Extech Instruments 48VTR User Manual page 11

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The CYCLE TIME (Ct in 3
percentage change. For the above diagrams, the Ct = 10, therefore every 10 seconds the
controller adjusts the output percentage. The controller decides the output percentage
based on the proximity of the PV to the SV. The further the PV is from the setpoint, the
higher the percentage. The closer the PV is to the setpoint, the lower the percentage.
Three Cycle Time examples are shown in the diagrams.
In Diagram 1 (above), the controller is calling for an output of 80% power. This is because
the process variable (PV) has a long way to go to get to the setpoint (SV). So, in this case,
for the 10-second cycle time period, the output will be ON for 8 seconds (80% of the 10-
second cycle time), and OFF for 2 seconds. In Diagram 2, we're much closer to the SV
and controller supplies 50% power. The output will be ON for 5 seconds and OFF for 5
seconds. In Diagram 3, the PV is very close to setpoint so the output power is now 10%
power. Output is ON for 1 second and OFF for 9 seconds.
The cycle time setting is critical and must be set to the longest time possible without
causing process oscillations. The longer the Cycle Time the longer the relay life span.
DC Trigger Output Control
Read the above paragraphs for ON-OFF and Time Proportioning Control actions since the
DC trigger operates in the same fashion. The only difference between a relay output and a
DC trigger output is that the DC trigger supplies its own power (available on terminals 8
and 9). The relay, on the other hand, must be wired in series with an AC source to produce
power. The DC trigger output is approx. 0VDC for 0% power and 29VDC (max) for 100%
power. Wire the DC trigger as described in the Wiring section of this manual. Caution:
When a DC trigger output is installed in the controller, never connect AC voltage to
terminals 8 or 9.
Analog Output Control (4 to 20mA)
With an analog output, the amount of current that the controller outputs depends on the
proximity of the PV to the SV; the further the PV is from the setpoint, the higher the current
output. As the PV approaches the SV the current output decreases. 100% output = 20mA,
0% output = 4mA, 50% output = 12mA, and so on. For proper operation of an analog
output, Cycle Time (parameter Ct in the 3
Controller Output% vs. Proportional Band (Pb) Setting
The 48VTR output percentage is largely determined by the proximity of the PV (Measured
variable) to the SV (Setpoint). However, the Proportional Band and the High/Low Limit
settings also have an effect on the Controller Output.
When the Proportional band is set to a small number (narrow band), the output gain is
higher - meaning that the process is allowed to rise to setpoint rather quickly. When the
Proportional band is set to a large number (wide band), the output gain is lower - meaning
that the process is restricted from rising to setpoint too quickly. Every process is different
and the Proportional Band must be set to match the characteristics of each individual
application. Auto Tune is the best way to do this. In the example diagrams below, a WIDE,
a NARROW, and a CORRECT Proportional Band are illustrated.
The HIGH and LOW LIMIT parameters (HiLt and LoLt in the 3
also important since the Proportional Band size is dependent on the size of the SPAN
(High Limit minus Low Limit). For example, a 10% Proportional Band setting with a wide
SPAN is larger than a 10% Proportional Band setting with a narrow SPAN.
Note that in the example diagrams below, the controller's are configured for HEATING
(selection 1 under Act in the 3
the SV from above.
NOTE: 'Pb' (Proportional Band) is automatically set when the Auto Tune function is used.
Refer to the section on Auto Tuning for details. 'Pb' can also be manually adjusted.
Menu Level) dictates how often the controller makes an output
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Menu Level) must be set to '0'.
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Menu Level). For COOLING, the PV would be approaching
11 Model 48VTR - Version 5.0 – January 2006
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Programming Level) are