Tcont_S; Description Tcont_S - Siemens SIMATIC S7-1200 Function Manual

Instructions
9.4 PID basic functions
9.4.5

TCONT_S

9.4.5.1

Description TCONT_S

The TCONT_S instruction is used on SIMATIC S7 automation systems to control technical
temperature processes with binary manipulated value output signals for actuators with
integrating behavior. The functionality is based on the PI control algorithm of the sampling
controller. The step controller operates without a position feedback signal.
Application
You can also use the controller in a cascade control as a secondary position controller. You
specify the actuator position via the setpoint input SP_INT. In this case, you must set the
process value input and the parameter TI (integration time) to zero. An application might be,
for example, temperature control with heating power control using pulse-break activation and
cooling control using a butterfly valve. To close the valve completely, the manipulated
variable (ER*GAIN) should be negative.
Call
The instruction TCONT_S must be called equidistant. To achieve this, use a cyclic interrupt
priority class (for example, OB35 for an S7-300). The sampling time is specified at the
CYCLE parameter.
If you call the instruction TCONT_S as a multiple instance DB, no technology object is
created. No parameter assignment interface or commissioning interface is available. You
must assign parameters for TCONT_S directly in the multiple instance DB and commission it
via a watch table.
CYCLE sampling time
The CYCLE sampling time match the time difference between two calls (cycle time of the
cyclic interrupt OB taking into account the reduction ratios).
The controller sampling time should not exceed 10% of the calculated integration time of the
controller (TI). Generally, you must set the sampling time to a much lower value to achieve
the required accuracy of the step controller.
Required accuracy G MTR_TM
0.5 %
Start-up
The TCONT_S instruction has an initialization routine that is run through when input
parameter COM_RST = TRUE is set. Following execution of the initialization routine, the
block sets COM_RST back to FALSE. All outputs are set to their initial values. If you require
initialization when the CPU restarts, call the block in OB100 with COM_RST = TRUE.
See also
Block diagram TCONT_S (Page 501)
496
CYCLE = MTR_TM*G
10 s 0.05 s
Comment
The sampling time is determined
by the required accuracy of the
step controller.
Function Manual, 03/2017, A5E35300227-AC
PID control