US3206621A - Switching techniques - Google Patents

Description

sePL 14, 1965 G. s. s'ruBBs 3,206,621

SWITCHING TECHNIQUES Sept 14, 1965 G. s. sruBBs 3,206,621

SWITCHING TECHNIQUES SWITCHING TECHNIQUES Filed April 14, 1959 4 Sheets-Sheet 4 Fig. 95?

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United States Patent O 3,206,621 SWITCHING TECHNIQUES Gilbert S. Stubbs, Levittown, Fa., assigner' to The Franklin Institute, Philadelphia, Ia., a corporation of Pennsylvania Filed Apr. 14, 1959, Ser. No. 806,238 Claims. (Cl. 36W- 149) This invention relates to switching techniques for varying the apparent values of fixed electrical parameters. These techniques are particularly useful but not necessarily limited to use in analog computers wherein circuit elements simulate analogous physical conditions which may vary from time to time. The use of the switching techniques makes possible apparent change in the circuit parameter so that variation in the physical condition may be simulated by variation in the circuit parameter.

In the copending U.S. application Serial No. 781,259, filed December 18, 1958, now Pat. No. 3,011,316, my co-invention with George P. Wachtell, there is described an analog computer in which heat capacity is simulated by electrical capacitance and heat flow impedance is simulated by electrical resistance. Under actual working conditions, the effective heat capacity, particularly that of the coolant, may change from time to time and the heat liow impedance may be materially modified by such things as surface iilrns. In simulating nuclear `reactors and heat exchangers where these factors become of substantial importance, it is necessary to have a means whereby the analog parameters in the computer can either actually or apparently be varied. The present invention provides the means for varying the apparent values of fixed-value parameters without material modification of the general arrange-ment of the computer circuitry.

The techniques involved in accordance with the present invention, for example, permit the simulation of a resistance whose value can be modified at will. A similar effect is possible using capacitance. Another possible effect is the simulation of a variable current source. In each case, the control is accomplished by use of a pulse train which provides a switching effect upon the circuit involved. In certain instances, a particular type of switch has been found particularly advantageous but the invention is not limited to any one type of switching element.

In accordance with the method of the present invention and in all circuits of the present invention, it is highly desirable that the pulse repetition time be sufficiently small in comparison with the response times of the analog circuit in which the parameter is located that the effect appears to be steady. In the usual situation, an averaging is achieved which gives the effect of a fixed value of the parameter for any given pulse train. For example, a resistor which is periodically connected and disconnected to a voltage source produces a lower average current in the resistor than would occur if it were permanently attached. The effective resistive load on the voltage source is of a value higher than the fixed resistor. By changing the ratio of the time connected to the voltage source to the time disconnected from the voltage source, the effective resistance can he changed. The effective incremental capacitance of a fixed capacitor may also be adjusted by the use of a periodic switching technique. Additionally, the effective current from a current source may be modified by a periodic switching technique.

For a better understanding of the present invention, reference is made to the following schematic circuit diagrams and voltage and current diagrams, in which FIG. 1 illustrates a section of an analog computer circuit in which one form of the present invention is employed;

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FIG. la shows a voltage pulse train for actuation of one of the switches shown in FIG. l;

FIG. 2 illustrates a circuit for simulating variable resistance;

FIG. 2n shows the voltage applied at the terminals of the circuit of FIG. 2;

FIG. 2b shows the switching voltage employed to actuate the switch and the circuit of FIG. 2;

FIG. 2c shows the resulting current output pulses from which is derived average current in the circuit of FIG. 2;

FIG. 3 illustrates a preferred form of switching device useful with the present invention;

FIG. 3a illustrates a wave-form for application to the positive terminal of the network of FIG. 3;

FIG. 3b shows a waveform for application to the negative terminal of FIG. 3;

FIG. 4 illustrates an alternative arrangement to the circuit of FIG 3 wherein electron tubes are substituted for semi-conductor diodes;

FIG. 5 illustrates a circuit for obtaining the variable current effect;

FIG. 5a illustrates a constant input voltage applied to the circuit of FIG. 5;

