GB2035561A

GB2035561A - A photoelectric level switch

Info

Publication number: GB2035561A
Application number: GB7936260A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-11-06
Filing date: 1979-10-18
Publication date: 1980-06-18
Also published as: DE2940799A1; AT382967B; GB2035561B; ATA793878A

Abstract

A photoelectric level switch comprising at least one light source having a beam which is conveyed to one out of a number of photocells or the like, depending on the optical density of a medium in a standpipe, characterised in that a first photocell 3 is disposed in the path of a beam 10, 11 reflected by the inner surface 18 of the standpipe when the pipe is empty and a second photocell 4 is disposed in the path of the transmitted light 13, 14, 15 when the standpipe 1 is full of fluid. <IMAGE>

Description

SPECIFICATION A photoelectric level-switching device The invention relates to a photoelectric level switch comprising at least one light source having a beam which is conveyed to one out of a number of photocells or the like, depending on the optical density of a medium in a standpipe.

In the case of known photoelectric level switching devices for liquids, use is made of a light source transmitting beams through a transparent standpipe and a photocell which is not disposed in the beam from the light source unless the stand pipe is either empty or full. The electrical resistance of the photocell changes in dependence on the amount of light received. Normally, use is made of phototransistors which interrupt the current when not illuminated, whereas when they are illuminated they transmit a current which depends on the operating voltage and the intensity of illumination. The components used must be small, to ensure that even slight changes in the level of the liquid can be evaluated in the form of switching pulses.Consequently, the light sources are weak and an amplifier must be connected downstream of the phototransistor, so as to react to small differences in voltage. The amplifier has to be accurately adjusted owing to environmental conditions, e.g. temperature and the changes in the optical properties of the standpipe and liquid when dirty. A disadvantage of this kind of level switching devices, when used industrially, is that they often have to be readjusted and therefore break down easily.

In the case of another known level switch of the initially-mentioned kind, a number of photocells are disposed around the standpipe. The object is to distinguish between liquids having different indexes of refraction. This known device has substantially the same disadvantage as the previously-described devices. As a result of the comparatively slight deflection of the light beam when the standpipe is empty or full, the response of the known device varies with slight changes in the environmental conditions, so that the device can break down easily.

An object of the invention is to give a photoelectric level switch optical and electric properties such that it operates substantially independently of its surroundings. To this end, a first photocell is disposed in the path of a beam reflected by the inner surface of the standpipe when the pipe is empty and a second photocell is disposed in the path of the transmitted light when the standpipe is full of fluid. Other features will be clear from the following description of an embodiment, in conjunction with the claims.

Figure 1 shows the path of rays when the standpipe is emtpy, Figure 2 shows the path of rays when the pipe is full, Figure 3 is a block diagram of the electric circuit of the photocells, and Figure 4 shows a variant having two light sources. The standpipe shown in Figs. 1 to 4 has an annular cross-section, but the crosssection can be any geometrical shape which enables light barriers to produce a path of rays as described hereinafter.

Fig. 1 shows a transparent standpipe 1 and a liquid at a level 6 below the line 5 joining the light source 2 to the photocell 3. A beam 8 from source 2 is refracted on transit from air to the optically denser material of pipe 1, and then travels in the direction 9 inside the pipe wall. When it strikes the inner surface 18 of the pipe, beam 9 divides into a reflected beam 10 and a transmitted beam 1 2. The reflected beam 10 is again diffracted at the outer surface 1 7 of the pipe on transit from the denser material to air. It then travels in direction 11 and strikes photocell 3. The signal 1 2 travelling through the empty pipe 1 does not reach a photocell.

Fig. 2 shows standpipe 1 holding liquid at a level 7 which is above the line 1 6 from the light source 2 to photocell 4. Beam 8 from source 2 is reflected on transit from air to the optically denser stand pipe material, and then travels in direction 9. Standpipe 1 is filled with a transparent fluid having an optical density which is usually considerably different from that of air but only slightly different from that of the standpipe. The beam portions 9, 13, 1 4 through the full standpipe 1 experience very slight changes in direction. The only appreciable refraction occurs when beam portion 1 4 strikes the outer surface 1 7 of the pipe, as a result of the transit from the denser pipe material to air. The refracted beam 1 5 strikes photocell 4.

