INTEGRATED EMBEDDED PROLOG PLATFORM FOR RULE-BASED CONTROL SYSTEMS
G.J.NALEPA, P. ZI ˛ECIK AGHUNIVERSITYOF SCIENCEAND TECHNOLOGY,POLAND
KEYWORDS: Intelligent Control, Embedded Systems, Prolog, XTT, Rule-Based-Systems, EPP, GNU/Linux
ABSTRACT: The paper presents the idea of using an integrated, Prolog-based platform for an efficient implementation of embedded control systems. A principal idea consists in separating an embedded control system into two layers. The first one is the logic control layer, where the control procedures are implemented in Prolog. The second one provides means for communication with particular devices. The paper describes the concept of an integrated platform allowing for implementation of Prolog-based control routines. The platform itself is built on top of a GNU/Linux based embed- ded runtime, suited for several hardware architectures. In the paper key architectural and implementation issues of the platform are discussed. They are presented using a control system example. The paper also discusses practical hardware implementation on the Palm III platform based on the Motorola/Freescale DragonBall CPU.
INTRODUCTION tem. This would allow for practical deployment of a XTT-based control logic into an embedded system. The The rule-based programming paradigm [2, 3] plays an platform is composed of a Prolog interpreter embedded important role in number of engineering domains, includ- into a GNU/Linux environment controlling the hardware. ing intelligent and real-time control. However, an effi- The complete rule-based system development cycle pro- cient design of rule-based systems, including the knowl- posed in this approach is shown in Fig.1. It includes the edge engineering process, encounters number of prob- following phases: lems [2, 3]. In recent years rule-based systems design benefited from advanced design and implementation tools, 1. Rule-based control logic design and analysis using that use number of techniques taken from the software en- a high level knowledge representation, the XTT. gineering [12]. In this paper an integrated approach to the design and implementation of such systems, suitable for 2. Automatic translation of the XTT rulebase into an embedded applications is put forward.1 executable Prolog-based prototype. This paper follows the approach presented in detail in [7, 3. The deployment of the prototype into an embed- 6]. In this approach the design of the rule-base of the ded control system, using the integrated Embedded control system is supported by an integrated design, anal- Prolog Platform (the EPP). ysis and implementation process. The process is centered around a new visual knowledge representation method This paper mainly focuses on the third phase of the above for rule-based systems called XTT. It combines selected cycle. It presents the architecture and implementation important features of decision tables and decision trees of the platform. Results of some practical experiments, in order to support the design and implementation pro- including the deployment an the Palm III on Motorola cess efficiently. The XTT-based design framework, the DragonBall CPU platform are also provided. Mirella environment, provides a formal Prolog-based means to analyze the designed rulebase [9, 10]; some critical PLATFORM ARCHITECTURE properties such as determinism or completeness can be automatically verified. A rule-based system designed with In the approach presented in this paper the control sys- XTT is implementation agnostic. The design tools pro- tem logic is written in Prolog. The language is a good choice thanks to its advanced features such as high ex- vided with Mirella allow for translation to different rule- pressiveness and conceptual logic-based knowledge rep- based implementations. They also allow for fast proto- typing of rule-based systems using Prolog [1]. The de- resentation. However, there are some serious obstacles signed rulebase is automatically translated into a prede- in using Prolog for practical implementation of real-life fined Prolog form, which serves as an executable rule- control systems. The main problem is, that Prolog is a based system prototype. high level language, and generic implementations do not provide robust means for hardware communication and This paper extends these ideas by presenting a new ap- control. This is why an integrated platform combining proach to practical implementation of embedded rule-based systems. A principal idea is to provide an integrated plat- Prolog-based control layer with lower level runtime is form for embedding Prolog into a hardware control sys- proposed. In this approach the platform (see Fig. 2) contains three 1Research supported from AGH University Grant No.: 11.11.120.44 main layers: Vague system description Conceptual design business rules
