Light driven remote system and power supply therefor

A light driven remote system and power supply therefor that includes a laser diode that provides pulses of light to a fiber optic cable. The pulses of light are detected by a remote photodiode that is connected in parallel with a step-up transformer. The output of the step-up transformer is filtered to provide power for a remote system. Data transmission from the remote system is synchronized with transmission of light to the photodiode.

BACKGROUND OF THE INVENTION 
The present invention relates to light powered systems, and in 
particularly, to light powered remote systems and a power supply therefor. 
In remote systems, a sensor is typically located remote from its power 
supply and from the device monitoring the sensor's output. In electrically 
noisy environments, this physical separation practically ensures that the 
power supplied to the sensor will be noisy, causing the sensor to operate 
in an unknown and unpredictable manner. As a result, the sensor's output 
may not accurately reflect the parameter being sensed. This problem is 
compounded because the output of the sensor must also be routed through 
the electrically noisy environment, causing further noise to be added to 
the sensor's output. 
One approach to the noise problem is to employ a power supply located close 
to the sensor. An example of such a power supply is found in battery 
powered sensors. However, batteries have limited lifetimes. This makes 
batteries unacceptable for many applications. 
Another approach uses a remote power supply together with fiber optics to 
route the sensor's output to the device monitoring the output. This 
approach, however, solves only half of the problem. 
An additional approach eliminates the power supply altogether, and employs 
passive or electromechanical sensors. A fiber optic links the passive 
sensor to the device monitoring its output. Such passive sensors alter 
some property of light so that a parameter can be detected, i.e., 
transduced. These passive sensors, however, are expensive and can be used 
only to measure a limited number of parameters. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a light driven remote 
system for operation in an electrically noisy environment. It is another 
object of the present invention to provide a remote system driven by 
pulses of light. 
It is a further object of the present invention to provide a light driven 
remote system using conventional sensors. 
It is still another object of the present invention to provide a light 
driven remote system having sensors operated synchronously with pulses of 
light powering the system. 
It is still a further object of the present invention to provide a light 
driven remote system having sensors transmitting data synchronously with 
pulses of light powering the system. 
To achieve the above and other objects, the light driven remote system of 
the present invention comprises alight source means for providing pulses 
of light; a receiving means for receiving the pulses of light and for 
providing, a response to the pulses of light; a pulse signal having a 
magnitude; transform means for increasing the magnitude of the pulse 
signal and for providing a pulse supply signal that varies in accordance 
with the increased magnitude of the pulse signal; and a filter means for 
receiving and filtering the pulse supply signal and for providing an 
output responsive to the filtered pulse supply signal. 
In a preferred embodiment of the present invention, the light source means 
can comprise a laser diode driven at, for example, a 50% duty cycle. The 
receiving means can comprise, for example, a fiber optic cable and a 
photodiode. In a preferred embodiment of the present invention, the 
transform means can comprise a transformer connected in parallel across 
the photodiode, and the filter means can comprise a simple diode/capacitor 
network that filters the output of the transformer and provides a small 
d.c. output.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a block diagram of an embodiment of a power supply in accordance 
with the present invention. In FIG. 1, a drive circuit 10 drives a laser 
diode 15, causing the laser diode 15 to emit light. The drive circuit 10 
can drive the laser diode 15 with a 50% duty cycle pulse, having a 
frequency of 8 Khz, and a 90 ma peak. This particular drive is, of course, 
only an example, and other drive signals can be used. Light from the laser 
diode 15 illuminates the photodiode 20. I the embodiment shown in FIG. 1, 
a fiber optic cable 25 optically connects the photodiode 20 with the laser 
diode 15. 
