CPI’s LVDT Based Linear Transducer Design

As all engineers know, design is really a process of making a never ending series of tradeoffs as one seeks to achieve a balanced engineering goal:  Tradeoffs between cost and functionality; tradeoffs between performance and reliability; tradeoffs between size and power or environmental durability.  It’s the dream of most engineers to create something that moves most or all engineering capabilities forward while somehow lowering costs and improving performance.

Such is now the case for hydraulic position sensor technology. The better mousetrap made by CPI is the Patented design of our LVDT based SL Series Linear Position Sensors, replacing and outperforming the rod-type magnetostrictive sensors, and linear potentiometers of yesteryear.

CPI’s LVDT based SL Series Linear Position Sensors find applications everywhere from the robust high temperature environment of a steel mill motion control, to the corrosive undersea world of hydraulically operated blowout protectors on massive underwater drilling operations. Not to mention everywhere in between where harsh environments, maximum durability and reliability are a concern.

What Is An LVDT?

LVDT stands for “Linear Variable Differential Transformer”.

Diagram of LVDTAn LVDT is essentially a non-contacting transducer that can convert the rectilinear motion of an object to which it is mechanically coupled to an analog signal which can be used to determine the displacement of the object. They are extremely accurate, robust, and proven through years of use in high pressure hydraulic environments. LVDTs are extremely accurate, capable of resolving movements as small as a few millionths of an inch, however only have a stroke range of approximately 5 inches. But using CPI’s highly developed sensor core, our sensor converts the relatively long stroke of hydraulic cylinders to the short stroke of the LVDT via a patented linear to rotary to linear mechanism that is unique in the industry.

The LVDT technology used by CPI is extremely accurate, tough, and reliable. LVDT linear position sensors are available that can measure movements as small as a few millionths of an inch up to several inches, and others are capable of measuring positions up to ±120 inches (±0.5 m).

How Does An LVDT Work?

Graph of LVDT PerformanceAs shown in the figure above, the LVDT is essentially a differential inductive sensor. The magnetic core moves through the energized windings, producing both a voltage and a phase. The DC output is typically 4-20ma and is proportional to the core movements. The signal once filtered and rectified also indicates its direction positive or negative from a central point. A distinct advantage of using an LVDT displacement transducer is that it is non-contacting (vs. resistive implementations) making it ideal for high cycle applications. In summary, the distinct advantages of using an LVDT displacement transducer are long life, high reliability, extreme accuracy and robustness. In CPI’s SL Series sensors, the magnetic core is attached to the movable hub of the cable spool as shown in the figure below.

LVDT sensor internal diagramAs the cable extends, the spool turns on a micrometer-like threaded hub. The hub translates axially, providing a precise linear motion for the LVDT core which is proportional to the cable translation.

Precise position feedback comes fromthe LVDT’s micron level resolution coupled with the close tolerance manufacturing standards of the SL Series mechanism.

Still using Rod-Type Magnetostrictive sensors? Really?

For many years, rod-type linear position sensors were the standard implementation for position measurement in a hydraulic cylinder or hydraulic accumulator. Their installation requires a bore through the center of the piston and rod assembly to accommodate the sensing rod, and either linear potentiometer or magnetostrictive sensor technology is used to sense piston position along the rod. In the case of the piston accumulator position sensing, the rod-type linear position sensor creates another unwanted seal point around the sensor rod. Other shortcomings of rod-type sensors:

  • Not robust to shock and vibration.
  • Cannot be mounted horizontally on long stroke applications, as the rod sags and becomes damaged.
  • End of stroke dead zones
  • Cannot withstand operation in high temps; rod-type sensors have an upper temperature limit of 185 degrees, vs. 300F for the CPI SL Series sensor. 185F is remarkably easy to exceed when you consider the operation of mobile hydraulics on a hot day, plus the heat passed to the hydraulic fluid by the hydraulic pumps themselves over long periods of operation. If you consider the ambient environment of say a Steel Mill or plastics assembly line, system temperatures are so high, magnetostrictive sensors can experience early failure, despite expensive cooling systems.
  • Contact type linear potentiometers have limited endurance. (See discussion of issues with use of Linear Potentiometers in sensor design). High vibration and dither cause contacts to wear creating dead zones
  • Each sensor must be made to specific stroke length (one CPI sensor will work for all strokes within the sensor range)

LVDT use In Linear Transducers made by CPI – A Revolutionary Evolution.

Our LVDT Position Sensor is neither Rod-Based, nor is it a “string-pot” in the classic sense. By combining LVDT technology with a more generalized implementation of draw-wire sensors, we have created (and patented) something truly unique.

CPI LVDT Linear Position Sensors have the following characteristics:

  1. Our Linear Position Sensor Technology can be installed into any hydraulic cylinder with no gun drilling.
  2. The CPI LVDT Linear Position Sensor is available for field replacement of Rod-Type Sensors or can be installed into any cylinder with no gun drilling required. It fits and operates perfectly in the existing center bore that the failed Rod is removed from.
  3. A single CPI sensor part number can retrofit a failed rod-based sensor of any length, lowering the costs of maintaining field inventory for a variety of stroke lengths.
  4. The LVDT Technology uses no sensitive electronic components and is certified to operate in temperatures to 300F.
  5. The CPI LVDT Linear Position Sensor supports Long Stroke Position Sensor requirements up to 23 feet as a drop in replacement for broken rod type sensors.
  6. The technology delivers absolute position measurements and requires no homing on startup.
  7. The CPI Linear Position Sensor design is inherently immune to shock and vibration.

CPI sensor technology incorporates the best of proven LVDT design and draw wire sensor technology but in a new way that has none of the drawbacks of traditional sensors in this class.

If you have not seen our technology in action, see this video to understand how transformative this sensor technology could be to your operations and how easy it is to retrofit existing Rod-Type Sensors.


Contact Our Engineering Sales Team for More Information at (973) 887-9400