Abstract:
The invention relates to a measuring device having a transmitter side and a receiver side, arranged for signal transmission from the transmitter side to the receiver side via an optical connection. The transmitter side comprises members for combining an incoming measuring signal with a signal from a reference voltage source to two different signals, arranged to be transmitted to the receiver side via each of two parallel optical channels. The receiver side comprises means for receiving the two transmitted signals and for controlling the amplification in the signal paths with a view to obtaining compensation for variation of parameters included in the two channels and other elements in corresponding signal paths. In addition, the receiver side includes a device which, from the two transmitted signals, forms a signal (U ut ) independent of the input measuring signal of the transmitter side.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a measuring device having a transmitter side and a receiver side, said device being arranged for transmitting signals from the transmitter side to the receiver side via an optical connection. 
     2. Prior Art 
     It is known that measuring signals and other light signals can be transmitted via an optical link. Such a link comprises a light transmitter, light-transmitting members and light-detecting members. The light transmitter comprises members for modulation of the emitted light in a suitable manner such that the emitted light contains information about the present value of the measuring signal. The light-transmitting member, which may consist of one or more light conductors (optical fibers), transmits light signals to the detecting member, where information about the measuring signal is transformed into a suitable signal, for example into an electric voltage proportional to the measuring signal. A measuring device of this kind has several advantages. Galvanic separation is automatically obtained between the transmitter side and the receiver side, and therefore these sides may be located at completely different potential levels. The optical line is wholly insensitive to electromagnetic disturbances. In addition, no sparks or short-circuits may occur, which is of value, for example, in an explosive environment. 
     In a measuring device of the above-mentioned kind it is of considerable importance to reduce the power consumption on the transmitter side as far as possible, since the energy supply to the members of the transmitter side may present a problem, for example in those cases where these members are located at a high potential level. 
     In measuring at a high potential, it is a desire to dispense with the current and voltage transformers for high voltages which have been commonly used so far, but the transmission of energy from the receiver side to the transmitter side continues to be a problem. 
     In a measuring device for which a patent application has been filed in the United States by the same inventors (U.S. Ser. No. 75,873, filed Sept. 17, 1979) a measuring signal is transmitted from a transducer over a certain distance and/or a certain potential difference. The measuring device has a transmitter side, arranged in conjunction with the transducer, which is connected to a receiver side by an optical transmission link. The transmission side includes comparing members which compare measuring signals with a feed-back signal transmitted from the receiver side. The comparison result is transmitted to the receiver side, where it controls a regulator, the output signal of which constitutes the feed-back signal as well as the output signal of the measuring device. The measuring device further includes members for automatically maintaining the amplification in the transmission link for the feed-back signal constant. This device is a solution to the above-mentioned problems, and since only minor error signals are transmitted from the transmitter side to the receiver side the dynamic range of such transmission is low. The dynamic range of the total transmission is wholly determined by the dynamic range in the transmission of the feed-back signal from the receiver side to the transmitter side. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to supplement the aforementioned device and still solve the above-mentioned problems for those cases where only a certain limited power is available on the transmitter side. It is another object of the invention to provide a certain dynamic range on the transmitter side. 
     The present invention provides a solution to the problems mentioned above and is characterised in that the transmitter side comprises members for combining incoming measuring signals (U in ) with reference signals (V ref ) into at least two different signals, arranged to be transmitted to the receiver side via at least two parallel optical channels, the receiver side comprising means for receiving the transmitted signals and for forming from these signals an output signal (U ut ) dependent on the measuring signals (U in ) of the transmitter side and independent of drift and instability in the optical channels and the associated optoelectronics. In this way a reliable and simple equipment is obtained involving few optical fiber transmissions a simple electronics system. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The above objects, features and advantages of the invention are exemplified in the accompanying drawings, wherein. 
     FIG. 1 shows an optically coupled, analog measuring device according to the invention, in which the output signal is formed from the difference between the transmitted optical signals; 
     FIG. 2 shows a modified embodiment of the invention for linearising the transmission involving a lower power consumption on the transmitter side; 
     FIG. 3a shows an optically coupled, analog measuring device, in which the output signal is obtained through the formation of a quotient; and 
     FIG. 3b shows a power supply for the transmitter of FIG. 3a. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows an optically coupled measuring device according to the invention for transmitting and transforming an analog electric input signal U in  into an analog electric output signal U ut . The measuring device consists of a transmitter side S and a receiver side M, which are interconnected by means of at least two light conductors 8, 9. The transmitter and receiver sides may be separated a distance of up to several kilometers, and they may be located at different electric potentials. 
