Transducer apparatus utilizing fiber optics for data transmission

A transducer apparatus utilizing fiber optics for data transmission is disclosed in which an output optical signal is encoded with information representative of a parameter being measured and the encoded optical signal transmitted to a remote measurement and control apparatus site for decoding. The transducer is adapted to receive an input optical signal, and converts the input optical signal into electrical energy to power a driver. An electrical drive signal, generated by the driver, has a selectively variable frequency representative of the parameter being measured. Using the electrical drive signal, the output optical signal is generated. The driver includes a variable frequency oscillator whose frequency of oscillation is dependent upon the parameter being measured.

CROSS REFERENCE TO RELATED APPLICATIONS 
Passive Transducer Using Fiber Optics for Data Transmission, Ser. No. 
371,830, filed on Apr. 26, 1982 invented by James F. Lockett, Jerry F. 
Janecka, Thomas O. Mehrkam, William F. Wiseman and Robert L. Remels and 
assigned to the assignee of the present application; Transducer for 
Optical Data Transmission, Ser. No. 371,951, filed on Apr. 26, 1982, 
invented by James F. Lockett, Jerry F. Janecka and Thomas O. Mehrkam and 
assigned to the assignee of the present application. 
BACKGROUND OF THE INVENTION 
This invention relates generally to transducers, and more particularly to a 
transducer which utilizes fiber optics for data transmission. 
Physical parameters, such as temperature, pressure, force, stress, 
displacement, flow, and so forth, are utilized daily for monitor and 
control of processes. To monitor such parameters, transducers have 
generally been used to transform the parameter to be measured into an 
electrical signal, and the resulting electrical signal transmitted over 
electrical wires from the point of measurement to a second point at which 
measurement and control apparatus are located. And, in most instances, 
electrical power to operate the transducer is brought to the point of 
measurement from the measurement and control apparatus location over 
electrical wires. 
Such systems, however, are susceptible to numerous problems when used in 
hostile or explosive environments. For example, the electrical wires used 
to transmit the electrical signal representative of the parameter from the 
transducer to the measurement and control apparatus location, and the 
electrical wires used to transmit electrical power to the transducer from 
the measurement and control apparatus location are susceptible to 
electromagnetic interference. Such electromagnetic interference can 
produce undesired impulses on the electrical wires which can result in 
transmission of incorrect data. Additionally, shorting due to insulation 
failure and the like can result in explosion caused by generation of an 
electric arc in the explosive environment, or by heating an element past 
the flash-point of the explosive environment. 
SUMMARY OF THE INVENTION 
According to a preferred embodiment of the present invention, an optical 
signal representative of the parameter to be measured is produced by the 
transducer, and the optical signal is then transmitted from the transducer 
to the remote measurement and control apparatus site. Electrical power may 
be supplied to the transducer by an optical input signal transmitted to 
the transducer from the remote site, thereby eliminating all electrical 
wire connections between the transducer and the remote measurement and 
control apparatus site. 
According to one preferred embodiment of the present invention, an input 
optical signal transmitted from the remote measurement and control 
apparatus site is received at the transducer and converted into an 
electrical signal for powering the transducer. So powered, the transducer 
generates an electrical drive signal representative of the parameter being 
measured. The electrical drive signal is then converted into an optical 
signal representative of the parameter, and this optical signal 
transmitted back to the remote site.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a transducer apparatus includes an optical coupler 92 
which receives an input optical signal 10 and transmits an optical signal 
12. Optical signal generator 100 receives optical signal 12 and generates 
modulated optical signal 18 in such a manner that the modulated optical 
signal is representative of a parameter 30 that is monitored by modulator 
100. The modulated optical signal is received by optical coupler 94 which 
in turn transmits optical output signal 22 from the transducer. 
FIG. 2 illustrates the optical signal generator 100 in block diagram form. 
Referring to FIG. 2, optical signal 12 is received by converter 108 which 
converts the optical signal into a first electrical signal 40. The 
electrical signal 40 is used to power a driver 120 which monitors 
parameter 30 and generates an electrical drive signal 42 representative of 
the parameter. Electrical drive signal 42 drives a converter 150 which 
produces generated optical signal 20, the generated optical signal being 
representative of parameter 30. 
