Detecting Fill Levels With Direct Contact and Non-contact Capacitive Sensors

Capacitive sensors are commonly used in level detection applications. Specific capacitive sensors can supply better solutions than others depending on the type of media you may be detecting and if the sensor will be in direct contact with that media. Keep reading to decide which type works best for different application solutions.

Non-contact capacitive sensors

Capacitive sensors are great for monitoring the fill level of non-conductive materials. In many cases, the capacitive sensor doesn’t need to physically touch the media it is detecting; rather, it can sit outside a thin, non-metal container or pipe. As the level rises or falls, the capacitive sensor can signal if the medium is there. Since non-contact capacitive sensors sit outside the medium, there shouldn’t be any interference or false readings from direct contact with the material.

Selecting the correct capacitive sensor for these applications is important. While you don’t have to risk contaminating the sensor face (and getting a false read) in non-contact applications, you need to keep in mind other factors that can cause a sensor to false trip. One thing that is important to keep in mind with externally mounted capacitive sensors is that viscous materials can still leave a layer of residue on the inside walls of tanks or basins. While the sensor face is not covered, if you select the wrong type of sensor this build up on the wall can cause a false reading (such as reading as reading the tank as full when it is actually half-empty).

Another thing to keep in mind when selecting the correct capacitive sensor for a non-contact application is foam. In applications such as bottling beer in glass bottles, most standard capacitive sensors will detect presence once that layer of foam reaches the sensor face. While the foam may be at the sensor face, the bottle could still be only half way full of actual liquid. Making sure you select a sensor that can account for things like foam is something to keep in mind as well.

There are many benefits when using non-contact capacitive sensors in fill level applications. Not every application requires direct contact with the medium, and not every application even allows for the medium to be touched directly. There are many capacitive sensors in many form factors that are used every day for fill level applications, but making sure the right sensor is selected is important.

Contact with media capacitive sensors

In certain applications, the capacitive sensor will only be able to detect the fill level of a container, pipe, or tank if it is in direct contact with the media it’s trying to sense.

For various reasons, a sensor must be in direct contact with a media like oil, paint, powder, or paste. You may need to place a sensor directly in a tank because the tank is made of metal, or possibly because the walls of the tank are too thick for a capacitive sensor to sense through. Direct contact applications can be difficult to find solutions for if you are not aware of what capacitive sensors are capable of.

There is a way to fix issues such as false tripping in sticky substances.

Advanced technologies allow for capacitive sensors that mask residual build-up or foam when sensing in direct media contact. These level-sensing capacitive sensors are great for applications in the food and beverage industry and for detecting practically all the same materials as non-contact capacitive sensors. In areas of detection where adhesive substances may stick to the sensor face is a perfect application for direct contact capacitive sensors. Some typical direct-contact applications include areas such as vegetable oil or ketchup container fill levels, hydraulic oil levels in a hydraulic cylinder, or even the amount of flour in a container.

For instance, if you stick a capacitive sensor inside a tank of oil to monitor the fill level, the sensor face will get covered in the oil. As the level in the tank drops below the sensor face, that oil will remain on the face. So, even if the tank is empty, the sensor will always detect something. With specialized capacitive sensors that ignore build-up, adhesive or viscous media that typically influence detection is no longer a concern.

Another use for capacitive sensors that allow for direct media contact is for leak detection. If a tank, pipe, or tub is known to leak, there are capacitive sensors that can be mounted to the ground in the area that puddles form. In some instances you know a machine could potentially leak, and puddles form in an area you can’t regularly see, which is where these sensors are perfect for application. Depending on the situation, some of these sensors can be mounted a couple millimeters to an inch off the ground waiting for a leak. As a puddle forms and reaches the sensor’s switching range, maintenance can be alerted of the issue and work to fix it.

