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可编程控制器的数据库系统--中英文翻译资料.doc
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可编程控制器的数据库系统--中英文翻译资料.doc
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Database Systems for Programmable Logic Controllers
In this paper, we identify the database issues associated with programmable logic
controllers (PLC), special-purpose computers used in scientific and industrial
applications, e.g. in factories in manufacturing environments. We propose as a PLC
database system a single-user,real-time, scalable main-memory-only relational
database system with a two-level architecture having historical data modeling and
manipulation capabilities, and query processing techniques incorporating time- and/or
error-constrained query evaluation. We revise the ladder logic language, the most
common PLC language, to incorporate data manipulation language instructions. We
add a separate time component into the PLC processor scan time to handle database
updates, backup, integrity enforcement and data archival issues.
1. Introduction
A programmable logic controller (PLC) is a special-purpose computer used
within real-time scientific computing systems, and industrial control systems, say, the
automated control of a factory's machinery - the running example used in this paper.
This paper is a position paper that proposes a PLC database system and discusses its
features. In doing so, we touch bases with a number of basic database topics, and, thus,
frequently refer to other work for details.
PLCs are mostly used in manufacturing environments - hence, the choice of our
running example. However, PLCs are also used in scientific applications for signal
data gathering and preliminary data processing. Thus, we think that for some
scientific applications, a PLC database may also serve as a local/transient part of a
larger scientific database.
With the rapid advances in computer hardware and falling memory prices, in
recent years, the capabilities of the new PLCs in the marketplace have been increasing
dramatically. This paper is a position paper that argues that a PLC software can now
contain a database system to greatly increase its functionality. We propose the
architecture in Figure 1 as the architecture of an environment where real-time data
gathering (from multiple sensors) and real-time data manipulation takes place.
We now list the advantages of having a database system directly inside a PLC.
(1)Data Modeling Techniques : The input and output buffers represent a rather
unorganized transient model of the real world, and hence, carl be modeled better
using the traditional data modeling techniques of databases.
(2) Historical Databases : PLCs routinely deal with different versions of data
over time. Therefore, historical data modeling techniques as well as historical data
manipulation techniques can replace the ad hoc ways of manipulating historical data
in PLCs.
(3) User-Friendly Interfaces : Presently in the marketplace, the PLC software and
industrial terminals allow a limited display of messages and variable-data information
in memory. For example, the contact histogram function displays the on/off history of
a specific main memory.
(4) Handling Large Volumes of Data : With the added capabilities of a database
and a query language, the PLC may analyze much larger volumes of data.
(5) Data Reduction and Compaction at the PLC Level : Presently, for further
analysis or simply due to various regulations, data collected by PLCs get transmitted
and stored into a host computer file using an architecture shown..
Since the present PLCs cannot satisfactorily analyze most data, they simply
transmit data to the host computer. In some scientific experiments and applications,
the data gathered is so large that arguments have been raised for "processing the data
on-the-fly" during the execution of an experiment/transaction [SSDB 86]
(a)Real-Time Database : The data in the input buffer must be scanned within
reasonably short "real-time" intervals ranging from microseconds to seconds.
Therefore,responses to queries must be guaranteed to be less than a certain "realtime"
time bound, almost always less than 5 to 10 seconds.
(b)Main Memory Database : Microseconds/seconds query response restrictions
necessitate main-memory-only databases.
(c) Scalable Database : Once the environment of a PLC and the requirements of
the associated application program are determined, the possible query types to the
database stay fixed for a reasonably long period of time. Since the response time of
queries is of utmost importance, the DBMS should be scaled so that only the needed
routines/functions (e.g., access methods, data structures, etc.) are incorporated.
In section 2, we discuss the general characteristics PLCs, and briefly present the
ladder logic language. Section 3 discusses the features of the proposed database
system for PLCs.
In general, the PLC hardware is mostly custom-built with occasional
off-the-shelf hardware, and consists of a CPU (or multiple CPUs), main memory, an
"industrial terminal", and high- and medium-speed data communications hardware.
The size of the main memory ranges from 16K bytes (of 5 to 10 years ago) to 8M
bytes (of the present time).Although the CPU has an instruction set similar to those
found in CPUs of 16-bit and 32-bit machines, it is especially equipped with fast bit
manipulation instructions. The industrial terminal comes with a special keyboard to
make the programming of the PLC easier and/or to intervene with the application
program.
The PLC software consists of an operating system (ranging from a very
simplistic monitor (of ten years ago) to a sophisticated real-time operating system (of
the recent time)), high-speed communications software for communicating with I/O
processors,medium-speed communications software to the industrial terminal and to
other "intelligent" devices.
Both general-purpose computers and PLCs are used for industrial control [Star
87].However, they differ in the programming languages that they use, environmental
specifications, and their user types. PLCs are rugged, and work in hostile
environments with no special climate controls, tolerating extremes of temperature
(60° C), humidity (95%) and air contamination. Users of PLCs include the original
programmers of the application programs, as well as plant electricians and
maintenance personnel, who are accustomed to relay-type controlling environments.
A rung is an ordered set of PLC instructions drawn on a single line. Instructions
on a rung are classified as input instructions (those that monitor the input buffer) and
output instructions (those that set the output buffer), and are executed from left to
right, sequentially (Please see figure 4). A PLC application program consists of a
main program and a set of subroutines, each of which containing an ordered set of
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