Pidb Algorithm - VXI VT1422A User Manual

PIDB Algorithm

Setpoint
Σ
Variable
+
-
alarm limits
status .B6
Clipping Limits
Alarm Limits
Appendix G
Figure G-2 shows the block diagram of a more advanced algorithm that is
favored in process control because of the flexibility allowed by its two
differential terms. The "D" differential term is driven by changes in the
process input measurement. The "SD" differential term is driven by changes
in the setpoint variable value.
SD_factor
Variable
SD_out
variable
clip limits
status .B3
P_factor
Variable
Error
clip limits
variable
status .B1
I_factor I_out
variable
Variable
clip limits
status .B2
D_out
variable
D_factor
Variable
Figure G-2. The Advanced Algorithm "PIDB"
The PIDB algorithm provides clipping limits for its I, D, SD terms and the
value sent to <outp_chan>. Values for these terms are not allowed to range
outside of the set limits. The variables that control clipping are:
I term limits;
D term limits;
SD term limits;
<outp_chan> limits;
The PIDB algorithm provides Alarm Limits for the process variable PV
and the Error term variable Error. If these limits are reached, the algorithm
sets the value of <alarm_chan> true and generates a VXIbus interrupt.
The variables that control alarm limits are:
Process Variable (from <inp_chan>); PV_max and PV_min
Error term alarm limits;
The max and min limits for clipping and alarms are set to 9.9E+37 and
-9.9E+37 respectively when the algorithm is defined. This effectively turns
the limits off until the values are changed with the ALG:SCALAR and
ALG:UPDATE commands as described in "Pre-setting PID Variables and
Coefficients" later in this section.
Man_out
Variable
+ +
manual
Σ
+
+
auto
Man_state
Variable
status .B4
I_max and I_min
D_max and D_min
SD_max and SD_min
Out_max and Out_min
Error_max and Error_min
Example PID Algorithm Listings
slew rate limited by
Man_inc variable
<outp_chan>
process
channel
clip limits
status .B0
<inp_chan>
channel
493