SCADA Applications & Installations:
Friant Water Users Authority Selects TeleSAFE and Lookout
When the engineers at Friant Water Users Authority decided to upgrade the current system controlling the 14 checks and 9 open ditch turnouts of the Friant Kern Canal, they quickly discovered that this was not going to be an easy task.
The chosen RTU must control the level of a pool either upstream or downstream of the check structure by opening or closing the gates. The problem lay in the fact that the level of the pool must be controlled to within two one-hundredths of a foot, and the large volume of water being controlled contains an incredible amount of inertia. Once the water level is either falling or rising too rapidly, changing its direction without over-correcting is almost impossible. If over-correction occurs, effective control of the water level is lost for the next few hours. For these reasons, it was quickly discovered a typical PID control scheme was not the answer.
For about thirty years the canal has been controlled automatically via an electromechanical device of cams, switches, and egg-timers, affectionately called the 'Littleman'. The 'Littleman' was essentially a two-stage bang-bang controller with an anti-hunting mechanism called a 'Mark Twain'. Since this device had successfully controlled the canal in the past, it was decided it would be best if the system chosen could do in software what the old system did in hardware.
After evaluating and reviewing a number of competing products, the engineers chose the TeleSAFE 6000 RTU from Control Microsystems. It was decided that, without the TeleSAFE 6000's accessibility and programmability, this project would have been impossible to accomplish in an economical or timely manner. Within two months though, a successful automatic control strategy replacing the 'Littleman' was written, tested, certified, and then put into operation using the TeleSAFE 6000 RTU. This automatic control strategy functions as a four stage bang-bang controller with a braking feature. The novelty of this braking feature is that it keeps track of the rate of change of the water level and will reverse the controlling action if the level is changing too quickly toward the target level. This eliminates the over controlling and oscillations previously experienced with the 'Littleman'. The four stages of control provide a responsiveness to large flow changes which was previously not possible.
The TeleSAFE 6000 RTU was chosen over other control devices because it provides an accessibility to the user not normally found in other RTUs, and because of the flexibility in programming not normally found in PLCs. In addition, not only does the TeleSAFE 6000 RTU provide the Authority with improved control of the Canal over a wide range of flows and level changes, but it also provides telemetry and remote control capability at the Authority's central office in Lindsay, CA. There are currently four sites which have the TeleSAFE 6000 installed, and the remaining sites will receive their RTUs over the next twelve months.
Once the RTU requirements had been determined, all that remained was for the Authority to decide what SCADA software to use on the central computer. A large number of programs were evaluated, and it once again became obvious that this would not be an easy task. After getting past the glitzy graphics, these programs were not considered "user friendly." Also, none of them dealt effectively with the problem of real-time SCADA communications. All the SCADA programs that were evaluated required a hardware solution - a communications processor of some kind - that would cost as much as the computer itself.
These problems remained an issue until the Authority learned of a host package called Lookout which was specifically developed for the SCADA market by a municipal consultant. Lookout uses Object-Oriented programming to handle a variety of communications methods such as radios, leased lines and dial-up either a polled or reported-by-exception environment. Only a leased line modem was required to connect to the phone system rather than the expensive, proprietary communications processor offered by other vendors. Lookout also contains a Modbus object that handles all communications with the TeleSAFE RTU's Modbus protocol emulation, allowing the two products to dovetail seamlessly.
As the system grows and develops over time, another unique capability of Lookout will prove to be beneficial: Lookout allows the process file to be edited while it is running. Because of this feature and the built-in flexibility of Control Microsystems' products, new RTUs are being added monthly with no operational downtime.
Thanks to Christopher Oldziewski, Electronics Technician at Friant Water Users Authority, for submitting this article.
[Editors note: Since this paper was published, all RTUs have been implemented.]
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