CAPACITOR MONITORING CIRCUIT

Abstract: The capacitor monitoring circuit for measuring for example humidity, comprises a voltage driving circuit (22) providing two varying voltages (20, 21) of opposite phase, a first measuring circuit supplied with one of the voltages and including a first resistance (Ra) and a fixed capacitor (Cf), and a second measur­ ing circuit supplied with the other of the voltages and including a second resistance (Rb) and a variable, humidity sensitive, capacitor (Ch). The resistances (Ra, Rb) are of generally the same magnitude and the capacitors (Cf) are of the different capacitive values. An adjustable resistance (Rx) is connected to one of the measuring circuits to provide null calibration at output (25) to a load (31), such as a meter.

Description: In capacitor monitoring or measuring types of circuits it has been common to use capacitor bridges typically having four legs. A four-legged type of bridge normally is fed with power across two legs and an output is differentially measured across the other two legs. In this type of a bridge, particularly one that utilizes capacitive elements in the bridge circuit, the nulling of the bridge or the measuring of a capacitive element causes an interaction which in turn normally requires trimming or adjustment of more than one element. A capacitance measuring system is known from GB2058364 in which oppositely phased pulses are fed to the unknown capacitor and reference capacitor means, the outputs of which are combined. The reference capacitor means are two capacitors, equal to the maximum and minimum possible values of the unknown capacitor, and one or the other is selected on each pulse to achieve a long-term null output. The cycle number selection ratio gives the unknown capacitance. US-A 3 135 916 discloses a capacitor monitoring circuit having two RC branches driven in phase from a common pulse source whereby the difference between the voltages on the capacitors is the output signal. It is an aim of the present invention to provide a simple sensing circuit which has a single output relative to ground rather than the usual outputs from a four-legged type of bridge. According to the invention, there is provided a capacitor value measuring circuit comprising a voltage driving means providing first and second varying voltages of opposite phase; a first measuring circuit receiving the first voltage and including a capacitor whose value is fixed and different from the value of the capacitor whose value is to be measured; a second measuring circuit receiving the second voltage and including the capacitor whose value is to be measured, said first and second measuring circuits being joined to provide an output of the capacitor value measuring circuit, said capacitor value measuring circuit characterized by a first resistance in the first measuring circuit through which passes current of the first voltage from the driving means for the capacitor whose value is fixed and in the second measuring circuit a second resistance whose value is substantially the same as that of the first resistance and through which passes current of the second voltage from the driving means for the capacitor whose value is to be measured, and an adjustable resistance connected between ground and the connection between the resistor and the capacitor in the measuring circuit including the capacitor of larger value, and having a value at which the signal at the output is nulled, said output of the capacitor value measuring circuit being at a common node between said capacitors. In this way, the present invention is distinguished over e.g. US-A-3135916 by providing that the branches are driven in anti-phase, the differencing being simplified and the output voltage being referred easily to ground. An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, which is a circuit of a humidity responsive circuit according to the present invention. Referring to the drawing, the humidity responsive circuit includes a signal generator 10 providing a square wave output. The output of the voltage generator 10 is provided on a conductor 11 to a NOT gate 12 where the square wave in turn is inverted and supplied to a conductor 13. The conductor 13 separates at 14 into two voltage paths at conductors 15 and 16. The conductor 15 is connected through a NOT gate 17 to a conductor 18. The conductors 16 and 18 provide two varying voltages that are opposite phases of a single wave form as indicated by wave forms 20 and 21. The output conductor 18 is connected to a resistor Ra, while the conductor 16 is connected to a resistor Rb. For reasons that will become apparent later on, the resistor Ra and Rb are of the same magnitude. The resistor Ra is connected at 23 to a variable resistor Rx. The variable resistor Rx is connected to ground at 24 and can be any type of variable resistance element ranging from a potentiometer to a resistor that is trimmed in a hybrid circuit application. The junction 23 is further connected to a capacitor Cf which is a fixed capacitor having a very low dissipation factor thereby appearing as substantially a pure capacitance with negligible resistance. The capacitor Cf in turn is connected at a common point 25 to a second capacitor Ch which has been shown as a variable capacitor. The capacitor Ch is a condition responsive capacitor, such as a humidity responsive polymide type of capacitor. The capacitor Ch could be any type of variable capacitor that varies in response to a condition (other than humidity) or could be manually adjusted, if called for by the circuit application. The capacitor Ch in turn is connected to the resistor Rb. The common junction 25 forms an output for the circuit disclosed, and is connected at terminal 30 to a load that is shown at 31 as a resistance type of load (such as a meter) connected to a further terminal 32 which in turn is connected to ground 24. The adjustable resistance Rx is adjusted in such a manner as to be capable of nulling the voltage at the circuit output 25. For a low cost circuit, it is easiest to drive the bridge elements from signal outputs that are being driven to saturation by potential values near the power supply values supplied by the signal generator 10. With a regulated power supply, the output to the bridge elements, Ra, Rb, Cf, and Ch are then essentially regulated. In order for a single output to null relative to ground, the two legs must be driven with opposite phases of a signal, such as the square wave signals 20 and 21. This is accomplished readily from a single voltage generator 10 by the use of the NOT gate or inverter amplifier 17 in the top leg of the circuit. In order to understand how the null is obtained through the adjustment of resistor Rx, the following considerations and assumptions are made. It is assumed that capacitor Cf is a fixed, stable capacitor which is in the order of 50 percent larger than the variable capacitive humidity element Ch. If this is the case, we can define a parameter A equals Cf/Ch where A will be approximately 1.5. Now, as the voltage at 18 goes to a value of E (the power supply voltage), the value of voltage at 16 goes to 0. When this happens, the instantaneous voltage change across the resistance element Ra of the top leg must equal that of the bottom leg, since the magnitude change of the voltages at 16 and 18 are equal. For this reason, and the fact that the instantaneous current change in Ra equals Rb, resistance Rb of the bottom leg is designed equal to Ra of the top leg. This selection can reasonably be made to provide Ra sufficiently close to Rb for all practical purposes without the need for calibrations on their values. With Ra equalling Rb, the instantaneous current change in Ra equals Rb at the time the driving voltage switches because the voltage cannot change instantly across the series connected capacitors Cf and Ch. Since the instantaneous change in current through Ra all flows through Cf, no instantaneous change in current occurs in Rx to add an instantaneous voltage change to Cf of the top leg. Because of this there is not an instantaneous voltage change at terminal 30 when the driving voltage switches. At null, it is also necessary that the time constant of the impedance driving capacitor Cf is equal to that of the capacitor Ch since the currents in Cf and Ch must remain equal without causing a voltage change at terminal 30. In order for these time constants to be equal, the resistor Rx must be selected. The equivalent series resistance driving capacitor Cf is Rx in parallel with Ra or RxRa/Rx+Ra. For the two time constants to be equal, Rx·Ra/Rx+Ra times Cf must equal Rb·Ch. Now, since Rb already equals Ra, and Cf equals (A) Ch then Rx·Ra/Rx+Ra times A·Ch must equal Ra·Ch. By substitution Rx·A equals Rx + Ra; or Rx equals Ra/A-1. With this circuit then, a capacitor Cf is applied which is some factor larger than Ch, but not of a precise calibrated value. Calibration is then obtained by selecting Rx equals Ra/A-1 where A is the factor of the capacitor Cf relative to the capacitor Ch at the point we desire null. The resistor Rx can be determined by observing the output between the capacitor Cf and Ch at point 25 in the circuit while the adjustment is being made. The present circuit can be used by measuring or amplifying the off null output voltage at terminal 30 with respect to terminal 32 (ground 24) as humidity changes the circuit from the null condition. Or, the present circuit can be used by calibrating a potentiometer making up Rx that defines null points for the entire humidity range. The present invention discloses a very simple circuit that is capable of being used with a capacitive sensing element in which a null can be created by the simple adjustment of a single resistive element. This arrangement highly simplifies the design of a sensing circuit and provides for a very inexpensive circuit that can be readily adapted to condition control applications.

