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How to Develop and Qualify Sensors for Arctic Conditions

The climatic conditions in the arctic are characterized by cold winters and short summers. There is a large variability in climate and weather: Some regions face permafrost and are ice covered year-round with temperatures down to -40°C / -40°F (and lower), other land areas face the extremes of solar radiation up to +30°C / +86°F in summer.

As oil and gas exploration, as well as renewable energy (e.g. cold climate versions of wind turbines) move into arctic areas, the need grows for sensors designed to deal with the extreme conditions and to perform reliably over their whole life cycle.

One option is the implementation of a Highly Accelerated Life Test (abbreviation HALT) in the development process. The basic idea of HALT is the accelerated aging of electronic products (including sealing gaskets, potting compound, housing etc.) with the aim of detecting their possible weak spots as early as possible and to correct them at the development stage.

Example of a sensor in the HALT test facility (Balluff magnetostrictive linear position sensor BTL)

The item under test is subjected to higher and higher thermal and mechanical stress in order to cause failures. The limits where the product will fail functionally or be destroyed are determined in order to push these limits as far out as possible, and so achieve a higher reliability for the product.

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Product operational specs = data sheet values

The HALT procedure – in brief:

a) Analysis of weaknesses already known, definition of failure criteria, establishing the stress factors

b) Stressing the test specimen beyond the specification to find the “upper and lower operating limits”, and the “upper and lower destruct limit” for temperature, rapid change of temperature, vibration, combined vibration and temperature stress

c) Determination of the causes of failure

d) Devising a solution to eliminate the weaknesses

e) Repeating of steps b) to c)

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Example: Temperature step test – cold and hot
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Example: Combined vibration test and rapid temperature changes
Cowan
Example: Cowan Dynamics E2H Electro-Hydraulic valve actuator Photo: Cowan Dynamics (Canada)

In contrast to other environmental tests, HALT is not qualification testing according to specific technical standards (as  ISO/IEC etc.), but it applies stimuli to the items under test until they fail, so weak spots will be revealed. A HALT test is not an exam you can pass!

However, if sensors are implemented into more complex automation systems that will be operated in remote areas, this method may help to prevent major faults in the field and is therefore also used in the aircraft and automotive industry.

For more information about Balluff testing methods and the laboratory, please visit www.balluff.com or download our brochure “The Balluff Testing Laboratory”.

 

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Difficulties Faced in Externally Mounted Magnetic Transducer Applications

In a previous blog post, we looked at the basic operating principle of magnetostriction and how it is applied in a linear position transducer. In this post we’re going to take a more in-depth look at this popular sensing technique and the factors that can contribute to its usability in transducer applications.

magnetostricive response
Figure 1: Magnetostrictive response (λ) as the absolute value of the Magnetic Field (H) changes.

As we’ve seen, the magnetostrictive effect is produced using a permanent magnet that is fixed either around or mounted some distance away from the ferromagnetic alloy that is being used. Now, if you’re like me and you’ve spent countless nights lying awake wondering how the effect of magnetostriction in a transducer is changed by the strength of the applied magnetic field and how that pertains to the output of a magnetostrictive transducer, then there isn’t need to look much further than this blog post. The strength of the magnetic field (which is directly related to distance through an inverse cubed relationship) projected onto the ferromagnetic material plays an incredibly critical role in the effect of magnetostriction. In Figure 1 you can see a depiction of the how the magnetostrictive effect varies at differing values of magnetic field strength.

Since magnetostriction does not behave perfectly linearly, some consideration must be taken into account when choosing a magnet and mounting system for your transducer. Potential problems occur if the magnet is mounted too far away from or too close to the transducer (thus producing either too much or too little magnetic field). But before we explore why too much or too little magnetic field is a problem, let’s compare it to the most desirable magnetic conditions.

Figure 2: Standard Application in which a magnet is in a fixed position therefore, allowing no variance in the strength of the magnetic field seen by the ferromagnetic material.

Ideally, the goal is to choose a magnet and mounting distance which can produce a magnetic field that will fall within the green region from Figure 1. In this region, the magnetostrictive effect can be approximated as linear, thus being used to produce a strong and consistently predictable output response. As for what actual magnetic field strength values fall within this range, that is almost entirely dependent on the properties of the ferromagnetic alloy that you are using. In a standard application, for instance, one in which a magnet and transducer is fixed inside of a hydraulic cylinder where the magnet is mounted in a fixed position relative to the transducer rod (as shown in Figure 2.), this magnetic field value will not vary so there is no reason to worry about how the magnetostrictive response could vary. The device is designed to operate in Figure 1’s green region.

Figure 3: Externally mounted magnets that are not fixed to the transducer typically need to have stronger magnetic properties for the field to reach the device. However, if it is too strong the magnetostrictive effect can saturate and cause a loss of output.

