Basic PLC - Introduction to Control System
CONTROL SYSTEM
A control system consists of three sections: input, processing and output.
1.1.1 Input section
Input signals usually come from various sensors that convert physical quantities into electrical signals. These sensors may be simple push-button switches, limit switches or proximity sensors, etc. Depending on the sensor uses this information may be an on/off(binary) or a continuous (analog) representation of the input quantity
1.1.2 Processing section
From the input signals the automatic control system has to produces the necessary output signal in accordance to the control plant built into the processor. The control plant can be implemented into two different ways, using either hard – wired control system or programmable control system.
Hard-wired control systems have the control function fixed permanently when the system components are connected together (eg. Electrically).Whereas in a programmable control system the control function is programmed and stored within a memory unit. This program can be changed or modified when necessary.
| Hard – wired systems | Programmable systems |
| Relays | Computer |
| Electronic logic | Microprocessor |
| Pneumatic logic | PLC system |
| Hydraulic logic |
|
1.1.3 Output section
Motors, cylinders, relays etc are output devices which convert signals from the control system into other necessary quantities. A pneumatic cylinder, for example, converts pneumatic signals into linear motion.
1.2.2 Advantageous of using PLC
- Shorter project implementation time
- Easier modification without cost penalty
- Project cost can be accurately calculated
- Shorter training time required
- Design easily changed using software
- A wide range control application
- Easy maintenance
- High reliability
- Standardization of Controller Hardware
- Able to withstand Harsh Plant Environment
1.2.3 Type of PLC
1. ABB
2. Mitsubishi
3 Siemens
4 FESTO
5 Omron
6 Keyence
1.2.4 PLC operation
1 CHECK INPUT STATUS-First the PLC takes a look at each input to determine if it is on or off. In other words, is the sensor connected to the first input on? How about the second input? How about the third... It records this data into its memory to be us
2 EXECUTE PROGRAM-Next the PLC executes your program one instruction at a time. Maybe your program said that if the first input was on then it should turn on the first output. Since it already knows which inputs are on/off from the previous step it will be able to decide whether the first output should be turned on based on the state of the first input. It will store the execution results for use later during the next step.
3 UPDATE OUTPUT STATUS-Finally the PLC updates the status of the outputs. It updates the outputs based on which inputs were on during the first step and the results of executing your program during the second step. Based on the example in step 2 it would now turn on the first output because the first input was on and your program said to turn on the first output when this condition is true.
1.2.5 Application of PLC:
- CNC machines
- Industrial Robot
- Automatic transport
- Process Control in petrochemical
