OPTICAL POSITION SENSOR

Abstract: The patent describes an optical sensor for measuring the position of a remote moving body, which converts said position into a phase modulation of an optical measure signal and in which the false phase variat­ ions induced by the variations in the environmental conditions are compensated. The compensation is effected by generating an optic­ al reference signal, which travels together with the optical measure signal, and by making a phase comparation between the two signals.

Description: The present invention refers to an optical position sensor and more particularly it relates to an optical sensor able to measure the position of a moving body with respect to a fixed reference element, said moving body and said fixed reference element being remote from the generator of the local optical signal, in which the displacement of the remote moving body is converted into a phase variation of the optical signal transmitted to the remote area through an optical fiber. As known, there are apparatuses which ― for their operation ― require a determination of the displacement of moving bodies with respect to fixed reference elements situated in zones distant from the apparatus and in environments where the atmospheric conditions are severe and changeable. For instance, the central control unit of an aircraft, situated in the pilot compartment, must know the position of the ailerons and it is evident that the environmental conditions around the aircraft, such as temperature, pressure, humidity and so on, may be extremely variable. It is also known that the most reliable instruments to measure such displacements are those of the optical type which, besides being simple and compact, show a low attenuation of the signal and immunity to electro-magnetic disturbances. Further, optical position sensors are known, which make use of a local, amplitude modulated monochromatic light source which, through an optical fiber, is conveyed to the remote area comprising the moving body whose position is to be determined. In said sensors, the modulated light outcomes from the free end of the optical fiber, strikes against the surface of the moving body and is again reflected into the optical fiber. In this way the light reflected by the moving body is phase displaced by an angle which is proportional to the distance between the moving body and the end of the optical fiber. The phase displaced light runs through the optical fiber again and is locally compared to the modulating signal. In this way the phase displacement of the reflected light with respect to the modulating signal, and therefore the position of the moving body, is determined. The above described optical sensors show the substantial drawback that the phase displacement measured between the reflected light and the modulating signal does not represent only the position of the remote moving body, but also involves all of the improper phase variations induced by the environmental conditions where the optical fiber is operating. In fact, such variations in temperature, pressure, humidity and so on, together with the length of the optical fiber, which can reach some tens of meters, induce variations in the transmission characteristics of the optical fiber, which result in improper phase displacement of the optical signal. In particular, when the displacements to be measured are very small, of the order of some tens of microns, the false phase displacements induced by the ambient may distort the measure of the position of the remote body. Consequently, it is the aim of the present invention to overcome the above indicated disadvantages and to provide an optical sensor able to determine the position of a remote moving body, in which the false phase variations induced by the variations of the environmental conditions where the optical fiber operates, are compensated. To achieve said aim, the object of the present invention is an optical position sensor, which comprises: a generator for generating an optical monochromatic measure signal, a modulator for the amplitude modulation of said optical measure signal by a radio-frequency electric signal, an optical fiber for the transmission of the optical measure signal to a remote area and the reception of the reflected optical measure signal from the remote area, a device positioned in the remote area for the conversion of the position of a moving body positioned in the remote area, with respect to a fixed reference element positioned in the remote area, into said reflected optical measure signal, a device for the conversion of the optical measure reflected signal into an electric measure signal having a frequency equal to the frequency of the modulation electric signal, and a phase comparator, characterized in that it further comprises: a second generator for generating an optical monochromatic reference signal having a wave length different from that of the optical measure signal, also said reference signal being amplitude modulated by said radio-frequency electric signal and being conveyed by said optical fiber, and a second device for the conversion of said reflected optical reference signal