Pepperl+Fuchs Proximity Sensors
Selecting the appropriate sensor depends on the application and the material of the object to be detected. If the object is metal, we recommend an inductive sensor. If the object is plastic, paper or fluid (oil or water-based), granular or powder, we recommend a capacitive sensor.
Inductive Proximity Sensors
- 6 Key Attributes for Inductive Sensor Selection
- Defining an Inductive Sensor's Mounting Conditions
- Metal Detection Using Inductive Sensors
- What Makes an Inductive Sensor Weld Immune
Capacitive Proximity Sensors
- The Basics: Capacitive Sensors and Dielectric Constants
- Selecting the Best Inductive or Capacitive Sensor for Your Application
- Mounting Brackets for Cylindrical Inductive or Capacitive Sensor
Detecting the presence or absence of a metal may seem like a fairly simple concept, but narrowing your selection to a specific sensor can sometimes be a difficult project. These 6 key attributes can help get you moving in the right direction when it comes to selecting an inductive sensor.
1. Housing design
Choosing a housing design is a great place to get the selection process started, as it simply entails looking at the application and deciding what kind of room you have for a sensor. There are many options to choose from, including rectangular/cube sensors, cylindrical sensors, and ring/slot proximity sensors, to name a few. Making sure you select the correct housing style may seem like a simple task, but it is an important decision to ensure your application is handled correctly.
2. Operating distance
Arguably the most important parameter for an inductive proximity sensor is the sensing range. Making sure that you have the correct sensing range can be the difference between sensing a target and missing it altogether. There are a few variations of sensing ranges, but the most straightforward is the nominal sensing range. The nominal sensing range is a standard value for defining the operating distance of a sensor.
3. Mounting conditions
Determining whether your inductive sensor needs to be flush or nonflush mounted is also an important decision. Flush-mounted sensors are great because they can be installed without leaving a space between the mounting surface and sensing face. Another plus for flush-mounted sensors is the fact that they are better protected and less prone to damage than a nonflush mount. But a nonflush mount has its own advantages. One advantage is that they have the greatest sensing range relative to the sensor diameter. For this higher range to be functional, a metal-free space needs to remain around the sensing face so that no incorrect objects are detected.
4. Electric data
Another important criteria is the type and level of supply voltage. An overview of the output circuits for our sensors can be found on our website, in the Sensors and Systems catalog. Our product range includes AC, DC, and AC/DC in terms of supply voltage.
5. Connection types
Inductive proximity sensors are available with quick disconnect connection or with a factory-attached cable (pigtail). The quick disconnects enable easy connection, maintenance, and replacement. Cables for the quick disconnect sensors are available in PUR, PVC, POC, and STOOW for your application needs. The factory-attached cables have advantages also. Because the cable is embedded in the sensor, water, oil, or dust cannot penetrate the sensor, making it less susceptible to failure. These cables come in 2 m lengths and are offered with PVC or PUR jackets.
6. General specification and special features
The general specification section of the selection process is an application-specific portion that really depends on any factor not mentioned in one of the first four steps. It can involve the operating or residual current, what degree of protection the sensor needs to be (i.e., IP69K), or if you need a special sensor feature for a special application. Some examples of special-application sensors would be reduction factor 1, weld-immune, and Pile Driver sensors. This section is the last, yet still important, piece to the puzzle of selecting an inductive sensor.
Selecting the most appropriate sensor depends on the application and the material to be detected, and if that material is metal, an inductive sensor is most likely the right choice. These 6 steps should help you select the best inductive sensor for your specific application!
Part One of Two: Defining the ways sensors can be mounted (embeddable, non-embeddable, quasi-embeddable)
Inductive sensors are often specified based on their sensing distance and housing geometry. But you must also be aware of any mounting restrictions that might apply to the specific sensor chosen for the application. Various types of sensors require different considerations when they are mounted to a bracket or machine component. Mounting conditions may restrict the close proximity of any non-target metal in the area around the sensor mounting zone. The effect of the surrounding metal, located within the electromagnetic field generated by the sensor, may result in unintended consequences for the operation of the equipment. Depending upon the sensor, this metal may cause pre-damping of the sensor oscillator resulting in:
- False triggering
- Temperature instability
- Sensor latch-on effects
Several mounting options are available:
Flush mount = embeddable mounting = shielded sensor
These sensors incorporate an electromagnetic shield positioned around the inductive coil that minimizes the effect of surrounding metal. They normally have the widest range of mounting options and the best immunity to mutual interference of nearby sensors.
