Abstract
Machine tool, which is the basis energy consumed device in manufacturing system, its energy consumption model and energy efficiency evaluation are the prerequisites for energy saving in manufacturing. Considering the multi-energy-sources features，analysed the mathematical model of a CNC continuous generating grinding machine tool from the view of energy constitute. The energy sources of a CNC continuous generating grinding machine tool are classified into gear grinding system, grinding wheel dressing system and auxiliary system. Based on the power balance equations of energy flow, the energy flow system of a CNC continuous generating grinding machine tool is established. And then the energy consumption mathematical model of a CNC continuous generating grinding machine tool is set up by combining the power balance equations with the operating features of three kinds of energy sources. The case study shows that the proposed energy consumption model can provide fundamental support for energy consumption forecasting, energy efficiency analysis and energy-saving optimisation during the machine tool operation process.
Introduction
Machine tools are the basic energy consumption devices in manufacturing, whose energy saving cannot be neglected (Liu, Wang, and Liu Citation2013). The energy yearbook published by the US energy information administration in 2012, showed that machine tools electricity consumption occupied 75% of manufacturing electricity consumption, and it was an important part in manufacturing electricity consumption. To achieve the green production of the machine tools industry, the International Organisation for Standardisation drafted the standard ‘environmental evaluation of machine tools’ in 2010 (ISO14955-1, 2014) (Zhou et al. Citation2016). It focused on energy consumption test procedure of metal cutting and design methodology for energy-efficient machine tools. Predictably, the energy-saving specification will be an important index of the machine tools product in the future. The Japanese Standards Association (JSA) published the TSB0024 standards (Test Methods for Electric Power Consumption) to measure the power consumption of machine tools, and this prompted the development of efficient, environmental and friendly machine tools (Yoon et al. Citation2014). The ‘Made in China 2025’ proposes the full implementation of green manufacturing to accelerate green upgrade and build efficient, clean, low carbon, recycling green manufacturing system. It aims to reduce energy consumption and pollutant emissions. Meanwhile, growing energy demand and rising energy prices force manufacturing to seek for high energy efficiency and low-cost solutions. China’s machine tools have the characteristics of large amount of usage, wide application and huge energy consumption, however, the effective energy utilisation is low. And so there is a great potential to realise energy saving for machine tools (Wang and Liu Citation2013).
Many domestic and abroad institutions and scholars conducted extensive studies around energy consumption, energy efficiency and the corresponding monitoring method of machine tools or workshops. Assumed that the output power is a function related to the spindle speed and the input power, Murari (Citation1977) used the approach of multiple linear regression analysis to determine their quantitative relationship and proposed a model of power loss and energy efficiency. According to the experimental analysis, Draganescu, Gheorghe, and Doicin (Citation2003) established a mathematical model of the machine energy efficiency and process parameters to predict the machining energy. And the design of high-speed dry gear hobbing for green manufacturing and technology of dry cutting was studied by Li (Citation2003). Liu (Citation2005) discussed systematically the theory and technology of green manufacturing, in terms of the introduction of green manufacturing, theoretical system, key technology, related environmental management system standards and regulations. Gutowski, Dahmus, and Thiriez (Citation2006) collapsed the specific electrical energy requirements for a wide range of manufacturing processes into a single plot. The results showed that the process rate is the most important variable in estimating energy requirement. Dietmair and Verl (Citation2009) found an energy consumption model of machining system, using it able to study the energy consumption of any given machining systems. Shi et al. (Citation2010) established power balance equations about the CNC machine tools’ main transmission system driven by variable-frequency adjustable-speed motor. In addition, the power transition features of motor, mechanical transmission system and the energy loss of each section were also analysed. Balogun and Mativenga (Citation2013) studied the effects that machine modules, auxiliary units and machine codes have on power and energy consumption during machining, and measured the current electrical consumption. This research logically contributes towards the development of a new mathematical model and predicting direct electrical energy demand in machining toolpaths. As can be seen from the research status of the machine tools energy consumption, the current study focuses on the energy consumption characteristics and energy flow models of general machine tools. However, there is a lack of energy consumption model and energy efficiency evaluation for CNC continuous generating grinding machine tools. Compared with common machine tools, the structure and energy features of CNC continuous generating grinding machine tools have occurred great changes. The machine tool with 10-axis numerical control and 5-axis linkage uses the classical vertical structure layout form. The big column moves to realise the radial feed and each axis is independently driven, as shown in Figure . It owns unique energy characteristics, namely close energy correlation, complicated energy consumption regularity and multiple energy consumption components. The energy consumption of main transmission system accounts smaller for the proportion of the entire machine tool energy consumption. The rated power of main transmission system motor is 38 kW, less than 30% of the total rated power of the whole machine tool. Thus, the energy consumption study of single main transmission system and general equipment cannot fully reflect the CNC continuous generating grinding machine tool energy consumption characteristics. In order to acknowledge the whole machine tool multi-source energy flows and energy efficiency evaluation, study each subsystem of machine tools consumption characteristics and energy flow is necessary. Based on the characteristics of multiple energy flows, many energy flow links and complicated flow process, this paper intensively studied the mathematical model of multi-source energy flows of a CNC continuous generating grinding machine tool.
