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 21st 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.
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