Intelligent signal transmitter

Intelligent signal transmitter Prof Dr sc techn M . Seifart Technical University Otto von Guericke, Magdeburg, DDR The following paper presents the hardware and software concept of a multisensorbased intelligent signal transmitter which is equipped with an isolated 2-wire field bus. Communication on the serial bus enables signal transmission, variation of parameters, selection of prefabricated signal processing routines and remotecontrolled diagnostics. Finally, some trends on intelligent modules resulting from advanced efficiency of electronic components are reported. Keywords:

Data acquisition, signal transmitter, field bus.

Introduction

Basic concept

In industrial instrumentation and process control, the availability of low-cost microprocessors and single-chip microcomputers (SCMC) leads to intelligent multipletransducer data acquisition structures and systems (Seifart, 1983; Seifart et al, 1986; Bradshaw, 1984; Allen, 1983). Such systems consist of remotely located intelligent modules, which are controlled by a central unit; for instance, by a personal computer. We may distinguish between four difTerent structures of sensor-signal acquisition systems (see Fig 1). For small distances between the sensors and the central microcomputer (a few metres only), structure 1 is preferred. This structure is often realised in data acquisition systems which are plugged into empty slots of PCs. The sensor signals are transmitted as analogue signals to the data concentrator (analogue multiplexer). The digitised data are transmitted via a parallel bus. The performance of such data acquisition systems may be increased by application either of microprocessors or SCMCs. The main advantages are an increase of accuracy and flexibility, simplification of the interface, correction of errors, autocalibration, preprocessing of data and selfdiagnostics (Seifart et al, 1985). For large distances (some 10, 1(X) or 1000 metres), structure 4 is much more advantageous. This structure is the basis of SCMC-based signal transmitters designed for remotely located intelligent measuring data acquisition using the party-line technique. This concept has several advantages over existing systems with analogue signal transmission. It leads to a remarkable reduction of cable costs, because all signal transmitters of the system are connected to a single two-wire bus (field bus), which transmits the signal bidirectionally up to 19 200 baud. The following text refers to this type of transmitter.

Starting from practical demands, the intelligent signal transmitter has been equipped with seven analogue/ binary direct sensor inputs, one counter input and output, four binary outputs, one analogue signal output which can be scaled to any range from 0-20 mA or 0-10 V and with an isolated serial output/input (UART). Signals of ^256 transmitters may be transmitted bidirectionally by means of this serial output/input over a two-wire bus (field bus). This bus also serves as an interface to further signal processing devices. Parameters of the intelligent transmitter, such as range (offset, full scale), alarm levels, configuration of the transmitter, are adjustable either remotely or by means of a programmable array and EPROM. The unit processes analogue signals with a resolution of 12 bit. Communication on the serial bus enables remote-controlled diagnostics (access to all registers of the SCMC). The SCMC allows preprocessing of sensor signals, which is easily software-adaptable to the demands of special applications. Examples are routines for linearisation of sensor characteristics, correction of off"set and drift errors, averaging, elimination of noise (filtering) and control of limits. The SCMC in combination with the EPROM leads to some additional flexible properties: ability for the solution of new tasks by interchanging the EPROM, selection of prefabricated basic signal processing routines by proper wiring the programmable array or remotecontrolled by means of the function field in the communication protocol, self-diagnostics (program run, supply), signalling of errors.

Prof. Dr. sc. techn. Manfred Seifart, Sektion Auto-matlslerungstechnik und Elektrotechnik, Technical University Otto von Guericke, OOR-ZOyO Magdeburg, PSF 124 Measurement Vol 5 No 3. Jul—Sep 1987

Circuit design

Fig 2 shows the block diagram of the signal transmitter. Main elements are the SCMC, the programmable sensor signal amplifier with multiplexer, the ADC and DAC, the bus coupler with isolated output stage, the programmable array (matrix) with 6-4 programmable wired connections and a transverter for isolated power supply. 107

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Measurement Vol 5 No 3, Jul—Sep 1987

Seifart The address of the transmitter is selectable by a DIL switch in connection with the programmable array and a decoder. The programmable array allows the user to select and combine special statements, program starts and software conditioning without variations on hardware and software. Each of the seven input channels accepts either analogue or binary signals without change of hardware. To achieve high resolution of the signal processing, the sensor signals are amplified to a level of lOV. This amplifier realises auxiliary functions: eg, common mode rejection, filtering, adjustment of supply voltage or current for sensors, generation of offset. OfTset, full-scale voltages and the gain of the software programmable amplifier are adjusted automatically by the SCMC. One problem often arising in data acquisition systems with several sequentially sampled analogue channels is the limited settling time of the amplifier, especially after a large spike or noise pulse. This settling time is proportional to the gain and inversely proportional to the bandwidth of the amplifier. Typical values are some 100//s if the gain equals 100. Fig 3 shows a circuit with a much smaller settling time. The basic idea is to maintain constant loop gain of the whole system by switching the feedback factor and the gain of an auxiliary amplifier simultaneously (Dmitriev, 1985). The settling time of this circuit is independent of the gain and amounts approximately to 5 /is. The ADC is based on the successive approximation principle. The algorithm of successive approximation is processed by the SCMC. Conversion time amounts to 260 /is. To minimise hardware and power consumption, the 12-bit D A C C 5 6 5 D (RFT) is time-shared both for the A/D and D/A conversion. During the A/D conversion the analogue output signal is held by a sample-and-hold circuit. The number of channels, the organisation of the analogue/binary inputs and the selection of sensor types

are adjustable to the special demands of application by means of an interchangeable auxiliary board (80120 mm^). The hardware of the main board (160120mm^) remains unchanged for the various applications of the transmitter. A stabilised supply current (1 mA) is switchable to each input via an additional multiplexer. This enables seven resistive sensors to be connected to the seven inputs without additional circuitry.

