By using ladder logic simulation, users can test complex operating conditions that cannot be achieved by other methods, thereby reducing potential costs and hazards for users.
In the most basic programmable logic controller ( PLC ) training courses, the buttons and indicator lights built into the training device are typically used to illustrate the different types of instructions used by the PLC software platform. Advanced courses focus more on the techniques used in programming, such as automatic sequencing, component tracking, and other system functions.
Getting all elements to work together in a program can be a daunting task. Different types of routines are interconnected. Contacts representing machine or sequence states are easily tested. Internal memory bits can represent automatic/manual modes, automatic loops, and even faults.
However, inputs and outputs are another matter. In larger machines or systems, they represent many different types of sensors or output devices. Training equipment often used in training courses lacks sufficient buttons, switches, and indicator lights to replace actual equipment. Furthermore, input devices (such as buttons, switches, and potentiometers) do not provide real-time automatic responses to sequential and output commands.
Here, simulation routines can be useful. Using appropriate output logic, the inputs and outputs are saved to memory bits as "aliases" instead of actual input/output (I/O). In a real machine, the Z-Axis_Lower_SV output variable would typically activate automatically. Since this isn't a real sensor on a solenoid-driven cylinder, we need to simulate a sensor that's being manufactured.
Timer circuit and memory bits
Ensure the timer circuitry is functioning correctly. Note that memory bits are required to simulate inputs. Input memory bits can also be used in automatic sequences to transition from one sequence state to the next. EnableOut is required to simulate faults. If the bit is disabled, it's as if the output is active but no input is ever detected. The fault timer will time out and lock the fault condition.
It's also important to note that the "lock" or "set" bit is used for input. This is especially important for solenoid valves that close when the sequence moves to the next step. When the output is off, the simulated sensor will remain active.
Ideally, all simulation circuitry should be placed in a single routine. If the program can be used for both training and actual machines, the simulation routine can be removed or disabled later. Simulated I/O can also be replaced with real components later. For analog values, a timer is used in the simulation. In this case, the tank level increases by 5 every 20 milliseconds; both the timer value and the tank level increase can be adjusted to achieve the desired effect. More modulation is needed to simulate a real tank, but the diagram shows a general concept. To drain water from the tank, a subtraction instruction is needed. This can also be used to test proportional-integral-derivative (PID) instructions.
During the design phase of a project, actual equipment is often unavailable. Simulations allow programmers to test critical code before deploying it to a computer. With the help of Human-Machine Interfaces (HMIs), programmers can even visualize processes through animated objects.
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