Concepts Attributes
Formal conceptualization ARD Reformulation Input/Output Specification
Structure Refinement Formal logical design Formal logical Logical design XTT analysis Prolog Verification
Physical design Prolog prototype
emProlog Linux embedded hardware runtime
The Embedded Prolog Platform Fig. 2. The platform architecture Fig. 1. The system development cycle interpreter as a Task in Ada has been put forward. In or- Prolog layer, including an advanced Prolog inter- der to do this a small Prolog interpreter was developed in preter (and compiler), that executes the control logic Ada and integrated with a larger Ada system. This ap- code, proach does not address the practical construction of an a supervising middleware layer, used for integra- embedded environment. It also relies on the low-level tion of the Prolog interpreter with the operating Ada runtime implementation. In contrast, the approach system environment, presented in this paper offers a complete solution, from the high level logic design, to the actual hardware imple- an embedded multiplatform operating system with mentation. strong hardware support via number of drivers and real-time extensions, providing the main runtime PLATFORM IMPLEMENTATION environment. The approach discussed in this paper is based on the idea The main control logic is contained in a declarative Pro- of controlling Prolog-based logical core, using a ANSI-C log program. It is executed by the Prolog compiler (all based real-time supervisor. This is made possible due to advanced Prolog „interpreters” actually precompile the advanced Prolog-to-C interfaces found in multiple Pro- code). There are number of advanced Prolog implemen- log compilers. However, there are some implementation tations available, namely the SWI Prolog and GNU Pro- problems, related to different Prolog-to-C APIs. log, with different features (e.g. Constraint Logic Pro- One has to decide whether to choose a single specific gramming). This is why an extra Prolog abstraction layer Prolog implementation, or try to support several of them. has been provided (see next section) in the Prolog layer. The former approach seems to be more desired, since in The Prolog layer is controlled in by the real-time super- practice different Prolog implementations have some im- visor middleware, using Prolog-to-C interface. It is com- portant features; e.g. SWI Prolog has rich external proce- posed of several elements, including event management, dure library and a clean API, GNU Prolog provides CLP and hardware drivers communication and control. This (Constraint Logic Programming) capabilities and good layer provides unified driver interface for hardware de- performance, YAP Prolog has CLP and SWI interface. vice drivers provided by the operating systems. The su- So, a decision has been made to provide means to work pervising layer is implemented in pure ANSI C language with different Prolog platforms. for speed and efficiency. In order to do this, a Prolog abstraction layer has to be The foundation for this architecture is a multiplatform provided. The SWI Prolog API has been chosen as the embedded GNU/Linux-based environment with possible reference one. It seems to be the most flexible and well real-time extensions, providing low-level hardware drivers. established. In fact some other Prolog compilers, namely The idea of using Prolog as the high-level knowledge YAP Prolog provide SWI compatibility API. Currently specification language for control systems is not com- in our approach both SWI and YAP Prolog are supported. pletely new though. In [11] an idea of combining Pro- GNU Prolog compatibility is in the works, and is planned log knowledge representation with real-time features of as a future extension. the Ada language have been presented. However, this approach differed largely to the one presented in this pa- per. In [11] the proposal of developing the RT Prolog a simple Init facility, providing basic system setup and software upload mechanism, a simple system shell for program execution, some extra shell utilities may be needed for other Unix-specific functions. The Linux Kernel used comes from the Clinux Embed- ded Linux/Microcontroller Project (uclinux.org), which provides an operating system including Linux ker- nel releases for 2.0 2.4 and 2.6 as well as a collection of user applications, libraries