As shown in Fig. 2, production equipment of complex electromechanical product assembly line through a specific motion logic, sequentially install multiple parts on the workpiece of the vehicle, and finally complete the processing of the product. The assembly unit is the basic component unit of the production line, and is also the basic unit of the production line structure design. This paper takes the equipment unit as the research object for modeling method research.
3.1 Hierarchical decomposition design
Considering the actual process behavior characteristics of the assembly unit, the assembly unit is divided into the following modules by function: (1) clamping function (2) part conveying function (3) workpiece conveying function. Among them, the clamping function is divided into equipment trunk adjustment and end execution functions, the part conveying function is divided into part storage, ejection and flow functions, and the workpiece conveying function is divided into workpiece flow and positioning functions. The part conveying conveys the material from the storage point to the loading point by the single free motion of the mechanism, and the workpiece flow carries out the operation of flowing, positioning and turning of the processed parts by the single free motion of the mechanism. The clamping function mainly includes the following work: (1) Installing the parts from the loading table to the specified position of the workpiece. (2) Multi-degree-of-freedom spatial movement of the part during part convey. (3) Spatial multi-freedom spatial movement of the part during the workpiece flow. In the flow of Fig. 3-a, firstly, the single-degree-of-freedom moving parts are centered, and the geometric entities are merged into groups to form motion objects by relying on parent-child and contact relationships, and then the interrelated motion objects are merged into groups according to their corresponding functions, and each functional object is divided into groups in turn until the whole assembly unit. Figure 3-b shows the result of the hierarchical division of the assembly unit.
3.2 Model design and escapsulation
As shown in Fig. 4, the static, dynamic and control information of the model is extracted and encapsulated into components and fused into a unified model. The model can represent any twin body in the scene, and the model contains three types of components: entity component, dynamic component and control component. The entity component records the geometric and physical information of the model, the dynamic component records the motion logic information of the model, and the control component records the signal communication and parsing mechanism of the model. The entity component need to be assembled according to a certain spatial configuration, and based on the actual model information of the twin, it is determined whether the configuration of dynamic and control components is required. Each twin model has the potential to become a child object of another twin model, so component pointers are designed to map this composition relationship. The next step is to design the specific modeling details for each component of this model.
3.2.1 Static information model based on association matrix
Formula 1–1 is the static information model of all the objects at a certain level of the twin unit at a certain moment. X11, X22, X33, etc. represent the geometric and physical information of the object's sub-component entities, including geometric information such as size and shape, and physical properties such as material and mechanical performance parameters, etc. X12, X23, etc. represent the relationships between entities, including assembly, force and position relationships.
\(X=\left[ {\begin{array}{*{20}{c}} {{X_{11}}}&{{X_{12}}}&{...}&{{X_{1j}}}&{...}&{{X_{1{\text{n}}}}} \\ {{X_{21}}}&{{X_{22}}}&{...}&{{X_{2{\text{j}}}}}&{....}&{{X_{2{\text{n}}}}} \\ {...}&{...}&{...}&{...}&{...}&{...} \\ {{X_{{\text{i1}}}}}&{{X_{{\text{i2}}}}}&{...}&{{X_{{\text{ij}}}}}&{...}&{{X_{{\text{in}}}}} \\ {...}&{...}&{...}&{...}&{...}&{...} \\ {{X_{{\text{n2}}}}}&{{X_{{\text{n2}}}}}&{...}&{{X_{{\text{nj}}}}}&{...}&{{X_{{\text{nn}}}}} \end{array}} \right]\) (1–1)
Where:
X ——static information of a hierarchical object
X ij (i = j) ——static information of subcomponent entities
X ij (i ≠ j) ——interrelationship between subcomponent entities
Figure 5 shows the static model encapsulation of the clamping mechanism, whose geometric structure is shown in Fig. 5-a. According to the principle of hierarchical division, the division process is shown in Fig. 5-b, and it is merged into groups according to the relationship of parent-child and contact. Figure 5-c shows the encapsulated hierarchical structure, and Fig. 5-d shows the data encapsulation format of the model.
3.2.2 Behavioral information modeling based on finite state machine
Dynamic information describes the twin model response to external drives and disturbance effects, and Fig. 6 shows the state model of unit equipment behavior based on Moore-type finite state machine. Different assembly units have different specific processes and different sequences of parallel and serial execution. Considering that all assembly processes will eventually affect the machined workpiece, a unit process state tracking method based on the information of machined part properties is designed, and the main process is as follows.
(1) The information of the machined workpiece is abstracted into a ternary tuple(trans,rot,type), where trans and rot record the overall position posture information of the workpiece and type records the type of the mounted part.