Figure 1. The structure of CNC continuous generating grinding machine tools.
Energy consumption features and energy flows
CNC continuous generating grinding machine tool integrates mechanical, electrical, hydraulic and interdisciplinary technology, which obviously improves machining accuracy and work efficiency. Overall, the energy consumption consists of three parts, including mechanical, electrical and hydraulic consumption (Chen Citation2013), as shown in Figure .
Figure 2. Energy consumption of CNC continuous generating grinding machine tools.
Energy consumption system of CNC continuous generating grinding machine tools is a dynamic process of input and output, which has three characters. First character, is composed of many energy consumption components and energy consumption is huge. The energy consumption components include spindle motor, feed motor, inverter, servo driver, hydraulic cooling, lubrication, chip removal, flushing, lights, fans and other auxiliary system components. Taking YW7232CNC grinding machine tools for an example, the total motor amount has reached 15 even more, as shown in Table . Second character, is that has plenty of energy flow links and complex energy flows. The generating motions consist of wheel rotational motion, feed motion along Z-axis and work-piece rotational motion. Each motion contains no-load energy consumption, machining load energy consumption, additional energy consumption and the friction energy consumption between various transmission chains, respectively. Third character, is that the start process and machining process are relatively specific. The start of grinding machine tools is usually the pattern of Starting-Standby-Empty-Grinding. Each stage of the pattern brings out the certain part of the energy consumption. Therefore, the corresponding phase transition of power is relatively stable. According to the requirements of gear processing level, the selection of processing of gear grinding changes less and thus, the energy consumption regularity presents a consistency. It is beneficial for different grinding machine tools to apply the same monitoring methods, which contributes to enhancement of the universality and practicability of monitoring system.
Table 1. Energy sources of CNC continuous generating grinding machine tool (YW7232CNC).
Since, CNC continuous generating grinding machine tool possesses complicated energy consumption regularity and multiple energy consumption components, it is necessary to divide the energy sources into several categories. In establishing energy consumption model, the energy sources are distributed into three classes, as showed in Figure : gear grinding system (including main transmission system and feeding system), wheel dressing system and auxiliary system. The energy source of gear grinding system provides the direct energy for grinding and power transmission, which is related to the machining load. The energy source of wheel dressing system supplies the energy required for the profile modification of the worm wheel and the power transmission. The energy source of auxiliary system is used to support the completion of gear grinding tasks. Further, the energy source of auxiliary system can be broken down into machining, dynamic and others three categories. The machining energy source of auxiliary system is started when the machine tool is working. The dynamic energy source of auxiliary system is synchronised with the open or closed status of the power system. Other energy source of auxiliary system generally turns on according to machine tool general power.
Figure 3. Energy flows of CNC continuous generating grinding machine tool.
The model of energy flows
The energy from input to output is seen as an energy flow during the grinding process. It is hard to study every energy consumption component because of the features of huge and complex energy consumption. By thinking of the CNC continuous generating grinding machine tools as a large system, the total energy flow is divided into three parts, namely energy flows of gear grinding system (including main transmission system and feeding system), wheel dressing system and auxiliary system.
(1)
where PM = the total power input of grinding machine tool; PS = the input power of main transmission system; = the input power of certain feeding system; Py = the input power of wheel dressing system;
= the input power of certain auxiliary system; m = the total number of feeding systems; n = the total number of auxiliary systems.
