10.17 Pre-Tuning Operation; 1Variable Constants - Mitsubishi FX1S Series Programming Manual

FX Series Programmable Controllers

10.17 Pre-tuning operation

10.17.1 Variable Constants
The Pre-tuning operation can be used to automatically set values for the following variables:
- The direction of the process; Forward or Reverse (S
- The proportional gain constant; K
- The integral time constant; T
- The derivative time constant; T
Setting bit 4 of S
set by the pre-tuning operation: the sample time, Ts (S
Derivative gain, K
The Pre-tuning operation measures how fast the system will correct itself when in error.
Because the P, I, and D equations all react with differing speed, the initial error must be large
so that effective calculations can be made for each type of equation. The difference in values
between SP and PV
is not the case, then please change SV to a suitable value for the purpose of pre-tuning.
The system keeps the output value (MV) at the initial value, monitoring the process value until
it reaches one third of the way to the Set Point. At this point the pre-tuning flag (bit 4) is reset
and normal PID operation resumes. SV can be returned to the normal setting without turning
the PID command Off.
During the course of normal operation, the Pre-tuning will NOT automatically set new values if
the SV is changed. The PID command must be turned Off, and the Pre-Tuning function
restarted if it is necessary to use the Pre-tune function to calculate new values.
• Caution: The Pre-tuning can be used as many times as necessary. Because the flag resets,
the set bit can be turned On again and new values will be calculated. If the system is
running an oven heater and the SV is reduced from 250 to 200 C, the temperature must
drop below 200 or the "Forward/Reverse" flag will be set in the wrong direction. In addition,
the system error value must be large for the pre-tune variable calculations to work correctly.
• Note: Set the sampling time to greater than 1 second (1000 ms) during the pre-tuning
operation. It is recommended that the sampling time is generally set to a value much
greater than the program scan time.
• Note: The system should be in a stable condition before starting the pre-tuning operation.
An unstable system can cause the Pre-tuning operation to produce invalid results. (e.g.
opening a refrigerator door, adding ingredients to a mixture, cold start, etc.)
• Note: Even though Pre-tuning can set the above mentioned variables, additional logic may
be needed in the program to "scale" all operating values to those capable of being
processed by the special function devices being used.
(S
I 3
D
+1 starts the pre-tuning process. Before starting, set all values that are not
3
(S +5); the Set Point, SV (S
D 3
must be a minimum of 150 for the Pre-tuning to operate effectively. If this
nf
+1, bit 0)
3
(S +3)
P 3
+4)
(S +6)
3
+0); the input filter
3
); and any alarm or limit values, (S
1
Points Of Technique 10
FX FX FX
1S 1N 2N
α
(S
3
10-27
FX
2NC
+2); the
3
+20-23).