FI84666C - MAETKOPPLING FOER SMAO KAPASITANSER. - Google Patents

Description

5,84666

Low capacitance stripping connection Mätkoppling för smä kapasitanser

The invention relates to a circuit for measuring small capacitances, which circuit comprises an oscillator, such as an LC or RC oscillator, the output frequency of which is a function of the capacitance to be measured connected between the input terminal of the oscillator and the capacitance of the oscillator to be measured.

Several different oscillator circuits with a LC or RC circuit as a frequency-affecting part are already known. These oscillator-15 circuits are known to be used for capacitance measurement, because the oscillator frequency can be determined using the calculation formulas of each oscillator circuit when the values of the other components of the circuit are known.

20 Various LC and RC oscillators are also known, the second pole of the capacitance C being connected to ground. However, in the measurement of small capacitances, the disadvantage and problem of the latter connections is that different uncontrolled stray capacitances are connected in parallel with said capacitance C of the measurement oscillator, whereby the measurement accuracy suffers. These problems are greater the smaller the capacitances being measured.

Measurement of small capacitances is quite common in various sensor technology and telemetry, e.g. in radiosondes, where different capacitive sensors with relatively small capacitance are used to measure 30 meteorological quantities, typically in the range of 1-100 pF. For these known measuring methods and connections, reference is made to the following patents of the applicant: US-4,295,090, US-4,295,091, FI-54664 and FI-57319.

35 2 84666 1 The object of the present invention is to avoid the above-mentioned drawbacks and to provide a measuring oscillator connection which is very suitable for measuring small capacitances and also for measuring arrangements using a changeover switch which alternately connects different sensor capacitances and reference capacitances in the measurement sequence.

It is an object of the present invention to provide a measuring oscillator circuit by means of which small capacitances, typically of the order of 0 to 100 pF, can be measured more accurately without the interfering effect of stray capacitances and the like.

It is a particular object of the present invention to provide a measuring circuit suitable for use in radiosondes in connection with capacitive sensors for measuring meteorological parameters.

In order to achieve the above and later objects, the invention is mainly characterized in that the other measurable capacitances with respect to the capacitance to be measured between the oscillator input terminal and ground are connected to a common line for the capacitance and that said voltage follower is an operational amplifier, the output of which is connected to its inverting input, to which the non-measured capacitances 30 are connected so that no voltage differences and thus no charging currents occur at their terminals.

When the input of a voltage follower, preferably the input of an operational amplifier, is connected according to the invention to the poles of capacitances which are not under measurement, the opposite poles of these capacitances will be at the same potential, so that there are no charging currents of these capacitances Various stray capacitances and DC leakage currents may occur in the circuit within the framework of an operational amplifier acting as a voltage follower or the like.

In the application of the invention, stray capacitances may also occur on the side of the common line between the capacitances to be measured and the input of the measurement oscillator. The effect of these stray capacitances can be eliminated by using a shield in this line, which is connected to the output of the voltage follower. The principles set out in the applicant's FI patent No. 57319 may otherwise be used in said protection.

The circuit according to the invention is generally most advantageous when the measuring oscillator is a sine oscillator, because then the speed requirement of the voltage follower is the lowest. The amplifier of the voltage follower must be sufficiently amplified, because otherwise the amplitude will be smaller in the output than it is in the input, and then the stray capacitances will start to adversely affect the measurement accuracy.

In the following, the invention will be described in detail with reference to some application examples of the invention shown in the figures of the accompanying drawing, to the details of which the invention is in no way narrowly limited.

Figure 1 shows a prior art oscillator circuit 25 according to the invention.

Figure 2 shows a measuring oscillator circuit according to the invention for measuring three small capacitances, which are alternately connected to the input of the measuring oscillator.

30

Figure 1 shows an RC oscillator known per se using a hysteresis inverter 10. For the measurement of small capacitances and probe telemetry of this circuit and its application, reference is made to Applicant's FI Patents Nos. 54664 and 57319 and U.S. Pat. Nos. 4,295,090 and 4,295,091. At the input terminal 16 of the oscillator according to Fig. 1 and thus at the C-terminals of the capacitor, a voltage Ue (t) acts on the ground, the waveform of which is drawn in connection with Fig. 1.

«84666 1 The curve of the voltage UQ (t) consists of the discharge and charge curves of the capacitor C and the stray capacitance C connected in parallel.

B

for parts with time constant t = 1 / R (C + C „). The oscillator

o H

the rectangular wave U ^ (t) is obtained from the output terminals 18 of the circuit, the frequency 5 f 1 / T of which is a function of the capacitance C + C connected between the input terminal 16 of the oscillator circuit and ground.

