阅读说明：本技术 一种面向制造车间的离散事件动态仿真引擎及其实现方法 (Discrete event dynamic simulation engine facing manufacturing workshop and implementation method thereof ) 是由 朱海平 甄国辉 蒋猷明 沈洌政 关辉 于 2021-07-13 设计创作，主要内容包括：本发明公开了一种面向制造车间的离散事件动态仿真引擎及其实现方法,属于虚拟仿真技术领域。包括：仿真建模模块,用于生成制造车间的静态仿真模型；仿真时钟模块,由仿真时钟和仿真扫描时刻表构成,仿真扫描时刻表,用于记录每个仿真事件的触发时刻；仿真时钟,用于在仿真扫描时刻表的驱动下,离散推进到下一时刻点；仿真事件模块,由仿真事件表、事件表初始程序和事件例程解释执行器构成,仿真事件表,用于记录所有仿真对象关联事件类型及其判断和处置逻辑；事件表初始程序,用于各类事件初始化；事件例程解释执行器,用于自动解释执行事件判断和处置逻辑。本发明优化了时钟推进机制,实现了事件自动解析执行,提高了仿真引擎工作效率和灵活性。(The invention discloses a manufacturing workshop-oriented discrete event dynamic simulation engine and an implementation method thereof, and belongs to the technical field of virtual simulation. The method comprises the following steps: the simulation modeling module is used for generating a static simulation model of the manufacturing workshop; the simulation clock module consists of a simulation clock and a simulation scanning timetable, wherein the simulation scanning timetable is used for recording the triggering time of each simulation event; the simulation clock is used for discretely advancing to the next time point under the driving of the simulation scanning timetable; the simulation event module consists of a simulation event table, an event table initial program and an event routine interpretation executor, wherein the simulation event table is used for recording the associated event types of all simulation objects and judgment and disposal logics thereof; the event table initialization program is used for initializing various events; and the event routine interpretation executor is used for automatically interpreting and executing the event judgment and handling logic. The invention optimizes the clock propulsion mechanism, realizes the automatic analysis and execution of events, and improves the working efficiency and flexibility of the simulation engine.)

1. A manufacturing shop oriented discrete event dynamic simulation engine, comprising: the simulation system comprises a simulation modeling module, a simulation clock module and a simulation event module;

the simulation modeling module is used for generating a static simulation model of the manufacturing workshop according to the layout of the manufacturing workshop, the equipment capability and fault data, the process data, the production logistics data and the order data;

the simulation clock module consists of a simulation clock and a simulation scanning timetable, and the simulation scanning timetable is used for recording the predicted triggering time of the event associated with each simulation object; the simulation clock is used for discretely advancing to the next event triggering time point under the driving of the simulation scanning timetable;

the simulation event module consists of a simulation event table, an event table initial program and an event routine interpretation executor, wherein the simulation event table is used for recording the associated event types of all simulation objects in the manufacturing workshop and the judgment and handling logics of the associated event types; the event table initial program is used for initializing various events in the simulation event table; the event routine interpretation executor is used for automatically interpreting judgment and handling logic of the executed events and generating new events of the simulation objects.

2. The discrete event dynamic simulation engine of claim 1, wherein the simulation modeling module builds a graphical simulation object model for equipment, robots, production lines, transportation vehicles, buffers, warehouses, gates, parts, workers, handling tools, and data tables, triggers, analysis charts, communication interfaces within the physical plant, defines attributes, input/output data, and basic logic of the simulation object, and builds a static simulation model of the manufacturing plant.

3. The discrete event dynamic simulation engine of claim 1, wherein the simulation event table comprises: the system comprises an object name, an event type, an event response preposition logic, an event processing logic and an event response postposition logic, wherein the event response preposition logic refers to the condition that the event can respond and the preprocessing operation needs to be carried out, the event processing logic refers to the simulation operation of the creation, processing, assembly, loading and unloading, moving, transporting, fault and repair of the workshop object and the initialization, pause, recovery and stop operation of the simulation engine, and the event response postposition logic refers to the operation after the event is processed.

