Trane CNT-APG002-EN, PID Control - page 20

Calculating the sampling frequency

CNT-APG002-EN 15

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6. Calculate two-thirds (66%) of the change in measured variable deter-

mined in step 4. Add this value to the initial temperature to deter-

mine at what point two-thirds of the total change occurs.

In the example, 0.66 × 50°F = 33°F, so two thirds of the total change

occurs at 70°F + 33°F = 103°F (0.66 × 28°C = 18°C; 21 + 18 = 39°C).

7. Again, set the analog output to 0% and allow the measured variable

to stabilize.

The measured variable stabilizes at 70°F (21°C).

8. Control the output to the value used in step 3 and record the time it

takes to reach the two-thirds point determined in step 6. This is the

system time constant.

The time it takes to reach 103°F (39°C) is 2.5 minutes (150 seconds).

9. Divide the system time constant by 10 to determine the initial sam-

pling frequency.

150 seconds ÷ 10 = 15 seconds.

Figure13 illustrates the procedure described above.

Figure 13: Determining the system time constant

Note:

The system time constant is the time it takes to reach 63.21% of

the difference between the start point and the end point. How-

ever, two-thirds (66%) is accurate enough for most purposes.

System tim e

constant

Time (minutes)

Space temperature (°F)

Initial value (valve closed)

Final value (valve open)

2/3 of total change

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