Linear Position Sensor Technology Choices

When considering the implementation of a linear position sensor, there are number of technologies to choose from. Since CPI’s own sensors focus on the needs of hydraulic accumulator designers it is instructive to review the technologies used and the associated tradeoffs of each.

Magnetostrictive Linear Position Sensors

These types of sensors represent the main solution for cylinder piston sensing. Magnetostriction is a property of a ferromagnetic waveguide, which twists in the presence of a magnetic field, affecting the time for a pulse to travel from one end of the waveguide to the magnet. To create a non-contacting solution to the problem of linear position detection, LPS is using this technology. The solution is known for being highly accurate and suitable for relatively long measurement ranges (6-120 in).  Inherent in this solution is the use of a waveguide (the “rod”) which must run the length of the cylinder and requires core drilling of the cylinder for insertion.

Sagging can be an issue for longer strokes with magnetostrictive sensors.  The rods are also open to vibration which can cause bending and misalignment. These alterations will require the rods to be replaced. And finally the technology has “dead zones” at the cylinder limits which can be minimized, but not eliminated.

Draw Wire Sensors

Draw-wire displacement sensors measure linear movements using a spring loaded spool, an extremely flexible steel cable, and a sensor mechanism of some type.  The cable drum is attached to the sensor element which provides a proportional output signal.  Draw wire sensors can measure linear position with great accuracy and do not need centered linear alignment.  As such they offer installation flexibility and can be configured for wet, dirty, or high vibration outdoor environments and applications. Weaknesses relate more to the sensor mechanism employed, typically a potentiometer (hence the term “string pot) or an optical encoder.

Potentiometer Centered Position Sensing

One of the cheapest and most conceptually simple detection mechanisms is the use of a variable resistor attached mechanically to a slider on the piston. A big variable resistor with a “wiper” attached to the moving piston is used. This solution requires real friction between the wiper and the variable resistive element.

The main concern with this sensor is their reliance on mechanical contact to provide feedback causes output signal degradation (dead zones) over time. MTBF is quite short in a rugged environment and thus their applications are limited to moisture free, moderate temperature environments where there are smaller stroke lengths and low cost is a primary design consideration.

LVDT based Linear Position Sensors (Traditional)

To determine position, LVDT use magnetic induction and thus a completely non-contacting sensor solution. They contain coils wound around a coil form through which a high permeability Nickel-Iron core passes, which is attached to the moving member. To infer the direction and displacement of the piston, piston displacements are used which generate both a phase angle and differential voltage. And for ease of integration with control systems, signal conditioning and support electronics convert the output to DC voltage or current. LVDT’s can be engineered with special sealed packaging for industrial and aerospace applications allowing for extreme reliability, high precision and extended temperature range. Disadvantages can include, feasibility for measuring large displacements, the support electronics and cost has historically been higher due to the complexity of production and the support electronics. Modern layer winding techniques and low cost ASIC’s have in recent years however allowed cost reductions. These solutions can cost effectively displace potentiometer and traditional magnetostrictive solutions in many applications.

Optical Encoders

As the name suggests, optical encoders read optical gratings which are then decoded to provide either absolute or relative position. Low cost versions provide resolutions of 10-12 bits while high end devices can provide up to 18 bits of resolution.  These devices can determine speed, direction, and position quickly, with high accuracy. In order to operate however, these sensors require more complex support electronics and are generally suitable for labs and indoor applications where their relative fragility and high cost can be tolerated.

Hybrid Solutions

CPI’s SL series is one of the newer hydraulic cylinder / piston accumulator position sensor technologies. The series fits in the class of draw wire sensors but replace the traditional core sensor with a patented sealed LVDT implementation for reliable position measurement in harsh environments. Using a unique linear to rotary to linear mechanism the LVDT can detect displacement at the spool on either the oil or the gas side of the piston.  Moreover, LVDT that is completely vented, this combination yields a solution that is immune to high pressures, as well as shock, vibration, magnetic field interference and has an extended operating temperature range up to 125C.

The overall system is very accurate and has high MTBF, while being easy to field retrofit in a wide array of cylinders and stroke lengths including telescoping cylinders. CPI’s SL series LVDT based Linear Position Sensors are commonly used in long stroke hydraulic cylinders and accumulators where harsh operating environments must be tolerated.

Another significant advantage of the CPI sensor is that a single part number can replace sensors in cylinders of widely varying stroke lengths. No more needing to stock a replacement rod sensor in every single length needed, now one or two part numbers cover them all.

CPI Linear Position Sensors

Since 1997, CPI has been producing state of the art linear position sensors. The current sensor models are the most evolved and robust solution to position measurement. They are deployed throughout the world in Marine, Ocean Drilling, Mobile hydraulics, and mining applications where only a tough solution will do.

Is your hydraulics application is tough enough for our sensor? Visit http://www.cpi-nj.com/ for more information.