|Title of Invention||
CAPACITANCE TRANSDUCER CIRCUIT
|Abstract||"CAPACITANCE TRANSDUCER CIRCUIT" ABSTRACT There is disclosed a capacitance transducer circuit comprising an oscillator having its outputs connected to a first and a second combinations, each said combination comprising a sensor capacitor, a measuring inductor shunted by a resistance, and each said combination being connected to the input of respective envelope detector, which in turn are connected to low pass filters, whereby the difference between the outputs of the said low pass filters represents the magnitude and variation of physical quantity, which is being measured.|
The present invention relates to a capacitance transducer circuit which may be used in any of the capacitance based transducers, for example in sensing the size of yarn or roving : in spinning mill.
Capacitance Transducers are characterised by low signal levels, high source impedance, easy contamination of signal by extraneous influences and associated difficulties of processing the output and extracting the useful component of the signal.
In a widely used circuit using capacitance transducers, the signal to be measured (it can be displacement, change in the dielectric medium, or change in any of the physical properties associated with the dielectric medium like humidity), manifests itself, as a change in capacitances in differential mode. A centre-tapped transformer together with the capacitance pair forms a ratio arm-bridge. The output of the bridge is a measure of the change in the differential capacitance. It is demodulated by multiplying with a reference signal and filtering it. The circuit is in wide use for the past few decades. But, the circuit is cumbersome, costly and difficult to adjust. Further it requires a well-matched differential capacitance pair at the sensor stage itself.
In an alternative and less commonly used circuit, the oscillator output is given to an inductance-capacitance circuit where the capacitor is a sensor capacitor and the inductor is a. measuring inductor. The output across the
inductor is put through an envelope detector. In the absence of any signal, the standing zero signal level value of the capacitance itself causes a steady output. This output is nullified by generating an almost equivalent output from the oscillator itself. This circuit is comparatively simpler; however, it is prone to be affected by a variety of factors. Change in oscillator frequency due to change in power supply or other reasons will affect the operation.
Change in the voltage drop across the inductor is used as a measure of the physical variable being measured. Such use of inductor makes the circuit more sensitive than the use of a resistor. However, change in oscillator parameters affect the performance more adversely here.
The circuit parameters of the oscillator, supply voltage change, or change in environmental conditions, can affect the oscillator frequency as well as amplitude. All these factors in turn affect the sensor circuit and lead to erroneous output. Providing the null output for zero signal condition is difficult. Here consistency on a unit-by-unit basis cannot be ensured easily. Further, often a number of sensors are used at physically close locations and supplied from the same main power supply. In such cases also, the sensors interfere with each other making it impossible to sift out individual signals - and ensure a satisfactory level of fidelity.
The main objective of the present invention is to provide a circuit for capacitance transducers of low capacitance levels such as of the order of IpF.
Another objective of the invention is to have a capacitance transducer circuit by which location, change in mounting etc. produce least effect.
Yet another objective of the invention is to provide a capacitance transducer circuit least affected by temperature, humidity and similar environmental conditions.
A further objective of the invention is to have a capacitance transducer circuit with very low drift and error due to drift in the oscillator frequency, drift in the oscillator amplitude and drift in power supply.
A yet further objective of the present invention is to provide a circuit which will work with a high level of fidelity when operating in a multi-sensor environment.
The invention has a further objective to discriminate between capacitance change representing the signal to be * sensed and drift due to extraneous reasons.
The invention has a still further objective to have a capacitance transducer circuit which will give output from zero frequency (representing very very slow changes in the signal) to a frequency level decided by the end application.
The present invention accordingly provides a capacitance
transducer circuit comprising an oscillator having its
/combination outputs connected to a first/and a second combination, each
said combination comprising a sensor capacitor connected in
series with a measuring inductor shunted by a resistance,
said inductors being of equal value, said resistances being
of equal value, and each said combination being connected to
the input of a respective envelope detector, the said
envelope detectors being connected to respective low pass
filters, whereby the difference between the outputs of the
said low pass filters forms the output voltage signal Vo, the
said output voltage signal Vo representing the physical
quantity whose magnitude and variations manifest as
corresponding magnitude and variations of Vo.
