The essential function of presence detectors is to inform the processing unit of an automation equipment on the state of the machine or the installation, on the presence or absence of products being produced or transfer. These devices play a preponderant role in the flow of processes by providing “all or nothing” information at predetermined fixed points. Photoelectric sensors, inductive and capacitive proximity sensors, electromechanical position switches are classified in the field of data acquisition.
Which detectors for which applications?
Detecting is an essential function in automatic systems. In all processes, it is indeed necessary to be informed at a given moment of the presence or not of an object, a person, a machine position …
In their acquisition role dedicated to information processing, detectors precisely control the presence, absence, positioning, passage, scrolling, jamming, counting of various objects …
The main types of detectors
There are different families of presence detectors:
– Photoelectric detectors, to detect objects from 1/10 of a mm up to several tens of meters.
– Inductive proximity detectors, to detect metal without physical contact and at short distances.
– Position switches, actuated by direct contact with objects.
– Capacitive proximity detectors, to detect conductive or insulating objects without physical contact and at a short distance.
– Ultrasonic detectors to detect any object by overcoming its color, opacity, nature (powder, glass, liquid, etc.), without physical contact.
– Pressure switches or vacuum switches, to detect a pressure level.
Approach to choosing a detector
The proposed choice takes place in two stages.
The flowchart below illustrates this approach which leads to selecting a family of detectors on the basis of simple criteria.
Phase 1: it consists of determining the family of detectors best suited to the application by answering the following questions:
– Nature of the object to be detected: solid, liquid, gas, metallic or not.
– Possible contact with the object.
– Object / detector distance, object mass.
– Scroll speed.
– Maneuver rates.
– Space for integrating the detector into the machine.
Phase n ° 2: it aims to determine the type and reference of the desired detector.
This second phase takes into account:
– The environment: temperature, humidity, dust, various projections, etc.
– From the power source: AC or DC.
– From the output signal: electromechanical, static.
– The type of connection: cable, terminal block, connector.
Photoelectric sensors
Photoelectric detectors allow the detection of objects of all kinds (opaque, transparent, reflective, etc.) in the most diverse industrial and tertiary applications. Detection is based on the following five basic systems:
– Barrage.
– Reflex.
– Polarized reflex.
– Proximity.
– Proximity with background suppression.
Principle of optical detection
A photoelectric detector detects a target (object or person) by means of a light beam. Its two basic constituents are a light emitter and a receiver.
– 1 Light emitter.
– 2 Light receiver.
– 3 Signal processing stage.
– 4 Output stage.
Detection is effective when the target enters the light beam and changes the amount of light received by the receiver enough to cause the output to change state.
It is carried out according to two methods:
– Beam blocking by the target.
– Return of the beam to the receiver by the target.
All photoelectric sensors have a light emitting diode (LED) emitter and a phototransistor receiver.
Depending on the detector models and the application requirements, the emission takes place in infrared non-visible light (most common case), in ultraviolet (detection of luminescent materials), in visible red or green light (marker readers) and in laser red (long range and short focal length).
To insensitize the systems to ambient light, the current through the emitting LED is modulated so as to obtain a pulsed light emission.
Only the pulsed signal will be used by the phototransistor and processed to control the load.
The emitted light beam has two zones:
–
A recommended operating area in which the beam intensity is high enough to ensure normal detection.
Depending on the system used (barrier, reflex or proximity), the receiver, reflector or target must be located in this area.
– An area in which the beam intensity is no longer sufficient to guarantee reliable detection.
The two detection methods
Photoelectric detectors detect a target using two methods:
– By blocking the beam by the target.
– By returning the beam to the receiver by the target.
Beam blocking
In the absence of a target, the light beam hits the receiver. When a target enters the beam, it blocks the beam:
–
No light on the receiver = detection
Three basic systems operate according to this process based on the absorption properties of the objects to be detected: barrier, reflex, reflex
polarized.
Beam forwarding
In the absence of a target, the light beam does not reach the receiver. When a target enters the beam, it sends the beam back to the receiver:
– Light on the receiver = detection
Two basic systems operate according to this process, which is based on the light reflection properties of the objects to be detected: proximity, proximity with background suppression.
The five basic systems
Dam system
Transmitter and receiver are located in two separate housings. This is the system that allows the longest ranges, up to 100 m in laser technology.
With the exception of transparent objects which do not block the light beam, this system can detect objects of all kinds (opaque, reflective, etc.), with excellent precision thanks to the cylindrical shape of the useful area of the beam.