Fig. 3 is a block circuit diagram showing the basic electric circuit of photocells 3 and 4, which are e.g. phototransistors. A positive voltage or current is supplied by line 1 9 and a negative current by line 20. Phototransistors 3 and 4 are connected in series, ie. the input line 21 of photocell 3 is connected to the positive supply line 1 9 and the output line 22 of photocell 3 together with the input line 23 of photocell 4 and the output line 24 of photocell 4 are connected to the negative supply line 20.

An amplifier 33 of known construction is connected via line 25 to the positive line 1 9 and via line 26 to the negative line 20.

Amplifier 33 is adapted to amplify the current through photocells 3 and 4. The input point 34 of amplifier 33 is connected via line 27 and junction 32 to the output line 22 of photocell 3 and to the input line 23 of photocell 4.

When standpipe 1 is empty, the beam from light source 2 travels via 8, 9, 10 and 11 to photocell 3 and the electric photocurrent flows in the direction of arrow 29. When pipe 1 is filled with liquid, the beam from source 2 travels via 8, 9, 13, 14, 1 5 to photocell 4 and the electric photocurrent flows in the direction of arrow 30. The equalizing the closedcircuit currents, depending on the operating voltage and the surrounding conditions, flow in the direction of arrow 31 and thus equally influence photocurrent 29 or 30 when pipe 1 is empty or full. When the level of a liquid rises, the changeover from an empty to a full pipe in the neighbourhood of light barriers 2, 3 and 2, 4 is transmitted in the form of a switching pulse from the output point 35 of amplifier 33 along the output line 28.The switching pulse is a change in the voltage 36 between lines 1 9 and 28 or in voltage 37 between lines 20 and 28. Since photocells 3 and 4 are connected in series and input line 27 of amplifier 33 is connected to input line 23 of one photocell and output line 31 of the other photocell at junction 32, equalizing currents of the photocells dependent on the environment, flowing in the direction of arrow 31, cannot appreciably influence the output voltages 36 and 37 of amplifier 33. Since, furthermore, current 31 has the same effect on current 29 when the standpipe is empty as on current 30 when the standpipe is full, there will be not change in the average values of voltages 36 and 37.

Amplifier 33 can be any circuit capable of converting small voltage differences between lines 25, 27 or 26, 27 into larger voltage differences between lines 19, 28 or 20, 28.

Fig. 4 shows a variant with two light sources. Each source is associated with only one photocell. When standpipe 1 is empty, the photo-current is generated by photocell 3 if the beam from source 2 travels along 8, 9, 10 and 11. When pipe 1 is filled with liquid the photocurrent is generated by photocell 4 if the beam from the second light source 38 travels along 39, 40 and 41. The photocells have substantially the same electric circuit as shown in Fig. 3.

Claims

1. A photoelectric level switch comprising at least one light source having a beam which is conveyed to one out of a number of photocells or the like, depending on the optical density of a medium in a standpipe, characterised in that a first photocell is disposed in the path of a beam reflected by the inner surface of the standpipe when the pipe is empty and a second photocell is disposed in the path of the transmitted light when the standpipe is full of fluid.

2. The photoelectric level switch according to claim 1, characterised in that the photocells are connected in series and the input line of the first photocell, the output line of the second photocell and the input line of the downstream amplifier are connected to a common junction.

3. The photoelectric level switch according to claim 1 or 2, characterised in that each photocell is associated with a separate light source.

4. A photoelectric level switch constructed and arranged substantially as herein described and as shown in Figs. 1, 2 and 3 of the accompanying drawings.

5. A photoelectric level switch constructed and arranged substantially as herein described and as shown in Fig. 4 of the accompanying drawings.