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.

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

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.

To learn more about capacitive sensor technology visit www.balluff.com.

Level Detection Basics – Where to begin?

Initially I started to write this blog to compare photoelectric sensors to ultrasonic sensors for level detection. This came to mind after traveling around and visiting customers that had some very interesting applications. However, as I started to shed some light on this with photoelectrics, sorry for the pun but it was intended, I thought it might be better to begin with some application questions and considerations so that we have a better understanding of the advantages and disadvantages of solutions that are available. That being said I guess we will have to wait to hear about ultrasonic sensors until later…get it, another pun. Sorry.

Level detection can present a wide variety of challenges some easier to overcome than others. Some of the questions to consider include the following with some explanation for each:

  • What is the material of the container or vessel?
    • Metallic containers will typically require the sensor to look down to see the media. This application may be able to be solved with photoelectrics, ultrasonics, and linear transducers or capacitive (mounted in a tube and lowered into the media.
    • SmartLevelNon-metallic containers may provide the ability for the sensors look down to see the media with the same technologies mentioned above or by sensing through the walls of the container. Capacitive sensors can sense through the walls of a container up to 4mm thick with standard technology or up to 10mm thick using a hybrid capacitive technology offered by Balluff when detecting water based conductive materials. If the container is clear or translucent we have photoelectric sensors that can look through the side walls to detect the media. You can get more information in our white paper, SMARTLEVEL Technology Accurate point level detection.
  • What type of sensing is required? The short answer to this is level right? However, there are basically two different types of level detection. For more information on this refer to the Balluff Basics on Level Sensing – Discrete vs. Continuous.
    • Single point level or point level sensing. This is typically accomplished with a single sensor that allows for a discrete or an on-off signal when the level actuates the sensor. The sensor is mounted at the specific level to be monitored, for instance low-low, low, half full (the optimistic view), high, or high-high. These sensors are typically lower cost and easier to implement or integrate into the level controls.
    • Example of in-tank continuous level sensor
      Example of in-tank continuous level sensor

      Continuous or dynamic level detection. These sensors provide an analog or continuous output based on the level of the media. This level detection is used primarily in applications that require precise level or precision dispensing. The output signals are usually a voltage 0-10V or current output 4-20mA.  These sensors are typically higher cost and require more work in integrating them into system controls.  That being said, they also offer several advantages such as the ability to program in unlimited point levels and in the case of the current output the ability to determine if the sensor is malfunctioning or the wire is broken.

Because of the amount of information on level detection this will be the first in a series on this topic. In my next blog I will discuss invasive vs non-invasive mounting and some other topics. For more information visit www.balluff.us.

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

Std_Switch_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

TwoPoint_Switch_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

Window_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
Analog_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
ere
.

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.

cap1
Principle of a capacitive sensor
cap4
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.

cap2
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!

To learn more about Balluff capacitive sensors visit www.balluff.us.

There’s more than just one miniature sensor technology

As I discussed in my last blog post, there is a need for miniature, precision sensors. However, finding the right solution for a particular application can be a difficult process. Since every sensor technology has its own strengths and weaknesses, it is vital to have a variety of different sensor options to choose from.

The good news is that there are several different technologies to consider in the miniature, precision sensor world. Here we will briefly look at three technologies: photoelectric, capacitive, and inductive. Together these three technologies have the ability to cover a wide range of applications.

Photoelectric Sensors

MiniPhotoelectricPhotoelectric sensors use a light emitter and receiver to detect the presence or absence of an object. This type of sensor comes in different styles for flexibility in sensing. A through-beam photoelectric is ideal for long range detection and small part detection. Whereas a diffuse photoelectric is ideal for applications where space is limited or in applications where sensing is only possible from one side.

Miniature photoelectric sensors come with either the electronics fully integrated into the sensor or as a sensor with separate electronics in a remote amplifier.

