measurements is a platinum resistance type sensor which has a non-linear ~ -temperature-resistance characteristic approximately represented by the relationship R(x) = Rl(l+~ X +~ X ), where R(x) is the resistance of the sensor at X C, Rl is the resistance at O C, and o~and ~ are constants. The non-linea~ity of the sensor is an_unfavourable property which must b~ compensated for in the measuring device. The measuring device must further be capable of be mg calibrated simply in such a way that when it is in a state of balance `
the increment of a predetermined balancing resistor corresponds to the actual temperature change in the environment of the sensor.
According to the invention this is achieved by the employment of a ~ . ...
` 20 balanced Wheatstone bridge containing a temperature sensor R(x) which, depend- -: :.
ing upon the measured temperature, generates an output voltage which is `
balanced by a variable voltage ~ provided by ohmic ratio arms in such a way ~ -` that also if the sensor R(x) has a non-linear characteristic, there will I still be a linear relationship such that x = const. ~, the inventive feature ; being that the fixed ratio arms of the balanced Wheatstone bridge are formed in a manner known per se by an inductive potential dividex and that for correcting the non-linearity of the sensor the voltage across the variable ohmic ratio arms is a predetermined ratio (03) of the voltage feeding the bridge.
The manner in which a bridge according to the invention functions ~
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~03S~1 will now be described with reference to Figures 1 and 2 of the drawing in which:
Fig. 1 is a schematic diagram of the network, and Fig. 2 is a schematic diagram of an exemplary embodiment of the network.
The first part of the network (Fig. 1) contains a Wheatstone bridge a comprising an inductive potential divider with two windings N and N' and an ohmic potential divider with the resistance Rl and a variable resistor R(x) which depends on the input variable x (temperature). This Wheatstone bridge a is suppLied at connections e and f with the voltage US and produces the output voltage ub as the difference between voltages u2 and ul. The bridge is grounded at d. --~
The second part of the network (Fig. 1) is a further potential ~`
divider b with the resistor Ro and a variable resistor R(y) provided with a scale with the subdivision y. The potential divider b is fed by the secondary winding N2 f a transformer, the primary winding Nl of which is connected to the supply voltage us. The output voltage uT~compensates the output voltage Ub of the bridge a. Indication is accomplished by means of a zero detector k. The foot of the potential divider b is grounded at c. It can be shown that the output voltage of a Wheatstone bridge a containing a resistance `
R(x~=Rl+x . ~ R which varies in proportion to a variable quantity x can be ~ ~ `
balanced by a potential divider b containing a resistance R(y)=y , ~ R which `
varies in proportion to a quantity ~ in such a way that x = const. '2- In ,~
the network illustrated in Figure 1 the following equation holds for the out-put voltage ub of the Wheatstone bridge Rl + x ~ Rx 1 US ~ R
b 1 2 S 2Rl + x~Rx 2) 2 '2Rl + x ~R

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whereas the voltage uT at voltage divider b is defined by - ~
Y a R `` -.
T Nl Ro + y ~ Ry If in this instance N2 . US = US is set, then the condition of Nl 2 balance will be simplified to X /\ RX y ,~ R
- Ro + Y ~ Ry or xl~ RXRo + X ~ Rx.y ~ Ry y/\ R~, 1 x y x~ RXRo = y ~ Ry 1 F

~ Ry2Rl X = y. 8 con5t-X O ' ' The relationship between the input variable x and the balancing quantity y is therefore linear. It is also worthy of note that the increments R of the balancing resistor Rty) can be freely chosen although the choice of the resistor (Rx) (temperature sensor) determines the magnitude of the increment ~ R and of Rl. The balancing potential divider b can therefore conveniently be composed of resistors which can be cheaply manufactured to comply with the necessary tolerances. Ano~her major advantage is that when the properties ~ R +~ of the resistor Rtx) are fixed and the scale R of the resistor (R ) is likewise fixed, the constant (const.) in the equality x = const. 'y can still be varied by adjustment of R in the potential divider b. This permits the arrangement to be easily adapted to resistors R(x) having different properties. It should also be emphasised that the position of the measuring range is determined exclusively by Rl, whereas the potential divider b merely determines the width of the range. Particularly when this is small ..

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it need not therefore satisfy the same stability requirements as the resistor If the bridge is so modified that for a ratio of the bridge divider of 1:2 the transformation ratio 2 ~ 0.5 is chosen then the conditions of balance will be Nl y.Nl ~, Ry.4Rl ~.
A R ~Ro + ~R ~ R (1 - N2)-Y

It will be seen that the second term in the denominator does not 10vanish and dapends on ~; in other words, the relationship between the input -variable x and the balancing quantity ~ is not then linear. This circumstance is utilised to compensate any non-linearity of the sensor, as represented by ~ `
the equation R(x)=~l~ x+~x ) for a platinum resistance sensor.
Fig. 2 shows a Wheatstone bridge a with the two windings N and N', the temperature-dependent resistor (R(x) = Rl (1 + ~ x + ~ x ) and the refer- `
ence resistor R2 adjustable within a small range. The bridge a is grounded at d and is supplied at connections e and f with the voltage us. The winding N has a divider g~ The bridge a is balanced for a reference point, e.g. ~
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Furthermore, Fig. 2 shows a potential divider with a step resistor having the conductivity Gty) = Gy . y and an adjustable resistor f" ,. '' `
with the conductivity Go~ The potential divider h is grounded at i and generates the voltage uT, when it is supplied via the divider ~ of winding N.
The output voltage ub is compensated with the voltage uT by adjustment of resistor G(y). Indication of compensation is accomplished by means of zero detector k. ~ ;
The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The embodi-ment described and shown is therefore to be considered in all respects as illustrative and not restrictive.

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Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A temperature measuring circuit comprising in combination:
a) an electrical bridge comprising:
1) a temperature responsive resistor and a reference resistor in two of its branches respectively;

2) first and second windings of a transformer in the other two of its branches respectively;
b) means for supplying a voltage input across the junction points between said resistors and said transformer windings;
c) a null detector;
d) means for connecting said null detector to the output of said bridge taken between the junction of said transformer windings and a common connecting point between said temperature responsive and reference resistors;
e) a voltage divider comprising a fixed resistor and a variable resistor;
f) means for supplying a portion of said voltage input across said fixed and said variable resistors;
g) means for connecting said null detector to a junction between said fixed and variable resistors whereby said variable resistor may be adjusted so as to supply voltage to said null detector until the voltage from said bridge has been nullified; and h) a linear graduation scale associated with said variable resistor having a predetermined temperature indicating scale;
i) the ratio of the supply voltage for said potential divider to the supply voltage of said bridge being such that when balanced there is a linear relationship between the actual temperature change sensed by said temperature responsive resistor and the incremental change of the value of said variable resistor necessary to nullify the voltage from said bridge as determined by said null detector.