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7.4.5
CV Bias Term
The CV Bias term (word 9 in the reference array) is an additive term separate from the PID inputs. It
may be useful if you are using only Proportional gain (Kp) and you want the CV to be a non-zero value
when the PV equals the SP and the Error is 0. In this case, set the CV Bias to the desired CV when the
PV is at the SP. CV Bias can also be used for feed forward control where another PID loop or control
algorithm is used to adjust the CV output of this PID loop.
If a non-zero Integral rate is used, the CV Bias will normally be 0 as the integral term acts as an
automatic bias or reset. Just start up in Manual mode and use the Manual Command word (word 14
of the reference array) to set the desired CV, and then switch to Automatic mode. This will
immediately calculate the required value for the integral term.
7.4.6
CV Amplitude and Rate Limits
The PID block does not send the calculated Output directly to CV. Both PID algorithms can impose
amplitude and rate of change limits on the output Control Variable. If the Minimum Slew Time (word
12 of the reference array) is non-zero, the rate of change (slew rate) limit is determined by dividing
the maximum CV value (32,000) by the Minimum Slew Time. For example, if the Minimum Slew Time
is 100 seconds, the rate limit will be 320 CV counts per second. If the solution interval was 50 ms, the
new CV output cannot change more than 320*50/1000 or 16 CV counts from the previous CV output.
The CV output is then compared to the CV Upper Clamp and CV Lower Clamp values (words 10 and
11 of the reference array). If CV is outside either limit, the CV output is clamped to the appropriate
limit value. When the CV output is modified to impose either slew rate or amplitude limits (or both)
the stored integral term would normally accumulate a large value over time. This phenomenon is
known as reset windup. Reset windup introduces errors in CV after the PID output no longer needs to
be limited. For example, windup would prevent the CV output from moving off a clamp value
immediately.
There are two optional methods for preventing reset windup. If the Anti-reset-windup Action bit (bit
4) of Config Word (word 13 of the reference array) is zero (the default), the integral term is adjusted
at each PID solution to match the error input and limited CV output exactly. When PV changes while
CV is clamped, or when CV is both rate and amplitude limited in a particular PID solution, this option
assures that a smooth transition will always occur after CV is no longer limited.
If the Anti-reset-windup Action bit of Config Word is set, then the integral term stored on the
previous PID solution is simply retained as long as CV is limited. This option was added to assure
compatibility with existing PID applications when the default action described above was introduced.
This option is not recommended for new applications.
Finally, the PID block checks the Output Polarity (bit 2 of the Config Word) and changes the sign of
the output if the bit is 1.
CV = – (Clamped PID Output) if Output Polarity bit set, or
CV= (Clamped PID Output) if Output Polarity bit cleared.
If the block is in Automatic mode, the final CV is placed in the Manual Command (word 14 of the
reference array). If the block is in Manual mode, the PID equation is skipped because CV is set by the
Manual Command, but the slew rate and amplitude limits are still checked. This assures that the
Manual Command cannot change the output above the CV Upper Clamp or below the CV Lower
Clamp, and the output cannot change faster than allowed by the Minimum Slew Time.
GFK-2950C
Chapter 7. PID Built-In Function Block
February 2018
357
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