Low-energy wireless SCADA used for water application
Low-power wireless technologies help supervisory control and data acquisition (SCADA) communications at the Bogotá, Colombia, water and sewage company, Empresa de Acueducto y Alcantarillado de Bogotá (EAAB). The utility provides services to nearly 700,000 users across the Colombian capital of Bogotá and 11 neighboring municipalities, for a total of more than 7 million inhabitants. EAAB sought a SCADA solution to monitor water pressure in underground pipes with:
- Report-by-exception (RBE) data acquisition
- Real-time sequence of events (SOE)
- Low-energy consumption
The region was divided into five zones; each zone contained 70-80 underground sites. Each station needed to measure water pressure (primary interest), battery voltage, temperature, intrusion, and inundation. When any value is not within normal range, an alarm code should be generated.
Choosing the correct remote terminal unit (RTU) was an essential part in a project of this scale. Compax International did a market study; the project subcontractor awarded the solution to an extremely low-power-consumption (2 mA at full operation) solution that supports data logging and a protocol allowing the usage of time stamps.
Each station cabinet houses three main devices, the RTU, a cellular modem as the primary means of data transmission, and a WiFi server as a backup means to retrieve data. Each site is housed underground so physically accessing the RTU to retrieve data was impractical.
Each site was designed to operate at the most efficient manner possible for energy usage, physical costs, and data costs. To achieve these goals, the RTU takes measurements every 10 minutes and stores the data in its logging memory. Data transmits once every 6 hours. When a transmission is initiated, the RTU turns on the modem, data is transmitted, and the modem is turned off. In the case of an out-of-range value, the rule of transmitting every 6 hours is bypassed, and data transmits immediately to the control station. Due to this report-by-exception based protocol, a sequence of events analysis can be generated.
To reduce the amount of data and related costs associated with transmitting the data via the cellular network, Rugid Computer and Compax International collaborated to devise a means by which multiple floating-point values were scaled and packed into one 16-bit integer. Additionally, Compax International used Rugid Computer’s RUG9 communication protocol, instead of MODBUS, due to the better data efficiency. To interface with the HMI software, Wizcon (now part of emation AG), at the zone control station, required Compax International to develop a custom driver. The driver enables the connection of unlimited devices over TCP/IP without communication error or system overload. Additionally a designated SQL server was assigned. The SQL server receives the data from each zone server, and authorized users can access the system from anywhere through a standard web browser.
-Cheryl Melchior is vice president of visual design and engineering with Rugid Computer Inc., Olympia, Wash., and Meidad Vaknin is with Compax International (93) Colombia LTD, Bogota, Colombia. Edited by Mark T. Hoske, CFE Media; posted by Chris Vavra, Control Engineering, www.controleng.com