Figure 3 shows test bench to implement our closed-loop control strategy for metering the
chemical injection rate of the RRV. It is composed of a direct injection unit, flowmeter, two pressure
regulators, and a controller. The EC sensor mounted between mixing chamber and nozzle in injection
unit was used to indicate the concentration of the spraying solution, it could also be used to verify
the control effect.
The flowmeter is based on the thermodynamic principle. When a liquid medium flows through,
the sensor generates a heat pulse internally. The heat is then conducted away by the medium flux
resulting in a cooling down of the sensor. The temperature within the sensor is measured and
compared with the temperature of the medium, and the flow rate can be derived from the
temperature difference. Theoretically, the flowmeter is independent of the viscosity of the
agricultural chemicals.
The chemical applied in the experiments was not real, and it was simulated by a solution
consisting of (in proportion by weight) 10% LUVITEC® powder, 3% salt and 87% water. The
LUVITEC® powder was used as additive to adjust the solution viscosity in the range of 230–240
mPa·s. Salt made the solution electrical conductive, so the EC sensor could detect the concentration of the solution. The
more electroconductive the solution at the nozzle was, the more chemical the RRV injected. Moreover,
the stability of EC values indicated the stability of the chemical flow rate.