Reduce time and costs associated with manual level-checking

Another application for a capacitive sensor with direct media contact capabilities is within the automotive industry. Inside the painting process of an assembly plant, for example, you must be able to monitor the fill levels of the e-coat, the primer, the base coat, and the clear-coat paint tanks. Without a sensor to determine the fill levels, the time and energy and dollars it can cost the workforce to manually check the fill levels can be high.. Luckily, these contact-capacitive sensors can monitor viscous media like paint, reducing the time and costs associated with manual level-checking.

While non-contact and contact capacitive sensors perform the similar functions, they are used in different applications. Some applications allow a sensor to sit outside a container or tank and detect through the walls, while others require direct contact. Now that you understand the differences and their strong points of application, you can determine which sensor is best for you.

Capacitive, the Other Proximity Sensor

What is the first thing that comes to mind if someone says “proximity sensor?” My guess is the inductive sensor, and justly so because it is the most used sensor in automation today. There are other technologies that use the term proximity in describing the sensing mode, including diffuse or proximity photoelectric sensors that use the reflectivity of the object to change states and proximity mode of ultrasonic sensors that use high-frequency sound waves to detect objects. All these sensors detect objects that are in close proximity to the sensor without making physical contact. One of the most overlooked or forgotten proximity sensors on the market today is the capacitive sensor.

Capacitive sensors are suitable for solving numerous applications. These sensors can be used to detect objects, such as glass, wood, paper, plastic, or ceramic, regardless of material color, texture, or finish. The list goes on and on. Since capacitive sensors can detect virtually anything, they can detect levels of liquids including water, oil, glue, and so forth, and they can detect levels of solids like plastic granules, soap powder, sand, and just about anything else. Levels can be detected either directly, when the sensor touches the medium, or indirectly when it senses the medium through a non-metallic container wall.

Capacitive sensors overview

Like any other sensor, there are certain considerations to account for when applying capacitive, multipurpose sensors, including:

1 – Target

    • Capacitive sensors can detect virtually any material.
    • The target material’s dielectric constant determines the reduction factor of the sensor. Metal / Water > Wood > Plastic > Paper.
    • The target size must be equal to or larger than the sensor face.

2 – Sensing distance

    • The rated sensing distance, or what you see in a catalog, is based on a mild steel target that is the same size as the sensor face.
    • The effective sensing distance considers mounting, supply voltage, and temperature. It is adjusted by the integral potentiometer or other means.
    • Additional influences that affect the sensing distance are the sensor housing shape, sensor face size, and the mounting style of the sensor (flush, non-flush).

3 – Environment

    • Temperatures from 160 to 180°F require special considerations. The high-temperature version sensors should be used in applications above this value.
    • Wet or very humid applications can cause false positives if the dielectric strength of the target is low.
    • In most instances, dust or material buildup can be tuned out if the target dielectric is higher than the dust contamination.

4 – Mounting

    • Installing capacitive sensors is very similar to installing inductive sensors. Flush sensors can be installed flush to the surrounding material. The distance between the sensors is two times the diameter of the sensing distance.
    • Non-flush sensors must have a free area around the sensor at least one diameter of the sensor or the sensing distance.

5 – Connector

    • Quick disconnect – M8 or M12.
    • Potted cable.

6 – Sensor

    • The sensor sensing area or face must be smaller or equal to the target material.
    • Maximum sensing distance is measured on metal – reduction factor will influence all sensing distances.
    • Use flush versions to reduce the effects of the surrounding material. Some plastic sensors will have a reduced sensing range when embedded in metal. Use a flush stainless-steel body to get the full sensing range.

These are just a few things to keep in mind when applying capacitive sensors. There is not “a” capacitive sensor application – but there are many which can be solved cost-effectively and reliably with these sensors.

Why Use Ultrasonic Sensors?

by Nick Smith

When choosing what sensor to use in different applications, it is important to first look at how they operate. Capacitive sensors generate an electrical field that can detect various liquids or other materials, such as glass, wood, paper, ceramic, and more at a close. Photoelectric sensors emit a light beam that is either received by a light sensor or bounced back to the emitter to detect an object’s presence or measure the distance to an object. Ultrasonic sensors bounce a sound wave off objects to detect them, which can make them a good solution for a surprising variety of uses.

