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Technology of Fiber-Optic Sensors

Fiber-optic sensors detect objects and conditions by directing light to a test object and evaluating the intensity change of the returning light. They can detect very small objects, are particularly flexible to mount and are extremely resistant in harsh environments – even in high temperatures, humidity or wet media. 

What Are Fiber-Optic Sensors?

Fiber-optic sensors use the physical properties of light when transmitting it via fiber-optic cable with glass or plastic fibers to detect objects. They consist of a fiber-optic amplifier and fiber-optic cables with or without optics. The fiber-optic amplifier contains the light source and the receiving element as well as the processing unit of the sensor. The fiber-optic cables are only intended to transmit and receive light. Since fiber-optic cables do not contain electronic components, fiber-optic sensors are particularly suitable for applications in confined spaces, harsh environments or where other sensors cannot be used.

   

How Do Fiber-Optic Sensors Work?

Fiber-optic sensors measure different light sizes such as wavelength and intensity in order to derive other measured values from them. In industrial automation, the energetic principle is often used. The emitter, usually an LED light source, couples light into a fiber-optic cable. The light exits at the end of the fiber-optic cable and either hits an object which reflects it back (sensing/reflection principle) or it is detected directly by a receiver (through-beam principle). The returned light is then directed to the analysis module, where a photodiode measures the amount of light received. The electronics constantly compare this amount of light with a defined threshold value and switch the output of the sensor accordingly.

What Are the Advantages of Fiber-Optic Sensors?

Flexible Installation

Fiber-optic sensors are extremely compact and ideal for installation in confined industrial environments. In addition, the high flexibility and low attenuation of the fiber-optic cables also make larger transmission distances possible.

High Reliability

Fiber-optic sensors are extremely durable and ensure reliable performance even under harsh ambient conditions such as high temperatures, humidity and aggressive media such as cooling lubricants or cleaning agents. 

Electromagnetic Compatibility

In fiber-optic cables, signal transmission is purely optical, which eliminates the challenges associated with EMC for fiber-optic sensors. They are also extremely insensitive to electromagnetic interference.

Fiber-Optics vs. Small Photoelectrics: Technology Overview

 
Fiber-optic cable Small photoelectrics
Measurement range
 
Ambient temperature
 
Mounting effort
Detection of transparent objects
 
Small parts detection
Flexibility and customization
 
Measurement range
 
Fiber-optic cable
Small photoelectrics
Ambient temperature
 
Fiber-optic cable
Small photoelectrics
Mounting effort
Fiber-optic cable
Small photoelectrics
Detection of transparent objects
 
Fiber-optic cable
Small photoelectrics
Small parts detection
Fiber-optic cable
Small photoelectrics
Flexibility and customization
Fiber-optic cable
Small photoelectrics

What Are Fiber-Optic Amplifiers?

Fiber-optic amplifiers, also known as optical amplifiers, are components that amplify signals in optical communication systems and play a key role in fiber-optic communication. Here, they increase the transmission range.

In the context of industrial automation, fiber-optic amplifiers are sensors that use fiber-optics such as glass fibers or plastic fiber-optics to measure various physical variables such as pressure, temperature, expansion and the presence or position of objects. They utilize the ability of fiber-optics to transmit light, detecting changes in the spectrum or amount of light.


What Does Multi Unit Mean?

A sensor network, also known as a multi unit, consists of several sensors that can communicate directly with each other. The sensors do not interfere with each other, even if they are close to each other or opposite each other and inspecting the same object. This enables efficient coordination and collaboration between the sensors. In addition, the sensor network minimizes the need for cabling, as only one cable is required for the connection to the IO-Link master. The signal levels and switching channels of all connected sensors are transmitted via the IO-Link process data, a connection cable and a port on the IO-Link master. This optimizes data transfer and significantly reduces cabling and installation effort.

Do the Sensors Influence Each Other?

Mutual interference can occur with fiber-optic cables installed close together or aligned at the same point. wenglor fiber-optic sensors are equipped with various technologies to reduce or suppress these effects. The P1XD0 and P1XD1 series synchronize via their light beams, while the P1XD2 series sensors, in multi-unit mode, are synchronized via the internal bus, which eliminates mutual interference. It is recommended to avoid mounting the multiple sensor heads too closely or aligning them to the same test object. Alternatively, sensors from the P1XD2 series can be used.

