The authors have proposed Vapor Pressure Deficit (VPD) control methods for a solar-light-type plant factory (greenhouse) with a fog-cooling system. In particular, a proportional feedback controller with a variable gain is adopted in an actual system. In the system, however, there is some dead time—was experimentally confirmed by vision-based observation—which brings vibration to the value of VPD. This paper proposes a VPD control considering the dead time of the fog generation by using a communication disturbance observer with a high pass filter (HPF-CDOB). The stability of the whole control system is verified by the small gain theorem. The effectiveness of the proposed control method was demonstrated via experiments. In the experimental result during the high-temperature phase, the proposed method achieves a 81% reduction of the average error and a 20% reduction of the standard deviation in comparison with the conventional method.
