PLC software once written must be easy and intuitive to follow. PLCs are an integrated part of the domain systems, advances in technology of the system will effect the requirements of the PLC software. PLC software must therefore be maintainable and extensible.
From the figure above, the block diagram configurations can be obtained from this set up.
1. A closed-loop control system for constant speed operation, configured with speed feedback. The induction motor drives a variable load, is fed by an inverter and the PLC controls the inverter output.
2. An open-loop control system for variable speed and variable frequency operation. The induction motor drives a variable load and is fed by control mode. The PLC is an inverter constant in activated.
3. The standard variable speed operation. The induction motor drives a variable load and is fed by a constant voltage-constant frequency standard three-phase supply.
The open loop configuration can be obtained from the close loop configuration by removing the speed feedback. On the other hand, operation results if the entire control is bypassed.
PLC programming is based on the logic demands of input devices and the programs implemented are predominantly logical rather than numerical computational algorithms. Most of the programmed operations work on a straightforward two-state” on or off” basis and these alternate possibilities correspond to “true or false” (logical form) and “1 or 0” (binary form), respectively. PLCs offer flexible programmable alternative to electrical circuit relay-based control systems built using analog devices.