IO-Link Measurement Sensors Solve Application Challenges

In industrial distance and position measurement applications, one size definitely does not fit all.  Depending on the application, the position or distance to be measured can range from just a few millimeters up to dozens of meters.  No single industrial sensor technology is capable of meeting these diverse requirements.

Fortunately, machine builders, OEM’s and end-users can now choose from a wide variety of IO-Link distance and position measurement sensors to suit nearly any requirement.  In this article, we’ll do a quick rundown of some of the more popular IO-Link measurement sensor types.

(For more information about the advantages of IO-Link versus traditional analog measurement sensors, see the following blog posts, Solving Analog Integration Conundrum, Simplify Your Existing Analog Sensor Connection, and How Do I Make My Analog Sensor Less Complex?)

 

Short Range Inductive Distance Sensors

These sensors, available in tubular and blockScott Image1.JPG style form factors are used to measure very short distances, typically in the 1…5 mm range.  The operating principle is similar to a standard on/off inductive proximity sensor.  However, instead of discrete on/off operation, the distance from the face of the sensor to a steel target is expressed as a continuously variable value.  Their extremely small size makes them ideal for applications in confined spaces.

Inductive Linear Position Sensors

Inductive linear position sensors are available in several block style form factors, and are used for position measurement over stroke lengths up to about 135 mm.  These types of sensors use an array of inductive coils to accurately measure the position of a metal target.  Compact form factors and low stroke-to-overall length factor make them well suited for application with limited space.

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Magnetostrictive Linear Position Sensors

IO-Link Magnetostrictive linear position sensors are available in rod style form factors for hydraulic cylinder position feedback, and in external mount profile form factors for general factory automation position monitoring applications.  These sensors use time-proven, non-contact magnetostrictive technology to provide accurate, absolute position feedback over stroke lengths up to 4.8 meters.

Laser Optical Distance Sensors

 

Scott Image 4.JPGLaser distance sensors use either a time-of-flight measuring principle (for long range) or triangulation measuring principle (for shorter range) to precisely measure sensor to target distance from up to 6 meters away.  Laser distance sensors are especially useful in applications where the sensor must be located away from the target to be measured.

 

Magnetic Linear Encoders

IO-Link magnetic linear encoders use an absolute-codedScott Image 5 flexible magnet tape and a compact sensing head to provide extremely accurate position, absolute position feedback over stroke lengths up to 8 meters.  Flexible installation, compact overall size, and extremely fast response time make magnetic linear encoders an excellent choice for demanding, fast moving applications.

IO-Link Measurement Sensor Trends

The proliferation of available IO-Link measurement sensors is made possible, in large part, due to the implementation of IO-Link specification 1.1, which allows faster data transmission and parameter server functionality.  The higher data transfer speed is especially important for measurement sensors because continuous distance or position values require much more data compared to discrete on/off data.  The server parameter function allows device settings to be stored in the sensor and backed up in the IO-Link master.  That means that a sensor can be replaced, and all relevant settings can be downloaded from master to sensor automatically.

To learn about IO-Link in general and IO-Link measurement sensors in particular, visit www.balluff.com.

Measurement Fundamentals: Position Measurement vs. Distance Measurement

Continuous measurements on industrial machines or the materials that these machines are making, moving, or processing can be categorized into two main types of sensors:  position measurement sensors, and distance measurement sensors.  It’s a somewhat subtle distinction, but one that is important when evaluating the best measurement sensor for a particular application.

Position Measurement: When we speak in terms of position measurement, we’re typically talking about applications where a the sensor is installed onto a machine, and mechanically coupled to the moving part of the machine – or is installed into a hydraulic cylinder that is moving the machine – and is reporting the continuous position of the machine.  In a positioning application, the questions that need to be answered are: “Where is it?  Where is it now?  And now?”.

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Examples of position measurement sensors include magnetostrictive linear position sensors and magnetically encoded linear sensors.  With each of these sensor types, either the sensor itself, or the position marker, is typically attached to the moving part of the machine.

Distance Measurement: Distance measurement sensors, on the other hand, are used in applications that require accurate measurement of a target that is typically no part of the machine.  A good example would be an application where parts or components are moving along a conveyer belt, and the position of those parts needs to be accurately measured.  In this example, it wouldn’t be practical, or even possible, to attach a sensor to the moving part.  So its position needs to be measured from a DISTANCE.  In a distance measuring application, the question being answered is: “How far away is it?”.