Problems can arise when dealing with externally mounted transducers in which the user has a significant amount more freedom in the selection of magnet and its mounting design. These instances could look incredibly different depending on the application. However, Figure 3 shows how this sort of design would come into play in a “floating” magnet application. At greater distances or when using weaker, non-standard magnets, the magnetic field will fall into the first red zone on Figure 1. In this region, the magnetic strength is too weak for a signal to be processed as an output by the transducer. This is primarily caused by the effect of magnetic hysteresis which is essentially a “charge up” zone and creates an exponential relationship in this zone, rather than the desired linear relationship. On the other hand, if you attempt to use a strong, rare-earth magnet too close to the transducer, it will produce too strong of a magnetic field and move beyond the green region into the second red zone. The problem here is that the magnetostriction will saturate and can potentially cause output loss. Also, similarly to the weaker magnetic field strength, the behavior cannot be approximated using the same linear relationship that we see in the green zone of Figure 1. As with the green region of the graph, the magnetic field strength values that lie in the red zones are dependent on the properties of the material as well as the response processing electronics of the transducer.

Due to these issues, there are many things to consider if you’re using an externally mounted transducer and choosing a non-standard mounting method in your application. You need to be careful when mounting a magnet too far away or using a rare-earth magnet too close to the transducer.

For more information visit www.balluff.us.

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Quick field replacement for linear sensor electronics

Micropulse Transducers BTL 7 Rod-style with Rapid Replacement Module
Micropulse Transducers BTL 7
Rod-style with Rapid Replacement Module

When maintenance technicians replace linear position sensors (also known as probes or wands) from hydraulic cylinders, it can leave a terrible mess, waste hydraulic oils, and expose the individual to harmful hot fluids.  Also, the change out process can expose the hydraulic system to unwanted contaminants. After the sensor replacement has been completed, there can also be more work yet to do during the outage such as replacing fluids and air-bleeding cylinders.

Hydraulic linear position sensors with field-replaceable electronics/sensing elements eliminate these concerns.  Such sensors, so-called Rapid Replacement Module (RRM) sensors, allow the “guts” of the sensor to be replaced, while the stainless steel pressure tube remains in the cylinder.  The hydraulic seal is never compromised.  That means that during the replacement process there is no danger of oil spillage and no need for environmental containment procedures. There is also no need to bleed air from the hydraulic system and no danger of dirt or wood debris entering the open hydraulic port. Finally, there is no danger of repair personnel getting burned by hot oil.

The RRM is an option for Balluff’s BTL7 Z/B Rod Series used in applications for the lumber industry, plastic injection and blow molding, tire and rubber manufacturing, stamping presses, die casting, and all types of automated machinery where a continuous, absolute position signal is required.  Applications in industries such as Oil & Gas and Process Control are especially critical when it comes to downtime.  For these applications, this Rapid Replacement Module capability is especially advantageous.

You can learn more about linear position sensors with hazardous area approvals, by visting http://www.balluff.com/local/us/products/sensors/magnetostrictive-linear-position-sensors/

The video below shows a demonstration of the Rapid Replacement Module in action.

 

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The Pressure to Step Up Performance

PressureSensorAppWe all know the roles of mechanical pressure gauges and switches. They either give us a visual indication of hydraulic pressure, or open or close a switch when reaching a certain threshold pressure. Electronic pressure transducers do the same, but more effectively and with a single component instead of two or three. Furthermore, an electronic pressure transducer provides more output variations, longer life, and greater accuracy.

Mechanical pressure gauges and switches still have their place in fluid power, but with more features and greater accuracy and life, transducers are being specified in a wide variety of applications. In an article recently published in Hydraulics and Pneumatics these applications and their conditions are discussed in greater detail. You can read the entire article on the Hydraulics and Pneumatics website.

 

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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”

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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.

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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

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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.
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2 Solutions for Preventing Catastrophic Metal Forming Events

Have you ever heard the phrase, “Nine pounds of stuff in a one pound bag?”, or otherwise known as the “Blivet Effect?”

I’ve recently experience this, actually four incidences in three different companies to be exact. It revolves the wrong shut height.  When the recipe in a press doesn’t match die dimensions, or when the die dimensions are estimated, some bad things can happen.

In all of these companies, stamping presses of various tonnage ratings were run with a die that was over shut height dimension (the first hit caused a kaboom!).  Dies were locked up so badly, that they had to be torched, cut, and/or mechanically coaxed out.  In all cases, it took several days for this process to take place, causing lost production and significant down time (not to mention the financial loss and aggravation for a multitude of employees).

In order to eliminate these situations, here are two off-the-shelf electronic solutions that can be installed:
Continue reading “2 Solutions for Preventing Catastrophic Metal Forming Events”

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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.

Continue reading “3 Steps to Choosing the Right In-Cylinder Position Sensor”