SE428971B - OPTICAL SENSOR - Google Patents

Description

In a particularly preferred embodiment, the transducer elements consist of photodiodes and LEDs and the resonant circuits consist of parallel or series-connected inductances or capacitances, the capacitance values or the inductance value being a measure of the input variable (quantity). . The sensor is fed with. optical energy of defined frequency content, eg in pulse form.

Compared with other optical sensor principles, the sensor thus has the following advantages according to the invention: It can in several cases be constructed with. using commercially available electronic components.

It has an uncomplicated optical structure because the signal information can be transmitted in the form of an intensity-independent and wavelength-independent modulation frequency.

The same optical transmission link can be used for several sensors by frequency multiplexing.

The invention is further exemplified below and in the accompanying figures, of which Fig. 1 shows the basic principle of the sensor, Fig. 2 shows an embodiment of a complete system, Figs. 3, 4 and 6 some alternative designs of the sensor and Fig. 5 a design with two resonant circuits common -optic feeding.

The principal function of the sensor (see Fig. 1) is as follows: A light pulse 1 is incident on the sensor's receiver unit 2, which consists of one or more photodiodes. The resulting electrical voltage drives a current through the transmitter unit 5, for example consisting of an LED. This current is modulated by the resonant circuit 4, the emitted light having an oscillating intensity with a ringing frequency fr which is mainly determined by the resonant circuit 4. This is designed so that the variable to be measured or detected gives a effect on the resonant frequency fr. Alternatively, the input variable is allowed to influence the Q value or attenuation of the circuit (Q = wo E), which, however, places a higher lmav on the components included in the system. See the flow / / time curves on the left in Fig. 1, where in the upper one the relationship between øín and time is shown and in the lower øut and time.

Fig. 2 shows an embodiment of a complete system, based on the sensor principle. For signal transmission, an optical fiber 5 with two, branches 10 15 20 25 ßO 35 8107188-8 6 and 7 is used. The LED 8 emits a light pulse of high intensity and short duration (generally shorter than the period time of the resonant frequency). The process obtained from the sensor unit 9 is detected by the photodiode 10, the photocurrent of which amplifies in the amplifier 11. the signal path filter (12) to reduce the effect of noise and other disturbances, for example electromagnetic switching from the excitation pulse to the LED 8. The filtered the signal constitutes an input signal to a so-called PLL circuit 15 Qhase Locked _I¿oop, locked loop). This consists of a phase comparator 14, a low-pass filter and a voltage controlled oscillator 16 (V00). In the phase comparator 14, any variations in the frequency of the sigrxal are converted into electrical voltage variations, which “lock” the frequency of the voltage-controlled oscillator 16 to the input signal frequency. The output signal from the PLL circuit is applied to a frequency divider or counter 17, which divides the frequency by an even multiple, for example four or eight. This signal goes on to a mono flip-flop 18 to determine the duration of the pulse, and a drive stage 19 to obtain sufficient output power to the LED 8. As an output signal to a possible signal processing unit, presentation unit or effector means, either the analog output signal from the low-pass filter 15 or the frequency-modulated signal from the voltage-controlled oscillator 16 can be used.

It is of course also possible to excite the resonant circuit with an input signal of another form, for example a sinusoidal signal. In this case, the amplitude, frequency and phase position of the output signal can be used to provide information about the resonant frequency nf. Fig. 3 shows some alternative designs of the resonant circuit 4. In Fig. 1a a parallel resonant circuit is used, where the variable to be measured / detected affects the capacitance C of the circuit. The resonant frequency is given by fr = 1/2 7FV_.

In Fig. 3b it is the inductance L which is affected by the input variable. In Figs. 3st and šd, series resonant circuits have been used with modulation of capacitance (C) and inaurance (L), respectively.

In Fig. 4, the LC circuit has been integrated using thin film or thick film stelmology. The inductance and capacitance are distributed parameters here and made as conductive layers. two. plates 20, 21. The conductive layers have been given patterns such as two. flat coils 22. The resonant frequency becomes dependent on the distance between the plates, and the device can thus with a suitable mechanical design, for example, detect an applied force 23. 10 15 20 25 50 8107188-8 Further possibilities for alternative designs of the resonant circuit are the use of the mechanical the resonance in a piezoelectric crystal, such as quartz, or elements based on acoustic surface waves.

Fig. 5a shows how two sensor elements can be combined to, for example, temperature compensate or use the same optical link to transmit two. independent measurement signals. Two extra fiber branches 24, 25 compared to. Fig. 2 has been introduced. Through this device, the two resonant circuits are excited by the same pulse 26 (Fig. Sh). If the two resonant frequencies are assumed to satisfy the condition Af <4 fr, where Af is the difference in resonant frequency, an output signal øut is obtained as shown in Fig. Sb. The exponentially decaying ringing process in Fig. 1 now has a superimposed hovering frequency = Aff. A difficulty in realizing such a system, however, is a much greater requirement for a high Q-value of the resonant circuits to enable a sufficiently accurate determination of Afa. In more general multiplex applications, the different input resonant frequencies should be so separated that they can are excited independently with suitably selected waveforms on the excitation signal.

