12-CHANNEL PNEUMO-ELECTRICAL INTELLIGENT CONVERTER

    Your factory uses pneumatic output signal devices for different purposes, like fire and explosion safety, or for other reasons, bound with production process. Your operating rooms are overloaded by hundreds of pneumatic transmitters. They take up too much room and require permanent technical maintenance and repair, which are too expensive. You would give up outdated equipment and exclude human factor from the production process.

    But the purchase of expensive electro-transmitters of explosion-proof type has its own risks and difficulties. The replacement of outdated devices might stop for a time all production process.

    Another solution of this problem might be the use of analog pneumatic converters with standardized current output, compatible with computer facilities. But as a rule, these analog pneumatic converters are too unwieldy. And, above all, these converters require one more operation of data processing, i.e. conversion of current signal in a digital form. It essentially reduces measurement accuracy and increases data line cost half or twice as much again.

    KRUG has solved this problem and since 1999 successfully produces 12-channel pneumo-electric converters (PPM), having the State Standard Certificate of Russian Federation No.18020-98 of the State Registration from 5.01.1999. The PPM is designed for proportional conversion of standardized pneumatic signal (0,2-1,0 kg/cm2) into electric digital one, which is compliant by its electric performance with requirements of EIA RS-485 standard. The used logic data exchange algorithm is MODBUS protocol compliant.

    The PPM converter consists of a pullout module provided with an individual front panel (81 x 130 mm), on which are mounted pneumatic assembly connectors, interface and 24V supply ports and indicators. The front panel is also fitted with a processed plate, to which are connected 6 "mezzanines" (or removable small-size plates) by means of special ports with gilt contact. Each "mezzanine" carries 2 piezoresistive pressure transmitters. Such construction allows easy replacement of transmitters when necessary (for example, by repair or technical maintenance of the device). For ease of mounting and mechanical protection the pullout PPM modules are set in a composite rack DIN 19`` (Fig.3). One composite rack contains up to fife 12-channel PPM converters; also it might contain up to 60 pneumatic input channels. The dimensions of such composite rack are 364 x 483 x 130 mm overall.

    One of the most significant features of the PPM is availability of a built-in microcontroller that has enabled to get not only high metrological performance, but also full automation of metrological maintenance of the device. The calibration and checking procedure of the device has been considerably simplified thanks to the specially elaborated by KRUG software, which is provided together with PPM device.

    First PPM converters have been applied at May 1999 at the Tuapse Oil-Refinery, where are employed today 24 PPM devices at all. At present over 140 PPM converters are successfully employed at least at 15 plants of Russia. Among them are such companies as "Lukoil-Permnefteorgsynthese" (over 40 PPM converters), "Novokuibyshevsk Oils- and Additions-making Plant", "SANAK" (Moscow), "KNPZ-Krasnodareconeft", "Omsk Rubber", etc. For more details see "Reference List".

    Experience has proved reliability and stability of metrological characteristics of PPM device. Thanks to absence of switches and removable elements, there is no need of preventive maintenance.

    The PPM has following performance specifications:

    The operating principle of the PPM is based on conversion of input signals by strain gauges 1:12 into electrical millivolt range signals, which are fed on inputs of analog-digital converters ADC1 - ADC4 accordingly. Each ADC has 3 multiplex inputs (MUX1 - MUX4). 16-bit code signals from ADC outputs are read by microcontroller. The microcontroller digitally corrects offset and sensitivity of strain gauges and reduces complementary error, resulted from temperature deviation inside the rack. All individual calibration coefficients for each measurement channel are stored in power-independent Flash-memory. Corrected values of measured pressure from all 12 channels and inside temperature value are transferred in a digital form to the communication line through the RS-485 interface converter with galvanic isolation. The device can be powered with any AC/DC power source between 18 and 30V through a built-up voltage converter with galvanic isolation (made by TRACO). Each PPM receives an individual system address by means of system address switch. The function block diagram is given on Figure 1.

Figure 1. Function block diagram
1 : 12 - piezoresistive pressure converters PDI-Sh1
MUX 1 : MUX 4 - multiplexers
F1:F3 - self-recovering fuses

    The PPM converters can be configured in the local trunk network with a fixed control center via multipoint interface RS-485. It may be connected with up to 31 PPM converters. The length of the trunk line may reach 1000 meters (Figure 2).

Figure 2. Connection of PPM modules in a local network

    To prevent emergence of echo signals on the ends of the data line, implemented in form of a shielded twisted pairs of wire, should be set up terminal resistors with nominal resistance 100-200 Ohm, equal to wave impedance of the data line. The active party of the network, i.e. Master-module, sends requests to a specific address and then passes into "suspend" mode, waiting for response. The function of Master-module might be performed by a PC; the PPM converters can be regarded as Slave-modules. Each Slave-module is usually in "suspend" mode, waiting for request from Master-module. If the requested address corresponds to its own one and contains no errors, the Slave-module responds. The requests, sent to other address or containing errors, are ignored by the Slave-module.

    The PPM can be connected to PC by using "KRUG-2000" program packet via OPC-server (provided together with PPM device) or by writing a new exchange driver (protocol description is given in the "Operating instructions", "Appendix K").

Figure 3. Composite rack DIN 19"

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