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Examples of distance measuring sensors include photoelectric (laser) sensors and inductive distance sensors.  These types of sensors are usually mounted on the machine, or in the immediate vicinity of the machine, and are aimed at a point or a path where the object to be measured is, or will be, located.

In summary, while both position and distance sensors do much the same thing – provide continuous indication of position – the applications for each are generally quite different.  Gaining an understanding of the application and its requirements will help to determine which type of sensor is the best choice for the task.

For more information on position and distance measurement sensors, visit www.balluff.com.

Absolutely Incremental – Innovations in Magnetic Linear Encoder Technology

Linear encoders – absolute or incremental?  Incremental encoders are simple, inexpensive, and easy to implement, but they require that the machine be homed or moved to a reference position.  Absolute encoders don’t require homing, but they’re usually more expensive, and implementation is a bit more involved.  What if you could get an incremental encoder that also gave you absolute position?  Would that be great, or what?  Read on.

IncrementalEncodersIncremental encoders are pretty simple and straightforward.  They provide digital pulses, typically in A/B quadrature format, that represent relative position movement.  The number of pulses the encoder sends out correspond to the amount of position movement.  Count the pulses, do some simple math, you know how much movement has occurred from point A to point B.  But, here’s the thing, you don’t actually know where you are exactly.  You only know how far you’ve moved from where you started.  You’ve counted an increment of movement.  If you truly want to know where you are, you have to travel to a defined home or reference position and count continuously from that position.

AbsoluteEncodersAbsolute encoders, on the other hand, provide a unique output value everywhere along the linear travel, usually in the form of a serial data “word”.  Absolute encoders tell you exactly (absolutely) where they are at all times.  There’s no need to go establish a home or reference position.

So absolute is better, yes?  If that’s so, then why doesn’t everyone use them instead of incremental encoders?

It’s because incremental encoders typically cost a lot less, and are much easier to integrate.  In terms of controller hardware, all you need is a counter input to count the pulses.  That counter input could be integral to a PLC, or it could take the form of a dedicated high-speed counter module.  Either way, it’s a fairly inexpensive proposition.  And the programming to interpret the pulse count is pretty simple and straightforward as well.  An absolute encoder will usually require a dedicated motion module with a Synchronous Serial Interface (SSI, BiSS, etc.).  These interfaces are going to be both more expensive and more complex than a simple counter module.  Plus, the programming logic is going to be quite a bit more involved.

So, yes, being able to determine the absolute position of a moving axis is undoubtedly preferable.  But the barriers to entry are sometimes just too high.  An ideal solution would be one that combines the simplicity and lower cost of an incremental encoder with the ability to also provide absolute position.

Fortunately, such solutions do exist.  Magnetic linear encoders with a so-called Absolute Quadrature interface provide familiar A/B quadrature signals PLUS the ability to inform the controller of their exact, absolute position.  Absolute position can be provided either on-demand, or every time the sensor is powered up.

How is this possible?  It’s really quite ingenious. You could say that the Absolute Quadrature encoders are “absolute on the inside, and incremental on the outside”.  These encoders use absolute-coded magnetic tape, and the sensing head reads that position (with resolution as fine as 1 µmeter and at lengths up to 48-meters, by the way).  But, during normal operation, the sensor head outputs standard A/B quadrature signals.  Remember though, it actually knows exactly where it is (absolute inside…remember?), and can tell you if you ask.  When requested (or on power-up, if that’s how you have it configured), the sensor head sends out a string, or burst, of A/B pulses equal to the distance between the home position and the current position.  It’s as if you moved the axis back to home position, zeroed the counter, and then moved instantly back to current position.  But no actual machine movement is necessary.  The absolute burst happens in milliseconds.

So, to sum it up, Absolute Quadrature linear encoders provide a number of advantages:

  • Economical: Compatible with standard A/B incremental interfaces – no absolute controller needed
    • No need to upgrade hardware; can connect to existing control hardware
    • Get the advantages of absolute, but maintain the simplicity of incremental; eliminate the need for homing
  • Easy implementation: Simple setup, no (or very minimal) new programming required
  • Accurate: Resolution down to 1 µm, over lengths up to 48 meters

If you’d like to learn more about linear encoders with Absolute Quadrature, go to: http://www.balluff.com/local/us/news/product-news/bml-absolute-quadrature/

IO-Link Sensors in Tire Manufacturing

Much has been written here on Sensortech about IO-Link, and the advantages that an IO-Link-based architecture offers. In this article, we’ll take a look at a specific application where those IO-Link advantages are clear.