A number of realization opportunities are also offered in terms of the input variable's impact on. the resonant frequency fr. The table below gives some of these.

Input variable Active element y Mechanism Heavy element, magnet, Connection and disconnection of parallel mechanical switch, or series-connected L or C. contacts, relay.

Position (on / off) 'Variation of eg plate spacing for G, position of the ferrite chains L.

Mechanical converter element. position (continuous), L, C force, pressure, fluid level, flow Bimetallic switch On or off L or C.

Temperature '(on / off) Variation of C (space charge temperature (continuous, fetoaioa nuerlig) range) with temperature.

Voltage, current, C Voltage dependent capacitance, magnetic field capacitance diode.

IQ (on / off), I. mutual inactivity formed by magnetic field eddy currents in nearby metal objects.

Current, magnetic field L Saturation of inductance core.

Claims (17)

8107188-8 The design of the receiver unit 2 and the transmitter unit 3 can also be varied within the scope of the invention. If photodiodes and LEDs are used, these should of course be combined in this way. that the wavelength band of the LED 8 corresponds well with the maximum of the photodiode 2 in spectral response. The same should apply to the LED 5 and the photodiode 10, the wavelength bands of which should be suitably offset from the previous ones in order to enable optical filtering out of reflections in branches and joints. Although these have no direct disturbing effect, since the signal is bandpass filtered, they make an undesirable 'contribution to the noise in the photodiode 10. An elegant design would be to use one and the same opto-component for both mottegoing (2) and fee-toning (3) . This ezonie is possible with an e-photoluminescence diode 27 (Fig. 6). The function of the component is described in more detail in Swedish patent specification 8004602-2. The invention can be varied in many ways within the scope of the following claims. PATENTRAV Optical sensor for detecting or measuring quantities such as position, force, pressure, liquid level, flow, temperature, voltage, current and magnetic field, containing means (2, 5, 27) for converting optical energy into electrical energy and vice versa, characterized in that 1, Benson, includes a measuring or detecting device and at least one electrical resonant circuit (4), said device being arranged to influence the resonant frequency or Q-frequency of the resonant circuit measured in accordance with any measurement measured ( aetertoret) »atvat-ae. 2. An optical sensor as claimed in Claim 1, characterized in that the optical energy is arranged to be transmitted to and from the sensor by means of at least one optical fiber (5). Optical sensor according to claim 1, k 'a'. n n e t e c k n a d of the fact that the optical energy is arranged to be supplied in the form of pulses (1). 4. An optical sensor as claimed in Claim 5, characterized in that the duration of the pulses (1) is shorter than the period time of the resonant frequency. 8107188-8 6 5. In an optical sensor according to claim 1, characterized in that the optical energy is arranged to be supplied in the form of a sinusoidal signal. 6. An optical sensor as claimed in claim 1, characterized in that the resonant circuit consists of at least one series- or parallel-connected inductance and capacitance. 7. An optical sensor according to claim 6, characterized in that the capacitance or inductance value is a measure of. the quantity to be detected or measured. Optical sensor according to claim 1, characterized in that the means (2) for converting optical energy into electrical energy contains at least one photodiode. IN Optical sensor according to claim 1, characterized in that the means (5) for converting electrical energy into optical energy consists of an LED. 10. An optical sensor according to claim 1, characterized in that the means for converting optical energy into electrical energy and vice versa is a photoluminescence diode (27). Optical sensor according to claim 1, characterized in that the resonant circuit (4) consists of a distributed inductance and capacitance network (22). 12. An optical sensor as claimed in Claim 1, characterized in that the resonant circuit (4) contains tongue elements, bimetallic elements, contactors, relays and / or mechanical switches for connecting and disconnecting inductances or capacitors. 13. An optical sensor according to claim 1, characterized in that the resonant circuit contains at least one voltage-dependent capacitor. 14. An optical sensor as claimed in Claim 1, characterized in that the resonant circuit contains at least one temperature-dependent capacitance. 15.. Optical sensor according to Claim 1, characterized in that the resonant circuit (4) contains at least one piezoelectric crystal. 8107188-8 16. Optical sensor according to claim 1, characterized in that the resonant circuit (4) contains at least one element based on. acoustic surface waves. 17. Optical sensor according to claim 1, characterized in that the resonant circuit (4) contains an inductance, the inductance value of which can be affected by a nearby metallic object. 19. Optical sensor according to Claim 1, characterized in that the resonant circuit (4) contains an inductance with a core, the permeability number of which can be influenced by an external magnetic field.