How ultrasonic sensors operate

Ultrasonic sensors operate by emitting an ultra-high frequency sound wave that ranges from 300 MHz to 3 GHz, which is well above the 15-17 kHz range that humans can hear that bounces off the target object. The sensor measures the amount of time that sound wave takes to return to calculate the distance to the object. Ultrasonic sensors send these sound waves in a wider beam than a photoelectric uses, so they can more easily detect objects in a dusty or dirty environment. And with a greater sensing distance than capacitive sensors, they can be installed at a safe distance and still function effectively

Common applications for ultrasonic sensors

These capabilities together make ultrasonic sensors a great choice for tasks like detecting fill level, stack height and object presence. Sound waves are unaffected by the color, transparency, or consistency of an object or liquid, which makes it an obvious contender in the packaging, food, and beverage industry and many other industries with similar manufacturing processes.

So to monitor glass bottles as they travel on a conveyor, an ultrasonic sensor could be a good choice. These sensors will consistently work well detecting clear or reflective materials such as water, paint, glass, etc., which can cause difficulties for photoelectric sensors. Another benefit of these sensors is the ability to mount them further away from their targets. For example, there are ultrasonics that can be mounted between 20 to 8000 mm away from the object. After tuning your setup, you can detect very small objects as easily as larger, more visible items.

Another common application for ultrasonic sensors is monitoring boxes. Properly implemented ultrasonic sensors can detect different sizes of boxes as they travel on a conveyor belt by constantly emitting and receiving sound waves. This means that each box or object will be measured by the sound wave. Different photoelectric and capacitive sensors may fail to detect the full presence of an object and may only be able to detect a specific point on an object.

When it comes to all types of different fill-level applications, there are many ways a sensor can monitor various liquids and solids. The width of an ultrasonic beam can be increased to detect a wider area of solid material in a hopper or decreased to give a precise measurement on liquid levels. This ability to detect a smaller or larger surface area gives the user more utility when deciding how to meet the requirements of an application. Although capacitive sensors can detect fill levels very precisely as well, factors like beam width and sensing distance might make ultrasonic a better choice.

With so many different sensor technologies available and factors like target material and sensing distance being such important factors, choosing the best sensor for an application can be demanding. A trusted expert who is familiar with these different technologies and the factors related to your applications and materials can help you confidently move toward the smart factory of the future.

Capacitive Prox Sensors Offer Versatility for Object and Level Detection

When you think of a proximity sensor, what is the first thing that comes to mind? In most cases it is probably the inductive proximity sensor and justly so because they are the most widely used sensor in automation today. But there are other types of proximity sensors. These include diffuse photoelectric sensors that use the reflectivity of the object to change states and proximity mode of ultrasonic sensors that use high frequency sound waves to detect objects. All of these sensors detect objects that are in close proximity of the sensor without making physical contact.

One of the most overlooked proximity sensors on the market today is the capacitive sensor. Why? For some, they have bad reputation from when they were released years ago as they were more susceptible to noise than most sensors. I have heard people say that they don’t discuss or use capacitive sensors because they had this bad experience in the past, however with the advancements of technology this is no longer the case.

Today capacitive sensors are available in as wide of a variety of housings and configurations as inductive sensors. They are available as small as 4mm in diameter, in hockey puck styles, extended temperature ranges, rectangular, square, with Teflon housings, remote sensing heads, adhesive cut-to-length for level detection and a hybrid technology that is capable of ignoring foaming and filming of liquids. The capability and diversity of this technology is constantly evolving.

Capacitive sensors are versatile in solving numerous 1applications. These sensors can be used to detect objects such as glass, wood, paper, plastic, ceramic, and the list goes on and on. The capacitive sensors used to detect objects are easily identified by the flush mounting or shielded face of the sensor. Shielding causes the electrostatic field to be short conical shaped much like the shielded version of the inductive proximity sensor. Typically, the sensing range for these sensors is up to 20 mm.