Non-flush mount = non-embeddable mounting = unshielded sensor
Sensors in this group have no electromagnetic shielding present around the inductive coil system and, therefore, are strongly affected by surrounding metal. These sensors normally have the longest sensing ranges, but require the largest amount of metal-free area around the mounting zone.
Semi-flush mount = quasi-embeddable
These sensors have partial electromagnetic shielding present around the inductive coil. This partial shielding helps to minimize the effect of surrounding metal. Semi-flush mount sensors provide longer range than flush-mount sensors for the same housing size, but have some mounting restrictions that limit the metal allowed in the mounting area.
Notes about this image:
- The red area shows the metal-free mounting zone required by different sensor types
- Unshielded sensors have a wider radiated magnetic field
- Flux lines have a higher field density nearer the coil system
- Mounting metal closer to the coil will have a more significant effect on pre-damping
Part two of two: How to mount the different sensor types (embeddable, non-embeddable, quasi-embeddable)
In our previous blog post on this topic, we took a look at the different sensor types and their definitions. Now we'll deal with the preferred methods for mounting each type.
Because their construction limits the electromagnetic flux path, shielded sensors allow the user to mount the sensor flush with the mounting surface. For cylindrical sensors, this means having the mounting surface even with the sensing face; for cubical sensors the mounting surface can extend up to the face of the sensor.
Note: For cubical sensors the mounting, metal-free zone includes the area on all sides of the sensor.
Embeddable sensor mounting conditions
Mounting metal must not extend beyond the sensing face or pre-damping of the sensor will occur.
- Cylindrical sensor face must not be recessed into metal
- Cubical sensor must not have metal beyond sensing face of on any of the four sides of the device
Because of their construction, unshielded sensors allow the electromagnetic flux path to freely radiate from the coil system area. This requires that strict mounting conditions be observed with regard to metal in the mounting area. For these sensors it means having the mounting surface below or out of the range of the radiated field pattern.
Note: Non-flush mount sensors are very sensitive to the metal near the sensor mounting zone because as stated earlier, the flux density is highest near the coil system.
Non-embeddable sensor mounting conditions
- Mounting metal or mounting fasteners must not be in the metal-free zone
- For cylindrical, unshielded sensors the typical metal-free zone extends to an area of two times the sensing range of the device
Note: These sensors provide a longer sensing range with the added benefit of a full sensor housing length providing side impact protection near the sensor face.
Quasi-embeddable or semi-flush mountable
Quasi-embeddable sensor mounting conditions:
Standard embeddable/flush-mount sensors can be mounted so that the sensing face is flush to the mounting surface or fastener nut. Semi-flush mounting requires that the sensor face be extended by a small amount beyond the metal mounting surface or fastener nut as shown below.
Mounting metal or mounting fasteners must not be in the metal-free zone, otherwise pre-damping or sensor activation can occur.
In conclusion, selection of the appropriate sensor for the application should not only include evaluation of the sensor housing, sensing range, and electrical output type, but consideration should also be given to the mounting environment for the sensor. Always check the appropriate catalog or data sheet when concerned about having mounting-related brackets or other metal in the mounting zone.
Inductive sensors are a type of proximity sensor that use an electromagnetic field to detect metal. The concept involves an oscillating electromagnetic field generated by the sensor that drive eddy currents through a metal target. This is commonly known as damping of the oscillation. When damping occurs the sensor is able to detect the change in the electromagnetic field.
There are several factors to keep in mind when using inductive proximity sensors, including the size, distance, and material of the object. Some very small objects do not provide enough electromagnetic losses to be detected, while large objects are easily detected. The nominal sensing range or commonly referred to as the Sn value, is a standard value for defining the operating distance. The Sn value does not take into account production tolerances or changes through external influences such as voltage and temperature. Conductivity, permeability and other electromagnetic properties determine how well the metal can create losses in the electromagnetic field. These properties affect how well the sensor can detect the metal. Typically ferrous metals have more electromagnetic losses than nonferrous metals which is a key difference.
Ferrous (FE)/Nonferrous (NE) Standard Inductive Proximity Sensors
Metals can be sorted into two categories: ferrous and nonferrous, otherwise known as magnetic and nonmagnetic. The most ferrous metals are iron, cobalt, nickel, and manganese. These metals have stronger electromagnetic properties than most, so the electromagnetic fields created by the eddy currents are typically stronger than those in other metals.
