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

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).


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

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.


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.


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.

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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 utilize 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 other sensing technologies such as:

  • 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 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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IP Ratings and ECOLAB Basics

WashdownSensorsIntegrating sensors in washdown applications can be confusing when considering the various approvals.  So what do they all mean?  If a sensor is an IP69K rated sensor does that mean it will survive everything?  In the world of sensors there is IP54, IP67, IP68 and IP69 so if my sensor is IP69K that means it is the best right?  The short answer is no.  Let’s take a brief look at the differences.

IP ratings will generally have two digits with the first digit referring to the solid particle protection.  The second digit indicates the level of protection against the ingress of water.

Sensors rated for IP54 indicates they are dust protected, meaning that dust can get inside the sensor, however, it cannot be enough to interfere with the operation of the equipment –  this is designated by the 5.  The 4 indicates that the sensor withstands splashing water on the housing from any direction with no detrimental effect.  The test for the splashing of water lasts at least five minutes with a water volume of 2.64 gallons per minute with a pressure of 7.25 to 21.76 PSI.

IP67 rated sensors are the most commonly used sensors on the market.  Even most electrical enclosures used in automation are IP67 rated.  The 6 indicates these devices will not allow the entry of dust.  The 7 indicates that the sensor can be immersed in water to a depth of 1 meter for 30 minutes.

IP68 rated sensors are dust tight sensors that can be immersed in water continuously under conditions specified by the manufacturer.  Typically the depth of the immersion is 3 meters.

The IP69K rating is based on a dust tight sensor that can withstand high pressure sprays.  The devices are sprayed with a pressure of 1,160 to 1,450 PSI.  The water temperature can be as high as 176°F with a flow rate of 3.7 to 4.2 gallons per minute.  The distance from the nozzle to the device is 4 to 6 inches.  The sensor is placed on a rotary table that rotates at 5 revolutions per minute and the sensor is sprayed for 30 seconds at four angles 0°, 30°, 60°, and 90°.

The ultimate sensor would have a rating of IP67/IP68/IP69 indicating that it will survive submersion and high pressure washdown.  Also, some of these sensors are 316L stainless meaning they have low carbon content and are more corrosion resistant than other stainless steel grades.  Are all IP69K sensors stainless steel?  No, some sensors utilize polycarbonate-ABS thermoplastic.

Usually during washdown applications in the food and beverage industry the spray is not just water but some sort of cleaning chemical or disinfectant.  These aggressive cleaning and disinfecting agents can attack different housing materials.  This is addressed by the ECOLAB certification.

The ECOLAB test consists of testing the housing and sensor materials to exposure to these aggressive cleaning and disinfecting agents.  The devices are tested for 14 to 28 days at a room temperature of 68° F.  During this time the sensor is visually inspected for swelling, embrittlement, or changes in color.

Don’t forget that even though the sensor has the correct IP rating for your application that the mating connector has to meet the same specifications.  For example, if the sensor is IP69K rated and a IP67 mating cable is used then the lower IP rating has precedence.

If you are interested in what sensors and cables meet washdown requirements, please visit

Liquid Handling Solutions in Action

On Sensortech, we have posted several entries about the trend toward miniature sensors including, Let’s Get Small: The Drive Toward Miniaturization and Trending Now: Miniature Sensors. At the end of January Balluff attended SLAS in San Diego, CA and saw this trend firsthand. Automation in the clinical lab is growing by leaps and bounds. Bioscience engineers are facing pressure to reduce cost, increase the number of samples run, and improve the speed at which lab tests are performed.

As an exhibitor at the event, we were able to showcase our solutions with a great functional demo. Below is a brief video of the demo with our Life Science Industry Manager, Blake DeFrance explaining the technology.

For more information on solutions for the Life Science Industry visit

Multiple Sensing Modes for Miniature Capacitive Sensors

MiniCapacitiveIn a previous blog post we discussed miniature capacitive sensors and their use for precision and small-part sensing. Here we will discuss the different sensing modes available with separately amplified miniature capacitive sensors.

Standard Switching Mode


This is the most commonly used teach method for most sensing applications. As an object is placed statically in front of the sensor at its desired detection point, the amplifier is triggered to teach-in this value as its switch point (SP1). Once the value is taught, the output will then switch when the switch point is reached.

Two-Point Switching Mode


As the name sug
gests this teach method has two separate teach-in points, a switch-on point (SP1) and a switch-off point (SP2). These points can be taught wide apart or close together, depending on the application need. One application example is for fill-level control by teaching in min. and max. fill-level points.

Window Function Mode


This teach method creates a window between two separate switch points (SP1 and SP2). If the sensor value falls inside this window, the output will switch on. If the sensor value is outside of this window, the output remains off. An application example is material thickness (or multiple layer) detection. If the material is too thin or too thick (i.e., sensor value is outside the window) the output remains off; however, if the material is at the correct thickness (i.e., sensor value falls inside the window) the output switches on.

Dynamic Operation Mode

This mode only responds to moving objects and ignores static conditions. This mode is commonly used to ignore a close background, and only detect objects moving in front of the sensor.

Analog Output Mode

Additionally, an analog output (either voltage or current) is available. To utilize the whole analog range, two separate teach points are needed. SAHi, analog signal high, and SALo, analog signal low, are taught accordingly to obtain the full range. An application example would be continuous fill-level detection across the sensing area.

For more information on capacitive sensors and their remote amplifiers, click h

Miniature Capacitive Sensors for Small Part Detection

As discussed in a previous blog post, miniature sensors are an ongoing trend in the market as manufacturing and equipment requirements continue to demand smaller sensor size due to either space limitations and/or weight considerations. However, size and weight aren’t the only factors. The need for more precise sensing — higher accuracy, repeatability, and smaller part detection — is another demanding requirement and, often times, the actual main focus point.

