Photoelectric sensors are electronic devices that use light to detect the presence or absence of an object. With a long sensing distance, fast response time and high resolution, they can detect virtually any object without touching it. This makes them an essential component in a wide range of industries. Modern photoelectric sensors come with a precise lens and intelligent electronics, both integrated into robust housing made of resistant plastic or metal. In this article, we will discuss the different types of photoelectric sensors and their working principles as well as their advantages and disadvantages.
What is a Photoelectric Sensor?
The photoelectric sensor is an electrical device that responds to a change in the intensity of the light falling upon it. Modern photoelectric sensors offer a precise lens and intelligent electronics as a standard. Both components are integrated into robust housings made of resistant plastic or metal.
The photoelectric sensor has a long sensing distance, fast response time, and high resolution. It can be used to detect virtually any object without touching it.
How Does a Photoelectric Sensor Work?
The function of a photoelectric sensor is to use light to detect the presence or absence of an object. A photoelectric sensor consists of the sensor, control unit, and output. The source is a light-emitting diode that emits a powerful beam of infrared or visible light. The detector is typically a photodiode that senses the presence or absence of light.
Basic components of a photosensor:
- Source LED
- Lens (for source and detector)
- Power Circuit (modulator and demodulator)
- Output Circuit
There are many different types of photoelectric sensor technologies. The most popular technologies are through-beam, reflective, diffuse, and background suppression.
A through-beam method consists of a transmitter and receiver mounted opposite each other. Each time an object interrupts the direct path between transmitter and receiver the electrical response of the receiver transistor or the receiver diode changes. This change can be used to detect the presence of an object using electronics and can be signaled via an output stage.
Thru-beam detection generally provides the longest range and provides higher power at a shorter range to penetrate steam, dirt, or other contaminants between the source and the detector. The alignment of the source and detector must be accurate.
The effective beam area is that of the column of light that travels straight between the lenses. Because the light from sources is transmitted directly to a photodetector, thru-beam sensing offers the following advantages:
- It has the longest range for sensing.
- It has the highest possible signal strength.
- It has the greatest light/dark contrast ratio.
- It has the best trip point repeatability.
Disadvantages of thru beam sensing are:
- It requires the wiring of the two components across the detection zone.
- It may be difficult to align the source and detector.
- If the object detected is smaller than the effective beam diameter, an aperture over the lens may be required.
The diffuse sensing method requires that the source and detector are installed on the same side of the object to be detected and aimed at a point in front of the sensor. When an object passes in front of the source and detector, light from the source is reflected from the object’s surface back to the detector, and the object is detected. The main strength of diffuse sensors is the detection of very small objects at close distances.
The optimum range for the diffuse and reflex sensor is more significant than the maximum range. The optimum range is best shown by an excess gain chart.
The detecting zone is controlled by the type, texture, and composition of the object.
Advantages of diffuse sensing:
- Installation and alignment are simple and involve wiring on one side.
- It can detect the difference in surface reflectivity.
Disadvantages of diffuse sensing:
- It has a limited sensing range.
- The light/dark contrast sensing range depends on the target object’s surface reflectivity.
Background suppression enables the diffuse sensor to have excess gain to a predetermined limit and insufficient excess gain beyond that range where it might pick up objects in motion and yield false detection. By using triangular ranging, sensor developers have created a sensor that emits light that reflects on the detector from two different target positions.
The signal received from the distant target is subtracted from the closer target, providing high excess gain for the closest target.
The reflective sensing method requires that the source and detector are installed on the same side of the object to be detected. The light beam is transmitted from the source to a retro-reflector that returns the light to the detector. When an object breaks a reflected beam, the object is detected.
The reflective method is widely used because it is flexible and easy to install and provides the best cost-performance ratio of the three methods. The object to be detected must be less reflective than a retroreflector.
Since the light travels in two directions (hence twice the distance), reflex sensors will not sense as far as thru-beam sensors. However, reflex sensors offer a powerful sensing system that is easy to mount and does not require that electrical wire be run on both sides of the sensing area. The main limitation of these sensors is that a shiny surface on the target object can trigger false detection.
Below, you can see some advantages and disadvantages of each technology.
Through-beam: Most accurate, longest sensing range, very reliable / Must be installed at two points on
the system: emitter and receiver. Costly – must purchase both emitter and receiver.
Reflective: Cost less than through-beam, Only slightly less accurate than through-beam, Sensing range better than diffuse. Very reliable. / Must be installed at two points on the system: sensor and reflector. Slightly more costly than diffuse. Sensing range less than through-beam.
Diffuse: Only install at one point. Costs less than through-beam or reflective. / Less accurate than through-beam or reflective. More setup time is involved.
Background suppression: Effective with reflective backgrounds. / Cost more than diffuse, reflective, or through-beam. Most setup time is required.
Photoelectric sensors have wide application areas in the industry:
- Detecting tab threads.
- Counting packages.
- Determining and detecting the orientation of the IC chip.
- Detecting jams on a conveyor.
- Detecting caps on bottles.
- Detecting components inside the metal can.
- Detecting items of varying heights.
- Controlling the height of a stack.
- Counting boxes anywhere on a conveyor.
- Verifying liquid in vials.
- Detecting labels with transparent backgrounds.
- Verifying screws are correctly seated.
- Detecting reflective objects.