US2093745A - Electric constant temperature device - Google Patents

Description

Sepf. 21,1937. EJP. L. WESTELL ELECTRIC CONSTANT TEMPERATURE DEVICE Filed llay 26, 1936 3 Sheets-Sheet 1 Sept. 21, 1937.

E. P. WESTELL Y ELECTRIC CONSTANT TEMPERATURE DEVICE Filed May 26, 1936 3 Sheets-Sheet 2 //V VEN TOR ATTORNEY Sept. 21, 1937. E. P. L. WESTELL ELECTRIC CONSTANT TEMPERATURE DEVICE Filed May 26 1936 3 Sheets-Sheet 3 Fig. 3.

lr-HT Patented Sept. 21, 1937 Application May 26,

ELECTRIC CONSTANT TEMPERATURE DEVICE Edgar Philip Lawrence Westell, Coventry, England, assignor to The General Electric Company Limited, London, England 1936, Serial, No. 81,969

In Great Britain May 31, 1935 5 Claims.

This invention relates to constant temperature device of the type in which the thermally sensitive element is a Wheatstone bridge whose arms are constituted so that the bridge balances at one temperature only.

In such constant temperature device a supply is usually connected across one diagonal and a responsive element (for instance, a relay) across the other. The responsive element controls a current heating the constant temperature device. It has been proposed to include a thermionic amplifier in the responsive element, and, in particular, to connect the grid and cathode of a thermionic triode directly across the diagonal and to use the anode current to control, directly or through a relay, the heating of the constant temperature device. The object of the invention is to provide an improved modification ofsuch known arrangements, which avoids all moving parts within the controlled space and in which the sensitivity of control is substantially greater than hitherto achieved. v The principle on which the invention depends is that, if the input of an amplifier is connected across one diagonal of a bridge and the output oi the amplifier across another, there will be no coupling between input and output through the bridge so long as the bridge is balanced; but when the bridge is unbalanced, there will be coupling through the bridge. In order that the amplifier may be capable of self-maintained oscillations, it

is necessary that there should be more than some limiting amount of coupling between its input and output. If it is arranged that this limit is attained when the bridge is unbalanced, but not when it is balanced, unbalance of the bridge due to departure of the constant temperature device from the desired temperature will set the amplifier into oscillation. The presence of these oscillations can be made to supply or subtract heat from the constant temperature device so as to restore it to the desired temperature.

One simple application of this principle is illustrated in Figure 1 of the accompanying drawings;

Figure 2 shows a more complete diagram and Figure 3 another modification;

In Figure I the bridge has two opposite arms R of one material and two other opposite arms R1; of another material, these materials having different temperature coefiicients of resistance.

The bridge is constituted so that it is balanced at some desired temperature To of the constant temperature device. At this temperature there will be no coupling through the bridge and no oscillation of the amplifier. When the constant temperature device is at a temperature T sufilciently different from To, the bridge is unbalanced, and a voltage 11. across the output of the amplifier 2 produces a voltage v across its input. If v /v. is sufiiciently large and or the right sign (according to known principles), this coupling between input and output will cause the amplifier 2 to oscillate and the relay 1 to operate.

It is to be observed, however, that if v /vt is of the right sign when T is greater than To, it will generally be of the wrong sign when T is less than To and vice versa. Accordingly the amplifier will oscillate and the relay will operate only for changes in temperature in one direction. 'Even if this limitation could be removed and the relay made to operate for changes in temperature in either direction, there would be nothing to dis tinguish operation caused by oscillations generated by T becoming greater than To from operation caused by T becoming less than To. Accordingly it is impossible to arrange that when T becomes greater than To, the relay cuts off heat so as to oppose the change and also to arrange that when T becomes less than To the relay supplies heat to oppose the change. The arrangement can compensate change in one direction only.

This feature appears to be inherent in the use of the principle characteristic of the invention. It must, therefore, always be understood that the constant temperature device is one in which T tends to depart from To in one direction only, that is to say, to rise above To but not to fall, or to fall but not to rise.

In Figure 1 the relay 1 starts a current in the circuit 8 fed by the supply I and containing the heating element 9 by which heat is supplied to the constant temperature device. The constant temperature device must, therefore, be one in which T tends to fall below To and the bridge must be arranged so that, when T falls below To, v /v. is of the right sign to enable the amplifier to oscillate. If the constant temperature device is one in which T tends to rise above To, two changes must be made. First the sign of 'Ug/Da for a given temperature change must be reversed; this can be achieved by reversing the connections between (3, 3') and (4, 4) or those between (5, 5) and (6, 6). Second, the relay 1, when it operates, must cut ofl the heating current from the element 9.

The control of the heating by the oscillation of theamplifler is indirect in Figure 1; for the heating and amplifier circuits are connected only through the relay 1. But if the amplifier is able to iurnish sumcient power, the relay may be abolished and the control made direct, that is to say, the heating current may be derived from the amplifier circuit. In this case the heating element through which the heating current flows need not be a separate resistance, as shown at 9; it may be constituted by some or all of the arms of the bridge. Constant temperature devices are already known in which the same resistances serve both as a thermally sensitive bridge and as a heating element. Direct control has the advan:

tage that it may be made continuous and not intermittent.

