JPH0116005Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical sensor that detects the state of an optical fiber by inputting light from one end of the fiber and measuring the amount of light reflected from the other end.

Optical fibers have various advantages such as being unaffected by surrounding electromagnetic fields, having a thin cross-sectional dimension and being lightweight, having no problem with sparks, and having good heat resistance and corrosion resistance. It is used as a sensor for process control.

FIG. 1 is a diagram showing the main parts of a reflected light optical sensor used in a conventional liquid level gauge using an optical fiber. In FIG. 1, light emitted from a light emitting device 1 (usually a light emitting diode or a laser diode) propagates through optical fibers 2 and 3 and enters a reflecting member 4. In FIG.
The reflecting member 4 is a glass rod whose tip is polished into a conical shape or a corner reflector shape. When this reflective member 4 is in the air above the liquid level, the incident light is almost totally reflected, and when it is in the liquid below the liquid level, the difference in refractive index between the liquid and glass is much smaller than in the case of air and glass. As a result, the proportion of light rays reflected at the tip is significantly reduced. Therefore, the reflected light is detected by the photodetector 7 via the optical directional coupler 5 and the optical fiber 6, and the output is compared with the reference voltage level Vref by the voltage comparison circuit 8, thereby determining the liquid level position. is above the tip of the reflecting member 4 or not.

However, in such a conventional device, an optical fiber directional coupler is inserted in the middle, which makes the device configuration complicated and expensive. Further, it has the disadvantage that fluctuations in temperature characteristics and the like of the directional coupler directly affect the measurement accuracy of the optical sensor.

The present invention improves on this point, making it possible to simplify the device structure, reduce the cost, and
Moreover, it is an object of the present invention to provide a reflective optical fiber sensor with good measurement accuracy.

The present invention is based on a semiconductor light emitting device (LD,
LED, etc.) can be used as a photodetector that generates a photocurrent when it receives optical input at its P-N junction in a non-operating state (non-emitting state), and the propagation speed of an optical signal in an optical fiber. This takes advantage of the fact that it takes a relatively long time, approximately 100 ns, to make a round trip over a length of 10 m.

The present invention includes a conversion element that converts an electrical signal into an optical signal and a conversion element that converts the optical signal into an electrical signal, and an optical fiber having one end optically connected to the conversion element, and an optical fiber that is optically connected to the conversion element at one end. A drive circuit that sends out an optical signal by driving a conversion element that converts a signal into an optical signal in a pulsed manner using an electric signal, and a comparison result that compares the received signal converted into an electric signal by the conversion element with a comparison reference signal. In the optical fiber sensor equipped with a comparison circuit that outputs a The present invention is characterized in that it is constituted by one conversion element and includes means for inhibiting the output of the comparison result of the comparator circuit during the time period during which an optical signal travels back and forth through the optical fiber in synchronization with the output of the drive circuit.

An embodiment of the present invention will be described based on the drawings.
FIG. 2 is a block diagram of the main parts of the first embodiment of the present invention. That is, the output of the drive circuit 11 is guided to the conversion element 10 and the timing control circuit 12. Light emitted from this conversion element 10 is guided via an optical fiber 3 to a reflecting member 4 attached to the other end. The reflected light from the reflecting member 4 returns to the conversion element 10 via the optical fiber 3 again. The output of this conversion element 10 is guided to a resistor 13 and also to an analog gate circuit 14. The output of the timing control circuit 12 is led to this analog gate circuit 14, and the output of this analog gate circuit 14 is led to the voltage comparison circuit 8. Moreover, in FIG. 2, 15 indicates an optical connection part (optical connector).

With such a circuit configuration, an optical signal is provided from the drive circuit 11 to the conversion element 10 in the form of an intermittent signal, for example, a pulse signal having a constant repetition rate and pulse width, and the light emission is transmitted through the optical fiber 3. is incident on the The light propagated within the optical fiber 3 is reflected from the reflecting member 4 and enters this reflected light or the same conversion element 10 . At this time, the voltage applied to the conversion element 10 becomes a negative value (in principle, it may be zero) by the drive circuit 11, and the conversion element 10 changes from the light emission mode to the light reception mode, so that the voltage applied to the conversion element 10 changes according to the amount of light received. A voltage is generated across the load resistor 13. This light reception signal is input to the voltage comparator circuit 8 via an analog gate circuit 14 controlled by a timing control circuit 12.

