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.

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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Is Your Spidey Sense Tingling?

The July 2011 issue of Hydraulics and Pneumatics magazine featured an interesting application story about how hydraulics systems were designed and used in the “Spiderman: Turn off the Dark” Broadway musical.  The article describes some of the challenges faced by motion systems designers, and how those challenges were solved.

One particularly challenging aspect of the hydraulic motion systems was the requirement that multiple hydraulically driven platforms had to be raised and lowered simultaneously.  The motion of the platforms had to be very precisely controlled, making hydraulic component selection critical.

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The Power of 2

In numerous types of analog position sensors, resolution is expressed in terms of bits, e.g, 8-bit, 12-bit, 16-bit, etc.  But what does that really mean?  In a previous entry, I discussed what I called Digitally Derived Analog Signals, which provides a basic overview of how Digital-to-Analog Converters (DAC’s) are used to generate analog sensor signals.  You may recall from that entry that when someone says a sensor has “16-bit resolution”, what they really mean is that the sensor employs a 16-bit DAC, which is capable of processing 216 discrete values, and representing any one of those values as a corresponding analog signal.

To help better understand what these binary numbers actually mean, I thought it might be helpful to provide a quick-reference chart showing the equivalent decimal values of numbers from 20 to 232.

(click to enlarge)The values in bold represent some of the more commonly used DAC’s for industrial sensors.

Thinking Outside the Cylinder

In a previous entry, I discussed how linear position sensors are used with hydraulic cylinders to provide continuous position feedback.  While this is certainly one of the most common ways linear position sensors are used, there are many applications for linear position sensors that either don’t involve a hydraulic cylinder at all, or that involve a cylinder only indirectly.

Linear position sensors for external use (not installed into a hydraulic cylinder) offer some very tangible benefits when compared to in-cylinder sensors.  Let’s explore a few of those benefits:

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Linear Transducer Installation Considerations


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In my last entry, I talked about using a magnetostrictive linear position sensor in a hydraulic cylinder.  I received a few questions about that application, and I wanted to take this opportunity to answer one of them.

Q.  Why is it necessary to use a non-ferrous spacer to attach the magnet ring to the face of the hydraulic cylinder’s piston? 

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High Precision Positioning with Electro-Hydraulic Motion Control

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Peter Nachtwey of Delta Computer Systems has written an excellent primer on electro-hydraulic motion control.   In addition to many design and component selection tips, he highlights the benefits of magnetostrictive linear displacement transducers (MLDT) for position feedback to the controller.  Check out the article in the July edition of Design World online, called “A Second Look at Electro-Hydraulic Motion Control Systems.”

Hydraulic Cylinder Position Feedback

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Today, we’re going back to basics with one of the most common applications for linear position sensors: hydraulic cylinder position feedback.

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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