and tool chains for number of processor architectures, including chips with no Memory Management Units (MMUs). The Unix LibC library for embedded systems is provided Fig. 3. Supervisor architecture by the Clibc Project (uclibc.org). The Clibc is Real-Time Supervisor much smaller than the GNU C Library, but nearly all ap- plications supported by GLibC also work perfectly with The real-time supervisor’s task is to control Prolog-based Clibc. It currently runs on standard Linux and MMU- core in real time. It provides basic time handling features, less systems with support for number of CPUs. including event handling by Prolog procedures. The struc- The msh shell, a part of the BusyBox Project (busybox. ture of the supervisor is shown in Fig. 3. net), is used as interpreter for the Init script and few The Prolog Call Manager provides the communication core utilities (such as mount, expand) needed for the sys- with the Prolog interpreter. Its main goal is to monitor tem setup. Software upload is provided by Base64 en- the events and execute Prolog-based registered event han- coder/decoder and other shell scripts. BusyBox combines dlers. It is also responsible for active monitoring of the tiny versions of many common Unix utilities into a single Prolog layer. In case of an error (e.g. in a event handler) small executable. It provides replacements for most of it provides appropriate emergency procedures. the utilities you usually find in GNU fileutils, shellutils, The Event Manager manges all of the events registered etc BusyBox itself requires a MMU or XIP (eXecution and handled in the system. All of the events are scheduled In Place) support, which is not present on all platforms. according to their priority. In case an event handler is However, msh itself does not have such requirements. provided by the Prolog layer, the Event Manager sends Using the components described above a prototype the execution request to the Call Manager. The events not GNU/Linux runtime has been built. It has only about having handlers are ignored by default. However, some 100kB in size, stored in compressed form in ROM or generic handling procedures can also be provided. Flash memory. It is sufficient for a complete system boot The Driver Manager manages the unified device drivers. and setup, as well as executing the Prolog runtime. It is responsible for registering new drivers, and the I/O procedures they provide. It also routes the appropriate REAL-TIME CONTROL ASPECTS event information the the corresponding device. When it comes to real-life control systems, the real-time The Supervisor also provides a virtual timer driver. It aspect of the control has to be considered. Prolog lan- provides number of generic timers, allowing the Prolog guage itself does not provide any explicit time-related layer to have the access the the time-related information. facilities. However, the solution presented in the paper GNU/Linux Runtime introduces a possibility of Prolog-based control in real- time. It is based on the idea of extending the Supervisor The whole runtime has been built using widely available with a real-time features. GNU/Linux components. GNU/Linux has been chosen The Prolog Logic Controller process is controlled by the because of its availability, high customization capabili- Supervisor. The Supervisor itself runs in the GNU/Linux ties, including the right the modify and extend the source environment on top of the Linux Kernel. The current code, thanks to the GNU GPL license, and good hard- Linux kernel v2.6 has some well-tested real time fea- ware support. It is also important to point out, that the tures and extensions such as: low latency control, ker- Linux kernel provides number of real-time related fea- nel preemption, and number of external real-time exten- tures, such as low-latency, kernel preemption and real- sions. The include (but are not limited to): RTLinux time extension (e.g. the RT Linux). (www.rtlinux.com) a hard-real time OS running Linux, The following software components constitute the and RTAI a comprehensive Real Time Application In- GNU/Linux runtime: terface (www.rtai.org). Currently for test purposes Linux kernel v2.0 has been used in the prototype (due to the Linux kernel configured and optimized for an memory constraints of the Palm IIIx). On superior hard- embedded system, ware all of the results can be reproduced with kernels 2.4 the LibC library providing system functions, and 2.6. The current version of the Supervisor and Logic Con- choose_direction :- troler has some basic time-related features, such as: