Smart hydraulic cylinder measures force

The trend toward “intelligent” components, those with highly integrated sensors and control capabilities, is becoming increasingly important in hydraulic applications. Drive-systems with electronic control systems help maximize performance, energy efficiency and ease of use. However, control systems need to acquire information from various processes, which is often challenging, as sensors must be protected against harsh environments typical on construction, mining and agricultural sites.

Supporting cylinder with integrated force measurement from Weber-Hydraulik is used to help stabilize mobile cranes.
Weber-Hydraulik, based in Güglingen, Germany, has been at the forefront of developments involving highly integrated, reliable and reasonably priced sensors for hydraulic cylinders. In a presentation at this year’s IFK Fluid Power Conference held in Aachen, Germany, authors Roman Weidemann, Mehdi Javdanitehran, Torsten Winkler and Torsten Boldt discussed the company’s work on an optical position measurement system for hydraulic cylinders—now in mass production—and ongoing research into in-cylinder integrated force measurement.

The integrated measuring sensor detects cylinder position by optical recognition of a binary barcode applied on the piston rod, which has a unique code structure in every position. A redundant version meets more strict safety requirements.

The compact sensor includes an exposure unit, a sensor unit to capture the image and an electronic module for image processing and communication. The exposure unit momentarily illuminates a section of the code which the sensor then detects. This takes place at specific intervals adapted to suit the maximum traversing speed of the rod. Images are analyzed individually to determine the respective position.

The sensor mounts on the cylinder outside the pressure chamber and directly behind the wiper. It is thus an external sensor, albeit highly integrated into the cylinder design. In many cases the sensor doesn’t increase the retracted length of the cylinder. Due to the design, component reliability and the safety measures implemented, the sensor meets requirements corresponding to PLe (EN ISO 13849). The maximum measuring length for the current system is approximately 6 m.

Weber-Hydraulik engineers are also actively involved in application-specific solutions for integrated cylinder force measurement. To monitor the stability of supported mobile vehicles such as cranes, aerial ladders and concrete mixer pumps, it would be advantageous to have information about conditions in outriggers and supporting arms.

In practice, the load capacity of the supporting base is often unknown because surrounding conditions don’t allow for a complete outrigging of supporting plates, or the load carrying capacity of the ground is limited. An electronic monitoring system that measures both outrigger positions as well as the supporting load of each supporting cylinder would increase safety.

The force of a supporting cylinder is commonly measured indirectly via internal pressure, but this method is not particularly precise and measurements can deviate by more than 20% under transversal loading conditions. To ensure safe operation, sizeable safety factors are required which consequently reduces the available load capacity of the system.

The new design involves a supporting cylinder with an integrated force measurement system in the piston rod: force is measured directly on the ball head. Measurement signals are transmitted through the cylinder and end cap to the crane control system using a standard interface. Consequently, sensitive cable connections outside the piston rod are not required.

Various methods like strain gauges and piezoelectric crystals have been developed to measure loads. Strain gauges are widely used in industrial and mobile applications, but the contacting point in this technique is a major disadvantage, especially when the component is overloaded. A contactless measurement system developed by Trafag GmbH, based on the magnetic induction principle, is beneficial because the measuring cell does not touch the component. The sensor is not directly subjected to forces, so the possibility of mechanical damage due to the loading (especially shock and vibration) decreases drastically. Consequently, the design prevents degradation of performance due to fatigue.

Planar coils generate magnetic fields which penetrate in the component and receive the resulting magnetic field as a measurement signal processed directly in the electronics. Ferromagnetic materials are principally suitable for contactless inductive measurement. The magnetic properties of the material change as the part deforms under load. This alters magnetic flux in the signal coils which, in turn, affects the induced current. The sensor calculates the force by measuring these changes. By applying multiple sensors in a suitable arrangement, one can measure multiaxial loading conditions (such as compression and bending).

In preliminary tests, the sensor was adapted to measure the forces of a hydraulic cylinder with a 110 mm piston diameter and 90 mm rod diameter. Two different versions were considered. In one, the sensor housing attaches to the piston rod next to the rod head. Up to four sensor modules are placed circumferentially orthogonal in the housing to consider the effects of the lateral forces. There’s a gap of up to 1 mm between the rod surface and the sensor modules. In a second version, the sensor housing mounts at the cylinder gland while the piston rod slides freely in the housing. This assembly is beneficial in many applications as the signal/power transmission line doesn’t have to compensate for length changes during cylinder extension and retraction.

Functional tests with sensors attached to the piston rod have shown promising results in terms of linear deviation and repeatability. Rest magnetism and irregularities in the piston rod hamper the short-term implementation of the cylinder gland execution, which is beneficial from various aspects. Despite the disturbing factors, researchers plan to follow up and investigate different compensation methods.

The goal of further developments of this system is mainly to reduce sensitivity to transversal loads as well as enhance robustness in the field. An exemplary goal is increasing allowable transversal loads from 10% to 20% of the axial supporting load.

In addition, hydraulic cylinders with integrated force measurement are not solely suited for supporting applications. There are other applications that can benefit, such as force-controlled manufacturing processes that enhance finish quality. Thus, the company is not only investigating a new generation of supporting force measurement systems, but also the development of a universal integrated solution of force measurement for cylinder applications.

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