FIG. 5b illustrates the voltage wave effect created by variable pulse width modulation oi the input voltage in the circuit of FIG. 5;

FIG. 5c represents the resulting effective current in branch a of the circuit of FIG. 5;

FIG. 6 illustrates a circuit for simulating a Variable capacitance;

FIG. 6a shows a constant current signal which is injected at the input terminal of the circuit of FIG. 6;

FIG. 6b shows a pulse train applied to actuate the switch of FIG. 6;

FIG. 6c illustrates the capacitor voltage with and without the pulse train;

FIG. 7 illustrates another variable capacitor circuit in which all the switches are connected to ground or to a low impedance voltage source;

FIG. 7a shows the operating wave train for actuation of two ofthe switches of FIG. 7;

FIG. 7b shows the operating wave train applied to the other switch of FIG. 7;

FIG. 8 illustrates a conventional capacitance charging circuit;

FIG. 8a is a voltage time diagram showing capacitor voltage versus time in the circuit of FIG. 8;

FIG. 9 illustrates the capacitor circuit of the present invention with a constant voltage input;

FIG. 9a represents the pulse train for actuating the switch of FIG. 9; and

FIG. 9b represents the capacitor output voltage from the circuit of FIG. 9.

Referring first to FIG. l, there is shown one portion of the computer circuit in the region denoted 10 or 10 in the above-mentioned copending application of Gilbert S. Stubbs and George I. Wachtell. Here the capacitor 10 simulates heat capacity of the heat exchanger or its ability to store heat. The resistances 11 and 12 simulate the flow impedance between the heat exchanger coolants and the center of the heat exchanger wall. Numbers 10, 11 and 12 in this drawing represent the corresponding parts of FIG. l of said application. The capacitor 13 simulates the heat capacity of the wall or its ability to store heat. In the copending application the resistances R1 and R2 simulate the heat flow impedance between the heat exchanger coolants and the center of the heat exchanger wall. In the present application, the resistors I4 and 15 in combination with switches 16 and 17 simulate variable heat tlow impedance in the same regions as R1 and R2. Another switching arrangement would permit variation of the effective capacitance of the heat exchanger coolants which in FIG. l of the copending application is represented by the fixed capacitors C1 and C2, here designatd 18 and 19. Switching elements 20 and 21 represent the switching by stepping relays described in the copending application. A typical wave form for the operation of switches 16 and 17 is shown in FIG. la.

FIG. 2 represents a basic circuit for simulation of a variable resistor. In this circuit a fixed resistor 23 is connected in series with switch 24 in a circuit across terminals 25 and 26. A voltage e, which is constant relative to the wave form shown in FIG. 2b, is impressed across terminals 25 and 26. A voltage pulse train es, shown in FIG. 2b, is impressed on switch 24 through suitable means 27 to effectuate alternate opening and closing of the switch. As a consequence, actual current in the form of the pulses, shown in FIG. 2c, ows through resistor 23. The response times of the circuits to which terminals 25 and 26 are connected are sufficiently large in comparison with the pulse repetition time To that the current pulses in FIG. 2c are effectively averaged. As a consequence of this averaging, the appearance to the circuit external of terminals 25 and 26 `is that of a fixed resistance whose value is greater than the actual resistance 23 and whose value is determined by the ratio of the pulse width T to the pulse repetition time T0.

Examples of switches which can be used in FIGS. 1 and 2 are shown in FIGS. 3 and 4. The operation of these switches is known to the art. These switches present the particular advantage that they enable the relatively rapid switching required to effectuate the small pulse repetition times required.

