98
amount (low flame). A controller output opens the so-
lenoid valve when the process requires additional heat
(high flame).
Also refer to Modulating Valves and Time Proportioning
Control.
Tare
Type: Indicator Parameter
When an Indicator’s Tare function has been enabled,
the operator can set the current Process Variable input
value to be displayed as zero. This function may be
used to easily eliminate any offset on the input signal,
e.g. when a transducer output is not giving a true zero
value. It may also be used in applications displaying
the weight of a product, to remove the weight of a con-
tainer before starting. When Tare is activated, the in-
strument automatically sets the PV Offset to an equal,
but opposite value to the current measured value.
Display code = tArE, default setting = diSA (dis-
abled).
Also refer to Indicator, Process Variable, and Offset.
Three Point Stepping Control
Type: VMD Controller Denition
Refer to Valve Motor Control.
Time Proportioning Control
Type: Controller Denition
Time proportioning control is accomplished by cycling
the output on and off, during the prescribed cycle time,
whenever the process variable is within the proportion-
al band. The control algorithm determines the ratio of
time (on vs. off) to achieve the level of output power
required to correct any error between the process val-
ue and setpoint. E.g. for a 32 second cycle time, 25%
power would result in the output turning on for 8 sec-
onds, then off to 24 seconds. This type of output might
be used with electrical contactors, Solid State Relays
Time proportioning control can be implemented with
Relay, Triac or SSR Driver outputs for either primary
(Heat) or secondary (Cool) outputs depending on hard-
ware configuration.
Also refer to Current Proportioning Control, Cycle Time,
PID, Primary Proportional Band, Process Variable, Sec-
ondary Proportional Band, Setpoint, SSR and Triac.
Tuning
Type: Controller Denition
PID Controllers must be tuned to the process in order
for them to attain the optimum level of control. Adjust-
ment is made to the tuning terms either manually, or
by utilizing the controller’s automatic tuning facilities.
Tuning is not required if the controller is configured for
On-Off Control.
Also refer to Automatic Reset, Auto Pre-Tune, On-Off
control, PID, Pre-Tune, Primary Proportional Band,
Rate, Self-Tune and Secondary Proportional Band.
Triac
Type: General Denition
A small internal solid state device, which can be used
in place of a mechanical relay in applications switching
low power AC, up to 1 amp. Like a relay, the output
is time proportioned, but much faster switching cycle
times are also possible, leading to superior control. As
a solid-state device, a Triac does not suffer from con-
tact degradation when switching electrical currents. A
triac cannot be used to switch DC power.
Also refer to Cycle Time, SSR and Time Proportioning
Control.
Valve Close Limit
Type: VMD Controller Parameter
When Valve Position Indication is to be used on Valve
Motor Controllers, this parameter
provides a “clamp” on the upper valve position, which
the controller will not attempt to drive the valve past.
It can be set between 0 (fully closed) and the Valve
Open Limit value –1, expressed as a percentage of the
valve’s “fully open” position.
The Auxiliary Input must correctly scaled using the Set
Valve Open and Closed parameters before using this
parameter.
Display code = PiUL, default setting = 0.
Also refer to Auxiliary Input, Set Valve Closed Position,
Set Valve Open Position, Valve Open Limit, Valve Motor
Control and Valve Position Indication.
Valve Motor Drive Control
Type: VMD Controller Denition
Valve Motor Drive Controllers are designed to control
Modulating Valves using a special “Open Loop” Valve
Motor Drive (VMD) PI control algorithm. Output signals
are provided to move the valve further open, or further
closed when the process is higher or lower than the de-
sired setpoint. When on setpoint, no output is required
to maintain control unless load conditions change. This
known as Three-Point Stepping control.
Valve Position or Flow Indication is possible if an Aux-
iliary Input option module has been fitted and config-
ured for this purpose.
Also refer to Auxiliary Input, Modulating Valve, Open
Loop VMD, PI Control, PID, Setpoint and Valve Posi-
tion Indication.
Valve Position or Flow Indication
Type: VMD Controller Denition
The Valve Motor Drive Controllers do not require any
kind of position feedback in order for the PID algorithm
to correctly control the process. However, where feed-
back or flow level signals are available, they can be
displayed as a percentage (0 to
100) of the possible
valve opening or flow level. Valve Position Indication is
shown in the Operator Mode lower display in place of
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