4.1 Development and ontology-based knowledge model
The regional ontologies are constructed to offer a general representation of knowledge in the several fields. Based on the classifications of ontology [52] and the identification of ontology features [54], the schema of regional ontology can be represented as
(Reg_O)m = [Nm, Pm, Lm, Cm, Tm], m∈(u, v, w) (1)
Where u, v, and w represent component, process proposal, and fabrication respectively. Notion group Nm reveals the traditional notions applied in the mth regional field; Property group Pm indicates some homogenous properties of a definite notion to assure it is intelligible and specified; Relation group Lm depicts the relations between notions in the mth regional field; Case group Cm shows the actual examples for the notions; Criterion group Tm refers to the specific criteria, which depict the restrictions of the property values for notions or properties, and the approaches of assigning property values for notions or properties.
To express the ontology structure of non-traditional manufacturing-related element clearly, the partial notion hierarchies of regional ontology for the three specific fields are illustrated in Fig. 5. The notion hierarchy of component ontology is applied for depicting and modeling fundamental sorts of component-specific messages. The notion hierarchy of process ontology is the connection between component and fabrication two information and its superior tier depicts the specified process information. The notion hierarchy of fabrication ontology is utilized to depict fabrication-specific messages. The notions in the three fields are not restricted to the ones depicted above. By using the representation of ontology, it not only permit adding field-specific notions, but do farther categorization. In regional ontology, each notion has particular properties, such as identifier (id), title, content, etc. In addition to its own properties, the inferior-notion also has the general properties inherited from superior-notion.
4.2 Approach of ontology-based knowledge integration
As illustrated in the central portion of Fig. 5, the regional ontologies are able to be merged as a universal ontology by criteria of relevance, which can promote reuse and share of messages. Regarding the relations and restrictions among various fields, they can be represented as criteria of semantic and element relevance, which are translated in semantic web rule language (SWRL). In col-NTMT environment, each component is fabricated by at least one process proposal, and each manipulation in process proposal applies at least one item of non-traditional manufacturing. In the phase of initiatory design, each case of notion has the specific identifier (i.e. id), so the criteria of semantic relevance can be constructed to deduce the original relevance among cases of component, process proposal and non-traditional fabrication by the consistence of identifier. When the cases of process proposal and component have the identical id of component, is_fabricated_by, a relation, is able to be reasoned to depict that the component is fabricated through the process proposal, and the SWRL is defined as:
Component(?i),component_id(?i, ?j),ProcessProposal(?k), component_id(?k, ?j) -> fab(?i, ?k) |
When the cases of manipulation and nt-manufacturing have the same nt-fabrication id, a relation, utilize, can be reasoned to depict that the manipulation utilizes the nt-fabrication, and the SWRL is defined as:
Maniuplation(?i),nt-fabrication_id(?i, ?j),Fabrication(?k), nt-fabrication_id(?k, ?j) -> utilize(?i, ?k) |
Here, ?i, ?j, and ?k represent various variables, which indicate cases of component, id number, and process proposal based on the past principle [55].
As soon as the original relations among cases have been built, the cases with detailed elements are improved by engineers. When the itemized elements of cases have related restrictions in three fields, e.g. geometric type, material property, and availability of fabrication, the restrictions are constructed as the criteria of element relevance, and restriction, a relation, is assigned the relationship between two items. For instance, restriction(Rm, Rn) represents that specific elements of item Rn are decided through the relevant elements of Rm. Meanwhile, item Rm and Rn belong to two various fields which are linked by relations is_fabricated_by or utilize. If the relevant elements of item Rm are known variants, the criteria of element relevance can deduce specific element values for item Rn based on the elements of item X. In addition, the relation restriction is added between item Rm and Rn.
The elements are defined in ontology of component should be converted into a group of manipulations defined in ontology of process. Besides, the elements of component may be restricted by process capability. There is an element through-hole, for example, on the component, its accuracy requirements: Dh (diameter) = 4.5 mm; IT (international tolerance) grade = 4 ~ 6; Ra (surface roughness) = 1.25 ~ 2.5 µm. To achieve the above-mentioned demands, the process proposal for this through-hole should be Drilling, Fine boring, and HS-WEDM (High speed-wire electrical discharge machining) in order. The criteria of element relevant can be depicted as:
Component(?u),Through-hole(?th),has_element(?u,?th),Diameter(?th,?d),IT(?th,?it), SR(?th,?sr),equal(?d,4.5),less_than_or_equal(?it,6),greater_than_or_equal(?it, 4),less_than_or_equal(?sr,2.5),greater_than_or_equal(?sr,1.25),Process Proposal(?v),is_fabricated_by(?u,?v),Manipulation(?i),is_component_of(?i,?v),Manipulation(?j), is_component_of(?j,?v),Manipulation(?k),is_component_of(?k,?v),priors_to(?i,?j),priors_to(?j,?k) -> Drilling(?i),Fine-boring(?j),HS-WEDM(?k),restrictions(?th, ?i),restrictions(?th,?j), restrictions(?th,?k) |
Here, the item “?th” is an element through-hole of component “?u” and the “?u” is fabricated by process proposal “?v”. The “?v” includes three manipulations”?i”, “?j”, and “?k”. Hence, the deducted truth is that “?i”, “?j”, and “?k” are restricted through “?th”, and the cases are Drilling, Fine boring, and HS-WEDM separately. Also this mode can be applied to depict the restrictions of process proposal on element of component.