What Is the Alignment Mode?

Fiber-optic cables must be aligned precisely to the target for reliable object detection. Especially when using the through-beam principle, the emitter and receiver should be positioned axially as closely as possible to each other. Since the amplifier or analysis module is often installed in the control cabinet or outside the field of vision, the setup is often based on the operator’s view and assessment. The alignment mode visualizes the signal strength by pulsing the transmitting light. Similar to parking sensors in the car, the pulse frequency increases the stronger the signal received. The sensor head is adjusted in its angle and axis until the optimal alignment with the maximum signal is achieved. This enables efficient and precise setup even with greater distances between the emitter and receiver.
What do you need a DIN rail adapter for?
The amplifier unit is usually mounted on standardized DIN rails. Installation is tool-free by simply and quickly snapping the amplifiers onto the rail. When using the multi-unit mode, several fiber-optic amplifiers can be arranged next to each other in the control cabinet in a space-saving and non-slip manner.

What Are the Advantages of Different Light Sources?

Depending on the specific requirements of the application, wenglor fiber-optic sensors use red, blue, pink or infrared light.

  • Red LEDs (633 nm) offer high process stability, even with very bright or white test objects.

  • Blue LEDs (455 nm) are particularly suitable for precise measurements on glowing, glossy or dark surfaces, as they penetrate less deeply into the test object.

  • In pink light mode, red and blue LEDs are activated simultaneously to increase light output and improve the range of the sensors.

  • Infrared light (over 750 nm) is invisible to the human eye, preventing visual distractions and manipulation – ideal for moving sensors on robot grippers or autonomous vehicles. It also enables a greater range due to its higher power.

What Are Fiber-Optic Cables?

Fiber-optic cables are optic fibers consisting of a light-conducting core and a jacket, each having a different refractive index. In this process, the light is transported through the core with virtually no losses due to total reflection on the jacket. When exiting the fiber-optic cable, the light is scattered at an aperture angle of approximately 60 degrees.
 

What Is the Refractive Index?

The refractive index describes how much light rays change direction when they enter from one medium to another. It is defined by the ratio of the light velocity in the vacuum c to the light velocity in the considered medium v. The refractive index n is dimensionless and varies depending on factors such as the temperature and wavelength of the light.

The following physical formula is used to determine the refractive index:
 
n = v/c

What Is an Aperture Angle?

The aperture angle refers to the angle at which light exits the optical fiber. A large aperture angle offers the advantage that it enables reliable detection of even heterogeneous objects at a short distance. It is also easy to handle, as the orientation of the device is not important. However, the light output quickly spreads over a large area, reducing the range as the light does not stay focused.

To control this wide aperture angle, lenses are used that focus or collimate the light as required. This enables the detection of very small objects or significantly increases the range of the fiber-optic cables.

Optical Fibers in Comparison

Plastic fiber-optic cables are ideal for object detection in applications requiring little space. Glass fiber-optic cables, on the other hand, prove themselves in demanding ambient conditions with high temperatures and offer chemical resistance. These and other advantages of these materials open up numerous application possibilities to meet a wide range of requirements.

Glass Fiber-Optic Cables

Transmission of visible light and infrared light
Tolerant to extreme temperature ranges
Suitable for corrosive or wet industrial environments
Particularly low attenuation in the area of the infrared light
Risk of breakage due to excessive or repeated bending

Plastic Fiber-Optic Cables

Transmission of visible light
Less tolerant to extreme temperature ranges
Not suitable for corrosive or wet industrial environments
Particularly low attenuation in the visible light area
Repeat bends possible due to high flexibility

Parallel Fibers

With this type of reflection, the fibers run parallel to each other to transmit light signals. This fiber arrangement is available as both plastic and glass fiber-optics and is used in most standard applications.


 

Coaxial Fibers

The coaxial reflection type is a high-precision measurement method consisting of a core (emitter) and a surrounding area (receiver). With this type, the direction of entry of the test object into the measuring range is irrelevant for the position of the fiber-optic sensor.

 

Mixed Fibers

The mixed reflection type refers to a fiber-optic structure in which many transmitting and receiving fibers are arranged without separation. The position and distance of the fiber-optic cable to the object are less relevant here. The image area is very small or not present.