Referring specifically to FIG. 3, the switch is constructed primarily about a bridge composed of four rectifiers 30, 31, 32 and 33 connected at terminals 34, 35, 36 and 37. The terminals 34 and 35 are switching terminals whereas the terminal 36 is connected to the circuit element to be effected and the terminal 37 is preferably connected to ground or some low impedance voltage source. The diodes are arranged so that current can flow from terminal 34 through diodes 30 and 31 to terminals 37 and 36, respectively, and from terminals 36 and 37 through diodes 32 and 33 to terminal 35. When the voltages esl and esg are of the polarities indicated in FIG. 3 at terminals 38 and 39, the voltages across the diodes will be of the proper polarity to cause conduction. If the resistances of the diodes as Well as the resistances 4t) and 41 are equal, or nearly equal, the diode bridge will be essentially at a balanced condition, in which case the voltage at terminal 36 would be essentially equal to the voltage at terminal 37. The diode bridge thus in effect forms a short circuit between terminal 36 and the ground terminal 37. When the voltages esl and esz applied to terminals 3S and 39 are opposite to the polarities indicated in FIG. 3, the resulting voltages on the diodes will be of such polarities to inhibit conduction. Under this condition the effective resistances of the diodes are so high that terminal 36 may be considered as effectively disconnected from the signal esl voltages and ground. The effective state of the diode switch is open The wave forms shown in FIG. 3a and FIG. 3b illustrate possible switching signals which may be applied to terminals 38 and 39, respectively, in order to obtain periodic switching effects employed in this invention.

FIG. 4 illustrates another switching arrangement suitable for use in connection with the present invention employing a pair of vacuum triodes 43 and 44. The cathode of one tube is connected to the anode of the other in cach case at terminals 45 and 46. Terminal 46 is connected ground or to a low impedance voltage source, and terminal 45 is connected to the circuitry in which the switch is employed. The grids of the tubes are connected to leads 47 and 48 upon which are impressed similar switching signals esl. When these signals are sufficiently positive either of the tubes may conduct, and terminal 45 will effectively be connected to terminal 46 and be at essentially the same potential as terminal 46. When the signals impressed on the grid leads 47 and 48 are not sufficient to produce conduction, however, terminals 45 and 46 are essentially isolated from one another so that an open Circuit is effected.

In addition to electronic switches of the type described in FIGS. 3 and 4, it is possible also to employ mechanical type switches whose switching time may be more limited than the electronic switches but which may be employed in circuits whose relative response times are much greater.

FIG. 5 illustrates ya circuit in which a variable current source is simulated. In this circuit, a trigger oscillator 5f) is used to actuate a variable pulse width circuit 51. Each trigger pulse from circuit 50 causes circuit 51 to put out a Voltage pulse of Width T proportional to the input signal el shown in FIG. 5a. The repetition of pulses in the output of circuit 51 produces the pulse train wave-form shown in FIG. 5b. The wave-form in this instance has a maximum pulse voltage designated E0 which is large compared to the computing network voltages es and eb. The starting level of the pulses in e0 is well below the voltages ea and eb of the computing network 52 in order that the diodes 53 and 54 do not conduct in the absence of a pulse from the variable pulse width circuit. Voltage differences Eo--es and Ell-eb are impressed on the resistors Rs and Rb `respectively design- ated 55 and 56, during the time interval T of the voltage pulse. The voltages es and el, are small compared to the voltage E0 so that the currents fiowing in the pulse interval T are essentially independent of the computing network voltages. The wave-form of the current is flowing through resistor R,l is shown in FIG. 5c. This current which occurs in the form of pulses is effectively average by the computing network whose response times are large compared to the pulse repetition time T0.

The circuit of FIG. 5 is capable of generating only positive values of the currents il, and is. However, it is possible to construct a circuit using voltages of opposite polarity and diodes oppositely oriented to generate negative currents. It is also possible to construct a circuit having resistor-diode branches in which both positive and negative currents may be generated.

In passing, it may be observed that the circuit of FIG. 2 may alternatively be employed to simulate a variable current source. When this is done, terminal 25 may be considered as connected to a high voltage source and terminal 26 connected to a computing network. The high voltage source in this case would be constant rather than varying as e0 does in FIG. 5b.

Referring to FIG. 6, a circuit for simulation of a variable incremental capacitance is illustrated. In this case, capacitors 59 and 64I, connected in parallel, constitute the fixed capacitance Whose effective value is to be varied by switching technique using a single-pole double-throw switch 61 in conjunction with the D.C. amplifier 62. Otherwise capacitor 59 takes no part in the variable capacitance simulation except that its capacitance value is effectively added to that of capacitor 60.

