【文章內(nèi)容簡介】 s the PLC modules and the rectifierinverter are installed in a control panel. The program is downloaded into the PLC from a personal puter PC and an RS232 serial interface. V. SOFTWARE DESCRIPTION PLC’s programming is based on the logic demands of input devices and the programs implemented are predominantly logical rather than numerical putational algorithms. Most of the programmed operations work on a straightforward twostate ―on or off‖ basis and these alternate possibilities correspond to ―true or false‖ (logical form) and ―1 or 0‖ (binary form), respectively. Thus, PLCs offer a flexible programmable alternative to electrical circuit relaybased control systems built using analog devices. The programming method used is the ladder diagram method. The PLC system provides a design environment in the form of software tools running on a host puter terminal which allows ladder diagrams to be developed, verified, tested, and diagnosed. First, the highlevel program is written in ladder diagrams, [33], [34]. Then, the ladder diagram is converted into binary instruction codes so that they can be stored in randomaccess memory (RAM) or erasable programmable readonly memory (EPROM). Each successive instruction is decoded and executed by the CPU. The function of the CPU is to control the operation of memory and I/O devices and to process data according to the program. Each input and output connection point on a PLC has an address used to identify the I/O bit. The method for the direct representation of data associated with the inputs, outputs, and memory is based on the fact that the PLC memory is anized into three regions: input image memory (I), output image memory (Q), and internal memory (M). Any memory location is referenced directly using %I, %Q, and %M (Table III). The PLC program uses a cyclic scan in the main program loop such that periodic checks are made to the input variables (Fig. 3). The program loop starts by scanning the inputs to the system and storing their states in fixed memory locations (input image memory I). The ladder program is then executed rungbyrung. Scanning the program and solving the logic of the various ladder rungs determine the output states. The updated output states are stored in fixed memory locations (output image memory Q). The output values held in memory are then used to set and reset the physical outputs of the PLC simultaneously at the end of the program scan. For the given PLC, the time taken to plete one cycle or the scan time is 0, 18 ms/K (for 1000 steps) and with a maximum program capacity of 1000 steps. The development system prises a host puter (PC) connected via an RS232 port to the target PLC. The host puter provides the software environment to perform file editing, storage, printing, and program operation monitoring. The process of developing the program to run on the PLC consists of: using an editor to draw the source ladder program, converting the source program to binary object code which will run on the PLC’s microprocessor and downloading the object code from the PC to the PLC system via the serial munication port. The PLC system is online when it is in active control of the machine and monitors any data to check for correct operation. A. PLC Speed Control Software In Fig. 4, the flowchart of the speed control software is illustrated. The software regulates the speed and monitors the constant speed control regardless of torque variation. The inverter being the power supply for the motor executes this while, at the same time, it is controlled by PLC’s software. The inverter alone cannot keep the speed constant without the control loop with feedback and PLC. From the control panel, the operator selects the speed setpoint and the forward/backward direction of rotation. Then, by pushing the manual start pushbutton, then the rotation stops. The corresponding input signals are interfaced to the DIM and the output signals to the DOM as shown in Table IV. The AIM receives the trip signal IS from the stator current sensor, the speed feedback signal from the tachogenerator, and the signal from the control panel. In this way, the PLC reads the requested speed and the actual speed of the motor. The difference between the requested speed by the operator and the actual speed of the motor gives the error signal. If the error signal is not zero, but positive or negative, then the PLC according to the putations carried out by the CPU decreases or increases the V/F of the inverter and, as a result, the speed of the motor is corrected. The implemented control is of proportional and integral (PI) type (., the error signal is multiplied by gain KP, integrated, and added to the requested speed). As a result, the control signal is sent to the DOM and connected to the digital input of