what is fiber optic sensor?
Fiber Optic Sensors: Detection Principles, Setup and PLC Integration
A fiber optic sensor uses an optical fiber as part of a detection or measurement system. On production machinery, the term usually describes a photoelectric sensor with a remote fiber head: the electronics stay in an amplifier while a small optical tip reaches the target. The optical fiber carries light between those locations, allowing sensing where a complete electronic sensor may be difficult to install. [1,2]
That industrial arrangement should not be confused with fiber Bragg grating sensors or distributed fiber measurement. A photoelectric detector may answer “is the component present?”; an FBG system can measure strain using a dedicated reader. Sharing the word fiber does not give these instruments the same capabilities. [1,10,12]
What is inside the sensing system?
The typical amplifier contains a light source, optical receiver and signal-processing electronics. Light travels through the transmitting fiber to the sensing location. The receiving path returns light from an opposed tip or from the target, and the electronics evaluate the change. The passive tip does not provide an electrical PLC output. The amplifier does. Here, “fiber amplifier” means an industrial sensor unit, not a telecommunications optical-line amplifier. [1,2,5]
Choose the optical arrangement first
Through-beam, or opposed mode: Separate emitting and receiving tips face one another. An object is detected by the reduction in received light as it crosses the beam. For opaque targets, surface color is generally less influential than in diffuse sensing. Alignment, beam geometry and the smallest target must still be considered together. [1,2]
Diffuse reflective: Transmitting and receiving paths share a sensing head, which collects light returned by the object. Access from one side is convenient, but dark surfaces, shiny finishes, target angle and background reflections can change the result. A range specified using white reference material is not a guaranteed range on every production part. [2,3,7]
Retroreflective and specialist heads: A compatible head can monitor light returning from a separate reflector. Other designs restrict the active sensing region or improve detection of transparent materials. A reflector is not the same as the target in a diffuse arrangement. Clear film and glass need the right optical configuration, not simply a more sensitive threshold. [1,3]
Where fiber detection earns its place
Typical jobs include counting small components, checking a part inside a narrow fixture, detecting thin edges and confirming component presence in assembly. A pin-in-place check is one example. The small head handles the optical task while the adjustment controls remain accessible elsewhere on the machine. [1,4]
Special heads can detect liquid presence in tubing or a level at a particular point. Neither result automatically measures litres per minute; that requires an appropriate flow meter. Likewise, a switching event identifies a positional condition, not a continuous distance. For a value in millimetres, specify a measurement instrument, such as a suitable laser displacement sensor, rather than treating received light intensity as distance. [1,2]
Specify a compatible head, fiber and amplifier
When comparing fiber optic photoelectric sensors, check more than the mounting thread. The sensing method, working distance, beam size, fiber diameter, interface and amplifier compatibility all matter. The same fiber can have different sensing ranges with different amplifiers. Fiber cable length and sensing distance are separate specifications. Cutting or extending the fiber can also alter optical performance. [3,4]
Plastic and glass fibers provide different construction options. A free-cut plastic assembly must be trimmed using its approved tool and procedure; do not assume a glass bundle or factory-terminated assembly can be treated the same way. High-temperature heads are available, but their rating does not apply to the amplifier. For moving equipment, check suitability for repeated flexing, not merely whether the cable can be bent once. [1,6,8,17]
Teaching a threshold that survives real production
Teach-in establishes a switching threshold from representative sensing conditions. Depending on the model, those conditions can be presented while stationary or during motion. The objective is a reliable separation between states, not the largest possible display value. [5]
Consider an illustrative through-beam application showing 1,800 without a part and 350 with a part. A threshold of 1,000 could be a starting point for testing. These are example display counts, not millimetres or lux. Now change the part angle and position. If the two conditions occupy overlapping value ranges, no single fixed threshold can distinguish them reliably; improve the optical arrangement first.
Light-ON activates the output when sufficient light is received; Dark-ON activates it when received light falls. For an opposed beam, part presence commonly calls for Dark-ON. A diffuse presence check commonly uses Light-ON. When comparing this with NO/NC behavior, establish which condition is being treated as normal. None of these logic choices changes an NPN output into PNP. [5,7]
Connect the electrical output, not the fiber, to the PLC
Match the amplifier’s PNP or NPN output to the PLC input arrangement. Verify the supply range, common connection, output loading and terminal assignments in the device instructions. Wire colors and connector appearance are not sufficient evidence. A switching output is also not an analog measurement channel; the distinction between analog and digital inputs determines how the controller can use it. [5,6]
Some models or communication units expose received-light values and diagnostics in addition to detection status. An integration could show counts and a low-signal warning on an HMI, with stoppage records passed to SCADA. These are design options, not universal features. IO-Link, analog monitoring and network connectivity must be checked for the exact model. [3,5,6]
Will the complete system catch the shortest event?