Energy flow modelling of main transmission system
Electro-spindle is used in grinding machine tools as a spindle driver. The torque motor integrates motor and rotating parts with zero main driving chain, which is called zero transmission. The energy consumption of main transmission system comprises an inverter, a torque motor and the main transmission mechanical system.
The energy consumption of inverter can be calculated from the following relationship (Hu et al. Citation2009):
(2)
where Pa = the energy consumption of inverter; = the forward loss;
= the switching loss;
= the recovery loss
The energy consumption of the spindle motor is given by:
(3)
where Pb = the input power of main transmission motor; and
= the iron loss and copper loss; Pst = the stray loss; Em = electromagnetic loss;
= the kinetic energy loss; Pfs = the friction loss of the rotor; Pbs = the additional load loss; P1 = the output power of the main transmission motor.
The energy consumption of the main transmission mechanical system can be written as:
(4)
where Pc = the energy consumption of the main transmission mechanical system; Pm = the output power of the main transmission mechanical system, namely grinding power; = friction loss of the transmission parts;
= the kinetic energy loss of transmission parts; k = the total number of transmission parts.
Thus, according to Equations (2)–(4), the energy consumption of main driving system can be presented as follows:
(5)
Energy flow of feeding systems
The energy consumption of the feeding system consists of servo drivers, servo motors and the feeding mechanical system.
The energy consumption of AC servo driver can be written in the following form (Bland et al. Citation2001):
(6)
where Pe = the energy consumption of AC servo driver; = the switching loss of single IGBT module;
= the conduction loss of IGBT module; δ = the duty ratio; Ir = the heat rating current; Fsw = the nominal carrier frequency; Vorel = the modulation.
The balance equations of servo motor power can be established:
(7)
where Pf = the input power of feeding motor; = the output power of feeding motor.
The energy consumption of the feeding mechanical system be written as:
(8)
where Pg = the energy consumption of the feeding mechanical system; Pn = the output power of the feeding mechanical system.
The grinding process needs several feeding systems to work together because of the requirement of mechanical linkage. Thus, the energy flow of feeding system can be expressed as:
(9)
Energy flow of wheel dressing system
Wheel dressing is mainly used for trimming the profile and crest circle of the wheel. It is composed of servo driver, servo motor and the dressing mechanical system.
The computational model of energy consumption of wheel dressing system is similar to the feeding system as given by:
(10)
where Ph = the energy consumption of servo driver; Pk = the energy consumption of servo motor; Pl = the energy consumption of the dressing mechanical system.
Energy flow of auxiliary systems
The auxiliary systems of CNC continuous generating grinding machine tools can be divided into two classes. One is related to the motor, including hydraulic, cooling, lubrication, chip removal, flushing and others, and the other is opposite, including lights, fans, and computer and so on. As a result, the more energy consumption components, the more complex regularities of energy consumption are.
However, the energy consumption of auxiliary systems is relatively stable and less affected by the load (Hu Citation2012), so rated power can be used to approximately obtain actual input power.
(11)
where = the activation function of auxiliary systems， Hi(t) = 0 represents used situation, Hi(t) = 1 represents unused situation. Pi = the actual input power of certain auxiliary system.
The operation energy consumption of CNC continuous generating grinding machine tools consists of four parts, namely, the energy consumption of main transmission system, feeding system, wheel dressing system and auxiliary system. In summary, the mathematical model of multi-source energy flows can be written as follows:
(12)
Application analysis of the model of energy flows
Data supporting for different operating conditions, energy links and energy sources can be provided by monitoring the process of energy flows and counting energy consumption of each energy link. Besides, it also provides scientific basis for energy efficiency analysis, forecasting and energy optimisation.
Establish the evaluation model of energy efficiency
The energy consumption of CNC continuous generating grinding machine tools is closely related to operation time, machining parameters, the number of work pieces and the structure of machine tools. However, only the instantaneous power consumption and the total energy consumption were considered, it could not reflect the real energy utilisation efficiency of grinding machine tools. There are two main methods for evaluating the energy efficiency of machine tools which are instantaneous energy efficiency and process energy efficiency (Liu and Wang Citation2013).
