英語翻譯
When defining the rules,refer back to the list of field devices developed.Start with the output instructions and work backward to define the condition that produces the desired action.It is important to remember that emergency-stop switches should not be programmed in the programmable controller.Only a hardwired master control relay provides a reliable means for emergency shutdown.These emergency devices should therefore be documented as such.When writing,the text needs to use language that helps convert the operating characteristics to ladder logic.For instance,the AND logic function connects condition instructions in series on a ladder diagram rung,while the OR logic function connects inputs in parallel.
Addressing all elements of a ladder diagram involves identifying and labeling inputs and outputs with a letter/numerical designation.Use an “I” to indicate inputs and an “O” for outputs.(Designating O for zero will prevent confusion between O for output and the number zero).For instance,the first input device is addressed I/O,the second is I/1 and so forth.Outputs are addressed in the same fashion; O/O,O/1,O/2.The program is then downloaded into the programmable controller and ready to be tested.
Installing the Programmable Controller
Before installing the programmable controller,it is necessary to analyze the control cabinet currently housing the relays.Are all the field devices wired to terminal blocks?Or,are all the field devices brought into individual relays scattered throughout the panel?In essence,the engineer will have to mock-up a layout of the cabinet’s contents because the programmable controller typically takes up less space.This is important because if you pull relays out and install a small programmable controller in the middle of the panel,you will find that some of the wires may not reach.If this is the case,you may need to wire in terminal blocks to land the wires coming from the field and then wire from the terminal blocks to the programmable controller.This process is obviously easier when the relays are wired to terminal blocks because you do not have to disturb the field side of the terminal strip wiring -- all you have to do is modify the control side that connects to the programmable controller.At this point the field wires should be labeled with the device names,I/O address or some other identifying code to help document the installation and simplify troubleshooting.Color coding similar characteristics such as AC vs.DC devices further simplifies this process.