Sensors and Actuators
A sensor senses the various process variables and converts them into the electrical or optical signals. These sensors include temperature, pressure, velocity, flow, etc.Actuators convert the electrical signals to the mechanical means to gain control over processes. These include the relays, magnets, servomotors, etc.Some of the sensors and actuators have the capability to communicate with the industrial field communication buses which comes under the smart devices.
Programmable Logic controllers (PLC’s) also called as industrial computers are capable of being programmed to perform certain control functions. It consists of a CPU or processor, I/O modules (both analog and digital) to connect the various input/output devices and relay modules. These may be modular which is of fixed typeor integrated types to extend modules based on the inputs available.
Along with the PLC’s, conventional PC’s are used to control the process by online or by changing the programs. PLC’s comes with dedicated software to program the control strategy.
HMI(Human Machine Interface)
HMI’s offers the facilities like, displaying the information on computer screens and other displays, logging the results in the database, giving alarm signal, etc. It uses technologies like SCADA (Supervisory Control and Data Acquisition) and other visual based technologies.
In industries many sensors, actuators, controlling PC’s and other control devices are geographically distributed and interacting with each other via several data buses. There are three types of buses used in the industrial automation i.e., factory bus, process bus, and the field bus.
Field bus interacts between field instruments and the control devices while the process bus connects the supervising level computers to the control devices like PLC’s. Factory bus connects the higher level of the organization to the supervising level. Different protocols are used for the communications like RS-485, profibus, CAN control modbus,etc.