Performance, analysis and testing of a dual-acting controlled leak (DACL) irrigation canal offtake control : A thesis submitted in partial fulfillment of the requirements for the degree of Master of Applied Science in Lincoln University, New Zealand

Abstract

The Dual-Acting Controlled Leak (DACL) system is a mechanical-hydraulic control device developed for canal structures to control water level and flow rates. The system is powered by the head drop available and is therefore particularly well suited to applications in remote areas without electrical power. The dynamic performance of the DACL level and flow control system was analyzed using classical linear control theory and tested in the laboratory.

The mathematical analysis predicts that in normal closed-loop operation, the DACL system behaves as a second order system. The system response to disturbances is therefore a function of its damping ratio. An explicit expression for damping ratio was obtained and design recommendations for optimal response were made for similar DACL systems. Simplified expressions for steady-state errors are presented together with tuning recommendations.

The analytical model was verified by laboratory model tests in which the open-loop and closed-loop downstream reach water level responses to nominal step input changes in upstream water level and valves/regulator setting were measured. Test results showed that the final values of the actual response came to within about 1 mm of the final values of the predicted response. This corroborates previous findings that the DACL system can control water levels to within a very narrow band, in this case, ± 1 mm.

The study shows that a combination of mathematical prediction models together with laboratory and field tests should result in successful production designs of similar DACL systems that will provide the desired specified system output response in any given application.