Micromod MOD: 1800P - MOD 30ML Identity Module (Version 2) PID and Ramp/Soak Functions - Manual (Page 15)

Continuous Control Functions
PID CONTROL BLOCK
2-9
2.2.1 PID Control Block Operation
The PID calculation is performed provided that the mode is AUTO or CASCADE, and that all
required inputs are available and GOOD. The algorithm types that drive the PID calculation
are proportional on error or process, standard or MICRO-SCAN integral or manual reset, Pre-
Act on error or process. Anti reset-windup for the standard integral is handled by using an
external feedback input (if configured), or the limited output value.
The PID track status output is set TRUE when the output mode is not AUTO, or the setpoint
mode is LOCAL.
The PID calculation is initialized when changing from the TRACK/MANUAL mode to AUTO or
CASCADE, BAD to GOOD output qualities, first run after a power up, or when the microscan
integral is used and the output is limited or not selected. The input filters are forward
initialized using the current input values, and the outputs and integral values are initialized
either from the previous output or the track value, depending on the track signal.
For all attribute writes, the rules which apply to the configuration of those attributes apply to
values written to the attributes. For example, the setpoint mode cannot be set to REMOTE if
no remote setpoint is configured, and limit attributes cannot be written so that the high limit is
less than or equal to the low limit. Specific write message considerations are described as
follows:
•
Algorithm Tuning Attributes
These attributes are the base gain, reset and preact values, local manual reset value, and
controller action. When these values are changed and the controller mode is AUTO, the
PID algorithm will reinitialize the calculation.
•
Ramp values
Values that may be ramped from a user interface device for example base gain, reset or
preact, local setpoint or output will ramp up to the limit value. A value beyond the limit is
clamped to the limit value.
•
Mode attributes
The mode attributes, setpoint, controller and output, are each a pair of attributes; one
representing the actual present mode (status) and the other representing the mode set by
a user to which the status will return when leaving TRACK mode (command or return
value). Either mode attribute can be read, but only the status attribute can be written to.
Values written to the status attribute are limited to those which can be set by a user, i.e.,
REMOTE and LOCAL for setpoint, AUTO, MANUAL and CASCADE for controller mode.
Writes of mode values that cannot be supported will be NAK’d. For example, attempting to
put the controller mode to CASCADE will be NAK’d if a remote setpoint is not configured.
Writes to a mode will be capable of causing block errors. For example, changing a
setpoint mode from LOCAL to REMOTE when the remote setpoint source quality is BAD
will cause the PID block outputs to be set BAD (if bad inputs are not accepted).
•
Block Outputs
The active setpoint attribute can only be written if the setpoint mode is FAULT or LOCAL,
or if the block state is OFF or DEBUG. The output attribute can only be written if the
mode is MANUAL or FAULT, or if the block state is OFF or DEBUG. The track status
attribute can only be written if the block state is OFF or DEBUG or the output or controller
mode is FAULT. Adjusted setpoint, ratio and bias can be written anytime.
•
Limit attributes