Position sensors for miniature hydraulic cylinders and actuators

Compared with traditional methods of measuring displacement and piston position in hydraulic cylinders and valves (i.e. LVDTs and Magnetostrictive sensors), Micro-Epsilon's EDS series of sensors is more compact in both its length and diameter. It uses a non-ferrous aluminium outer sleeve as its target, which can be easily integrated into the piston rod. This enables the sensor body to be a solid rod rather than a traditional LVDT style with a hollow sensor body and plunger, making it easier for OEMs to assemble, and more robust and reliable in harsh environments.

The sensors are manufactured from a pressure-resistant stainless steel (up to 450bar) and can withstand extreme vibration and shock levels (up to 300g axial and 100g radial). The sensor electronics and signal conditioning are completely integrated in the sensor flange using compact electronics. This means the hydraulic cylinder does not have to be increased in length to accommodate large electronics or have remote electronics, which can become awkward to match to the sensor once fitted into the hydraulic cylinder. Compared with an LVDT with similar measurement range, EDS sensors are typically 50 per cent shorter.

The EDS series operates using the eddy current measuring principle. The long stroke displacement sensors transform a linear motion (e.g. piston in a hydraulic cylinder) into a linear electrical signal. An aluminium tube moving around the sensor rod is used as the target. Energy is transferred from the coil by inducing eddy currents in the aluminium tube. The integrated microelectronics convert the tube position into a linear electrical output signal. Due to the use of the eddy current principle, no permanent magnets need to be mounted inside the cylinder (similar to Magnetostrictive sensors). The sensor operates without any contact between the moving parts and is wear-free.

Chris Jones, Managing Director at Micro-Epsilon UK comments: "Whilst our standard EDS series of sensors with a body diameter of 10mm satisfies most customer applications, some OEMs require a smaller footprint or specific mounting flanges for certain applications. For example, we have custom-developed EDS sensors to include a high-pressure mounting flange, so that the sensor effectively becomes the rear part of the hydraulic cylinder. We've also recently developed the 3mm diameter version for miniature actuators and hydraulic cylinder applications [remote electronics are required]. In some applications, we can calibrate the sensor to use the piston rod as the target, thus eliminating the target sleeve altogether."

Micro-Epsilon's standard range of EDS sensors are available for measuring lengths from 100mm up to 630mm. OEM versions can be designed to almost any measurement range, from 10mm to 1m typically. Pressure resistance is up to 450 bar (sensor rod, flange) and temperature range is from -40degC to +85degC. The sensors have very short dead-bands at either end of the measurement range. For example, the 250mm sensor has a body length of just 298mm. The sensors are being used in a variety of hydraulics and pneumatics applications to measure displacement, position, distance, gaps, deflection, movement, stroke, fill level, immersion depth and spring travel.

Closed loop suspension control for mobile cranes

The EDS sensor is also suitable for use in hydraulic suspension cylinders on mobile cranes and other off-highway vehicles, where precise measurement of displacement is required. Most mobile cranes have an adaptive design of running gear, where the suspension of each independent axle can be adapted to different terrain. This is implemented using a suspension cylinder fitted with a displacement sensor. The sensor acquires the cylinder stroke and enables the best possible adaptation to the ground conditions. Active suspension becomes important when the crane moves from open terrain to normal road conditions, and this is where active closed loop control is critical. The axles must then be adjusted to a flat road by moving all the cylinders to their normal positions. To do this, the exact displacement measurement of the hydraulic cylinders is critical.

Micro-Epsilon has developed position sensors which are integrated into hydraulic cylinders for harsh environments such as subsea and oil & gas, where reliability is paramount, directly acquiring measurements of piston position for closed loop control. The sensor is not only protected against external damage by being positioned within the hydraulic cylinder, but it also provides a more compact and integrated sensing system.

Follow the link for more information about Micro-Epsilon's EDS pressure-resistant position sensors or email [email protected].