Optical sensors
There is often a misinterpretation of such a simple concept as the one mentioned in the title of the article. It is not a bad understanding of the title by the reader or a bad definition by the author. The very term “level sensor” can mean continuous measurement of the level, that is, gradual determination of its height, or indication of the level when a certain level is reached. The term optical level sensor should represent the conversion of information about the level into an optical signal (optical information).
May the creators of the Slovak language forgive me, but there are many technical problems if we want to express ourselves in Slovak. I am a supporter of taking terms from other languages if it helps to be precise in the technical field. Just as the Japanese, Americans or Chinese have no problem with the term robot, we will not look for substitutes in Slovak for the words sensor, actuator, or the already mentioned term continuous
In the article, we will focus on the sensor (sensor) of the discrete level value, that is, not on the measurement of the level height. The level is converted to an optical signal in the actuator. Subsequently, the signal is processed in the electronic part of the sensor, and at the output of the sensor we get a switched on or open transistor, which will be the carrier of information about the state of the liquid level.
In school, we learned that a light beam is reflected from the surface of objects at the same angle at which it hits the surface. Thanks to this, we have the possibility to see objects. However, if the object has such optical properties that the light passes through, then reflection does not occur, or only partially occurs. In that case, part of the light is reflected and part of the light passes through the object further, and partial refraction of light also occurs. How light is reflected, or proceeds further, it is determined by the optical properties of both objects (optical environments).
In Figure 2 we see an optical prism placed in a container. In this case, let’s consider that the container does not represent any obstacle for the light beam. Later we will understand the bird. A light beam, or in practice more often a bundle of light beams, is reflected on the optical prism according to fig. 2 back. The container is empty, so total reflection occurs without any light entering the container.
For now, let’s think about ideal conditions. In practice, almost 100% of the light is returned, possible losses are of no practical importance and therefore we can consider a perfect reflection.
If we fill the container with water, or another liquid, as can be seen in fig. 3, the beam of rays penetrates the container and its attenuation (absorption) occurs in the liquid, and not even a small part of this light gets back. The liquid has approximately the same optical properties as glass (refractive index) and therefore the beam will not be reflected, as shown in fig. 2, but penetrates the liquid.
What does this mean in practice? If the optical prism is outside the liquid, the optical beam will return, but if the prism is immersed in the liquid, the optical beam will not return. In this case, the color of the liquid does not matter, or its light transmission.
This is how we explained in detail the conversion of the presence of the liquid level into a light signal, in our case the principle of the actuator. Next, we convert the light signal into an electrical signal and we have a finished sensor.
Compared to other principles of level sensors, this principle has a number of advantages. It has no mechanical parts, which makes it one of the sensors with a long service life. It does not affect the composition of the detected liquid. Depending on the design (e.g. in Fig. 1), it resists high pressures. It is specifically intended for making adjustments for explosive environments.
As we have already mentioned, the mentioned principle has several advantages over other types of sensors. It also has disadvantages, which, however, can be solved. In fig. 4 we see an example of a liquid with a higher viscosity. After the liquid level drops below the level of the sensor, a residual drop of liquid remains on the optical prism, which causes a false signaling of the sensor, i.e. the sensor will indicate the presence of liquid.
In Fig. 5 this case is shown from the front view of the sensor. In Fig. 5, “V” is a light beam transmitter, “P” is a light beam receiver and “K” represents a drop of liquid.
In Fig. 5a) it can be seen that a drop of liquid covers the receiver “P” (a similar situation occurs if the sensor is rotated by 180° and the drop covers the transmitter “V”). Thus, the sensor will not work reliably with liquids with a higher viscosity. In fig. 5b) the solution to this problem can be seen when the sensor is rotated by 90°. In practice, this solution is not suitable, because the given position must be determined experimentally and it may not meet the condition for perfect sealing of the system.
In Fig. 5c) it can be seen that when two transmitters and two receivers are used, one pair (transmitter-receiver) is always functional. The system works reliably and a residual drop will not affect its proper function.
Another interesting problem arises when the mirror surface is brought close to the optical prism. In fig. 6 this problem is shown schematically. In practice, it is best to simply avoid this problem. However, this is not entirely possible in the beverage industry, as containers are made of stainless steel, which can have a shiny finish. This can be avoided in two ways: keep the minimum distance between the shiny surface and the sensor, or place the sensor in such a way that the possible reflection of the beam of light rays does not get back into the optical prism.
