PLC sensors, in detail

Sensors can be defined as electronic devices that convert a physical variable into a corresponding electrical value. This electrical value can be expressed in terms of current, voltage, or resistance. Sensors, in turn, belong to the data input elements of an automatic control system, and therefore the classification of input elements is as follows:
Classification of input elements
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Manual Activators
Sensors
Manual actuators are devices used to start or stop a production process. These actuators are buttons that may have normally open (NO) or normally closed (NC) contacts, or even one of each. These buttons can be push-button type or toggle switches that, once activated, require a special key to deactivate them.
Manual triggers are input elements that generate a discrete type signal, that is, it is either pressed (“logic 1”) or at rest (“logic 0”).

Figure 4.3 Examples of manual actuators.

Manual activators are indispensable elements that cannot be omitted from automated industrial processes, because human intervention is always necessary, for example, when activating mechanisms by means of a button at the start of the workday, or stopping the process when an unexpected event occurs, or simply to stop the processes because the workday is over.

The next input elements that we will describe are called sensors; these devices are classified into two categories:

Sensor classification

• Discreet
• Analogues

Discrete sensors simply tell us whether they are detecting an object or not; that is, they generate a logic "1" if they detect it or a logic "0" if they do not. This information is primarily generated by the presence or absence of voltage, although in some cases the information is reported through an electrical current flow. Discrete sensors can operate with both direct current (DC) and alternating current (AC) voltage signals.

Analog sensors can produce an infinite number of values, representing the different magnitudes of a physical variable. Therefore, their operation is represented by ranges, for example, from 0V to 1.5V. Within this range of possible values that the sensor signal can acquire lies the measurement range that the sensor is permitted to use to measure a physical variable. In analog sensors, the signal delivered can be represented by variations in a voltage signal or by variations in a resistance value.

DISCREET SENSORS
Presence or limit switch sensors.- These sensors are based on the use of switches that can open or close contacts, depending on the application assigned to them. For example, when used as presence detectors, they indicate when an object is placed on them, and through the pressure exerted, the switch is pressed, allowing a certain voltage to be sent to the control system (which in this case is suggested to be a PLC). Obviously, when the object is not present, the reported voltage will be zero.

When these sensors are tasked with detecting a limit switch or area boundary, it's because they are working in conjunction with an actuator that produces a mechanical displacement. Therefore, when that mechanical part reaches its limit, its movement must stop to avoid damaging any part of the automated process. When the actuator reaches its permitted displacement limit, it activates the contacts of a switch, which can either open or close them. Figures 4.4 and 4.5 show examples of presence and limit switch sensors, respectively.

Figure 4.4 Presence Sensor

Figure 4.5 Limit switch sensors.

Inductive Sensor – This type of sensor, by its very nature, is dedicated to detecting the presence of metals. Internally, the inductive sensor contains an electronic circuit that generates a magnetic field. This field is calibrated to measure a certain amount of electrical current without any metal present. However, when metal is brought near, the magnetic field is altered, causing the current that generates it to change value. This, in turn, triggers a response from the sensor to the control system, indicating the presence of metal. One application of this sensor is, for example, on conveyor belts where metallic materials, such as cans, travel. The sensors are installed at the points where these cans are to be placed, and without the need for physical contact, the sensor reports when a can is nearby.

Figure 4.6 Inductive Sensor.

Magnetic Sensor – A magnetic sensor indicates the presence of a magnetic field nearby. It has an internal circuit that responds when a magnetic field strikes it. This sensor can range from a simple reed switch to a more complex circuit that reports the presence or absence of a magnetic field via a voltage signal. The response must be sent to the control system for further processing. One application of this type of sensor is found in linearly moving actuators. These actuators can have magnets attached to their ends so that when they reach the magnetic sensor, the magnetic field is detected, and the actuator stops and ceases its movement.

Figure 4.7 Magnetic Sensor.

Capacitive Sensor - This type of sensor is designed to detect materials with a dielectric constant greater than one (1). The capacitive sensor operates based on the electric field that can be stored in a capacitor. Depending on the dielectric material, the stored charge will be very large or small. Air has a dielectric constant of 1, while other materials such as plastic, glass, water, cardboard, etc., have a dielectric constant greater than 1.
To detect a material other than air, the capacitive sensor must be calibrated to know which material to detect. One example of using this type of sensor is on a production line where transparent containers, whether glass or plastic, are filled with a liquid, which may also be transparent.

Figure 4.8 Capacitive Sensor

Optical Sensor – The optical sensor creates a barrier by emitting an infrared light beam. This is why this sensor is used to detect physical interference, identify colors, and measure distances. This sensor relies on an infrared light-emitting diode (LED), which is imperceptible to the human eye. The LED emits the light beam, and through reflection, this beam is sent back to be captured by a photodiode or phototransistor. This phototransistor then sends a signal indicating the presence or absence of the infrared light beam.
Similarly, colors can be identified, since reflection on a surface can be total or partial because materials can absorb the infrared light beam, depending on the surface's odor. To measure distances, the time it takes for the light beam to return can be measured, and the distance can be calculated using a very simple formula: v = d/t, where time can be measured, and v is the speed at which light travels. Therefore, the distance d can be calculated. The applications of this type of sensor are very broad, as it can be used as a barrier to detect when an operator places their hands in a hazardous area and could have an accident, or to detect when the light beam is interrupted by a material passing through it while traveling on a conveyor belt, among other applications.