Just as there are non-flush or unshielded inductive sensors, there are non-flush capacitive sensors, and the mounting and housing2 looks the same. The non-flush capacitive sensors have a large spherical field which allows them to be used in level detection. Since capacitive sensors can detect virtually anything, they can detect levels of liquids including water, oil, glue and so forth and they can detect levels of solids like plastic granules, soap powder, sand and just about anything else. Levels can be detected either directly with the sensor touching the medium or indirectly where the sensor senses the medium through a non-metallic container wall. The sensing range for these sensors can be up to 30 mm or in the case of the hybrid technology it is dependent on the media.

The sensing distance of a capacitive sensor is determined by several factors including the sensing face area – the larger the better. The next factor is the material property of the object or dielectric constant, the higher the dielectric constant the greater the sensing distance. Lastly the size of the target affects the sensing range. Just like an inductive sensor you want the target to be equal to or larger than the sensor. The maximum sensing distance of a capacitive sensor is based on a metal target thus there is a reduction factor for non-metal targets.

As with most sensors today, the outputs of a capacitive sensor include PNP, NPN, push-pull, analog and the increasing popular IO-Link. IO-Link provides remote configuration, additional diagnostics and a window into what the sensor is “seeing”. This is invaluable when working on an application that is critical such as life sciences.

Most capacitive sensors have a potentiometer to allow adjustment of the sensitivity of the sensor to reliably detect the target. Today there are versions that have teach pushbuttons or a teach wire for remote configuration or even a remote amplifier. Although capacitive sensors can detect metal, inductive sensors should be used for these applications. Capacitive sensors are ideal for detecting non-metallic objects at close ranges, usually less than 30 mm and for detecting hidden or inaccessible materials or features.

Just remember, there is one more proximity sensor. Don’t overlook the capabilities of the capacitive sensor.

Do Your Capacitive Sensors Ignore Foam & Condensation for True Level Detection?

Capacitive sensors detect any changes in their electrostatic sensing field. This includes not only the target material itself, but also application-induced influences such as condensation, foam, or temporary or permanent material build-up. High viscosity fluids can cause extensive delays in accurate point-level detection or cause complete failure due to the inability of a capacitive sensor to compensate for the material adhering to the container walls. In cases of low conductive fluids such as water or deionized water and relatively thin container walls, the user might be able to compensate for these sources of failure. Potential material build-up or condensation can be compensated for by adjusting the sensitivity of the sensor, cleaning of the container, or employing additional mechanical measures.

However, this strategy works only if the fluid conductivity stays low and no other additional influencing factors like temperature, material buildup, or filming challenge the sensor. Cleaning fluids like sodium hydrochloride, hydrochloric acid, chemical reagents, and saline solutions are very conductive, which cause standard capacitive sensors to false trigger on even the thinnest films or adherence. The same applies for bodily fluids such as blood, or concentrated acids or alkaline.

Challenges of this type of application are not obvious. This is especially true when the sensors performed well in the initial design phase but fail in the field for no obvious reason. An example of this would be when the sensors on the equipment are setup with deionized water however, the final process requires some type of acid  Difficult and time-consuming setup procedures and unstable applications requiring frequent readjustment are the primary reasons why capacitive level sensors have been historically avoided in certain applications.

Today, there are hybrid technologies employed in capacitive sensors for non-invasive level detection applications that would require little or no user adjustment after the initial setup process. They can detect any type conductive water based liquid through any non-metallic type of tank wall while automatically compensating for material build-up, condensation, and foam.

This hybrid sensing technology helps the sensors to distinguish effectively between true liquid levels and possible interferences caused by condensation, material build-up, or foaming fluids. While ignoring these interferences, the sensors still detect the relative change in capacitance caused by the media but use additional factors to evaluate the validity of the measurement taken before changing state. These sensors are fundamentally insensitive to any non-conductive material like plastic or glass, which allows them to be utilized in non-invasive level applications.

