CA2036352A1 - Optical process variable transmitter - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
OPTICAL PROCESS VARIABLE TRANSMITTER
A light input (26) energizes an optical transmitter (10) providing a light output to a waveguide (16) indicating a process variable. The transmitter (10) has a process variable sensor (21) generating a sensor output. An electrical circuit receives the sensor output and generates an electrical transmitter output indicating the process variable adjusted by a changeable parameter stored in the electrical circuit.
The electrical circuit has an input (42,44) energizing the circuit. An LED (58) converts the electrical transmitter output to the light output. A gallium arsenide photodiode (42,44) converts a first portion of received light into electrical energy provided to the energization input (42,44) and converts a second portion of the received light into an electrical output controlling the stored parameter.

Description

~3~35;2 OPTICAL PROCESS VARIABLE TRANSMITTER

BACKGROUND OF~ 3~15LY
This invention relates to process variable transmitters which are optically energized and provide optical outputs representing the process variabl.e.
: The terms light, optic, and radiation as used herein refer to visible and invisible electromagnetic radiation with wavelenyths shorter than about 100 10microns (100,000 nanometers); the terms electric, electrical used herein refer to lower frequency phenomenon such as commonly occur in electronic circuits operating at frequencies below 100 MHz. The term "process variable" used herein refers to a variable such ~5 as pressure, temperature, f].ow, velocity, specific gravity, etc. sensed by a transmitter such as a process control or aerospace instrument.
UMMARY OF THE INVENTION
In the present invention, light coupled into a light input of a transmitter energizes electrical ci.rcuitry in the transmitter. The light coupled into the light input is modulated with commands which control l'operation of the transmitter inclucling commands which program one or more reprogrammable transmitter output parameters. The transmitter comprises sensor means which sense a process variable, and the transmitter transrn.its a programmed optical O-ltpUt inclicatir)g the process variabl.e to a medium or waveguide whlcll carries the opti.cal output.. The sensor means yenerate an e].ectrical sensor output representative of the process variable. The electrical sensor output is coupled to clrcuit means in the transmitter ~hich generate an , . . , . - :

;2 033~S2 ~2--electrical transmitter ou-tput indicating the process variable adj~lsted or programmed by a changeable or programmable parameter stored in the circuit means. The circuit means comprise an energization input for energizing the circuit means. The transmitter further comprises con~ersion means for converting the programmed electrical transmitter output to the programmed optical output. The conversion means further includes receiver means for receiving light to convert a first portion of the received light into electrical energy provided to the energization input and for con~erting a second portion of the received light into an electrical output controlling stored changeable parameters in the circuit means, thus providing programming of the transmitter 1.5 output.
The transmitter is coupled via a medium or waveguide means to an interface coupled to an electrical data bus. The interface comprises light generating means coupled to the waveguide means and including first means for generating a programming light component modulatecl to program the transmitter's generation of an optical output by adjustinc; the stored changeable parameter. The light generating means further comprise second means for generating an energi.ziny light component to energize the transmitter. The interface further comprises control means coupled to the light genera-ting means for electrica1.ly controlli.ng the modulation as a function of a firs-t reference recei~ed ~rom the bus and for electrically controlling amplltude oE the energi.zing light component as a functlon of a second reference in -the control means. The interface further comprises rece.i.ver mealls for pro~iding ~n ,:

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electrical output to the bus representative of a ~rogrammed op-tical output received from the transmi,tter.
In a preferred multidrop arrangement, the waveguide couples to a plurality of optical process variable transmitters and the light generatiny means further comprise means for generating a programming light component modulated to program generation of optical outputs by the plurality of addressable transmitters. The programming liqht component is preferably modulated according to a serial data protocol.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. l shows an embodiment o~ an optical transmitter coupled via waveguides to an interface.
FIG. 2 shows a second embodiment of an optical transmitter coupled via a waveguide -to an interface.
FIG. 3 shows an embodiment o~ a DC to DC
converter circuit used in an optical transmitter.
FIG. 4 shows an embodiment of an optical transceiver coupled to a waveguide.
FIG. 5 shows a second embodiment of a optical transceiver coupled to a waveguide.
FIGS. 6, 7 and 8 show three embodiments of multidrop optical transmitters coupled to a waveguide.
DE'r~ILED DESCRIPTION OF T~E PREFERRED EMBODIMENTS
In F'IG. l, optical pressure transmitter lO
senses process variable 12 and communicates via waveyui,des :l4, 16 with interEace 18. Inter~ace 18 intereaces the optical signals on waveguicles l4,16 to electrical bus 22 connectiny to control system 24 so that two-way communication is establislle~l between transmitter lO and control system 24, with electrically ' -: ' 2~363~2 insulating waveguides providing galvanic isolation between the transmitter 10 and bus ~.2.
Sensed process variable 1~ can be pressure, temperature, flow, pH, or the like. Waveguides 1~, 16 ean be single or multiple strand fiber optic cable and ean extend from a short distance up to thousands of meters in length as desired for a particular installation. Transmitter 10 is electrica]ly isolated ~rom control system 24 by waveguicles 19, 16 which conduct light but not eleetric current, thus preventing undesired coupling of electrical energy between control system 24 and transmitter 10.
Liyht generator 26, which can be a laser, couples light to transmitter 10 via waveguide 14. Light generator 26 generates a ~irst light component which is modulated to program transmitter 10 so that the transmitter's output on waveguide 16 is a programmed output. The modulation of the first light component will normally be a serial data string in a standard serial communication protocol using FSK techniques such as the HART Brand Communication Protocol of Rosemount Inc. or other standard protocol. Programminc~ can comprise storing the settings for span, zero, range, or the like in memory in the transmitter 10 for scaling the transmitter's output. The first light component can al~o be modulated ko interroyate the transm:Ltter. The transmitter responds to the interrocJation wi-th previously stored data such as transmitter location, the transmitter's materials of construction, diagnostic data, compensation and linearization clata, anfl the li)ce.
Light generator 26 generates a second light component ~/hich eneryizes transmitter 10, and this component is normally not modulatecl. Controller 28 .. ~ . :

