Introduction to photoelectric sensors
- Housing – various shapes, sizes and materials of construction
- Basic sensor element – varies depending on the technology, but always includes a lens system
- Electronics -- evaluates what the sensor detects
- Electrical connection – provides power and signal(s)
To best apply photoelectric sensors, it is helpful to understand the electromagnetic radiation spectrum. ifm photoelectric sensors operate in the visible (primarily red) and infrared frequency range.
|Visible red light|
|is the best “all around” light type and is recommended for most applications. The majority of ifm sensors use visible red light.|
Easy to see at short ranges, making it useful as a set-up aid
Color dependent at longer ranges
Color independent over most of the sensing range
Good choice for dirty environments – it has the ability to “burn through” dust, mist, steam, etc.
Invisible to the human eye making set-up more difficult
Ability to detect small targets at long ranges
Small light spot allows for precise switch points
Bright red visible beam can be used as a set-up aid
Laser LEDs are generally more expensive than standard visible red or infrared LEDs
Modulated light – light sent by the transmitter is pulsed at a frequency unique to each sensor family. The receiver is tuned to detect light modulated at this frequency and ignore ambient light from other sources.
Switching frequency – maximum speed at which a sensor will deliver discrete pulses as the target enters and leaves the sensing field. Simply, it is how fast the sensor can switch on and off when a target passes by.
Contrast – the difference in color and brightness between two objects. White is the easiest color to detect and black is the hardest to detect.
Beam spot (or light spot) – the diameter of the transmitted light at a given distance. This dimension is usually shown on datasheets at the maximum range and it is a function of the transmitter lens angle of aperture.
Effective beam – the area of the light beam that must be completely interrupted for the sensor output to change state. Sensors that switch when the light beam is broken (i.e., through beam and polarized retroreflective sensors) have effective beams. Sensors that bounce light directly off the target (i.e., diffuse sensors) do not have effective beams.
Light operate (or light-on) – the output changes state when the receiver detects light.
Dark operate (or dark-on) – the output changes state when the receiver does not detect light.
Excess gain – the ratio of light energy actually received by the sensor to the light energy required to change the output state. A gain value of 1 is the minimum required to switch the output. Anything above this threshold is considered excess gain. It is useful in determining proper operation of the sensor in contaminated areas.
|Maximum Excess Gain Required||Operating Environment|
|1.5X||Clean air: No dirt build-up on lenses or reflectors.|
|5X||Slightly dirty. Slight build-up of dust, dirt, oil, moisture, etc. on lenses or reflectors; lenses are cleaned on a regular schedule.|
|10X||Moderately dirty: Obvious contamination of lenses or reflectors, but not obscured; lenses deaned occasionally or when necessary.|
|50X||Very dirty: Heavy contamination of lenses; heavy fog, mist, dust, smoke, or oil film, minimal deaning of lenses|
Through beam sensors
Also known as through beam / thru-beam pairs. The transmitter and receiver are 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.
The effective beam is uniform in diameter and is approximately equal to the diameter of the transmitter and receiver lenses. So long as the target is at least as big as the effective beam, the output will switch when the target breaks the beam.
Outputs for a thru-beam pair:
- Light operate outputs turn on when the target is not present.
- Dark operate outputs turn on whe the target is present.
When mounting multiple thru-beam pairs, take care so that the transmitted beam of one sensor does not interfere with other receivers. A simple solution is to alternate transmitters and receivers as shown.
A highly reflective object passing through a beam may reflect light onto an unrelated receiver causing a false signal. A simple solution is to place barriers between the sensors to block any stray reflections.
Because sunlight contains the same wavelengths of light as photoelectric transmitters, very bright ambient light can often fool the receivers. This is commonly seen when photoelectric sensors are used for home garage door openers and sunlight at a certain angle can interfere with the door operation. Possible solutions include angling the sensors, adding a barrier or reversing the transmitter and receiver.
