VersaPro Temperature Controller
Page 14
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6.2
Time Proportioning Dual (TD)
This mode is used when there are two processes to control that have complementary effects; like heat and
cool. The time proportioning dual mode uses two control outputs; one for heat and one for cool. There is
never a time when both outputs are on simultaneously. The control loop computes a percent output from -100
to +100%. When positive, the proportioning action applies to the forward output. When negative the
proportioning action applies to the reverse output.
6.3
Time Proportioning with Complement (TC)
This mode is identical to the time proportioning mode except that both control outputs are used. The second
control output is the complement of the first. That is when the first output is ON then the second is OFF and
vice versa. This mode is used with single action motorized valves that open quickly when a voltage is applied
to one terminal and close quickly when voltage is applied to the other terminal.
6.4
Position Proportioning (PP)
This mode is used with motorized valves that do not have slidewire feedback. This mode is sometimes
referred to as "bump" mode because it "bumps" the valve slightly more open or closed. This mode uses both
control outputs; one to drive the motor forward (open) and the other to drive it reverse (closed). The control
output is the difference between the new percent output and the last percent output. If the difference is
positive than the valve motor is driven open for that percentage of the cycle. If negative it is driven closed by
that percentage of the cycle time.
For example if the new percent out is 60% and the old was 45% then the motor is driven open for 15% of the
cycle time. If the cycle time is set to the time that the motor takes to move from fully closed to fully open, then
the flow is theoretically increased by 15%. Two special cases exist. If the control output is computed at 100%
then the motor is driven continuously in the open direction. Likewise if the control output is computed as 0%
then the motor is driven continuously closed.
There is a built in deadband for this control based on the length of the cycle time. The comparison between
the previous and current output values are made at the end of each cycle time. Faster comparisons can be
made by shortening the cycle time assuming that a 100% command output is a continuously close control
contact.
6.5
ON/OFF (OF)
ON/OFF control is exactly what it implies, the control action is either ON or OFF. With true ON/OFF control the
control output is ON whenever the process is below the setpoint value and OFF when the process is at or
above the process value. In many real world applications this simple control method will cause "contact
chatter" because of noisy signals which will switch the ON and OFF states rapidly. Also since the control
action does not turn OFF until the setpoint is reached, the process will overshoot due to lags in the control
action.
Marathon controllers incorporate two features that prevent these problems from occurring; hysteresis and
deadband. Hysteresis provides a delay between the control on point and the off point. Noise will not cause
the control output to “chatter” with this gap applied. Hysteresis is ±20% of the deadband value.
