Relay-Based ON/OFF ControllerIntroductionON/OFF control is a type of control method commonly applied to process control applications such as liquid level control. It is the simplest and often least expensive type of automatic process control. ON/OFF control, or two-position control, examines the value of a process variable and gives an output that is either fully on (I00%) or fully off (0%). There are no intermediate settings with ON/OFF control.
How it works in a level control system?A solenoid-operated valve controls the output flow from the tank. Two level switches are positioned at a high and low level limits. The control circuit maintains the level between the level limits.
When the level falls below the low level limit, the low level switch send a signal to the control circuit, causing the control circuit to close the solenoid valve. The tank level then rises until it reaches the high level limit. At that point, the high level switch sends a signal to the control circuit, causing it to open the solenoid valve and allow the tank level to fall. The tank level then drops until it reaches the low level limit, and the cycle repeats. When the level is between the upper and lower level sensors, there is no change in the control circuit's output. Since there is no change in the output, there is no change in the position of the final control element (valve). For this reason, the area between the high level and low level is called the dead band, or neutral zone. If the dead band is too narrow, the control element turns off frequently, causing reduced life. Therefore, an ON/OFF control system should be set up so that the dead band is wide enough to prevent frequent oscillation and still control the process variable within acceptable limits.
As you can see, ON/OFF control systems cannot achieve a specific setpoint but instead provide an output range between a high and low level limit.
How is connected the circuit for the Relay-Based ON/OFF Controller?This is the circuit to determine the level of water in the process tank using Relay-Based ON/OFF control.
How the operation of relay-based ON/OFF control system is?To explain how it operates, the images below will show a ladder diagram (left-side figure) and a pictorial diagram (right-side figure) for a visual concept of what happens with relays and the control system.
Electromechanical relay control systems were one of the first types of control systems used in the process control industry to perform ON/OFF control. This is a typical relay-based control system that is used to control liquid level in the tank. This circuit uses two level switches to maintain the level within a given range. To understand how the circuit works, first, when control power is applied (SS1 ON), the pump motor is energized. This causes the tank to begin to fill.
When the water level reaches the low level switch (LS1), the switch opens. However, the pump continues to fill the tank because the current to coil CR2 is sealed in by the CR2 N.O. (Normally Open) contacts on rung 3.
When the water reaches the high level, the N.C. (Normally Closed) contacts of switch LS2 open causing the coil CR2 to de-energize and the pump to turn off. The pump stays off until the water drains below the low level switch.
When the water in the process tank drains below LS1, the switch closes, the pump is again energized, and the tank fills. The process continues until the control power is removed (SS1 OFF). It is relevant to mention that the solenoid valve (SV2) that drains the tank is always energized as long as there is control power.
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