US 3555263 A Abstract available in Claims available in Description (OCR text may contain errors) United States Patent lnventor Jean C. Lejon Paris, France Appl. NO. 633,772 Filed Apr. 26, 1967 Patented Jan. 12, 1971 Assignee Controle Bailey (Societe Anonyme) a company of France Priority Aug. 4, 1966 France 72,046 SOLID-STATE ANALOG COMPUTING DEVICE FOR CONTROLLING A PHOTO-RESISTOR IN NON-LINEAR RELATIONSHIP TO INPUT 3 Claims, 6 Drawing Figs. US. Cl 235/ 194, 235/ 195 Int. Cl G06g 7/ 16 Primary Examiner-Eugene G. 8012 Assistant Examiner-Joseph F. Ruggiero Attorney-John Fv Luhrs ABSTRACT: A solid-state device for generating an impedance varying in functional relation to a variable input signal. VARIABLE VOLTAGE SOLID-STATE ANALOG COMPUTING DEVICE FOR CONTROLLING A PHOTO-RESISTOR IN NON-LINEAR RELATIONSHIP TO INPUT BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is in the field of analogue computers of the electrical type. 2. Description of the Prior Art Some presently available analogue computers incorporate a servomotor adjusting an impedance such as a resistor or movable core transformer, others incorporate a photoresistor, the illumination of which is caused to vary through a luminous source responsive to a controlling voltage. The servomotor, a mechanical element found in the first type, is a source of trouble. Computers of the second type have been unsatisfactory because of their dependence on ambient temperature, nonlinearity and long-term instability. SUMMARY OF THE INVENTION A device generating an output signal or signals varying in functional relation to an input signal comprising a feedback amplifier, a light flux transmitter fed by said amplifier and driving a photoresistor cell formed by two or more substantially identical photoresistors, the first of which generates a signal which is subtracted from the input signal to generate an error signal driving the feedback amplifier and the others of which provides the output signal or signals. The main object of the present invention is to overcome the difficulties indicated in the prior art discussion and to provide a simple and therefore inexpensive solid-state analogue computing device enabling an impedance to continually take a value varying in functional relation to a variable electric input signal, the impedance being uncoupled from the circuit to which is applied the variable electric input signal. FIG. 1 represents diagrammatically a device according to the invention enabling a photoresistor to take a value which is a linear function of an input voltage; FIG. 2 represents diagrammatically a device according to the invention enabling a photoresistor to take a value which is a hyperbolic function of an input voltage; FIG. 3 represents diagrammatically a device according to the invention enabling a photoresistor to take a value which is a function other than the previous ones, of an input voltage; and FIGS. 4, 5 and 6 are general diagrams of photoresistive cells comprising at least two photoresistors mounted on a common support, which can be used in the device according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS I The simplest device according to the invention will be explained with reference to FIG. 1. Across the input terminals 10 of an amplifier 3 there is applied an error voltage Ve which is the difference between a signal voltage V, and a negative feedback voltage V',. Voltage V, is the voltage at the terminals of a controlled photoresistor cell 1 fed by the constant current I,, originating from a current source 4. The amplifier 3 with a high gain A delivers, at its output, a voltage V, which feeds a light source 5. Source 5 illuminates cell 1. The variable luminous flux emitted by source 5 determines the present ohmic resistance R, of said photoresistor cell 1. Adjacent to cell 1 is a second photoresistor cell 2 which is also illuminated by source 5 and is selected in such a manner that it is as identical as possible with the first and is arranged like the first with respect to the light source. Cell 2 assumes a value R very near R,. Under these conditions: B being a relatively complicated function of V, and depending both on the law of variation of the luminous flux as a function of V, and on the ohmic values R, and R, as a function of the common illumination of the two photoresistive cells. However this may be, by eliminating V, and V, from the above equations, the following equation is obtained: once B Al,, is sufficiently large with respect to unity. In this manner, therefore, a variable photoresistor 2 having a value R is obtained, the ohmic value of which remains proportional to the value of a voltage V,, which may be used in numerous ways for analogue calculations based on the quantity V,. Let it be assumed that the previous device is modified slightly so as to bring it into accordance with that of FIG. 2 which only differs therefrom in the following respects. The photoresistive cells 1 and 2 which are substantially identical and are illuminated by the same light source in identical conditions and therefore have ohmic values R, and R substantially equal are serially connected respectively with fixed resistors 6 and 7 having values R and R substantially equal with