Software concept Complexity and efficiency of software are strongly dependent on the characteristics of the SCMC (memory size). The program memory (4 Kbyte) of the transmitter contains a library of various input/output types and ranges and the usual process functions. This enables the user to field program features required for his special application. The program consists of the following three modules: (1) Initialisation; (2) Bus-communication; and (3) Process software. The process software realises acquisition of analogue and binary signals, their arithmetic and logic processing as well as output of data and control signals. Transfer of data on the serial bus is realised asynchronously in Manchester Il-Code or NRZ-Code with a communication protocol similar to the PDV-bus. The address byte is followed by one function byte, and data bytes with cyclic redundancy check or parity check. Only a complete protocol received without errors is processed in the SCMC.

Single-chip microcomputer The performance of the transmitter is strongly influenced by the single-chip microcomputer, which solves numerous important tasks, among others: Automatic zero and full-scale calibration Automatic range selection Preprocessing of acquired data (arithmetic, logical, supervision of limits, concentration of data) Display of data in appropriate form Processing of complex algorithms Realisation of interfaces to other devices and bus communication Organisation of the system Increase of accuracy by reduction of noise Increase of reliability by self-test and self-diagnostics Simplification of service by automatic supervision of failures Now there are three generations of single-chip microcomputers on the market which are characterised, for instance, by the products 8048 (1), 8051 or Z 8 (2) and 8096 (3). We apply the SCMC U 882/884 (RFT) which is equivalent to Z 8 and has an on-chip UART.

Advantages, application A BC Fig 3 Amplifier with settling time independent of gain Measurement Vol 5 No 3, Jul—Sep 1987

The universal concept of the intelligent transmitter leads to a broad field of applications. The flexible system structure involves several advantages concerning project work and operation of automation systems. Some advan109

Seifart tages are: • Saving cable costs by party-line principle • High reliability of communication and signal acquisition by digitising close to the sensor • Fixed hardware structure for different tasks, quick alteration of the application program (EPROM, programmable array, function field) • Combining and preprocessing of sensor signals in the transmitter.

Trend In the near future we expect a growing efficiency of SCMC and more types of CMOS-ICs. This will lead to advanced modules. The progress in the performance of SCMC is a strong drive for the propagation of distributed intelligence. We will find more types of SCMC designed especially for process automation purposes like the examples SAB 80515 (Siemens) with eight analogue channels, 8-bit ADC (15 n%\ sample and hold and /iPD 78312 (NEC) with four analogue channels, 8-bit ADC, 8-Kbyte ROM, CMOS. Simplification of programming is realisable with an on-chip BASIC interpreter as used in the 8052 AH (Intel). In addition, this SCMC includes a real-time clock. Of course, further increased efficiency will surely result by the use of 16-bit SCMC. Interesting concepts will be realisable by the application of EEPROMs, for instance: • Variation of programs without the need to take the memory out of the device • Variation of parameters • Modifiable firmware • Adaptive systems

• Loading of programs from a central computer into decentralised modules • Variation of data by service and supervision work An important step forward will result in the design of a CMOS-gate array which realises the address recognition and serial transmission. This is a way to reduce considerably the amount of power consumption, because only one transmitter (the addressed one) must work and the others are in standby. To increase the efficiency of programming, much effort must be undertaken to develop powerful applicationoriented higher programming languages. We expect a trend towards SCMC-based multifunction converters, which also act as stand-alone intelligent modules.

References Allen, Ch, 1983. 'How distributed should your controlsystem be?', Measurement and Control, 16 (5), 174-179. Bradshaw, A. T. 1984. 'Smart pressure transmitters' Measurement and Control, 17 (9), 353-357 Dmitriev, N. V. 1985. 'Usilitel' povysennoj tocnosti i bystrodejstrija s cifrovym programmirovanniem koefficienta usilenija', Izmeritel'naja technika-Moskva, 1, 56-57. Seifart, M. 1983. 'Mehrkanal-MelJwerterfassungssystem mit seriellem Bus', Wiss Zeitschr TH Magdeburg, 11 (1/2), 81-85. Seifart, M., Hertwig, B., and Hanisch, H. 1985. Intelligente Analogwerterfassungsplatine fiir das Mikrorechnersystem K 1520,' Wiss Zeitschr TH Magdeburg, 29 (7) 105-106. Seifart, M., Beikirch, H., Baumecker, D., and Fincke, U. 1986. 'Systeme zur Sensorsignalerfassung und -verarbeitung', Wiss Zeitschr TH Magdeburg, 30 (5).

Contributions Contributions are invited on all aspects of the research, development and application of the science and technology of measurement. Manuscripts should be sent to: Mr M. J. Yates, Secretary, The Institute of Measurement and Control, 87 Gower St, London WC1 E 6AA, UK. Guide notes for contributors

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are to be found on the inside back cover.

Measurement Vol 5 No 3, Jul—Sep IBS';