Adaptive control system with knowledge sever in Intellegent CNC system
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07-08-2009, 05:06 PM


In an ideal scenario of intelligent machine tools the human mechanist was almost replaced by the controller. During the last decade many efforts have been made to get closer to this ideal scenario, but the way of information processing within the CNC did not change too much.The paper summarizes the requirements of an intelligent CNC evaluating the advancement of technology in this field using different adaptive control systems. In this paper a low cost concept for artificial intelligence named Knowledge Server for Controllers (KSC) is introduced.
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31-03-2010, 11:03 PM

ADAPTIVE CONTROL SYSTEM WITH KNOWLEDGE SERVER IN INTELLEGENT CNC SYSTEM

ABSTRACT:

In an ideal scenario of intelligent machine tools the human mechanist was almost replaced by the controller. During the last decade many efforts have been made to get closer to this ideal scenario, but the way of information processing within the CNC did not change too much. The paper summarizes the requirements of an intelligent CNC evaluating the advancement of technology in this field using different adaptive control systems. In this paper a low cost concept for artificial intelligence named Knowledge Server for Controllers (KSC) is introduced. It allows more devices to solve their intelligent processing needs using the same server that is capable to process intelligent data. The KSC concept is used in an open CNC environment to build up an intelligent CNC.
1. INTRODUCTION:
There are many definitions of the intelligent machine tools. In a well known book Wright and Bourne said that We must therefore acknowledge that the degree of intelligence can be gauged by the complexity of the input and/or the difficulty of ad hoc in-process problems that get solved during a successful operation. Our unattached, fully matured intelligent machine tool will be able to manufacture accurate aerospace components and get a good part right the first time. They told that an intelligent machine tool had the CAD data, the materials and the set-up plans as inputs and could produce correctly machined parts with quality control data as outputs.
2
It is clear that adaptive control techniques are necessary to apply if one wants intelligent CNC machine, but - of course - the usage of them is not adequate in intelligent behavior.
Table 1 summaries the features of an intelligent CNC (Wright and Bourne collected them more than ten years ago) and shows two further things: the positive changes done in the recent years and the still existing gaps where - according to the scientific community “ adaptive control systems offers solutions with its information processing methods.
Analyzing the above list, it is clear that many features do not require direct adaptive control methods. We can state that the main reasons of the advancement were:
(1) The development of the hardware elements (more sensitive sensors, more precise actuators, quicker and stronger computers etc.) even in higher requirements.
(2) The development of the software and the methodology mainly in the preparation phases of the manufacturing (in design, planning, scheduling, resource management etc.) and in the user interface issues (more comfortable and informative 'windows-like' screens and Menus).
TABLE 1. COMMERCIAL NEEDS FOR THE INTELLIGENT MACHINE TOOLS
Features (forecasted in 1988) Big advance by 2001
Artificial intelligence methods still needed
1. Reduce the number of scrap parts following initial setup.
2. Increase the accuracy with which parts are made.
3. Increase the predictability of machine tool operations.
4. Reduce the manned operations in the machine tool environment.
5. Reduce the skill level required for machine setup and operations.
6. Reduce total costs for part fabrication.
7. Reduce machine downtime.
8. Increase machine throughput.
9. Increase the range of materials that can be both setup and machined.
10. Increase the range of possible geometries for the part
11. Reduce tooling through better operation planning
12. Reduce number of operations required for setup
13. Reduce setup time bydesigning parts for ease of setup
14. Reduce the time between part design and fabrication
15. Increase the quantity of information between the machine
16. control and part design operations




















Even there is a big advance in the technology of the CNCs, the
Knowledge processing and other adaptive control methods have not appeared within the Intelligent Open CNC System Based on the Knowledge Server Concept 3controllers, so in some points there is no real development. Special heuristic rules, problem-solving strategies, learning capabilities and knowledge communication features are still missing from the recent controllers available on the market. It is also true for many new, open or PC-based CNC s, where DSP add-on-boards provide the necessary computation power and speed. Further requirements of intelligent CNC s can be find out and defined.
(Table 2.): The second column indicates whether the different adaptive control techniques (mainly rule base systems, neural nets and fuzzy logic) would provide methods and solutions. One can find positive answers to all these issues in the resent literature:
Table2. Future requirements of an intelligent CNC:
Table 2. Future requirements of an intelligent CNC
Further features
Artificial intelligence methods would help
¢ Model based on-line path generation
¢ Automatic tool selection
¢ Technological based settings of the operational parameters
¢ Automatic compensation of machine limits
¢ Automatic back-step strategies
¢ Detection and compensation of geometrical deflection
¢ On-line selection of control algorithms
¢ Intelligent co-operation with other devices to solve problems
together
¢ Detection and correction of tool wear and breakage
¢ Automatic handling of rejected work pieces
¢ Detection and managing of emerging situations of the machine tool
¢ Complex self-diagnostics












