Reinventing Draw Wire Sensors Part 1 – Mechanical Revolution

Subsea Hydraulic Piston Linear Position Sensor

We have to look at the CPI sensor as having two subsystems:  1.) The mechanical draw-wire mechanism which reduces the long travel of the draw wire to a highly accurate, directly proportional short stroke, and 2.) the electronics that very accurately track and transduce the short stroke. The unique capabilities of the CPI SL Series sensors come from how they use existing field proven technologies and evolve them into elegant linear position measurement systems that are better than the sum of their parts.

CPI’s Draw Wire Sensor – An Evolution in Material Science and Engineering

In order for our draw wire sensor to meet the needs of incredibly challenging operational environments, including those at pressure thousands of feet below the ocean, a new approach to both spooling design and material science was needed. To solve this challenge CPI developed and patented a unique linear-to-rotary-to-linear translation mechanism that effectively translates the long linear motion of the draw wire to a short linear motion along a precision threaded axis.

This translation mechanism is really the mechanical heart of the sensor. The linear motion of the piston which can be 10 meters or more is translated to a rotary motion on the spool, which in turn drives a secondary linear motion of a greatly scaled down length along a fine threaded rod of only a few inches. Measurement of travel along this precision engineered rod can be achieved either inductively by using an LVDT, or by using an extremely short length magnetostrictive sensor.

But the design evolution doesn’t stop there. Early on CPI knew that if their design was to survive at depths of 5000 feet or more in highly corrosive salt water, there could be no sealed cavities in the device; it would have to be capable of being flooded with water, seawater, hydraulic fluid or gases during operation.  Sometimes it would be mounted inside the hydraulic cylinder or accumulator, sometimes outside completely exposed in 5000 feet of dirty seawater.

Linear Position sensor for subsea and intrinsic safety rated.

Since the heart of the system involves essentially a micro threaded hub running continuously up and down a threaded lead, entirely stainless steel components would not work as stainless steel on stainless steel in the presence of water will seize up faster than you can say Jack Robinson.  To solve this problem, CPI conducted extensive materials testing to select a bushing material that would allow continuous threaded movement on the stainless steel lead without any seizing, even fully saturated in seawater. The specialized bronze alloy selected has been tested to 1 million full stroke cycles while submerged in aerated salt water, in the absence of any lubrication at all.  Thus this endurance can be achieved in air, or any other gas atmosphere presented by the mounting location of the sensor.

Another common problem with traditional draw wire sensors was their fishing reel like behavior — the way they force the line to move from right to left overlaying the line in an arbitrary fashion on the spool. CPI completely redesigned this mechanism so that in our model, the spool moves, not the line.  In this way we completely eliminate off axis motion, enhancing both the reliability and the accuracy of the sensor.

No String Pot Here

Yet another world of analysis concerns the actual wire used in our draw wire sensor. Make the cable too thick and its rigidity may cause binding during retraction.  Make the cable too thin and it may break under the strain of sudden forceful piston or object motion. After extensive analysis and field qualification, CPI uses multi strand, pre-formed, braided stainless steel, with a braid and strand count specifically selected for the requirements of the hydraulics.   A 7×19 cable is common (7 braids, 19 strands each) though different combinations may be chosen for the specific application.  The retraction mechanism achieves speeds that meet or exceed the fastest impulse reaction of the piston, while staying well within the operating limits of our retraction springs.  In this area, our sensor is thoroughly overdesigned, with part and material selection that allows our sensor to exceed 1 million cycles without significant wear or loss of retractive force, regardless of stroke length or impulse response requirements.

Seawater performance

The frame components of our sensor are made from hard anodized 6061 aluminum making them virtually impervious to rust and corrosion. But salt water adds the element of galvanic corrosion, so a zinc anode may be specified. Furthermore the magnetostrictive sensor used in our submersible solution can be specified with an industry standard subsea (Seacon) connector.

A “High Tech” Mechanical Design

The CPI sensor takes mechanical design for a draw wire mechanism to the peak of performance and endurance. If you read this far and you are an engineer of any kind, we know you understand this. But what make our sensor unique doesn’t stop there. In our next segment we will talk about the marriage of a unique mechanical design with an even more unique and hardened transducer design.

Stay tuned.