Measuring Distance: Should I Use Light or Sound?

Clear or transparent sensing targets can be a challenge but not an insurmountable one. Applications can detect or measure the amount of clear or transparent film on a roll or the level of a clear or transparent media, either liquid or solid.  The question for these applications becomes, do I use light or sound as a solution?

photoelectric.png
An application that measures the diameter of a roll of clear labels.

In an application that requires the measurement of the diameter of a roll of clear labels, there are a number of factors that need to be considered.  Are the labels and the backing clear?  Will the label transparency and the background transparency change?  Will the labels have printing on them?  All of these possibilities will affect which sensor should be used. Users should also ask how accurate or how much resolution is required.

Faced with this application, using ultrasonic sensors may be a better choice because of their ability to see targets regardless of color, possible printing on the label, transparency and surface texture or sheen.  Some or all of these variables could affect the performance of a photoelectric sensor.

Ultrasonic sensors emit a burst of short high frequency sound waves that propagate in a cone shape towards the target.  When the sound waves strike the target, they are bounced back to the sensor. The sensor then calculates the distance based on the time span from when the sound was emitted until the sound was received.

In some instances, and depending on the resolution required, a time of flight sensor may solve the above application. Time of Flight (TOF) sensors emit a pulsed light toward the target object. The light is then reflected back to the receiver. The elapsed time it takes for the light to return to the receiver is measured, thus determining the distance to the target. In this case, the surface finish and transparency may not be an issue.

Imagine trying to detect a clear piece of plastic going over a roll.  The photoelectric sensor could detect it either in a diffuse mode or with a retroreflective sensor designed for clear glass detection.  But what if the plastic characteristics can change frequently or if the surface flutters.  Again, the ultrasonic sensor may be a better choice and also may not require set up any time the material changes.

So what’s the best solution?  In the end, test the application with the worst case scenario.  A wide variety of sensors are available to solve these difficult applications, including photoelectric or ultrasonic. Both sensors have continuous analog and discrete outs.  For more information 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.

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

Meeting the Challenges of Precision Sensing: High Acceleration Machinery

Challenge: High Acceleration Machine Movement

Fundamental application problem: Anything mounted to the moving mechanism must be low mass

  • Added mass reduces acceleration capability of a given motor & drive system
  • Added mass increases motor and drive size requirements to meet acceleration specs, driving costs higher
  • Larger motors increase energy consumption, which makes the machine less competitive in the market
  • Any space taken up by sensors reduces space available for tooling and work-in-process
  • Conventional prox sensors and brackets are much too large and heavy to address these requirements

Solution: Incredibly miniaturized, self-contained inductive proximity sensors

  • Tiny size = inherently low mass
  • Correspondingly tiny mounting brackets = inherently low mass
  • Totally self-contained electronics = zero space taken up by separate amplifier
  • Miniaturization of sensors allows no-compromise installation in compact tooling
  • Additional tooling sensors enhance the level of high-end machine automation/control that can be achieved

Stay tuned to this space for more precision sensing challenges and solutions. Miniaturized sensors are also available in photoelectric, capacitive, magnetic cylinder, ultrasonic, and magnetic encoder. Click here to see the whole mini family.

GIZMOS

Plural of Giz-mo.  A noun.  Defined as a gadget, one whose name the speaker does not know.  Customers call us and ask for this or that “gizmo” all the time!  I think we should consider creating a product category simply called “GIZMOS”.

I like to call these things “Enablers” because these devices are very much helping hands that optimize the function of sensors.  A sensor of any brand and manufacturer performs only as well as it’s mounted, matching the fixture to the demands of the application at hand. But how often does this happen in a price-driven world?  They often end up in below-par mounting that fails with regularity, in both pristine environments as well as in hostile environments.  Some examples:

Here’s one example below. These inductive proximity sensors in plastic brackets, showing an exposed coil on one, with corroded mounts on the sensor caused by being beaten to death during parts loading and heat.

gizmo1      gizmo2

With a few “Gizmos” like an application-specific quick change mount, some care in gapping the sensor and guarding the cable/connector system, it could look much different. Check out the examples below.

gizmo4 gizmo5

Photoelectric sensors can suffer the same fate.  In this case, a plastic bodied photoelectric sensor, originally used to replace a fiber optic thru beam pair also suffered abuse. With a little extra beefy mounting, these photoelectric sensors can be expected to last a long time without failure.

gizmo6 gizmo7

There are literally hundreds of these mounting “ENABLERS”, off-the-shelf, cost-effective application specific mounts, guards, actuators and entire systems to help protect your sensor investment.  All categories of products have these “enabling” accessories for Magnetic Field (air cylinder), Inductive Proximity, Capacitive, Ultrasonic, Connectivity, Linear Transducer and Photoelectric product categories.

The Blind Zone – Understanding the Principles of Operation

When an application calls for an Ultrasonic sensor it is very important to understand the principles of operation. The most important question in sensor selection… what is the operating distance needed? How far away can I be from the target? Understating “The Blind Zone” will be the key to selecting the proper sensor for your application.

“The Blind Zone” is the shortest permissible sensing range. This means that no objects or targets are permitted within the minimum working area (“Blind Zone”) as this would false trigger the sensor. For example we have an application where we need to see our target at 80mm away. We could select a sensor that has an operating distance of 20….150mm. This means we can see our target down to a minimum distance of 20mm and a maximum range of 150mm. anything below the 20mm is the “Blind Zone” and out of our working range of 20…150mm.

The shortest permissible sensing range
This is determined by the blind zone of a sensor. No objects or are permitted within the blind zone, since this would cause faulty measurements or readings.

blindzone

So as you can see it is very important to understand your minimum working area and where the “Blind Zone” begins within the working range of the senor. If you have any questions on this topic or other questions on Ultrasonic sensor selection, please leave a comment below.

The 3-Tiered Position Sensing Hierarchy

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There are three general classes of position sensors that – taken together – form a position sensing hierarchy.  This hierarchy applies to any underlying sensing technology, for example inductive, capacitive, ultrasonic, or photoelectric.  Going from the most basic to the most advanced sensor operation, the hierarchy includes:

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