We have now to consider more closely how the oscillations are to control the heating. Here it 16 may be assumed that, in the present state of the art, the amplifiers will always include thermionic valves.

The simplest method in principle would be to use the alternating component of the amplifier outputs to operate the relay, if indirect control were used, or to feed the heating element, if direct control were used. (Direct control could then be used only in constant temperature devices in which T tends to fall below To.) But it is generally more convenient to use direct current for control. This direct current may be the mean anode current of one or more of the valves. It might be made to change when the valve starts to oscillate by causing the representative point oi the valve in the non-oscillating state to lie on a curved part of its characteristic But preferably the oscillations are made to feed a rectifier by which grid bias is generated; this grid bias applied to one or more of the valves changes its mean anode current. The advantage of this method is that the sign of the grid bias generated and therefore the sign of the change in mean anode current produced by the oscillations are easily adjusted to the needs of the particular case.

When compensation can be applied only when T departs from T0 in one direction, it is known to be desirable that compensation, once started by variation of T, should continue even after (TTo) has been made less than the value which originally started the compensation. Since this condition always obtains in constant temperature devices according to the invention, it is desirable that appropriate means for achieving such continuation should be provided. For this reason also control by means of grid bias generated as aforesaid is desirable; for by applying a suitablegrid bias the degree of coupling (and therefore the value of TTo) necessary in order that 55 the amplifier may oscillate can be reduced. But other means obvious to experts are possible. Thus, if indirect control is used, the relay may be biased towards the operated state by oscillations once started; or, conversely, the operation of the relay may apply a potential (e. g. a grid bias) to the amplifier biasing it towards oscillation. X

Preferably the amplifier is tuned to a single frequency, the arms of the bridge are non-reactive, and the amplifier is such that during oscillation the phase difference between input and output is 0 or 180. But this is not essential; all that is necessary is that at the desired temperature the bridge shall be balanced at the same time for all frequencies with which the amplifier can oscillate, and shall be unbalanced for some at least of these frequencies for all temperatures on some range on that side of the desired temperature towards which the constant temperature device is apt to depart.

Two embodiments of the invention, which include some features not mentioned in the foregoing account of general principles, will now be described with reference to Figures 2 and 3 of the accompanying drawings. Both figures are conventional circuit diagrams; Figure 2 shows indirect control, Figure 3 direct control.

In Figure 2, the bridge I, constructed as in Figure 1, is enclosed in the constant temperature chamber The points 4, 4' areconnected (as in Figure 1) to 3, 3', which are the terminals oi the primary of an input transformer l2, whose secondary is connected between grid I3 and the cathode M of the thermionic triode IS; the connection to the cathode includes the source of constant grid bias I6 and the part II of a potentiometer resistance |8 which (as explained below) provides a variable grid bias IS. The source of constant bias is a resistance in the heating circuit of cathodes l4 and 26 (see below) fed from the L. T. ( low tensioni'supply 50, 5|, of which the negative terminal is joined to the-H. T.

I (high tension) negative. A regulating resistance It is included in this heating circuit. A smoothing condenser is connected across the part I! of the potentiometer H3. The anode l9 of the "triode I5 is connected through the choke 20 and one winding of the relay 2| to the positive terminal of the supply whose negative terminal is connected to earth or grounded. The anode is also connected through the coupling condenser 22 to a tapping" on the choke 23, which forms the tuning element in conjunction with the condenser 43 shunted across it. One end of this choke is connected to the grid 24 of another thermionic triode 25, the other end at a point on the circuit by which the cathode 26 ofthis triode, as well as I4, is heated. The anode 29 of the triode 2 5 is connected through the primary 42 of a transformer 52 and through the other winding of relay 2| to the positive terminal. Condensers 3| are provided as usual to bye-pass A. C. components of the anode current, and prevent them passing through the windings of the relay 2|. 400 P. P. S. (periods per second) is a suitable natural frequency for the tuned element.

The transformer 52 has two secondaries 33, 34. The secondary 33 is split into two halves, the inner ends of which, shunted by a condenser 53, are connected to the plus and minus sides of a high tension circuit H; T+ and H. T;.the outer ends, which correspond to 5, 5' in Figure 1, are connected to the diagonal points 6, 6' of the bridge. The bridge is thus fed with D. C. current (for a purpose that will appear presently) and, when the amplifier oscillates, with A. C. current. Secondary 34 feeds a rectifier 35 which in turn supplies current to the potentiometer Ill.

The relay 2| has contacts 38 which, on closing,

, enable current to flow through the heating element 36, enclosed in the chamber H, from the supply 31.