In fig. 6 shows the estimated values of light radiation per receiver. For example in this case, 20% may be a sufficient value for a wrong sensor evaluation. Of course, it is very difficult to estimate the situation in advance, and therefore in this case there is room for the application of processors directly in the sensor. But we’ll talk more about that later.
A very interesting solution is shown in Figure 7.
Figure 7a) schematically shows a classic sensor. “V” means transmitter, “P” means receiver as in Fig. 5. “H” represents the optical prism and “S” are the light guides that distance the transmitter and receiver from the optical prism as shown in fig. 7b). This solution makes it possible to position the transmitter and receiver in a different environment than the optical prism. For example the optical prism can operate at a significantly higher temperature than the rest of the sensor. Likewise, e.g. if the sensor is used in an explosive environment, its higher resistance class can be achieved.
In this section, we have shown how to troubleshoot an optical liquid level sensor. From this it can be seen that it is possible to solve a number of problems that are not even related to the shortcomings of the sensor, but on the other hand, they favor this principle over other types of sensors.
In the 70s of the last century, new elements in control began to be used – microprocessors. At that time, Electronics magazine announced a competition for the best application of a microprocessor in practice. The winner was the use of a microprocessor in an apple slicer. Even then it was clear how close microprocessors would be to human needs. Since then, I have seen many applications with microprocessors, often (micro)processors were used unnecessarily, but often their use increased the quality and functionality of the product.
The use of processors in the sensor should also lead to its higher quality and functionality. So far we have considered only liquids with the same simple properties. We took into account its different viscosity and said that the liquid can be colored differently. However, these are not all its features.
Next, we will describe various applications in which we cannot do without the use of a processor, or the solution would be significantly more complex. We will not discuss the specific details of the use of the processor in the sensor (e.g. the program structure), as these may differ depending on the manufacturer. In the end, even the scope of this post does not allow us to do so.
In fig. 8 shows the detection of the beer level. It is clear from the figure that even if the beer is below the level of the optical prism, a significant part of the light radiation is reflected back. In any case, however, it is possible to evaluate this state and find out that the beer level has dropped even if the foam level is still above the optical prism.
It is up to the user whether he decides to sample beer without foam or with foam. In practice, the requirement is to measure the beer level and not the foam. These decision variants are already suitable for the function of the processor
In fig. 9 shows the case where the liquid contains fat – e.g. milk. In this case, even if the prism is flooded with liquid, there is a partial reflection of light radiation back. This is due to the oily liquid forming a shiny film on the surface of the optical prism, which behaves like a mirror. In the early days of using this principle in the beverage industry, this type of sensor was considered unsuitable for dairy products. And the processor did its job again and made it possible to use the principle of sensing the level using an optical prism even with liquids containing fat.
The example of foamy liquid and oily liquid is contrary to the principle of use. In practice, such cases occur only rarely. But let’s imagine an oil that contains water. This is a common case in industry. If we want to get the water out of the oil, we start heating the mixture of oil and water. Since water has a significantly lower boiling point than oil, it will begin to evaporate at a lower temperature. However, this will cause the entire mixture to foam. And we have a request for sensing the level of an oily liquid that is foamy. An almost unrealistic requirement can be solved using the processor in the sensor. However, if we have a processor in the sensor, it can solve other tasks, as we have mentioned so far. It depends on the performance of the processor and the ability of the designer. Basically, if the processor is already in the sensor, then all its other activities are actually free (they do not increase production costs), but they increase the useful value of the sensor.
In conclusion, it would be appropriate to mention where sensors of this type can be used everywhere. In the previous text, we discussed some properties of sensors when measuring the levels of liquids such as beer or milk. It is therefore most likely to use optical level sensors in the beverage industry. It is only a short step from the beverage industry to the medicine and pharmaceutical industry. There, the materials used for the production of sensors must be taken into account. These sensors have no influence on the composition of the liquid and do not affect it in any way. Due to their high reliability, they are increasingly used in ordinary industry as well. They have a significant contribution to increasing the level of automation and are used in air and sea transport for their resistance to the environment.
Author: Ing. Štefan Ploskoň, PLOSKON AT, s.r.o
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