Tire manufacturing machinery in general, and tire curing presses in particular, incorporate numerous sensors and indicators that contribute to machine efficiency. As an example, tire curing presses often use magnetostrictive linear position sensors for feedback and control of mold open/close. Overwhelmingly, sensors that provide an analog, 4-20 mA signal are used. But maybe there’s a better alternative to typical analog feedback.

As discussed HERE and HERE, migration away from typical analog sensor signals to network-capable IO-Link interfaces makes a great deal of sense in many areas of application.

In a tire manufacturing operation, there are typically numerous, essentially identical curing presses, lined up in a row, all doing essentially the same job. Each press uses multiple analog position sensors that need each need to be connected to the press control system. As with pretty much analog device, the use of individual shielded cables is critical. Individual shielded cables for every sensor is a costly a time-consuming proposition. An Engineering Manager at a machine builder told us recently that wiring each press requires around 300 man hours(!), a significant portion of which is spent on sensor and indicator wiring.

Which brings us to IO-Link. Replacing those analog sensors with IO-Link sensors, allows feedback signals from multiple machines to be consolidated into single cable runs, and connected to the network, be it Ethernet/IP, EtherCat, Profinet, or Profibus. The benefits of such an approach are numerous:

  • Wiring is simple and much more economical
    • Eliminates need for shielded sensor cables
  • Integrated diagnostics allow remote machine status monitoring
  • Reduces more expensive analog IO on the controller side
  • Over-the-network configuration and the ability to store those configurations reduces setup time

And, by the way, the IO-Link story doesn’t end with position sensors. The ever-growing list of IO-Link enabled sensors and indicators allows the benefits to be rolled into many areas of machine automation, such as:

  • Intelligent IO-Link power supplies with HeartBeat technology that monitor their own “health” and report it back over the network (think Predictive Maintenance)
  • Highly-configurable IO-Link stack light alternatives that can be set up to display a number of machine and process condition states
  • IO blocks, memory modules, pressure sensors, discrete (on/off) sensors of all type, and more

To learn more about IO-Link, visit Balluff.com

IO-Link Hydraulic Cylinder Position Feedback

Ready for a better mousetrap?  Read on…..btl_io-link

Some time ago here on Sensortech, we discussed considerations for choosing the right in-cylinder position feedback sensor.  In that article, we said:

“…….Analog 0-10 Vdc or 4-20 mA interfaces probably make up 70-80% of all in-cylinder feedback in use…..”

And while that 70-80% analog figure is still not too far off, we’re starting to see those numbers decline, in favor a of newer, more capable interface for linear position feedback:  IO-Link.  Much has been written, here on Sensortech and elsewhere, about the advantages offered by IO-Link.  But until now, those advantages couldn’t necessarily be realized in the world of hydraulic cylinder position feedback.  That has all changed with the availability of in-cylinder, rod-style magnetostrictive linear position sensors.  Compared to more traditional analog interfaces, IO-Link offers some significant, tangible advantages for absolute position feedback in hydraulic cylinders.

Connectivity

First and foremost, the story of IO-Link is that it offers easy, simple connection of sensors and IO to nearly any industrial network.  You can read more about that here.

Simplicity

Another big advantage of IO-Link is the ability to connect sensors to the network using standard, simple, unshielded M12 connectors and cables.  Compared to analog systems, which require shielded cabling, and sometimes unusual or proprietary connectors, connecting IO-Link sensors to the network is simpler, and usually less costly.

Visibility

Unlike their traditional analog counterparts, position sensors with IO-Link offer built-in diagnostic capabilities.  Sensor status can be monitored over the network, greatly simplifying troubleshooting and fault detection.

Flexibility

This is where IO-Link position sensors really start to shine.  Traditional analog position sensors provide one thing: position feedback in the form of an analog signal (obviously).  IO-Link position sensors provide position feedback, of course…but wait, there’s more.  In addition to position feedback, IO-Link sensors can provide velocity/speed information, temperature, and differential position (the difference between two position magnets).  And the best part?  All of this functionality can be freely configured over the network.  Plus, sensor configurations can be stored and subsequently downloaded to a replacement sensor if necessary.

Suitability

It’s worthwhile to point out that IO-Link linear position sensors are ideal for most positioning or position monitoring applications.  Just as with analog sensors though, they’re probably not suitable for high-performance closed-loop servohydraulic motion control applications.  In those applications, interfaces that are capable of providing super-fast, deterministic data, such Synchronous Serial Interface (SSI) or even Ethernet/IP are more suitable.

To learn more visit www.balluff.us

You can also learn more in this overview flyer.