The primary winding 3, 3' is split and the inner ends, shunted by a condenser 54, are connected through a D. C. instrument 56, which thus receives the D. C. out of balance current from the bridge I. The instrument 56 may be a measuring instrument, which will then act as a distant reading thermometer. Alternately or additionally it may be a relay which operates an alarm and breaks part of the circuit, if the apparatus ceases to function and the temperature of the bridge departs widely from that desired. But such devices do not form part of the invention.

' flowing through the bridge.

The windings'oi' relay 2i oppose each other.

' and are adjusted so that, when the amplifier is not oscillating, the anodev currents of triodes ll ondary 34 produce a grid bias in the part ll of the potentiometer making the grid II more 7 positive. The anode current of the trlode II increases, the balance of the windings of relay 2i is upset, relay 2| operates and contacts 38 close. Current is therefore supplied to the heating element 36 which causes the temperature to rise till the oscillations cease.

v In this arrangement the positive grid bias applied to the grid l3 makes the amplifier more ready to .osciliate and therefore fulfills the purpose, above mentioned, of continuing oscillations after the originating disturbance has been renewed. In order to increase this effect, the valve It may be a varlable-mu valve.

Figure 3 shows an arrangement for direct control in which there is no relay, and compensation is effected by the anode current of a valve Parts represented by the same reference numeral in Figures 2 and 3 perform the same function. Accordingly only the differences need be mentioned.

Relay 2| and the circuit and heating coils 36, 31, 38 are absent. The effective output terminals 5, 5' are now connected with the anode and cathode of the triode 25. This anode circult, which includes the bridge between 6 and 6', is fed by the supply 31, of which the negative terminal is grounded; the supply 31 may be the same as that supplying the valve ii. The primary 42 of the transformer which feeds the secondary 34 and rectifier 35 is connected in series with a condenser 4| across the output terminals. A condenser 46 and a choke 41 prevent alternating current from reaching the supply. A blocking condenser 48 is inserted in the leads between the input and the bridge.

The heating circuits for cathodes l4 and 26 are not shown. Grid 24 has now a source of constant grid bias 21 which, like it, is shown as a lmttery; both grid 24 and grid l3 receive a variable bias from the potentiometer It; the grid i3 is fed from the part l1 as before, the grid 24 from the part 23. Accordingly when the oscillations start, the mean anode current through both valves i5 and 25 increases; the current through valve 25 alone flows through the bridge; the object of increasing the positive bias on valve it has been explained already.

It will usually be desirable to supply the bridge, which is now the heating element, with a constantcurrent, independent of the compensation. This can be drawn from they supply 31 through the choke 44 and variable resistance 45, one end of which is grounded.

Thermostats according to the invention are particularly suitable for accurate control of temperatures but little above atmospheric. Thus they can be used to control a temperature of C. to within i 0.05 C. if the supply voltage is reasonably constant.

a; aoeans relay; for amplifiers can be designed whose gain is almost independent of the supply voltage. But fullindependence of the supply 31 feeding the heating element is not secured either in Figure 2 or Figure 3.

I claim:-

1. A constant temperature device comprising a thermally sensitive Wheatstone bridge arranged to balance at a selected temperature and to be come unbalanced by a change from said temperature, a trlode tube amplifier having its input and output terminals connected to the ends of opposite diagonals of said bridge so that a coupling is established when the bridge is unbalanced, circuits and connections adapted to start and maintain oscillations in the amplifier when the bridge becomes unbalanced by a change from said selected temperature, heating means for controlling the temperature of the bridge and means controlled by oscillations of the amplifier for operating said heating means to restore the balance of the bridge and stop said oscillations.

2. A constant temperature device comprising a thermally sensitive Wheatstone bridge arranged to balance at a selected temperature and to become unbalanced by a change from said temperature, a trlode tube amplifier having its input and output terminals connected to the ends of opposite diagonals of said bridge so that a coupling is established when the bridge is unbalanced, circuits and connections adapted to start and maintain oscillations in the amplifier when the bridge becomes unbalanced by a change from said selected temperature, an electric heating coil for restoring the temperature of the bridge when it drops below the selected temperature and a relay adapted to be operated by oscillations of said amplifier for throwing in said heating coil, the relay I is unbalanced, means causing the amplifier to oscillate when the bridge becomes unbalanced by a change in temperature, a rectifier adapted to be operated by the alternating component in the output of said amplifier to provide grid bias for at least one of said tubes for controlling the oscillations and means rendered effective by said bridge and stopping the oscillations.

4. A constant temperature device as in claim 3, wherein the means for restoring the temperature of the bridge comprises an electric heating coil adjacent the bridge which coil is adapted to be operated by a relay controlled by said oscillations in the amplifier.

5. A constant temperature device as in claim 3, wherein resistance coils in the arms of the Wheatstone bridge comprise heating coils and the current flow established through the bridge by omillations, in the amplifier output connections heat said coils and restore the bridge to the seaegted temperature, thereby stopping said oscilla- EDGAR PHILIP LAWRENCE WESTELL.

oscillations for restoring the temperature of the

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