GE Fanuc Automation - Platforma Internetowa ASTOR

The actuating value y i is clamped to zero value (if the result of calculation is a negative value) or
directly to the PWMPeriod value (if the result is above the MaxTemp value) otherwise it is
converted into a value of the duty cycle of the PWM period.
Y i = y i / MaxTemp * PWMPeriod
The value of the MaxTemp depends on the thermocouple type.
To suppress overshoots during the warm-up phase, the following two non-linearities are
implemented in the control algorithm:
• The PLC unit sets the (linear) Control Zone value in units of 0.1 °C. For example, if this value is
250 (that is, 25°), the linear Control zone lies within ± 25°C around the Set point temperature.
The contribution of the integral term is temporarily suspended if the current temperature is
outside the Linear Control Zone and its content is frozen. The PWM output value is set up either
to 100% (below the lower Control Zone point) or to 0% (above the upper Control Zone point).
• Within the linear Control zone the regulation error is limited to the Error Limit value, which is
another parameter sent by the PLC unit and expressed in 0.1 °C. Such a limitation of the
regulation error is equivalent to a control system which tracks the moving Set Point. Since the
Set Point is ahead of the current temperature only by a relatively low value, even higher values of
the PID constants can be used without a danger of overshoots.
The following figure shows a situation when the new Set Point value is much higher than the
current or the starting temperature (ST). The Error limit value is deliberately set up too high in
order not to influence the regulation error.
Power [%]
100%
0%
ST
ST - Starting temperature
LCZp - Lower Control Zone point
a - contribution of the proportional term Pp
b - contribution of the integral term Ip
c - combined contribution of Pp + Ip
GFK-1466 Chapter 4 Operation and Field Wiring Information 4-7
b
LCZp Setpoint
c
a
4
Temp [°C]