GB2113835A

GB2113835A - Sensor with optically excited resonant circuit

Info

Publication number: GB2113835A
Application number: GB08234234A
Authority: GB
Inventors: Morgan Adolfsson; Bertil Hok
Original assignee: ASEA AB
Current assignee: ABB Norden Holding AB
Priority date: 1981-12-02
Filing date: 1982-12-01
Publication date: 1983-08-10
Also published as: SE8107188L; GB2113835B; DE3243074A1; JPS58105398A; SE428971B

Abstract

A sensor (9) for detecting or measuring quantities such as position, force, pressure, liquid level, flow, temperature, voltage, current and magnetic field, which comprises opto- electric members for converting optical energy into electric energy and vice versa is characterised in that at least one electrical resonant circuit, the resonant frequency or Q-value of which is arranged to be influenced by the quantity to be detected or measured is included in the electrical circuit of said opto-electric members. As shown, the sensor enables conversion of an electrical pulse input 8, derived from a phase locked loop 13, to a ringing electrical output from 10 which is fed to the phase comparator 14 of the loop 14. The branch 7 of optical fibre coupler 5 may be further divided for simultaneous use of a plurality of sensors with different resonant frequencies. Light emitting and photo diodes 8 and 10 may be replaced by a single diode performing both functions. <IMAGE>

Description

SPECIFICATION Sensor with optically excited resonant circuit Technical field The present invention relates to an optical sensor for sensing (i.e. detecting or measuring) quantities such as position, force, pressure, liquid level, flow, temperature, voltage, current and magnetic field, which includes opto-electric members for converting optical energy into electrical energy and vice versa.

One problem with known optical sensors of the above-mentioned kind is that they typically have a relatively complicated optical structure, and do not lend themselves to the multiplexing of a plurality of sensors to one and the same optical transmission link.

The invention seeks to provide a solution to the above-mentioned probiem and other problems associated therewith. An optical sensor according to the invention is characterised in that it comprises, in addition to the above-mentioned opto-electric members, at least one electrical resonant circuit, a characteristic (e.g. the resonant frequency or Q-value) of which is adapted to be influenced by the quantity to be detected or measured. By Q value is meant-as is well known-the factor of merit or the value of Q=oxL/R, where L is the inductance and R the resistance in the circuit and w, is the resonant frequency. In relation to previous optical sensors a sensor of the kind envisaged by this invention has an uncomplicated optical construction involving the possibility of multiplexing as mentioned above.

In a specially preferred embodiment, the electro-optical members consist of photo-diodes and light-emitting diodes (LEDs) with resonant circuits of parallel- or series-connected inductors or capacitors, whereby capacitance values or inductance values may be varied in proportion to the input variable (the quantity to be sensed). The sensor can be fed with optical energy of a defined frequency content, for example in pulse form.

Advantages over prior art Compared with other known optical sensors, an optical sensor according to the invention thus has the following advantages: The sensor can be constructed from commercially available electronic components.

The sensor has a simple optical structure allowing the signal information to be transmitted from the sensor in the form of amplitude-dependent or wavelength-dependent modulated frequency signals.

A common optical transmission link can be employed for a plurality of optical sensors by frequency-division multiplexing.

Brief description of drawings The invention will now be exemplified in greater detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 a illustrates the input and output pulses to and from a sensor according to Figure 1 b, Figure 1 b illustrates the basic principle underlying a sensor according to the invention, Figure 2 shows, schematically, one embodiment of a complete parameter-sensing system based on the sensor of Figure 1 b, Figures 3a to 3d, 4 and 6 show some alternative embodiments of sensor according to the invention.

Figure 5a shows one embodiment of sensor according to the invention with two resonant circuits, which sensor is fed from a common optical fiber, and Figure 5b the fludtime graphs of the input and output signals to and from the sensor of Figure 5a.

Description of preferred embodiments The fundamental mode of operation of a sensor according to the invention is as follows: A light pulse 1 (see Figure 1 a) is directed onto a receiver unit 2 of a sensor according to the invention, the receiver unit 2 consisting of one or more photodiodes. In the case illustrated in Figure 1 b, three photo-diodes are shown but this is purely by way of example and more than three, or less than three, can be used. The electrical voltage generated by the photodiode(s) 2 drives a current through a transmitter unit 3, which may, for example, be an LED. The current flowing through the unit 3 is modulated by a resonant circuit 4, whereby the light intensity emitted by the unit 3 acquires an oscillating amplitude with a ringing frequency fr which is substantially determined by the resonant circuit 4.The resonant circuit 4 is constructed so that the variable parameter, which is to be measured or detected, influences the resonant frequency fr of the circuit 4. Alternatively, the variable parameter influences the Q value (Q=o,L/R) or attenuation of the resonant circuit 4. However, these alternatives are not normally preferred modes of operation since higher demands are then imposed on the components included in the parameter-sensing system. The upper flux/time curve in Figure 1 a, shows the ratio between the input light flux j and time and the lower curve in Figure 1 a shows the relationship between the output flux tout (from the unit 3) and time.

