Description of Major Linear Position Sensor Technologies for Hydraulics

1.    Potentiometer based position sensing

Use of a variable resistor attached mechanically to a slider on the piston is one of the cheapest and most conceptually simple detection mechanism’s there is. The Primary issue with this sensor is their reliance on mechanical contact to provide feedback, causing output signal degradation over time especially in high vibration environments. MTBF is relatively short and their applications are limited to moisture free, moderate temperature environments where low cost is a primary design consideration.

2.    Magnetostrictive Linear Position Sensors

These position sensors represent the dominant solution for cylinder position sensing in use today. Magnetostriction is a property of certain transition metals whereby they expand or contract in the presence of a magnetic field. LPS’s using this technology take advantage of this property to create a non-contacting solution to the problem of linear position detection. 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. Additionally the rods are susceptible to vibration which can cause bending and misalignment which requires them to be replaced. And finally the technology has “dead zones” at the cylinder limits which can be minimized, but not eliminated.

3.    Draw Wire Sensors

 Draw-wire displacement sensors measure linear movements using a highly flexible steel cable, a spring loaded spool, 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 high accuracy and do not need precise linear alignment.  As such they are ideal for wet, dirty, or high vibration outdoor environments and applications. Weakness relate more to the sensor mechanism employed typically a potentiometer aka “string pot” or an optical encoder.

4.    LVDT based Linear Position Sensors (Traditional)

LVDT’s (Linear Voltage Differential Transformers) use magnetic induction to determine position and thus are a completely non-contacting sensor solution. They contain coils wound around a high permeability Nickel-Iron core which is attached to the moving member. Piston displacements generate both a phase angle and differential voltage which are used to infer the direction and displacement of the piston. Signal conditioning and support electronics convert this output to DC voltage or current for ease of integration with control systems. 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, 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.

5.    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.

6.    Hybrid Solutions, CPI Extensometer

One of the newer hydraulic cylinder position sensor technologies available fits in the class of draw wire sensors but replaces the electronics with a patented 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 cylinder without the need for the transformer to encapsulate the cylinder. Using an LVDT that is completely sealed, this combination yields a solution that is immune to vibration, magnetic field interference and has an extended operating temperature range up to 125C. The 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.