Types of Photosensitive Sensors
Photosensitive sensors use light to detect changes in surface conditions and items. They are widely used in material handling, packaging and food and beverage manufacturing industries.
There are many types of photosensors, but three of the most common are photoresistors, photodiodes and phototransistors. All of these work in a similar way, detecting a change in light intensity and letting an amount of current flow through the circuit.
The Emitter
The emitter of a photosensitive sensor emits a beam of light to detect an object. There are many different types of photosensitive sensors. These include diffused, through-beam, retro-reflective, and distance-settable sensors. The type of sensor used depends on the application.
The main differences between these types of sensors are the radiation pattern, effective beam size, and alignment. Opposed-mode sensors, for example, require that the emitter and receiver be aligned to focus all of the radiated energy in a single point. This can be a difficult task for older opposed-mode sensors, but newer modulated designs and alignment systems make it easy to achieve.
Through-beam (Modulated) Sensors operate by emitting a series of pulses at fixed intervals and are typically designed with mutual interference protection. This eliminates the effects of external light interference, allowing the sensor to detect objects at far distances. These sensors are also a good choice for applications that require fast response times, but have limited mounting space or other constraints.
Another important factor to consider when selecting a photosensitive sensor is the output light bandwidth. Standard LEDs emit a range of visible light, but some special applications may require a narrower bandwidth to reduce stray light that can interfere with the sensor’s detection.
A photosensitive sensor’s output light bandwidth is measured in nm, and this can vary depending on the device’s wavelength. The most commonly used light-emitting diode (LED) has a spectrum of 30 nm to 100 nm, while phototransistors can respond to longer wavelengths such as red and infra-red. Other devices, such as a photoconductive cell, have a spectral response curve that closely matches that of the human eye. Cadmium sulphide is one of the most popular materials for photoconductive cells, and it has an excellent spectral response in the yellow-orange region. The sensitivity of a photoconductive cell can be adjusted by an on-board potentiometer, which increases the detection range of the sensor. A light-dependent resistor is another type of photosensitive sensor that can be used to measure ambient brightness and light intensity. Its sensitivity can be adjusted by turning an on-board potentiometer, and it has a suitable supply voltage for use with microcontrollers and logic circuits.
The Receiver
The receiver is the part of the photosensitive sensor that detects changes in light. It contains a light source (LED), a signal converter, and an amplifier. When the light reaches the sensor, the receiver (a phototransistor) analyzes it, verifying that it comes from the LED and triggering a pre-defined output.
There are three primary types of receivers in photoelectric sensors: diffused, retro-reflective, and through-beam. In photosensitive sensor each, the receiver uses a different technique to identify an object.
Diffused sensors emit a beam of light that diffuses in all directions, filling a sensing area. When an object enters the beam, it disrupts it and causes a corresponding change in light intensity. In a diffused-type sensor, the emitter and receiver are located together in the same housing.
In retro-reflective sensors, the transmitter component emits a beam of light that is reflected back into the receiver. When the beam is interrupted, it triggers the pre-defined response. These sensors have the longest ranges of any type, and are commonly used in industrial safety systems for large spaces.
Background suppression is a technique that reduces the amount of light that can be reflected from an object. This eliminates the possibility of false detection, and helps maintain a reliable, long-term operation.
Depending on the application, the sensor may use a polarizing filter that rotates the light 90 degrees, or the phototransistor can be filtered to only sense certain wavelengths of light. Both of these techniques can increase the sensitivity and reduce the number of false alarms.
Another sensitivity adjustment is the use of a phototransistor, which is a bipolar NPN transistor with its base region electrically unconnected. The collector region is reverse biased and when light falls onto the base more electron/hole pairs are formed, causing current to flow and amplifying the output voltage of the transistor.
Phototransistors are very versatile light sensors that can turn their base flow “ON” or “OFF” in nanoseconds, and they have an excellent sensing range to size ratio. They are widely used in cameras, lighting meters, CD and DVD-ROM drives, TV remote controls, scanners, fax machines, copiers etc.
Mechanical Background Suppression
A photosensitive sensor detects targets in range based on the amount of light reflected back to it. However, when the target and background have similar reflectivity (such as light reflected back by dark targets against a lighter, more reflective background), it can be difficult for the sensor to distinguish the target from the background.
To overcome this problem, manufacturers have developed diffuse-sensor background suppression that measures the angle of incidence rather than the amount of reflected light. This technique is called triangulation, and it offers improved stability when color variations or reflective backgrounds are problematic.
First-generation fixed-field background suppression uses an emitter to send a beam of light to two receivers: one is focused on the optimal target distance, and the other is focused on a more distant background. A comparator contrasts the light intensities from these receivers, and the sensor outputs when the light collected by the focused-receiver is brighter than that from the more distant receiver; otherwise it remains inactive.
Second-generation diffuse-sensor background suppression has an array of receivers, each with an adjustable sensing distance. These sensors can photosensitive sensor be set to operate at their preset focal spot, or can use a potentiometer to electronically adjust the sensing distance for small part recognition.
In addition, these diffuse-sensor background suppressors exhibit better target-area cutoff tolerances and color sensing than the first generation of sensors. However, a variety of target qualities degrade their results, including glossiness and reflective objects outside the sensing area.
Third-generation diffuse-sensor background suppression combines these advantages with the technique of triangulation, which measures the angle of incidence rather than the amount reflected back to it. This allows these devices to detect targets in range, even against a light or glossy background, and provides a more reliable method of background suppression when color variations and reflective backgrounds are problematic.
Compared with traditional photoelectric sensors, these new devices are relatively small and can be installed in tight places. They also offer a high level of reliability, accuracy, and repeatability. In addition, they are easy to install and use. They can be used in a wide variety of applications, including packaging and industrial automation.
Electronic Background Suppression
The most common photoelectric sensor for use on a production line is a through-beam sensor that features a separate transmitter and receiver unit arranged to face each other. When an object obstructs the light beam, it triggers the receiver. The resulting signal can be used to determine whether the object is present or not. However, these sensors have a number of limitations.
First, they have a relatively short sensing range. This is due to the fact that a target obstructing the light beam must be very close to the sensor in order to generate a signal. This minimum sensing distance is often under 10% of the full sensor range.
Second, they are prone to a phenomenon known as the cross-eyed effect. When a specified target is smaller than the LED light spot of the background suppression sensor, it can cause the majority of the transmitted light energy to be reflected off the background and back to the sensor. This causes the sensor to believe there is no target present.
Another disadvantage of these sensors is that their sensing ranges are dependent on the color of the target. This means that a black target will typically reduce the sensor’s sensing range by about 190mm.
Fortunately, this problem can be solved by using an electronic diffused mode sensor with background suppression. Diffused mode sensors with background suppression employ different sensing components that distinguish between objects close together without picking up background noise.
These sensors are available in both fixed and adjustable range models, with adjustable models allowing for sensitivity adjustments through an external potentiometer. This is a more flexible option than fixed-range sensors, which only have a stationary focal plane.
Additionally, these models are more tolerant of target surface qualities such as glossiness and can also be used to detect small parts. This is especially true if the receivers are electronically adjusted.
Other advantages of background suppression include higher switching accuracy, better target-area cutoff tolerances, and improved color sensing. These sensors are more expensive than standard diffused models, but they do offer greater reliability.
