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| Scheme: Photoelectric sensor |
A photoelectric sensor is an electronic equipment used to detect absence or presence of an object, and to measure the distance. By using a light transmitter (mostly infrared) and photovoltaic receiver, predicts the distance of a particular object, their presence and absence. And they are often used extensively in the manufacturing industry.
The emitter in photoelectric sensors is for emitting the light and the receiver for receiving light. When emitted light is interrupted or reflected by the sensing object, it changes the amount of light that arrives at the Receiver. The Receiver detects this change and converts it to an electrical output. As mentioned earlier, the light source for the majority of Photoelectric Sensors is infrared or ordinary visible light (generally red, or green/blue for identifying colors).
How a photoelectric sensor does detects an object?
A photoelectric sensor uses light as a medium to detect the object. It captures change in light depending on the object of detection. i.e, identifies objects using the difference in the amount of light received by the receiver when an object is present and when it is not.
For example, consider we are to detect a bottle, when the bottle is not there, there is no reflected light and therefore there is a very limited quantity of light received i.e zero. When a bottle is present, there is a significant quantity of reflected light, which means that there is a difference in light quantity between the two states. Using this principle photoelectric sensors detect the presence or absence of objects.
Therefore, selecting a sensing type (Through-beam, Retro-reflective and Diffuse-reflective) or light source color (red, or green/blue) that makes a greater difference in light quantity and adjusting the sensitivity so that the difference in light quantity can be identified.
Classifications of photoelectric sensor:
There are three main types of photoelectric sensors
- Through-beam sensor,
- Retro-reflective sensor and
- Diffuse-reflective sensor.
i. Through-beam sensor:
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| Through-beam sensor: Keyence |
The Emitter and Receiver are installed opposite each other in a same straight line to enable the light from the Emitter to enter the Receiver. When a target object passing between the Emitter and Receiver interrupts the emitted light, it reduces the amount of light that enters the Receiver. This reduction in light intensity is used to detect an object.
Some models, called slot sensors, are similar in sensitivity to thru-beam sensors and are configured with an integrated emitter and receiver.
ii. Retro-reflective sensor:
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| Retro-reflective sensor: Keyence |
The Emitter and Receiver are installed in the same housing and light from the Emitter is normally reflected back to the Receiver by a Reflector installed on the opposite side. When the sensing object interrupts the light, it reduces the amount of light received. This reduction in light intensity is used to detect the object.
iii. Diffuse-reflective sensor:
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| Diffuse-reflective sensor: Keyence |
The Emitter and Receiver are installed in the same housing like retro-reflective type, and light normally does not return to the Receiver. When light from the Emitter strikes the sensing object, the object reflects the light and it enters the Receiver where the intensity of light is increased. This increase in light intensity is used to detect the object.
Photoelectric sensor wiring:
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| Photoelectric sensor wiring: Omron |
The picture shows how to connect a 3 wire sensor in circuit. It has three wires Blue (0V), Brown (+12v), Black (Output). Before connection sensor read the technical specifications to avoid malfunctions. Do not use a voltage in excess of the operating voltage range. Applying a voltage in excess of the operating voltage range, or applying AC power (100 VAC or greater) to a DC Sensor may cause explosion or burning.
Also do not reverse the power supply polarity or otherwise wire incorrectly. Doing so may cause explosion or burning.
Applications of photoelectric sensors:
1. Auto-operation doors:
In shopping malls, public transport, Hotels and Administrative offices the photoelectric sensors detects when someone is standing near by the door.
2. Materials handling:
Automated warehouses with robot pickers or trucks are used photoelectric sensor to detect position and object, operate efficiently and safely.
3. Measuring distance:
With diffuse-reflective sensors, and can be used to accurately determine distance, for example, to check the level of liquid in storage tank of manufacturing systems.
4. Detecting colors:
In printing and packaging industries photoelectric sensor are used to identify a particular object by scanning independently with red, green and blue light beam.
5. Checking objects on conveyors:
Photoelectric sensors can detect items sizes to spot any errors, or simply spot their absence, as well as picking up problems like misaligned caps on bottles. They are widely used in the food and pharmaceutical industries, and in packaging plants.
6. Food & Beverage Industry:
For example, in a soft drink manufacturing company the bottle cap fitting machine must have each bottle cap properly aligned and oriented. If there are any faults in the fitting, photovoltaic sensors will help detect them to allow the plant to run smoothly.
7. Pharmaceutical Industry:
Pharmaceutical industry applications such as packaging of medicines also use photoelectric sensors. During packaging, sensors can be used to avoid discrepancies such as empty packaging due to medicine tablets not being present on the line, etc.
Features:
a. Long Sensing Distance:
A Through-beam Sensor, can detect objects more than 10 m away. This is impossible with magnetic, ultrasonic, or other sensing methods.
b. No Sensing Object Restrictions:
These Sensors operate on the principle that an object interrupts or reflects light, so they are not limited like proximity Sensors to detecting metal objects. This means they can be used to detect virtually any object, including glass, plastic, wood, and liquid.
c. Fast Response Time:
The response time is extremely fast because light travels at high speed and the Sensor performs no mechanical operations because all circuits are comprised of electronic components.
d. High Resolution:
The incredibly high resolution achieved with these Sensors derives from advanced design technologies that yielded a very small spot beam and a unique optical system for receiving light. These developments enable detecting very small objects, as well as precise position detection.
e. Non-contact Sensing:
There is little chance of damaging sensing objects or Sensors because objects can be detected without physical contact. This ensures years of Sensor service.
f. Color Identification:
The rate at which an object reflects or absorbs light depends on both the wavelength of the emitted light and the color of the object. This property can be used to detect colors.
g. Easy Adjustment:
Positioning the beam on an object is simple with models that emit visible light because the beam is visible.
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