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Chapter 4 Applications
34 CNT-APG002-EN
Cascade control—first stage
A PID loop can be used to automatically determine a discharge-air tem-
perature setpoint. Other programs or control systems can then make use
of this calculated setpoint. This type of control, called cascade control,
results in very tight control of space temperature. Calculating the dis-
charge-air temperature setpoint is the first stage of cascade control.
Figure 24 illustrates how a PID loop calculates the discharge-air temper-
ature setpoint. The calculated discharge-air temperature is changed
based on the difference between the space temperature setpoint and the
space temperature.
Figure 24: Calculating the discharge-air temperature setpoint
Cascade control requires two sensors, one for the space temperature and
one for the discharge-air temperature. The PCL or TGP program must be
programmed to respond to a failure of either of the sensors. Suggested
failure conditions are:
• If the space temperature sensor fails, set the discharge-air tempera-
ture setpoint to the space temperature setpoint. Other limits for the
discharge-air temperature setpoint may need to be defined. Inform
the operator of the sensor failure.
• If the discharge-air temperature sensor fails, control the hot or
chilled-water valve as appropriate to the climate in your region.
Inform the operator of the sensor failure.
Table 13 shows a PCL program to calculate the discharge-air tempera-
ture setpoint, and Figure 25 on page 35 shows the same program in TGP.
Most of the programming occurs in the DDC Loop Parameters screen. Use
the values shown in Table 14 on page 35.
Σ
PID calculation
Space
setpoint
Error
+
–
Space
temperature
Discharge-air
temperature setpoint
Table 13: PCL program for discharge-air temperature setpoint in cascade control
Line Result 1st Arg Operator 2nd Arg Description of Statement
---- -------- --------- -------- -------- --------------------------------------
1 DATSTPT = SP_TEMP DDC:1 SP_STPT Call the PID function
2 *IFT = SP_TEMP FAIL If the space sensor has failed
3 DATSTPT = SP_STPT set the disch air setpt to space setpt
4 *END = (Note: the discharge-air temp sensor is
checked in another program.)