into an electric reference signal having a frequency equal to that of the modulation electric signal, said phase comparator comparing the phases of said electric measure signal with said electric reference signal, and in that said fixed reference element comprises an optical filter which allows the passage of said optical measure signal and reflects said optical reference signal. In a preferred embodiment of the invention, the fixed reference element is represented by a semi-reflecting mirror. In a further preferred embodiment the reflected optical signals are separated from the transmitted optical signals by means of a directional coupler and the reflected optical signals are separated from one another by means of two semi-reflecting mirrors. Further aims and advantages of the present invention will be more clearly apparent from the following detailed description and by the appended drawing, given only by way of non-limiting example, which represents a general diagram of the optical position sensor forming the object of the invention. The diagram illustrates a source 1 of monochromatic light having a wave length λ₁, which generates a so-called "reference" optical signal, and a source 2 of monochromatic light, having a wave length λ₂ which generates a so-called "measure" optical signal. The meaning of such terms will be more clearly understood in the course of the description. The optical measure signal S₂ and the optical reference signal S₁ are amplitude modulated by an IGHz radio-frequency electric signal generated by a modulator 3. The measure signal S₂ and the reference signal S₁, amplitude modulated, are sent to a same input of a directional coupler 4. An optical fiber 5 outcomes from an output of the directional coupler 4; said fiber extends as far as a remote area where is situated a moving body 6 whose displacements are to be measured. To the free end of the optical fiber is connected a lens 7, used to focalize the light beam coming from the optical fiber 5, and a semi-reflecting mirror 8, which acts as an optical filter, allowing the full passage of the optical measure signal S₂ and reflecting completely the optical reference signal S₁. The moving body 6 has a surface 9, facing the semi-reflecting mirror 8, which reflects light in a complete manner. An optical fiber 10, to which are connected two semi-reflecting mirrors 11 and 12, departs from another output of the directional coupler 4. The semi-reflecting mirror 11 acts as an optical filter, allowing the full passage of the optical reference signal S₁ and blocking completely the optical measure signal S₂. Analogously, the semi-reflecting mirror 12 acts as an optical filter, allowing the full passage of the optical measure signal S₂ and blocking completely the reference signal S₁. Two photodiodes 13 and 14 are respectively facing the semi-reflecting mirrors 11 and 12; they convert the optical signals into electric signals. The electric signals coming from the two photodiodes 13 and 14 are respectively amplified by two operational amplifiers 15 and 16 and are then sent to the two inputs of a phase comparator 17. A variable phase shifter 18 is also connected to the phase comparator 17. In operation, the amplitude modulated measure signal S₂ and reference signal S₁ pass through the directional coupler 4 without being attenuated and, after having run through the optical fiber 5 along its whole length, reach the remote area where the semi-reflecting mirror 8 acts as a fixed reference for the moving body 6. The optical reference signal S₁ is completely reflected by mirror 8 and is therefore reflected exactly on the fixed reference. On the contrary, the optical measure signal S₂ passes completely through mirror 8, strikes against the reflecting facing surface 9 of the moving body 6 and goes again on mirror 8, wherefrom ― through the focusing lens 7 ― re-enters the optical fiber 5. In this to-and-from travel with respect to the fixed reference element 8, the optical measure signal S₂ suffers a phase displacement with respect to the optical reference signal S₁, which is proportional to the distance existing between the same fixed reference element 8 and the facing surface of the moving body 6. Consequently, the phase displacement between the reference signal S₁ and the measure signal S₂ represents the distance of the moving body from the fixed reference 8. The reflected optical signals, namely the reference signal S₁′, and the measure signal S₂′, phase shifted with respect to the reference signal, run through the whole optical fiber again to reach the directional coupler 4 where they are deviated on the optical fiber 10. The semi-reflecting mirrors 11 and 12 separate the two reflected optical signals S₁′ and S₂′, allowing the only passage of the measure signal S₂′ and the reference signal S₁′, respectively. Photodiodes 13 and 14 convert the reflected optical reference signal S₁′ and measure signal S₂′ into two electric signals v₁ and v₂, which have the same frequency as the modulating electric signal and which maintain the same phase relation of the reflected optical signals; the operational amplifiers 15 and 16 amplify the electric signals