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controls the operation of light generator 26. A first reference applied to analog buf~er 30 controls modulation of the ~irst light component. A second reference in controller 28 controls amplitucle or magnitude of the second light component so that a controlled amount of optical power is provided to waveguide 14. Controller 28 generates a modulatecl electrical output which is applied to a laser diode in liyht generator 26 to modulate the li.c3ht output. Bus 22 provides the first re-Eerence to buffer 30, typically comprising serial digital words indicating changes to the programming of transmitter lnl and indicating interrogation commands to be sent to transmitter 10, as well.
Interface 18 receives transmitter lO's light output from waveguide 16 at receiver 32, which can comprise a photodiode detector. Receiver 32 converts the received light output to an electrical signal for transmission to bus 22.
Control system 24 comprises a control 3~ which can comprise a computer coupled to bus 22 via modem 36.
Control computer 34 provides the reference to the bus which is amplified ~y analog buffer 30. Control computer 34 receives in~ormation contained in the transmitter's light output about the process variahle from bus 22 and uses the information to control a process (not shown) or parameter of the process generating the process variable 12.
In transmitter 10, coupler 3~ splits li.yht from light yenerator 26, couplinc~, in one preferrecl embodiment, approximately 1% to convertor 92 and coupliny the remainder, less losses in the co~pler, to convertor 44. Convertors 42, 44 can comprise :
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photodiodes, and preferably comprise gallium arsenlde photodiodes-to provide preferred high voltage output and high conversion efficiency. Convertor 44 is coupled to a power converter circuit 46 which converts, in one preferred embodiment, the relatively low voltage output (about 0.9 volts) of convertor 44 to a higher voltage output 48A, 48B (3.5 to ~ volts) suitable for energizing MOS circuits. Alternatively, converter 44 can comprise several photodiodes connected in series to provide the higher voltage, making power convertor 46 unnecessary.
The output 48A, 48B is applied to sensor circuit 50 for energizi.ng the sensor circuit. Sensor circuit 50 preferably comprises MOS circuitry and a process variable sensor sensing process variable 12. Sensor circuit 50 provides electrical output 52A, 52B
indicative of a magnitude of process variable 12.
Convertor 42 senses the modulated component of light on waveguide 14 and couples an electrical signal representative of the modulation to circuit 50 vi.a line 54. A driver circuit 56 is energized from lines 48A, 48~ and controlled by output on ].ines 52A, 52B to modulate emitter 58, which can be a ~ight emitting diode. Emitter 58 couples an optical or light output of transmitter 10 along waveguide 16 back to interface 18.
The emboditnent shown in FIG. 1 enables control. system 24 to program transmitter 10 with span, Zero, temperature correction data, and the like, and recei.ve a programmed transmitter output back from transmitter 10 without the need for electrical connections or p~wer sources of any kind other than through waveguide 14 to transmitter 10.
All of the power for transmitter 10 is provided by the opti.cal waveguide 1~.