B

As can be seen from the above, the stray capacitances directly affect the output frequency f1 of the oscillator circuit, which spoils the measurement accuracy of small capacitance dances. These problems are further exacerbated if a previously known changeover switch is placed on the input side of the circuit according to Fig. 1, with which different capacitive sensors and reference capacitances are connected in turn to a measuring oscillator whose output frequency is a function of the capacitance to be measured f = F (C.,).

M o M

15

Figure 2 shows an example of a measuring circuit according to the invention, with which the above-mentioned problems are solved.

The circuit according to Figure 2 comprises capacitances C 1, C 2 and C 2, of which 20 e.g. C 1 is a well-known reference capacitance and and capacitances to be measured e.g. sensors measuring radlosond pressure (F), temperature (T) and / tal relative humidity (U) . The poles of the capacitances C1, C2 and the other poles are connected together and together on the line 19 further to the input terminal 16 of the inverter 10. The opposite poles of the capacitances ^ ^> ^ 2 and ^ 3 25 are connected to the switches 11,12 and 13. Said switches 11,12 and 13 are two positions and in their second position the capacitances 3a ^ 3 are connected to the ground via the contact 15 and in the second position of the switches 11, 12 and 13 the capacitances C 1, C 2 and connected to a conductor 21 or a contact connected to the output terminal of the voltage-following 30 amplifier. The input + terminal of the voltage follower 20 is connected to the input terminal 16 of the inverter 10.

According to the connection method of the voltage follower known per se, the output of the operational amplifier 20 is connected to its input terminal, which is described by the conductor 22. 35 The characteristics of the voltage follower and its operational amplifier 20 or other similar component include that the input of the voltage follower 5 8 4 6 6 6 1 ® la output resistor R * 0 Ideally.

out

Then the output voltage of the voltage follower is IJ ^ = U.

According to Fig. 2, the capacitance Cj is connected between the ground and the inverter terminal 16 of the inverter 10 g, whereby the output frequency Fq F (Cj ^) of the measuring oscillator. The capacitances C 1 and are connected via conductors 1A and 21 to the output of the voltage follower 20, where the voltage = Uq prevails. Thus, there is no voltage difference between the capacitances and the terminals, so no charge currents occur.

10

Switches 11, 12 and 13 are controlled by the control unit 17 so that each capacitance and the other pole are in turn connected to ground and thus affect the input of the measuring oscillator, at which point the other capacitances are connected between the input + terminal output of the voltage follower 20.

15

Scattered capacitance can also occur on the common line 19 of the capacitances to be measured. The effects of these stray capacitances can be prevented by providing the wire 19 with a shield 19a which is connected to the output of the voltage follower by a conductor 23.

20

The invention has been described above only in an application using an RC oscillator. However, within the scope of the invention, any other suitable oscillator circuit known per se can be used, e.g. an LC oscillator, which is advantageous due to the good sine wave it provides.

In the following, the claims are set out, within the scope of the inventive idea defined by which the various details may vary and differ from those set forth above by way of example only.

30 35

Claims (8)

A coupling for measuring smA capacitances (C1, C2, C3), which includes an oscillator such as an LC or RC oscillator whose output frequency (F0) is a function of the capacitance to be measured and coupled between the oscillator input coil (16) and the earth, and alternately coupling capacitances (C1, C2, Cj) to the oscillator input coil (16), characterized in that one pole of the other capacitances to be measured in -containing to the capacitance to be measured and connected between the input coil (16) of the oscillator and the earth connected to a common line (19) for the capacitances (C the input of said voltage follower is coupled to the input coil (16) of the oscillator coupling, and said voltage follower is an operational amplifier (20) whose output is coupled to its inverting input to which the acitances (C2, C3) that are not subjected to measurement are coupled so that voltage differences do not appear in their poles and hence no charge currents. 25 2. Measurement coupling according to claim 1, characterized in that one of the capacitance electrode (C) to be measured is connected for the time of measurement to the earth (15) by means of a coupling (11). 30 3. Measuring circuit according to claim 1 or 2, characterized in that an inverting amplifier (10) with hysteresis is used in the measuring oscillator coupling, which amplifier is coupled back to a resistor (R). 35 4. Measurement coupling according to claim 1 or 2, characterized in that the measuring oscillator is a sine oscillator. 9 84666 Measurement coupling according to any of claims 1-4, characterized in that the common line ¢ 19) for the capacitor dances (C1, C2, Cj) is provided with a cover (19a), e.g. a protective jacket coupled to the output of the voltage sequencer (20). Food coupling according to any of claims 1-5, characterized in that the food coupling comprises two or more capacitances (C1, C2, C3) to be measured, at least one of which is a reference capacitance and other capacitances which shall be measured. Cap coupling according to claim 6, characterized in that the capacitances to be measured are a radio probe of capacitive sensors measuring meteorological quantities such as pressure (P), temperature (T) and / or relative humidity (U). 8. Measurement coupling according to any of claims 1-7, characterized in that the capacitances to be measured are of the class 0-100 pF. 20