FIG. 3 illustrates a measurement system having a transducer as illustrated 
in FIG. 2. Light source 90, photodiode 96, and measurement circuit 98 are 
located at a remote measurement and control apparatus site. Input optical 
signal 10, generated by light source 90, is transmitted to the transducer 
by optical fiber 92. Optical signal 12 impinges on photovoltaic cells 109 
which, in turn, produce a first electrical signal 40 across capacitor 112. 
The first electrical signal is used to power integrated injection logic 
(I.sup.2 L) oscillator circuit 124, as well as optional divider 130 and 
optional pulse generator 132. 
Oscillator circuit 124 is preferably a variable frequency oscillator whose 
frequency of oscillation is controlled by crystal 126. The crystal is 
disposed with respect to a moving mechanical member 128 such that pressure 
32, the parameter being measured, inpinges upon member 128. In turn, 
member 128 inflicts a force upon crystal 126 thereby changing the 
frequency at which the crystal oscillates proportionally to applied 
pressure 32. Alternatively, any circuit component which can be used to 
vary the frequency of the oscillator, for example a resistance element, an 
inductive element, or a capacitive element, may be used in place of 
crystal 126. When a parameter other than pressure is being monitored, the 
particular parameter is inflicted upon the variable circuit element in 
such a manner that the oscillator circuit frequency is varied in response 
to the parameter. 
Preferably, the crystal should be fairly low frequency, for example 
approximately 10 kHz. The output of oscillator circuit 124 may be used to 
directly drive light emitting diode (LED) 152. Alternatively, the 
frequency of the output signal from oscillator circuit 124 may be reduced 
by a divider 130 and the output of the divider coupled to pulse generator 
132 which, in turn, produces the electrical drive signal for LED 152. 
Light emitting diode 152 is thus driven by an electrical signal which is a 
function of the parameter being measured. Consequently, the light produced 
by LED 152, generated optical signal 20, is representative of the 
parameter measured. The generated optical signal is transmitted to the 
remote site over optical fiber 94. At the remote site, output optical 
signal 22 is measured by photodiode 96 and measurement circuit 98. 
The oscillator circuit 124 may comprise any suitable oscillator circuit, 
for example, a complimentary metal oxide semiconductor (CMOS) oscillator 
or an I.sup.2 L oscillator. The latter, however, is preferred since an 
I.sup.2 L oscillator works on a lower voltage than a CMOS oscillator. 
In the embodiment illustrated in FIG. 3, the capacitor 112 is used to 
provide LED current during LED pulsing. The LED is pulsed with short 
pulses approximately 1 microsecond in duration. Advantageously, the 
crystal frequency of the embodiment illustrated in FIG. 3 may be 
approximately 16 kHz and the divider 130 may divide by 2.sup.10. And, as 
will be apparent to those skilled in the art, photovoltaic cells 109 and 
LED 152 may be disposed in a single array and a single optical fiber used 
in place of optical fibers 92, 94. 
In still another embodiment of the present invention a battery or other 
suitable electrical storage device may be included in the transducer 
thereby eliminating the need for converting any of the optical signal 
received by the transducer into a first electrical signal to power the 
transducer. The battery may advantageously be charged by a solar cell. 
Referring to FIG. 4, one such embodiment includes only one optical 
coupler, optical coupler 94, because no input optical signal need be 
transmitted to the transducer. Such a transducer includes a battery or 
other suitable power member 116 which is self-contained at the location of 
the transducer and which produces the first electrical signal 40 used to 
power driver 120. The remaining portions of the circuit illustrated in 
FIG. 4 perform identically as previously described. 
The principles, preferred embodiments, and modes of operation of the 
present invention have been described in the foregoing specification. The 
invention which is intended to be protected herein, however, is not to be 
construed as limited to the particular forms disclosed, since these are to 
be regarded as illustrative rather than restrictive. Moreover, variations 
and changes may be made by those skilled in the art without departing from 
the spirit of the present invention.