Position-sensing hydraulic cylinder
Encyclopedia
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.
’s piston rod
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
rod add substantial 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 — accuracy overkill, if you will.
rod. Instead, an external sensing “bar” utilizing Hall-Effect technology senses the position of the hydraulic cylinder
’s piston. This is accomplished by the placement of a permanent magnet within the piston
. The magnet propagates a magnetic field through the steel wall of the hydraulic cylinder
, providing a locating signal to the sensor. The external LDT has the following advantages over the internal LDT.
Internal LDT
In-cylinder linear displacement transducers (LDTs) have been used with limited success in mobile equipment to achieve these goals. A limitation to most in-cylinder LDTs is that the hydraulic cylinderHydraulic cylinder
A Hydraulic cylinder is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in engineering vehicles.- Operation :...
’s piston rod
Piston rod
In a piston engine, a piston rod joins a piston to a connecting rod.Many internal combustion engines, and in particular all current automobile engines, do not have true piston rods, and the term piston rod is often used as a synonym for connecting rod in the context of these engines.All engines...
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
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
rod add substantial 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 — accuracy overkill, if you will.
External LDT
External linear displacement transducers (LDTs) eliminate the need for a hollow hydraulic cylinderHydraulic cylinder
A Hydraulic cylinder is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in engineering vehicles.- Operation :...
rod. Instead, an external sensing “bar” utilizing Hall-Effect technology senses the position of the hydraulic cylinder
Hydraulic cylinder
A Hydraulic cylinder is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in engineering vehicles.- Operation :...
’s piston. This is accomplished by the placement of a permanent magnet within the piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
. The magnet propagates a magnetic field through the steel wall of the hydraulic cylinder
Hydraulic cylinder
A Hydraulic cylinder is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in engineering vehicles.- Operation :...
, providing a locating signal to the sensor. The external LDT has the following advantages over the internal LDT.
- Typically, lower overall cost (i.e., no need to gun-drill cylinder rod)
- 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 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 cylinderHydraulic cylinderA Hydraulic cylinder is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in engineering vehicles.- Operation :...
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
- OEM’s can prepare hydraulic cylinderHydraulic cylinderA Hydraulic cylinder is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in engineering vehicles.- Operation :...
s with magnets so end users can add sensing at a later time, if so desired