Two important considerations when looking at the integration of any hydraulic linear position sensor are the underlying technology and the packaging of the transducer. These principles effect size, life, maintainability, and above all, durability.

The underlying technology used influences the linearity, repeatability, resolution, temperature immunity, maximum stroke length, emi immunity, and ability to achieve safety certifications. Things like signal conditioning are also affected and the need for sensitive, complicated electronics local to the sensor must be considered a drawback in most cases.

With respect to packaging, a transducer’s internal and external construction should be rugged enough to stand up to physical abuse. Internal mounting maximally protects sensor components but then co-existence with the cylinder operation must be considered. Certain locations or attitudes subject the transducer to severe vibration that can cause catastrophic failure. Vibration and orientation of the cylinder are important as long rod-type magnetostrictive sensors for instance have a significant failure rate when deployed horizontally or subject to vibration.

Enter CPI draw wire sensors in 2017 and you have a reinvented sensor tech that addresses most if not all of these concerns in the harsh duty world of offshore or underwater hydraulic operations for oil & Gas. Often referred to as “In-cylinder extensometry” CPI was the first to deploy a non-contacting, internally mountable draw wire using a linear-to-rotary-to-linear (LRL) mechanism to translate the long stroke of a hydraulic cylinder to the relatively small motion of a short-acting sensor, such as an LVDT or a very short magnetostrictive sensor.

The LRL mechanism is a micrometer-like assembly, which forms the axis of a recoil spool mechanism. The spool uses a polymer-coated, stainless-steel cable to form a reliable, repeatable coupling between the piston and the sensor. Anchoring the connector to the piston causes the cable to wind or unwind from the spool as the piston rod retracts or extends, respectively.

The big advantage to this technology is ease of installation – no core drilling of the piston rod is required. The assembly also offers extreme immunity to shock, vibration, and environmental factors, regardless of stroke length and no sensitivity to EMI as electronics may be removed from the sensor area.

The CPI SL2000 sensor provides absolute output, and, unlike other sensor technologies it is both non-contacting, and pressure tolerant. The CPI sensor is designed to be mounted inside the cylinder housing with access provided through the protrusion of a short stroke magnetostrictive sensor. Developed initially for mobile equipment applications, our in-cylinder extensometers fill a void in the deployment of advanced hydraulic control feedback for a wide range of machinery, including offshore drilling equipment, underwater hydraulics, mining excavators, and other large scale agricultural and marine equipment. It is also one of the few sensor technologies suitable for telescoping cylinder applications.

Original content posted on https://www.cpi-nj.com/blog/when-old-technology-becomes-new-again-part-ii/

 

Interestingly when one looks back at the history of hydraulic position sensing technology some of the earliest ideas from as far back as the 1950’s revolved around both potentiometer based implementations and draw wire systems.  Potentiometer based systems with their friction based wiper technology and inherent operational limitations were mostly relegated to the lab as straightforward easy to implement if non-robust solutions to measuring linear displacement.

Early draw wire sensors or “String Pots” as they were called suffered a similar fate. Also a straightforward method of mechanically determining linear displacement, it was perhaps too simple a solution to be considered elegant, or mechanically robust. Early versions after all, did use actual strings and crude spooling encoders to measure the length of the displacement.

Eventually rod-type sensors were developed taking advantage of the physics of magnetostriction. By inserting a waveguide inside of a cylinder, small differences in magnetic field reflections could be detected as the piston traveled up and down. The rod (or waveguide) was carefully inserted through a center bore in the piston, running the entire stroke length.  A long stroke cylinder therefore required a long rod. With longer rods came the problem of making a longer, perfectly straight center bore in the piston and making sure they fit perfectly in center of the cylinder.

Despite these challenges, magnetostrictive technology is scientifically elegant, simple to manufacture, and can be highly accurate under the right conditions. So called “Magnetostrictive Sensors” became the most well used linear position sensing technology by hydraulic cylinder manufacturers.

Draw Wire Sensors – The Sequel

Other hydraulic linear position sensor technologies have entered the market over the years achieving some success for niche applications where their cost to performance ratio is acceptable. Microwave sensors, variable induction sensors, linear encoders, LVDT based sensors, MVDT Sensors and even new and improved versions of potentiometric sensors.

Draw wire sensors, like the other technologies has seen a major reinvention as well. Satisfying a niche for extreme high endurance, draw wire sensors were reimagined by CPI in a way that retains the conceptual simplicity of the solution, but adds the technology and elegance that makes it a 21^st century solution to any advanced hydraulic position sensing needs.

In Part 2 of this blog we’ll talk about the key considerations in choosing a sensor and define the places where draw wire technology excels. Stay tuned.

Original content posted on https://www.cpi-nj.com/blog/draw-wire-reinvented-part-1