4. A discrete event dynamics simulation engine as recited in claim 1, wherein the simulation event types include: initialization/pause/resume/stop of simulation engine, part creation/deletion, part access current location, part machining, part assembly, equipment failure and repair, line flow, transport vehicle calling and handling service, worker calling and operating service, robot loading and unloading service, and timed trigger program.

5. A method for implementing the manufacturing plant-oriented discrete event dynamic simulation engine according to any one of claims 1 to 4, wherein the method comprises:

firstly, before a simulation engine is started:

(S1) the simulation modeling module generating a static simulation model of the manufacturing plant based on the layout, equipment capability and fault data, process data, production logistics data and order data of the manufacturing plant;

(S2) the event table initial program scans all simulation objects of the static simulation model of the manufacturing plant, and initializes the simulation event table according to the object attributes and the expected behaviors;

(S3) the simulation clock module automatically creates a simulation clock, initializes a simulation scanning timetable by adopting the definition data of the static simulation model of the manufacturing shop, and collocates the initial scanning time of the simulation engine to be 0;

secondly, after the simulation engine is started:

(T1) at the current simulation scanning time, the simulation event module scans the simulation event table one by one, identifies the executable event of the current simulation clock, the event routine explains the execution of the event processing routine by the actuator, generates the new event of the simulation object, records the state data of the simulation process of the manufacturing workshop, updates the simulation scanning time table and the simulation event table, and repeats T1 until the simulation clock has no executable event, and then the step (T2) is carried out;

(T2) the simulation event module obtains the next scan time from the simulation scan time table and the next time exists, the simulation clock discretely advances to the next scan time, step (T1) is entered, otherwise, the simulation is finished.

6. The method of claim 5, wherein the step (S2) includes:

(S21) the event table initial program scans all simulation objects in the static simulation model of the manufacturing shop to generate a simulation event table;

(S22) initializing the specific implementation routines of the event response pre-logic, the event processing logic and the event response post-logic according to the simulation event table.

7. The method of claim 5, wherein executing the event handling routine by the event routine interpretation executor to generate new events for the simulation object, recording manufacturing plant simulation process state data, and updating the simulation scan schedule and the simulation event table until the simulation clock has no executable events comprises:

(1) the event routine interpretation executor starts working, interprets and executes event processing logic, acquires simulation parameter data, and outputs execution result data of each event, and comprises the following steps: event type, logistics object, current part information and operation time;

(2) calculating next scanning time according to the requirements of operation time, clamping time, transportation time, fault interval and maintenance time, if the scanning time is random time, performing random simulation, generating time data meeting random distribution characteristics, and adding the time data into a simulation clock scanning table;

(3) generating a new event of the simulation object, adding the new event into the simulation event table, and deleting the responded event;

(4) the event routine interpretation executor continues to operate, interpreting the post-logic that executed the event response, and returning to step (T1).

8. The method of claim 5, wherein during the time that the simulation engine is suspended: the event routine interpretation executor automatically executes a pause routine of the simulation engine, backups a simulation event table, a simulation scanning time table and the state data of the simulation process of the manufacturing workshop, and the simulation clock saves the clock value of the pause time and stops working.

9. The method of claim 5, wherein the simulation engine is to resume from paused to resumed: and the simulation clock module restarts the simulation clock and recovers the simulation clock to a clock value before suspension, and the event routine interpretation executor automatically executes a recovery routine of the simulation engine to recover the simulation event table, the simulation scanning timetable and the state data of the simulation process of the manufacturing workshop.

Technical Field

The invention belongs to the technical field of virtual simulation, and particularly relates to a manufacturing workshop-oriented discrete event dynamic simulation engine and an implementation method thereof.

Background

Virtual simulation techniques have been used to study the behavior and performance of manufacturing plants. The method not only converts complex logic, process, scheduling and control relations in a manufacturing workshop into more visual model description, but also can fully consider the influence of external and internal random factors of the system; the static model of the manufacturing system can be established, the dynamic characteristics of the manufacturing system can be described, and the workshop simulation is operated to help to select a workshop design scheme, predict the performance of the manufacturing workshop and formulate a production scheduling strategy.

The simulation of the manufacturing plant belongs to the field of discrete event dynamic simulation, and is characterized in that the system state change is discrete, namely, the events causing the state occur at discrete time. In order to support the simulation operation of the manufacturing shop, a discrete event dynamic simulation engine needs to be established to realize the functions of simulation modeling, simulation operation, simulation analysis and the like.