Polarized retroreflective sensors
As with thru-beam sensors, 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.
The effective beam is of polarized retroreflective sensors is cone-shaped. Near the sensor, the beam is approximately the size of the transmitter lens. Near the reflector, it is the size of the reflector. This means that smaller objects can be detected when close to the sensor, but not necessarily when close to the receiver.
Outputs for a polarized retroreflective sensor:
- Light operate outputs turn on when the target is not present.
- Dark operate outputs turn on when the target is present.
Prismatic reflectors are required for polarized retroreflective sensors. By their design, these reflectors rotate the incoming light beam by 90 degrees. The sensors are equipped with polarizing filters over the lens so light waves are oriented in one direction only. The reflector rotates the light waves to match the orientation of the filter on the receiver.
Shiny targets may return high intensity light to the sensor, but since the light is not properly oriented, the shiny targets will not cause a false signal.
The transmitter and receiver in a diffuse sensor is located in the same housing. The transmitted light reflects back to the sensor from the target and the receiver evaluates it. It is important to carefully consider the characteristics of the target and the background behind the target when selecting the correct solution for an application. Diffuse sensors have much less excess gain than thru-beam pairs, but typically more than polarized retroreflective types.
The sensitivity of diffuse sensors is very high. Only 2% of the transmitted light energy reflected off the target will cause the output to switch.
Larger objects reflect more light resulting in greater sensing range.
With visible red sensors, lighter colors can be detected at longer range than darker colors. Target color has much less effect on infrared sensors. Shiny surfaces can be sensed at longer range than flat or matte surfaces.
Smooth surfaces have better reflective quality than rough surfaces. A smooth blue plastic target, for example, will reflect more light than a blue velvet target.
Flat targets perpendicular to the sensor will reflect more light than flat targets at an angle. Also, non-flat targets tend to deflect light away from the sensor resulting in a loss of energy and sensing range.
A diffuse sensor detects all light reflected into the receiver, regardless of its source. Light reflecting off the background appears the same as light from the target and is especially troubling when the background is more reflective than the target and when the target and background are very close together.
To reduce the detection of the background:
- Modify it by painting it with a dark, flat paint.
- Change the angle of the sensor relative to the background.
- Reduce the sensitivity of the sensor to “tune out” the background.
- Use a diffuse sensor with built in background suppression.
Background suppression sensors
These sensors are specially designed diffuse sensors that eliminate false tripping on the background behind the target. Several technologies suppress backgrounds including: Fixed range, Triangulation principle, Diode array, PMD time-of-flight
The position of the transmitter and receiver lenses are angled to create a detection zone. Objects in the detection zone reflect light into the receiving lens and are sensed. Objects outside the detection zone (either too close or too far) do not have the correct geometry to return light to receiver. This method is normally used for short range and is not adjustable.
This technology uses two receiving elements to obtain background suppression. Using a potentiometer for adjustment, a mirror is mechanically positioned to determine the point where one receiver detects the target and the other detects the background. The sensor is then adjusted halfway between these two points. The sensor evaluates the angle of the received light to determine if the light comes from the target or the background.
This method is similar to the triangulation principle, except the receivers are a 63-diode array. The additional receivers allow for precise background suppression (i.e., the target and background can be very close). Diode array sensors are equipped with a microprocessor and programmed electronically via pushbuttons.
PMD (Photonic Mixer Device) determines the distance between the sensor and object (and the sensor and the background) by measuring the time it takes for the light to travel from the sensor to the target and back again.
A laser diode generates a modulated laser beam. The light reflected by the target is directed onto a photosensitive chip (PMD Smart Pixel) via a lens. The chip then compares the incoming light waves and draws conclusions about the distance of the target.
This proprietary technology provides:
- Robust detection of small reflective targets
- Quick installation due to color and angle independence
- Measured distance information via IO-Link
ifm’s ODG, O1D, O5D and OID laser distance sensors all use this technology.