Abstract : This paper introduces a monitoring scheme for variable frequency speed control in a water treatment system using a host computer, Siemens S7-200 series PLC, and MMV frequency converter. The main components and characteristics of the system are analyzed. Keywords : Frequency converter; PLC; PID control; USS protocol; PPI protocol; Configuration software 0 Introduction A certain water treatment experimental system requires constant pressure and flow rate, and needs to dynamically store a large amount of data for analysis. Manually adjusting the pressure and flow rate is not only cumbersome but also prone to large errors and energy waste. This system uses a host computer, PLC, frequency converter, and configuration software to form a constant pressure water supply system control scheme, realizing automatic monitoring of the system and meeting the needs of the experimental system. 1 Hardware Configuration The system uses one Siemens S7-216 PLC, two Siemens EM231 modules, one Siemens MMV420 frequency converter, and a standard host computer (due to the favorable test environment). It includes 11 solenoid valves, two flow transmitters, and three pressure transmitters. The monitoring software is KingView 6.5. The system composition is shown in Figure 1, and the electrical schematic is shown in Figure 2. [b]2 Program Design 2.1 Human-Machine Interface[/b] The configuration software is KingView 6.5 from Beijing Yacon. The system has seven interfaces: "Main Interface," "Alarm," "Report," "Pressure Curve," "Flow Curve," and "Parameter Setting." The "Main Interface" simulates the on-site status; the "Alarm" interface displays system alarms; the "Report" interface saves and prints reports; the "Pressure Curve" displays real-time and historical pressure changes; the "Flow Curve" displays real-time and historical flow changes; and the "Parameter Setting" interface sets and selects system parameters. [b][align=center]For details, please click: Application of Frequency Converter PLC in Water Treatment Experimental Monitoring System[/align][/b]