Figure 2 shows one embodiment of a complete measuring/sensing system, employing an optical sensor 9 according to the invention. For signal transmission an optical fiber 5 with two branches 6 and 7 is used for transmitting light signals to and from the sensor. An LED 8 emits a light pulse of high intensity and short duration (generally shorter than one period at the resonant frequency fr) The ringing output derived from the sensor 9 is detected by a photo-diode 10, the photo-current of which is amplified in an amplifier 1 The signal is band-pass filtered at 1 2 to reduce the effect of noise and other disturbances, for example receipt of electromagnetic radiation from the excitation pulse generated by the LED 8.The filtered signal leaving the filter 12 constitutes an input signal to a Phase Locked Loop (PLL) circuit 13. The PLL 1 3 consists of a phase comparator 14, a low pass filter 1 5 and a voltage-controlled oscillator (VCO) 1 6. In the phase comparator 14, any variations in the frequency of the input signal are converted into electrical voltage variations, which "lock" the frequency of the VCO 1 6 to the input signal frequency. The output signal from the PLL circuit 13 is supplied to a frequency divider or counter 1 7, which reduces the frequency by one even multiple, for example four or eight.This reduced frequency signal passes to a monostable flip-flop 1 8 for determining the duration of the pulse, and an exciter 1 9 for obtaining sufficient output power to energise the LED 8. The output signal from the low pass filter 1 5 or the frequency-modulated output signal from the voltage controlled oscillator 16, can be used as the input to a possible signal processing unit, presentation unit or effector member (not shown).

It will be appreciated that it is also possible to excite the resonant circuit 4 using an input light pulse of some other shape, for example a sinusoidal signal. In that case, the amplitude, frequency and phase position of the output signal can be utilized to provide information about the prevailing resonant frequency fr of the resonant circuit 4 (and thus information about the parameter to be sensed/measured).

Figures 3a to 3d show four alternative forms for the resonant circuit 4. In Figure 3a a parallel resonant circuit is used, in which the variable to be measured/detected influences the capacitance C of the resonant circuit. The resonant frequency is given by fr=1/27r < . In Figure 3b it is the inductance L that is influenced by the input variable. In Figures 3c and 3d series resonant circuits are used with the variable influencing the capacitance EC) or inductance (L), respectively.

Varying the inductance L can be effected by varying the position of a metallic object adjacent to the inductance coil or varying the permeability of a core of the inductance by means of an external magnetic field.

In Figure 4, the resonant LC circuit has been integrated, utilizing thin film or thick film technology.

The inductance and the capacitance are here shown as distributed parameters and arise by virtue of conducting layers on two plates 20, 21. The conducting layers shown in Figure 4 are in the form of two flat coils 22 but other patterns may, of course, be used. The resonant frequency will be dependent on the separation of the plates 20, 21, so that with a suitable mechanical linkage, the sensor shown in Figure 4 is able to detect, for example, an applied force 23.

Additional possibilities for alternative embodiments of the resonant circuit involve the use of the mechanical resonance in a piezo-electric crystal, for example quartz, or the use of elements based on acoustic surface waves.

Figure 5a shows how two sensor elements 9', 9" may be combined, for example, in order to temperature-compensate or utilize the same optical fiber link to transmit two independent measuring signals. Compared with the right-hand end of the fiber link shown in Figure 2, two additional fiber branches 24, 25 are employed. With this arrangement, the two resonant circuits 9' and 9" are excited by means of a common input pulse 26 (see the upper graph in Figure 5b). If the resonant frequencies of both sensors 9' and 9" are assumed to fulfil the condition Af fr, where Af is the difference in resonant frequencies between the two sensors, an output signal (ilout is obtained, as shown in the lower graph in Figure 5b. The exponentially decaying ringing output shown in the lower graph of Figure la now has superposed on it an oscillating frequency=Af. In practice, one difficulty in utilizing a system such as that shown in Figure 5 is the considerably greater demand for a high Q value of the resonant circuits to make possible a sufficiently accurate determination of the frequency Af.

In more general applications of multiplexing, the different resonant frequencies employed should be separated in such a way that they may be excited independently of each other with suitably chosen curve shapes of the input excitation signal.

A number of possibilities exist for making the resonant frequency fr dependent on the parameter to be sensed/measured (input variable). The following table shows some of these possibilities.

Table Active element for

modifying the resonant In put variable frequency Mechanism employed Position (on/off) Tongue element, Coupling in and out of paralle mechanical switch, or series-connected inductor contactor, relay or capacitor Position (continuous), force, L or C Variation of, e.g. plate separation pressure, liquid level, flow rate in a capacitor, position of a ferrite core in an inductor.