The list may be continued with the learning capabilities and others. In the users' point of view these features rough in a controller, that "recognizes the problem" and "efficiently and reasonable solves them" with minimal disturbance of the environment of the controller.
4. RESULTS IN INTELLIGENT CNC s:
On the one hand in the recent literature one can find many different topics related to intelligent CNCs. Unfortunately they often do not mean intelligent behaviors but the application of intelligent methods. Sometimes it is the case that authors call their devices intelligent" if one module of the system contains based method. On the other hand (2) the key controller vendors leave everything to the users or machine tool builders offering PC/Windows based CNCs. With these systems any software modules (e.g. even adaptive control based ones) can be coupled into the controller but they do not offer J real solutions or methodologies, but only software possibilities.

The following list summarizes the most important active research topics in this field. A real intelligent CNC would contain most of these issues.
1) Fuzzy logic based concurrent control of some operating parameters
(E.g. cutting speed, depth of cut, feed rate) independently from the
Given tool and the work piece.
2) Neural nets and fuzzy rules in the CNC's control algorithms.
3) Optimal path planning, real-time correction of the trajectory.
4). Compensation of temperature (and other) deformations.
5) Life time management of the tools and other parts of the machine
Tool including self-diagnostics.
6) Tool breakage detection (maybe forecasting) and tool wear
Monitoring (maybe compensation) with AI methods.
7) The utilization of CNC management (setup, orders, etc.) via
Intelligent agents.

Intelligent parts of a CNC can be classified into three groups, namely:
(1) Tool monitoring,
(2) operation/machine tool modeling and
(3) Adaptive control.


A general problem in all the three groups is that the adaptive control based solutions are typically limited and valid only in a very narrow field. If one changes some parameters of the operation or the environment, the earlier successful methods become false.
A special type of adaptivity partly helps on this hard and well-known problem. If it is possible to replace the different modules of the controller time by time, than one can guarantee, that a given adaptive control module can run within its limitation, and over it another module (e.g. a much simpler one) covers the same functionality. It can be realized (among others) if the controller is open to allow this replacement.
5. Adaptive Control System :
In adaptive control, the operating parameters automatically adapt themselves to conform to new circumstances such as changes in the dynamics of the particular process and any arise.
The adaptive would check load conditions, adapt an appropriate desired braking profile (for ex: antilock brake system and traction control), and then use feed back to implement it. With advanced adaptive controllers the gain may vary continuously with changes in operating condition.
Purpose of adaptive control:
1. to optimize production rate
2. to optimize product cost
3. to minimize cost
The functions common to adaptive control systems are the following:
1. Determine the operating conditions of the process, including measures of performance. Thos typically achieved by using sensors which measure process parameters (such as force, torque, vibration, and temperature).
2. Configure the process control in response to the operating conditions. Large changes in the operating conditions may provoke a decision to make a major switch in control strategy. More modest alterations may be the modifications of process parameters (such as changing the speed of operation or the feed in manufacturing).
3. continue to monitor the process, making further changes in controller when and
As needed.
In an operation such as turning on lathe, the adaptive control system senses real “time cutting forces, torque, temperature, tool-ware, tool chipping or tool fracture, and surface finish of the work piece. The system then converts this information into commands that modify the process parameters on the machine tool to hold them constant (or with in certain limits) or to optimize the cutting operation.