When the constant current wave-form of FIG. 6a is impressed on terminal 63 and the switch position is varied in accordance with the pulse train of FIG. 6b, :a waveform corresponding to the step-like wave of FIG. 6c is generated. A constant current source and with the switch at position b, the wave-form is a constant-slope ramp as shown in FIG. 6c. The level positions of the step-like wave-form in FIG. 6c occur when the switch 61 is in position a. When the switch is in this position, the voltage at 63 is essentially clamped at the voltage level which existed at node 63 at the instant before the switch was changed to position a. During the time the switch is in position a, the voltage on capacitor 60 remains essentially at its initial value because the amplifier connected to the capacitor is designed to draw very little current at its input terminal. As a result of the unity gain of the amplifier, the voltage at terminal a of the switch is always essentially equal to the voltage on capacitor 60. The ramps in the step-like wave-form of FIG. 6c occur during the time intervals that switch 61 is connected to terminal b. During these time intervals, the voltage ec increases at a constant rate determined by the current i and the capacitors 59 and 60. In the case illustrated by FIG. 6, the effective capacitance when the switch is operated is higher than the sum of capacitors 59 and 60 by a factor T a Tz-T1 The variable capacitor circuit of FIG. 7 is essentially identical in its overall operation to that of FIG. 6. The essential feature of FIG. 7 is that it permits use of switching elements which can be connected only to a low impedance source or to ground. This variable capacitor circuit would be particularly well suited to the use of the switches described in FIGS. 3 and 4. The capacitor 65 plays a role similar to that of capacitor 59 in FIG. 6 in that it prevents the voltage at terminal 67 from varying when the switch 68 is opened. The capacitor 66 plays a role similar to that of capacitor 60 in the circuit of FIG. 6. This capacitor, in combination with capacitor 65, determines the total capacitance at terminal 67 when the switches are in the positions indicated in FIG. 7. Because of the requirement that the switches must be connected either to ground or to the low impedance output of an amplifier, the means for clamping the capacitor voltage employed in FIG. 7 must be different from the means employed in FIG. 6. The clamping circuit of FIG. 7 includes two D.C. amplifiers of unity gain, designated 70 and 71, two switches, ldesignated 72 and 73, and a capacitor, designated 74. The inputs of the amplifiers 70 and 71 lare 4designated as 7S and 76, and the outputs are designated respectively as 77 and 78. The clamping of the voltage at terminal 67 is initiated by opening switches 68 and 72 and simultaneously closing of switch 73. This switching action connects the output of amplifier 71 to terminal 67, effectively disconnects capacitor 66 so that it will not charge, and disconnects capacitor 74 from the output of amplifier 77. At the instant of switching the Voltage of capacitor 74 is equal to the voltage of terminal 67. This capacitor voltage working through amplifier 67 and switch 73 effectively clamps the voltage at terminal 67. The switching is effected as indicated by the pulse trains shown in FIGS. 7a and 7b.

Referring to FIG. 8, the circuit shown is a conventional circuit employing a constant voltage source 80 to charge a capacitor 81 through a resistor 82 when switch 83 is closed. The wave-form which appears at terminals 84 and 85 of capacitor 81 as the charge on the capacitor increases is shown in FIG. 8a.

The circuit of FIG. 9 is essentially the same as the circuit of FIG. 6 except that the capacitor 81 is replaced by a variable capacitor circuit whose effective capacitance is equal to that of capacitor 81. The effective capacitor circuit corresponds to the circuit of FIG. 6 and similar number designators are employed, with the addition of primes thereto, to indicate similar parts. Likewise, the circuit elements corresponding to those of FIG. 8 are designed with similar designators, with the addition of primes thereto. The pulse train of FIG. 9a represents the switching operation of switch 61', an the wave-form of FIG. 9b represents the voltage at the terminals of the variable capacitor circuit 81.

The specific embodiments of the present invention have been described in some detail and modifications thereof have been suggested. Additional modifications and similar circuits will occur to those skilled in the art. All such modifications and variations within the scope of the claims are intended to be within the scope and spirit of the present invention.

