4-176 Series 90-30/20/Micro Programmable Controllers Reference Manual – September 1998 GFK-0467K
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Table 4-5. PID Parameters Details
Data Item Description
Loop Number
(00) This is an optional parameter available to identify a PID block. It is an unsigned integer that
provides a common identification in the PLC with the loop number defined by an operator interface
device. The loop number is displayed under the block address when logic is monitored from the
Logicmaster 90-30/20/Micro software.
Algorithm (01) An unsigned integer that is set by the PLC to identify what algorithm is being used by the
function block. The ISA algorithm is defined as algorithm 1, and the independent algorithm is
identified as algorithm 2.
Sample Period
(02) The shortest time, in 10 millisecond increments, between solutions of the PID algorithm. For example,
use a 10 for a 100 millisecond sample period. If it is 0, the algorithm is solved every time the block is
called (see section below on PID block scheduling).
The PID algorithm is solved only if the current PLC elapsed time clock is at or later than the last PID
solution time plus this Sample Period. Remember, that the 90-30 will not use a solution time less than 10
milliseconds (see Note on page 4-171); so sweeps will be skipped for smaller sweep times. This function
compensates for the actual time elapsed since the last execution, within 100 microseconds. If this value is
set to 0, the function is executed each time it is enabled; however, it is restricted to a minimum of 10
milliseconds as noted above.
Dead Band
(+/—)
(03/04)
INT values defining the upper (+) and lower (–) Dead Band limits in PV Counts. If no Dead Band is
required, these values must be 0. If the PID Error (SP – PV) or (PV – SP) is above the (–) value and
below the (+) value, the PID calculations are solved with an Error of 0. If non-zero, the (+) value must be
greater than 0 and the (–) value less than 0 or the PID block will not function. You should leave these at 0
until the PID loop gains are setup or tuned. After that, you may want to add Dead Band to avoid small
CV output changes due to small variations in error, perhaps to reduce mechanical wear.
Proportional
Gain–Kp
(05)
This INT number, called the Controller gain, Kc, in the ISA version, determines the change in CV in CV
Counts for a 100 PV Count change in the Error term. It is displayed as 0.00 %/% with an implied decimal
point of 2 . For example, a Kp entered as 450 will be displayed as 4.50 and will result in a Kp*Error/100
or 450*Error/100 contribution to the PID Output. Kp is generally the first gain set when adjusting a PID
loop.
Derivative
Gain–Kd
(06)
This INT number determines the change in CV in CV Counts if the Error or PV changes 1 PV Count
every 10 milliseconds. Entered as a time with the low bit indicating 10 milliseconds, it is displayed as 0.00
Seconds with an implied decimal point of 2. For example, a Kd entered as 120 will be displayed as 1.20
Sec and will result in a Kd * delta Error/delta time or 120*4/3 contribution to the PID Output if Error was
changing by 4 PV Counts every 30 milliseconds. Kd can be used to speed up a slow loop response, but is
very sensitive to PV input noise.
Integral Rate
Gain–Ki
(07)
This INT number determines the change in CV in CV Counts if the Error were a constant 1 PV Count. It
is displayed as 0.000 Repeats/Sec with an implied decimal point of 3. For example, a Ki entered as 1400
will be displayed as 1.400 Repeats/Sec and will result in a Ki * Error *dt or 1400 * 20 * 50/1000
contribution to PID Output for an Error of 20 PV Counts and a 50 millisecond PLC sweep time (Sample
Period of 0). Ki is usually the second gain set after Kp.
CV Bias/Output
Offset
(08)
An INT value in CV Counts added to the PID Output before the rate and amplitude clamps. It can
be used to set non-zero CV values if only Kp Proportional gains are used, or for feed forward control of
this PID loop output from another control loop.