Linear Measurement Sensors for Short Stroke Applications

We’ve posted numerous articles here on the Sensortech blog about linear position sensors used for applications such as hydraulic cylinder position feedback, plastic injection molding machinery, tire manufacturing machinery, etc.  What all of the applications have in common is that we’re generally talking about fairly long linear travels, usually longer than 12″, sometimes up to 300″.spindle

But in applications such as spindle clamp positioning on machine tools or positioning of
linear movements on automated assembly machinery, travels are sometimes only a couple of inches, and the available space to mount a position sensor is extremely limited.  Fortunately, there are highly capable linear position sensors that are perfectly suited for such applications.

For example, there are sensors that use an array of inductive coils to detect the bips
precise linear position of a simple metal target.  These sensors, with working strokes ranging from < 1″ up to around 5″ have are extremely compact, with very little dead zone.  That means they fit into very tight spaces, where other type of linear position sensors simply couldn’t.

Typically, these types of sensors provide a position signal in the form of an analog voltage (0-10V) or current (4-20 mA).  Increasingly though, IO-Link interfaces are gaining in popularity, offering simplified wiring, better noise immunity, built-in diagnostics, and the ability to easily get the position data into virtually any industrial field-bus architecture.

For more information, visit www.balluff.com

External Linear Position Sensors: Floating or Captive Magnet?

External Linear Position Sensors:  Floating or Captive Magnet? 
PFMagnetsLinear position sensors that are designed to be mounted externally on a machine (as opposed to those designed to be installed into a hydraulic or pneumatic cylinder) are available in a variety of form factors that suit a variety of different applications and application requirements.  One of the most common form factors, particularly for magnetostrictive linear position sensors, is a rectilinear aluminum extrusion that houses the sensing element, or waveguide, and the processing electronics.  Commonly, you’ll hear these referred to as profile-style linear position sensors.

CaptiveVSFloating
Captive magnet (left) and floating magnet (right)

With these types of sensors, the moving part of the machine to be measured or monitored is attached to a position magnet.  The position magnet can be either captive or floating (see image to the right).  Each of these magnet configurations offer some inherent advantages.  We’re going to take a closer look at each.

Captive Magnet

A captive magnet glides along in a track that is an integral part of the extruded aluminum sensor housing.  The magnet is attached to the moving part of the machine via a mechanical linkage.  Advantages of a captive magnet arrangement include:

  • Mechanical flexibility: The magnet usually incorporates an articulating swivel or ball joint that is attached via a linking rod to the moving machine part.  That means the sensor doesn’t need to be perfectly in line with the axis of movement.
  • Protection from damage – In some cases, it is necessary to move the sensor out of harm’s way (e.g., extreme heat, caustic chemicals, strong electromagnetic fields, etc.). The linkage can be as long as necessary in order to connect to the sensor, which will be located in a more hospitable environment.

Some things to consider when choosing to use a captive magnet configuration:

  • Binding of the magnet: A high-quality magnetostrictive sensor is going have a near-zero drag coefficient between magnet and extrusion.  The magnet should not bind or drag.  But in some applications, dirt, grease and particulates can accumulate and cause issues.  For these applications, a floating magnet may be a better choice.
  • Mechanical overtravel:  In a captive magnet arrangement, if the machine travel exceeds the physical length of the sensor, the magnet will (of course) fall off the track.  If this is a concern, consider a floating magnet instead.

Floating Magnet

In a floating magnet arrangement, the sensor is located adjacent to the moving machine part.  The magnet is attached to that machine part, usually on a rigid arm or bracket.  Advantages of a floating magnet include:

  • No mechanical contact: The magnet never makes contact with the housing.  This could be important in applications where dirt, grease or particulates tend to collect on the sensor (see photo below)
    harshenvironment
  • Machine overtravel: Since the magnet is completely uncoupled from the sensor, machine overtravel isn’t a problem.  Obviously, if the magnet leaves the sensor, position feedback is lost, but the sensor will resume normal operation once the magnet re-enters the sensor’s range.

Some things to consider when choosing a floating magnet configuration:

Magnet-to-sensor gap:  In some cases the movement of the machine does not allow a consistent magnet-to-sensor gap to be maintained.  In some sensors, this can lead to inconsistent or erratic sensor operation.  Fortunately, there are sensors available with innovative technology that automatically compensates for such gap fluctuations and maintain full performance and specifications even as the gap varies.  Click below to see such technology in action.

Ultimately, the choice between a floating magnet and a captive magnet arrangement is going to be driven by the requirements of your particular application.

