Dynamic Assessment of Sustainable Manufacturing Capability for CNC Machining Systems in Cloud Manufacturing

. Sustainability has been attracted extensive attention in manufacturing industry for the increasing excessive resource consumption and serious environmental pollution. The assessment of sustainable manufacturing capability is a vital factor, especially in cloud manufacturing, in which the capability assessment of the manufacturing resources is the premise of selecting the optimal manufacturing services. While the CNC machining tools are the basic manufacturing resources in production process, so the dynamic assessment of their sustainable manufacturing capabilities is investigated in this paper. A set of indicators is established considering manufacturing sustainability and a new dynamic assessment model is proposed based on a modified “vertical and horizontal” method. The model is verified through a case study and it can comprehensively evaluate the capability performance of the CNC machining systems, and also effectively follow the dynamics of their production activities.


Introduction
Currently, cloud manufacturing becomes a new manufacturing model under the promotion of manufacturing informatization. The capabilities of manufacturing resources are presented in the form of services consumers can share together globally. Selecting the optimal manufacturing resource from the global shared resource pool becomes a vital issue under cloud manufacturing. Meanwhile, sustainability has been the focus of heated discussion in recent years. There're many definitions about sustainability. The most acceptable one is defined at 1987 World Commission on Environment and Development [1], which states, "Meeting the needs of the present without compromis-ing the ability of future generations to meet their own needs." Generally, sustainable manufacturing is the reflection of sustainability in manufacturing. Meanwhile, manufacturing informatization makes digital design and manufacture become a way to improve manufacturing capability. CNC machining tools are widely used and they can solve many parts processing problems (e.g., high-precision, small batch and multi varieties production). They are the key to parts processing, which will greatly affect the processing quality, cost, time, energy, wastes, etc. So the assessment of manufacturing capability for CNC machining systems is of great significance.

Related Works
The concept of manufacturing capability was first proposed by Skinner in 1969 [2]. He stated that manufacturing capability is a kind of ability for enterprises to achieve manufacturing strategy and is composed of costs, quality, time and the relationship between these elements. Corbett [3] pointed out that manufacturing capability is consist of quality, cost, delivery speed, flexibility and innovation. Reference [4] defined manufacturing capability as time, quality, cost, technology, service, corporate performance .etc. when selecting virtual enterprise partners. In the field of sustainable manufacturing, some scholars have applied sustainability assessment to manufacturing decision making from economic, environmental and social aspects [5,6]. The relative assessment methods mainly have two categories: One is the subjective method which has certain subjectivity. The other is the objective method which cannot reflect the preferences of decision makers. So far, most assessment models are static. The dynamic assessments have already made some achievements [7,8], which have two classifications. One is a twice-weighted method which confirms the weights at different times; the other is dynamically dealing with the time series data, by which the weights are updated along with the update of data. For the former, reference [9] proposed a dynamic method based on Time Entropy to assess manufacturing capability, while it only considers subjective weights ignoring the effect of objective data.
Overall, the existing relative researches mainly have two shortages. Firstly, the researches solely focus on economic benefits without considering environmental impact and resource consumption. Secondly, most assessment methods are static which cannot reflect the capability dynamically. Therefore, this paper establishes a set of indicators system of sustainable manufacturing capability for CNC machining systems. A dynamic assessment method is proposed, which combine the advantages of subjective and objective assessment methods. Finally, "time-value coefficients" are introduced to aggregate data to obtain comprehensive capability value.

Assessment Model
For CNC machining systems, the indicator values are changing dynamically. Therefore, the issue of manufacturing capability assessment is a multidimensional decision making problem of three dimensions with indicators space, object space and time space. It can be supposed that there are n alternatives 12 , , , the dynamic time series data, the connotation of manufacturing capability and sustainable manufacturing, this paper mainly concern economic and environmental aspects. A dynamic assessment model is proposed as Fig.1.

Table 1. Dynamic timing database
Alternatives

Assessment Indicators
According to the connotation of manufacturing capability and the demands of sustainable manufacturing, besides quality, time and cost, the basic elements of manufacturing capability, resource consumption and environmental impact are also considered in this paper. Given the actual processing of machining system, the five factors can be decomposed to establish the indicator system shown in Table2.

Subjective Weight Calculation Based on AHP
Analytic hierarchy process (AHP) was first put forward by Saaty as multi criteria decision tool that quantifies the qualitative analysis [10]. It is used in this paper to determine the subjective weights. The method consists of the following steps:

Obtaining Time-value Coefficient
The system capability of different time () ik yt has been obtained from Eq. (2). The linear comprehensive assessment function can be defined as . .

5
A Case Study and Implementation

A Case Study
The case about selecting the machining system which has the optional sustainable manufacturing capability for machining a batch of shafting parts is presented. x surface machining precision; 3 x shape machining precision ), time( 4 x processing time; 5 x auxiliary time), cost( 6 x operation expenses; equipment depreciation), resource consumption( 8 x electricity), environmental impact( 9 x noise; 10 x cuttings; 11 x liquid wastes). The original data are shown in Table 3.   Table 4 by the expansion without affecting the ranking. From the results, we see 1 s is the optimal. x are the key factors of capability. More attention should be attached to them to improve the sustainable manufacturing capability. 4.The time-value factor  can affect the comprehensive assessment value. Take  as 0, 0.2, 0.4, 0.6, 0.8, and 1 respectively, the corresponding capabilities are shown in Fig.3. How to adjust the time-value coefficients to forecast the capability of machining system at next time is the next research content.

Conclusion
This paper focuses on the dynamic assessment of sustainable manufacturing capability for CNC machining systems. A set of indicators system is established from quality, time, cost, resource consumption and environmental impacts. Then a dynamic assessment method based on "vertical and horizontal" method and AHP considering "time-value coefficients" is proposed to assess the capability dynamically and effectively. Its application in a case study verified its advantages and effectiveness. The dynamic assessment model is also applicable to other dynamic decision making problems, such as economic decision-making, performance assessment of systems, etc.