(PSI Technics, 9/2013)
Innovative techniques helped determine various optimization approaches for a vertical material lift used in the automotive industry. An ideal optimization scenario was assumed to minimize energy consumption and include time-optimized trajectories that would not affect throughput, while keeping mechanical stresses to a minimum. Since the vertical material lift combines mechanical, electrical and PLC control engineering with closely interacting core components, the entire system had to be taken into account. Motion analysis was used to determine the system's current motion profile, which provided a basis for determining optimization potentials. All existing loading conditions and motion sequences were simulated in a virtual model using finite element methods (FEM), multi-body models and controller modeling. The results were verified against the actual operation of the system and illustrated the benefits of using an intelligent positioning controller.
The primary objective of the analysis was to reveal and demonstrate optimization potentials as well as approaches and solutions for harnessing those potentials. PSI Technics used motion analysis and virtual modeling for the analysis. Both methods support the design of automated industrial vehicles and conveyor systems with regard to modernizations and new installations.
The analysis was performed for a vertical material lift used in the Volkswagen AG factory in Wolfsburg, Germany. The system transports car parts and vehicle bodies to and from different floors of a body painting workshop. The parts are delivered and retrieved in a particular order, from the highest to the lowest workshop level. The unloaded system then travels back to its starting position. The system's motion sequences are controlled by a PLC. No positioning controller is used. The material lift reaches the individual loading and unloading positions at creeping speed and is stopped using proximity switches. Only the frequency converter drive has a speed control. Thus, the system is a typical vertical material lifting system commonly used in this industry.
The first step consisted of a motion analysis to determine the following aspects and to enable virtual modeling that would reveal initial optimization potentials:
The motion analysis was based on individual, particularly relevant conditions, such as frequently recurring loading conditions and/or loading conditions likely to cause a high amount of mechanical stress (Table 1). To enable a comparison between the current system and a system equipped with a positioning controller, two additional loading conditions that are specific to positioning control were required (Table 2).
During the motion analysis, the system's travel profiles were recorded using a high-resolution optical distance meter and evaluated. The results of the motion sequence analysis are shown in Figure 2.
During this analysis, the system's energy consumption was measured and evaluated for different loading conditions using an Accura 100 Nicolet S oscilloscope, a Chavix Arnoux clamp meter, a Contac 900 differential probe and the Flexpro analysis software v6.0.33. A total of 17 measurements, each lasting approximately 50 seconds, were performed to enable a qualified assessment of the energy consumption. The results are shown in Figure 3.
Virtual modeling uses mathematical models to create a virtual representation of existing factory logistics systems. Within certain limits these models simulate the operation of an existing logistics system. By using virtual modeling for designing controllers while simultaneously taking into account mechanical stresses, PSI Technics is able to obtain a very accurate representation of the actual system behavior. Virtual modeling has the crucial advantage of enabling a comparison between various system configurations in terms of mechanics, electronics, and open-loop or closed-loop control. Consequently, the stresses that are characteristic to a PLC-controlled system could be compared to those occurring in a system with closed-loop control.
To ensure a highly accurate mathematical representation of the existing vertical material lift, the entire system was divided into several sub-systems based on its individual components. The virtual modeling process essentially took place in several steps:
1. Finite Elements:
2. Multi-body model:
3. Closed-loop controller model:
The project was set up to enable a co-simulation between the multi-body model and the controller model following the creation of the virtual model.
For verification purposes, the data obtained during virtual modeling were verified against the operation of the system at run-time. Strain gauges were placed at relevant measurement positions on the vertical material lift. The strain gauge positions had previously been determined using static and dynamic finite element calculations. In addition, motion analyzing technology was used to measure the systemâ€™s mechanical stresses as well as the system kinetics.
Using motion analysis and virtual modeling, PSI Technics was able to provide the factory operator with comprehensive data for an optimized operation of the vertical material lift. The results obtained from the mathematical models were verified by placing strain gauges at the material lifting system at run-time. The verification data underlined the benefits of using a positioning controller.
Virtual modeling revealed significant potential improvements with regard to
Creeping speed can be completely eliminated and cycle times can be reduced by 5% by using a positioning controller. In addition, cycle times can be dynamically adapted to match the production process. Buffer zones can be reduced to ensure that the transported material is available just in time, at all times.
The elimination of creeping speed and shorter cycle times will reduce the system's energy consumption for each duty cycle. The system can be fitted with modern and highly efficient drive technology to enable energy recovery. Based on the load cycles of the modeled system, energy savings of 354% are possible. The same applies to CO2 emissions: The CO2 emissions for every single material lift can be reduced by 1894.2 lbs. (859.2 kg) per year.
Positioning control, improved drive technology and improvements to the loading and retrieval cycles will reduce peak mechanical stresses by approximately 10% (Figure 6). Moreover, oscillations would be significantly reduced, considerably relieving stress on individual system components which results in lower maintenance and an increased lifespan.
The results were used as a basis for creating a factory standard for the retrofitting of similar material lifts. The factory operator already advised the planned retrofitting of the first system. Moreover, the motion analysis and the virtual modeling approach will be used for future planning and for the design and implementation of new systems.
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Links>Application Reports >Modernization / Retrofitting >ARATEC & Positioning Modules >FLP6000EOS Energy Optimizing Software >FLP6000AOC Advanced Oscillation Control >MA6000MC Motion Analysis Kit >Performance Analyses >Custom Analyses - Virtual Modeling / Finite Element Method (FEM)
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