Can A Draw Wire Sensor Really Be High-Tech?

Over a decade ago, our sister company CPI patented the first version of their unique linear-to-rotary-to-linear draw wire, LVDT based sensor. Over that time, the product line has evolved specifically for high-endurance, large scale hydraulic cylinder applications, hydraulic accumulators, and subsea applications; CPI are almost always in the mix in discussions of how to implement linear position sensors in this class of hydraulics.

But draw wire sensors have been around for over half a century, making their debut in places like Bell Labs in the 60’s, used to measure linear displacement in conjunction with a resistive potentiometer. These early draw wire sensors resembled nothing so much as modified fishing reels, spooling out or reeling in a string or cable and dragging a resistive wiper along for the ride to give a crude and friction-rich implementation that didn’t last long, wasn’t water resistant, and needed a stable, solid, and level platform to operate correctly.

But engineers will tinker. Fast forward a half century and the early draw wire position sensor, despite many improvements, has given way to shinier objects in many applications. Technologies such as potentiometric-based devices have given way to non-contacting variations including magnetostrictive rod-type sensors, microwave sensors, optical encoders, LVDT sensors, Fiber-Optic sensors, and many variations within these types. 

How has the modern draw wire sensor fared in the face of so many competing linear position measurement technologies? Does it still have a role to play in linear position measurement outside the lab? Have the mechanics of draw wire operation kept up? Or is it essentially the same fancy fishing reel it was 50 years ago with a fresh coat of paint?

Bringing Draw Wire Position Sensing Into the 21^st Century

When CPI conceived of an extreme duty version of a draw wire sensor, it was because no other technology was addressing this market robustly.

1. The most popular hydraulic cylinder sensor technology, magnetostrictive “rod-type” sensors, while extremely accurate, had a fatal flaw in long-stroke measurement applications.  It required a long rod (used as a waveguide) which was prone to warping and bending under vibration. The rod needed to have its entire length mounted into the cylinder by gun drilling a hole into the piston rod the entire length of the stroke to be measured. The longer the stroke, the more difficult and problematic the gun drilling and the more likely that the rod would suffer the adverse effects of vibration, like warping, which essentially renders the sensor useless.
2. Existing draw wire designs and materials would never withstand the extreme operating environments presented by mining, drilling and construction equipment. Heavy duty cylinders and accumulators in these applications commonly present extremes of temperature, pressure, and vibration that would cause existing draw wire sensors to fail badly and often.
3. It was determined that the core measurement technology for any modern sensor in an aggressive, high endurance environment must be non-contacting.  Existing methods like rotary spool encoders contained sensitive electronics that were just not robust enough to withstand extreme environments.

CPI SL Series Reinvents the Draw-Wire Sensor.

The CPI sensor can be seen 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 CPIs 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.

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 the new model, the spool moves and not the line.  In this way off axis motion is completely eliminated which enhances both the reliability and the accuracy of the sensor.

No String Pot Here

Yet another world of analysis concerns the actual wire used in the 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 7x19 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 the retraction springs.  In this area, CPI sensors are thoroughly overdesigned, with part and material selection that allows them 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 CPI sensors 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 the submersible solution can be specified with an industry standard subsea (Seacon) connector.

Draw Wire Sensor Measurement Electronics

CPI currently offers two options for sensor measurement electronics: one is an LVDT-based measurement system which can provide a sealed, non-contacting solution that can be completely enclosed within a hydraulic cylinder or hydraulic accumulator.  In this case, signal conditioning electronics are remotely located, so the sensor can operate in higher temperatures, and is highly tolerant of temperature induced error.   The other option is an extremely short magnetostrictive sensor probe which can be mounted through the cylinder wall.  This approach leverages all of the technology advances behind magnetostrictive sensors, including intrinsic safety ratings, redundancy, and suitability for subsea deployments.

A comparison of the two transducer solutions follows:

Applications For The CPI High Endurance Draw Wire Sensor

Over a 15 year period since our sensor was introduced, it has evolved, improved, and now enjoys wide acceptance in high endurance, long stroke hydraulic applications across dozens of industries and military applications.

Navy Vehicles – Navy subcontractors have used customized versions of our LVDT sensor for sea-going applications on surface ships and submarines.  While some of these uses are proprietary, you can read here about our use of smart cylinders on the navy’s interdiction vessel, the Sea Slice.

Offshore and Subsea Drilling, Mining and Refining Operations – Large scale hydraulic cylinders used in heave compensation systems and riser tensioners are typical high-endurance applications.  Our subsea rated sensor can also monitor pre-charge pressure in hydraulic accumulators like those commonly used in blowout preventers at the well head. For applications above the surface or on the drilling platform, our SL 1490MS hybrid sensor is fully ATEX Intrinsic safety rated.

Oil & Gas – Rig raiser systems now enjoy much improved reliability, as our internal mount sensors replace external draw wires on the mast cylinder.

Commuter Trains – Telescoping pneumatic cylinders on commuter trains required sensors for a closed-loop feedback system to assist in rail car coupling.

Onshore Mining – Remote hydraulic valve actuators can be reliably monitored with use of our external-mount 1M sensor, which easily mounts onto any NFPA cylinder.

In-Plant Chemical Processing – External draw wire applications on linear and valve actuators include hazardous chemical, paper processing, as well as oil refinery installations.  Internal applications range from hydraulic roller tensioners in paper manufacturing to telescoping cylinders used in pharmaceutical processing.

Telescoping Cylinders – CPI sensors are one of the only high endurance solutions in the world for this class of hydraulics. Our sensors have the ability to mount either inside or outside the cylinder, in any type of hydraulic fluid or gas atmosphere, with stroke lengths available up to 15m.

Infrastructure, Hydroelectric – Dam gate actuators are typically long-stroke.  In some instances, these are oriented horizontally – this is where our draw wire solution overcomes deflection issues associated with long rod-type sensors.

Specifying Your Sensor

We works closely with CPI and with each engineering team of our customers to understand and define the particular needs of each sensor application.  While some sensors can be used “off the shelf”, Custom solutions are always the best route. Make sure you contact us to discuss your requirements.