Complicated control tasks accomplished with a highly automated control system may be in the form of Programmable Controller (PLC). Signal interfacing to field devices (such as an operator panel, motors, sensors, switches, solenoid valves) capabilities in network communication enables large scale implementation and process co-ordination besides providing greater flexibility in distributed control systems.
Role of Programmable Controllers (PLC)
In an automated system, the PLC is commonly regarded as the heart of the control system. Within a control application program (stored within the PLC memory), the PLC constantly monitors the state of the system through the field input devices feedback signal. Control is based on the program logic to determine the course of action to be carried out at the field output device.
A PLC can be used to control a simple or repetitive task, or a few them can be interconnected together with other host controllers through a sort of communication network in order to integrate the control of a complex process.
Intelligence of an automated system is dependent on the ability of the PLC to read in the signals from types of automatic sensing and manual input field devices.
Push buttons, keypad and toggle switches which form the basic man-machine interfaces are types of manual input device. For detection of work pieces, monitoring of moving mechanisms, checking on pressure and or liquid levels and many others, the PLC will tap the signal from specific automatic sensing device such as a proximity switch, limit switch, photoelectric sensor and level sensor and so on.
Types of input signal to the PLC would be of ON/OFF logic or analogue. Input signals are interfaced to the PLC through various types of PLC input modules.
An automatic system is incomplete and a PLC system is virtually paralysed without means of interface to the field output devices. Most commonly controlled devices are motors, solenoids, relays, indicators and buzzers. Through the activation of motors and solenoids, the PLC can control from a simple pick and place system right up to much complex servo positioning systems. Like input signal interfacing, signals from output devices are interfaced to the PLC through a wide range of PLC output modules.
What is a Programmable Controller?
A PLC consists of a Central Processing Unit (CPU) containing an application program and Input & Output Interface modules which is connected to the field I/O devices. The program controls the PLC so that when an input signal from an input device turns ON, the appropriate response is made.
PLC Scan Time
The process of reading inputs, executing the program and updating the outputs is known as a scan. The scan time is normally a continuous and sequential process of reading the status of inputs, evaluating the control logic and updating outputs. Scan time specification indicates how fast the controller can react to the field inputs and correctly solve the control logic.
- Disadvantage of Conventional Control Panel
- There is too much wiring work in the panel
- Modification can be quite difficult
- Troubleshooting can be quite troublesome as the engineer would have to have skilled knowledge of the control panel
- Power consumption can be quite high as the coil consumes power
- Machine downtime is usually long when problems occur, as it takes a longer time to troubleshoot the control panel
- Drawings are not updated over the years due to changes, causing longer downtime in maintenance and modification
Programmable Controller (PLC) Control Panel & Their Advantages
With the arrival of PLC’s the control design and concept improved significantly.
Advantage of PLC Control
- The wiring of the system is usually reduced by 80% compared to conventional relay control systems
- The power consumption is greatly reduce as PLC’s consume much less power
- The PLC self diagnostic functions enable easy and fast troubleshooting of the system
- Modification of control sequences or applications can easily be done by programming through the console or computer software without changing of I/O wiring, if no additional Input or Output devices are required.
- In a PLC system, spare parts for relays and hardware timers are greatly reduced as compared to conventional control panels.
- The machine cycle time is improved tremendously due to the speed of PLC operation is a matter of milliseconds.
- It costs much less compared to conventional systems in situation when the number of I/Os is very large and control functions are complex
- The reliability of the PLC is higher than the mechanical relays and timers
- An immediate printout of the PLC program can be done in minutes (Therefore hardcopy of documentation can be easily maintained).
A Systematic Approach of Control System Design Using a Programmable Logic Controller (PLC)
The concept of controlling a control system is a very simple and easy task. It involves a systematic approach by following the operation procedure.
Determine The Machine Sequence of Operation
Firstly the user should decide which equipment or system they want to control. The purpose of the PLC is to control an external system. The system is to be controlled can be a machine equipment or process.
The movement of the controlled system is constantly monitored by the input devices which gives a specified condition and sends a signal to the programmable controller.
The programmable controller outputs a signal to the external output devices which controls the movement of the controlled system as specified and achieves an extended control action. In simplicity, a user needs to determine the sequence of operation by drawing out a flowchart.
Assignment of Inputs & Outputs
All external input and output devices to be connected to the programmable controllers must be determined. The input devices are the various switches, sensors. The output devices are the solenoids, electromagnetic valves, motors and inductors.
The actual wiring will follow the number of the programmable controller. The assignment of input and output numbers must carried out before writing the ladder diagram because the number dictate what is the precise meaning of the contact in the ladder diagram.
Writing of the program
Next: write the ladder diagram program by following the control system sequence of operation as determined by step one.
Programming into Memory
Apply power to the programmable controller. Depending on the type of programmable controller (PLC), a user may have to do an I/O generation to prepare the system configuration. After that a user can enter the program in the memory either by a programming console or by computer aided ladder software tool. After completion of the programming, users should check for any coding errors by means of diagnostic functions and if possible simulate the whole operation to see that it is alright.
Running the system
Before the start push button is pressed, thoroughly ensure that the Input and Output wiring are correctly connected according to the I/O Assignment. Once confirmed, the actual operation of the PLC can now be started. Users may need to debug along the way and fine tune the control system if necessary. Test run thoroughly until it is safe to operate by anyone.
For more information about PLC Programming, installation & advice, contact More Control on 0345 00 00 400.