This article was printed in CONTROL's October 2009 edition.
By Tom Pagakis
The scenic beach town of Carlsbad, Calif., just north of San Diego is known for its beautiful beaches, upscale homes and quaint shopping district. The SCADA system employed by the city monitors current status and provides remote control abilities for all of Carlsbad's water and reclaimed water. The key components for these systems are SNAP controllers from Opto 22 (www.opto22.com). The city began using the hardware as an experiment. Years ago, outdated technologies like tone telemetry were in place, and then later the city used a mix of I/O modules and systems from Grayhill (http://grayhilloem.com) and other vendors. Special drivers had to be created to make everything communicate, and the system became very complicated and unwieldy.
The SNAP systems—which communicate with each other and back to their command and control center through a combination of radio, serial and Ethernet links—are used to monitor, control and acquire data, as the City of Carlsbad's Municipal Water Department regulates all water transport, and an independent water treatment plant and controls the chemical dosing of ammonia and chlorine that cleans the water. In total, these activities incorporate more than 1,000 I/O points distributed across the control center and numerous remote sites. The department's responsibilities center on two distinct areas of operations: sourcing, transport and distribution of purchased water from lakes and reservoirs to the city, and reclamation of treated water.
Water Transport and Distribution
Water purchased from San Diego County's water authority is transported through piping to Carlsbad, where it's distributed to businesses and residences. Delivery of the potable water requires little to no pumping, as the water travels via a gravity feed that brings it from storage tanks as large as 9 million gallons located at higher elevations (around 700 feet) all the way down to sea level.
The SCADA system has been carefully configured to control equipment and monitor the water of both systems throughout this entire process. Specifically, analog and digital input and output modules connect to:
- Level transducers that track water levels both in large reservoirs and tanks,
- Pressure transducers in pipes that measure pressure within each zone, and ensure that water is pumped and is flowing efficiently as it's being delivered,
- Flowmeters that measure how many gallons of water per minute are being pumped,
- Pressure-sustaining, pressure-reducing and various other valves that open and close incrementally to adjust the rate at which the water flows.
Currently, the SCADA system encompasses three large reservoir impounds, 14 reservoirs and 20 pressure reducing stations used to monitor more than 500 miles of distribution pipeline.
Significantly, all of the monitoring and data acquisition functions are performed, not by a single (or even multiple) central controllers, but by multiple local I/O processors (or "brains") distributed at pump stations and several other points across the city's infrastructure. These brains communicate to I/O located on the same rack and perform functions normally reserved for an expensive PLC. For example, the brain takes valve readings from the analog input modules, and performs scaling calculations to convert a voltage measurement into a percentage. (Thus, for a typical 4-20mA analog input module, a measurement of 12 volts would result in a reading of 50%.) To have this scaling and engineering unit conversion take place locally at the I/O level relieves the main controller from extra processing, while also giving the control center personnel the convenience of being able to view and interpret easy-to-understand units of measure (for example, percentages as opposed to voltages.)
Distributing intelligence in this manner also provides a safeguard in the event there's an interruption in the operation of one of the main controllers. The brain can be instructed to continue to record measurements and perform calculations, and operators can continue to view this data from the control center's HMI.
The SCADA system's HMI is Wonderware's (www.wonderware.com) InTouch, which is integrated neatly with the Opto 22 hardware, and allows control center personnel to view operational variables and conditions relating to pressure, flow, levels, equipment status and more. The department also uses InTouch to generate bar and line graphs (indicating such things as tank levels, pump status and valve positions) and also for performance and historical trending. For example, the system generates a 24-hour trend for each storage tank along with reports for the city on how much water is pumped each day, week and month.