v₁ and v₂ and at last the phase comparator 17 measures the phase displacement existing between the two electric signals v₁ and v₂, which represents the distance of the moving body 6 from the fixed reference 8. The variable phase shifter 18 has the task of calibrating the phase comparator 17. A signal indicating said distance might then start from the phase comparator 17 to reach a circuit controlling the position of the moving body 6. The above described sensor is able to determine the axial position of a moving body with respect to a fixed reference; moreover, by simple modifications, it can also determine the angular position of a revolving shaft. From the foregoing description it is clear that the optical sensor forming the object of the present invention reaches the aim to compensate the false phase variations induced on the optical signal measuring the position of the moving body by the variations in the environmental conditions. In fact, when these variations take place, the optical fiber changes its trans mission characteristics and induces exactly the same phase displacements on both the modulated optical measure and reference signals, so that their phase difference is always and solely that due to the position of the moving body. In other words, in the conventional optical sensors the reference signal is represented by the electric signal of the modulator and remains "in loco",whilst the optical signal which measures the position of the moving body, travelling along the optical fiber, is influenced by the improper phase variations; on the contrary, in the optical sensor according to the present invention, an optical reference signal is generated which, travelling together with the optical signal which measures the position of the moving body, suffers the same improperphase variations suffered by said measure signal so that, when the phase difference is effected, these improperphase displacements are compensated and therefore eliminated.

Claims: 1. An optical position sensor, which comprises: a generator for generating an optical monochromatic measure signal (S₂), a modulator for the amplitude modulation of said optical measure signal (S₂) by a radio-frequency electric signal, an optical fiber for the transmission of the optical measure signal (S₂) to a remote area and for the reception of the reflected optical measure signal (S₂′) from the remote area, a device positioned in the remote area for the conversion of the position of a moving body positioned in the remote area, with respect to a fixed reference element positioned in the remote area, into said reflected optical measure signal (S₂′), a device for the conversion of the reflected optical measure signal (S₂′) into an electric measure signal (V₂) having, a frequency equal to the frequency of the modulation electric signal, and a phase comparator, characterized in that it further comprises: a second generator (1) for generating an optical monochromatic reference signal (S₁) having a wave length (λ₁) different from that (λ₂) of the optical measure signal (S₂), also said reference signal (S₁) being amplitude modulated by said radio-frequency electric signal and being conveyed by said optical fiber (5), and a second device (13) for the conversion of said reflected optical reference signal (S₁′) into an electric reference signal (v₁) having a frequency equal to that of the modulation electric signal, said phase comparator (17) comparing the phases of said electric measure signal (v₂) with saidelectric reference signal (v₁), and in that said fixed reference element comprises an optical filter (8) which allows the passage of said optical measure signal (S₂) and reflects said optical reference signal (S₁). 2. An optical sensor as in claim 1, characterized in that said fixed reference element comprises a focusing lens (7). 3. An optical sensor as in claim 1, characterized in that said moving body (6) has a surface (9) facing said fixed reference element and reflecting light. 4. An optical sensor as in claim 1, characterized in that it comprises a directional coupler (4) which separates said transmitted optical signals (S₁, S₂) from said reflected optical signals (S₁′, S₂′) and conveys them on a second and a third optical filter (11, 12), which separate said reflected optical measure signal (S₂′) from said reflected optical reference signal (S₁′). 5. An optical sensor as in claims 1 and 4, characterized in that said optical filters (11, 12) are light semi-reflecting mirrors. 6. An optical sensor as in claim 1, characterized in that it comprises a first amplifier (15) and a second amplifier (16) which amplify said electric measure signal (v₂) and said electric reference signal (v₁). 7. An optical sensor as in claim 6, characterized in that said phase comparator (17) is connected between the outputs of said first (15) and second (16) amplifier. 8. An optical sensor as in claim 7, characterized in that it comprises a variable phase shifter (18) connected to said phase comparator (17) for the calibration of the latter.

Inventors: BOERO, PAOLO
Applicants: PIRELLI CAVI S.P.A.
Application year: 1989
Main classification: G01B11
IPC codes: G01D5