The instantaneous energy efficiency is the rate of the effective energy change and input energy at a certain moment of a mechanical manufacturing system. For the grinding machine tool, its instantaneous energy efficiency can be defined as:
(13)
where η(t) = the instantaneous energy efficiency; Pm(t) = the instantaneous grinding power; PM(t) = the instantaneous total power input.
Process energy efficiency is the ratio of the effective energy consumption of a manufacturing process or a period time and the total energy input of a mechanical manufacture system. Because CNC continuous generating grinding machine tool is mainly used for grinding metal, its effective energy consumption refers to the grinding energy. In the case of a work period from T1 to T2, the process energy efficiency of grinding machine tools can be given by:
(14)
Not only can get the value of energy efficiency, but also can analyse influential factors of the energy efficiency, the influence degree of the energy flows and energy sources on energy efficiency. Therefore, it can provide theoretical data for energy saving and optimisation of grinding machine tools.
The prediction of energy consumption
Using a certain CNC continuous generating grinding machine tool as an example and assuming that the machine tool adopts single-step processing. If the machining time is ts, the time of power system operation is tk, the time of wheel dressing is td and the total machining time is tm, as showed in Figure .
Figure 4. The operation period of CNC continuous generating grinding machine tools.
The calculation method of energy consumption can be written as follows:(15)
where Pcl = the power of cooling system; Pcr = the power of chip removal system; Pfl = the power of flushing system; Pos = the power of oil mist separation system; Pfa = the power of fans; Phy = the power of hydraulic system; Plu = the power of lubrication system; Pli = the power of light system.
Since the processing adopts numerical control, each production time can be obtained from the programmes. The power of main driving system, feeding systems and dressing system can be measured by power sensors. Other power parameters can refer to brochure of machining. This application analysis provides solutions of the prediction of energy consumption, but the resolution process has yet to be detailed.
Conclusion
Considering the multi-energy-sources features，analysed the mathematical model of a CNC continuous generating grinding machine tool from the view of energy constitute. The conclusions can be drawn from this work:
| (1) | Introduced the structure and energy consumption features of grinding machine tools in detail. There are more than 15 motors and energy consumption is huge and complicated. | ||||
| (2) | Divided the energy sources into three classes: gear grinding system (including main transmission system and feeding system), wheel dressing system and auxiliary system. Then established the power balance equation of each energy flow and obtained the mathematical model of multi-source energy flows. | ||||
| (3) | Research on the energy efficiency evaluation method of CNC continuous generating grinding machine tools. Discussed two methods to evaluation the energy efficiency and highlight the significance of using the mathematical model of energy efficiency to evaluate energy utilisation efficiency of grinding machine tools. | ||||
| (4) | Through the case analysis, provided specific and feasible method for the prediction of energy consumption. | ||||
Disclosure statement
All the authors have no conflict of interest.
Funding
This work is supported by the National Natural Science Foundation of China for ‘Research on the Five-Axis Form Grinding Method for Special Shaped Helicoid Based on Envelope to a Two-Parameter Family of Point Vectors’ [grant number 51375512]; and Foundation item: Project supported by the National Science and Technology Support Program, China [grant number 2014BAF08B02].
Notes on Contributors
Jiang Ping, is a postgraduate student at Chongqing University, China. Her main research interests are in precision grinding and computer numerical control.
Li Guolong, PhD, is a professor at State Key Laboratory of Mechanical Transmission, Chongqing University, China. His research interests include intelligent numerical control technology and system, precision machining technology and equipment and complex curved surface grinding technology. Previous publications have appeared in the Chinese Journal of Mechanical Engineering, China Mechanical Engineering, Computer Integrated Manufacturing System, and others.
Liu Pengxiang, is a postgraduate student at Chongqing University, China. His research is centered on precision grinding technology. Previous publications have appeared in Chinese Journal of Mechanical Engineering, Computer Integrated Manufacturing System.
Jiang Lin, PhD, works at State Key Laboratory of Mechanical Transmission, Chongqing University, China. His research is CNC worm grinding gear machine precision grinding technology. Previous publications have appeared in the Chinese Journal of Mechanical Engineering, China Mechanical Engineering, and others.
Li Xiaozhuo, is a postgraduate student at Chongqing University, China. His main research is CNC machine reliability analysis.
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