Like: moving the machine in a certain direction to see if it hits a certain position sensor (digital input). Now there are situations where you don't see any input at all, then you must activate the machine in such a way that you start searching for a state. Then you need to think for the next state transition and see what the inputs are so it can go to the next state. It doesn't know in which state it is at that moment. The machine powers up and starts, goes to the start state, and then what? When you -think- you're finished, scrutinize this state machine and think of possible problems that can occur and are yet not in this state machine/SFC.Īnd here's a -very- nice one to think about: Such states I often call asynchronous states, since normally, the states run in a synchronous way from 1 state to the other. alarm condition state (temperature too high) emergency stop pushed, then it moves immediately to the emergency stop state (often with all outputs disabled=off) The machine can only move between states, but there can be asynchronous emergency states: So you only have states (that drive the outputs) and transitions (triggered by the inputs). In every state it drives the analog or digital outputs in a certain way. It moves between the states with changing conditions of the analog or digital input states. This flowchart of how the machine works (often called an SFC or a state machine) has many states in which the machine can position itself. When that is ok and you (and the customer, in this case also you, haha) agree upon the functional specification, start translating that into a technical specification, for example a flowchart. mechanical), otherwise they didn't hire you and did the PLC job themselves. This is a specification that can be understood by the customer with another background (i.e. So first make a functional specification of what the PLC program is going to do, with what sensors and actuators.
So don't start programming before you know almost 100% certain what you need to make. What is most important, is that you know beforehand -what- you need to program.
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