Hydraulic cylinder vibrates to a 400 Hz sine.

DYNA-MESS and Hänchen reach new heights of performanceModern quality standards mean that components are nowfacing more varied and useful tests than at any time before.DYNA-MESS Testing Systems specialises in testingmachines for dynamic component and material testing. Withits broad and varied range of standard and special testmachines, the Aachen-based company exploits thepossibilities of modern fluid technology to the best advantage.Its pièce de resistance is a special testing machine forelastomers which, with the help of standard hydrauliccylinders from Hänchen, realises controlled vibrations of 400Hz in order, for instance, to test the damping of bearingelements made of rubber-metal composites for theautomotive industry.Hydraulics is cutting-edge technologyAccording to Dr. Marcus Jarchow, Managing Director of DYNA-MESS, "these test units areexposed to extreme stresses with a nominal force of 10 kN and a frequency of anything from 0to 400 Hz. It has been extremely difficult to find a suitably high-performance hydraulic cylinderon the hydraulics market", because the test device depends on the perfect stick-slip behaviourof the hydraulic system. Cylinders with hydrostatically supported piston rod guides are normallyused for this type of application, but the technical expenditure for this solution is enormous. "Wehave nevertheless kept our ears to the ground to try and find out who can offer some help withfinding a solution" stated the Managing Director of DYNA-MESS. "Everybody hesitated whenwe mentioned the technical requirements, with the exception of Hänchen Hydraulik. A furtherargument in their favour was that they had their own test equipment and would be able to basetheir results on extensive experience. Ultimately, the hydraulic specialists from Ostfildern, nearStuttgart, offered a standard cylinder with patented, floating annular gap seal which isconsiderably more cost-effective than drives with hydrostatically supported piston rod guides".The PZL assembly series from Hänchen, however, also provides these high-end cylindersThe PZR floating annular gap sealThe patented, floating annular gap seal from Hänchen permits the use of cost-effective PZRhigh-end test cylinders, in which a steel sleeve is positioned through a choke gap and therebycreates a contact-free sealing gap of just a few 1/100 mm. This process can, however, onlyfunction correctly with a production accuracy of only a few μm, as leaks would otherwise lead toan unacceptable loss of hydraulic fluid. This PZR assembly differs only by the slightly lesserabsorption of the lateral forces from a cylinder with hydrostatically supported piston rod guide,but yields a considerable cost advantage. Thanks to their negligible friction, however, PZRcylinders offer maximum positioning and repeat accuracy and are stick-slip free, making themideally suitable for extremely slow as well as fast movements and minimum travel over rangesof a very few μm. As lateral forces are not experienced in this 400 Hz DYNA-MESS testequipment, the decision was made in favour of these cylinders.High-quality peripheralsThe cylinder is equipped with a special inductive position measuring system and a forcetransducer with an extremely low natural frequency and yield measuring strips. Both sensorssupply analog output signals which are digitalised with a resolution of 16 Bit and tested withmore than 20 kHz. They supply the control with the necessary data, which must be convertedpractically in real time by a control card with its own high-performance processor. Tried-andtestedDYNA-MESS software modules were adapted to the special conditions to facilitateprogramming. A PC forms the basis for the Windows user interface. The valves represented yetanother technical challenge, as they have to work with a control frequency of 16 kHz. The fluidcircuit has been designed for 300 bar and operates at 40 l/min. This high power density enableda low design size, because a high frequency with a wide frequency spectrum places extremelyhigh demands on the mechanical construction: the frame must not have any natural resonanceover the whole of the frequency range.CommissioningCommissioning proved to be a time-consuming exercise, particularly due to the need to tune into the various frequency ranges. The frequency was increased in steps of 10, 20, 30 … up to400 Hz, with the resonance having to be minimised at every stage. The complete control wasalso tuned during this time, in that the runtime delays between the reaction of the processor andits execution by the hydraulic system are captured and included in the control. Finally thecontrol quality and curve form were optimised, this operation being followed by fine tuning withthe help of a friction-free and loss-free spring, which measures the dynamic without phasedisplacement and so enables material tests to be carried out under clearly defined conditions.After up to 400 Hz had been reached, Dr. Jarchow’s team carried out further tests up to afrequency of 1,000 Hz. "We were surprised at what outputs were possible, up to double thenominal output at 800 Hz", said the hydraulic specialist. "It was only above this frequency thatthe limits of what is technically feasible at the moment were clearly reached. The hydrauliccylinder has more than matched the requirements placed on it, and that was with the use of acatalogue cylinder and at a very attractive price”. The finished system was then commissionedduring the first quarter of 2002.Practical applicationHänchen’s PZR hydraulic cylinders are now being used by a Daimler-Chrysler supplier to testvibration dampers for engine and gearbox suspensions for load carrying ability, dampingbehaviour and service life. These components are made from rubber-metal composite materialwith embedded passive hydraulic damping. The sealed glycol circuit in this component is usedto dampen the vibrations. A new test bench enables the developers to optimise the dampingcharacteristics. Sweep tests with a pre-selected load which corresponds to the engine orgearbox weight are used to determine at which frequency damping is most effective. For thistest the frequency is superimposed over the static load with the aim of achieving an optimiseddamping over the whole frequency range of up to 400 Hz."Our engineering competence, the experience and evaluation of our suppliers as well as theability to develop test solutions together with our customers were the deciding factors",explained Dr. Jarchow, whose team performs all calculations, is responsible for design,software, electronic development, control, configuration as well as installation and realises thecomplete system in co-operation with the suppliers. "Suppliers whose products conform to ourown high standards play a critical role. That is why we are working together with HänchenHydraulik".Jörg