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In FIG. 2, a further preEerred embodiment oE
an optical communieation system is shown. Components with reference numbers corresponding ~o components previously described perform the same functions. In FIG. 2, a single waveguide 62 couples transmitter 60 to interface 66 whieh in turn couples to control system 68.
Interfaee 66 ineludes a eoupler 70 which eouples light generator 26 and light reeeiver 32 to waveguide 62.
Light output by light generator 26 preferably is at a wavelength (e~g., 800 nanometer) different from the wavelength (e.g., 660 nanometer) of light output from transmitter 60. In this ease, coupler 70 is preferably a dichroic mirror which enhances the optieal ~hroughput of the system and optical filter 72 can be used to filter out residual light originating from the light generator 26.
In FIG. 2, a deviee 6~ eouples to single waveguide 62 to reeeive light from light generator 26 and also transmits the transmitter light output to interface 66 through waveguide 62. Deviee 64 comprises a gallium arsenide photodiode which provides an eleetrical output on line 74 eoupling to power converter 46. The output on line 74 is also capacitively eouplecl alony line 76 to provide the modulated eomponent of received liyht to the sensor eircuit 50. Deviee 6~
further eornprises a liyht emitting diode driven by clriver 56. The embocliment ln ~IG. 2 provicles communieation and energization as in FIG. 1, however, in FIG. 2 this is achieved with a sincJle waveguide between :30 transmitter 60 and interfaee 6~. Various eor,~binatiolls of uses of eouplers and extraetinc~ modulation shown in FIGS. 1 and 2 ean be used to acllieve the same resultiny energization and communication. In FIGS. 1 and 2, the : , , .

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light generator 26 provides all of the energi~ation for the transmi~ter, and ther~ is no lleed for separate sources of power such as wires, batteries, or solar cells. Electrical circuitry in the transmitter and liyht modulation in the transmitter i.s so].e:ly powered by light received from a waveyuide.
In FIG. 3, a circuit diagram of po~er converter 46 is illustrated coupled to a gallium arsenide photodiode ~30 receivi.ng light for energization from waveguide 82. Photodiode 80, in turn, energizes a free running multivibra-tor circuit 84. The multivibrator circuit 84 generates a pair of oscillatory outputs 86A, 86B which are electrically out of phase with one another. Photodi.ode 80 also energizes a step up transformer type power supply ~8. The outputs 86~, 86B are coupled to transistors 90A, 90B respectively driving a primary winding of transformer 92. A
secondary winding 92A of transformer 92 is coupled to full wave rectifier 94 to provide a voltage output 96 (3.5 to 5 volts~ higher than the voltage used to energize the converter 46, typically 0.~ volts from gallium arsenide photodiode 80. Transistors 90A, 90B, 98 can be germanium type transistors to achi.eve operation at even lower voltages.
In F:[G. 4, a solid state o:r semiconcluctor device 6~A is shown providing the function of device 64 oE 1~'IG. 2 or the function of convertors ~2, ~ an~
coupler 38 in FI.G. 1. In FIG. fi, a liyht emi~ting diode 106 cJenerates a light output at a first wavelenyth.
Layers 102, 104 are formed of materials which are substantially transparent at the first wavelenyth. I.i.ght output from light emitting cliode 106 couples through layers 102, 104 to waveguide 100 forming the light -~3635:~:

output ~rom the transmitter. Layer 104 comprises a photodiode sensor for sensing modulation and corresponcls to convertor 42 in FIG. 1. Layer 102 comprises a sensor for providing electrical energization and corresponds to convertor 44 in FIG. 1. The light for energlzation is at a second wavelength different than the first wavelength, and the light for modulation is at a third wavelength differenk from the first' and second wavelengths. The sensors in layers ]02, 104 are correspondingly wavelength selective so that separate modulation and power outputs are generated on leads 108.
Alternatively, energi~ation and modulation can be at the same wavelength, diode 104 can be eliminatecl and modulation and power can both be detected by a single photodiode on la~er 102.
In FIG. 5, another device 110 ~or use in transceiving ]ight in a transmitter is shown which receives and transmits light to a waveguide 112. Gallium arsenide photodiode 114 receives light for energizing the transmitter and converts it to electrical energy.
Photodiode 116 receives light and provldes the modulation signal. As explained above, photodiode 116 can be left out when the modulation signal is taken from photodiode 114 as in FIG. 2. The light output o~ the transmitter can be yenerated by a licJht emitting diode 118 disposecl at location 118A, 118B, or 118C. For location 118B, a port 120 throucJh the pho-todiode 11~
allows liyht from the light em;tting diode at 11.8B to reach the waveyuide 112. For location 1l8C, the licJh-t from IJED 118 is reflected from the photodiode 11~ into the waveguidc 112. Tlle shapes of the photodiode surfaces 114, 116, 118A and their arranyement can be any arrangement convenient for coupling light to and from waveguide 112. Preferably, the active elements substantially fill the light capture angle (numerical aperture) of the waveguide to reduce losses. Active areas can be concentric rings, sectors of a circle or an arbitrary shape.
In FIG. 6, a "multidrop" arrangement of transmitter 60A, 608, 60C coupling to a single waveguide 62 is shown. Interface 66 (shown in FIG. 2) couples enough light to the waveguide 62 to provide all of the energization for multiple transmitters. The liyht outputs of e~ch of the transmitters is formed as a digital word which includes an address identifying the transmitter to the interface 66. Likewise, the modulated output from light converter 26 (shown in FIG.
2) comprises digital words which include an address identifying the transmitter which is to receive and respond to the digital word. In ~IG. 7, a further alternative embodiment of a multidrop arrangement of a plurality of transmitters 60D, 60E, 60F coupling along a single waveguide 62 to an interface 66 (shown in FIG.
2). The light outputs from each -transmitter can comprise serial digital data in a selected multidrop digital protocol such as the HART digital protocol of Rosemount Inc.
2~ In ~IG. 8, a further embodiment of a multidrop arrangement of transmitters 60G, 60~1, 60J coupling via waveyuide 62 to interface 66 (shown in FIG. 2) is shown.
In F:[G. 8, wave:Length div:ision couplers 130A, 1308, 130C
in each transmitter provide optically separatecl coupling paths for the excitation and tnodulation at one wavelength and the transmitter light OlltpUt at a second wavelenyth diEferent than the first. Many known wavelength coding and decoding arrangements can be used, z~ :