For nonferrous metals such as aluminum, copper, and brass a reduction factor should be implemented for standard inductive proximity sensors. A reduction factor is a number usually ranging from 0-1, that describes how well metals can be detected by the sensor. As seen in the chart below, a standard inductive proximity sensor has a reduction factor of 0.3 when sensing copper. This means that when detecting copper the sensing distance is reduced to 0.3 of the effective sensing range which is a 70% decrease.
FE-Only and NE-Only Sensors
If a sensor needs to detect only ferrous metals, a ferrous only sensor can be used. Since ferrous metals have increased electromagnetic properties compared to other metals they are easily able to be identified by the sensor.
If a sensor is needed to detect only nonferrous metals, a nonferrous sensor can be used. The electromagnetic properties of these metals have less resistance and they do not create electromagnetic losses as well. The sensor can look for these specific characteristics in the power loss and only give an output if a nonferrous metal is within sensing range.
Reduction Factor 1 Sensors—Nonferrous or Ferrous Metals
Pepperl+Fuchs Reduction factor 1 series sensors is able to detect ferrous and nonferrous metals at the same distance without a reduction factor. The sensors are able to do this by implementing a two coil, air core system to detect eddy currents and magnetic losses to determine the trip point. The conductivity, permeability, and other electromagnetic properties of different metals do not affect sensing distance but the size of the target is still a factor. Since the sensor doesn’t have limitations on metal type, it will give an output at its Sn value.
For detection of unique metal like gold, titanium, alloys, and others, the reduction factor 1 sensors should be able to do the job with consistent results for all metals. If a standard inductive sensor is chosen there might be a reduction factor involved depending on the metal type. If a ferrous only or nonferrous only sensor is needed, determining the properties of the unique metal is imperative.
Weld-immune inductive sensors are designed for use in weld-sensing applications. Inductive sensors installed in harsh welding environments are exposed to strong magnetic fields as well as sparks and weld slag. So what exactly is a weld-immune inductive sensor?
There are three main features that make a sensor weld-immune:
- Magnetic-field-immune electronics
- Weld slag-resistant coatings
- High-temperature sensing face materials
Let’s take a look at these three features individually:
The strong electromagnetic fields common to the welding process can cause an inductive sensor to provide false triggers or lock on the output. The use of magnetic-field-immune electronics allows the sensor to function without false signals in these environments, ensuring accurate detection of metal targets.
During the welding process, molten fragments are expelled and can resolidify on surrounding metal surfaces. This is called weld slag. Weld-immune sensors feature a high-temperature coating to prevent weld slag from accumulating on their housings. Weld slag build-up on the face of the sensor can cause the sensor to lock on indefinitely. Slag can also be a problem for the body of the sensor if it accumulates on the threads, making removal of the sensor extremely difficult. The image below shows the effect of weld slag on an uncoated sensor:
High-Temperature Sensing Face Materials
High-temperature-resistant plastics are used to protect the sensor face from the extreme temperatures common to welding applications. Some models feature a ceramic-coated sensor face for better protection against weld slag. It is very important to protect the sensing face from melting or eroding in these harsh environments.
Weld-immune sensing options are not limited to standard cylindrical inductive sensors. Pepperl+Fuchs also offers weld-immune products in the following inductive families:
Weld-immune surface mount sensors are available in various housing options including flat pack, limit switch, Rhino (cube), and some low-profile designs. The surface mount sensors have a longer sensing range than the cylindrical styles.
Pepperl+Fuchs’ metal-face inductive sensors are also available in weld-immune options. This is our most durable inductive sensor design and used in the most demanding applications. The weld-immune version is an all stainless steel housing coated with BlackArmor™. BlackArmor is an extremely tough, slag-resistant coating that ensures survivability in very aggressive welding applications.
Reduction Factor 1
Reduction Factor 1 weld-immune sensors detect both ferrous and nonferrous targets without any reduction in the sensing range. These are available in cylindrical, Rhino (cube), and flat pack housing styles.
Gripdicator sensors are used in power clamps to confirm gripper extension and retraction. These are designed to be universally compatible with products from a variety of manufacturers. The Gripdicator sensors are weld-field-immune and offer extremely accurate gripper position indication.
Inductive Cylindrical Position Sensors
Weld-field-immune inductive cylindrical position sensors provide reliable end-of-stroke sensing in hydraulic or pneumatic cylinders. These sensors are rated for pressures up to 3,000 PSI and can be ordered in a variety of probe lengths ranging from 0.835 to 4.562 inches. They are available in two different probe diameters and with multiple connectivity options.
