TT301 – Operation and Maintenance Instruction Manual
2.4
Functional Description - Software
Refer to the block diagram (Figure 2.2). The function of each block is described below.
Input
Calculates the actual mV or Ohm from the value sensed by the input circuitry.
Digital Filter
The digital filter is a low-pass filter with an adjustable time constant. It is used to smooth noisy signals.
The Damping value is the time required for the output to reach 63.2% for a step input of 100%.
Input Trim
Here, the value obtained by READING-TRIM is used to correct the transmitter for long term drift.
Standard Sensor Linearization & Compensation
Here, the mV and Ohm measurements are linearized and cold-junction compensated according to the
sensor characteristics stored in the CPU. The CPU contains data about most standard sensors
available.
Ranging
It is used to set the process values corresponding to the output 4 and 20 mA in transmitter mode or
process variable 0 and 100% in PID mode. In transmitter mode the LOWER-VALUE is the point
corresponding to 4 mA, and UPPER-VALUE is the point corresponding to 20 mA. In PID mode, the
LOWER-VALUE corresponds PV = 0% and UPPER-VALUE corresponds to PV = 100%.
Time Generator (Optional)
Counts the time to be used by the Setpoint generator function. It may be paused by using PAUSE or
set to any value inside the table.
Setpoint (Optional)
Here, the setpoint is adjusted in PID. In this block, Setpoint tracking may be activated in SP-TRACK.
The setpoint may also be generated automatically by turning the SP-GENERATOR on. When running,
the setpoint generator will ramp and dwell the setpoint according to a table (recipe) configured in SP-
TABLE.
PID (Optional)
First the error is calculated as SP-PV or PV-SP depending on which action (direct or reverse) is
configured in ACTION. When Calendar Van Dussen function is used, the SP table points number is
limited to 14 points instead to 16.
TABLE POINTS
This block relates the output (%) with the input (%) according to a 16 table points. The output is
calculated through the interpolation of these points.
Auto/Manual (Optional)
The Auto/Manual mode is toggled in PID. In Manual, MV may be adjusted by the user in the INDIC
option. The SP-POWER-ON option is used here to determine in which mode the controller should be
upon powering it on.
Limits (Optional)
This block makes sure that the MV does not go beyond its minimum and maximum limits as
established by the HIGH-LIMIT and LOW-LIMIT. It also makes sure that the Rate-of-Change does not
exceed the value set in RATE-CHNG.
Output
Calculates the current proportional to the process variable or manipulated variable to be transmitted on
the 4-20 mA output depending on the configuration in OP-MODE. This block also contains the constant
current function configured in OUTPUT. The output is physically limited to 3.6 to 21 mA.
Current Trim
The 4 mA TRIM and 20 mA TRIM are used to make the transmitter current comply with a current
standard, should a deviation arise.
Display
Alternates two indications as configured in DISPLAY. The engineering unit for the process variable can
).
1
(
dt
dPV
Tdedt
Tr
eKpMV
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