Sensors - selection and importance for automation technology (12 4963)

Collecting data is a central aspect of modern facilities, especially in the context of digitalization and Industry 4.0. Sensors make this possible. They provide input data and monitor and adapt processes. But what types of sensors are there? How does one select the right sensor for a specific application? This article presents the mode of action and possible applications for various sensors, as well as selection criteria for finding the right sensor.

What are sensors?

Sensor systems deal with the use of sensors for measuring and controlling changes, e.g. in technical systems. Sensors acquire various measurands, which can be qualitative and also quantitative in nature. Measurands can be chemical (e.g. pH value), biological (e.g. enzyme presence), or physical (e.g. temperature, humidity, current). Sensors, also called detectors, transducers, or probes, convert an input signal into an output signal. The input signal is usually not an electrical metric, while the output signal is an electrical signal such as current or voltage.

Find more information on metrology in our article: Metrology - Quality Control Through Measuring Methods.

How it works

Sensors operate in a system together with actuators. Signals from sensors are usually forwarded to a control unit, which analyzes and evaluates the data accordingly and sends commands to actuators as required. Actuators then implement the instructions through physical actions. For example, a signal is sent to the actuator if the temperature in a warehouse is too high. The actuator can be a heater that is now dialed down so that the room can cool down to the desired temperature.

Types of sensors

There are active and passive sensors. The measurand acquisition method determines whether the sensor is an active or passive sensor. Active sensors themselves (actively) generate a signal to acquire the measurand. The determined data is then output as an output signal. Generally, a power supply is required to generate the signal to acquire the measurand. Typical examples of active sensors are: Laser distance meters, IR motion detectors or ultrasonic sensors.

Passive sensors do not generate an active signal for acquiring the measurement data of a measurand. They contain passive elements whose property (e.g. conductivity) changes in response to a measurand (e.g. temperature). A power supply for generating a signal required for the measurement is therefore not necessary. However, auxiliary power is required to record the change, since the input and output signal is compared to determine the measurement results. Passive sensors are installed relatively frequently because they are suited for acquiring static measurements. Examples of passive sensors are: PIR (passive infrared) sensors, resistance thermometers, or strain gauges.

Sensors are further subdivided into switching and measuring based on the type of output signal. Measuring sensors continuously record physical or other values and provide data records for detailed monitoring of processes. Switching sensors, on the other hand, detect when a measurand deviates from the target value and actively react by switching an output, which e.g. triggers an actuator.

Sensors can be further subdivided with regard to their operating principle or measurement principle, for example into:

• Mechanical: React to mechanical motion (e.g. deflection), example: Pressure sensors.
• Resistive: React when electrical resistance changes, example: Strain gauges.
• Thermoelectric: React when there are differences in temperature that are converted to electrical energy, example: Temperature sensors.
• Piezoelectric: React by converting pressure into electrical energy, example: Piezo-ceramic in ultrasonic sensors.
• Inductive or electromagnetic: React to a change in magnetic flux, example: Speed sensors.
• Capacitive: React to changes in capacitance, example: Humidity sensors.
• Optical: React/record light or other optical phenomena, example: Light barriers, photoelectric sensors.
• Acoustic: React to sound waves, example: Noise level meter.
• Chemical: React to chemical changes, example: pH sensors.

We will now look at the operating principles of some selected sensors in detail:

Inductive sensors

Inductive sensors contain a coil through which current flows. An electromagnetic field is generated for measuring in the measurement direction. A workpiece or material is then introduced, which causes a change in the magnetic field and induces a voltage in the coil. A circuit detects this voltage and outputs a corresponding signal. Inductive sensors only work with magnetic workpieces/materials.

Capacitive sensors

The capacitance indicates how much charge two electrically conductive bodies, which are separated from one another by an insulating medium, can absorb when voltage is applied. This capacitance changes as a function of the measurand.

A capacitive sensor consists of two electrodes between which an electric field is created. The latter changes when an object approaches; the sensor contactlessly detects the material in its active zone. It then converts the electrical field into an electrical signal.

Proximity Sensors

Proximity sensors detect when objects or people are in their proximity. They send out a beam or field and then measure the changes in the beam or field reflected by the object or person.   This allows them to estimate distances and to act on the corresponding trigger. In industry, sensors can, for example, detect the presence of workpieces on conveyor belts and control corresponding operational processes.

Selection of sensors: When to use what sensor?

Some preliminary considerations must be made when selecting sensors. Since sensors are integrated directly into control systems in automation systems, the user must for example verify that the required interfaces are available. Interfaces can be, for example:

• Analog interfaces such as analog outputs and inputs
• Digital interfaces such as TTL, RS-232, SPI
• Wireless interfaces such as Wi-Fi
• Integrated or external signal processing

Environmental conditions also play a role in selecting a suitable sensor. Is the sensor exposed to extreme temperatures? Or vibrations? In damp environments, for example, the sensor should be waterproof regardless of the type and, if necessary, have a corresponding protection rating, such as IP67. In environments with aggressive chemicals, the material used for housings and seals must be resistant to these.

Identifying the correct sensor, step by step

The following list provides a summary of the most important steps for selecting a sensor:

• Determine the measurand and measuring range: Which physical quantity is to be measured and is the maximum and minimum expected value covered by the measurement and/or can the sensor also withstand the maximum values?
• Determine accuracy: Do higher or lower requirements apply to accuracy?
• Analyze environmental conditions: Under what conditions is the sensor used? Are there extremes in terms of temperature, humidity, dust exposure, etc.? Is the sensor exposed to chemicals?
• Select output format and interfaces: The entire control system plays a role here: What type of signal can be processed (e.g. analog or digital)?
• Consider the specific application: Are there any special requirements that arise from the specific application? Do special standards apply? (see also safety standards in mechanical engineering). For example, sensors in cleanrooms must meet elevated cleanroom requirements. It is also possible that the sensor is installed in a difficult-to-reach location and should therefore be particularly low-maintenance and durable. Sensors can also be used for quality control and must meet certain criteria, see also the article "Metrology - Quality Control by Measuring Methods".
• Response time: How quickly does the sensor need to respond to changing conditions?
• Observe space requirements: Can the sensor be easily integrated into the existing system and is there enough space?
• Clamping pieces, standardized sensor rails or sensor holders can also be helpful for optimized integration of the sensor into existing systems.

You can also be inspired by our selection of sensors.

Sensors in automation technology: Smart sensors

The ongoing development of sensors has also been heavily influenced by digitized production in the context of Industry 4.0. So-called smart sensors represent the key ingredients for this. These sensors are the cornerstone for monitoring and controlling industrial processes. In the overall system of actuators and controls, smart sensors provide machines with all the information they need to make production more efficient and accurate. Automation technology has become increasingly autonomous as a result.

Today’s sensors have such high resolution, speed, and small size that they can be used directly in the areas where physical effects are generated. They take on numerous functions in leading edge equipment. They are no longer only responsible for the mere measurement of data, but can now also perform self-diagnosis, communication and signal processing tasks. Sensors can detect and correct abnormalities before they become a problem in production. This process is then called sensor-controlled processing. Find more details in our blog article Computer Numerical Control - What actually is a CNC?