In a PLC-based control system, a programmable computer receives input signals from the equipment being controlled, processes this information using the structure and rules entered into the program and then controls outputs that operate equipment.
Input signals can be either digital or analog. A digital input signal must be either on or off. These signals are received from controls such as level switches, pressure switches, and temperature switches. An analog input signal is an actual numeric value and is interpreted by the PLC. Examples of analog input signals are pressure transducers, pH transmitters and flow meters.
Output signals can also be either digital or analog. Digital output signals turn equipment such as pumps, solenoid valves and alarm lights off or on. Analog outputs can vary over a range of values (0 to 100%) and are used to control rates or speeds.
PLC-based control systems are flexible in the range of operations they can control and are easily modified, enhanced or altered, even after the system is installed. Changing the logical sequence of events in the operation of a system to adjust to changing operating conditions or adding new equipment to be controlled is usually as simple as a small amount of programming. These systems also offer a range of user communication options.
When designing a PLC-based control system, it is important to consider the number and types of input and output signals, the type of power available, and the required level of remote communication.
MLE Equipment designs and builds PLC-based control systems. PLC modules used include DirectLogic, EOS, Schneider, Allen-Bradley, and Siemens. These systems are built in a UL-approved facility, and can be built with UL, CSA, MET, or ESA approval. Please see the provided specification sheets and manuals for more detailed information on PLC-based control systems.