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and many known electrical communication protocols, includin~ hal~ duplex and full dupleY. arrangements can be adapted for use in optical commurlication over the optical media.
Although the present invention has been described with reference to prefe:rred embodiments, workers skilled in the art wi.ll recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

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Claims (18)

1. A transmitter having a light. input energizing the transmitter and providing a light output to an optical medium indicating a process variable, comprising:
sensor means for generating an electrical sensor output representative of the process variable;
circuit means receiving the sensor output for generating an electrical transmitter output indicating the process variable adjusted by a changeable parameter stored in the circuit means, the circuit means having an energization input for energizing the circuit means; and conversion means for converting the transmitter output to the light output, the conversion means further including receiver means receiving light for converting a first portion of the received light into electrical energy provided to the energization input and for converting a second portion of the received light into an electrical output controlling the parameter.

2. The transmitter of Claim 1 wherein the changeable parameter comprises a changeable measurement range of the process variable.

3. The transmitter of Claim 1. wherein the receiver means receive the received light from a waveguide.

4. The transmitter of Claim 3 wherein the conversion means include an emitter surface emitting light and the receiver means include a receiving surface for converting the received light.

5. The transmitter of Claim 4 wherein the waveguide receives and transmits light within a light capture angle.

6. The transmitter of Claim 5 wherein the emitting surface and the receiving surface substantially fill the light capture angle of the waveguide.

7. The transmitter of Claim 6 wherein the receiving surface is disposed along a light path between the waveguide and the emitting surface, the receiving surface being arranged to permit light to pass along the path from the emitting surface to the waveguide.

8. The transmitter of Claim 7 wherein the receiving surface has a port therethrough for passing the light from the emitting surface.

9. The transmitter of Claim 7 wherein the receiving surface is at least partially transparent to pass light from the emitting surface.

10. The transmitter of Claim 7 wherein the receiving surface reflects light from the emitting surface to the waveguide.

11. The transmitter of Claim 1 wherein the received light represents' a function of a desired span setting and the parameter stored in the circuit means is a function of the desired span setting programmed by the received light.

12. The transmitter of Claim 1 wherein the received light represents a linearity correction of the transmitter output and the parameter stored in the circuit means is a function of the linearity correction programmed by the received light.

13. The transmitter of Claim 1 wherein the received light represents a temperature correction of the transmitter output and the parameter stored in the circuit means is a function of the temperature correction programmed by the received light.

14. An interface between an optical medium communicating with a remote optical process variable transmitter and an electrical bus, comprising:
light generating means coupled to a waveguide and including first means for generating a programming light component modulated to program the transmitter's generation of an optical output, and second means for generating an energizing light component energizing the transmitter;
control means coupled to the light generating means for electrically controlling the modulation as a function of a first reference received from the bus and for electrically controlling amplitude of the energizing light component as a function of a second reference in the control means; and receiver means for providing an electrical output to the bus representative of a programmed optical output received from the transmitter.

15. The interface of Claim 14 wherein the waveguide couples to a plurality of optical process variable transmitters and the first means further comprises means for generating a programming light component modulated to program generation of optical outputs by the plurality of transmitters.

16. The interface of Claim 15 wherein the energizing light component energizes the plurality of optical process control transmitters.

17. The interface of Claim 16 wherein the programmed optical output is coupled along the waveguide.

18. The interface of Claim 17 wherein the receiver converts the programmed optical outputs from a plurality of optical process transmitters to a common electrical bus.