Click the link for more information on external-mount linear position sensors.

Benefits of Non-contact Linear Position Sensing Technology

Linear position sensors that provide continuous, typically analog, feedback are used extensively in a variety of applications in many different industries and markets.  Linear position sensors employ various technologies, but at the most basic level the technologies can be classified as being either non-contact or contact based.

For the purpose of this article, when we talk about contact based technology, the example we’re using is resistive linear potentiometers.  And for non-contact technology, we’re talking about magnetostrictive sensors.

In industrial linear position sensing applications, both ultimately do the same job; provide variable analog signals that represent the linear position of a machine or a process.  The difference is how the signal is derived.

Resistive linear potentiometers employ a resistive element upon which a spring-loaded contact rides:01_Potentiometers

The output of the sensor represents the position of the slider along the resistive element and typically ranges from 0-10Vdc or -10 to +10Vdc.  Out of the box, performance and accuracy is pretty good.  But after repeated cycles, wear can start to place that affects the connection between the resistive element and the contact.  The end result is signal anomalies and worsening performance over time, as can be seen in the image below.

02_WorseningPerformance

Other external factors, such as dirt and/or moisture only serve to accelerate this declining performance.

03_Waveguide

Non-contact technology, such as is incorporated into magnetostrictive linear position sensors, isn’t vulnerable to mechanical wear and subsequent performance degradation.

Unlike, resistive sensors, magnetostrictive sensors operate on the principle of magnetism.  Interacting magnetic fields define the output value, which changes as a moving magnet travels along a sensing element, called a waveguide.  There is no mechanical contact, so there is no mechanical wear.  The result is greatly enhanced life expectancy and consistently excellent performance

Cost Considerations

Generally, resistive linear position sensor cost a bit less than magnetostrictive sensors.  However, that doesn’t tell the whole story.  True cost of ownership has to be considered.  For a more complete discussion about cost of ownership, take a few minutes to review the Sensortech article The True Cost of Low Cost.

Hydraulic Cylinder Position Feedback, Revisited

In a previous Sensortech post entitled “Hydraulic Cylinder Position Feedback“, we discussed the basic concept of hydraulic cylinder position feedback.  In case you might have missed that post, here it is for an encore appearance.

Magnetostrictive linear position transducers are commonly used in conjunction with hydraulic cylinders to provide continuous, absolute position feedback.  Non-contact magnetostrictive technology assures dependable, trouble-free operation.  The brief video below illustrates how magnetostrictive position sensors are used with hydraulic cylinders.

Continue reading “Hydraulic Cylinder Position Feedback, Revisited”

Requirements for Sanitary Fill Level Sensors

In a previous entry here on the SensorTech blog, we discussed the concept of liquid level sensing, and the difference between discrete liquid level detection and continuous liquid level monitoring.  In this entry, we are going to talk about the requirements for liquid level sensors that are used to measure or monitor liquid products that will ultimately be consumed by humans.

In these applications, it is necessary and critical that sanitary standards be met and maintained.  Sensor designed for sanitary applications are usually designed from the ground up to meet these requirements.

Basically, there are two key criteria that come into play when considering the suitability of a sensor to be used in a sanitary environment:

  • Cleanability – Sanitary filling systems typically need to be regularly cleaned and/or sterilized to prevent the growth of potentially harmful bacteria. It is desirable in most cases that the cleaning/sterilization process be done as quickly and as easily as possible, without having to remove components (including sensors) from the system.  For this reason, many sanitary fill sensors are designed to withstand “cleaning-in-place” (CIP).  Factors such as water-tightness, and ability to withstand elevated cleaning solution temperatures come into play for CIP suitability.
  • Mechanical Sensor Design – Sensors for sanitary fill applications are usually designed such that there are no mechanical features that would allow liquid or debris to collect. Crevices, grooves, seams, etc. can all act as collection points for liquid, and can ultimately lead to contamination.  For this reason, sanitary sensors are designed without such features.  The physical make-up of the sensor surface is also important.  Exterior surfaces need to be very smooth and non-reactive (e.g. high-grade stainless steel).  Such materials also contribute to cleanability.

Consistent standards for sanitary equipment, products, and processes are defined and maintained by 3-A SSI, a not-for-profit entity that provides consistent, controlled, and documented standards and certifications for manufacturers and users of sanitary equipment, particularly in the food, beverage, and pharmaceutical industries.  Equipment that meets these sanitary standards will usually display the 3-A symbol. For more information on this solution visit the Balluff website.