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Photoelectric Methods of Operation

Photoelectric sensors vary in their operating principles and can be used in a variety of ways, depending on the application. They can be used to detect whether an object is present, determine its position, measure level, and more. With so many types, it can be hard to narrow down the right sensor for your application while accounting for any environmental conditions. Below will give a brief overview of the different operating principles used in photoelectric sensors and where they can be best used.

Diffuse

Diffuse sensors are the most basic type of photoelectric sensor as they only require the sensor and the object being detected. The sensor has a built-in emitter and receiver, so as light is sent out from the emitter and reaches an object, the light will then bounce off the object and enter the receiver. This sends a discrete signal that an object is within the sensing range. Due to the reflectivity being target-dependent, diffuse sensors have the shortest range of the three main discrete operating principles. Background suppression sensors work under the same principle but can be taught to ignore objects in the background using triangulation to ensure any light beyond a certain angle does not trigger an output. While diffuse sensors can be affected by the color of the target object, the use of a background suppression sensor can limit the effect color has on reliability. Foreground suppression sensors work in the same manner as background suppression but will ignore anything in the foreground of the taught distance.

diffuse

Retro-reflective

Retro-reflective sensors also have the emitter and receiver in a single housing but require a reflector or reflective tape be mounted opposite the sensor for it to be triggered by the received light. As an object passes in front of the reflector, the sensor no longer receives the light back, thus triggering an output. Due to the nature of the reflector, these sensors can operate over much larger distances than a diffuse sensor. These sensors come with non-polarized or polarizing filters. The polarizing filter allows for the sensor to detect shiny objects and not see it as a reflector and prevents any stray ambient light from triggering the sensor.

retroreflective

Through-beam

Through-beam sensors have a separate body for the emitter and receiver and are placed opposite each other. The output is triggered once the beam has been broken. Due to the separate emitter and receiver, the sensor can operate at the longest range of the aforementioned types. At these long ranges and depending on the light type used, the emitter and receiver can be troublesome to set up compared to the diffuse and retro-reflective.

throughbeam

Distance

The previous three types of photoelectric sensors give discrete outputs stating whether an object is present or not. With photoelectric distance sensors, you can get a continuous readout on the position of the object being measured. There are two main ways the distance of the object is measured, time of flight, which calculates how long it takes the light to return to the receiver, and triangulation, which uses the angle of the incoming reflected light to determine distance. Triangulation is the more accurate option, but time of flight can be more cost-effective while still providing good accuracy.

Light type and environment

With each operating principle, there are three light types used in photoelectric sensors: red light, laser red light, and infrared. Depending on the environmental conditions and application, certain light types will fare better than others. Red light is the standard light type and can be used in most applications. Laser red light is used for more precise detection as it has a smaller light spot. Infrared is used in lower-visibility environments as it can pass through more dirt and dust than the other two types. Although infrared can work better in these dirtier environments, photoelectric sensors should mainly be used where build-up is less likely. Mounting should also be considered as these sensors are usually not as heavy duty as some proximity switches and break/fail more easily.

As you can see, photoelectric sensors have many different methods of operation and flexibility with light type to help in a wide range of applications. When considering using these sensors, it is important to account for the environmental conditions surrounding the sensor, as well as mounting restrictions/positioning, when choosing which is right for your application.

For more information on photoelectric sensors, visit this page for more information.

Smart IO-Link Sensors for Smart Factories

Digitizing the production world in the age of Industry 4.0 increases the need for information between the various levels of the automation pyramid from the sensor/actuator level up to the enterprise management level. Sensors are the eyes and ears of automation technology, without which there would be no data for such a cross-level flow of information. They are at the scene of the action in the system and provide valuable information as the basis for implementing modern production processes. This in turn allows smart maintenance or repair concepts to be realized, preventing production scrap and increasing system uptime.

This digitizing begins with the sensor itself. Digitizing requires intelligent sensors to enrich equipment models with real data and to gain clarity over equipment and production status. For this, the “eyes and ears” of automation provide additional information beyond their primary function. In addition to data for service life, load level and damage detection environmental information such as temperature, contamination or quality of the alignment with the target object is required.