A step-up transformer 30 has a primary coil 35 connected in parallel with 
the photodiode 20. Since the laser diode 15 is driven with a signal having 
less than a 100% duty cycle, pulses of light are provided to the 
photodiode 20. The photodiode 20 detects the pulses of light and generates 
current pulses in the loop comprising the photodiode and the primary coil 
35. The transformer 30 transforms the voltage across the photodiode 20 so 
that a larger voltage appears across a secondary coil 40 of the 
transformer 30. The voltage and current induced in the secondary coil 40 
by the photodiode 20 is filtered by a circuit comprising a diode 45 and a 
capacitor 50 that are connected in parallel across the secondary coil 40. 
The laser diode shown in FIG. 1 can comprise, for example, a laser diode 
manufactured by Mitsubishi and having part number 6101. The fiber optic 
cable can be a 400 um fiber. The photodiode 20 shown in FIG. 1 can 
comprise a photodiode manufactured by Hamamatsu and having a model number 
S874-18K. The transformer in FIG. 1 can have a ratio of primary turns to 
secondary turns in the order of 1:12.5. The capacitor 50 can have a 
capacitance of 1 .mu.F. 
FIG. 2 is a graph of the output of the power supply system shown in FIG. 1. 
In FIG. 2, waveform A represents the power in a load (identified by the 
right hand ordinate ledged) versus the resistance of a load. Waveform B 
represents the d.c. voltage across a load resistance (identified by the 
left hand ordinate legend) versus the resistance of a load. Referring to 
FIG. 2, at the point C, with 5 volts across a load resistance of 30 Kohm, 
a current of 170 .mu.A flows through resistance. With reference to 
waveform A, it is seen that the 5 volts across the load with a resistance 
of approximately 30 Kohm provides a near maximum power in the load. 
A trigger signal (TR) is provided by the drive circuit shown in FIG. 1. The 
trigger signal (TR) is provided in synchronism with pulses of light being 
generated by drive circuit 10. 
A Schmitt trigger or comparator 55 is connected to the output of the supply 
also shown in FIG. 1. The Schmitt trigger detects the magnitude of the 
output voltage and provides another trigger (TR.sup.1) signal when the 
output voltage reaches a specified value. The Schmitt trigger/comparator 
55 need not be employed, but does provide the advantage of being able to 
synchronize operation of the circuitry driven by the power supply with 
reception of light by the power supply. 
FIG. 3 is a schematic block diagram of a remote system embodying the 
present invention. In FIG. 3, the laser diode 15 transmits light pulses 
through the fiber optic cable 25 to the photodiode 20. A power conditioner 
60 corresponds to, for example, the transformer 30, diode 45 and capacitor 
50 shown in FIG. 1. The power conditioner 60 drives low power electronics 
65. A conventional sensor 70 is connected to the low power electronics 65. 
The low power electronics senses a parameter of the sensor (e.g., 
resistance) and drives a light emitting diode 75 in accordance with 
changes in the sensed parameter. The light emitting diode 75 transmits 
narrow pulses of light, having a period proportional to the sensed 
parameter, over a fiber optic cable 80 to a receiver 85 that includes a 
photodiode 90. The light pulses are kept as narrow as possible with 
respect to the overall period so that the power draw from the supply is 
minimized while still maintaining the ability to detect the pulses. A 
typical pulse width would be in the range of 1% to 5% of the period. 
In the system of FIG. 3, the power conditioner 60 is not affected by the 
electrical noise of the hostile environment since it is driven by pulses 
of light supplied by the fiber optic cable 25. In addition, the signal 
that varies in accordance with the sensed parameter is not affected by the 
electrically hostile environment because it too is transmitted from the 
remote system to the receiver 85 over the fiber optic cable 80. 
FIG. 4 is a schematic diagram of an embodiment of the low power electronics 
65 shown in FIG. 3. In FIG. 4, NAND gates 95 and 100 are connected to form 
an oscillator. Variations in the resistance of a temperature sensor 105 
cause the frequency of the oscillator to vary. The output of the 
oscillator is applied to a wave shaping circuit 110. The wave shaping 
circuit 110 provides a narrow pulse to a NAND gate 115. The NAND gate 115 
drives a transistor 120 that controls emission of light by the photodiode 
75. The frequency of light pulses provided by photodiode 75 varies in 
accordance with the resistance of the temperature sensor 105. 
