Download 2. Control and data acquisition system topology

Control and Data Acquisition System For
Laboratory Analytical Instrumentation and Experiments
M. Kaucsar, H. Bendea, D. Vonica
1. Introduction
In laboratory instrumentation and also in physical or in chemical experimental process
studies the computerised control and data acquisition systems are one of the most important
upgrade in our research activity. This allows translating the communication between man and
instrument or man and process to a much higher level of human intelligence than before
microcomputers were extended. In physics and in chemistry we deal with complex processes
where the useful information results after acquiring and processing of a very large set of data.
The use of personal computers (PC) is not only recommended but also indispensable.
2. Control and data acquisition system topology
Combining the features of the PC's with the one of distributed programmable
microsystems in the studying of experimental processes or in the development of new scientific
instruments we obtain many advantages. Distributed processing provides us speed and better
reliability for the entire system. Depending on system requirements, a compromise between
speed and reliability could be accomplished with an adequate hardware and software
engineering. The topology of a complete control and data acquisition system is shown in figure
1. Multiple feedback control loops are closed using distributed programmable microsystems. A
powerful PC controls the entire distributed microsystem structure. In this complex hardware
and software configuration the PC is the master and the microsystems are the slaves. Interfacing
an experimental process or a scientific instrument with such a master-slaves computing
configuration is one of the most demanding steps in the whole system development. The first
task is a logical separation of all functional units in the target controllable system. Depending on
the acquired or/and controlled parameter each functional subassembly is equipped with proper
sensors, transducers and actuators. Many controlled systems and instruments could be damaged
when one or more functional parameter exceeds the maximum rating values. In order to avoid
such undesired situations, apart from sensors used in feedback loops, the system is equipped
with other transducers. These are intended to watch the maximum or minimum allowed limiting
values. Therefore the entire controlled process or instrument will be completed with two very
important functional blocks: one is the block of sensors and the another is the block of
actuators. These are separated in the logical diagram but in reality they are integrated with the
controlled system. All the sensors and actuators must be installed according to the theoretical
background and mathematical model of the entire controlled system.
Fig. 1. Simplified block diagram of the control and data acquisition system for
laboratory analytical instrumentation and experiments
The type of microcomputer was chosen to be appropriate to the task complexity of the
correspondent loop and the speed’s demands for proper real time functionality. For medium
complexity and medium speed is recommended to use microcontrollers, which allow an easy
implementation of a local feedback loop with minimum hardware effort. For higher speed and
complexity the most suitable are microprocessors (CPU’s) from Intel or Zilog. In special
situations with very high-speed signals, the choice is DSP (Digital Signal Processor).
The master PC correlates all functional tasks executed individually by the slave
microsystems and also implements a friendly interface between the controlled system and the
operator. For this purpose is essential to implement a proper communication protocol between
the master PC and the distributed slave microsystems. We prefer to use the standard serial
communication protocols such as EIA/TIA-232-E, RS-485 or the instrumentation standard
communication GPIB-488. It is possible to apply a special bilateral data transmission with the
parallel LPT port but only if it has EPP or ECP capability.
3. Microcomputer with extended input/output capability – the hardware
The general block diagram of the microsystems and the main computer is represented in
figure 2. Every PC is equipped with standard input/output hardware, but in order to use them
in complex feedback loops extra input/output hardware must be added. The microsystem with
extended analog and digital input/output capability is fitted with dedicated input/output module
cards. Depending on the operating principle of the detectors and actuators the signals involved
in the whole system could be analog or digital. The majority of the sensors and transducers
provide us analog signals and only a few of them have digital output. The second case is typical
for transducers specialised mainly for detecting the level of a parameter. When its output is a
logical true level the system works properly. Actuators also need analog or digital control signals,
corresponding to their operating principles.
