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12 CASE STUDY Test Facilities With its 9.1 square meters of jet surface space, the NWB facility is the second largest low-speed wind tunnel within the organization. At the facility, wind speeds of up to 300 kilometers per hour are achieved, roughly comparable to the speed at which a commercial aircraft takes off and lands, at a power consumption of 1.6 megawatts. On commission of aeronautical industry and research organizations, studies are conducted primarily into the dynamic load behavior of aircraft, helicopters and propulsion units. Apart from this, the NWB is also a test partner for the motor sport sector, to include commissioning contracts from the Formula 1. The wind tunnel experts in Brunswick have even tested such unusual items as bicycle helmets, lights and a complete sailboat with rigging in their tunnel. Manipulation – the right way! In aircraft development, it is of key importance to – at a very early stage – be able to evaluate the handling characteristics and the level of dynamic loads that occur. This is the only way to keep development risks low and, thereby, keep close track of development costs. Consequently, taking measurements of model aircraft movement in the wind tunnel is an important field of activity at the NWB. Models of the new giant A380 passenger jet “flew” in the wind tunnel in order to furnish designers with data on the aircraft’s dimensioning, as well as load estimates of the structural design during flight maneuvers and wind gusts. In most wind tunnels, model planes are moved in the measurement path using an elaborate mechanical lever construction, whereby the stem supporting the model extends beyond the measurement path, and is kept in a Cartesian arrangement from outside of the air stream with the help of mechanisms arranged in series. This solution, however, has the drawback that the required complex movements cannot be conducted with the desired speed and dynamics at the necessary level of rigidity. Special parallel kinematics in the tunnel Some of these drawbacks can be avoided with the help of the so-called Stewart platform, which is used in flight simulators for pilot training. As of 2000, NWB has also been using 6-axis parallel kinematics with hydraulic bars of variable length. The results, however, have never been entirely convincing. First, due to the operating principle, the hydraulic bars lack rigidity, while the inertia of the valves limits the dynamics and accuracy of the model’s movement. Second, the continuously switching valves excited vibrations in the models, thereby corrupting the results of the measurements. The solution based on the flight simulator also proved unsuitable for the motor racing measurements. The hydraulic platform stood on the floor, therefore precluding the customary wind tunnel measurements during which the vehicle model is held from above, while a conveyor belt a very short distance beneath it was moved. However, this arrangement can scarcely be implemented in any other way to simulate the relative movement of the vehicle and track. In addition, the hydraulic system was costly to maintain, and bore the risk of hoses bursting or becoming loose and soiling the measured section with oil. The kinematics and operator interface were developed for the DNW technicians in accordance with the special needs of flight tests Loss of control over the valuable models, which are “packed” with measurement technology, must be avoided at all costs All pictures: P. Koerber motion world 1/2007
13 New concepts from the machine tool world The “wind makers” of TAO Technologies managed to find a solution in cooperation with PKsystems (former employees of the ZFS, the center for manufacturing technology in Stuttgart which accompanied the project). Together, a parallel kinematic mechanism was developed, the so-called Model Positioning Mechanism, or MPM for short. The MPM kinematic mechanism also consists of one Stewart platform, which is connected to the wind tunnel via six rigid bars and six carriages positioned on two parallel guide rails. Given the fact that it is difficult to distinguish natural oscillations from the aerodynamic effects when evaluating the dynamic forces and torques, linear motors are employed to move the carriages. First choice: linear motors from Siemens Six Simodrive 1FN3 linear motors from Siemens form the heart of the MPM 1FN3 linear motors from Siemens were the first choice when selecting the drives, which are also popular at the ZFS for parallel kinematic designs. The natural oscillations of the test facility now no longer pose a problem: “With the MPM, the motion is a good order of magnitude better than previously was the case with the hydraulics,” as Thomas Löser, project supervisor at DNW-NWB, confirms. Three carriages are mounted on one axle respectively. As a result of the two-guiderail configuration, the design requires fewer components, thus cutting costs. All of the carriages can be shifted independently of one other, enabling movements with six degrees of freedom, and providing a work space of 1,100 millimeters in the wind direction, 300 millimeters laterally and 500 millimeters in the lift direction. The design achieves accelerations of up to 2.5 g, with the rigidity remaining virtually constant throughout the entire work area, as well as angle settings with an accuracy of less than 0.005 degrees. Tailoring the coordinate transformation To allow the engineers at DNW to program the forms of aerodynamic motion in a Cartesian coordinate system with amplitude data, frequencies as well as freely definable axes of rotation, a Sinumerik 840D was selected to serve as a control platform to support the standard coordinate transformation. The OEM interface of the Sinumerik 840D enables the integration of the 54 parameters needed for the kinematic description of the transformation into the control, which are incorporated into the NC core of the basic control using compile cycles. The operator interface was also configured according to test engineer designs. Safety first in the tunnel As a result of the fact that the masses set in motion, and thus the kinetic energy, are very extensive, and the model aircraft also very expensive, Sinumerik 840D is equipped with the Safety Integrated components to fulfill safety-relevant requirements, also enabling the safety-oriented functions of “safe speed,” “safe stop positions” and “integrated brake test.” The brake test is particularly essential, because there is no counterweight for the highly dynamic axles. At the high traversing speed of the utilized linear motors, this function enables the prompt signaling of safety-relevant errors in accordance with category 3, thereby ensuring a high degree of protection for man, machine and last, but not least, also models. “We must always keep in mind that a highly sensitive strain gauge scale is built into the model and can be overloaded very easily. If control over the linear motors is lost, the weight scale could be damaged, which is valued at 100,000 euros,” Löser explains. Measurable success The test facility has been up and running since the end of 2004 without any faults. Measurements, for example, on the new Airbus A380, have revealed that the expectations of DNW regarding positioning speed and accuracy have been completely fulfilled. With the help of the new dynamic MPM, it is now for the first time possible to realistically simulate flight maneuvers in a wind tunnel. More information: www.siemens.com/sinumerik E-mail: thomas.wissert@siemens.com motion world 1/2007
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