The development of a bond graph model for a self-propelled irrigation system is presented. The bond graph integrates the mechanical, electrical and hydraulic subsystems of linear moving and center pivot irrigation systems into a comprehensive simulation model. The model includes the effects of the engine used to drive the well pump and electric alternator. Simulations using ENPORT/PC are used to analyze mobile irrigation systems with variable water flow requirements. Scenarios that result in variable water flow requirements included end guns, cornering systems and the use of Global Positioning Systems (GPS) to spatially vary the amount of water applied. Under each scenario, extreme system pressure variations resulted from the variable water demand if the well pump was not properly matched to the application. In addition, systems using existing engines and pumps previously designed for high pressure applications were found to be the most susceptible to the high pressure fluctuations.
To address the pressure fluctuation problems associated with variable water demand, the bond graph model was used to evaluate the potential of a control system to maintain constant system pressure as water flow requirements varied from an irrigation system. Results indicated that small changes in engine speed as flow requirements changed could prevent the large pressure variations.
Using the simulation results, a prototype engine speed control system was developed and field tested over the summer of 1995. The control system varied engine throttle position to maintain a given (input) system pressure as water demand varied. The throttle controller was installed on a Caterpillar 3208 naturally aspirated engine driving a Gould ten stage deep well turbine pump originally installed for a high pressure, high flow application (a traveling gun irrigation system). The engine/pump combination was currently being used to supply a low pressure center pivot irrigation system with a high output end gun which was operated approximately 70% of the irrigation cycle. Prior to installing the engine controller, system pressure increased from 300 kPa to 1000 kPa when the 700 1/min end gun cycled off. With the engine controller, system pressure varied within the range of 300 to 360 kPa with the end gun both on and off. The field results also indicate that a savings of approximately 5% in fuel consumption was obtained with the engine control system.