【正文】 ystem is to control 2 elevators in a 9storey residential building. For each elevator, there is a sensor located at every floor. We can use these sensors to locate the current position of the elevator car. The elevator car door can be opened and closed by a door motor. There are 2 sensors on the door that can inform the control system about the door?s position. There is another sensor on the door can detect objects when the door is closing. The elevator car?s up or down movement is controlled by a traction motor. Every floor, except the first and the top floor, has a pair of direction lamps indicating that the elevator is moving up or down. Every floor, has a seven segment LED to display the current location of the elevator car. The first step for the development of the elevator control is to define the basic requirements. informally, the elevators behavior is defined as follows. (1) Running with a single elevator Generally, an elevator has three operation states: normal mode, fireprotection mode and maintenance mode. The maintenance mode has the highest priority. Only the maintenance mode is canceled can the other operation modes be implemented. The next is fireprotection mode, the elevator must return to the bottom floor or base station immediately when the fire switch acts. The elevator should turn to normal operation mode when the fire switch is reset. Under normal operation mode, the control system?s basic task is to mand each elevator to move up or down, to stop or start and to open and close the door. But is has some constraints as follows: Each elevator has a set of 9 buttons on the car control panel, one for each floor. These buttons illuminate when they are pressed and cause the elevator to visit the corresponding floor. The illumination is canceled when the corresponding floor is visited by the elevator. Each floor, except the first and the top floor, has two buttons on the floor control panel, one to request an up elevator, one to request a downelevator. These buttons illuminate when they are pressed. The illumination is canceled when an elevator visits the floor, then moves in the desired direction. The buttons on the car control panel or the floor control panel are used to control the elevator?s motion. The elevator cannot pass a floor if a passenger wants to get off there. The elevator cannot stop at a floor unless someone wants to get off there. The elevator cannot change direction until it has served all onboard passengers traveling in the current direction, and a hall call cannot be served by a car going in the reverse direction. if an elevator has no requests, it remains at its current floor with its doors closed. (2) Parallel running with two elevators in this situation, there are two elevators to serve the building simultaneously. it runs at 7am to 9am and 5pm to 7pm every day. When an elevator reaches a level, it will test if the stop is required or not. it will stop at this level when the stop is required. At the same time, to balance the number of stops, the operation of two elevators will follow a certain dispatching principle. An elevator doesn?t stop at a floor if another car is already stopping cooperation of its electric power driving system and logic control system. 3. Software design Due to the random nature of call time, call locations and the destination of passengers, the elevator control system is a typical realtime, random logic control system. Here we adopted collective selective control method with siemens PLC S7200 CPU226 and its extension modules. There are 46 input points and 46 output points in the system. The i/O points are showed in Tablei and Table 2. About software designing, we adopt the modularized method to write ladder diagram programs. The information transmission between modules is achieved by intermediate register bit of PLC. The whole program is mainly posed of i0 modules: hallcall registration and display module, carcall registration and display module, the signal bination module, the hallcall cancel module, the elevatorlocation display module, the floor selection module, the moving direction control module, the door open/close module, the maintenance operation module and the dispatching module under parallel running mode. The design of the typical modules is described as follows: . Hallcall registration and display There are two kinds of calls in an elevator: hallcall and carcall. When someone presses a button on the floor control panel, the signal will be registered and the corresponding lamp will illuminate. This is called hallcall registration. When a passenger presses a button in the elevator car, the signal will be registered and with the corresponding lamp illuminated. This is called carcall registration. Figure2 shows the ladder diagram of up hallcalls registration and display. The selflock principle is used to guarantee the calls ?continuous display. . The collective selection of the calls Here the collective selection control rules are used. As showed in Figure3, , and are auxiliary relays in PLC. They denote the stopping request signal of ist to 9th floor respectively. The auxiliary relay denotes the elevator driver?s operation signal. When there is a call in a certain floor, the stopping signal of corresponding floor will output. When the elevator is operated by the driver, the hallcalls will not be served. And the elevator cannot pass a floor at which a passenger wishes to alight. . The cancellation of the calls The program of this module can make the elevator response the hallcalls which have the same direction as the car?s current direction, and when a hallcall is served,its registration will be canceled. The ladder diagram of up hallcalls?cancellation is showed in Figure4. Figure3 The bination of the calls Figure4 The canclellation of up call