What is a Sensor?
In the context of Industrial applications, Sensors are devices that detect physical properties of a system and convert these properties into measurable electrical signals. Let us take an example of an Engine that is driving a water pump. In this case ne and pump
Physical parameters can be pump vibration, discharge pressure, discharge flow
Sensor can be a pressure sensor with a measurement range of 0 to 400 bar[1], and an output range of 4 to 20mA
Shown below are the building blocks of a pressure sensor
Let us examine the building blocks of a sensor with an example of a Pressure sensor as shown above. As we can see, there are 3 building blocks
Sensing Element that is in direct contact with the medium for pressure measurement. In this example, the sensing element is a diaphragm that moves based on the fluid pressure.
Transducer that converts one form of signal to another form. In this example, the transducer is a capacitor with the sensing element (Diaphragm) as one of the conductive plates. Any movement of the diaphragm causes a change in the distance between the two conducting plates, there by changing the capacitance. Capacitance = (Dielectric constant * Area of the plates) / Distance between the plates
Signal conditioning unit that converts the capacitance (in micro farads) into standard electrical output. In this example, the standard electrical output is 4 to 20 mA. For instance, if the sensor has a measuring range of 0 to 400 bar, then 0 bar corresponds to 4 mA and 400 bar corresponds to 20 mA. The relationship between the measurement (pressure) and output is linear. Pls note that all sensors do not have a signal conditioning unit by default. Sensors with a signal conditioning unit are also called as Transmitters. In the context of industrial applications, Sensors with signal conditioning units have different standard output types such as Analog outputs (4 to 20mA, 1 to 1V, 0 to 20mA, 0 to 10V), Field protocol interfaces (Modbus, IO-link, ASIbus), Wireless interfaces using BLE, Wireless HART
Why are Sensors needed?
In the industrial context, sensors are required to
Understand operating condition of machines and equipment
Measure performance, process effectiveness, inventory availability and so on
Enable closed loop control processes like pressure control, level control, combustion control
Monitor compliance to environmental regulations like emission norms, effluent discharge norms
Enable safe operations of plants and equipment
What are some of the commonly used sensors ?
Given below are some of the commonly used sensors industrial applications.
Table 1 : Sensor Types
The world of sensors is very large and complex, and the above table provides only an overview of the sensors that are commonly used in Industrial applications. Thankfully, there are numerous proven sensor manufacturers in the market who can help in supplying the right sensors for your IIoT application.
For these vendors to help you with the right sensors, it is imperative to define the sensor requirements properly. Let us define the sensor requirements with an application example.
Defining your sensor requirements – Application Example
Let us consider a case where we need to measure the Engine coolant outlet temperature[2] of an engine that is used to drive an Industrial heavy-duty pump.
Let us specify the requirements for a temperature sensor for this application.
Table 2 : Sensor requirements
Above table gives you the requirement specification for a temperature sensor for measuring engine coolant outlet temperature. This requirement specification is divided into multiple sub headings.
Measurement requirements are about the parameter that needs to be measured and its associated attributes such as measurement range, accuracy, medium that needs to be measured and so on. Measurement requirements help the sensor manufacturer decide the type of sensor required for your application. For this example of measuring coolant temparature, RTD (Resistance Temperature Detector) based temperature sensor would be suitable considering the measurement range, medium, and accuracy requirements. If the measurement range were higher, Thermocouple based temperature sensor would be more suitable
Outputs speaks about the kind of outputs that are required from the sensor. In this example, we have two outputs: 4 to 20mA and IO-Link[3]. 4 to 20 mA is a standard analog output signal used in many sensors and transmitters. IO-Link is a popular short-range field bus for communication with sensors and actuators. IO-Link is based on IEC 61131-9 standard
Mechanical requirements are about the process connection, sensor dimensions and material composition. Specifying this depends on where and how the sensor will be installed. In our example, the sensor will be installed using a threaded connection. Temperature sensors best measure the temperature when the probe is in the middle of the pipe carrying the medium. In our case, we have given a probe length of 25 mm considering a 50 mm pipe
Electrical requirements speak about the supply voltage required for the sensor and the type of cabling required for connecting the sensor. In this example, we have asked for a prefabricated cable to ensure a proper and professional wiring connection, and a 24v DC electrical supply for powering the sensor. While specifying the electrical supply, one should consider the source of this supply. In our case, the 24v DC supply will come from the equipment battery
Operating requirements are about the environmental conditions in which the sensor will operate. Ingress protection (IP rating) is about protecting the sensor from dust and water. In this example the first digit 6 denotes “Total protection from dust ingress” and the second digit 9K denotes protection against close-range high-pressure water spray. This kind of ingress protection is required for sensors that are installed in harsh outdoor environments. Operating pressure is the maximum pressure of the measurement medium. Ability to withstand 5g vibration is required considering the proximity to the engine and the operation of the pump
We have asked for a factory Calibrated sensor where 0 to 150 Deg C corresponds to 4 to 20 mA with a linear relationship between the measurement and output
These are the minimum attributes that are needed for properly specifying sensor requirements. With this type of sensor requirements, you can reach out to reputed sensor manufacturers to select and buy the right sensors for your IIoT application.
Acquiring sensor data
So far, we have looked at the basic building blocks of a sensor, got ourselves familiarized with different types of sensors, and defined the requirements for a type of sensor (Temperature sensor). Now let us look at acquiring data from this temperature sensor.
Shown below are the solution components required for acquiring coolant temperature data from the temperature sensor.
As shown above, the 24v DC power supply provides the required power for the operation of the sensor, and the power supply, temperature sensor, and the IoT gateway are connected in series in a 4 to 20 mA current loop.
If you refer to table 2 ( Sensor requirements), the temperature sensor provides a 4 to 20 mA output corresponding to 0 to 150 degrees temperature with a linear relationship between the measured temperature and the analog output (i.e.) 4 mA corresponds to 0 Degrees and 20 mA corresponds to 150 degrees.
IoT gateway takes this analog current input through using its analog input module. Typically, this analog value is converted to a raw input count based on the A/D converter[4] used in the analog input module of the IoT gateway. For instance, if we consider a 16-bit converter, then 4 to 20 mA will correspond to a raw input count of 0 to 65535 with a linear relationship.
So far, “0 to 150 deg” has been converted to “4 to 20 mA” by the temperature sensor, which in turn is converted to “0 to 65535” by the analog input module of the IoT gateway. Well, there is one more conversion to go. Within the IoT gateway, this raw input count of 0 to 65535 is scaled back to 0 to 150 degrees to be used in the application program. This last scaling typically happens either in the firmware of the IoT gateway or is done with a small application program. Typically, this is how simple sensor data acquisition happens in an Industrial IoT application.
As we saw, there are a wide variety of sensors available for different measurement requirements. However, there are cases where direct sensor measurements are not viable due to multiple reasons. This is where “Derived measurements” come in handy.
Check out my next blog post on derived measurements ...
[1] Bar is a metric unit of pressure; 1 Bar corresponds to 1.019 Kg/cm2
[2] Engine coolants are used to maintain the engine temperature at the right levels. Coolant temperature is a critical parameter that shows how hot your engine is operating. High coolant temperature requires immediate measures for cooling down the engine.
[3] For more information about IO-link, please refer https://io-link.com
[4] Analog to Digital converter
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