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.

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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.

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.

Open- vs. Closed-loop Control

Several previous articles here on SENSORTECH have mentioned closed-loop control (Servo-Hydraulic Showcase, Linear Feedback Sensor Applications: The Three M’s). But exactly what does “closed-loop control” mean? How does it compare to open-loop control? I recently ran across an article in Control Engineering magazine that does an outstanding job of answering those questions.

Click over and have a look at this excellent article.

Servo-Hydraulic Showcase

48959254_woodbannerIn a previous installment here on SENSORTECH, we explored the three M’s of linear position feedback application (Linear Feedback Sensors – The Three M’s).  One of those three M’s stands for Motion Control.  When we talk about motion control applications for industrial linear position sensors, we’re often referring to closed-loop servo-hydraulics.  In these applications, the linear position sensor, which is usually installed into a hydraulic cylinder, plays a key role in the ability to accurately and reliably control the motion of very large, heavy loads.

Nowhere is closed-loop servo hydraulics more prominently utilized than in primary wood processing – where raw logs are transformed into all manner of finished board lumber.  Applications such as saws, edgers, planers, along with many more, rely heavily on closed-loop servo-hydraulics.  In many cases, hydraulic actuators get the job done when other types -electric, pneumatic – simply can’t.

If you’d like to get a look at some of these application, or to learn more about how linear positions sensors are used in the applications, a good place to start would be at an event where many of the machinery builders and suppliers gather in one place for a few days.  Does such an event exist? (I hear you asking).

Well of course it does!  It just so happens this very thing will be taking place in Portland, OR in the middle of October 2014.  If you would like to learn more about these interesting applications in general, and how linear position sensors are used in particular, you might want visit Balluff at the Timber Processing and Energy Expo.  Click the link below for more information.

Timber Processing and Energy Expo, October 15th through October 17th

Improved Feedback for Valve Actuators

Here on SensorTech, we frequently talk about the need for high performance, rugged, reliable position feedback in modern industrial applications. A recent article in Valve Magazine, entitled “The Case for Magnetostrictive” illustrates how linear feedback transducers using non-contact magnetostrictive technology help to improve the performance and reliability of valve actuators used in the petrochemical industry.

It’s worth a read. See a variety of linear feedback transducers here.

Illustration of Magnetostrictive Linear Displacement Transducer (MLDT) inserted into a gun-drilled cylinder.
Illustration of Magnetostrictive Linear Displacement Transducer (MLDT) inserted into a gun-drilled cylinder.

Position Monitoring with EtherCAT

Much has been written here on SensorTech about the value of industrial networking in the machine automation realm.  As the trend towards industrial networking continues to expand, we see more and more network-capable sensors coming to the fore.  Linear position sensors are no exception.

Network-connected linear position sensors take the concept of continuous, absolute linear position feedback a step or two forward by allowing the position sensor to be directly connected to the network, and also providing additional information in the form of sensor-level diagnostics.

Two such examples of network-connected linear position sensors are the newly introduced Micropulse EtherCAT position transducers.

Available in two varieties, one for basic position monitoring, and one capable of closed-loop positioning tasks, the Micropulse EtherCAT transducer is a good example of the continuing evolution of basic sensors towards more “intelligent” network-capable sensors.

For more information on industrial networking products, start here.

Linear Feedback Sensor Applications: The Three M’s

Applications for linear feedback sensors are numerous and varied.  Likewise, linear feedback sensors are available in numerous form factors and with a wide variety of performance characteristics.  Matching your application to the most appropriate sensor can be a daunting proposition.

When choosing the right linear feedback sensor, it is helpful to first define the job the sensor is being tasked to do.  One way to do this is to think in terms of the three M’s: (M)easuring, (M)onitoring, and (M)otion control.  Linear feedback sensor characteristics that are critical for one of these jobs may not matter as much for another job.  We’re going to take a look at each of these jobs and discuss some of the more important linear feedback selection criteria associated with each.

Measuring:  In measuring applications, the linear feedback sensor is asked to perform the job of an “electronic ruler”.  That is, the sensor is a measuring device used to gauge the size (length, width, thickness, etc.) of the part being produced or processed.  Examples of measuring applications include cut-off saws, or any other cut-length applications.  In such applications, it is absolutely critical for the feedback sensor to provide 1) high accuracy (low non-linearity), and 2) fine resolution. Other factors, such as a fast update rates and highly rugged enclosures are typically not as important in most measuring applications.

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3 Steps to Choosing the Right In-Cylinder Position Sensor

I recently ran across an interesting article that explored some of the factors involved in selecting hydraulic cylinders.  The article, entitled “3 Steps to Choosing the Right Hydraulic Cylinder” was very informative and helpful.  But what if you need a “smart cylinder”, i.e. a cylinder that can provide absolute position feedback?  Just as it’s important to select the proper cylinder to match the mechanical requirements of your application, it’s also important to select the right sensor to meet the electrical requirements.

So, to that end, I’d like to piggyback on the cylinder selection article with this one, which will look at 3 steps to choosing the right in-cylinder position sensor.  In particular, I’ll be talking about rod-type magnetostrictive linear position sensors that are designed to be installed into industrial hydraulic cylinders to provide absolute position feedback.

Before we get to step 1, let’s talk about the cylinder itself.  So-called smart cylinders are typically prepped by the cylinder manufacturer to accept a magnetostrictive position transducer.  Prepping consists of gun drilling the cylinder rod, machining a port on the endcap, and installing a magnet on the face of the piston.  For more information about smart cylinders, consult with your cylinder supplier.

Step 1 – Choose the Required Stroke Length

The stroke length of the position sensor usually matches the stroke length of the cylinder.  When specifying a position sensor, you usually call out the working electrical stroke.  Although the overall physical length of the sensor is going to be longer than the working electrical stroke, this is usually not a concern because the cylinder manufacturer accounts for this added length when prepping the cylinder.

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