FFT of the recorded vibration level at medium speed. The eigenfrequency of the portal robot is approx. 4Hz 

Context in source publication

Context 1

... object under investigation in this work is a portal robot used for the production of large fibre reinforced structures. Fig. 1 e.g. is showing the Computer-Aided Design of the portal robot. In this blueprint also the effector and the feeding tray are included. Fig. 2 shows the realisation of this portal robot. The dimension of the portal robot is approx. 7m x 4m. The achieved prod- uct quality and the production speed of the device are influenced by the vibration and the damping behaviour of the system. The required process time is determined by kinematic parameters, i.e. speed and acceleration of the system and the dead time required for the vibration level to decay to a threshold level given by the required precision of process. In the first step, the scheduled values of the machine are compared to the real achived values. Fig. 3 shows the scheduled machine parameters. The red curve shows the x-position of the effector, the blue curve represents the speed of the effector. Fig. 4 shows the difference between the scheduled values and the real machine data output. In the next step, a Design of Experiments is used in order to identify the ideal combination of acceleration, speed and length of the z-axis (the lever of the effector). At the optimum, the sum of the drive time and the dead time reaches a minimum for a given precision of 0.5mm peek-to-peek of the oscillation ringing for this process. Fig. 5 shows the process time at the effector position of -480mm. This effector position means that the lever of the effector is maximal. The blue bars represents the driving time, the orange bars are equal to the dead times of the vibration level to decay to the threshold level of 0.5mm peek-to-peek. The sum is the resulting process time. Fig. 6 and Fig. 8 show the vibration level during two exemplary processes. Fig. 6 shows the process at a medium speed level of approx. 800 mm s . During the phase of constant velocity the vibration levels out. A vibraton level beneath 0.5mm peek-to-peek is reached after approx. 15s (the color of the curve is switched from blue to black to visualize the vibration level is gone below the limit of 0.5mm peek-to-peek). Fig. 7 shows the FFT of the recorded vibration level at medium speed. The eigenfrequency of the portal robot is approx. 4 Hz. The eigenfrequency is changing with di- versified x-positions. Fig. 8 shows the process at a high speed level of approx. 1000 s . During the phase of constant velocity the vibration is not leveling out. But a vibraton level beneath 0.5mm peek-to-peek is reached after approx. 13 s. Fig. 9 shows the FFT of the recorded vibration level at high spped. The eigenfrequency of the portal robot seems to be approx. 4Hz. Fig. 10 shows the process time for the effector position of +480mm. This means that the lever of the effector is minimal. It is easy to see, that the dead time for the vibration level to decay to the threshold level of 0.5mm peek-to-peek is much lower at minimal lever. The blue bars represents again the driving time, the orange bars once again the dead times caused by the decay of the vibration. The sum of both is again the resulting process time. Fig. 11 and Fig. 13 show - analog to the ivestiga- tion with the maximal lever -, the vibration level during two processes with same velocity and acceleration as in the first part of the investigation. So Fig. 11 shows the process at the high speed level of approx. 1000 mm s . During the phase of constant velocity the vibration is again not leveling out. A vibraton level beneath 0.5mm peek-to-peek is reached after approx 14 s. Fig. 12 shows the FFT of the recorded vibration level at high speed. The eigenfrequency of the portal robot has changed to approx. 5Hz. This time the second Figure - Fig. 13 - shows the process at the medium speed level. Also during the phase of constant velocity the vibration levels out. A vibraton level beneath 0.5mm peek-to-peek is reached already after 10 s. Fig. 14 shows the FFT of the recorded vibration level at medium speed. The eigenfrequency of the portal robot has changed to approx. 5Hz. In the second step, an experimental Modal Analysis is performed in order to identify the potential for an optimization of the structure. Fig. 15 shows the result of the experimental modal analysis. The first eigenfrequency of the system is recognized at 4.89 Hz with a damping ratio of 0.99%. After the experimental investigations a finite element model was developped to describe the dynamic behaviour of the portal robot. Fig. 16 shows the result of a Finitie Element Simulation of the portal robot. In this model the first eigenfrequency of the system is determined at 5.31Hz with a damping ratio of 1.11%. Due to the results of the experimental analysis further inevstigations are carried out. The process has to begin and end at the maximal lever position (Pos# 1). There exsist several diffent possible ways to move the effector from its endposition to Pos# 1 are possible. Fig. 17 shows these options. The effector can be moved directly from the endposition to Pos# 1 (1). Another possibility is to first lift up the effector than move it translational and finaly lift the effector down to Pos# 1 (2). Also the effector can be lifted up or down diagonal (3) and (4). Fig. 18 - Fig. 21 show the result of the different moving options at maximum speed and maximum acceleration of the portal robot. At least, Option 1 takes about 12 s, Option 2 needs approx. about 7 s, Option 3 approx. about 10 s and Option 4 approx. about 14 s. Option2 seems to be the best driving way to achieve minimal process times. Experimental investigations concerning the dependance of process time and process parameters have been carried out. An increasing lever size leeds to an increasing time for vibration to level out. An experimental modal analysis has been performed and the portal robot was implemeted as a finite element model. This model can be used for virtual prototyping to evaluate the possible changes in the dynamic behaviour of the portal robot if the boundary conditions are changing. Finally an opti- mized driving way has been found to minimize the process ...