I claim:

1. An analog computer having a circuit branch representing a variable resistance, said circuit branch comprising a circuit for simulating a resistance of variable size consisting of at least one fixed resistor in series with a switch across a potential difference in the computer circuitry, said branch being analogous to representation by a fixed resistor of a specific larger resistance value in a specific application, means for sensing variation in some preselected variable parameter elsewhere in the computer and means responsive to the sensed variations for oper ating the switch so that it opens and closes to interrupt the flow of current through the resistor circuit branch and thereby presents a flow of current pulses to the computer such that the average current through the resistor is the same as it would be if the resistance itself had been varied to simulate a resistor of larger size.

2. An analog computer comprising a receiving network and a circuit branch for simulating a variable current source comprising a variable pulse-train-modulated high voltage source connected in a series circuit with a rectifier and resistor to the receiving network and means responsive to a variable parameter elsewhere in the cornputer and operable on the variable high voltage source to produce variations therein proportional to said variable.

3. The computer of claim 2 in which the interruption of current is achieved by changing the level of the output of the high voltage source such that the network voltage across the series resistor and diode combination is of such a polarity that the diode would prevent current iiow.

4. A variable capacitance simulating circuit for use in connection with a circuit requiring a variable capacitance comprising a unity gain amplifier, a fixed Value capacitor, a single pole double throw switch and a voltage source interconnected so that the pole of the switch is alternately connected to the capacitor terminal and the amplifier output terminal, and the capacitor and the amplifier are connected together.

5. A variable capacitance simulating circuit for use in connection with a circuit requiring a variable capacitance comprising a fixed value capacitor, a switch between the capacitor and ground, two unity gain amplifiers, and amplifier whose input is connected to the capacitor and whose output is connected through a switch to a second fixed value capacitor, a second amplifier whose input terminal is connected to the second capacitor and whose output terminal is connected through a switch to the first fixed value capacitor, wherein when the circuit is in one state the first two switches are closed and the third is open and the pulse train changes the switches from this state to a second state where the first two switches are open and the third switch is closed.

References Cited by the Examiner UNITED STATES PATENTS 2,473,414 6/ 49 Darlington 320-1 2,775,715 12/56 Tuttle 307-96 2,812,413 1l/57 Brown 307-96 LLOYD MCCOLLUM, Primary Examiner.

MILTON O. HIRSHFIELD, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent N6i 3,206,621 september 14, 1965 Gilbert S. Stubbs It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 37, for "average" read averaged column 4, line 69, for "positions" read portions column 5, line 69, for "an" read and column 6, line 5l, for "and", second occurrence, read one --c Signed and sealed this 24th day of May 1966.

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J. BRENNER Commissioner of Patents

Claims (1)

1. AN ANALOG COMPUTER HAVING A CIRCUIT BRANCH REPRESENSTING A VARIABLE RESISTANCE, SAID CIRCUIT BRANCH COMPRISING A CIRCUIT FOR SIMULATING A RESISTANCE OF VARIABLE SIZE CONSISTING OF AT LEAST ONE FIXED RESISTOR IN SERIES WITH A SWITCH ACROSS A POTENTIAL DIFFERENCE IN THE COMPUTER CIRCUITRY, SAID BRANCH BEING ANALOGOUS TO REPRESENTATION BY A FIXED RESISTOR OF A SPECIFIC LARGER RESISTANCE VALUE IN A SPECIFIC APPLICATION, MEANS FOR SENSING VARIATION IN SOME PRESELECTED VARIABLE PARAMETER ELSEWHERE IN THE COMPUTER AND MEANS RESPONSIVE TO THE SENSED VARIATIONS FOR OPERATING THE SWITCH SO THAT IT OPENS AND CLOSES TO INTERRUPT THE FLOW OF CURRENT THROUGH THE RESISTOR CIRCUIT BRANCH AND THEREBY PRESENTS A FLOW OF CURRENT PULSES TO THE COMPUTER SUCH THAT THE AVERAGE CURRENT THROUGH THE RESISTOR IS THE SAME AS IT WOULD BE IF THE RESISTANCE ITSELF HAD BEEN VARIED TO SIMULATE A RESISTOR OF LARGER SIZE.

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