Linear Position Sensor Technology Choices
There are a number of technologies to choose from when considering the implementation of a linear position sensor. As CPI’s own sensors focus on the needs of hydraulic cylinder and hydraulic accumulator designers, it is instructive to review the various technologies being used in this market and the relative tradeoff’s of each, as compared to CPI’s LVDT based Linear Position Sensors.
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. A large variable resistor is used with a “wiper” attached to the moving piston. This is a contact based solution requiring real friction between the wiper and the variable resistive element.
The primary issue with this sensor is their reliance on mechanical contact to provide feedback, causing output signal degradation (dead zones due to wear) over time especially in high vibration environments. MTBF is relatively 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.
Magnetostrictive Linear Position Sensors
These position sensors represent the dominant solution for cylinder position sensing in use today. 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. 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.
With magnetostrictive sensors, sagging can be an issue for longer strokes. 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.
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 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.
LVDT based Linear Position Sensors (Traditional)
LVDT’s (Linear Variable Differential Transformers) use magnetic induction to determine position and thus are 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. 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.
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 Linear Position Sensor
As one of the newer hydraulic cylinder / piston accumulator position sensor technologies available, CPI’s SL series sensors fit 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. Using an 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 length’s. No more needing to stock a replacement rod sensor in every single length needed, now one or two part numbers cover them all.
About CPI Linear Position Sensors
CPI has been producing state of the art linear position sensors since 1997. Our current models represent the most evolved, and robust soution to position measurement available today. They are deployed throughout the world in Marine, Ocean Drilling, Mobile hydraulics, and mining applications where only the toughest solutions will do.
If you think your hydraulics application is tough enough for our sensor, you should give us a call.
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