| 摘要 |
PURPOSE:To hold a controlled variable within a range between previously fixed upper and lower limits even when a load is dropped over a variable speed control range in a PID control system by applying ON/OFF control to the upper and lower limit values of the PID control system. CONSTITUTION:When the inflow of waste water 5 is sharply reduced under control only by a PID control system 8, necessary air is reduced less than the minimum quantity which can be controlled by the PID control system, so that the dissolved oxygen concn. of the waste water 5 sharply exceeds a DO set value. When control is applied by an ON/OFF control part 11, an OFF control signal is generated at a point of time when the dissolved oxygen concn. exceeds the DO upper limit value, so that the operation of an inverter 10 is forcedly stopped. Consequently, the dissolved oxygen concn. in the waste water 5 is gradually dropped and reached to the DO lower limit value. When the dissolved oxygen concn. exceeds the DO lower limit value, an ON control signal is generated from the ON/OFF control part 11 and the inverter 10 is driven, so that the rotation of a blower 2 is controlled again by the PID control system. |