Claims: A capacitor value measuring circuit comprising a voltage driving means (22) providing first and second varying voltages of opposite phase; a first measuring circuit (15, Ra, Cf) receiving the first voltage and including a capacitor (Cf) whose value is fixed and different from the value of the capacitor (Ch) whose value is to be measured; a second measuring circuit (16, Rb, Ch) receiving the second voltage and including the capacitor (Ch) whose value is to be measured, said first and second measuring circuits being joined (at 25) to provide an output of the capacitor value measuring circuit, said capacitor value measuring circuit characterized by a first resistance (Ra) in the first measuring circuit through which passes current of the first voltage from the driving means for the capacitor (Cf) whose value is fixed and in the second measuring circuit a second resistance (Rb) whose value is substantially the same as the first resistance (Ra) and through which passes current of the second voltage from the driving means for the capacitor (Ch) whose value is to be measured, and an adjustable resistance (Rx) connected between ground and the connection between the resistor and the capacitor in the measuring circuit including the capacitor of larger value, and having a value at which the signal at the output (25) is nulled, said output of the capacitor value measuring circuit being at a common node between said capacitors (Cf, Ch). A circuit as described in Claim 1, characterised in that said variable capacitor is a condition responsive capacitor preferably having a low dissipation factor. A circuit as described in Claim 1 or 2, characterised by said two varying voltages being square waves. A circuit as described in any one of the preceding Claims, characterised by said voltage driving means including a single wave form generator having two output circuits with a NOT gate in one of them to provide said varying voltages. A circuit as described in any one of the preceding Claims, characterised by said fixed capacitor being of the order of 50 percent larger than said variable capacitor.

Inventors: KOMPELIEN, ARLON D.
Applicants: HONEYWELL INC.
Application year: 1983
Main classification: G01R27
IPC codes: G01R27, G01N27