These capacitive sensors provide cost-effective, reliable point-level monitoring for a wide array of medical, biotechnology, life sciences, semiconductor processes, and other manufacturing processes and procedures. This technology brings considerable advantages to the area of liquid level detection, not only offering alternative machine designs, but also reduced assembly time for the machine builders.  Machine designers now have the flexibility to non-invasively detect almost any type of liquid through plastic, glass tubes, or other non-metallic container walls, reducing mechanical adaption effort and fabrication costs.

Discrete indication tasks like fluid presence detection in reagent supply lines, reagent bottle level feedback, chemical levels, and waste container overfill prevention are now a distinct competence for capacitive sensors. Reagents and waste liquids are composed of different formulas depending on the application.  The sensing technology has to be versatile enough to compensate automatically for changing environmental or media conditions within high tolerance limits. Applications that require precision and an extraordinary amount of reliability, such as blood presence detection in cardiovascular instruments or hemodialysis instruments, medical, pharmaceutical machine builders, equipment builders for semiconductor processes can rely now on these hybrid capacitive sensors

Capacitive Sensors: Versatile enough for most (but not all) detection applications

capacitive 1

Capacitive sensors are versatile for use in numerous applications. They can be used to detect objects such as glass, wood, paper, plastic, ceramic, and more. Capacitive sensors used to detect objects are easily identified by the flush mounting or shielded face of the sensor. This shielding causes the electrostatic field to be short and conical shaped, much like the shielded version of an inductive proximity sensor.

capacitive 2Just as there are non-flush or unshielded inductive sensors, there are non-flush capacitive sensors, and the mounting and housing look the same. The non-flush capacitive sensors have a large spherical field which allows them to be used in level detection, including detection of liquids and granular solids. Levels can be detected either directly with the sensor making contact with the medium, or indirectly with the sensor sensing the medium through a non-metallic container wall.

Capacitive sensors are discrete devices so once you adjust the sensitivity to detect the target while ignoring the container, the sensor is either on or off. Also remember that the sensor is looking for the dielectric constant in the case of a standard capacitive sensor or the conductivity of a water based liquid in the case of the hybrid technology.

Recent technology advances with remote amplifiers have allowed capacitive sensors to provide an analog output or a digital value over IO-Link. As previously mentioned, these sensors are based off of a dielectric constant so the analog value being created is dependent on the media being sensed.

While capacitive sensors are versatile to work in many applications, they are not the right choice for all applications.

Recently a customer inquired if a capacitive sensor could detect the density of an substance and unfortunately the short answer is no, though in some applications the analog sensors can detect different levels of media if it can be separated in a centrifuge. Also, capacitive sensors may not detect small amounts of media as the dielectric constant of the media must be higher than the container that holds the media.

There are three important steps in applying a capacitive sensor — test it, test it and test it one more time. During your testing procedures be sure to test it under the best and worse conditions. Also like any other electronic device temperature can have an affect although it may be negligible there will be some affect.

For more information on capacitive sensors visit www.balluff.com.

Back to the Basics: Object Detection

In the last post about the Basics of Automation, we discussed how humans act as a paradigm for automation. Now, let’s take a closer look at how objects can be detected, collected and positioned with the help of sensors.

Sensors can detect various materials such as metals, non-metals, solids and liquids, all completely without contact. You can use magnetic fields, light and sound to do this. The type of material you are trying to detect will determine the type of sensor technology that you will use.

Object Detection 1

Types of Sensors

  • Inductive sensors for detecting any metallic object at close range
  • Capacitive sensors for detecting the presence of level of almost any material and liquid at close range
  • Photoelectric sensors such as diffuse, retro-reflective or through-beam detect virtually any object over greater distances
  • Ultrasonic sensors for detecting virtually any object over greater distances

Different Sensors for Different Applications

The different types of sensors used will depend on the type of application. For example, you will use different sensors for metal detection, non-metal detection, magnet detection, and level detection.

Detecting Metals

If a workpiece or similar metallic objects Object Detection 2should be detected, then an inductive sensor is the best solution. Inductive sensors easily detect workpiece carriers at close range. If a workpiece is missing it will be reliably detected. Photoelectric sensors detect small objects, for example, steel springs as they are brought in for processing. Thus ensures a correct installation and assists in process continuity. These sensors also stand out with their long ranges.