Figure 3 shows a time chart of this operation. In other words, Fig. 3a shows the pulse for driving the transmission light,
b indicates a received pulse, and timing control circuit 12
Therefore, the gate circuit 1 is activated within the range of the timing T _W required for measurement, which is delayed by T _D from the transmitted light pulse a.
A signal b is given to voltage comparator circuit 8.
is input. The voltage comparison circuit 8 compares this signal with the reference voltage signal Vref to detect the reflection level at the reflection member 4.

In addition, the optical connection part (optical connector) 1 shown in FIG.
If there is a reflective element such as 5, a noise component as shown in _FIG . That interference is eliminated.

Although this first embodiment shows the case where the reflected signal input to the voltage comparator circuit is controlled, the voltage comparator circuit is generally a circuit that determines the relative relationship between two input signals, and as shown by the broken line in FIG. The same purpose can be achieved by controlling the reference voltage input Vref using the transmitted light pulse signal.

FIG. 4 is a block diagram of the main parts of the second embodiment of the present invention. In this second embodiment, the signal voltage across the load resistor 13 is directly input to the voltage comparison circuit 8'. This results in a very simple configuration. However, the voltage comparator circuit 8' has a strobe function in its input or output circuit, and as in the first embodiment, the timing control circuit 11 has a strobe function.
It is used to control the voltage comparison function by the output signal S _C of No. 2.

In FIG. 5, the voltage comparison circuit 8' portion of FIG. 4 is realized by a general voltage comparison circuit 8 and a logic gate circuit 16.

Note that although the above embodiments have only described liquid level sensors, the present invention is not limited to this, and can be applied to any type of reflective light that performs detection using the variation in the level of reflected light from a part of an optical fiber. It can be commonly used for fiber sensors.

As explained above, according to the present invention, an element that can operate by switching between a light emitting mode and a light receiving mode is used as a conversion element serving as a light source, and the detected received signal is compared in a comparison circuit, and the timing at which the comparison output is determined is It synchronizes with the time that the optical signal travels back and forth through the fiber, and masks the comparative output during that time.
It is possible to perform accurate measurements without outputting erroneous detection results due to noise detected during the round trip time.

Furthermore, since only one conversion element is required, there is no need for a directional coupler of optical fibers as in the prior art, and the device can be simplified and manufactured at low cost.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of the main parts of a conventional device. FIG. 2 is a diagram showing the main parts of the first embodiment of the present invention. FIG. 3 is an operation time chart of the above embodiment. FIG. 4 is a block diagram of the main parts of the second embodiment of the present invention. FIG. 5 is another configuration diagram of the voltage comparison circuit portion of FIG. 4. 1... Light emitting element, 2, 3, 6... Optical fiber,
4... Reflection member, 5... Optical directional coupler, 7...
Photodetector, 8... Voltage comparison circuit, 10... Conversion element, 11... Drive circuit, 12... Timing control circuit, 13... Load resistor, 14... Analog gate circuit, 16... Logic gate circuit.

Claims (1)

[Claims for Utility Model Registration] An optical fiber comprising a conversion element that converts an electrical signal into an optical signal and a conversion element that converts the optical signal into an electrical signal, and one end of which is optically connected to the conversion element; A drive circuit that drives a conversion element that converts an electric signal into an optical signal among the elements in a pulsed manner with an electric signal to send out an optical signal, and a drive circuit that sends out an optical signal by driving a conversion element that converts an electric signal into an optical signal, and compares a received signal converted into an electric signal by the conversion element with a comparison reference signal. and a comparison circuit that outputs a comparison result, wherein the conversion element converts an electrical signal into an optical signal and converts an optical signal into an electrical signal by switching between a light emission mode and a light reception mode. It is comprised of one conversion element capable of bidirectional conversion, and includes means for inhibiting the output of the comparison result of the comparison circuit for the time period during which the optical signal travels back and forth in the optical fiber in synchronization with the output of the drive circuit. Characteristic optical fiber sensor.