The elements are defined in ontology of component may decide the variables of the utilized nt-fabrication. If a fixture, for example, is applied in the manipulation for a component, the sizes (length and width) of the upper pressure plate are decided by increasing 37 mm length and 12.5 mm width on the existing sizes of component, and it can be represented as:
Component(?u),Dimension(?i),has_element(?u,?i),Length(?i,?j),Width(?i,?k),Process Proposal(?v),is_fabricated_by(?u,?v),Manipulation(?x),is_component_of(?x,?v),nt-fabrication(?y),utilize(?x,?y),Upperpressureplate(?upp),has_fixture_unit(?y,?upp), Upperpressureplate-variable(?var),has_fixture_variable(?upp,?var),increase(?p,?j,37),increase(?q,?k,12.5) -> UpperpressureplateLength(?var,?j),UpperpressureplateWidth(?var,?k),restrictions(?i,?var) |
Here, the item “?i” is an element dimension for component “?u” and the “?u” is fabricated by process proposal “?v”. The “?x” is a manipulation of “?v”, nt-fabrication “?y” is applied by “?x”. The “?y” has upper pressure plate “?upp” and “?upp” has variable “?var”. It can deduced that the dimension “?i”, restrictions “?var”, and values of “?p” and “?q” are specified to “?var” to depict the length and width of “?upp”. Furthermore, availability of nt-fabrication may restrict the component element and can be depicted as the identical mode as noted above.
The elements are clarified in ontology of process proposal depict the fabrication demands beforehand and restrict the group of fabrication capable of implementing the process. If a Die-sinking EDM (wire electrical discharge machining) is applied in the rough machining of blind hole, then the total amount of electrical corrosion for the electrode is decided by the two electrical variables, pulse width and peak voltage. It can be depicted as:
Roughmachining(?rm),Manipulationvariable(?mv),has_manipulation_variable(?rm,?mv),Pulsewidth(?mv,?pw),Peakvoltage(?mv,?pv), EDM(?edm)utilize(?x,?edm),Electrode(?e),has_EDM_unit(?edm,?e),Totalamountelectricalcorrosion(?aec),has_EDM_variable(?e,?aec),union(?s,?pw,?pv) -> Totalamountelectricalcorrosion(?aec,?s),restrctions(?mv,?aec) |
Here, the rough machining “?rm” applies Die-sinking EDM “?edm”, “?rm” contains pulse width “?pw” and peak voltage “?pv”; “?edm” has “?e”, and “?e” includes variable “?aec”. The “?pw” and “?pv”, two relation restrictions, can be deduced, and the value “?s” is imported to “?aec” as the total amount of electrical corrosion for “?e”. Furthermore, the restriction of nt-fabrication availability on process element can be depicted as the same mode as noted above.
Different criteria of element relevance are extended in col-NTMT, which are usually elicited on the basis of existing experience and analytical knowledge from domain members (e.g. engineers, technicians, and experts, etc.) and documents (such as technical journals, manuals, articles, and so on). They also offers support of decision making with the reasoned truths except integration of information.
4.3 Approach of similarity mating
To enable the proper empirical information can be extracted for sharing to instruct relevant tasks, the semantic mating method also has to be developed except the representation and integration of information. In general, the procedure of ontology mating guided on similarities of titles and contents of notion and its relations [37, 56–57]. The contents of notion are represented as properties of notion. Furthermore, the messages regarding component, process proposal, and fabrication can be extracted with titles and default properties of notion. The extraction of ontology deals with a process of similarity for titles and properties of notions based on the previous method presented [58–59]. The procedure of similarity calculation is shown in Fig. 6 and the details are depicted below.
In this study, the similarity of notion title is computed through the past researches [60–61]. The titles of notion are split as item groups of bunch words. The similarity for item groups of two various notion titles is then computed by the following equation.
Sim N(Nq, Nu) = \(\left|{N}_{mg}^{q}\cap {N}_{mg}^{u}\right|\) / \(\left|{N}_{mg}^{q}\cup {N}_{mg}^{u}\right|\) = h / (i + j – h) (2)
Here, Nq means the query notion offered through the member; Nu indicates the title of notion in universal ontology; \({N}_{mg}^{q}\) shows the item group of the notion Nq; \({N}_{mg}^{u}\) means the item group of notion Nu; i and j represent the amount of \({N}_{mg}^{q}\) and \({N}_{mg}^{u}\) words respectively; h reveals the amount of similar groups between \({N}_{mg}^{q}\) and \({N}_{mg}^{u}\).
