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.

Solve Difficult Sensing Applications with Ultrasonic Technology

When reviewing or approaching an application, we all know that the correct sensor technology plays a key role in reliable detection of production parts or even machine positioning. In many cases, application engineers choose photoelectric sensors Image1as their go-to solution, as they seem more common and familiar. Photoelectric sensors are solid performers in a variety of applications, but they can run into limitations under certain conditions. In these circumstances, considering an ultrasonic sensor could provide a solid solution.

An ultrasonic sensor operates by emitting ultra-high-frequency sound waves. The sensor monitors the distance to the target by measuring the elapsed time between the emitted and returned sound waves.

Ultrasonic sensors are not affected by color, like photoelectric sensors sometimes are. Therefore, if the target is black in color or transparent, the ultrasonic sensor can still provide a reliable detection output where the photoelectric sensor may not. I was recently approached with an application where a Image2customer needed to detect a few features on a metal angle iron. The customer was using a laser photoelectric sensor with analog feedback measurement, however the results were not consistent or repeatable as the laser would simply pick up every imperfection that was present on the angle iron. This is where the ultrasonic sensors came in, providing a larger detection range that was unaffected by surface characteristics of the irregular target. This provided a much more stable output signal, allowing the customer to reliably detect and error-proof the angle iron application. With the customer switching to ultrasonic sensors in this particular application, they now have better quality control and reduced downtime.
Image3

So when approaching any application, keep in mind that there is a variety of sensor technologies available, and some will provide better results than others in a given situation. Ultrasonic sensors are indeed an excellent choice when applied correctly. They can measure fill level, stack height, web sag, or simply monitor the presence of a target or object. They can also perform reliably in foggy or dusty areas where optical-based technologies sometimes fall short.

For more information on ultrasonic and photoelectric sensors visit www.balluff.com.

Level Sensing in Machine Tools

Certainly the main focus in machine tools is on metal cutting or metal forming processes.

To achieve optimum results in cutting processes coolants and lubricants are applied. In both metal cutting and metal forming processes hydraulic equipment is used (as hydraulics create high forces in compact designs). For coolant, lubricant and hydraulic tanks the usage of level sensors to monitor the tank level of these liquids is required.

Point Level Sensing

For point level sensing (switching output) in many cases capacitive sensors are used. These sensors detect the change of the relative electric permittivity (typically a change of factor 10 from gas to liquid). The capacitive sensors may be mounted at the outside of the tank wall if the tank material is non metallic like e.g. plastic or glass. The installation may even be in retrofit applications yet limited to non metallic tanks up to a certain wall thickness.

When using metal tanks the capacitive sensors enter the inner area of the tank via a thread and a sealing component. Common thread sizes are: M12x1, M18x1, M30x1,5, G 1/4″, NPT 1/4″ etc. For conductive liquids specially designed capacitive level sensors may be used which ignore build up at the sensing surface.

Continuous Level Sensing

Advanced process control uses continuous level sensing principles. The continuous sensor signals e.g. 0..10V, 4…20mA or increasingly IO-Link deliver more information to better control the liquid level, especially relevant in dynamic or precise applications.

When using floats the magnetostrictive sensing principle offers very high resolution of the level value. Tank heights vary from typically 200 mm up to several meters. Another advantage of this sensor principle is the high update rate (supporting fast closed loop systems for level sensing)

In many applications the  requirements for the level control solutions are not too demanding. In these cases the ultrasonic principle has gained significant market share within the last years. Ultrasonic sensors do not need a float, installation on the top of the tank is pretty easy, there are even sensor types available which may be used in pressurized tanks (typically up to 6 bar). As ultrasonic sensors quite often are used in special applications, field tests during the design in process are recommended.

Finally hydrostatic pressure transducers are an option for level sensing when using non pressurized tanks (typically  connected to ambient pressure through a bore in the upper area of the tank). With the sensor mounted at the bottom of the tank the level is indirectly measured through the pressure of the liquid column above the sensor (e.g. 10m of water level resembles 1 bar).

Summary

Concerning level sensing in metalworking applications in the first step it should be decided whether point level sensing is sufficient or continuous level sensing is required. Having chosen continuous level sensing there are several sensor principles available (selection depending on the application needs and features of the liquids and tank properties). It is always a good engineering practice to prove the preselected sensing concept with field tests.

