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Product category: Level Sensors and Leak Detectors
News Release from: Micro-Epsilon UK | Subject: StrokeSensor
Edited by the Engineeringtalk Editorial Team on 07 June 2005

Sensor helps keep cranes on the straight
and level

Micro-Epsilon helped develop a customised, integrated sensor and suspension cylinder that enables Liebherr's mobile crane suspension system to continually adapt to different terrain.

Whenever a heavy load needs to be lifted quickly, for example, in the recovery of a broken down truck or lorry, a mobile crane is usually the most effective solution However, because these broken down vehicles become stranded in different terrain types, it is critical that the mobile crane can adapt to changing ground conditions, especially if the truck is in open terrain or rough ground rather than on an even road surface

The trick here is all in the crane's active suspension system, where sensors can play an important role.

Take Liebherr, a major manufacturer of mobile cranes based in Germany.

This company uses hydraulic suspension cylinders, which are manufactured by another German firm, Olhydraulik Altenerding.

In a recent application, sensor manufacturer Micro-Epsilon worked closely with Olhydraulik to develop a customised, integrated sensor and suspension cylinder that enables Liebherr's mobile crane suspension system to continually adapt to different terrain.

Most modern mobile cranes have an adaptive design of running gear, in which the suspension of each independent axle can be adapted to the relevant terrain.

This is implemented using a suspension cylinder that is fitted with a displacement sensor.

The sensor acquires the cylinder stroke and enables the best possible adaptation to the ground conditions.

Active suspension becomes critical when the crane moves from open terrain to normal road conditions and this is where active closed loop control comes in.

The axles must then be adjusted to the flat road by moving all the cylinders to their normal positions.

To do this, the exact displacement measurement of the hydraulic cylinders is critical.

Working with the customer, Micro-Epsilon developed a displacement sensor that is integrated into the cylinder itself, directly acquiring measurements of the piston position.

This has several key benefits.

First, the sensor is protected against external damage by being positioned within the hydraulic cylinder.

Second, because the installed length of the cylinder was specified, Micro-Epsilon had to ensure that the solution did not lengthen the cylinder.

Linde did not want to extend the cylinder design as Liebherr would not accept an extension for displacement measurement.

Hence, Micro-Epsilon's design had to be compact.

The sensor also had to withstand high peak pressures (up to 450bar) of the cylinder.

The finished sensor is a customised Micro-Epsilon eddy current long stroke, part of the company's StrokeSensor range.

The housing was customised and made smaller in diameter to suit the restrictions from Liebherr and units between 200 and 400mm stroke lengths are now being supplied, with a resolution of 0.1mm.

Temperature range is from -40 to +85C and the device is rated to IP67.

The sensor can also withstand a peak pressure, with hydraulic oil as the medium, of up to 450bar.

Euard Huber, Product Manager at Micro-Epsilon explains: "The customer considered using a sensor from a competitor that worked using the magnetostrictive principle".

"The two sensor systems offered comparable accuracy and precision, but our eddy current sensor was easier to integrate as it had a shorter 'dead zone' and could be integrated into the cylinder without extending the length of it".

The sensor operates using the eddy current loss principle, where eddy currents are induced using an aluminium tube embedded in the piston.

The aluminium tube acts as the measuring "target" and is attached to the piston, moving concentrically over the metal-encapsulated sensor coil.

There was another reason why Micro-Epsilon's sensor was deemed superior to the competition.

Huber points out that his company's sensor uses a unique temperature compensating method.

"Our sensor uses additional coil windings throughout the complete measuring range, so that we can measure more accurately the temperature differences across the cylinder", explains Huber.

"On the mobile crane, we can detect temperature variations, and compensate for this, if the engine or vehicle warms up because of a sudden change of terrain".

"Other sensor suppliers simply use coil windings at the beginning and end of the stroke length of the cylinder, which is less accurate and slower to compensate for temperature fluctuations".

As there is no mechanical contact between the sensor and the piston, the sensor is completely wear-free and requires no maintenance.

Micro-Epsilon's integrated electronic system (hybrid and SMD technology) converts the displacement measurement signal into an electrical signal, which is then used by the controller for closed-loop control.

The measuring method does not require a magnet and so the risk of collecting metal swarf is eliminated. Request a free brochure from Micro-Epsilon UK ...

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