Data Acquisition & Telemetry


Telemtry for Africa, from Africa

March 2000 Data Acquisition & Telemetry

Power utilities, water utilities and municipalities have been the primary consumers of telemtry systems, which keep control room operators in touch with outlying installations through the use of radio links, telephone links and fixed line networks. However, more industries are now turning onto the idea that powerful, reliable and fast telemtry systems need not be expensive, need not require major hardware installations at each end and need not require a degree in engineering to operate.

Technology changes and innovative designs are driving down the cost while pumping up the functionality. There are now products on the market that can be considered true multifunctional devices. These are equally at home in a factory monitoring and control environment - or stuck on a pole in the veld, at the mercy of the elements, powered from a solar panel and transmitting data via radio every now and again. Industrial data telemetry is a specialised field requiring expertise and experience to design, install, maintain and upgrade systems.

Telemetry systems in all the utility industries have traditionally used radios to communicate with out-lying stations, via remote terminal units (RTUs). However as bandwidth has become less available, engineers have sought alternative means of communicating. The four newish alternatives to the traditional UHF (ultra high frequency) radio are spread spectrum radios (where the energy level in the signal is lower than the minimum permissible under law), low power radios (which communicate in line of sight over short hops - typically 500 m), modem connections (using fixed telephone lines and cellular phone communications). All of the different communications links have their advantages and disadvantages and every installation must be evaluated on merit. Most larger systems employ a combination of any of these different links in making up the entire system.

One of the major benefits of the alternative links offers is that there is no waiting period for licence applications, installation costs are low, and the systems are generally maintenance free. In the case of dial-up modem technology, modems are available off the shelf and data transfers take place quickly as the files are small.

Omniflex, formerly the Conlog Industrial Division, provides a ready-made solution for fixed line networks, dial-up modem and radio telemetry applications using the modular Maxiflex RTU system, a powerful, yet cost-effective programmable remote I/O system. Maxiflex interfaces directly to any Telkom approved modem up to 33 600 baud and can be dialled by a host device (eg scada system) for data. It can also dial the host device itself at preset time intervals or on any data or derived variable trigger or alarm signal.

There are basically four components to the Omniflex dial-up telemetry system: the host computer or PLC, the host modem, the remote Maxiflex I/O and the remote modem. These are connected via the standard telephone network, which is usually available at most remote dam, pump-station and water treatment sites. Furthermore, because the system is configured as a dial up, there is no need to lease a dedicated line and data can be transferred a few times a day or on alarm.

The Maxiflex RTU can poll for data continuously, sampling inputs as fast as 100 ms for digitals or 1 s for analogs. However in practice most telemetry applications require samples to be taken every few minutes or hours and in many cases, the remote computer only dials up once a day to upload the entire day's readings. This scenario is typical where there are outstations at numerous remote dam sites and water levels need to be updated daily in the central control room.

Data acquired by the Maxiflex RTU are time-stamped at source and stored in a table, waiting for the host computer to dial up and extract the information. When this is done, the table is emptied and new data written into it. If there is a sudden, large amount of activity at the remote site which fills the table, then the Maxiflex RTU can be set up to overwrite the first or the last data, or to dial up and upload the contents of the table.

The Maxiflex RTU can also be configured to dial up the host device at preset time intervals (eg every 6 h) or under various conditions. These could be an alarm condition (eg a high bearing temperature), or when the data file is full, or if the remote I/O is expecting a dial-up from the host, but this has not been received for say 10 minutes after the expected time, or on an internal fault being detected etc. The power and programmability of the Maxiflex RTU means the configuration is practically unlimited.

One of the common tasks for a Maxiflex RTU system in the water industry is to monitor pump run-times for preventive maintenance purposes. At a pre-determined number of running hours, the system can be set up to send a specific message to the host device to advise that maintenance is now required, or alternatively, the running hours could be transmitted every time the remote I/O is accessed for tracking by the scada or maintenance management system.

A major benefit of the Maxiflex RTU system is that it can be configured as a distributed system at the remote site, via a Conet twisted pair local area network which links the different I/O racks via a twisted pair cable. Doing it this way means that the actual I/O at the treatment works or pump station can be positioned close to the individual motors, pumps and equipment that needs to be monitored, thus keeping input wiring to a minimum. These distributed networks can have up to 127 Maxiflex RTUs, each with 240 I/O (digital or analog) spread out over a 10 km cable. This is more than sufficient flexibility for even the largest installation.

The Maxiflex dial-up remote I/O systems use a Modbus protocol that has been modified to provide extended functionality. Any scada system with a standard Modbus Master driver is able to interface with the RTU via the dial-up modems and where enhanced functionality is required, drivers for most of the scada/MMI packages on the market are available.

Maxiflex is a versatile I/O system product that has PLC-style plug-in modules for most common industry inputs and outputs. These include: 12/24 V d.c. inputs, potential free contact inputs, relay outputs, process analog inputs (eg 4 to 20 mA), thermocouple inputs, RTD inputs, process analog outputs, and high speed counter inputs.

The system also has network interface modules (NIMs) which cater for every conceivable remote site expansion need using Conet and Modbus NIMs for interfacing to existing intelligent instrumentation on site via a serial port. For example water quality analysers and other specialised equipment can now have data transmitted back to a scada or other host system via the dial-up connection. Omniflex also manufactures different pieces of electronic monitoring equipment that can be connected to the Maxiflex RTU using Conet.

For really remote sites where there is no electricity, Maxiflex can be powered by a solar panel using a special plug-in power supply in the Maxiflex I/O system base. A standard phone or cellphone can be used for dial-up communications, assuming there is a signal at the remote site, as typical amounts of data acquired in a 24 h period will be transferred quickly, even at 9600 bits per second.

One of the growth areas in telemetry is to provide the remote information directly to the desktop and Omniflex's Conet OPC (Ole for process control) server has been designed specially to enable data from any Conet Network to be sent to MIS, scada, spreadsheet and other applications across the LAN.

Omniflex has a wide range of products for all applications, backed up by decades of experience in fixed line, dial up and radio telemetry.

Omniflex

(031) 207 7466



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