Biological Systems Engineering

 

Date of this Version

2015

Citation

Transactions of the ASABE, Vol. 58(4): 997-1008

Comments

Copyright 2015 American Society of Agricultural and Biological Engineers

Abstract

The goal of this research project was to further the development of an electromechanically controlled variable-orifice nozzle by creating an electronic control system and then evaluating that system based on step and ramp inputs. The control system was developed in a programming environment that combined an electronic data acquisition system and actuator with pressure and flow sensors. A proportional, variable-gain (based on system pressure) control system was developed to adjust nozzle flow rates to meet target application rates. The constraints were to achieve settling time of less than 1.0 s, overshoot of less than 10% of maximum flow (or minimum flow), and average absolute steady-state error of less than 2%. After several trials, the resulting control system achieved these objectives for full steps from maximum and

minimum flow rates (and vice versa) at carrier pressures from 140 to 410 kPa. Ramp response analyses revealed the maximum flow rate change (mL s-2) of the nozzle control system. Operation was considered successful if the average absolute error was less than 5% and the average absolute error +2σ did not exceed 10% of the desired flow, thereby ensuring that the nozzle operated within specifications 95% of the time. An additional goal was to maintain nozzle response lag times of less than 1.0 s based on input rate changes in the form of ramp signal input frequencies. Lag times were found to be less than 0.5 s (±0.05 s) over the carrier pressure range at input frequencies of up to 0.2 Hz. Further, these results indicated that for each carrier pressure, a maximum rate change frequency of 0.07 Hz ensured that system errors were within the design requirements. Lag times at this frequency were less than 0.38 s for all carrier pressures tested. The range of rate change achieved by the nozzle control system ranged from 2.97 to 6.39 mL s-2 for carrier pressures of 140 to 414 kPa, respectively. Thus, as operating pressure increased, the nozzle was capable of compensating for greater changes in the desired flow rate. While the turndown ratios (~2.4:1) over the range of carrier pressures were essentially stable, flow rates increased with carrier pressure.