OG photoelectric sensor technology
Industrial optical sensors use emitted light from a source, it gets sent out in to space, an objects interacts with emitted light, and finally a receiver detects characteristics about the object. The most common outputs from this system include simple presence of an object or not, distance measurement to the object, and reflective properties of the object.
Types of light
In the industrial marketplace the three most common types of light used are red, laser and infrared. Red light LEDs are inexpensive but optically powerful. Laser sensors are precise with small light spots. Infrared can penetrate through dusty environments more easily but are invisible to the eye.
Explanation of excess gain
Excess gain is the measurement of the amount of light on an optical receiving element above what is required to successfully operate the sensor.
In industrial applications it is critical to have enough light to burn through the surrounding environment. For example, steel plants have heavy amounts of dust in the air. Optical sensors need enough strength of light to “burn” through dust.
The curve to the right shows what is possible for a specific sensor at various distances to the target.
Technology comparison chart
Each product family within the OG photoelectric line includes part numbers for all three photoelectric technologies: diffuse, retroreflective, and through-beam.
| Diffuse with background suppression | Diffuse with no background suppression | Polarized retro-reflective | Through-beam | |
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| Ability to detect small targets | |
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| Conveying applications | |
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| Harsh environments (water and dust) |
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| Accuracy | |
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| Hardware cost | |
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| : Ideal application |
: Do not use in application |
: Proceed with caution |
Diffuse
Diffuse sensors do not require a reflector or second sensor to receive the light because the transmitter and receiver are located in the same housing. The transmitted light reflects back to the sensor from the target and the receiver evaluates it.
Diffuse with no background suppression
Diffuse sensors without background suppression are not as common in the industrial marketplace as the ones with background suppression technology. This is because most industrial applications have a background close to the target that the sensor needs to detect. However, diffuse technology without background suppression provide longer ranges on light colors without the need of a reflector or second receiving unit (through-beam).
Example of sensing ranges for diffuse sensors with and without background suppression
- O8H220 = 80 mm (with BGS)
- O8T200 = 180 mm (with no BGS)
Learn more about diffuse technology
Diffuse with background suppression
Diffuse sensors with background suppression are highly common in the industrial automation market. These sensors have the ability to sense a target but ignore a background when the target is not present. This functionality increases the number of applications the technology can solve because it only requires one device; can be fit into small spaces, and can detect small targets sitting on a surface but suppress sensing the surface.
How to read background suppression charts
The information about performance of background suppression technology can be found on the product datasheets. On the X-axis, distance from the sensor’s face to a target. On the Y-axis, number of millimeters behind the target that is needed for the sensor to successfully turn OFF when the target is not present. This is known as the minimum distance to the background.
Three different lines are plotted, these show results for black, gray and white targets.
Polarized retro-reflective
The transmitter and receiver are packaged in the same housing and mounted opposite a reflector. Light is sent from the transmitter lens, bounces off the reflector and returns to the receiver lens.
The output changes state when a target interrupts the beam and starves the receiver of light. As long as the target is large and solid enough to break the effective beam, the color, shape, angle, reflectivity and surface finish will not affect the application. This makes them more reliable than diffuse sensors, which depend on light reflecting off the target.
Choosing the right reflector
- Reflectors have small prisms that provide surfaces perpendicular to the sensing face. The more light that hits these perpendicular surfaces, the more reliable the sensing solution becomes.
- Use reflectors made for industrial applications. ifm has a full catalog of reflectors for industrial applications. (link)
- Reflectors for laser sensors have more prisms than standard optical sensors. These reflectors have even more smaller prisms due to the small light spot of a laser.
Through beam sensors
Through beam sensors, also known as thru-beam pairs have the transmitter and receiver packaged in separate housings and are mounted opposite each other. Light is sent from the transmitter lens and is picked up by the receiver lens.
The output changes state when a target interrupts the beam and starves the receiver of light. As long as the target is large and solid enough to break the effective beam, the color, shape, angle, reflectivity and surface finish will not affect the application. This makes them more reliable than diffuse sensors, which depend on light reflecting off the target.