The envelope detector is preferably a diode RC type of
The invention will now be described with reference to
the accompanying drawings, wherein -
Fig. 1 shows circuit diagram of a capacitance transducer
circuit of prior art; Fig. 2 shows circuit diagram of another capacitance
transducer circuit of prior art; Fig. 3 shows circuit diagram of one embodiment of the
capacitance transducer circuit of the present
Fig. 4 shows a preferred embodiment of envelope detectors
connected to low pass filters of the circuit of the
Fig. 1 shows a circuit using capacitance transducers
well known in the prior art. In this circuit, an oscillator
A has an input connected to main power supply P and an output
connected to a centre-tapped transformer T. Capacitor CI and
C2 are connected in series with the transformer. A
multiplier M has two input signals connected to the
transformer T and the capacitance transducers formed by
capacitors CI and C2. The output of multiplier M is
connected to a low pass filter LPF for filtering the output
to produce an output voltage signal Vo.
The signal to be measured may be a displacement or a change in the dielectric medium, or a change in any of the physical properties associated with the dielectric medium such as humidity. The signal manifests as a change in capacitances of capacitors CI and C2 in differential mode. The transformer T together with the capacitances of capacitors CI and C2 forms a ratio arm-bridge. The output of this bridge is voltage V^ which is a measure of the change in the differential capacitances of capacitors CI and C2. The output of the bridge is demodulated by multiplying it in multiplier M with -a reference voltage signal Vref and filtering it in low pass filter LPF. Although this circuit
is widely used, it is cumbersome, costly and difficult to adjust. Further, it requires a well-matched differential capacitance pair at the sensor stage itself.
An alternate circuit of prior art is shown in Fig. 2. This is less commonly used. In this circuit, the oscillator A has its output connected to an LC circuit comprising a sensor capacitor CS and a measuring inductor LI shunted by a resistance Rl. The output of the LC circuit is connected to a first envelope detector Bl. The oscillator is also connected to a second envelope detector B2. Detectors Bl and B2 are connected to respective low pass filters LPFl and LPF2, the outputs of which are connected to each other. The filters have second outputs at which output voltage signal Vo is formed.
The output of the oscillator is fed to the LC circuit. The output across the inductor LI is input to detector Bl. In the absence of any signal, the standing zero signal level value of the sensor capacitor CS itself causes a steady output voltage Vo. This output is nullified by generating an almost equivalent output from the oscillator through detector B2 and low pass filter LPF2.
Although this is a simpler circuit, it is prone to be affected by a variety of factors. For e.g., change in oscillator frequency due to change in power supply or other reasons will affect the operation. Change in the voltage drop across inductor LI is used as a measure of the physical variable being measured. Such use of the inductor makes the
circuit more sensitive than the use of a resistor. However, change in oscillator parameters affect the performance more adversely.
A preferred embodiment of the circuit of the present invention is shown in Figs. 3 and 4. In this circuit, the capacitor CU is connected to the output of the oscillator A. An inductor LU shunted by a resistance RU is connected in series with the capacitor CU and forms a first combination. This part of the circuit is similar to that described with reference to Fig. 2.
In addition, the same supply from the oscillator A is connected on the other side to a second combination comprising a capacitor CL connected in series with an inductor LL shunted by a resistance RL, which are identical to the first combination of capacitor CU, inductor LU and resistance RU. The capacitor CL can be within the circuit or in the printed circuit board of the circuit itself.
The output of the first combination i.e., the first half of the circuit, is demodulated through an envelope detector EDU. Similarly, the output of the other half of the circuit is demodulated through another envelope detector EDL. These detectors EDU, EDL may be of any type, for eg. , of the diode RC type, depending on the application of the circuit. When diode of RC type is used, the circuit is simple as shown in Fig. 4.
The outputs of the detectors EDU and EDL are low pass filtered in low pass filters LPFU and LPFL. The difference between the two demodulator outputs forms the output voltage signal Vo and this can be amplified for use.
The inductors LU and LL should be equal and resistances RU and RL should be equal. The capacitance of capacitor CL is made equal to zero signal value of the capacitance of capacitor CU at the time of initial adjustment. For this purpose, the voltage signal Vo is measured and the capacitance of capacitor CL is trimmed until the voltage signal Vo becomes zero.