Fig. 2. Generalised configuration the microcomputer with extended input/output capability
3.1. Digital input module
Digital signals can be very easy interfaced with any digital microcomputer, therefore the
use of digital data is preferred. On the other hand the transmission of digital data is much more
reliable than the one of analog transmission, especially in noisy environment. These
considerations make us to use transducers with direct digital output where possible. In order to
turn to the digital transmission, transducers with analog output placed at long distance from the
PC are equipped with local A/D converters. Digital data transmission uses both types of
transmission: parallel and serial. With digital parallel data we can accomplish higher bandwidth
but at a higher price. Serial formats lead to a lower cost and also offer a more simple way for
galvanic separation.
The standard logical signal levels must be applied in every digital transmission.
Therefore sender and receiver is equipped with level converters. At receiver the digital noise is
eliminated with Schmidt triggers. In special situations, when the sender and the receiver must be
galvanically separated, at receiver the input circuits are equipped with optocuplers.
3.2. Analog input module
A large number of sensors deliver information carried by analog signals. The main task
of this module is to convert the analog input signals to a digital one, making them compatible
with further digital processing with computers. If the sensor is not very far from the
microcomputer we prefer not to fit every sensor with an expensive A/D (analog to digital)
converter. Instead of it we use an A/D card with multiplexed analog inputs and only one
converter. In special cases when high acquisition speed and also high accuracy is needed, we
apply the expensive solution: each analog input equipped with its own A/D converter and
multiplexing in digital signal way. The A/D converter type is chosen according to the speed
demands. For higher speed signals we use successive approximation converters and for lower
speed dual slope converters. If high accuracy at medium speed is imposed a delta-sigma
converter is the best choice.
Analog signals delivered by transducers have various voltage and current levels,
depending on their constructive aspects. For optimum accuracy of the A/D conversion we
must match these levels with converter input specifications. In order to translate the output
range of a transducer to the input range of the A/D converters analog signal conditioners are
applied. These circuits accomplish filtering (antialiasing), amplifying and shifting tasks for the
given signal. The antialiasing filter is a low-pass filter with cut-off frequency matched with the
Nyquist frequency of sampled analog signal. The level of the signal is adjusted to the desired
value through software with digitally programmable gain amplifiers.
3.3. Digital output module
The digital output module is designated for actuators with digital control inputs. From
this category the most widespread are the bipolar on-off regulating elements. Output voltage
and current levels, from the basic components of the digital output module, usually don’t match
the actuators’ requirements. Therefore we use power drivers. Several drivers were realised: open
collector drivers for general purpose, relay output drivers and AC output drivers. The open
collector driver is realised also with optocupler galvanic isolation. The driver for AC lines is also
provided with optocupler separation and with a zero crossing circuit for minimum
electromagnetic switching noise.
3.4. Analog output module
The analog output module delivers control signals for analog input type actuators.
Complex waveforms, needed for testing and characterising the system, can be generated with
this module. The basic components of these modules are D/A (digital to analog) converters. In
order to assure required voltage and current levels needed by actuators and by signal generators,
high precision power amplifiers follow the D/A converters with increased output swing
capability.
4. The software
Because each feedback loop is closed through a local microcomputer system, proper real
time data acquisition and control routines are implemented on each microcomputer system. The
master-slave communication needs communication protocol routines. These are implemented
on the master PC and on each slave microsystem. On the master PC are implemented the most
sophisticated friendly operator interfacing routines.
Literature:
1. Duncan, F.G. – Microprocessor Programming and Software Development,
Prentice/Hall International Inc., London 1980
2. Tietze, U., Schenk, Ch. – Halbleiter schaltungstechnik, Springer Verlag, Berlin –
Heilderberg 1990
3. Norton, P., Eggebrecht, L.C., Clark, S.H.A. – Peter Norton’s Inside the PC, Sams
Publishing 1996
4. Schildt, H. – C++ The Complete Reference, Osborne Mc-Graw Hill 1995
5. Oppelt, W. – Kleines Handbuch technischer Regelvorgange, Verlag chemie GMBH,
Weinheim/Bergstrasse