6. DIFFERENT APPROACHES OF KNOWLEDGE
SERVERS:

The features of World Wide Web led to introduce knowledge server to easier solve the installation and version control problems of expert systems and to provide a web based interface of the knowledge base for the different users. Some advanced knowledge based systems are based on this concept. There are some applications of knowledge servers in manufacturing. In the HPKB (High Performance Knowledge Environment) some hundred thousand rules are performed in an intelligent knowledge environment. In this project and implimentation the different intelligent components are called knowledge servers. The components are communicating with each other via the OKBC (Open Knowledge Base Connectivity) protocol specified at Stanford.
On communication networks, the protocol named MAP uses a bus configuration, broad band transmission, a token passing access scheme and data Transmission rate of 10mbps. MAP is based on specification defined by the identical standard organizations (ISO) called the open system interconnection (OSI) reference model.

The seven layer structure of MAP standard the first four layers are connected with the inter connection functions, and the top three layers are connected with inter working functions. The application layer (seventh layer) is the highest layer in MAP at the time of this writing. It is possible that this layer may be sub divided into multiple layers as applications of communication protocol and computer techniques evolve in future.

7. KNOWLEDGE SERVER FOR CONTROLLERS:
Knowledge Server for Controllers (KSC) is defined as a server providing capability of intelligent data processing for other systems. It allows the basic system to reach external intelligent processing resources, because it does not have any. The KSC contains a high performance reasoning tool, and different knowledge based modules. All the modules have their special rules and procedures. The client system calls these modules, passes them specific data if necessary, and the KSC module can collect data if the knowledge processing requires. All the data acquisition and user interaction is done by the client system. It is clear that in KSC the clients have much more tasks than a simple browser based user interface and in the applications listed in the previous chapter.
It should be stated that KSC does not deal with fuzzy and neural net based adaptive control modules. The computing power and the necessary software costs and complexity of these methods are less than the rule or model based ones. (In the case of the neural nets it is true only if the net is not trained on-line.) .
The KSC allows the different modules to run independently, to cooperate as agents or to control each other. The third case means that one module is started by another one because either the second one uses the results of the first one or the inference of the first one led to the need of the second module.
Generally the resources of the KSC can use more clients (controllers) simultaneously. It leads to a cost effective AI solution, because one costly AI tool can solve all the intelligent problems in a distributed environment. The overhead of the KSC (network connection, one more computer, some delay etc.) is much less comparing to the advantages (adaptive control tool licensing, less computing power in the clients/controllers, one server module may used by more clients etc.).
Using the KSC together with the component based software technology (e.g. CORBA) gives a very adaptive software frame to solve complex problems.
In the Fig. 1 a CNC with an embedded PLC controls a machine tool. The modules of both controllers are open and some of them are also clients of a knowledge server (KSC). It means that these modules can run special adaptive control Intelligent Open CNC System Based on the Knowledge Server Concept methods during their work that is an independent service is implemented in the KSC.

8. PROTOTYPE INTELLIGENT CNC BASED ON KSC:

In an early prototype of the intelligent open CNC that is using KSC, Adaptive control system was implemented
An advance axis tester is put on the top of this. Axis Test module handles all the tests but it gets the necessary position and velocity values from a knowledge based general tester running as an application on the KSC. The KB tester determines some goal positions and motion speeds, that the Axis Test module executes with the axis using jog commands. The results (execution time, tuning in errors etc.) are sent to the KSC that analyses and qualifies the axis. In the prototype the modules are built in CORBA, the controller and the HMI is programmed in Java.

10. CONCLUSIONS:
The controllable parameters in machining by using micro controller i.e. adaptive control system are cutting force, torque, vibrations, feed and depth of cut. The controllable parameters in machining by using artificial intelligenceModel based
on-line path generation
¢ Automatic tool selection
¢ Technological based settings of the operational parameters
¢ Automatic compensation of machine limits
¢ Automatic back-step strategies
¢ Detection and compensation of geometrical deflection
¢ On-line selection of control algorithms
¢ Intelligent co-operation with other devices to solve problems
together
¢ Detection and correction of tool wear and breakage
¢ Automatic handling of rejected work pieces
¢ Detection and managing of emerging situations of the machine tool
By introducing an interface between micro controller of adaptive control system and the data base of artificial intelligence we can control all the above parameters by communicating with knowledge server. The features of KSC were discussed and an early prototype was introduced.

11. References:
Adaptive control system -------- Bernard wplrow,Samuel D.stearns
Manufactureing Engg and technology by Serope Kalpak Jain, Steven R. schmid Computer integrated manufacturing---------------- James A Regh, Henry w scrab Manufacturing system Engg -------------------- katsundo Hitoni


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