     Alternatively, for example where only a potential difference, which is not too great, is to be bridged, the transmitter and receiver sides may be connected to each other, in which case the light conductors with associated light-emitting diodes (LEDs) and photodiodes may consist of optic couplers. 
     FIG. 1 shows an optically coupled, analog measuring amplifier, in which the stabilization of the analog transmission takes place according to the &#34;bridge principle&#34;. The measuring signal U in  is input symmetrically to the middle node or midpoint of voltage divider 1. The signals +U in  /2 and -U in  /2 are then obtained at each respective end node of voltage divider 1. The signal +U in  /2 passes via an analog contact device, for example field effect transsistor 2, to a summation device 31 where this signal is added to a reference voltage V ref  from a constant voltage source, and the composite signal (+U in  /2+V ref ) passes to the input of the amplifier 4. In a corresponding manner, a composite signal (-U in  /2+V ref ) is obtained from the summation device 32 after -U in  /2 has passed the field effect transistor 3. Summation signal output of summation device 32 is supplied to amplifier 5. 
     The two signals thus combined, +U in  /2+V ref  and -U in  /2+V ref , enter two parallel signal paths, consisting of the following respective components: amplifiers 4 and 5, which are optically linearized by means of feedback photodiodes 27 and 28, light-emitting diodes (LED) 6 and 7, the two optical channels 8 and 9 (light conductor fibers), photodiodes 10 and 11, the controllable photo-amplifiers 12 and 13, and a variable amplifier 15 and an amplifier 14 with fixed amplification. The transmitter feedback circuits, which contain photodiodes 27 and 28, compensate for non-linearities of LEDs 6 and 7. The signal paths (the channels) differ from each other in that the amplifier 14 in the upper channel in FIG. 1 has fixed amplification or gain, whereas the amplification or gain of amplifier 15 in the lower channel is variable. The measurement procedure is divided with respect to time (time multiplexed) into two intervals, one in which the transmission in the two channels is balanced, and one in which the measurement itself is carried out. The signal paths of the measuring signals side in the transmitter portion S are controlled via transistors 23 by means of a control logic system 20 more fully described below. 
     During the balancing interval the transistors or switches 2 and 3 are open, and therefore the input signals to both channels have the same voltage, namely V ref . 
     The output signals from amplifiers 14 and 15, respectively, are compared in the summation device 33 and any deviation due to parameter variations (aging of the diodes, changed curvature of the light conductors, differences of the variable amplifiers, etc.) gives rise to an error signal, which is supplied to the regulator 17 which controls the amplification in the variable amplifier 15 by way of the sample and hold or track and hold circuit (T/H) 18. The balancing of the two channels causes the total transmission characteristic in the two channels 6-14 and 7-15, respectively, to substantially equal each other despite the variation of the parameters of the opto-components. The sum of the signals from amplifiers 14 and 15 is formed in the summation device 34 and is compared in a difference output former 35 with a constant voltage U O , and the difference is allowed to influence the amplifications in amplifiers 12 and 13 via a regulator 19. This method ensures compensation for the variations which occur in the two channels simultaneously. During the measuring interval, the amplification in amplifier 15 is held constant by the track and hold circuit (T/H) 18 operating in the hold position, and the voltages V ref  +U in  /2 and V ref  -U in  /2, respectively, are connected to the inputs of amplifiers 4 and 5, respectively. This time interval is called a measuring interval as opposed to that described above, which is called a calibrating interval. 
     During the measuring interval, the output signal is obtained as the difference between the output signals of amplifiers 14 and 15. The T/H circuit 16 makes it possible for the output signal to be held constant during the balancing interval (the calibrating interval). The T/H circuits 16 and 18 are controlled from the control logic system 20, which also operates the analog switches (transistors) 2 and 3 via an amplifier 21, an LED 22, a light conductor fiber 23, and a photo-current amplifier 25. 
