In-Cylinder Position Sensing in Electrically Conductive Hydraulic Fluids

The standard for hydraulic fluid in the industry is mineral oil, which is a dielectric medium that does not conduct electricity. Yet environmental concerns have led to the search for alternatives that are less harmful in case of leaks and spills. One development is biodegradable oils, typically with biological origins, often called “bio-oils” for short. They behave in many ways like mineral oil with a key difference in that they can be electrically conductive.

Another alternative hydraulic fluid is water-glycol mixtures, commonly known as the anti-freeze found in your liquid-cooled automobile engine. Water-glycol solutions are used for several reasons, including environmental concerns but more often conditions of extreme heat or extreme cold. They have much lower viscosity than oil, and there are several fluid power application considerations as a result, but water-glycol mixtures, like bio-oils, are electrically conductive.

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So, when it comes to cylinder position sensing, why should we care whether or not the hydraulic fluid is electrically conductive? Well, because it could come back to bite us if we put an incompatible position sensing technology into a cylinder that is filled with a conductive fluid.

I recently met an engineer who’d run into this exact situation. A hydraulic cylinder was ordered from the manufacturer with an “integrated position feedback sensor.” The feedback sensor turned out to be a resistive potentiometric type, in other words, a linear potentiometer or “pot.” The entire length of the resistive material is “wetted” inside the cylinder, along with the traveling “wiper” that moves with the piston. In typical applications with non-conductive, mineral-based hydraulic fluid, this works fine (although linear pots do tend to be somewhat fragile and do wear out over time). However, when the resistive material and wiper is wetted in a conductive liquid, all kinds of wrong start happening. The signal becomes very erratic, unstable, and lacks resolution and repeatability. This is because the fluid is basically short-circuiting the operation of the open-element linear potentiometer.

This caused quite a headache for the engineer’s customer and subsequently for the engineer. Fortunately, a replacement cylinder was ordered, this time with a non-contact magnetostrictive linear position sensor. The magnetostrictive sensor is supplied with a pressure-rated, protective stainless steel tube that isolates the electrical sensing element from the hydraulic medium. The position marker is a magnet instead of a wiper, which the sensor can detect through the walls of the stainless steel pressure tube. So, a magnetostrictive sensor is absolutely unaffected by the electrical properties of the hydraulic medium.

A magnetostrictive linear position sensor carries a lot of performance and application advantages over linear pots that make them a superior technology in most applications, but when it comes to conductive hydraulic fluids they are definitely the preferred choice.

To learn more about linear position sensors visit www.balluff.com.

Harsh Industrial Environments Challenge Plant Operators

Most industrial processes do not take place in a climate-controlled laboratory or clean room environment. Real-world industrial activity generates or takes place under harsh conditions that can damage or shorten the life expectancy of equipment, especially electronic sensors.

A cross-section of industrial users was surveyed about operating conditions in their facilities. The responses revealed that plant operators are challenged by a variety of difficult environmental factors, the biggest being heat, dust/dirt/water contamination, vibration, and extreme temperature swings.

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Over one-third of the industrial users surveyed reported that premature sensor failure is a problem in their operations. That is a surprisingly high percentage and something that needs to be addressed to restore lost productivity and maintain long-term competitiveness.

Many heavy industries are dependent on automated hydraulic cylinders to move and control large loads precisely. The cylinder position sensors are often subjected to damaging environmental conditions that shorten their life expectancy, leading to premature failure.

Fortunately, there are measures that can be taken to reduce or eliminate the occurrence of sensor-related downtime. Help is available in the form of a free white paper from Balluff called “Improving the Reliability of Hydraulic Cylinder Position Sensors”.

To learn more about this topic you can also visit www.balluff.us.

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.

 

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.

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

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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.