Capacitive Sensors

MiniCapacitiveCapacitive sensors use the electrical property of capacitance and work by measuring changes in this electrical property as an object enters its sensing field. Capacitive sensors detect the presence or absence of virtually any object with any material, from metals to powders to liquids. It also has the ability to sense through a plastic or glass container wall to detect proper fill level of the material inside the container.

Miniature capacitive sensors come with either the electronics fully integrated into the sensor or as a sensor with separate electronics in a remote amplifier.

Inductive Sensors

MiniInductiveInductive sensors use a coil and oscillator to create a magnetic field to detect the presence or absence of any metal object. The presence of a metal object in the sensing field dampens the oscillation amplitude. This type of sensor is, of course, ideal for detecting metal objects.

Miniature inductive sensors come with the electronics fully integrated into the sensor.

One sensor technology isn’t enough since there isn’t a single technology that will work across all applications. It’s good to have options when looking for an application solution.

To learn more about these technologies, visit www.balluff.us

Are all capacitive sensors for liquid level detection created equal?

When standard capacitive sensors are used for liquid level detection in an indirect level detection application, the sensor must be adjusted to the point where it ignores the container wall but reliably detects the capacitance change caused by the changing liquid level. Typically, a standard capacitive sensor can be adjusted to disregard a wall thickness of approximately 4mm. In addition, the dielectric strength of the liquid must be higher than the container wall for reliable level detection.

Capacitive sensors detect any changes in their electrostatic sensing field. This includes not only the liquid itself, but also application-induced influences such as condensation, foaming, temporary or permanent material build-up. High viscosity fluids can cause extensive delays in the accurate point-level detection or cause complete failure due to the inability of a standard capacitive sensor to compensate for material adhering to the container walls.

A perfect capacitive sensor for non-invasive level detection applications would not require any user adjustment after the initial setup process. It would detect the true liquid level of any type of water-based liquid through any non-metallic type of tank wall while automatically compensating for material build-up, condensation, and foam. While ignoring these interferences, the sensors would still detect the relative change in capacitance caused by the liquid but use additional factors to evaluate the validity of the measurement taken before changing state.

These sensors would be fundamentally insensitive to any non-conductive material like plastic or glass up to 10mm thick, which would allow them to be utilized in non-invasive level applications. The enhanced capacitive sensors only limitation is it would require electrically conductive fluid materials with a dipole characteristic such as water to operate properly. This is a great concept but does a sensor like this exist today? The short answer is yes, and it is called SMARTLEVEL!

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The Forgotten 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 a bad experience almost 10 years ago. With the advancements of technology this is no longer the case.

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”Well Jack Me Up!” – Error Proofing a Car Jack Kit

Picture this scenario.   You, your spouse, or one of your kids happens to be riding one night in the middle of nowhere when a tire blows on the car.  First, we can only hope that your loved one remembered the lesson they received on how to change a flat tire in a pinch (if we gave it to them in the first place), because on this particular night, there’s no cell coverage where they’re at, AAA isn’t going to get to them very quickly, there isn’t a can of Flat Fix in the trunk, and there isn’t much traffic on the road they’re traveling on for a good Samaritan to likely show up any time soon (the scenario is extreme, but not impossible).  The jack kit sitting under the spare tire is going to seem pretty doggoned important, don’t you think?

We take a lot for granted these days and for those of us who have been involved in the world of factory automation for many years, getting to work with customers to help solve Error-Proofing challenges on the plant floor is like one big “Class Trip” every single day!   It’s kind of like providing our customers with “toys for adults”.  And it’s a real hoot.  We get to see how stuff is made, get the opportunity to help manufacturers build better products through our Error-Proofing sensing technologies and learn over time which end products to buy and which ones to shy away from!  We also quickly realize the extreme importance of the DETAIL!  Like the components in the emergency jack kit!  What if the main handle was missing when you or your relative went to jack up the car?  What if there wasn’t any grease on the main lift shaft threads and the car couldn’t be raised?  What if other parts were missing from the kit? Not a good scenario.

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