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Twin-Control: a key tool for energy efficient machine tools

By Dominik Flum,
Philipp Schraml, Niklas Panten and Christoph Bauerdick

Researchers at the Institute of Production Management, Technology and Machine Tools (PTW) at the Technical University of Darmstadt.

Energy efficiency becomes an increasingly important quality attribute of modern machine tools. In recent years, the awareness of society, politics and industry on this subject has increased, too. One reason for this is the noticeable environmental impact, which yet again leads to both rising customer awareness and additional legislative regulations. The manufacturing industry, as one of the main consumers of global primary energy and producer of related emissions, represents a great lever to reduce the energy demand worldwide (IEA 2008). Furthermore, energy is an increasingly important cost factor. Studies have shown that the energy costs for machine tools are already responsible for up to 26% of the running costs – excluding labor, tooling and material costs. Due to rising energy prices, this share is expected to increase further and consequently the significance of the energy efficiency factor, as one target dimension for production machines besides the classic dimensions precision, power, investment costs and reliability will rise. Within production engineering, the manufacturers of production machines can notice a high customer demand for energy-efficient machines. In order to stay competitive and in liability towards our environment, the energy demand of machine tools must be significantly reduced without deteriorating the productivity and the quality of manufacturing.

Insufficient evaluation possibilities are one of the main obstacles for an implementation of energy efficiency measures. The energy efficiency module of Twin-Control seeks to determine the energy demand in the machine tool development process, procurement process as well as in its use phase. To gain a most widely transparency, the energy demand will be observed and simulated on a component level. The highest potential for a higher energy efficiency lies in the planning or design phase of machine tools. For that, a machine tool configurator for energy efficient machine tools based on simulation models will be developed. The simulation models are also applied for an energy monitoring, since real-time capable versions will be derived.

Simulation of energy demand

A model library within the simulation environment Matlab/ Simulink/ Simscape will be developed, which is then used as the basis for the configurator. It allows for an adaption of the models to a certain use case. The output of the configurator is the predicted energy demand of each energy relevant module and, by aggregating, of the whole machine tool for a given production process. This gives the opportunity for machine tool builders to compare different configurations, components, etc. in the machine tool design phase. For their customers, it is possible to use the configurator for an assessment in the procurement process of a machine tool. With the proposed models, based on Matlab/Simulink, every functional component relevant for the energy demand is mapped (example in Figure 1). Here, the physical input parameters are easily accessible information obtained from data sheets of the components.


Figure 1: Examples for hydraulic and electric system modelling using Simscape library

Energy monitoring

The first step towards energy demand reduction is taken by making the user aware of the dependencies of his actions (control inputs) on the power demand. The two energy monitoring approaches to be developed within Twin-Control provide the user with a clear view of the power demand in direct combination to changes of the state of PLC variables. For each machine tool component, the operator is informed about the current power demand and can react when the demand is above of what it was with a previous setup.

The objective of this module is to set up an energy monitoring system while simultaneously avoiding the extensive use of expensive hardware (hall) sensors. The result of both approaches is a drastic reduction in investment costs for an energy monitoring solution on component level.

Energy monitoring with simulation models:

Real time energy demand can not only be measured but also simulated. The combination of compiled Matlab/Simulink models and the ARTIS monitoring hardware will make this possible. Energy optimized strategies, by simulating the effect of parameter variations, can be tested as well.

Figure 2: Energy monitoring with simulation models

Figure 2: Energy monitoring with simulation models

It should be noted, that this is a straightforward approach, which makes use of the simulation models that will be developed for the Simulation of Energy Demand-Part of the Twin-Control project.

Energy monitoring by system identification:

The idea of this second approach is to use the logical control information coming from the machine. In this task a self-learning energy monitoring system will be developed, connecting PLC switching information with measurement data of a single power monitor at the main connection of the machine.

Knowing the exact point of time where dedicated machine components are switched on or off, the actual power demand can be mathematically derived. The approach is based on the fact that individual appliances have different characteristics for steady-state and transition states in both reactive and active power, so called “energy signatures”. The appliances’ loads are superimposed at the point of common coupling (PCC) and the individual curves can be extracted from the aggregate data by pattern detection algorithms (Figure 3).

Figure 3: Energy monitoring by system identification

Figure 3: Energy monitoring by system identification

By analyzing this individual power curves on a component level it is possible to use the energy data for condition monitoring. Every deviation from the normal energetic component behavior can be detected and adequate countermeasures can be taken.

About PTW

The Institute of Production Management, Technology and Machine Tools (PTW) is a research institute within the faculty of mechanical engineering at the Technical University of Darmstadt. PTW is particularly active in the field of sustainable production and has routed this research focus as a priority set in its own strategy. With research projects in the fields of energy efficient machine tools as well as energy and resource efficient production, PTW has experience in solutions which address all aspects of sustainable manufacturing. This includes the energetic optimization, simulation and the energetic assessment of machine tools and auxiliary devices and the development of energy and resource efficient process chains. Within the research project Twin-Control, PTW will contribute as a participant with its broad knowledge on energy efficient production systems, energy efficiency optimization measures and the experience in simulating the power and energy demand, demonstrated in a wide range of national and international research projects.

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