Process Meter ON/OFF ControllerIntroductionMany process meters have relay (discrete) outputs that can be programmed to perform ON/OFF control. The process meter measures the process using an analog sensor of some type and turns an output ON or OFF when the sensor's signal reaches a certain programmed level.
How it works in a level control system?A Honeywell UDI 1700 process meter can be used for ON/OFF control by connecting one of its output relays to a device such as a solenoid valve and setting the parameters. For example, in the image below, the process meter controls the level between 6.0 and 4.0 inches. The Process High Alarm 1 (PhA 1) is set to 6.0. The alarm 1 hysteresis (AHY 1) is set to 2.0. The hysteresis value is applied on the safe side of the alarm. For a high alarm, the safe side is below the high alarm value. Therefore, the control range is between 6.0 and 4.0 inches.
The output relay of the process meter is configured so the solenoid valve opens when the level in the tank reaches 6.0 inches and remains open until the level drops to 4.0 inches. When the level reaches 4.0 inches, the solenoid valve closes and allows the level to increase until it reaches 6.0. Then the cycle repeats.
How is connected the circuit for the Process Meter ON/OFF Controller?This is the circuit to determine the level of water in the process tank using the process meter and control the level in the tank using ON/OFF control.
This is the wiring diagram for the level circuit
Once connected the circuit, it proceeds Process Meter's programming.
What is the programming procedure for Process Meter ON/OFF Control?
For this system, the parameters and values that will be programmed are shown in the image below. The process pump is ON and the solenoid valve is de-energized. The process pump is connected to the normally open relay contact, and the relay is programmed reverse, non-latching. Therefore, the pump is running because the level is below the high level limit (6.0). Also, because the process variable (PY) is below the high level limit, the normally closed relay contact that is wired to the solenoid valve is open. Therefore, the solenoid valve is de-energized (closed), so the process tank is filling until the solenoid valve becomes energized (opens). This is determined by listening for a "click."
The solenoid valve will open and begin to drain the process tank when the level reaches 6.0 inches, which is the high alarm value. The process pump will be OFF after the level reaches the upper limit. Therefore, the relay contacts return to their normal state (normally open for the contact wired to the process pump). Consecuently, the process tank will drain because the solenoid valve is open. The process pump will turn ON and the solenoid valve de-energizes until reach the programmed initial level. This allows the tank to begin to fill again. NOTE: For much specific information about paramaters and programming them, please consult "BIBLIOGRAPHY & APPENDIX" section. |