Dam detectors have a very large profit margin. They are therefore particularly well suited to polluted environments (smoke, dust, locations subject to bad weather, etc.).
Reflex system
Transmitter and receiver are grouped together in the same box. In the absence of a target, the beam emitted by the transmitter is returned to the receiver by a reflector.
This consists of a multitude of trirectangle trihedra with total reflection, the property of which is to return any incident light ray in the same direction.
Detection is performed when the target blocks the beam between the emitter and the reflector. This system is therefore not suitable for detecting reflective objects which could reflect a greater or lesser amount of light on the receiver.
A reflex photoelectric detector can be used in a polluted environment. But due to a lower gain margin than that of a barrier system, it is essential to refer to the gain curve to define the working range which guarantees reliable detection.
Choice of reflector
The reflector is an integral part of a reflex detection system. Its choice, its installation and its maintenance condition the correct functioning of the detector associated with it.
A reflector should always be smaller than the object to be detected.
Polarized reflex system
Shiny objects do not block the beam but reflect some of the light back to the receiver. They cannot be detected by a standard SLR system. In this case, a polarized reflex system must be used. This type of detector emits visible red light. It is equipped with two opposing polarizing filters :
– A filter on the emitter that only allows rays emitted in a vertical plane to pass through.
– A filter on the receiver which allows only the rays received in a horizontal plane to pass.
In the absence of a target
The vertically polarized emitted beam is returned by the reflector after having been depolarized by the latter. The receiver filter allows reflected light to pass in the horizontal plane.
In the presence of target
The emitted beam is returned by the target without undergoing any modification. The reflected beam, polarized vertically, is therefore blocked by the horizontal filter of the receiver.
Proximity system
As for the reflex system, transmitter and receiver are grouped together in the same box.
The light beam is returned to the receiver by any sufficiently reflective object which enters the detection zone.
The range of a proximity system is generally less than 10 meters.
For this reason, its use in a polluted environment is not recommended.
This scope depends on :
– The color of the target and its reflective power (a light colored object can be detected at a greater distance than a dark colored object).
– Dimensions of the target (the range decreases with the dimensions).
Proximity system with background erasure
Proximity sensors with background erase are equipped with a range adjustment potentiometer. The latter makes it possible to “focus” on a detection area while avoiding any spurious reflection on the background.
Characteristics of photoelectric detection
Depending on the nature of the object and the wavelength of light emitted, only part of the light received from the object will be returned.
It depends on the reflection coefficient of the material.
The non-returned part of the light will be :
– Either absorbed by the material: it depends on the absorption coefficient of the material.
– Either transmitted through the material: it depends on the transmission coefficient of the material.
Coefficient of reflection
It is the ratio of the amount of light (luminous flux) reflected by the object to be detected by the amount of light (luminous flux) received by this object.
Objects reflect more or less the light they receive, depending on their nature and the wavelength of the light received.
In general, photoelectric sensors operate based on an amount of light energy received. In the case of proximity detectors, the useful (or actual) range of the detector will depend directly on the reflectance of the object to be detected.
Transmission coefficient of the object
Objects transmit light to a greater or lesser extent through their body, depending on their nature and the wavelength of the light.
In the case of a barrier, the ability to detect an object will depend directly on the transmission factor of the object to be detected; for example, the detection of a transparent bottle (high transmission coefficient) will be very delicate.
Conversely, in some cases, the ability to detect will depend on the transmission coefficient of the object. This is the case, for example, for detection through a glass door (glass transmission coefficient equal to 90%).
Absorption coefficient of the object
This phenomenon results in losses.
The coefficient is related to the nature of the materials and the wavelength of light.
Materials can be classified into two broad categories :
– “White” bodies: white bodies have the particularity of reflecting all of the visible and infrared light they receive. The reflection coefficient of white bodies in the visible and infrared spectrum is close to 1 (90%).
– “Black” bodies: Black bodies absorb all of the visible and infrared light they receive.
The reflection coefficient of black bodies in the visible and infrared spectrum is close to 0.
Inductive proximity sensors
Inductive proximity sensors are mainly used in industrial applications: machining or assembly machines, packaging machines, conveying installations, etc.
They detect any metallic object without contact: presence or absence control, passage detection, scrolling, jamming, positioning, coding, counting.
Principle of inductive proximity detection
An inductive proximity detector detects without physical contact the presence of any object made of conductive material.
It comprises an oscillator (1) whose coils constitute its sensitive face and an output stage.