each other to form two divider bridges l6 and 2-7. These bridges are fed from two sources of voltage, l6 by current source 8 of constant voltage V, and 27 by current source 9 of variable voltage V,. The negative feedback voltage V, is taken off at the terminals of resistor 6 and the output voltage V ut at the terminals 11 of resistor 7. The equations (1), (2) and (3) given above remain true. The equation (4) should be replaced by the following: The control has the effect of causing V, to tend constantly towards V, and R, and R are substantially equal so that it is possible to write: whereby R is a hyperbolic function of V,. On the other hand, the identity between the two divider bridges leads to: 1 on-7a i (8) which shows that the device functions as a multiplier of the two variable voltages V, and V,. Referring now to FIG. 3, it is easy to generalize the above in the following manner. It is assumed that the feedback chain includes a circuit 14 enabling the monotonous function: to be generated. Then the resistor R, has a value which is a function g (V,), 3 being the inverse function off, i.e. being obtained by solving equation (9) with respect to R,. The functions f and g are interrelated by the identities: In the case of Figure 1 VI z 1'=f( 1) o 1 whose inverse function is: Since the inverse function of a linear function is a liner linear function, R is a linear function of V,. In the case of Figure 2 whose inverse function is Vo- V, 12 R V! Since the inverse function of a homographic function is a homographic function, R, is a homographic function of V,. in the case of FIG. 3 if circuit (14) is an exponential circuit such that 1 z 1'= o (Rt/ then the inverse function is: 1 0 1 g (VD/V1) Since the inverse function of an exponential function is a logarithmic function, R is a logarithmic function of V,. Whatever the type of function realized, it is of prime importance that the two photoresistor cells 1 and 2 or the assembly of these photoresistor cells should initially be as identical as possible and remain so whatever their variations, even in the long term. The two photoresistors are mounted side by side on a common support 12. It results that the temperature gradient between the different resistors is constantly zero. The degree of symmetry in the resistors may be very high because they are made of one and the same material, for example of cadmium sulfide or selenide. Moreover the two resistors may be not identical but merely physically similar, that is to say have a constant relationship between their ohmic values for all illuminations, for example as a result of the fact that the key-patterns in which the sensitive layers are cut out are not of the same pitch (see F IG. 5), in which case the photoresistive and temperature coefficients are very slightly modified and although the degree of symmetry becomes less it remains sufficient. This possibility enables isolated controlled resistances to be obtained reaching several megohms for quite small illuminations and permitting, in particular, the direct control of the time-constantcell of an integrator. Finally, the resistors may number more than two, namely one resistor comprised in the feedback loop and the others (see FIG 6) uncoupled from one another and from the first, and capable of representing as many analogue quantities. Various changes may be made in the details of construction without departing from the spirit and scope of the invention as defined by the appended claims. 1. A device for effecting variations in the resistance of a photoresistor in predetermined nonlinear functional vrelationship to an analogue input signal, comprising: an amplifier having a pair of input terminals between which the analogue input signal is applied; a light source connected to the output of said amplifier; a photoconductive cell illuminated by said light source including a first photoresistor and a second photoresistor, the resistance of each of said photoresistors varying in substantially linear relationship with the intensity of illumination by said light source, said photoresistors being equally illuminated by said light source; circuit means including said first variable photoresistor to develop a nonlinear feedback signal, said feedback signal being applied between the input terminals of said amplifier in series opposing relationship with respect to the analogue input signal to form an error signal across the input terminals of said amplifier to control the intensity of said light source and thereby the resistance of said second photoresistor in predetermined nonlinear relationship with said input signaL 2. A device according to claim 1 wherein said first photoresistor has a variable resistance R, according to the equation R, g(V,'), said nonlinear feedback signal V, varies according to the equation V, =f(R,) and said second photoresistor has a variable resistance R according to equation R g(V,), where V, is said analogue input signal and g is the inverse function off v 3. A device according to claim 2 wherein said circuit means for electrically energizing said first variable photoresistor develops an exponential feedback signal V,'. Patent Citations
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