The simulation scheduling strategy is a key technology of a discrete event dynamic simulation engine, a common algorithm at present comprises an event scheduling method, an activity scanning method, a process interaction method, a message driving method and the like, the event scheduling method is most consistent with the characteristics of virtual simulation of a workshop, but the judgment of the event processing time is only based on time, the event is processed when the time is up, but in the actual situation, even if the event processing event is up, other conditions are not met, the event still cannot be processed immediately, so the event scheduling method is not flexible enough, the efficiency is low, and a large improvement space exists.

The propulsion algorithm of the simulation clock is also the core technology of the discrete event dynamic simulation engine, and the algorithms commonly used at present are a fixed-step event propulsion method and a next event time propulsion method. The former is simple to realize but has low efficiency, and the latter has high efficiency but is not easy to process the simulation of continuous actions such as production line flow, AGV movement and the like.

Disclosure of Invention

Aiming at the defects and improvement requirements of the prior art, the invention provides a discrete event dynamic simulation engine facing a manufacturing workshop and an implementation method thereof, aiming at optimizing a clock propulsion mechanism, realizing automatic analysis and execution of events and improving the working efficiency and flexibility of the simulation engine.

To achieve the above object, according to a first aspect of the present invention, there is provided a manufacturing plant-oriented discrete event dynamic simulation engine, comprising: the simulation system comprises a simulation modeling module, a simulation clock module and a simulation event module;

the simulation modeling module is used for generating a static simulation model of the manufacturing workshop according to the layout of the manufacturing workshop, the equipment capability and fault data, the process data, the production logistics data and the order data;

the simulation clock module consists of a simulation clock and a simulation scanning timetable, and the simulation scanning timetable is used for recording the predicted triggering time of the event associated with each simulation object; the simulation clock is used for discretely advancing to the next event triggering time point under the driving of the simulation scanning timetable;

the simulation event module consists of a simulation event table, an event table initial program and an event routine interpretation executor, wherein the simulation event table is used for recording the associated event types of all simulation objects in the manufacturing workshop and the judgment and handling logics of the associated event types; the event table initial program is used for initializing various events in the simulation event table; the event routine interpretation executor is used for automatically interpreting judgment and handling logic of the executed events and generating new events of the simulation objects.

Preferably, the simulation modeling module establishes a graphical simulation object model for equipment, robots, production lines, transportation vehicles, buffers, warehouses, entrances and exits, parts, workers, handling tools, data tables, triggers, analysis charts and communication interfaces in the physical plant, and defines attributes, input/output data and basic logic of the simulation object, thereby constructing a static simulation model of the manufacturing plant.

Preferably, the simulation event table includes: the system comprises an object name, an event type, an event response preposition logic, an event processing logic and an event response postposition logic, wherein the event response preposition logic refers to the condition that the event can respond and the preprocessing operation needs to be carried out, the event processing logic refers to the simulation operation of the creation, processing, assembly, loading and unloading, moving, transporting, fault and repair of the workshop object and the initialization, pause, recovery and stop operation of the simulation engine, and the event response postposition logic refers to the operation after the event is processed.

Preferably, the simulation event types include: initialization/pause/resume/stop of simulation engine, part creation/deletion, part access current location, part machining, part assembly, equipment failure and repair, line flow, transport vehicle calling and handling service, worker calling and operating service, robot loading and unloading service, and timed trigger program.

To achieve the above object, according to a second aspect of the present invention, there is provided a method for implementing the discrete event dynamic simulation engine for a manufacturing plant according to the first aspect, the method comprising:

firstly, before a simulation engine is started:

(S1) the simulation modeling module generating a static simulation model of the manufacturing plant based on the layout, equipment capability and fault data, process data, production logistics data and order data of the manufacturing plant;

(S2) the event table initial program scans all simulation objects of the static simulation model of the manufacturing plant, and initializes the simulation event table according to the object attributes and the expected behaviors;

(S3) the simulation clock module automatically creates a simulation clock, initializes a simulation scanning timetable by adopting the static data of the static simulation model of the manufacturing shop, and collocates the initial scanning time of the simulation engine to be 0;

secondly, after the simulation engine is started:

(T1) at the current simulation scanning time, the simulation event module scans the simulation event table one by one, identifies the executable event of the current simulation clock, the event routine explains the execution of the event processing routine by the actuator, generates the new event of the simulation object, records the state data of the simulation process of the manufacturing workshop, updates the simulation scanning time table and the simulation event table, and repeats T1 until the simulation clock has no executable event, and then the step (T2) is carried out;

(T2) the simulation event module obtains the next scan time from the simulation scan time table and the next time exists, the simulation clock discretely advances to the next scan time, step (T1) is entered, otherwise, the simulation is finished.

Preferably, the step (S2) includes:

(S21) the event table initial program scans all simulation objects in the static simulation model of the manufacturing shop to generate a simulation event table;

(S22) initializing the specific implementation routines of the event response pre-logic, the event processing logic and the event response post-logic according to the simulation event table.

Preferably, the executing the event processing routine by the event routine interpreting executor to generate a new event of the simulation object, recording the state data of the simulation process of the manufacturing plant, and updating the simulation scan time table and the simulation event table until the simulation clock has no executable event, including:

(1) the event routine interpretation executor starts working, interprets and executes event processing logic, acquires simulation parameter data, and outputs execution result data of each event, and comprises the following steps: event type, logistics object, current part information and operation time;

(2) calculating next scanning time according to the requirements of operation time, clamping time, transportation time, fault interval and maintenance time, if the scanning time is random time, performing random simulation, generating time data meeting random distribution characteristics, and adding the time data into a simulation clock scanning table;

(3) generating a new event of the simulation object, adding the new event into the simulation event table, and deleting the responded event;

(4) the event routine interpretation executor continues to operate, interpreting the post-logic that executed the event response, and returning to step (T1).

Preferably, during the time that the simulation engine is suspended: the event routine interpretation executor automatically executes a pause routine of the simulation engine, backups a simulation event table, a simulation scanning time table and the state data of the simulation process of the manufacturing workshop, and the simulation clock saves the clock value of the pause time and stops working.

Preferably, the simulation engine is resumed from pause: and the simulation clock module restarts the simulation clock and recovers the simulation clock to a clock value before suspension, and the event routine interpretation executor automatically executes a recovery routine of the simulation engine to recover the simulation event table, the simulation scanning timetable and the state data of the simulation process of the manufacturing workshop.

Generally, by the above technical solution conceived by the present invention, the following beneficial effects can be obtained:

(1) the invention provides a manufacturing workshop-oriented discrete event dynamic simulation engine, which supports the simulation modeling of various logistics objects and non-logistics objects of a workshop, establishes a cooperative working mechanism of a simulation clock module and a simulation event module, realizes the automatic processing of event logic and the automatic updating of events through an event routine interpretation executor, and can work more efficiently and flexibly.

(2) The invention provides a manufacturing workshop-oriented implementation method of a discrete event dynamic simulation engine, which optimizes a simulation clock propulsion mechanism in the running process of the simulation engine, realizes automatic analysis and execution of events, promotes a simulation process by continuously triggering, processing and updating the events, and records simulation process data in detail.

Drawings

FIG. 1 is a schematic diagram of a manufacturing shop oriented discrete event dynamic simulation engine according to the present invention;

FIG. 2 is a process flow of the discrete event dynamic simulation engine for a manufacturing plant according to the present invention;

FIG. 3 is a plant simulation model provided by the present invention;

FIG. 4 is a table of events for objects in logistics provided by the present invention;

fig. 5 is a simulation scan schedule provided by the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

As shown in FIG. 1, the manufacturing plant-oriented discrete event dynamic simulation engine comprises: the system comprises a simulation modeling module, a simulation clock module, a simulation event module and a data communication interface of the manufacturing workshop. The simulation modeling module realizes the simulation modeling of various physical objects of the workshop, including the static simulation modeling of logistics objects and non-logistics objects, and generates a workshop object simulation model. The simulation clock module is composed of a simulation clock and a simulation scanning timetable, under the drive of the simulation scanning timetable, the simulation clock realizes the automatic discrete propulsion of the workshop production simulation operation process, scans the simulation event table one by one, and identifies and activates related events. The simulation event module is composed of a simulation event table, an event table initial program and an event routine interpretation executor, completes the initialization and processing of the simulation event, realizes the automatic interpretation and execution of the event response preposition logic, the event processing logic and the event response postposition logic program, and updates the simulation scanning time table. The data communication interface is used for acquiring simulation input parameters such as orders and processes and recording simulation process data in detail, and the simulation process data can be used for performance analysis of a manufacturing workshop, such as: capacity, equipment utilization, etc.