Detecting Non-Metals

If you are trying to detect non-metal objects, for example, the height of paper stacks, Object Detection 3then capacitive sensors are the right choice. They will ensure that the printing process runs smoothly and they prevent transport backups. If you are checking the presence of photovoltaic cells or similar objects as they are brought in for processing, then photoelectic sensors would be the correct choice for the application.

Detecting Magnets

Object Detection 4

To make sure that blister packs are exactly positioned in boxes or that improperly packaged matches are sorted out, a magnetic field sensor is needed which is integrated into the slot. It detects the opening condition of a gripper, or the position of a pneumatic ejector.

 

Level Detection

What if you need to detect the level of granulate in containers? Then the solution is to use capacitive sensors. To accomplish this, two sensors are attached in the containers, offset from each other. A signal is generated when the minimum or maximum level is exceeded. This prevents over-filling or the level falling below a set amount. However, if you would like to detect the precise fill height of a tank without contact, then the solution would be to use an ultrasonic sensor.

Stay tuned for future posts that will cover the essentials of automation. To learn more about the Basics of Automation in the meantime, visit www.balluff.com.

The Evolving Technology of Capacitive Sensors

In my last blog post, Sensing Types of Capacitive Sensors, I discussed the basic types of capacitive sensors; flush versions for object detection and non-flush for level detection of liquids or bulk materials.  In this blog post, I would like to discuss how the technology for capacitive sensors has changed over the past few years.

The basic technology of most capacitive sensors on the market was discussed in the blog post “What is a Capacitive Sensor”.  The sensors determine the presence of an object based on the dielectric constant of the object being detected.  If you are trying to detect a hidden object, then the hidden object must have a higher dielectric constant than what you are trying to “see through”.

Conductive targets present an interesting challenge to capacitive sensors as these targets have a greater capacitance and a targets dielectric constant is immaterial.  Conductive targets include metal, water, blood, acids, bases, and salt water.  Any capacitive sensor will detect the presence of these targets. However, the challenge is for the sensor to turn off once the conductive material is no longer present.  This is especially true when dealing with acids or liquids, such as blood, that adheres to the container wall as the level drops below the sensor face.

Today, enhanced sensing technology helps the sensors effectively distinguish between true liquid levels and possible interference caused by condensation, material build-up, or foaming fluids.  While ignoring these interferences, the sensors would still detect the relative change in capacitance caused by the target object, but use additional factors to evaluate the validity of the measurement taken before changing state.

These sensors are fundamentally insensitive to any non-conductive material like plastic or glass, which allows them to be utilized in level applications.  The only limitation of enhanced capacitive sensors is they require electrically conductive fluid materials with a dipole characteristic, such as water, to operate properly.

Enhanced or hybrid technology capacitive sensors work with a high-frequency oscillator whose amplitude is directly correlated with the capacitance change between the two independently acting sensing electrodes.  Each electrode independently tries to force itself into a balanced state.  That is the reason why the sensor independently measures  the capacitance of the container wall without ground reference and the capacitance of the conductivity of the liquid with ground reference (contrary to standard capacitive sensors).

Image1

Up to this point, capacitive sensors have only been able to provide a discrete output, or if used in level applications for a point level indication.  Another innovative change to capacitive sensor technology is the ability to use a remote amplifier.  Not only does this configuration allow for capacitive sensors to be smaller, for instance 4mm in diameter, since the electronics are remote, they can provide additional functionality.

The remote sensor heads are available in a number of configurations including versions image2that can withstand temperature ranges of -180°C up to 250°C.  The amplifiers can now provide the ability to not only have discrete outputs but communicate over an IO-Link network or provide an analog output.  Now imagine the ability to have an adhesive strip sensor that can provide an analog output based on a non-metallic tanks level.

For additional information on the industry’s leading portfolio of capacitive products visit www.balluff.com.