According to the features of ontology for col-NTMT, there are three data sorts of properties classified to calculate similarity of property definitely. In the similarity of pure numerical property, suppose \({A}_{m}^{q}\)shows the mth pure numerical property for query notion Nq and \({A}_{m}^{u}\) means the same pure numerical property for notion Nu in universal ontology. The \({P}_{m}^{q}\) and \({P}_{m}^{u}\) are represented for values of \({A}_{m}^{q}\) and \({A}_{m}^{u}\) respectively. The equation of similarity is shown below:
Sim PN(\({A}_{m}^{q}\), \({A}_{m}^{u}\)) = 1 \(-\) \(\left|{P}_{m}^{q}- {P}_{m}^{u}\right|\) / \({P}_{c}\) (3)
Where Function SimPN( ) reveals the similarity of pure numerical property; Pc means the value gauge of property.
In the similarity of interim numerical property, suppose \({B}_{m}^{q}\)shows the mth interim numerical property for query notion Nq and \({A}_{m}^{u}\), the one in universal ontology, the \({B}_{m}^{q}\)and \({A}_{m}^{u}\) (interim numerical values) are [\({e}_{m}^{q}, {g}_{m}^{q}\)], [\({e}_{m}^{u}, {g}_{m}^{u}\)], respectively. The computation of similarity is listed below:
Sim IN(\({B}_{m}^{q}\), \({A}_{m}^{u}\)) = 1 \(-\left\{\frac{1}{2\text{max}\left(g\right)}\right\}\{{[{\left({e}_{m}^{u}-{e}_{m}^{q}\right)}^{2}+{\left({g}_{m}^{u}-{g}_{m}^{q}\right)}^{2}]}^{1/2}\)} (4)
Where SimIN( ) reveals the similarity of interim numerical property; the extreme value of the property, max(g), applied to regularized the messages. Furthermore, this value is decided through the technical viability.
In the similarity of cord property, suppose \({C}_{m}^{q}\)shows the mth cord property for query notion Nq and \({A}_{m}^{u}\), the one in universal ontology, the \({C}_{m}^{q}\)and \({A}_{m}^{u}\) (cord values) are \({CP}_{m}^{q}\), \({CP}_{m}^{u}\), separately. The computation of similarity is listed below:
Sim CP(\({C}_{m}^{q}\), \({A}_{m}^{u}\)) = \(\left\{\begin{array}{c}0, {CP}_{m}^{q}\ne {CP}_{m}^{u}\\ 1, {CP}_{m}^{q}={CP}_{m}^{u}\end{array}\right\}\) (5)
To compute the similarity of universal property for goal notion Nq and Nu in universal ontology, the closest method is used [58]. The similarity of universal property contains the above-mentioned equations (from Eq. (3) to Eq. (5)). Therefore, the equation can be represented as:
Sim U(Nq, Nu) = [\(\sum _{m=1}^{h}{w}_{m}\times {Sim}_{PN}({A}_{m}^{q},{A}_{m}^{u})]\)/\([\sum _{m=1}^{n}{w}_{m}]\) +
[\(\sum _{m=h+1}^{r}{w}_{m}\times {Sim}_{IN}({B}_{m}^{q},{A}_{m}^{u})]\)/\([\sum _{m=1}^{n}{w}_{m}]\) +
[\(\sum _{m=r+1}^{n}{w}_{m}\times {Sim}_{CP}({C}_{m}^{q},{A}_{m}^{u})]\)/\([\sum _{m=1}^{n}{w}_{m}]\) (6)
Here, wm means the weight for the mth property.
According to the previous approach presented [3], the eventual similarity between Nq and Nu is calculated by using the method of weighted sum and the equation can be revealed as:
Sim (Nq, Nu) = σ ⋅ SimN(Nq, Nu) + τ ⋅ SimU(Nq, Nu), σ, τ∈(0,1) (7)
Here, σ and τ indicate the weighting coefficients for similarity of notion title and property respectively. They could be computed in accordance with the steps (as shown in Fig. 7). Firstly, the third-order matrix of decision is built by applying the significance scale of property proposed [62–63].
D 3 =\(\left[\begin{array}{cc}1& 2\\ 1/2& 1\end{array}\right]\)
Afterward, by using the Eq. (8) to compute the weight coefficient w.
w m = \({w}_{m}^{\#}/{\sum }_{m}^{2}{w}_{m}^{\#}\), m ∈+N (8)
Here, \({w}_{m}^{\#}=\sqrt[2]{\prod _{n=1}^{2}{d}_{mn}}\), m∈+N
Next, the maximum eigenvalue \({\eta }_{max}\) is calculated through Eq. (9).
$${\eta }_{max}=\sum _{m=n=1}^{2}{w}_{m}\bullet {Sim}_{n}$$
9
Here,\({Sim}_{n}=\sum _{m=1}^{2}{d}_{mn}\)
Finally, the consistence test is essential for checking the weight coefficients. When \({\eta }_{max}\) ≥ \({\eta }_{th}\) (threshold eigenvalue), the values in the matrix of decision have to be modified, until the condition, \({\eta }_{max}\) < \({\eta }_{th}\), is satisfied. In the real utilization, the weight coefficient could be modified properly.
By the above computations, the notion with the similarity grade can be obtained. Besides, its relevant messages can also be tracked by the constructed relations in the universal ontology.