Effect of Fiber Diameter/Bundle Diameter

The larger the diameter of the light-conducting core, the more light can be transported through the cable. This leads to greater ranges and improved detection of deep black objects. For certain fiber-optic heads, such as fiber-optic cables, more fibers and consequently a larger diameter are therefore required.

What Does the Bending Radius Say?

The bending radius determines how much a cable can be bent without damaging it or affecting the signal quality. If a fiber-optic cable is bent excessively, there is a risk that the fiber jacket in the cable breaks and light escapes from the fiber core. This can lead not only to increased damping, but also to microcracks in the fiber core, resulting in permanent damage. Therefore, it is important to observe the bending radius, especially for glass fiber-optic cables.

What Is the Structure of Fiber-Optic Cables?

Plastic Fiber-Optic Cables

Glass Fiber-Optic Cables

What Types of Jackets Are there for Glass Fiber-Optic Cables?

Plastic, PVC

The most cost-effective variant. Suitable for standard applications that do not require special resistance to environmental influences.

Stainless Steel

Provides the highest level of protection against mechanical stress. Less flexible installation as larger bending radii are required. No protection against gasses or liquids.

Silicone

Maximum resistance to aggressive media. Absolutely tight, so fluids and gasses cannot penetrate the jacket and damage the optical fibers. FDA compliant.

What Are the Operational Principles of Fiber-Optic Sensors?

Reflex Mode

In the case of reflex mode, the emitter and the receiver are enclosed in a single housing. The light emitted by the emitter hits the test object and is returned to the receiver. The object is detected based on the amount of reflected light reaching the receiver of the fiber-optic cable.

Through-Beam Mode

The through-beam model consists of an opposing emitter and receiver. As soon as the test object passes through the space between the emitter and receiver, the light of the fiber-optic cable is interrupted. Detection is then performed by reducing the received light intensity.

Retro-Reflex Sensor

With the retro-reflex sensor principle, the emitter and receiver are located in a housing, while on the opposite side a reflector is positioned. The test object is detected when the light reflected back by the reflector is either completely interrupted or reduced. 

Fiber-Optic Cables

Fiber-optic cables are used to monitor areas. In contrast to spot-shaped light spots, which only monitor the presence of objects within one point, fiber-optic cables detect several centimeters. The sensor detects the object as soon as the signal is weakened or completely interrupted.

Comparison of Dynamic Readjustment and Jump Detection

Both dynamic readjustment and jump detection are suitable for reliable detection of objects under changing environmental conditions. In dynamic readjustment, a quasi-fixed threshold value is used, whereas jump detection does not require a threshold value and only evaluates signal changes instead.

Fixed Switching Point

The most common mode of operation of a sensor is based on a fixed switching point. The sensor determines the threshold value or the switching point during the teach-in process in accordance with a specified teach-in logic. In normal teach, this corresponds to 50% of the current signal, for example. If the ambient conditions and the objects to be detected are very constant, the mode of operation with a fixed switching point offers the highest insensitivity to interference, as external influences cannot change the switching point: If the signal is above the defined threshold, the output is activated; if it is below, the output remains inactive. However, if the signal is altered due to contamination, for example, this can lead to permanent malfunctions.

Dynamic Readjustment

The dynamic readjustment is particularly suitable for the reflex mode with static backgrounds as well as for the through-beam mode. The non-switched state should prevail, as the threshold value is only readjusted in this state. If there is contamination on the fiber-optic head or on the background, this is compensated by dynamic adjustment of the threshold value.

Jump Detection

The absolute signal values are irrelevant for jump detection. Instead, the direction of the signal change (negative, positive or both directions), the magnitude of the change and the observation period can be included in the evaluation. This enables the detection of highly variable objects (e.g. in color or surface finish) on non-static backgrounds (such as a slowly soiling conveyor belt) as well as the detection of objects without prior teaching (e.g. with changing batches).

Overview of Fiber-Optic Heads

Angled

Angled sensor heads are ideal for tight spaces where the optical axis and cable outlet must be oriented differently. Thanks to the thread, the sensor heads can be easily screwed into prepared holes or fastened to an angle or plate with two nuts.