wake- driver_elevator_get_level(L), up event, and a simple built in timer are provided. They level(X), X > L, allow for simple soft-real time operation. In the future retract(idle), these will be extended and integrated with advanced fea- driver_elevator_go_up. choose_direction :- tures the Linux kernel provides, in order to allow a com- driver_elevator_get_level(L), plete real-time environment, possibly even a hard real- level(X), X < L, time one. retract(idle), driver_elevator_go_down. CONTROL SYSTEM EXAMPLE choose_direction :- As a proof of concept a simple elevator control system assert(idle). has been implemented. Some illustrative excerpts of the The timer measuring the time to close doors is imple- Prolog implementation are included below. mented as follows: Every control logic begins with event handler registration predicates: time_to_close_doors :- driver_doors_are_blocked, /* Event namespace: driver_timer_run_once(
2 ?- bench(10). other hardware platforms, including the ARM family. An- 1336.93 lips for 10 iterations taking 3.71 secs other important issue that will be addressed is the incor- Yes poration of a real-time optimized Linux kernel. A full API making real-time extensions available to the Prolog Multiplatform Considerations control logic, is in the works too. Extending the hardware support of the embedded runtime on multiple CPUs and hardware architectures involves THE AUTHORS number of issues. Practical multiplatform implementa- Dr. Grzegorz J. Nalepa works in Institute of Automatics, tion needs to take into account the following: AGH University of Science and Technology Memory requirements At least 1 MB ROM and 2 MB Al. Mickiewicza 30, 30-059 Kraków, Poland RAM is needed. However, if a more powerful control is E-mail: [email protected] to be implemented, more memory is needed for Prolog and in stacks. An optimal amount is 4 MB of RAM: about 250– Institute of Physics, Kielce Pedagogical University 500 kB for the GNU/Linux runtime, 3 MB for userspace Ul. Swi˛etokrzyska15,´ 25-406 Kielce, Poland part (Supervisor and Prolog layer) and 2 MB ROM (for Mr. Piotr Zi˛ecikis a student at AGH UST, he writes his the Prolog libraries). The RAM memory usage can be Master’s Thesis under supervision of Dr. G. J. Nalepa reduced at the cost of ROM consumption, by using XIP E-mail: [email protected] (eXecute In Place) technology. Unfortunately it is not available on all platforms. REFERENCES [1] Ivan Bratko. Prolog Programming for Artificial In- telligence. Addison Wesley, 3rd edition, 2000. [2] Jay Liebowitz, editor. The Handbook of Applied Ex- pert Systems. CRC Press, Boca Raton, 1998. ISBN 0-8493-3106-4. [3] Antoni Lig˛eza. Logical Foundations for Rule-Based Systems. Springer-Verlag, Berlin, Heidelberg, 2006. [4] Motorola, Archived by Freescale Semiconductor, Inc. M68EZ328ADS v2.0, Application Development System User’s Manual, revision 1.0 edition, 19 jan- uary 2000. [5] Motorola, Archived by Freescale Semiconductor, Inc. MC68EZ328 DragonBall-EZ Integrated Pro- cessor User’s Manual, rev. 1 11/98 edition, 2005. [6] Grzegorz J. Nalepa and Antoni Lig˛eza. Con- ceptual modelling and automated implementa- tion of rule-based systems. In Tomasz Szmuc Krzysztof Zielinski,´ editor, Software engineering : evolution and emerging technologies, volume 130 of Frontiers in Artificial Intelligence and Applica- tions, pages 330–340. IOS Press, 2005. [7] Grzegorz J. Nalepa and Antoni Lig˛eza. A graph- ical tabular model for rule-based logic program- ming and verification. Systems Science, 31(2):89– 95, 2005. [8] Grzegorz J. Nalepa and Antoni Lig˛eza. Security systems design and analysis using an integrated rule-based systems approach. In Piotr Szczepaniak, Janusz Kacprzyk, and Adam Niewiadomski, edi- tors, Advances in Web Intelligence: 3rd Atlantic Web Intelligence Conference AWIC 2005, volume LNAI 3528. Springer-Verlag, 2005. [9] Grzegorz J. Nalepa and Antoni Lig˛eza.A visual edi- tion tool for design and verification of knowledge in rule-based systems. Systems Science, 31(3):103– 109, 2005. [10] Grzegorz J. Nalepa and Antoni Lig˛eza. Prolog- based analysis of tabular rule-based systems with the xtt approach. In to be published in: Proceedings of the 19th FLAIRS Conference, Melbourne Beach, Florida, USA. AAAI Press, 2006. [11] G. Scott Owen. Rt prolog: a real time prolog written in ada. In CSC ’88: Proceedings of the 1988 ACM sixteenth annual conference on Computer science, page 684, New York, NY, USA, 1988. ACM Press. [12] Ian Sommerville. Software Engineering. Interna- tional Computer Science. Pearson Education Lim- ited, 7th edition, 2004.