Figure 4.9 Optical Sensor

ANALOG SENSORS

Temperature sensor.- This is one of the most common sensors used in industrial processes, such as in the food, metallurgical, or plastic injection industries. It's necessary to maintain processes like cooking or melting at the appropriate temperature levels. Depending on the process being controlled, the temperature levels to be measured, and the required resolution, there is a sensor suited to the specific characteristics of the process.
In this case, a wide range of sensors are available to measure temperature, so we will proceed to describe the most common temperature sensors:
RTD - Its name stands for Resistance Temperature Detectors (RTD), also called metallic resistors. The main characteristic of these sensors is that they have a positive temperature coefficient (PTC), which means that when the temperature being sensed increases, there is an increase in the resistance of the materials that make up the RTD.
The response of these sensors is generally linear; that is, a change in temperature is reflected by a proportional change in resistance. The temperature measurement range is approximately -200 °C to 400 °C. This sensor requires a coupling circuit to transmit its information to the control system.

Figure 4.10 RTD.

Thermistors – Their name stands for Temperature Sensitive Resistance. This type of sensor has both positive temperature coefficient (PTC) and negative temperature coefficient (NTC) thermistors. This means that as the temperature being sensed increases, the resistance of the materials that make up the thermistor (PTC) increases, while in NTC thermistors, the resistance decreases as the temperature increases, and vice versa. The response of these sensors is not linear but exponential. This means that when the temperature changes, there is a sharp change in resistance. Therefore, this type of sensor is used to detect subtle changes in temperature. The temperature measurement range is approximately between -55 °C and 100 °C. This sensor requires a coupling circuit to transmit its information to the control system.

Figure 4.11 Thermistor.

Thermocouple – This sensor gets its name from the effect produced by the junction of two different metals. This junction generates a certain voltage depending on the temperature at the junction. The response of these sensors is in the form of small voltage magnitudes (between μV and mV) that correspond directly to the temperature being measured and can be considered a linear response. The main characteristic of thermocouples is that they are designed to measure high temperatures, even up to the melting point of metals. The temperature measurement range is approximately between -200 °C and 2000 °C. This sensor requires a coupling circuit to transmit its information to the control system.

Figure 4.12 Thermocouple.

Integrated Circuit Sensors: These sensors are used in environments that are not as demanding in terms of their operating conditions; for example, they don't need to measure the temperature of a boiler while directly exposed to a flame. Integrated circuit sensors contain an internal circuit based on the operation of a diode, which is sensitive to temperature effects. These diodes provide voltage values that correspond directly to the temperature being measured. The key feature of these sensors is their high accuracy. Furthermore, depending on the model and manufacturer, they are already calibrated in °C, °F, or °K. These sensors generally do not require a coupling circuit to transmit their information to the control system.

Strain gauges – These sensors can be considered suitable for measuring certain variables depending on the application, because their operating principle is based on the change in resistance value that occurs when the surface of these sensors is deformed. Of course, they cannot measure all variables, but they can measure those related to force, whose mathematical formula is:
f (force) = m (mass) * a (acceleration)
Therefore, depending on how the strain gauge is positioned, it can be used to measure: the acceleration of a moving object, speed, pressure or force, weight (mass), among other variables. Strain gauges are variable resistors whose resistance changes depending on the deformation present on their surface. These devices are very sensitive to physical changes on their surface and require a circuit to adjust their response so that it can be sent to the control circuit for further processing.

With the wide variety of both discrete and analog sensors reviewed here, a good number of measurable and quantifiable physical variables have been covered. In fact, the most common ones are included, but there are still more physical variables to consider, such as chemical ones (pH, CO2, etc.) or humidity levels, whether relative to the environment, the soil, or within a process. We could continue listing physical variables, but for each of these, there is a sensor that will adequately report the levels of its magnitude.

Furthermore, all sensors involved in a company's industrial processes are standardized; that is, the brand or manufacturer of these sensors is irrelevant, as they must all comply with the various standards governing automated systems. Examples of these standards include the following:

ANSI (American Standards).
DIN (European Standards).
ISO (International Standards).
IEEE (Electrical and Electronic Standards).
NOM (Mexican Standards).

All standards establish safety measures, voltage levels, physical dimensions of sensors, etc.
Finally, we want to remind you that sensors are important elements in the automation process, which is why they must be selected appropriately. Later, when designing the PLC program (which we normally use in ladder logic), we should represent the activity of the sensors using the symbols we have already reviewed in previous installments, which we list below as a reminder.

*.- Momentary input actuation (for the sensors).

To recap, it can be mentioned that the sensors represent the eyes of the automatic control system, while the other important part, which manipulates the process depending on the data fed into the control system, is known as "actuators".