Honeywell VRX180 - Manual (Page 80)

Programming and Operating Concepts
Video Recorder – User Manual 66
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.
VALVE
ACTUATOR
INSTRUMENT
PV 85
SP 95
OUT 73.5%
HOT
WATER
HOT WATER
VALVE
VALVE
ACTUATOR 4 TO 20 mA
(CAT)
COLD
WATER
COLD WATER
VALVE
4 TO 20 mA
(CAT)
WATER TANK
100 Ω
PLATINUM
RTD
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.