As shown in fig. 2, the present invention further provides a manufacturing shop-oriented discrete event dynamic simulation engine implementation method, which specifically includes the following steps:

step 1, establishing a static simulation model of a manufacturing workshop through a simulation modeling module.

The simulation modeling module generates a static simulation model of the manufacturing shop according to the layout, equipment capability and fault data, process data, production logistics data and order data of the manufacturing shop. The data can be obtained by manually dragging the component module or writing in a script.

Aiming at various logistics objects such as equipment, robots, production lines, transport vehicles, buffers, warehouses, entrances and exits, parts and the like in a physical workshop and various non-logistics objects such as data tables, triggers, analysis charts, communication interfaces and the like, a graphical simulation object model is established, and the attributes, input/output data and basic logic of the simulation object are defined, so that a static simulation model of the manufacturing workshop is established.

And 2, initializing a simulation event table through a simulation event module.

The simulation event table records the associated events of all plant simulation objects and the judgment and handling logics thereof, and the types of the events comprise: part creation, part entry and exit to and from a current location, part machining, component assembly, equipment failure and repair, production line flow, transport vehicle movement, robot start/end service, timed trigger programs, and the like. The method comprises the following specific steps:

step 201, before the simulation starts, aiming at the manufacturing shop static simulation model established in step 1, the simulation event module scans all simulation objects to generate a simulation event table, which includes attributes such as object names, event types, event response pre-logic, event processing logic and event response post-logic. The simulation event table is automatically generated without manual input. The event response pre-logic defines under what conditions the event can respond, and which pre-processing operations need to be performed, such as the pre-logic of "part leaves current location" is that "the specified length of stay of the part at current location has been reached, and the subsequent location is allowed to enter". The event processing logic refers to simulation operations of machining, loading and unloading, moving, transporting and the like of the workshop objects. The event response post logic refers to the operation after the event is processed.

Step 202, writing concrete implementation routines of the event response pre-logic, the event processing logic and the event response post-logic. These routines can be flexibly defined and automatically interpreted by the event routine interpretation executor according to the context information during the simulation process.

Event routines are mostly edited automatically by a simulation engine, and can be edited automatically if a user wants to optimize on some problems.

And 3, realizing automatic discrete propulsion of the simulation process through the simulation clock module: and (2) aiming at the static simulation model of the manufacturing workshop established in the step (1), running simulation, automatically establishing a simulation clock, reading data in a simulation scanning timetable in sequence by the simulation clock in the simulation process, discretely advancing to the next time point, circularly scanning a simulation event table, and identifying whether a simulation event can respond. The method comprises the following specific steps:

step 301, before simulation begins, a simulation clock is automatically created, the clock precision is millisecond, concurrent simulation is supported, namely, multiple times of simulation are run simultaneously, and mutual interference is avoided.

Step 302, if there are continuous moving objects such as production lines, AGVs, etc., next time data is generated at fixed step intervals and recorded in the simulation scanning timetable.

Step 303, during the simulation process, the simulation clock is automatically pushed forward in a discrete manner. And reading the first time data T after the current time from the simulation clock scanning table, if the T does not exist, ending the simulation process, otherwise, executing the step 304.

Step 304, the simulation clock advances to the time T, and the simulation event table is scanned in a traversing manner. And (4) judging whether the prepositive logic of each event response is satisfied, and if so, executing the step 4.

Step 305, in one traversal scan period of the time T, if there is an event actually executed in the simulation event table, a new traversal scan will be automatically started until there is no event in the simulation event table at the time to satisfy the response pre-logic, and then step 302 is repeatedly executed.