This post will look specifically at capacitive sensors and how smaller capacitive sensors can lead to better detection of smaller parts.

Principle of a capacitive sensor
Parallel-plate capacitor equation

Capacitive sensors provide non-contact detection of all types of objects, ranging from insulators to conductors and even liquids. A capacitive sensor uses the principle of capacitance to detect objects. The equation for capacitance takes into account the surface area (A) of either electrode, the distance (d) between the electrodes, and the dielectric constant (εr) of the material between the electrodes. In simple terms: a capacitive sensor detects the change in capacitance when an object enters its electrical field. Internal circuitry determines if the gain in capacitance is above the set threshold. Once the threshold is met the sensor’s output is switched.

Actuation of a capacitive sensor

When looking at small part detection, the size of the capacitive sensor’s active sensing surface plays a significant part. Now there isn’t a defined formula for calculating smallest detectable object for a capacitive sensor because of the numerous variables that need to be considered (as seen in the equation above). However, the general rule for optimal sensing is that the target size should be at least equal to the size of the sensor’s active surface. The reason behind this is if the target size is smaller than the sensor’s active surface, the electric field would travel around the target and cause unreliable readings.

Taking the general rule into consideration and comparing a miniature 4mm diameter capacitive sensor to a standard 18mm diameter capacitive sensor, it’s simple to determine that the 4mm diameter capacitive sensor can reliably detect a much smaller target (4mm) than the 18mm diameter capacitive sensor (18mm).

So when looking at small part detection, the smaller the sensor’s active sensing surface is, the better its ability for small part detection. Therefore, if an application requires detection of a small part, it’s best to start with miniature capacitive sensor.

For more information on miniature capacitive sensors click here.

The Often Overlooked Proximity Sensor

If someone says proximity sensor, what is the first thing that comes to mind?  My guess is inductive and justly so because they are the most used sensor in automation today.  There are other sensing technologies that use the term proximity in describing the sensing mode.  These include diffuse or proximity photo electric 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 or forgotten proximity sensors on the market today is the capacitive sensor.  Why?  Perhaps it is because they have a bad reputation from when they were released years ago as they were more susceptible to noise than most sensors.  I recently heard someone say that they don’t discuss capacitive sensors with their customers because they had this bad experience almost 10 years ago, however, with the advancements of technology this is no longer the case.

CapacitiveFlushCapacitive sensors are versatile in solving numerous applications.  These sensors can be used to detect objects such as glass, wood, and paper, plastic, ceramic, 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.

Capacitive Non-FlushJust as there are non-flush or unshielded inductive sensors there are non-flush capacitive sensors, and the mounting and housing 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 the can detect levels of solids like plastic granules, soap powder, sand and just about anything else.  Levels can be detected either directly, the sensor touches the medium or indirectly where the sensor senses the medium through a non-metallic container wall.

SmartLevelWith improvements in capacitive technology sensors have been designed that can compensate for foaming, material build-up and filming of water based highly conductive liquids.  Since these capacitive sensors are based on the conductivity of liquids they can reliably actuate when sensing aggressive acids such as hydrochloric, sulfuric and hydrofluoric acids.  In addition, these sensors can detect liquids through glass or plastic walls up to 10mm thick are not affected by moisture and require little or no cleaning these applications.

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 strength, 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.

Most capacitive sensors have a potentiometer to allow adjustment of the sensitivity of the sensor to reliably detect the target.  The maximum sensing distance of a capacitive sensor is based on a metal target thus there is a reduction factor for non-metal targets.

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 30mm and for detecting hidden or inaccessible materials or features.

Just remember, there is one more proximity sensor…the capacitive one!

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Direct vs Indirect Mounting of Capacitive Sensors

Direct sensing mount
Figure 1: Direct sensing mount

In liquid level sensing applications, capacitive sensors can be mounted directly in contact with the medium or indirectly with no contact to the medium.

Containers made of metal or very thick non-metallic tank walls (more than 1″) typically require mounting the sensor in direct contact with the medium (Fig. 1). In some instances, a by-pass tube or a sights glass is used, and the senor detects the level through the wall of the non-metallic tube (Fig. 2).

Indirect sensing mount
Figure 2: Indirect sensing mount

The direct mounting method could simplify sensor selection and setup since the sensor only has to sense the medium or target material properties. Nonetheless, this approach imposes certain drawbacks, such as costs for mounting and sealing the sensor as well as the need to consider the material compatibility between the sensor and the medium. Corrosive acids, for example, might require a more expensive exotic housing material.


The preferred approach is indirectly mounting the capacitive sensor flush against the non-metallic wall to detect the target material non-invasively through the container wall.  The advantages for this approach are obvious and represent a major influence to specify capacitive sensors.  The container wall does not have to be penetrated, which leaves the level sensor flexible and interchangeable in the application.  Avoiding direct contact with the target material also reduces the chances of product contamination, leaks, and other sources of risk to personnel and the environment.

The target material also has relevance in the sensor selection process.  Medical and semiconductor applications involve mostly water-based reagents, process fluids, acids, as well as different bodily fluids.  Fortunately, high conductivity levels and therefore high relative dielectric constants are common characteristics among all these liquids.  This is why the primary advantages of capacitive sensors lies in non-invasive liquid level detection, namely by creating a large measurement delta between the low dielectric container walls and the target material with high dielectric properties.

At the same time, highly conductivity liquids could impose a threat to the application.  This is because smaller physical amounts of material have a larger impact on the capacitive sensor with increasing conductivity values, increasing the risk of false triggering on foam or adherence to the inside or outside wall.  SMARTLEVEL sensors offered by Balluff will ignore foaming, filming and material build-up in these applications.

Learn more about Balluff’s capacitive solutions on our website at