     The power to the transmitter side is transmitted optically by a current passing through the LEDs, or the semiconductor lasers 28, and being transformed into light. The light is transmitted through one or more light conductor fibers 29 to the series-connected photodiodes 30, the output voltage of which constitutes the power supply p.s. for the electronics system. 
     Upon calibration (2 and 3 opened), the following output signal is obtained from the summation device 33: 
     
         k.sub.1 · V.sub.ref -k.sub.2 · V.sub.ref =(k.sub.1 -k.sub.2)V.sub.ref 
    
     k 1  and k 2  are the total amplification factors in the two channels. 
     The regulator 17 is adjusted so that k 1  -k 2  =0 and k 1  =k 2  =k. 
     The regulator 19 is adjusted so that: ##EQU1## 
     During measuring (2 and 3 closed) the following is obtained: ##EQU2## 
     FIG. 2 shows a device in which the direct feedback of the light from the LED 6 and 7 via a photodiode 27 and 28 respectively is replaced with a fiber optical feedback from LED 35 in case of failing power on the transmitter side S. An additional light fiber 36 is used for the feedback which takes place via the receiver portion where the remaining or other portion of the light from LED 35 is amplified to provide an output signal. This results in a transmitter portion (transducer module) which requires less power. 
     More specifically, light from the photodiode 6 passes via a light conductor fiber 8 to a photodiode 10 with an amplifier 14, which drives an LED 35. The light from the LED 35 results in an output signal via a photodiode 37 with an associated amplifier 38, as well as in a feedback signal to the amplifier 4 via the light conductor fiber 36 and the photodiode 38. 
     In FIG. 1 the analog switches 2 and 3 and photodiode 24 and photo-current amplifier 25 may be replaced with photo field effect transistors, which are controlled directly with the light from the light conductor fiber 23. 
     The measuring device according to FIG. 3 is an optically coupled, analog measuring amplifier, in which the stabilization of the transmission is made according to the bridge principle. The same oscillating signal of frequency f O  from the oscillator 39 via the summation devices 40 and 41 to the inputs of two amplifiers 42 and 43 is added to each of the two signals, the input signal U in  and the reference voltage V ref . The summed signals are transmitted in two parallel channels, consisting of amplifiers 42 and 43, LEDs 44 and 45, light conductor fibers 46 and 47, photodiodes 48 and 49, photo-current amplifiers 50 and 51, LEDs 52 and 53, photodiodes 54 and 55, and amplifiers 56 and 57 (cf. 14 and 15 in FIG. 1). Both channels are the same, except that amplifier 56 has fixed amplification and amplifier 57 has adjustable gain. To improve the linearity of the transmission, part of the light from LEDs 52 and 53 is conducted through light conductor fibers 58 and 59 and photodiodes 60 and 61 to summation points 40 and 41 at the inputs of amplifiers 42 and 43. The light is transformed in photodiodes 60 and 61 into electric signals and in this way forms linearizing feedback signals. The difference between the output signals from amplifiers 56 and 57 obtained by summation device 72 passes through a high-pass filter 62, where only components of the signal with the frequency f O , or higher, may pass. This signal component is rectified in the rectifier 63, is low-pass filtered in low-pass filter 64 and is supplied to a regulator 65 which adjusts the amplification or gain of the variable amplifier 57. The amplification or gain of amplifier 57 is changed in such a way that deviations in the transmission characteristic between the two channels are compensated. The output signals from the two channels are allowed to pass through low-pass filters 66 and 67, components of the frequency f O  thus being filtered. The quotient between the signals of the outputs to the low-pass filters is obtained by supplying the output signals to a division circuit 68. The resulting quotient is taken as an output signal and in this way compensation is obtained for variations in transmission which occur in both transmission channels. The signal of the frequency f O , which is used for balancing the transmission in the two channels, is formed by an oscillator 39 in the transducer module as shown in FIG. 3b. The supply to the transducer module is transmitted optically by the fact that the light, which is generated by the LEDs, or by the semiconductor lasers 69, is transmitted to one or more optical fibers 70 into series-connected photodiodes 71, which provide supply voltage to the electronics system in the transmitter portion. 
     The embodiments as described above may be varied in many ways within the scope of the following claims.