The oscillator creates an alternating electromagnetic field in front of the sensitive face with a frequency of 100 to 600 kHz depending on the model.
When a conductive object enters this field, it is the seat of circular induced currents that develop at its periphery. These currents constitute an overload for the oscillator system and therefore lead to a reduction in the amplitude of the oscillations as they approach the object, until they are completely blocked. The detection (2) of the object is effective when the reduction in the amplitude of the oscillations is sufficient to cause a change of state of the output (3) of the detector.
Electromagnetic field and zone of influence of an inductive detector
The intensity of this field decreases rapidly as one moves away from the sensitive face. The zone of influence, i.e. the area in which the field strength is sufficient for detection to take place, is therefore reduced.
It determines the distances to be respected between devices or between devices and metallic masses.
Curves and detection distances
The curves and detection distances are determined using a square measuring plate, 1 mm thick, in Fe 360 grade steel.
The side of this square is equal to the diameter of the sensitive face (cylindrical detector) or to 3 times the nominal range Sn (rectangular detector).
In order to ensure a comparison and a reliable choice of products, the IEC 947-5-2 standard defines different scopes such as:
Nominal range (Sn)
Nominal range used to designate the device. It does not take into account dispersions (manufacture, temperature, voltage).
Actual range (Sr)
The actual range is measured at the rated supply voltage (Un) and at the rated ambient temperature (Tn).
It must be between 90% and 110% of the nominal capacity (Sn): 0.9 Sn = Sr = 1.1 Sn.
Effective range (Su)
The useful range is measured within the allowable limits of the ambient temperature (Ta) and the supply voltage (Ub). It must be between 90% and 110% of the actual range: 0.9 Sr = Su = 1.1 Sr.
Working range (Sa)
This is the area of operation of the device. The working range is between 0 and 81% of the nominal capacity (Sn): 0 = Sa = 0.9 x 0.9 x Sn.
Differential travel or hysteresis (H)
The differential travel (H) or hysteresis is the distance between the switch-on point, when the measuring plate approaches the detector, and the release point, when the plate moves away from the detector. This hysteresis is essential to ensure stable operation of the product.
Reproducibility (R)
Reproducibility (R) is the accuracy of reproduction between two span measurements for specified time, temperature, and voltage intervals: 8 hours, 10 to 30 ° C, Un ± 5%. It is expressed as a percentage of the actual Sr.
Features of inductive proximity detection
In a large number of applications, the objects to be detected are made of steel and of dimensions equal to or greater than the sensitive face of the detector.
The operating range is the space in which detection of the object is certain.
Generally, the values indicated in the product catalogs are given for parts to be tested in steel and of dimensions equivalent to the sensitive face of the detector.
Any other case (small parts, different materials, etc.) requires a correction calculation.
Detectors that can be embedded in metal have a shield that blocks the lateral expansion of the magnetic field.
Their nominal range is less than that of detectors without shielding, which cannot be embedded in metal supports.
Working range of a detector
In practice, the parts to be detected are generally made of steel and of dimensions equal to or greater than the sensitive face of the detector.
For the calculation of the working range under different conditions of use, it is then necessary to take into account the correction factors which influence this range. These curves only give an order of magnitude of accessible range for a given application case.
Influence of ambient temperature
Outside the maximum temperature (25 ° C), the characteristics of an inductive detector deteriorate; a coefficient Kθ given by the curve below must therefore be applied to the parameters:
Material of the object to be detected
The fixed range models for ferrous and non-ferrous materials (Fe / NFe) allow the different materials to be detected at a fixed distance, by applying a correction coefficient Km to be determined according to the table below:
Position switches
Position switches have a role equivalent to that of inductive or photoelectric detectors for the detection of presence or passage in automation equipment.
They are used in a wide variety of applications due to their many qualities: operating safety (reliability of the contacts, positive opening operation), high precision (precision on the switching points from 0.1 to 0.01 mm depending on models), nominal thermal current (6 A for two contacts), natural immunity to electromagnetic disturbances, user-friendliness (easy to use, “visible” operation).
Composition of the position switches
Position switches are made up of the following three basic elements:
– An electrical contact.
– A body.
– A control head with its attack device.
Most of these devices are made up of different models of bodies equipped with electrical contacts, control head and attack device. This modularity greatly facilitates maintenance by easy exchange of one of the elements.
Electric contact
The common denominator is electrical contact.