To learn more 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.

Liquid Level Sensing: Detect or Monitor?

Pages upon pages of information could be devoted to exploring the various products and technologies used for liquid level sensing and monitoring.  But we’re not going to do that in this article.  Instead, as a starting point, we’re going to provide a brief overview of the concepts of discrete (or point) level detection and continuous position sensing.

 Discrete (or Point) Level Detection

Example of discrete sensors used to detect tank level
Example of discrete sensors used to detect tank level

In many applications, the level in a tank or vessel doesn’t need to be absolutely known.  Instead, we just need to be able to determine if the level inside the tank is here or there.  Is it nearly full, or is it nearly empty?  When it’s nearly full, STOP the pump that pumps more liquid into the tank.  When it’s nearly empty, START the pump that pumps liquid into the tank.

This is discrete, or point, level detection.  Products and technologies used for point level detection are varied and diverse, but typical technologies include, capacitive, optical, and magnetic sensors.  These sensors could live inside the tank outside the tank.  Each of these technologies has its own strengths and weaknesses, depending on the specific application requirements.  Again, that’s a topic for another day.

In practice, there may be more than just two (empty and full) detection points.  Additional point detection sensors could be used, for example, to detect ¼ full, ½ full, ¾ full, etc.  But at some point, adding more detection points stops making sense.  This is where continuous level sensing comes into play.

Continuous Level Sensing

Example of in-tank continuous level sensor
Example of in-tank continuous level sensor

If more precise information about level in the tank is needed, sensors that provide precise, continuous feedback – from empty to full, and everywhere in between – can be used.  This is continuous level sensing.

In some cases, not only does the level need to be known continuously, but it needs to be known with extremely high precision, as is the case with many dispensing applications.  In these applications, the changing level in the tank corresponds to the amount of liquid pumped out of the tank, which needs to be precisely measured.

Again, various technologies and form factors are employed for continuous level sensing applications.  Commonly-used continuous position sensing technologies include ultrasonic, sonic, and magnetostrictive.  The correct technology is the one that satisfies the application requirements, including form factor, whether it can be inside the tank, and what level of precision is needed.

At the end of the day, every application is different, but there is most likely a sensor that’s up for the task.

Ultrasonic Sensor Reflection Targets

In my previous posts (Ultrasonic Sensors with Analog Output, Error-proofing in Window Mode, and The Other Retro-Reflective Sensors) we covered the Ultrasonic sensor modes and how they benefit in many different types of applications. It is also important to understand the reflection properties of various materials and how they interface with the sensor selected. For example some Photoelectric sensors will have a very difficult time detecting clear materials such as glass or clear films as they will simply detect directly through the clear material detecting what is on the other side giving a false positive target reading. As we know, this is not an issue with an Ultrasonic sensor as they detect targets via a sound wave so clear objects do not affect the sensors function. When looking at sensor technologies it is import to understand the material target before selecting the correct sensor for the applied application such as an Inductive sensor would be selected if we are looking at a ferrous (metal) target at short range. Below are some examples of good and poor reflective materials when Ultrasonic sensors are used.

Good Reflective MaterialsUltrasonicApplication

  • Water
  • Paint
  • Wood
  • Metal
  • Plastic
  • Concrete/Stone
  • Glass
  • Hard Rubber
  • Hard Foam

Challenging Relective Materials

  • Cotton Wool
  • Soft Carpet
  • Soap Foams
  • Powders With Air
  • Soft Foam
  • Soft Rubber

So as you can see materials that are hard or solid have good reflective properties whereas soft materials will absorb the sound wave provided from the sensor making it much more challenging to detect our target. For more information on Ultrasonic sensors click here.

Ultrasonic Sensors with Analog Output

Many times in an application we need more than a simple discrete on/off output. For a more accurate detection mode we can utilize analog outputs to monitor position, height, fill-levels and part presence typically found in object detection assemblies. When utilizing Ultrasonic sensors with an analog output we can simply measure the distance value that is proportional to the distance of our target within the operating range of the sensor. Typically 0…10V or 4…20mA outputs are available with the option of rising or falling characteristics. Rising and falling is a way to invert the view of the output, so 0…10V would simply be inverted to 10…0V or 4…20mA would be 20…4mA.