A fast sensor cannot compensate for an unsuitable receiving input. Check switch-on time, switch-off time, input filtering and the program’s event-capture method. Short pulses may need a suitable high-speed counter input. An amplifier’s long-range setting may have a slower response than its high-speed setting. [6,7]
As a simplified calculation, a part extending 2 mm in the direction of travel crosses a narrow beam at 1 m/s. Its approximate interruption time is t = L / v = 0.002 s = 2 ms. A setting with a 5 ms response cannot be assumed to capture that event reliably. The clear interval between parts must also allow the system to reset. Actual pulse width depends on beam geometry and the switching threshold as well.
Installation checks before final adjustment
Route the fiber without crushing it or forcing it around sharp corners. Observe the assembly’s minimum bend radius. Where field cutting is permitted, make a clean cut, fully insert the ends and secure the clamp. Fit any required diameter adapter. Use a material-compatible cleaning method rather than an arbitrary solvent. [4,5,8]
Test with the darkest or shiniest production target, realistic angular variation, operating speed and neighboring optical sensors active. Check the IP protection rating of the head, connections and amplifier separately. Can the two optical states still be distinguished? Is the output logic correct? Does one part produce one controller count? These are useful acceptance questions after mechanical installation, not substitutes for it. [2,3,6]
Separate optical faults from signal-capture faults
A falling light reading points toward checks such as contamination, misalignment, loose fiber seating, cable damage or a changed target surface. A correct sensor output with missed PLC counts calls for checks of electrical compatibility, input filtering, pulse duration and software. This distinction helps keep troubleshooting focused rather than replacing optical parts without evidence. [3,5,6,8]
Fiber is not automatically the best technology. A short-range metal target may suit an inductive sensor; a suitable material or level application may suit a capacitive sensor. Where optical surface conditions are troublesome, an appropriate ultrasonic sensor is another possibility. Choose around the target and environment, not around a preferred sensor label. [13,14]
Beyond presence detection: intrinsic, FBG and distributed sensing
In an intrinsic sensor, the measurement effect occurs within the fiber itself. In an extrinsic arrangement, the fiber carries light to an external sensing region. Neither definition means that liquid flows inside the fiber. Conventional glass and plastic optical fibers are not hollow liquid-filled tubes. [1,9]
A fiber Bragg grating, or FBG, contains a periodic optical structure that reflects a particular wavelength. Strain and temperature shift that wavelength, which a dedicated optical interrogator measures. A strain measurement therefore needs temperature effects to be separated or compensated. An ordinary industrial photoelectric amplifier is not a substitute for that reader. [10,16]
Distributed sensing produces position-dependent measurements along a fiber. Raman-based DTS, for example, measures a temperature profile; other suitable systems map strain. Applications include power cables, pipelines and structural monitoring. The capability to monitor an extended installation belongs to the complete specialized system, not to every small fiber head used on a machine. [11,12]
Noise immunity does not establish a safety rating
The passive optical path offers an advantage around electromagnetic interference, but the amplifier and electrical wiring remain electronic equipment. Fiber alone establishes neither complete-system immunity nor ATEX suitability; optical radiation also needs assessment in an explosive atmosphere. A standard fiber photoelectric sensor is not a personnel-protection device. Do not replace a safety light curtain with a sensor that was never designed for that function. [1,5,15]
Technical references
[1] OMRON — Fiber Sensors · [2] OMRON — Photoelectric Sensors · [3] OMRON — Fiber Application Data · [4] Banner — Glass and Plastic Fibers
[5] Banner — DF-G1 User Manual · [6] KEYENCE — FS-N Specifications · [7] OMRON — Photoelectric Sensor Terms · [8] Banner — PIPS46U Cutting and Handling
[9] Udd — The Emergence of Fiber Optic Sensor Technology · [10] Sadık, Durak & Altuncu — FBG Sensor Interrogation · [11] Luna / LIOS — Distributed Temperature Sensing · [12] Luna — Fiber-Optic Strain Sensing
[13] OMRON — Proximity Sensor Principles · [14] OMRON — Ultrasonic Sensor Principles · [15] ISS — IEC 60079-28 Scope · [16] HBK — Fiber Bragg Grating Fundamentals