Inductive position sensor in miniature hydraulic F1 actuators

Moog Industrial Group is a designer, manufacturer and integrator of precision actuation components and systems. Martin Jones, Motorsport Market Manager at Moog, comments: "In the UK, we've been using the EDS 3mm sensor since 2011. It's a unique sensing technology that is compact, robust and lightweight, allowing it to fit inside our miniature hydraulic actuators used in F1 motorsport applications. We considered using LVDT sensors, but were concerned that they were not robust enough to survive the extremely high-temperature, high-vibration environment that is the norm in motorsport applications.

"We selected the 3mm EDS sensor for two main reasons: Micro-Epsilon's proven track record in supplying sensors for motorsport applications and the compact design of the sensor, which allows it to be easily incorporated into our hydraulic cylinders."

Jones continues: "Inevitably, motorsport teams continuously push the boundaries of technology to gain a competitive edge. For example, the hydraulics in F1 cars operate at extremely high temperatures, typically around 130degC. The hydraulic systems are typically cooled by the engine coolant, which itself has a temperature of around 115degC. Under these harsh conditions and with F1 teams demanding 100 per cent reliability of components and systems, the 3mm EDS sensor has performed exceptionally well for us over the years.

"Moog works very closely with Micro-Epsilon, with our engineers liaising directly with the design team at Micro-Epsilon. We've found them to be responsive and adaptable in developing custom sensors to meet our requirements. We've also collaborated with Micro-Epsilon on other development projects for motorsport customers, including the development of FIA-approved conditioning electronics modules."

Advantages over LVDTs

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 much 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 much 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 very 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.

Measuring principle and structure

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.

Whilst the 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. EDS sensors can be customised to include a high-pressure mounting flange, for example, where the sensor effectively becomes the rear part of the hydraulic cylinder. Other recent developments include customising the 3mm diameter version for miniature actuators and hydraulic cylinder applications. In some of these applications, the sensor has been calibrated 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 wide variety of hydraulics and pneumatics applications to measure displacement, position, distance, gaps, deflection, movement, stroke, fill level, immersion depth and spring travel.

For more information on inductive sensors, please visit www.micro-epsilon.co.uk.