Designed to perform consistently in the demanding environments common to welding applications, our sensors provide a durable, resilient, and reliable option for metal detection within the welding process.
Capacitive Proximity Sensors
Capacitive sensor basics
Capacitive proximity sensors are a noncontact option capable of detecting both metal and nonmetal targets. These sensors react to changes in capacitance caused by the presence of these targets. Capacitive sensors are available in cylindrical as well as surface mount housings and provide a sensing range from 1 mm to 50 mm. Many of these models feature a sensitivity adjustment potentiometer that allows the user to fine-tune the sensor to a specific application.
Capacitive sensors are commonly used for sensing targets such as plastics, liquids, powders, and granular materials. If chosen correctly, a capacitive sensor is able to sense through nonmetallic objects to detect a target behind them. This ability is very useful in level measurement applications when sensing a liquid or granular material through a container wall.
The sensitivity of the capacitive sensor is affected by the dielectric constant of the target material being detected. The higher the dielectric constant of the material, the more sensitive a capacitive sensor is to that target. Displayed in the chart below is the dielectric constant of some common materials:
Putting it together
As seen in the chart, liquid targets generally have a higher dielectric constant than dry granule targets. This makes liquid level measurement a very common application for a capacitive sensor because of the sensor’s extreme sensitivity to liquid targets.
On the right side of this chart are some common materials used for the containers that would allow the sensor to “see-through” to the material located within. Glass and plastics would be the best option for this purpose as they have a very low dielectric constant value which translates to a low sensitivity for the capacitive sensor.
Materials with a high dielectric constant are detectable at a longer distance than a material with a lower value. This also plays an important role in applications where the target is being detected through a container wall. The material being sensed behind the wall needs to have a dielectric constant that is greater than the material of the container. For ensured operation, the dielectric constant of the target material should be about double the dielectric constant of the container wall.
Application: Sensing water level through a sight glass
In this example, the desired target was much higher than the “see-through” material so the thickness of the glass would not be so critical. If the two values are more similar, then one would need to consider the thickness of the outside wall.
The dielectric constant of water is 80.4 and the glass is 3.7. Water has a dielectric constant of more than 20 times that of the glass so there would be no problem detecting the water while ignoring the glass.
Application: Sensing sand through a PVC container using a capacitive sensor with a fixed 5 mm range
The dielectric constant of sand is 4.8, while PVC is 3.4. Since these two values are very close, the thickness of the PVC becomes extremely important. The best way to determine the maximum thickness of the container wall would be through testing within the application. For this example, it was determined that the thickness of the PVC could be 3 millimeters or less. These results can vary depending on the make-up of the sand being used as a target.
The dielectric constant does have a significant effect on the operation of capacitive sensors. It is very important to consider this value when designing an application using this sensor type. When properly selected, capacitive sensors are an ideal choice for the following production applications:
- Level control (liquid or bulk solids)
- Plastic parts detection
- Wood detection
- Inspecting packaging procedures
Hundreds of different proximity sensors are available for use in automation applications. There are several points to consider when you look for a sensor, as your choices could affect your application later down the road. Let's take a look at why you would use an inductive or a capacitive sensor.
An inductive sensor is used to detect the presence of metals (such as steel, stainless steel, aluminum, or copper). An important factor to note is that our sensors have a different nominal sensing range depending on the material. The standard target is a mild steel plate (FE360) that is 1 mm thick with square dimensions the size of the sensor.
Our model number nomenclature such as the NBB4-12GM50-E2 has a rated operating distance of 4 mm (for an FE360 target), but will pick up aluminum only at about 1.6 mm because of the 0.4 reduction factor for aluminum.
If a longer sensing range is needed, we also have reduction factor 1 sensors that do not have a reduction factor for steel (FE360), stainless steel (304), copper, and aluminum. These have a model number that begins with NRB or NRN.
The common cylindrical inductives that we offer are 8 mm, 12 mm, 18 mm, and 30 mm. We do offer some smaller diameters as well such as 4 mm, 5 mm, and 6.5 mm models. Typically, a smaller diameter sensor has a higher switching frequency (number of times the sensors can change output states). This is an important feature to consider if the sensor will be used for a speed monitoring application involving a fast, rotating target. You can see an example in the image below.
The environment in which the sensor is located in plays a role as well. Most of our inductive proximity sensors have an IP67 degree of protection. An IP67 rating means that the sensor can resist the ingress of dust and water such that the sensor stays sealed even if submerged in stagnant, low-depth water for short periods of time. If the sensor has to be used underwater continuously, it requires an IP68 degree of protection. For high-pressure washdown, an IP69K degree of protection is needed. These ratings are listed on the specification sheet.
