IDEC MicroSmart - Manual (Page 366)

21: PID I NSTRUCTION
21-10 « FC4A M ICRO S MART U SER ’ S M ANUAL »
Set the AT sampling period to a long value to make sure that the current process variable is smaller than or equal to the pre-
vious process variable during direct control action (S2+0 is on) or that the current process variable is larger than or equal to
the previous process variable during reverse control action (S2+0 is off).
S1+20 AT Control Period
The AT control period determines the duration of the ON/OFF cycle of the control output (S2+6) during auto tuning. For
operation of the control output, see Control Period on page 21-8.
When using auto tuning, set a required value of 1 through 500 to specify an AT control period of 0.1 sec through 50.0 sec
to the data register designated by S1+20. When S1+20 stores 0, the AT control period is set to 0.1 sec. When S1+20 stores
a value larger than 500, the AT control period is set to 50.0 sec.
S1+21 AT Set Point
While auto tuning is executed, the AT output manipulated variable (S1+22) is outputted to the output manipulated variable
(S1+1) until the process variable (S1+0) reaches the AT set point (S1+21). When the process variable (S1+0) reaches the
AT set point (S1+21), auto tuning is complete and the output manipulated variable (S1+1) is reduced to zero. When PID
action is selected with operation mode (S1+3) set to 1 (AT+PID), the PID action follows immediately.
When the linear conversion is disabled (S1+4 set to 0), set a required AT set point of 0 through 4095 to the data register
designated by S1+21. When S1+21 stores a value larger than 4095, the AT set point is set to 4095.
When the linear conversion is enabled (S1+4 set to 1), set a required AT set point of –32768 through 32767 to the data reg-
ister designated by S1+21. The AT set point must be larger than or equal to the linear conversion minimum value (S1+6)
and must be smaller than or equal to the linear conversion maximum value (S1+5).
In the direct control action (see page 21-11), set the AT set point (S1+21) to a value sufficiently smaller than the process
variable (S4) at the start of the auto tuning. In the reverse control action, set the AT set point (S1+21) to a value sufficiently
larger than the process variable (S4) at the start of the auto tuning.
S1+22 AT Output Manipulated Variable
The AT output manipulated variable specifies the amount of the output manipulated variable (0 through 100) during auto
tuning. When using auto tuning, set a required AT output manipulated variable of 0 through 100 to the data register desig-
nated by S1+22. When S1+22 stores a value larger than 100, the AT output manipulated variable is set to 100.
While auto tuning is executed, the specified value of the AT output manipulated variable (S1+22) is outputted to the output
manipulated variable (S1+1), and the control output (S2+6) is turned on and off according to the AT control period
(S1+20) and the AT output manipulated variable (S1+22). To keep the control output (S2+6) on during auto tuning, set 100
to S1+22.
Auto Tuning (AT)
When auto tuning is selected with the operation mode (S1+3) set to 1 (AT+PID) or 2 (AT), the auto tuning is executed
before starting PID control to determine PID parameters, such as proportional gain (S1+7), integral time (S1+8), derivative
time (S1+9), and control action (S2+0) automatically. The
MicroSmart uses the step response method to execute auto tun-
ing. To enable auto tuning, set four parameters for auto tuning before executing the PID instruction, such as AT sampling
period (S1+19), AT control period (S1+20), AT set point (S1+21), and AT output manipulated variable (S1+22).
Step Response Method
The MicroSmart uses the step response method to execute auto
tuning and determine PID parameters such as proportional gain
(S1+7), integral time (S1+8), derivative time (S1+9), and control
action (S2+0) automatically. The auto tuning is executed in the
following steps:
1. Calculate the maximum slope of the process variable (S1+0)
before the process variable reaches the AT set point (S1+21).
2. Calculate the dead time based on the derived maximum slope.
3. Based on the maximum slope and dead time, calculate the four
PID parameters.
Process Variable (S1+0)
Dead Time
Maximum Slope
AT Set Point (S1+21)