There are many different styles of photoelectric sensors, including through-beam, reflective, diffuse, and background suppression. Let’s take a look at the types of photoelectric sensors.
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| Different styles of photoelectric sensors |
Types of Photoelectric Sensors
A photoelectric sensor (also known as a light-beam sensor) is an optical device that uses light to measure the distance to material by calculating the time it takes for emitted light to bounce off that material.
The most common type of photoelectric sensor is the reflective photoelectric sensor which sends out an infrared beam and measures how long it takes for this beam to return after bouncing off a reflective object. These devices are often used in object-sensing applications such as assembly lines, conveyor systems, and other robotics applications, which detect objects in front of them and automate switching operations accordingly.
Using a photoelectric sensor requires that it be paired with a light emitting diode (LED) or laser diode for emitting light. Some versions also have an infrared LED paired with the phototransistor to sense infrared light. The phototransistor receives photons from the emitter and converts them into electric current, which runs through the wires and is measured by the controller. The ratio of current to time = distance = object size/diaphragm diameter—which is proportional to object size and material type (as in wood, glass).
A reflective photo sensor is one of several light sensors used to measure distance. The simplest method uses a light source that generates light pulses at an established frequency. The sensor will reflect each pulse off its surface and measure the time it takes for the returning pulse to reach the sensor. Some systems use a pulsed laser beam in conjunction with a photodetector, in which case the distance to an object can be calculated by knowing the speed of light in air.
The reflective photoelectric sensors are largely used for industrial automation. They are widely used in robotic arms and industrial automation applications, paired with mechanisms that physically interact with objects. Common uses for reflective photoelectric sensors include machine vision systems, automated assembly lines, and conveyor tracking applications where objects are automatically identified before moving or processing.
Some recent advances have been made in using photodiodes and phototransistors in electronic sensors that are used with reflective photoelectric sensors. These devices operate at lower speeds and have a much faster response time, making them more suitable than traditional photoelectric sensors for some applications. However, they are still too slow to be used with high-speed robotic applications.
The primary disadvantage to using a reflective photoelectric sensor is that it cannot detect motionless objects—that is, it sees only those movements in which an object oscillates up and down or side-to-side within a certain range about its original position. Here, the term "motionless" means the object is not moving faster than a certain minimum speed, so an oscillation nontemplate can be applied to all things in front of the sensor.
Other disadvantages are that it takes more time for the light to bounce off the object and for the sensor to detect the returned pulse. The time difference is the inverse of the object's distance. Reflective photoelectric sensors are not always suitable with slower object speeds because they cannot detect small movements accurately. However, recent advances in photodiodes and phototransistors have made them ideal for many applications such as lab automation, medical equipment, and machine vision systems where the fast motion of objects is not needed but excellent accuracy.
Response time is also limited by ambient light levels in a given area. For example, if a reflective photoelectric sensor is used in a poorly lit room, it will more than likely require the light's source to be positioned directly above it. This kind of positioning does not scale well to larger applications.
Three basic types of reflective photoelectric:
The first type is the simple Phototransistor which consists of a photodiode and an amplified charge-pump transistor (typically BJT). Only very fast objects can be observed to minimize the detection distance over which reflections need to be taken to avoid excessive noise. High amplification and low input impedance are essential for immediate response at low light levels for these sensors. Therefore, transistor sensors are frequently used in small photodetection applications such as high-speed cameras.
Also known as the TTL photoelectric sensor, the second type uses a phototransistor and a full wave delay line (linear or nonlinear) to convert photocurrent pulses into voltage pulses. A fully digital design using three states enables or disables (short or open) the current paths between emitter and collector concerning the delay line. It is based on a bistable multivibrator, typically a D flip-flop. The basic principle allows for extremely simple design construction and high-speed analog/digital conversion of light pulses into electronic signals like TTL pulses.
The third type is a Phototransistor, Photodetector, and optionally an optical delay line, where the photoelectric sensor has a series of photodiodes (simple or full-wave) and a partially addressed amplifier.
In the case of multiple photodiode channels, which allow the detection of numerous fast-moving objects, this type of sensor can be improved using the Multiple Excitation (Mez) technique. An additional gain of the gate stage is achieved by using multiple sampling stages, which sample the input voltage at different time intervals (1 cycle/channel), in addition to the digital control stage such as Schmitt trigger or RTD (Racetrack Detector) circuit.
The digital sensor is needed to provide high speed, low cost, and low power supply for the illumination, which can be directly generated from the microcontroller.