One Sensor – Multiple Functions

This photoelectric sensor offers these benefits. Along with the switching signal, it also uses IO-Link to provide valuable information about the sensor status or the current ambient conditions. This versatile sensor uses red light and lets you choose from among four sensor modes: background suppression, energetic diffuse, retroreflective or through-beam sensor. These four sensing principles are the most common in use all over the world in photoelectric sensors and have proven themselves in countless industrial applications. In production this gives you additional flexibility, since the sensor principles can be changed at any time, even on-the-fly. Very different objects can always be reliably detected in changing operating conditions. Inventory is also simplified. Instead of four different devices, only one needs to be stocked. Sensor replacement is easy and uncomplicated, since the parameter sets can be updated and loaded via IO-Link at any time. Intelligent sensors are ideal for use with IO-Link and uses data retention to eliminate cumbersome manual setting. All the sensor functions can be configured over IO-Link, so that a remote teach-in can be initiated by the controller.

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Diagnostics – Smart and Effective

New diagnostics functions also represent a key feature of an intelligent sensor. The additional sensor data generated here lets you realize intelligent maintenance concepts to significantly improve system uptime. An operating hours counter is often built in as an important aid for predictive maintenance.

The light emission values are extremely helpful in many applications, for example, when the ambient conditions result in increased sensor contamination. These values are made available over IO-Link as raw data to be used for trend analyses. A good example of this is the production of automobile tires. If the transport line of freshly vulcanized tires suddenly stops due to a dirty sensor, the tires will bump into each other, resulting in expensive scrap as the still-soft tires are deformed. This also results in a production downtime until the transport line has been cleared, and in the worst case the promised delivery quantities will not be met. Smart sensors, which provide corresponding diagnostic possibilities, quickly pay for themselves in such cases. The light remission values let the plant operator know the degree of sensor contamination so he can initiate a cleaning measure before it comes to a costly production stop.

In the same way, the light remission value allows you to continuously monitor the quality of the sensor signal. Sooner or later equipment will be subject to vibration or other external influences which result in gradual mechanical misalignment. Over time, the signal quality is degraded as a result and with it the reliability and precision of the object detection. Until now there was no way to recognize this creeping degradation or to evaluate it. Sensors with a preset threshold do let you know when the received amount of light is insufficient, but they are not able to derive a trend from the raw data and perform a quantitative and qualitative evaluation of the detection certainty.

When it comes to operating security, intelligent sensors offer even more. Photoelectric sensors have the possibility to directly monitor the output of the emitter LED. This allows critical operating conditions caused by aging of the LED to be recognized and responded to early. In a similar way, the sensors interior temperature and the supply voltage are monitored as well. Both parameters give you solid information about the load condition of the sensor and with it the failure risk.

Flexible and Clever

Increasing automation is resulting in more and more sensors and devices in plant systems. Along with this, the quantity of transported data that has to be managed by fieldbus nodes and controllers is rising as well. Here intelligent sensors offer great potential for relieving the host controller while at the same time reducing data traffic on the fieldbus. Pre-processing the detection signals right in the sensor represents a noticeable improvement. A freely configurable count function offers several counting and reset options for a wide variety of applications. The count pulses are evaluated directly in the sensor – without having to pass the pulses themselves on to the controller. Instead, the sensor provides status signals, e.g. when one of the previously configured limit values has been reached. This all happens directly in the sensor, and ensures fast-running processes regardless of the IO-Link data transmission speed.

Industry 4.0 Benefits

In the age of Industry 4.0 and IoT, the significance of intelligent sensors is increasing. There is a high demand from end users for these sensors since these functions enable them to use their equipment and machines with far greater flexibility than ever before. At the same time they are also the ones who have the greatest advantage when it comes to preventing downtimes and production scrap. Intelligent sensors make it possible to implement intelligent production systems, and the data which they provide enables intelligent control of these systems. In interaction with all intelligent components this enables more efficient utilization of all the machines in a plant and ensures better use of the existing resources. With the increasing spread of Industry 4.0 and IoT solutions, the demand for intelligent sensors as data providers will also continue to grow. In the future, intelligent sensors will be a permanent and necessary component of modern and self-regulating systems, and will therefore have a firm place in every sensor portfolio.

To learn more about these smart sensors, visit www.balluff.com.

The Perfect Photoelectric Sensor – Imagine No More

In my last blog, Imagine the Perfect Photoelectric Sensor, I discussed the possibilities of a single part number that could be configured for any of the basic sensing modes: through-beam, retroreflective, background suppression and diffuse. This perfect sensor would also have the ability to change the sensing mode on the fly and download the required parameters for a changing process or format change. Additionally, it would have the ability to teach the sensing switch points on the fly, change the hysteresis, and have variable counter and time delays.

Tomorrow is here today! There is no need to imagine any longer, technology has taken another giant leap forward in the photoelectric world. Imagine the possibilities!