For use around high voltage sources or welding equipment, use models with a –C- or –C3, as these are weld field immune and have a protective coating that resists weld slag.
As an additional note for inductive and capacitive sensors, our models ending with a –V1 or -V3 or any -V number indicate a quick disconnect version that requires an additional cable to use. This is a preference, but many find it convenient when compared to a potted-in cable that can get damaged and take time to repair.
Capacitive sensors are capable of detecting plastic, wood, and other raw materials including metal. An inductive sensor can detect only metal. A common application is the detection of liquids, plastics, and grains. Capacitive liquid detection is used for level and presence detection.
Often times, this type of application involves sensing through a tube or tank to detect the liquid. Capacitive sensors have either a fixed sensitivity or an adjustable sensitivity. Depending on the material of the tube or sidewall, a capacitive can be selected to mount on the exterior of the tank/ tube and detect the fluid through the wall.
Factors would be the dielectric constant of the fluid and the material of the side wall. Tables with the dielectric constants of most materials can be found on the internet, or a brief list can be found in our catalog.
One typical application involves using a thin tube with a small capacitive wire tied to it to give an output when the fluid reaches the wire. This method of sensing can be useful in hydraulic cylinder applications.
Another popular solution utilizes our plastic sensor well that is drilled and fitted to the side of a tank. A cylindrical capacitive sensor can then be placed in this sensor well and used for level detection. This approach makes adjusting the sensitivity and removal of the sensor easy. Most cylindrical models have a potentiometer on the back for adjusting the sensitivity. Aside from the sensitivity adjustment and the ability to detect nonmetal targets, a capacitive sensor is similar to an inductive in most other aspects when considering housing size and range.
If you're using cylindrical inductive or capacitive sensors it's highly likely you're going to need mounting brackets for these sensors. A lot of times there may be more than one mounting bracket solution that could work for a particular setup. Selecting the correct bracket for your application is an important step, but knowing which one to select can sometimes be difficult. This breakdown of the different types of mounting brackets will help you decide which one is right for your setup.
Right angle bracket – AB Series brackets
Right angle brackets seem to be the most popular choice for customers to use. These stainless steel mounting brackets come in three different sizes: 12 mm, 18 mm, and 30 mm. Right angle brackets are versatile and easy to use in many applications. They provide the cheapest way to mount your cylindrical sensor. You use the locking nuts supplied with the sensor to secure and adjust it through the bracket opening.
Adjustable bracket – BF Series brackets
We offer adjustable mounting brackets for all diameter cylindrical sensors from 4 mm to 40 mm. These brackets are made up of tough Crastin, and offer more of a variety in sensor diameter options than the three right angle sizes. Some applications cannot use stainless steel brackets or require something closer to the surface than other options can provide. In those cases, the Crastin adjustable bracket is a great choice.
The BF5-30 mounting bracket can hold sensors from 5 mm up to 30 mm in diameter, and delivers 360 degrees rotation in 2 axes. This style of bracket is good for making quick adjustments at any angle. This bracket is a great choice for unique mounting applications, or if you need to change the setup periodically. This bracket offers more mounting options than the two more popular choices above.
Snap lever bracket – BF-F Series
Snap lever mounting brackets allow for quick installation or replacement of any cylindrical sensor. This is the quickest installation method that we offer for cylindrical type sensors. Snap the sensors into the bracket and let the stop shoulder position the switch perfectly each time. Two elongated mounting holes are used for modest range adjustments. Available in standard 8 mm, 12 mm, 18 mm, and 30 mm sizes.
Cushioned bracket – CM Series
Typically it’s best to use a cushioned mounting bracket when the sensor will be exposed to overtravel or contact with the target. A fiberglass reinforced nylon material protects the sensor’s face from contact with the target. It’s possible to have up to 10 mm of travel protection when using this setup. If there is a possibility of contact with the object and sensor in the application, this is the bracket you should be looking at using. Available in standard 8 mm, 12 mm, 18 mm, and 30mm sizes.
Exchanger bracket – EXG Series
Use this series when removing the sensor is a common practice. The locknut secures the sensor in place, eliminating the need for modification when the replacement sensor is installed. Available in standard 8 mm, 12 mm, 18 mm, and 30 mm sizes.
Mounting bracket options
Each type of bracket has its own different set of characteristics, but hopefully this will give you a better idea of some of the bracket options you can select.