And 4, utilizing a simulation event module to realize the triggering execution of the event: and 304, at the time T, when an event in the simulation event table meets the execution condition, executing the event processing logic, completing simulation tasks such as part machining, moving and the like, recording a simulation result, calculating to obtain the subsequent simulation scanning time, and updating the simulation scanning time table. The method comprises the following specific steps:

step 401, the event routine interpretation executor starts working, interprets and executes the event processing logic, accesses the data communication module, obtains the simulation parameter data, and outputs the execution result data of each event method, including: event type, logistics object, current part information, operating time, etc.

Step 402, calculating next scanning time according to actual production process data requirements such as operation working hours, clamping time, transportation time, fault intervals, maintenance time and the like, if the scanning time is random time, performing random simulation, generating time data meeting random distribution characteristics, and adding the time data into a simulation clock scanning table.

Step 403, generating a new event of the simulation object, adding the new event into the simulation event table, and deleting the responded event;

at step 404, the event routine interpretation executor continues to operate, interpreting the post-logic that executed the event response. Returning to step 304.

A simplified manufacturing plant is shown in fig. 3, where the lines between the objects represent the manufacturing process flow, the solid lines represent the non-consideration of the logistics and time, and the dashed lines represent the consideration of the logistics and time. Consists of a part inlet, a part to be processed area, a carrying manipulator, a machine tool and a part outlet, and A, B parts are sequentially produced. Parts were produced at the inlet, 0 seconds producing part a and 10 seconds producing part B. After the part is produced, the part immediately enters a to-be-processed area to wait, the part is conveyed to a machine tool by a manipulator to be processed, the service response time and the conveying time of the manipulator are respectively 5 seconds and 8 seconds, the time for processing A, B parts by the machine tool is a Normal distribution random number, wherein the processing time length of the part A is Noramyl (30,5), the Normal distribution is represented by Normal, the expected value (unit: second) of time is 30, the standard deviation (unit: second) is 5, and the processing time length of the part B is Noramyl (50, 10). If the machine tool is operating, the robot cannot handle the parts and must wait for the parts to enter the exit after they have been machined on the machine tool.

For the plant simulation model, an initialization simulation event table is shown in fig. 4, and the data table includes 5 columns of information: object name, event type, event response pre-logic, event processing logic, event response post-logic. The event types include "generate", "leave", "enter", "call", "service", and the like. Taking the data in the row 4 as an example, before a part leaves a to-be-processed area, whether a manipulator is ready is judged, if not, the part cannot leave, otherwise, the part enters the manipulator, and the state of the manipulator is set to be busy, so that a subsequent call request aiming at the manipulator automatically enters a service waiting queue of the manipulator.

The working process of the simulation engine is the process of continuously reading and updating the data of the simulation scanning timetable (as shown in fig. 5), advancing the simulation clock, judging the executable event and processing the event, generating a new event and updating the simulation event table, and the specific process is as follows: the 0 second and 10 second times are added to the simulated scan schedule based on the generation time of part A, B. And the simulation engine sequentially reads the data in the simulation scanning timetable and pushes the clock to perform event scanning until no data is readable, and the simulation is finished. Firstly scanning 0 second time, generating and leaving the inlet of a part A at the time of 0 second, entering a region to be processed, triggering an 'entering' event of the region to be processed, calling a manipulator, adding 5 seconds of predicted service time of the manipulator into a scanning time table, repeatedly scanning 0 time without other events, advancing a clock to 5 seconds, executing an event that the part A leaves the region to be processed, leaving the region to be processed, entering the manipulator, executing an entering event of the manipulator, setting the time that the part A leaves the manipulator to be 5+8 seconds to 13 seconds, adding 13 seconds into a simulation scanning time table, repeatedly scanning 5 seconds without other events, advancing the clock to 10 seconds, generating a part B, then advancing to 13 seconds, moving the part A into a machine tool for processing, triggering a machine tool entering event, and according to the random distribution Noraml (30,5) of the processing time of the machine tool, and generating a normal distribution random number 31, adding the time of 13+31 to 44 seconds into a simulation scanning time table, then advancing to the time of 44 seconds, leaving the part A from the machine tool, entering an outlet and finishing machining. And so on, until the time of 105 seconds, the part B enters the outlet, and the simulation is finished.

It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.