Below are just some of the features of this leading edge technology sensor. OEM’s now have the opportunity to have one sensor solve multiple applications. End users can now reduce their spare inventory.

To learn more visit www.balluff.com.

Imagine the Perfect Photoelectric Sensor

Photoelectric sensors have been around for a long time and have made huge advancements in technology since the 1970’s. We have gone from incandescent bulbs to modulated LED’s in red light, infrared and laser outputs. Today we have multiple sensing modes like through-beam, diffuse, background suppression, retroreflective, luminescence, distance measuring and the list goes on and on. The outputs of the sensors have made leaps from relays to PNP, NPN, PNP/NPN, analog, push/pull, triac, to having timers and counters and now they can communicate on networks.

The ability of the sensor to communicate on a network such as IO-Link is now enabling sensors to be smarter and provide more and more information. The information provided can tell us the health of the sensor, for example, whether it needs re-alignment to provide us better diagnostics information to make troubleshooting faster thus reducing downtimes. In addition, we can now distribute I/O over longer distances and configure just the right amount of IO in the required space on the machine reducing installation time.

IO-Link networks enable quick error free replacement of sensors that have failed or have been damaged. If a sensor fails, the network has the ability to download the operating parameters to the sensor without the need of a programming device.

With all of these advancements in sensor technology why do we still have different sensors for each sensing mode? Why can’t we have one sensor with one part number that would be completely configurable?

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Just think of the possibilities of a single part number that could be configured for any of the basic sensing modes of through-beam, retroreflective, background suppression and diffuse. To be able to go from 30 or more part numbers to one part would save OEM’s end users a tremendous amount of money in spares. To be able to change the sensing mode on the fly and download the required parameters for a changing process or format change. Even the ability to teach the sensing switch points on the fly, change the hysteresis, have variable counter and time delays. Just imagine the ability to get more advanced diagnostics like stress level (I would like that myself), lifetime, operating hours, LED power and so much more.

Obviously we could not have one sensor part number with all of the different light sources but to have a sensor with a light source that could be completely configurable would be phenomenal. Just think of the applications. Just think outside the box. Just imagine the possibilities. Let us know what your thoughts are.

To learn more about photoelectric sensors, visit www.balluff.com.

Photoelectric Basics – Distance Measuring

Some photoelectric applications require not only knowing if the object is present or not but exactly where the object is while providing a continuous or dynamic value representative of the objects location. For instance, if a robot is stacking a product is the stack at the correct height or how many additional pieces can be placed on the stack, how large is the coil or roll diameter of a product, and how high is the level or how much further can the product move before it is in position. Distance sensors can provide this dynamic information and in some case provide a digital output as well for alarms.

These sensors are normally based on diffuse sensing technology. However, in some cases retro-reflective technology is used for extremely long sensing distances. As with diffuse sensors there is only one device to mount and wire. However, due to the technology required for the higher resolutions, lensing, electronics and outputs these devices are typically much more expensive than a discrete diffuse sensor.

Similar to a diffuse sensor the distance sensor emits a pulsed light that strikes an object and a certain amount of light is reflected back to the sensor’s receiver. The sensor then generates an analog output signal that is proportional to the distance to the target. The technology that is utilized within the sensor to determine the distance is either Time of Flight or Triangulation.

Time of Flight sensors are more immune to target color and texture than light intensity based system because of the time component. These devices measure greater distances than the triangulation method however there is a sacrifice in resolution.

Triangulation sensors emit a pulsed light towards the target object. The light is then reflected back to the receiver. When the light reaches the sensor it will strike the photosensing diode at some angle. The distance between the sensor and the target determines the angle in which the light strikes the receiver. The closer the target is the sensor the greater the angle.
Triangulation based sensors being dependent on the amount of reflected light are more susceptible to target characteristics such as color and texture. These sensors are characterized by short to mid-range sensing distance however they provide higher resolutions than TOF sensors.

Output signals are either 0…10 volts, 1…10 volts or 4…20mA each of which has their pros and cons. Voltage outputs, 0 – 10 or 1- 10 volts, are easier to test and there is typically a broader offering of interface devices. However voltage outputs are more susceptible to noise from motors, solenoids or other coils and voltage drops of the wire. In addition generally voltage output cable runs should be less than 50 feet. Also since 0 volts is an acceptable output value broken wires, device failures, or power failures can go undetected.

