Trane CNT-APG002-EN, PID Control - page 40

Cascade control—first stage

CNT-APG002-EN 35

®

Figure 25: TGP program for discharge-air temperature setpoint in cascade control

If you use the settings shown in Table14, you should not have to tune the

loop. These values can be used in almost any cascade control application

without change. The proportional and integral gains are high to respond

aggressively to the error and change in error. The minimum and maxi-

mum output values keep the output between 50°F (10°C) and

120°F(49°C).

Figure26 on page 36 shows an example of the relationship between the

calculated discharge-air temperature setpoint and the space temperature.

The discharge-air temperature setpoint reacts strongly to small changes

in error because of the high proportional gain.

If space temperature sensor has

failed, switch control to space

temperature setpoint

Table 14: Settings for discharge-air temperature setpoint

PID setting Initial value

Proportional gain 20.0

Integral gain 5.0

Derivative gain 0.0

Error deadband 0.5

Action Reverse

Sampling frequency 60 seconds

Minimum output 50°F

Maximum output 120°F

Contents

Main Page Page Contents Chapter 1 Overview of PID control. . . . . . . . . . . . . . . . . . . . . . 1 Chapter 2 PID settings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Chapter 3 Programming PID loops. . . . . . . . . . . . . . . . . . . . . 23 Page Chapter 1 Overview of PID control What PID loops do How PID loops work PID calculations Proportional calculation Integral calculation Derivative calculation Page Velocity model Page Chapter 2 PID settings Throttling range Gains Calculating the gains Example Sampling frequency Page Calculating the sampling frequency To calculate the sampling frequency: Page Example Action Direct action Reverse action Determining the action Example 1 Example 2 Error deadband Typical applications Adjusting error deadband for modulating outputs IMPORTANT Adjusting error deadband for staged outputs To adjust the error deadband for staged outputs: Other PID settings Page Chapter 3 Programming PID loops Programming in PCL Follow these steps to program PID loops in PCL: Page Programming in TGP Follow these steps to program PID loops in TGP: Page Chapter 4 Applications Discharge-air temperature control Page Page Building pressure control Page Cascade controlfirst stage Page Page Staging cooling-tower fans Setting up the PID loop Page Page Page Page Determining the staging points To determine the staging points: Example 1: Two-stage fan system Example 2: Three-stage fan system Chapter 5 Troubleshooting Troubleshooting procedure Tips for specific problems Changing the sampling frequency Changing the gains Examples Example 1 Example 2 Page Example 3 Chapter 6 Frequently asked questions Why is the derivative gain usually zero? What is the difference between direct acting and reverse acting? When should I use proportional-only control? Why should I use PID control in staging applications? How can I tell if a PID loop is working well? Whats the best sampling frequency? Do I need to worry about the throttling range? Page Appendix A The math behind PID loops Velocity model formula Proportional control formula Page Glossary Page Page Page Index Numerics A B C E F G H I S T U V W