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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.

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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.

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Linear Position Sensors for Valve Actuators

Illustration of Magnetostrictive Linear Displacement Transducer (MLDT) inserted into a gun-drilled cylinder.Today’s petrochemical and process industries, like most industries, are striving to increase their capabilities of automation & control, coupled with condition monitoring, across their entire operation.  Demands for uptime are increasing and the focus on reliability through redundancy and prediction of pending maintenance requires new control and monitoring strategies.  This is nowhere more true than in the case of the sophisticated valves that form the most critical elements of the operation or process.  Operational readiness and confirmation of operation for these valves are indispensable to assure smoother and uninterrupted production…and safety.

Christian Dow has written an interesting article in Valve Magazine that highlights the benefits of linear position sensors when installed in the hydraulic actuators of these valves.  The benefits mentioned in the article don’t apply just for valves, though.  Many of the advantages can be obtained for almost any application where a hydraulic cylinder is the prime mover.

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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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Cable Length for Analog Sensors

A question came in recently concerning the maximum recommended cable length for analog sensors.  Even as digital interfaces gain popularity, sensors with analog interfaces (0-10V, 4-20 mA, etc.) still represent the overwhelming majority of continuous position sensors used in industrial applications.

The question about maximum cable length for analog sensors comes up pretty frequently.  Generally speaking, the issue is that electrical conductors, even good ones, have some resistance to the flow of current (signals).  If the resistance of the conductor (the cable) gets high enough, the sensor’s signal can be degraded to the point where accuracy suffers, or even to the point where it becomes unusable.  Unfortunately, there is no hard and fast answer to the question.  Variables such as wire gauge, whether or not the cable is shielded, where and how the cable is routed, what other types of devices are nearby, and other factors come into play, and need to be considered.  A discussion about all of these variables could fill a book, but we can make some general recommendations:

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E = IR: It’s Not Just a Good Idea, it’s the Law

I recently had a conversation with a customer that resulted in one of those forehead-slapping “duh” moments for me, and I thought it might be worth passing along. Here’s the story:

The customer had an application that required an analog linear feedback sensor that provided an output of 1 volt to 5 volts over the linear stroke range. Now, a 1-5V output is not very common, and the particular sensor he was interested in was only available with either a 0-10V or a 4-20 mA output. What to do? Perhaps the answer should have been obvious to me, but it was the customer who provided the solution this time: “couldn’t I use a 4-20 mA output and 250 ohm resistor to get my 1-5V output?” Why, yes….yes you could (smack…..duh!). And I know it will work, because we have the law on our side. Ohm’s Law, that is: E = IR, or voltage equals current x resistance.

Let’s check it:

4 (mA) x 250 (ohms) = 1 (volt)

20 (mA) x 250 (ohms) = 5 (volts)

So there you have it. Take a very common 4-20 mA output and drop it across a 250 ohm resistor and, lo and behold, you have your less common 1-5V signal. And, if you do this conversion right at the input to the controller, you get the added benefit of increased noise immunity of the 4-20 mA signal.

And, yes, I’m sure I knew of this little trick at one time. Maybe the part of my brain where this information was stored got overwritten by the names of the contestants on The Amazing Race or by the rollout plans for my million dollar consumer product idea: Dehydrated Water (just add water). But let’s keep that just between us, ok?

To learn more about analog feedback sensors visit www.balluff.us

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Intelligent Interfaces and IO-Link Innovation

I recently had the opportunity to attend Hannover Fair in Germany and was blown away by the experience… buildings upon buildings of automation companies doing amazing things and helping us build our products faster, smarter and cheaper.  One shining topic for me at the fair was the continued growth of new products being developed with IO-Link communications in them.

All in all, the growth of IO-Link products is being driven by the need of customers to know more about their facility, their process and their production.  IO-Link devices are intelligent and utilize a master device to communicate their specific information over an industrial network back to the controller.  To learn more about IO-Link, read my previous entry, 5 Things You Need to Know about IO-Link.

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Linear Position Sensor Terminology

Hysteresis, resolution, repeatability, non-linearity, null-point, temperature coefficient, accuracy.  These are but a handful of the many terms associated with linear position sensors.  To the uninitiated, it can be rather daunting.  And, unfortunately, there is a lot of room for ambiguity and confusion.

For example, let’s take a look at the term “accuracy”, as in “how accurate is this this linear position sensor?”  It seems like a fairly straightforward
question, right?  But in reality, it’s not that simple.   Whenever I get asked that question, my response is “what do you mean by accuracy?”  To which, I usually get a response something like “what do you mean what do I mean by accuracy?”  The fact is that the term “accuracy” means different things to different people.   The person asking the question may want to know the absolute straight-line, absolute positional accuracy (non-linearity) of the sensor.  Or, they may be referring to how accurately the sensor can repeat the same indicated value at the same position over subsequent moves (repeatability).  Or, perhaps what they’re really interested in is the smallest amount of position change that the sensor can detect (resolution).  So, as you can see, it’s not a simple question after all.

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