Current outputs, 4 – 20 mA, provide the best noise immunity, are not affected by voltage drop and the cables lengths can exceed 50 feet. Since the sensor will be providing 4mA at zero distance its lowest possible signal, if the sensor should fail, the cable damaged or a power failure the interface device can detect the absence of the signal and notify an operator. Current outputs are more difficult to test and in some cases are affected by temperature variations.

For more information about photoelectric sensors, request your copy of Balluff’s Photoelectric Handbook.

Photoelectric Output Operate Modes and Output Types

Photoelectric sensors are used in a wide variety of applications that you encounter every day. They are offered in numerous housing styles that provide long distance non-contact detection of many different types of objects or targets. Being used in such a variety of applications, there are several outputs offered to make integration to control systems easy and depending on the sensing mode when the output is activated in the presence of the target.

Diffuse Diffuse sensors depend on the amount of light reflected back to the receiver to actuate the output. Therefore, Light-on (normally open) operate refers to the switching of the output when the amount of light striking the receiver is sufficient, object is present. Likewise, Dark-on (normally closed) operate would refer to the target being absent or no light being reflected back to the receiver.

Retroreflective and through-beam sensors are similar in the fact they depend on the target interrupting the light beam being reflected back to the receiver. When an object interrupts the light beam, preventing the light from reaching the receiver, the output will energize which is referred to as Dark-on (normally open) operate switching mode or normally open. Light-on (normally closed) operate switching mode or normally closed output in a reflex sensor is true when the object is not blocking the light beam.

Outputs from photoelectric sensors are typically either digital or analog. Digital outputs are on or off and are usually three wire PNP (sourcing output) or NPN (sinking outputs). The exception to this is a relay output that provides a dry or isolated contact requiring voltage being applied to one pole.

Analog outputs provide a dynamic or continuous output that varies either a voltage (0-10 volt) or current (4-20mA) throughout the sensing range. Voltage outputs are easier to integrate into control systems and typically have more interface options. The downside to a voltage output is it should not be ran more than 50 feet. Current outputs can be ran very long lengths without worry of electrical noise. As additional advantage of the analog output is that it has built in diagnostics, at its minimum there will always be some current at the input unless the device completely fails or the wire is damaged.

Some specialty photoelectric sensors will provide a serial or network communication output for communications to higher level devices. Depending on the network, IO Link, for instance, additional diagnostics can be provided or even parameterization of the sensors.
Interested in learning more about photoelectrics basics? You can also request a copy of the new Photoelectric Handbook.

Photoelectric Basics – Light On or Dark On

Recently I was asked if light on and dark on for a photoelectric sensor was the same as normally open and normally closed. The short answer is yes, but I think it justifies more of an explanation. In the world of proximity sensors, capacitive sensors, and mechanical switches when the target is present the output changes state and turns on or turns off; there is no ambiguity.

With photoelectric sensors, instead of normally open or normally closed we refer to light-on operate or dark-on operate because we are referring to the presence or absence of light at the sensor’s receiver. The output of a light-on operate sensor is on (enabled, high, true) when there is sufficient light on the receiver of the sensor. Conversely, the output of a dark-on operate sensor is on when the light source is blocked or no light can reach the receiver.

There are three modes of operation with photoelectrics: diffuse, retro-reflective, and through-beam; and the sensing mode determines if the sensor is normally light-on or dark-on. Retro-reflective and through-beam sensors function as light-on operate sensors because under normal operating conditions there is a reflector or a light emitter providing a light beam back to the sensor receiver. If no object is blocking the light beam to receiver the output is on, normally closed. If the target or object is in between the reflector or emitter then the light beam can no longer reach the receiver causing the output to turn off.

Since the diffuse mode of operation requires the target or object to reflect the light source back to the receiver, it functions as a dark-on operate, normally open. If no object or target is placed in front of the sensor, no light will be reflected back to the receiver. When the object is present, the output changes state from normally open to closed.

The chart below should help to summarize the above:

Shedding the Light on Diffuse Mode Photoelectric Sensors

Photoelectric sensors have solved numerous industrial applications over the years. There are basically three different sensing modes. The first is diffuse or reflex mode, next is retro reflective, which requires a reflector, and the third is through beam, transmitted or opposed. These field devices provide an excellent means of detecting target at a distance without contacting the object. All of the sensing modes are based on the sensor’s ability to detect a change in light reaching the sensor’s receiver. In this posting, we will review the diffuse or reflex photoelectric sensor.

Continue reading “Shedding the Light on Diffuse Mode Photoelectric Sensors”

Tag: diffuse

Photoelectric Methods of Operation

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