L Type

The L type allows easy mounting with two screws and offers predefined positions of the optical axes. Exact alignment is not necessary due to the large aperture angle of the fiber-optic cables.
 

Flat

Flat sensor heads can be easily integrated into the base of the workpiece carrier. The flexibility of the cable outlet on the sensor head allows easy cable routing to the left, right or rear.

Flexible

The thin, long metal sensor lance can be adapted to the specific requirements of the respective application by simple bending.

Fiber-Optic Cables

Fiber-optic cables in through-beam mode are ideal for monitoring large areas. Reflex fiber-optic cables, on the other hand, are particularly effective for detecting heterogeneous objects and can also be used for measuring applications through the evaluation of the reflected light.

Miniature

Miniature sensor heads are particularly suitable for applications in the tightest of spaces.

Thread

Threaded sensor heads allow for quick and easy installation. They can either be screwed directly into pre-drilled holes or fixed to brackets or plates using two nuts.

Smooth

Smooth sensor heads are ideal for use in confined spaces and can be inserted or glued into prefabricated mounting brackets.

The Following Must Be Observed when Installing Fiber-Optic Sensors

To ensure reliable object detection and accurate measurement data, the following instructions should be observed when installing the sensor.

Length and Cutting

Fiber-optic cables are available in various lengths. Plastic fiber-optic cables can be cut to size by the customer, glass fiber-optic cables only industrially, as they must be ground and polished after cutting. The length has little effect on the detection range, but longer fiber-optic cables let less light through.


Tip: Select a suitable glass fiber-optic cable.

Detection Range

Due to the large aperture angle, fiber-optic cables have only a small detection range. Higher detection ranges can be achieved with larger fiber bundle/core diameters or with lenses that focus the light.


Tip: Use fiber-optic cables primarily for short ranges and detection of small parts.

Bending Radius

Fiber-optic cables are flexible, but minimum bending radii must be maintained to avoid damage and light loss. High-flex plastic fiber-optic cables are suitable for tight bending radii or moving installations. The following applies in general: Smaller diameters allow smaller bending radii.

Tip: Installation of high-flex fiber-optic cables.

Temperature

Plastic and glass fiber-optic cables differ in terms of temperature resistance. Above 85 °C, stainless steel or silicone coated glass fiber-optic cables should be used.

Tip: Thanks to individual lengths, the analysis module can also be placed in the control cabinet.

Sensor Orientation

In reflex mode, the emitter and receiver should be installed at a 90° angle to the test object when approaching from the side to ensure smooth switching on and off.

Tip: A planar orientation to the object leads to an offset with delayed on and off switching.

Cable with Dedicated Emitter

For fiber-optic heads with coaxial light emission and for certain light bands, it is essential to ensure the correct assignment of emitter on the fiber-optic head to emitter on the amplifier.

Tip: The amplifiers are marked with arrows for this purpose.

Sectors and Industries which Use Fiber-Optic Sensors

Metalworking Industry
In the production of metal profiles, the presence and dimensions of the objects must be detected before a clamping device secures them in place. Profiles can be black, white, chrome, glossy or matt. Glass fiber-optic light curtains based on the transmitter-receiver principle are used in confined spaces, together with a universal reflex sensor. The optical fibers are arranged in a single line to create a light band. The width is measured, the linear signal is output proportional to the glass fiber cover and the correct position is determined.

Electronics Industry
In the electronics industry, PCBs must be detected on transport modules at several stations in order to ensure safe transport. For this purpose, an angled fiber-optic cable with transmitter and receiver (through-beam principle) is attached to the side of each station in the narrow side panels. The fiber-optic cables are operated in a multiplex process on a central fiber-optic amplifier outside the conveyor line – this prevents reciprocal influence and ensures reliable detection of the PCBs at each individual station.

Which Objects Cannot Be Optimally Detected by Fiber-Optic Sensors?

  • Water and other clear liquids that absorb light strongly or change its path through refraction can lead to inaccurate measurements.
  • Highly transparent objects such as clear glass, which allow light to pass through completely without reflecting it, make detection more difficult.
  • Deep black objects that absorb the incoming light heavily and hardly or not reflect at all hinder the signal return to the sensor.
  • Extremely shiny objects that reflect light in unpredictable directions prevent accurate object detection.
     
 

 
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