In an automated installation, all the information needed to control it must be detected in order to be managed by the control systems. The "detection" function is therefore essential in all industrial processes, and knowledge of the various techniques is vital to choose the right detectors: they have to be able to operate in sometimes difficult environments and supply information that is compatible with the acquisition and processing systems. This document is aimed at those wanting to familiarize themselves with the field of Detection in Industrial Automation. After setting out the broad technical background to this field, each technology is analyzed in detail to provide a basic selection guide. This is complemented by an overview of related technologies, including Vision and RFID (Radio Frequency IDentification). Detection may seem complicated at first, but you will soon learn that it is simply "varied"! Happy reading!
Detection: A vital function
The detection function is vital because it is the first link in the chain of information (see Fig. 1) for an industrial process.
In an automatic system, detectors collect information about:
- All the events that are needed to control it, so that they can be taken into account by the control systems, using an established program.
- The progress of the various stages of the process when this program is executed.
The various detection function
Detection requirements are extremely varied. The most basic needs are as follows:
- Monitoring the presence, absence or position of an object.
- Verifying the passing, travel or obstruction of objects.
- Counting: These requirements are generally met using "discrete" devices, as in typical applications for detecting parts in production lines or handling activities, as well as for detecting people and vehicles.
There are other, more specific requirements, such as the detection of:
- Presence (or level) of gas or liquid
- Position (angular, linear)
- Tags, with reading and writing of coded information.
In addition to these, there are numerous requirements concerning the environment especially; depending on their location, detectors have to be resistant to:
- Moisture or immersion (e.g. watertight reinforced seal).
- Corrosion (chemical industries or even food and beverage installations).
- Extreme temperature fluctuations (e.g. tropical regions).
- All types of dirt (outside or inside machinery).
- And even vandalism...
In order to meet all these requirements, manufacturers have designed all sorts of detectors using various technologies.
The various detector technologies
Detector manufacturers utilize a number of different physical measurement principles, the most important being:
- Mechanical (pressure, force) for electromechanical limit switches.
- Electromagnetism (field, force) for magnetic sensors, inductive proximity sensors.
- Light (power or deflection) for photoelectric cells.
- Capacitance for capacitive proximity sensors.
- Acoustic (wave travel time) for ultrasonic detectors.
- Fluid (pressure) for pressure switches.
- Optical (image analysis) for vision.
These principles offer advantages and limitations for each type of sensor: for example, some are rugged but have to be in contact with the part being detected; others can be located in aggressive environments but can only be used with metal parts.
The aim of the description of these various technologies in the following sections is to explain the installation and operating requirements for the sensors available on the market in the automation and industrial devices sector.
Auxiliary Detector Function
Various functions have been developed to make detectors easier to use, self-teach mode being one. With this teach function, the effective detection range of the device can be defined simply by pressing a button: for example, learning the ultra-precise (± 6 mm for ultrasonic detectors) minimum and maximum ranges (suppression of foreground and background), and environment recognition for photoelectric detectors.
Electromechanical limit switches
Detection is achieved by means of physical contact (feeler or actuator) with an object or moving part. The information is sent to the processing system via an electrical contact (discrete).
These devices (actuator and electrical contact) are known as limit switches. They are used in all automated systems and in a wide range of applications because of the many inherent advantages in their technology.
A feeler or actuator may move in various ways (see Fig. 2), allowing it to detect in multiple positions and adapt easily to the objects to be detected:
Contact operating mode
The products available from manufacturers are characterized by the technology used to move the contacts.
The movement of the contacts is characterized by the phenomenon of hysteresis, in other words, by distinctly different tripping and reset points (see Fig. 3). The speed at which the moving contacts move is independent of the actuator speed. This feature means that satisfactory electrical performance can be obtained even at low actuator speeds. Increasingly, limit switches with snap-action contacts have contacts with a positive opening action: this relates to the NC contact and is defined as follows:
"A device meets this requirement when all its NC contact elements can be moved with certainty to their open position, in other words, with no flexible link between the moving contacts and the actuator to which the operating force is applied."
This relates to the electrical contact on the limit switch (see Fig. 3) but also to the actuator, which has to transmit the movement without deformation. The use of positive opening action devices is mandatory in safety applications.
Slow-action contact, also known as slow break
This operating mode is characterized by:
- Identical tripping and reset points.
- Moving contact travel speed equal or proportional to the actuator speed (which must not be less than 0.1 m/s = 6 m/min). Below these values, the contacts open too slowly, which is detrimental to the correct electrical operation of the contact (risk of arc being maintained for too long).
- The opening distance also depends on the actuator travel.
The design of these contacts means that they are inherently positive opening: the plunger acts directly on the moving contacts.
Inductive Proximity Sensors
Due to their physical operating principle, these sensors only work on metallic materials.
An inductive circuit (induction coil L) is the sensitive element. This circuit is connected to a capacitor with capacitance C to form a resonant circuit with a frequency, which is generally between 100 kHz and 1 MHz. An electronic circuit is used to maintain the system oscillations in accordance with the formula below:
These oscillations generate an alternating magnetic field in front of the coil. A metal screen positioned in the field emits eddy currents, which induce an additional charge, thereby modifying the oscillation conditions (see Fig. 5).
The presence of a metal object in front of the sensor reduces the quality factor of the resonant circuit.
Case 1, without metal screen:
Remainder: Q = = R =
Case 2, with metal screen:
Detection is achieved by measuring the variation in the quality factor (from 3% to around 20% at the detection threshold).
The approach of the metal screen results in a reduction in the quality factor and hence a reduction in the amplitude of the oscillations. The sensing distance depends on the nature of the metal being detected (its resistivity ρ and its relative permeability
Description of an inductive Sensor
Transducer: This comprises a multifilament copper wire (Litz wire) coil positioned inside a half ferrite pot which directs the field lines towards the front of the sensor.
Oscillator: There are many different types of oscillator available, including oscillators with a fixed negative resistance -R, which is equal in absolute value to the parallel resistance of the oscillating circuit at the nominal range (see preceding section).
- If the object to be detected is beyond the nominal range, || > |-R| so oscillation is maintained.
- Conversely, if the object to be detected is inside the nominal range, || < |-R| so oscillation is not maintained and the oscillator is blocked.
Shaping stage: This comprises a peak detector followed by a comparator with two thresholds (trigger) to prevent untimely switching when the object to be detected is close to the nominal range. It creates what is known as the sensor hysteresis (see Fig. 6).
Supply and output stages: The one allows the sensor to be powered across a broad supply voltage range (from 10 V DC to 264 V AC). The other, the output stage, controls loads from 0.2 A DC to 0.5 A AC, with or without short circuit protection.
Influence quantities in inductive sensing
Certain characteristics particularly affect inductive sensing devices, notably:
- Sensing distance: This depends on the size of the sensing area. Sn: Nominal range (on mild steel) varies from 0.8 mm (sensor diameter 4) to 60 mm (sensor 80 x 80).
- Sensors protected against magnetic fields generated by welding machines.
- Analog output sensors
- Sensors with a correction factor of 1, where the sensing distance is independent of whether the metal being detected is ferrous or nonferrous (see Fig. 7 )
- Selective sensors for ferrous and non-ferrous materials
- Rotation control sensors: These under speed sensors are sensitive to the frequency of passage of metal objects.
- Sensors for explosive atmospheres (NAMUR standards)
- Hysteresis: Differential travel (from 2 to 10% of Sn), which prevents bouncing on switching
- Frequency of passage of objects in front of the sensor, known as the switching frequency (maximum current 5 kHz).
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