The Basics of Optical Sensors and Common Types
Optical sensors are one of the most popular sensor types in industrial automation. This article covers optical sensor basics and commonly used types, including fiber optic, photoelectric, and optical encoders.
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Knowledge of the true nature of light took millennia to understand. For centuries, the debate centered around whether light was a wave or a particle. Finally, in 1678, Dutch scientist Christiaan Huygens proposed that light is a collection of waves vibrating up and down the direction of propagation. However, his contemporary, Sir Isaac Newton, proposed that light comprises small particles called photons.
In the end, it was proven that light is a wave that can propagate via electric and magnetic fields whistle still exhibiting some particle properties, such as momentum. These peculiarities make light an excellent medium for sensing devices.
Figure 1. Optical sensors often operate in the visible light spectrum but can also be IR or UV. Image used courtesy of SensoPart
Light waves exist in different ranges of frequency in the electromagnetic spectrum. For example, microwaves, infrared, visible light, and ultraviolet light can be found in the low-frequency band. This chapter focuses on optical sensors that operate in the visible light spectrum, such as photoelectric sensors.
Optical Sensor Types
Together with mechanical sensors, optical sensors are some of the most popular in industrial automation. This is because optical sensors are relatively simple to set up and calibrate and do not get much affectation from outside interferences. Also, they can offer long operating distances.
Presence and position detection are two of the most common applications in the industry. There are different types of optical sensors, some of which are covered next:
Figure 2. Wheatstone bridge electrical circuit.
Optical Bridge Sensors
These sensors employ a concept similar to the Wheatstone bridge electrical circuit. In electrical engineering, the Wheatstone bridge helps accurately measure resistance values by placing the unknown resistor as part of a simple series-parallel arrangement.
Four light detectors are connected in a bridge configuration in the optical bridge sensor. Each detector can sense very small changes in the positioning of the light beam emitted. The combined result from the four light detectors in the quadrant is processed to find the center of the power distribution.
Figure 3. A four-quadrant sensor photodiode. Image used courtesy of First Sensor
These sensors are good choices in presence and position measurements. Some applications include surface profilers, centering systems, tilt sensors, and many high-precision positioning applications.
In their natural condition, light waves travel in random directions and patterns. Light polarization means processing and filtering a light wave to remove randomness and orient it in a desired horizontal, vertical, or elliptical pattern.
Figure 4. A linear polarizer filters an unpolarized light wave. Image used courtesy of American Polarizers
There are different ways to achieve polarization. One way is employing linear polarizer filters, also known as Polaroid filters. These filters act similarly to camera focus lenses and are constructed with special materials that can filter out the unwanted planes of motion of the light wave. Polarization can also be achieved through controlled surface refraction and reflection.
The principle of polarized light is more like an enhancement to other sensor technologies rather than a sensor type. Polarized light offers numerous benefits and can be found in multiple industrial applications. They are an essential add-on to vision systems that help improve image contrast and reduce surface reflection.
Fiber Optic Sensors
Optical fiber is a versatile medium that can transmit data using light and function as a sensing device. Fibers can be modified to measure specific quantities via variations in intensity and wavelength. These sensors can measure temperature, pressure, vibrations, and more.
Fiber optic sensors are considered descendants of photoelectric sensors, which will be discussed next. Fiber optic sensors used as photoelectric sensors can be configured as through-beam, retro-reflective, and diffuse sensors.
Figure 5. A photoelectric diffuse sensor being used in small-part detection. Image used courtesy of SensoPart
Photoelectric sensors are among the most common sensors used in industrial automation. They have many possible applications, including presence detection and safety protection. In addition, they can offer relatively long operational distances for presence detection compared to other media.
In the basic configuration, photoelectric sensors consist of a light emitter and a receiver. Normally, light emitters employ light-emitting diodes (LEDs) that can produce modulated pulses. The receivers consist of photodiodes that can convert light at the input into electrical signals. These signals are then amplified and processed to be sent to the controller.
The three most common types of photoelectric sensors are the through-beam, retro-reflective, and diffuse reflection sensors.
Figure 6. A diagram of an optical encoder. Image used courtesy of Anaheim Automation
Optical encoders are another important application that uses light for motion detection. These sensors consist of a light source and receiver passed through a coded disk. The receiving sensor is then connected to a squaring circuit that converts the input into a pulse signal. Finally, the pulses are processed by a central controller and converted into a motion measurement.
Sensors in Industrial Automation
In the next article in our series, we will discuss laser sensing basics and the commonly used laser sensor types in industrial automation.