FIG. 5 is a schematic block diagram of an embodiment of the receiver 85 and 
photodiode 90 shown in FIG. 3. In FIG. 5, a photodiode receiver 125 
includes the photodiode 90 shown in FIG. 3. The photodiode receiver can 
comprise part No. HFBR2402 manufactured by Honeywell. The output of the 
photodiode receiver 125 is applied to a comparator 130. The output of the 
comparator 130 drives a set/reset flip-flop 135 through an inverter 140. 
The other input of the set/reset flip-flop 135 (TR) is supplied by the 
laser power supply 10 shown in FIG. 1. The output of the set/reset 
flip-flop 135 corresponds to the sensor data shown in FIG. 3. In the 
embodiment of FIG. 5, the sensor data drives a transistor 140 which 
control the illumination of an LED 145. The pulse width of the LED light 
is proportional to the temperature sensed by the temperature sensor 105. 
Alternatively, the output of the set/reset flip-flop 135 could be applied 
to a processor and converted to temperature by way of, for example, a 
simple look-up table. 
Since the set/reset flip-flop 135 is controlled by the timing signal (TR) 
from the laser power supply 10, the light signal from photodiode 75 that 
is responsive to the resistance of the temperature sensor 105, is 
synchronized with transmission of light pulses to the power conditioner 60 
shown in FIG. 3. Synchronizing the transmission of light by the photodiode 
75 with application of light pulses to the power conditioner 60 is 
optional. It is, however, efficient to synchronize these events in the 
manner described above where the light pulses applied to the power 
conditioner 60 function as both a power source and a timing reference. If 
such synchronization is not used the oscillator of FIG. 4 would be allowed 
to free run b eliminating the TR' connection, and the set-reset flip-flop 
135 of FIG. 5 would not be used. 
FIG. 6 is a schematic block diagram of another remote system embodying the 
present invention. In FIG. 6, a first remote system 150 and a second 
remote system 155 receive light pulses and transmit data over the fiber 
optic cable 25. The first and second systems (150, 155) are both driven by 
light travelling supplied by laser diode 15 via the fiber optic cable 25 
and directional couplers 160 and 165. Similarly, light data transmitted by 
the first and second systems (150, 155) is routed to the photodiode 90 and 
receiver 85 via the directional couplers 160 and 165. While two remote 
systems are shown here the concept could be extended to include any 
number. 
Since the first and second remote systems (150, 155) are identical, the 
following discussion refers only to the first remote system 150. Unlike 
the system shown in FIGS. 1 and 4, in FIG. 6, a transmitter/receiver logic 
circuit functions so that the LED 75 serve the purpose of receiving light 
from the laser driver 10, and transmitting light to the receiver 85. Using 
a photodiode in a dual capacity is known. An example of switching between 
receiving light pulses and transmitting data is to have the photodiode 
connected to the power conditioner most of the time. It normally receives 
codes which are superimposed on the pulsed light imposed on the fiber 
optic cable 25. When the remote system recognizes its code, the photodiode 
is switched to the transmit mode and the data is transmitted by means of 
pulses where frequency is related to the parameter being sensed. 
By means of illustration only, a simple encoder/decoder system can comprise 
the drive circuit 10 driving laser diode 15 with a greater than 50% duty 
cycle for enabling one of the systems (150, 155) and less than 50% duty 
cycle for enabling the remaining system (150, 155). For example, system 
150 would have in the transmitter/receiver logic circuit 170 a filter 
coupled to a comparator which detects when the duty cycle of the received 
light pulses is less than 50%. The comparator output would drive a, for 
example, CMOS transmission gate to switch the diode 75 to the transmit 
mode from the receive mode when the duty cycle (e.g. average voltage) of 
the received light pulses is less than 50%. The directional couplers 160 
and 165 shown in FIG. 6 can comprise, for example, part No. SM3C, 
manufactured by Canstar Communications.