Honeywell VRX180 user manual Split Output or Duplex Control

Programming and Operating Concepts

From Figure 3-18, the instrument’s AI1 function block will essentially process the 4 to 20 mA transmitter signal to generate a pH measurement. This measurement will be “AI1 OV” which, in turn, will be applied to LP1’s process variable input, “PV.” Before the 4 to 20 mA signal is applied to AI1, it will be converted to a 1 to 5 VDC signal with a 250 Ω shunt resistor. AI1 will be configured to generate a pH measurement in a range from 0 (RANGE LOW = 0) to 14 (RANGE HIGH = 14) in response to a voltage input between 1 (CKT LOW = 1) and 5 (CKT HIGH = 5) VDC. The PID algorithm of the control loop function block will adjust the value assumed by LP1 OV between 0 and 100%. This 0 to 100% signal will be applied to AO1, which will be configured as a DAT type analog output. The internal parameter of “IMPULSE TIME” in AO1 is the DAT analog output’s cycle time or period. With a specified impulse time of 150 seconds (an arbitrarily picked value), the DAT output will be ON for 75 seconds and OFF for 75 seconds when the input from LP1 is set to 50%. The ON and OFF times will be determined completely by the % output levels called for by LP1. Finally, to externalize the ON and OFF output states of AO1 to the outside world, the DO1 output relay, represented by the DO1 function block, will be programmed for AO1’s exclusive use. Hence, as AO1 switches between ON and OFF states in response to LP1 OV’s % output levels, so too will the DO1 output relay to generate the pulses required to drive the caustic reagent pump.

Split Output or Duplex Control

Split output or duplex control loops are typically used in heat/cool applications. Temperature is controlled through simultaneous use of both heating and cooling elements. If the instrument was to support a heat/cool control configuration, an example of the control scheme that might be dealt with is illustrated in Figure 3-19.

INSTRUMENT
PV	85
SP	95
OUT	73.5%
		VALVE
	4 TO 20 mA	ACTUATOR
	(CAT)
HOT
WATER		HOT WATER
		VALVE
	4 TO 20 mA
	(CAT)
		100 Ω
		PLATINUM
		RTD
		VALVE
		ACTUATOR
COLD
WATER		COLD WATER
		VALVE
		WATER TANK

Figure 3-19 Temperature Control Of Water Using Split Output Or Duplex Control

The instrument must be set up to produce two 4 to 20 mA control signals. By applying them to current- controlled valve actuators coupled to hot and cold water valves, these signals will regulate the amount of hot and cold water introduced to the vessel to maintain the water temperature at whatever set point will be programmed. The temperature of the water will be measured by means of a three-wire 100 Ω Platinum RTD. This process may be likened to manipulating hot and cold faucets regulate water temperature.

66 Video Recorder – User Manual