Under zero signal conditions, the current through capacitor CU and that through capacitor CL are equal. So, the voltage drop across inductors LU and LL are also equal. In turn, the outputs of envelope detectors EDU and EDL will be equal. As a result, Vo which is the difference between the output voltages of detectors EDU and EDL will be zero.
When Capacitance of capacitor CU changes due to change in the parameter being measured (for e.g., it increases), current through capacitor CU changes correspondingly (for e.g., increases). But, the current through capacitor CL remains unaltered. The change in current through capacitor CU is manifested as a corresponding change (for e.g. increase) in the output of the envelope detector EDU. In contrast, the current through Capacitor CL remains unchanged and as a result, the output across envelope detector EDL remains unaltered. The low pass filters LPFU and LPFL low
pass filter the respective outputs of envelope detectors EDU and EDL. The net difference forms the output voltage signal Vo. This is a measure of the change in capacitance of capacitor CU and in turn a measure of the change in the parameters which cause the change in the capacitance of capacitor CU.
The circuit of the present invention can be used with capacitance transducers in many applications. A typical application is in sensing the mass/cross-section of yarn/roving in a spinning mill.
In the circuit shown in Figure 3, Capacitor CU is basically formed by having two fixed metal plates of equal size, separated by a fixed distance. The air gap between the plates form the dielectric media. Area of plate, distance between the plates and dielectric medium together determine the total capacitance value.
Yarn is made to pass through the gap between the plates. Presence of yarn as well as its variation in mass/cross-section will alter the dielectric characteristic and hence the capacitance value which is basically used for quality monitoring and also corrective action in the process to improve quality.
The major advantages of the circuit are as follows: 1. The circuit is substantially simpler than the differential circuit of the ratio arm type.
2. The circuit can work with single capacitance type of sensors and does not require a differential capacitance arrangement. However, the circuit can be adapted for a differential capacitance circuit by replacing capacitor CL as the second half of the differential capacitance.
3. The circuit uses identical components on the two sides; as long as inductors LU and LL, resistances RU and RL, etc. track each other, changes due to them affect the two halves identically and nullify each other.
4. Increase or decrease in the oscillator frequency or amplitude in the oscillator, affects both halves in an identical manner; thus the differential output is once again immune to this change.
5. Change in the environmental conditions affect both halves identically and their effects are nullified in the circuit.
6. Any steady and slow change in capacitance of capacitor CU is reflected as a corresponding steady and slow change in voltage Vo. Fidelity of the circuit for low signal levels is thus ensured. This is not possible in the circuit of Fig.2 and it is not easily accomplished in the circuit of Fig. 1.
7. Often a number of transducers of this type are used at close enough locations. In such cases, use of independent oscillators from the same power supply, lead to interference through power supply. Despite substantial amount of filtering used in the power supply, such
interferences can cause beat frequency errors in the output. As the oscillator frequencies come closer to each other and are stabilised to a better and better extent, the nuisance value of such beat frequency interference increases. This is the major problem faced when using multiple sensors at close quarters. With the present invention, such beat frequency components affect both halves identically and thereby these effects are neutralized.
1. Capacitance transducer circuit comprising an oscillator (A) having its outputs connected to a first combination and a second combination. each said combination comprising a sensor capacitor (CU, CL) connected in series with a measuring inductor (LU, LL) shunted by a resistance (RU, RL), said inductors being of equal value, said resistances being of equal value, and each said combination being connected to the input of a respective envelope detector (EDU, EDL), the said envelope detectors being connected to respective low pass filters (LPFU, LPFL), whereby the difference between the outputs of the said low pass filters forms the output voltage signal Vo, the said output signal Vo representing the physical quantity whose magnitude and variations manifest as corresponding magnitude and variations of Vo.
2. Capacitance transducer circuit as claimed in
claim 1, wherein the envelope detector is a diode RC
type of demodulator.
3. Capacitance transducer circuit substantially as
herein described, particularly with reference to the
|Indian Patent Application Number||662/MAS/1997|
|PG Journal Number||30/2009|
|Date of Filing||03-Mar-1997|
|Name of Patentee||PREMIER POLYTRONICS LTD;|
|Applicant Address||304, TRICHY ROAD, SINGANALLUR, COIMBATORE-641 005|
|PCT International Classification Number||D02J1/00|
|PCT International Application Number||N/A|
|PCT International Filing date|