Sensing Types of Capacitive Sensors

Similar to inductive sensors, capacitive sensors are available in two basic versions.  The first type is the flush or shielded or embeddable version however with capacitive sensors they are sometimes referred to as object detection sensors.  The second type is the non-flush or non-shielded or non-embeddable version however again with capacitive sensors they are sometimes referred to as level detection sensors.

CapacitiveTypes1

The flush or object detection capacitive sensors are shielded and employ a straight line electrostatic field.  This focused field is emitted only from the front face of the sensor allowing the sensor to be mounted in material so that only the face of the sensor is visible.

The highly focused electrostatic field is perfect for detecting small amounts of material or material with low dielectric constant.  The typical range of a flush 18mm capacitive sensor is approximately 2 to 8mm depending on the objects dielectric constant.  As with any capacitive sensor the sensor should be adjusted after installation.

CapacitiveTypes2

If the sensors are mounted adjacent to each other the minimum gap should be equal to the diameter or the adjusted sensing distance whichever is less.  These sensors can also be mounted opposing each other however the distance should four times the diameter of the adjusted sensing distance whichever is less.

CapacitiveTypes3Shielded or flush capacitive sensors are perfect for detecting solids or liquids through non-metallic container walls up to 4mm thick.  If you are detecting liquid levels through a sight glass with the sight glass mounting bracket then the flush mounted sensor is the preferred choice.

CapacitiveTypes4The non-flush or level detection capacitive sensors are not shielded and employ a spherical electrostatic field.  This field is emitted from the front face of the sensor and wraps around to the sides of the sensor head.  Unlike the flush sensor this version cannot be mounted in material where only the face of the sensor is visible.  Non-flush sensors have better characteristics and better performance in applications with adhering media.

CapacitiveTypes5The spherical electrostatic field provides a larger active surface and is perfect for detecting bulk material and liquid either directly or indirectly.  The typical range of a flush 18mm capacitive sensor is approximately 2 to 15mm depending on the objects dielectric constant.  As with any capacitive sensor the sensor should be adjusted after installation.

CapacitiveTypes6If the sensors are mounted adjacent to each other the minimum gap should be equal to three times the diameter or the adjusted sensing distance whichever is less.  These sensors can also be mounted opposing each other however the distance should four times the diameter of the adjusted sensing distance whichever is less.

Shielded or flush capacitive sensors are perfect for detecting solids or liquids through non-metallic container walls up to 4mm thick.  If you are detecting liquid levels through a sight glass with the sight glass mounting bracket then the flush mounted sensor is the preferred choice.

Capacitive sensors are perfect for short range detection of virtually any object regardless of color, texture, and material.

To learn more about capacitive sensors visit www.balluff.com.

What is a Capacitive Sensor?

Capacitive proximity sensors are non-contact devices that can detect the presence or absence of virtually any object regardless of material. They use the electrical property of capacitance and the change of capacitance based on a change in the electrical field around the active face of the sensor.

Capacitive sensing technology is often used in various detection tasks:

  • Flow
  • Pressure
  • Liquid level
  • Spacing
  • Thickness
  • Ice detection
  • Shaft angle or linear position
  • Dimmer switches
  • Key switches
  • X-y tablet
  • Accelerometers

Principle of operation

A capacitive sensor acts like a simple capacitor. A metal plate in the sensing face of the sensor is electrically connected to an internal oscillator circuit and the target to be sensed acts as the second plate of the capacitor. Unlike an inductive sensor that produces an electromagnetic field a capacitive sensor produces an electrostatic field.

The external capacitance between the target and the internal sensor plate forms a part of the feedback capacitance in the oscillator circuit. As the target approaches the sensors face the oscillations increase until they reach a threshold level and activate the output.

Capacitive sensors have the ability to adjust the sensitivity or the threshold level of the oscillator. The sensitivity adjustment can be made by adjusting a potentiometer, using an integral teach pushbutton or remotely by using a teach wire.  If the sensor does not have an adjustment method then the sensor must physically be moved for sensing the target correctly. Increasing the sensitivity causes a greater operating distance to the target. Large increases in sensitivity can cause the sensor to be influenced by temperature, humidity, and dirt.

