The position-sensing feature in the position-sensing cylinder provides instantaneous analog or digital electronic position feedback information from the cylinder that indicates the amount of rod extension throughout the range of stroke.

"Smart" hydraulic cylinders

Internal LDT

In-cylinder Linear Displacement Transducers (LDTs) are used in mobile equipment. A limitation to most in-cylinder LDTs is that the

hydraulic cylinder A hydraulic cylinder (also called a linear hydraulic motor) is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in construction equipment (engineering vehicles ...

’s

piston rod In a piston engine, a piston rod joins a piston to the crosshead and thus to the connecting rod that drives the crankshaft or (for steam locomotives) the driving wheels. Internal combustion engines, and in particular all current automobile engin ...

must be bored through its center to accommodate certain elements of the LDT — usually the waveguide tube of a magnetostrictive transducer. The machining and additional production steps associated with “gun drilling” the

piston A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tig ...

rod can add cost to the finished cylinder. And although magnetostrictive LDTs provide extremely high accuracy, this accuracy usually is much greater than is needed for most mobile equipment applications. Installing linear position sensors into hydraulic cylinders complicates the production process. CPI has developed a system of sensors which can eliminate the need for gun drilling.

External LDT

External linear displacement transducers (LDTs) eliminate the need for a hollow hydraulic cylinder rod. Instead, an external sensing “bar” utilizing

Hall-Effect The Hall effect is the production of a voltage difference (the Hall voltage) across an electrical conductor that is transverse to an electric current in the conductor and to an applied magnetic field perpendicular to the current. It was discove ...

technology senses the position of the hydraulic cylinder piston. This is accomplished by the placement of a permanent magnet within the

.High Beam Article
/ref> The magnet propagates a magnetic field through the steel wall of the hydraulic cylinder, providing a locating signal to the sensor. Advantages of External LDT: * Full rod buckling strength is maintained * The cylinder is easier to assemble, install, and service * A wide variety of sensor inputs and outputs are available * The permanent

magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, ...

in the piston should never need to be replaced * The external sensor is readily accessible and easy to replace, if necessary * If the sensing bar is replaced, re-calibration is not necessary * The sensing bar is small, and its location along the outside of the hydraulic cylinder wall minimizes the potential for environmental damage * Positioning accuracy is +/- .5mm or better; more than adequate for most mobile equipment * There are no cables or wires used, and hence no concerns regarding stretching or breakage due to ice, plant limbs, or other environmental obstructions * Should machine power be lost and then recovered, the sensor will send the current position

References

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External links

High Beam ArticleHydraulic CylindersHydraulic Piping Systems
Hydraulic actuators