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O+P Fluidtechnik 7-8/2018

O+P Fluidtechnik 7-8/2018


IFAS JUBILÄUM FORSCHUNG UND ENTWICKLUNG 03 04 Secondary control – filling the forth quadrant Use of digital controls in fluid power drives research question to be addressed was the mechanism, its understanding and what impact the intensity has on typically used materials in fluid power components. Fig. 2 shows the effect at a nozzle for decreasing back pressure. It is taken from the dissertation of Werner Kleinbreuer in 1979 and hints can also be found in the 1983 thesis of Jörg Berger. What was learnt can be taken from the characteristics in the middle part. A jet was directed to a metallic surface using the test set up in the schematic on the right hand side. Over time a loss of mass appears and this loss is plotted via the nozzle sample distance and the normalized pressure. Inversed saddle curves are visible and maxima show up at certain counter pressures or distances to the nozzle. Kleinbreuer conducted the research with HLP oil and Berger with HFA fluids. It goes without saying that the intensity was much more aggressive with water based fluids. So in his dissertation the cavitation resistance of different materials is listed and they are still very valuable. The avoidance of cavitation must be the main thrust but in cases where it cannot be avoided the cavitation intensity must be minimized where the jet is directed to a surface letting the bubbles implode where no surrounding materials are in its way. The next example of major developments in the 1980s is devoted to the so called secondary control. Its origins go back to developments of Prof. Nikolaus at the Bundeswehrhochschule in Hamburg, publications by Kordak and the dissertation of Hubertus Murrenhoff in 1983. The idea was to fill the fourth quadrant in the drive matrix where you plot resistance control and displacement control horizontally and flow and pressure impressed supply vertically. Until successful research proofed otherwise it was considered impossible to set up a stable motor control fed by a pressure impressed supply. So research focused on the subject and the forth quadrant could be closed. Fig. 3 shows the general idea comparing a typical hydrostatic drive with a secondary controlled motor. In closed circuit the pressure difference changes its sign going forward to backward and during braking when recharging into the electric net or into a flywheel is possible. This is completely different in open circuit with secondary control. Here the volume flow changes its sign and the energy during braking can be fed completely back into the hydraulic net. This is a major advantage of the secondary control and an example will be provided later with the development of the STEAM system; treating recent developments. The advantage lies in the avoidance of energy transformation for the storage. Volume flow can be charged and discharged from an accumulator with minimum losses. No conversion of energy via hydraulic-mechanic or hydraulic-electric routes is necessary. However, a disadvantage in hydraulics is the missing of a continuously area adjustable cylinder. This limits the meaningful use of the forth quadrant to rotary applications in case a hydrostatic transformer is not considered for its conversion losses. So there is a huge demand to invent an adjustable cylinder and it can be seen as a challenge for engineering generations to come. For now it can only be approached with either multiple pressure nets or digital cylinders approaching the continuous stage in reasonable increments. 3 RD DECADE – THE 1990 TH The 90 th take us to the huge developments of electronic devices. Microprocessors became available after the introduction of the 8086 Intel processors and its successions. That way universities as well as industry were challenged to make use of this progress. Major breakthroughs happened as it became possible to calculate the whole control algorithm in and below one millisecond. This made it possible to compute state variables like speed and acceleration by applying intelligent derivative algorithms. An example depicting the idea is 40 O+P Fluidtechnik 7-8/2018

IFAS JUBILÄUM Reduction of static flow forces in spool valves Aachen IFAS hand with 11 degrees of freedom provided in Fig. 4. At discrete times the circuit is closed and all state variables are either taken from sensors as 05 digital signals or by deriving them by differentiation. This allowed state control and a lot of developments of control technology could be applied to hydraulic and pneumatic drives. This led to considerable improvements in its static and dynamic performance. Some reader might remember the active working group AK 13 within the fluid power section of VDMA. Next to many other dissertations Peter Ander’s thesis in 1986 was a trendsetter and guided the following scientific work in this field. It could be applied to hydraulic and pneumatic drive circuits and was responsible for the huge progress in servo-pneumatic drives for high performance pick and place assembly circuits. In later steps a field bus profile was developed within the VDMA pre competitive collaborative research work. The dissertations of Michael 06 Baldy in 1998 and Roland Bublitz in 2003 address relevant progress in this direction. We see it today incorporated into a lot of fluid power components and brand names like embedded systems or on board electronics dating back to these times. The next example deals with static flow forces appearing in hydraulic valves. They build a major disturbing force and the electro-mechanical drives need to overcome these forces. Today powerful CFD simulations are used but in the 1990s neither computing power nor the needed software was readily available. This made it necessary to use experiments to understand the underlying flow phenomena and the means required to compensate these forces. For economic reasons the goal was to use the smallest possible solenoid and that made it necessary to understand the interrelations. The dissertation of Joachim Feigel in 1992 addressed the relevant subject. Fig. 5 allows gaining some insight into his contributions towards the developments. Plexiglas models were built and flittering particles allowed following the flow at metering edges. He was able to show that guiding glands were able to almost completely compensate the forces created at the metering port. A typical characteristic can be seen in the lower left part and the results achieved were astonishing for just using experiments. Today we see compensations in all directly driven valves and samples are also depicted in the figure. The next contribution addresses the development of an anthropomorphic hand also known as the Aachen IFAS hand. The hand was driven by servo-pneumatic drives and it features 11 degrees of freedom. The underlying research work was conducted by Alexander Czinki and his thesis was published in 2001. Fig. 6 provides some insight into the development. For packaging purposes the hand was designed with just 4 fingers which are enough for all gripping tasks. Sideways movement was achieved by cylinder drives with levers and bending of the finger joints by either belt drives or rotary vane drives in the lower joint. Compared to competing mechanic solutions the finger force is much higher like human capabilities and on the other hand very dynamic for quick movements. This can be viewed in a video flipping a device. Another video shows the capability of turning a ball with just 4 fingers. Two fingers are in action for turning purposes and the other two reposition in that time. Videos are available at IFAS and for interested parties access can be granted. All steps taken are documented in the referenced thesis and further research is continued in Aachen. O+P Fluidtechnik 7-8/2018 41


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