(2) Design the ordered set of states of the process. As shown in Fig. 6, S0,S1,S2, etc. represent the individual process states of the unit, which form the finite state set of the unit; σ0,σ1, etc. represent the finite set of inputs, which are the trigger signals generated when the machined workpiece reaches the specified position; δ0,δ1, etc. are the state transfer functions; λ0,λ1, etc. are the state output sets of the machined workpiece, which represent the state of the workpiece after a process; ω0,ω1, etc. represent the output action of each state, which is specifically expressed as the specific mechanical movement of each process.
(3)Generation of twin units. First, the pose state of each part of the unit is initialized based on the state data of the real physical unit, and the parent-child relationship and physical constraints are added to each part model to give the same physical motion properties as the real part. Then the pose of the machined workpiece and the information of the installed parts are read in real time. When the workpiece arrives at the specified position, the process state transfer instruction σ0 is obtained by judging the information of the ternary. The unit migrates from S0 to S1 state through the transfer function δ0; at the same time, the process-driven part motion ω0 is executed, and the output λ0 is output after the state is executed. The processed workpiece is output to enter the next state of excitation, which is executed sequentially until all process states of a unit are completed.
Figure 7 shows the unit behavior hierarchy model, where each process activation will invoke the state of sub-levels layer by layer up to the action library. The behavior information of an object at a certain level includes the basic constituent actions and action invocation logic.
By analyzing the finite state machine model with behavioral information it is clear that any twin model containing dynamics can be decomposed into simple motion attributes of translation and rotation and a motion logic that coordinates these attributes. Therefore, the dynamic model information is divided into two modules of motion attributes and motion logic.
Taking the clamping function module as an example, the motion attribute escapsulation is shown in Fig. 8-a. Each serial number corresponds to the geometry of Fig. 5-a, containing rotation and translation library, with the following meanings: (1) Serial number 1 represents arm2. (2) Serial number 2 represents arm3. (3) Serial number 3 represents wrist4. (4) Serial number 4 represents actuator. Attitude adjustment and end execution correspond to the motion attribute escapsulation of two sub-functions, clamping is the entire motion attribute escapsulation of the mechanism. Figure 8-b represents the motion logic topology of the mechanism, where a,b,c,d,e,f represent the transfer points of each object state, the trigger of the stored action and the index of the specified action in the action library, and serial numbers 1,2,3,4 correspond to the serial numbers in Fig. 8-a, that is, represent the action sequence library of each subcomponent. The logic diagram runs as follows: (1) When the workpiece reaches the assembly point and the material arrives at the point to be assembled, point a opens and the clamping mechanism starts to operate. (2) b,c point open, function I-1 attitude adjustment begins to execute, after the execution, d point open, function I-2 end clamping begins to execute, in turn, after the execution of serial number 3 wrist action open e point, the execution of hand grip clamping parts. (3) continue to activate point b,c, while changing the index, call the corresponding animation. Execute iteratively until all specified actions are completed, and turn on point f to signal the completion of assembly to the downstream twin. Figure 8-c shows the dynamic data encapsulation format.
3.2.3 Interaction-oriented control information modeling
As shown in Fig. 9, the twin model receives control signals from two sources, a entity control signal from the workshop sensors and a simulation control signal between the upstream and downstream twins of the virtual scene. The physical control signal requires the construction of network communication protocols, and the information must also be transformed into information acceptable to the twin model. To ensure the normal communication and parsing of control signals, three principles are defined for the model to accept single, multiple signals and the case of accepting entity and simulation signals at the same time.
(1) Serial processing principle, the sequential relationship of adjacent twin information transfer, where the downstream twin cannot parse the signal when the upstream twin does not transfer the signal, and the upstream twin component cannot perform the signal transfer when the downstream twin component does not parse the signal.
(2) Parallel processing rules, when the same twin receives multiple signals, it needs to consider its own production behavior and system simulation rules to determine the order of signal processing.
(3) Entity priority principle, the entity object takes the initiative to send control signals to the twin object, and the entity control signal resolution priority is higher than the twin simulation signal. When the twin accepts the entity signal, it needs to mask the simulation signal, parse and execute the operation and then enter the accept signal wait state, if only the simulation signal is executed, execute the serial and parallel rules, parse and execute and then enter the signal wait state.
As shown in Fig. 10, the control model escapsulation of the clamping function module, for example, consists of two modules, external signal operation and internal signal operation. Separate the internal and external operations and constructe the signal resolution model. At the same time, the dynamic model transfer point is bridged, and its output signal is used as the control signal of the dynamic model. As shown in Fig. 10-a, the model needs to complete the configuration of the protocol information first, and its workflow mainly contains the following two points: (1) The external signal directly controls the transfer point of the twin entity through the module transformation, while the internal signal operation is suspended. (2) When the external signal stops transmitting, the internal signal operation starts to execute and the control point is operated according to the predefined simulation logic. Figure 10-b shows the encapsulated format of the control information.