Mechanical converter element Temperature (on/off) Bimetallic switch Coupling in and out of an inductor or capacitor Continuous temperature variations Diode, photo-diode Variation of the capacitance of a space charge region with changes in the temperature Voltage, current, magnetic field C Voltage-dependent capacitance, capacitance diode Position (on/off), Magnetic field L Mutual inductance produced by eddy currents in adjacent metallic objects Current, magnetic field L Saturation of the core of an inductor Also the form of the receiver unit 2 and the transmitter unit 3 can be varied widely within the scope of the invention. If photo-diodes and LEDs are used, these should of course be selected so that the wavelength band of the LED 8 accurately matches the maximum in the spectral response curve of the photo-diode 2. The same considerations also apply to the selection of the LED 3 and the photo-diode 10, the wavelength bands of which are suitabiy displaced from those of the LED 8 to make it possible to filter out reflections occurring in branches and joints of the optical fiber. Although such reflections do not have any disturbing effect, since the signal is band-pass filtered (e.g. in 12), they do give rise to an undesirable contribution in the noise in the output of the photo-diode 1 0.

One elegant solution would be to utilize one and the same opto-component for both reception of the input signal and transmission of the output signal. This is possible with a so-called photo-luminescence diode 27 (see Figure 6j. The function of a photo-luminescent diode is described in greater detail in EP-A-0043929.

The ihvention can be varied in many ways within the scope of the following claims.

Claims

1. An optical sensor for detecting or measuring quantities such as position, force, pressure, liquid level, flow, temperature, voltage, current and magnetic field, comprising first opto-electric means for converting optical energy into electrical energy and second opto-electric means for converting electrical energy into optical energy, characterised in that the sensor comprises at least one electrical resonant circuit and means arranged to influence a characteristic of said resonant circuit by the quantity which is to be sensed.

2. An optical sensor according to Claim 1 in which at least one optical fiber is employed to transmit optical energy to and from said opto-electric means.

3. An optical sensor according to Claim 1 or Claim 2, in which means is provided to supply the optical energy to the first opto-electric means in pulsed form.

4. An optical sensor according to Claim 3, in which the duration of each optical pulse is shorter than the reciprocal of the resonant frequency of said resonant circuit(s).

5. An optical sensor according to Claim 1 or Claim 2, in which means is provided to supply the optical energy to said first opto-electric means in the form of a sinusoidal signal.

6. An optical sensor according to any preceding claim, in which the resonant circuit(s) consist(s) of a series-connected inductor and capacitor.

7. An optical sensor according to any of Claims 1 to 5, in which the resonant circuit(s) consist(s) of a parallel-connected inductor and capacitor.

8. An optical sensor according to any preceding claim, in which the quantity to be sensed influences the capacitance of a capacitor in the resonant circuit.

9. An optical sensor according to any of Claims 1 to 7, in which the quantity to be sensed influences the inductance of an inductor in the resonant circuit.

10. An optical sensor according to any preceding claim, in which said first opto-electric means comprises at least one photo-diode and said second opto-electric means comprises a light-emitting diode.

11. An optical sensor according to any of Claims 1 to 9, in which said opto-electric means comprise photo-luminescence diodes.

12. An optical sensor according to any preceding claim, in which the resonant circuit includes distributed inductance and capacitance networks.

13. An optical sensor according to any preceding claim, in which the means to influence a characteristic of said resonant circuit includes a tongue element, a bimetallic element, a contactor, a relay or a mechanical switch for connecting and disconnecting an inductor or capacitor into the resonant circuit.

14. An optical sensor according to any preceding claim, in which the resonant circuit(s) comprise(s) at least one voltage-dependent capacitor, at least one temperature-dependent capacitor, at least one piezo-electric crystal or at least one element employing acoustic surface waves.

1 5. An optical sensor according to any of Claims 1 to 11, in which the resonant circuit(s) comprise(s) an inductor and a metallic object in the vicinity thereof, with means to move the metallic object in response to variations in the quantity to be sensed, whereby the inductance of said inductor is modified by said variations.

1 6. An optical sensor according to any of Claims 1 to 11, in which the resonant circuit(s) comprise(s) an inductor with a core, whose relative magnetic permeability may be changed by changes in an external magnetic field, and means, responsive to the quantity to be sensed, to change the value of said external magnetic field.

17. An optical sensor as hereinbefore described with reference to Figures 1 b, 2, 3a, 3b, 3c, 3d, 4, 5a or 6 of the accompanying drawings.

1 8. A method for multiplexing a plurality of optical sensors, characterised in that at least two sensors according to any preceding claim are connected to one and the same optical fiber via fiber branches.

1 9. A method for combining signals from two optical sensors, in which two sensors according to any of Claims 1 to 17, having closely identical resonant frequencies are connected to one and the same optical fiber via fiber branches.

20. A method for multiplexing a plurality of optical sensors, in which sensors according to any of Claims 1 to 17, having well separated resonant frequencies are selected and are connected to one and the same optical fiber via fiber branches.