PID Loop Operation
Maintenance
and Troubleshooting
8–26
PID Loop Operation
PID Loop Data Configuration
In choosing the Process Variable range and resolution, a related choice to make is
the data format of the three main loop variables: SP, PV, and Control Output (the
Integrator sum in V+04 also uses this data format). The four data formats available
are 12 or 15 bit (right justified), signed or unsigned (MSB is sign bit in bipolar
formats). The four binary combinations of bits 0 and 1 of PID Mode 2 word V+01
choose the format. The DirectSOFT32 PID Setup dialog sets these bits
automatically when you select the data format from the menu.
Process Variable V+03
Loop
Calculation
S
+
–
Control Output V+05Setpoint V+02
013456789101112131415 2Bit
12 bit unipolar
12 bit bipolar
15 bit bipolar
15 bit unipolar
0 to 0FFF (0 to 4095)
0 to 0FFF, 8FFF to 8001
(0 to 4095, –4095 to –1)
0 to 32767
0 to 7FFF, FFF to 8001
(0 to 32767, –32767 to –1)
Data formats
013456789101112131415 2Bit
PID Mode 2 Setting V+01
0 0
0 1
1 0
1 1
Select data
format using
bits 0 and 1.
= sign bit
LSB
The data format is a very powerful setting, because it determines the numerical
interface between the PID loop and the PV sensor, and the Control Output device.
The Setpoint must also be in the same data format. Normally, the data format is
chosen during the initial loop configuration and is not changed again.
Choosing the data format involves deciding whether to use unipolar or bipolar
numbers. Most applications such as temperature control will use only positive
numbers, and therefore need unipolar format. Usually it is the Control Output which
determines bipolar/unipolar selection. For example, velocity control may include
control of forward and reverse directions. At a zero velocity setpoint the desired
control output is also zero. In that case, bipolar format must be used.
Unipolar
Bipolar
Loop Parameter
Data Formats
Choosing Unipolar
or Bipolar Format
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