There are two categories of targets that capacitive sensors can detect the first being conductive and the second is non-conductive. Conductive targets include metal, water, blood, acids, bases, and salt water. These targets have a greater capacitance and a targets dielectric strength is immaterial. Unlike an inductive proximity sensor, reduction factors for various metals are not a factor in the sensors sensing distance.

The non-conductive target category acts like an insulator to the sensors electrode.  A targets dielectric constant also sometimes referred to as dielectric constant is the measure of the insulation properties used to determine the reduction factor of the sensing distance.  Solids and liquids have a dielectric constant that is greater than vacuum (1.00000) or air (1.00059). Materials with a high dielectric constant will have a longer sensing distance.  Therefore materials with high water content, for example wood, grain, dirt and paper will affect the sensing distance.

When dealing with non-conductive targets there are three factors that determine the sensing distance.

  • The size of the active surface of the sensor – the larger the sensing face the longer the sensing distance
  • The capacitive material properties of the target object, also referred to as the dielectric constant – the higher the constant the longer the sensing distance
  • The surface area of the target object to be sensed – the larger the surface area the longer the sensing distance

Other factors that have minimal effect on the sensing distance

  • Temperature
  • Speed of the target object

Sensing range

A capacitive sensor’s maximum published sensing distance is based on a standard target that is a grounded square metal plate (Fe 360) that is 1mm thick. The standard target must have a side length that is the diameter of the registered circle of the sensing surface or three times the rated sensing distance if the sensing distance is greater than the diameter.  Objects being detected that are not metal will have a reduction factor based on the dielectric constant of that object material. This reduction factor must be measured to determine the actual sensing distance however there are some tables that will provide an approximation of the reduction factor.

Rated or nominal sensing distance Sn is a theoretical value that does not take into account manufacturing tolerances, operating temperatures and supply voltages. This is typically the sensing distance listed in various manufactures catalogs and marketing material.

Effective sensing distance Sr is the switching distance of the sensor measured under specified conditions such as flush mounting, rated operating voltage Ue, temperature Ta = 23°C +/- 5°C. The effective sensing range of capacitive sensors can be adjusted by the potentiometer, teach pushbutton or remote teach wire.

Hysteresis

Hysteresis is the difference in distance between the switch-on as the target approaches the sensing face and switch-off point as the target moves away from the sensing face. Hysteresis is designed into sensors to prevent chatter of the output if the target was positioned at the switching point.

Hysteresis stated in % of rated sensing distance. For example a sensor with 20mm of rated sensing distance may have a maximum hysteresis of 15% or 3mm. Hysteresis is an independent parameter that is not a constant and will vary sensor to sensor. There are several factors that can influence hysteresis including:

  • Sensor temperature both ambient and heat generated by the sensor being powered
  • Atmospheric pressure
  • Relative humidity
  • Mechanical stresses to the sensor housing
  • Electronic components utilized on the printed circuit board within the sensor
  • Correlated to sensitivity – higher sensitivity relates to higher rated sensing distance and a larger hysteresis

How to determine a capacitive sensor’s sensitivity

Capacitive sensors have a potentiometer or some method to set the sensor sensitivity for the particular application. In the case of a potentiometer, the number of turns does not provide an accurate indicator of the sensors setting for a couple of important reasons. First, most potentiometers do not have hard stops instead they have clutches so that the pot is not damaged when adjusted to the full minimum or maximum setting. Secondly, pots do not have consistent linearity.

To determine the sensitivity of a capacitive sensor the sensing distance is measured from a grounded metal plate with a micrometer. The plate is grounded to the negative of the power supply and the target is moved axially to the sensors face. Move the target out of the sensing range and then move it towards the sensor face. Stop advancing the target as soon as the output is activated. This distance is the sensing distance of the sensor. Moving the target away and noting when the output turns off will provide the hysteresis of the sensor.

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