Ultrasonic sensor offerings are a great alternative as they can deal with difficult targets that are typically a challenge for other sensor technologies. They also offer very good resolution with the options of long and short range detection. Below is an example of a 4…20mA linear output. As you can see the closer our target gets to the sensor face it indicates an output closer to 4mA and the further away from the sensor it will provide and output closer to 20mA. For more information on Ultrasonic sensors, click here.

AnalogUltrasonic

Error-proofing in Window Mode – Ultrasonic Sensing

In my previous post, I talked about Ultrasonic Sensors utilizing reflected mode. Window mode is an extension of the reflective mode setting. When the sensor is set up in window mode the sensor is only activated when the object target is located within the detection limit setting defined within the detection range of the sensor. So, for example a sensor that has 65…350mm operating range could be set up to see a target within 100…150mm. This mode is commonly used when target sizes require wider tolerances or size such as detecting tall to short targets or simply detecting the correct size of the target. Additionally, if the sensor selected offers an Analog output you can receive output within the new defined window.

Continue reading “Error-proofing in Window Mode – Ultrasonic Sensing”

The Other Retro-Reflective Sensors

Most of the time when we think of Retro-Reflective sensors the first thing that comes to mind is a photoelectric sensor. Photoelectric offerings use a reflector to reflect light from the internally mounted emitter and receiver. Retro photoelectric sensors come in many form factors with light source options such as infrared, red light and laser types.

Ultrasonic sensors are commonly forgotten when reflective sensors are needed in a particular application. Ultrasonic sensors when set up in “Window Mode” are similar to a photoelectric sensor however the ultrasonic sensor can use an existing background as the reflective surface such as a metal plate or a solid background. The sensor simply returns a signal as soon as an object fully covers the reflector. This mode is ideal for detecting difficult targets that photoelectric sensor can have trouble with such as poorly reflective materials

ultrasonicThe example shows an Ultrasonic sensor set up in window mode. The sensor is sending a sound wave to the background (reflector) so a target can be detected when entering the detection area between the sensor and the reflector background.

For more information Ultrasonic sensors, click here.

Level Detection with Ultrasonic Sensors

Liquid measurement can be a demanding application therefore selecting the most effective sensor technology is imperative. Ultrasonic sensors are the ideal solution for distance measurement or position detection of granules, fluids and powders. They measure fill levels, heights and sag without making contact as well as count and monitor the presence of objects. They are extremely versatile, operate independently of color and surface finish, and are not affected by transparent objects that generate strong reflections. And because they are not affected by dust, dirt and steam, they are the ideal choice for critical applications.

Ultrasonic Sensors operate with propagation of sound waves providing a reliable detection source for level detection applications where liquid measurement monitoring is necessary. They will enhance the flexibility of the application with additional advantages of being a non-contact sensor so there are no mechanical floats or arms that can retain residual liquid build up. This residual material can cause machine application downtime as interruption of production is needed to maintain and clean the mechanical monitoring system. Because the distance to the object is determined via a sound transit time, ultrasonic sensors have excellent background suppression. With their transit time measurement, ultrasonic sensors can record the measured value with highly precise resolution (some sensors to even 0.025 mm). Ultrasonic Sensors can monitor
and detect nearly any liquid.

Level Detection with Ultrasonic Sensors2Level Detection with Ultrasonic SensorsAn ultrasonic sensor is not affected by the color, transparency or glossiness of a surface. They can reliably detect bulk materials, liquids and material surfaces. Good reflective materials include:

  • „„ Water
  • „„ Paint/varnish
  • Wood
  • Metal
  • Plastic
  • Stone/concrete
  • Glass
  • Hard foam rubber

Additional advantages ultrasonic sensors provide when monitoring liquid is their ability to have two independent outputs programmed into one sensor. This is a great way to monitor upper and lower liquid levels with one sensor. This sensor feature allows one independent output setting to indicate a minimum level and the second output to indicate a maximum level concurrently providing constant feedback along with a voltage or current output.