There are two versions depending on the manufacturer:
– Bipolar contact NO + NC snap action, offset NO + NC slow action, NO + NO snap action, NO + NO shift slow action.
– Three-pole NO + NO + NC snap action, NO + NO + NC shifted snap action slow action.
Snap action contact (snap action)
It is characterized by points of action and release that are not confused.
The speed of movement of the movable contacts is independent of the speed of the controller.
This feature makes it possible to obtain satisfactory electrical performance even at low speeds of movement of the control unit.
Slow action contact (slow break)
It is characterized by points of action and release confused.
The speed of movement of the movable contacts is equal to or proportional to the speed of the controller (which must not be less than 0.001 m / sec = 6 cm / min).
The opening distance is also dependent on the stroke of the actuator.
Snap action contacts block diagrams
Example: “NO + NC”
A – Maximum stroke of the actuator in millimeters or degrees.
B – Contact element action stroke.
C – Contact element release stroke.
D – Differential travel = B – C.
P – Point from which positive opening is assured.
Rectilinear movement
1 – Contact element release point.
2 – Contact element actuation point.
A – Maximum stroke of the actuator in millimeters.
B – Contact element action stroke.
C – Contact element release stroke.
D – Differential travel = B – C.
P – Point from which positive opening is assured.
Angular movement
1 – Release point of the contact element.
2 – Contact element actuation point.
A – Maximum stroke of the actuator in degrees.
B – Contact element action stroke.
C – Contact element release stroke.
D – Differential travel = B – C.
P – Point from which positive opening is assured
Body
Different versions are available: CENELEC standardized or compact, fixed or plug-in, metallic or thermoplastic, with one or more cable entries.
Control heads and attack devices
Many models can be associated with the body containing the contact element:
> Rectilinear motion head :
– Ball or end roller plunger, side with vertical or horizontal roller.
– Roller lever with horizontal or vertical action.
> Angular movement head :
– Thermoplastic or steel roller lever, fixed or adjustable length, angular position adjustable through 360 °, action in one or two directions.
– Rigid steel or polyamide rod, action in one or two directions.
– Spring or spring rod, action in one or two directions.
– Lyre one or two tracks with thermoplastic or steel roller, positions maintained.
– Multi-directional, flexible rod or rigid spring.
Examples of applications
Food industry, conveying glass bottles: Counting function
Specifications
- Bottles in white glass or dark glass.
- Rate of 3,600 bottles per hour.
- Duration of the “bottle presence alert”: 3.5 ms.
- Proximity detection distance: 2 cm.
- Detector subject to frequent passage by operators: surface mounting prohibited.
- Healthy atmosphere.
- Connection to a PLC.
Solution
- Photoelectric detector, aimed at 90 °.
- Metal case diameter 18.
- Proximity system, range 10 cm.
- Connection by M12 connector, 4 pins.
- Static output.
- IP 67.
Electrical industry sector, assembly machine: Presence control of mobile contacts
Specifications
- Presence control of 4 copper elements, very small size and very low weight.
- High cadence.
- No physical contact with the parts.
- Detection distance <0.5 mm. – Detector integrated in its support and compact. – Environment falling under normal requirements.
Solution
- Inductive detector embeddable in metal.
- Range 1 mm.
- Smooth metal case (brass), diameter 4 mm, length 30 mm.
- IP 67 waterproofing.
- Connection by cable length 2 m.
- 3-wire DC type detector, NO output.
Agri-food sector, packaging of gruyere wheels: Passage control on conveyor
Specifications
- Possible physical contact with the product.
- Weight of the product detected: 60 kg.
- Linear speed of the conveyor: 0.2 m / sec.
- Passage of a product every 10 sec.
- Inaccurate guidance with change of direction.
- Humid environment without runoff.
- A cable entry (11 cable gland).
- Control of a PLC input (“NC” contact).
Solution
Position switch equipped:
- A plastic body with a cable entry (PE 11) fitted with a double-pole “NO + NC” snap-action contact.
- A multidirectional movement control head.
- A spring-loaded flexible rod attack device.
Other detectors
Many other detectors exist:
- Industrial solution for object recognition by video camera (embeds the intelligence necessary to achieve detections that are impossible with traditional technology, configurable by learning the image control zones).
- Magneto-resistive proximity detectors: Application for position control on a jack.
- Capacitive proximity sensors for the detection of liquids and insulating and conductive materials.
- Ultrasonic detectors for the detection of any composite object, whatever its nature, color and surface.
- Sensors for pressure control …
