Page 2 - in Tracer Controllers
Applications Guide PID Control in Tracer Controllers CNT-APG002-EN October 2001 ®
Page 4 - Contents; Chapter 1
CNT-APG002-EN iii ® Contents Chapter 1 Overview of PID control . . . . . . . . . . . . . . . . . . . . . . 1 What PID loops do . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 How PID loops work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...
Page 5 - Chapter 4
® Contents iv CNT-APG002-EN Chapter 4 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Discharge-air temperature control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Building pressure control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....
Page 6 - Overview of PID control; What PID loops do
CNT-APG002-EN 1 ® Chapter 1 Overview of PID control This guide will help you set up, tune, and troubleshoot proportional, inte-gral, derivative (PID) control loops used in Tracer controllers. These con-trollers include the Tracer MP580/581, AH540/541, and MP501 controllers. This chapter provides an ...
Page 7 - How PID loops work
® Chapter 1 Overview of PID control 2 CNT-APG002-EN How PID loops work A PID loop performs proportional, integral, and derivative calculations to calculate system output. Figure 2 illustrates how a typical PID loop works. The sigma ( Σ ) symbol indicates that a sum is being performed. The plus (+) s...
Page 8 - PID calculations; Proportional calculation
PID calculations CNT-APG002-EN 3 ® PID calculations A PID loop performs three calculations: the proportional calculation, the integral calculation, and the derivative calculation. These calculations are independent of each other but are combined to determine the response of the controller to the err...
Page 9 - Integral calculation
® Chapter 1 Overview of PID control 4 CNT-APG002-EN Figure 4: The effects of proportional bias on system output Integral calculation The integral calculation responds to the length of time the measured vari-able is not at setpoint. The longer the measured variable is not at set-point, the larger the...
Page 10 - Derivative calculation
PID calculations CNT-APG002-EN 5 ® Figure 5: Integral output added to proportional output The value of the integral calculation can build up over time (because it is the sum of all past errors), and this built-up value must be overcome before the system can change direction. This prevents the contro...
Page 12 - Velocity model
Velocity model CNT-APG002-EN 7 ® Velocity model Trane controllers use a type of PID control known as the velocity model. The velocity model minimizes the problem of integral windup, which occurs when the sum of past errors in the integral calculation is too great to allow the controller to change th...
Page 14 - Chapter 2; PID settings; Throttling range
CNT-APG002-EN 9 ® Chapter 2 PID settings This chapter describes some of the key variables used to set up and tune PID loops. The variables discussed here are: • Throttling range • Gain • Sampling frequency • Action • Error deadband Throttling range The throttling range is the amount of error it take...
Page 15 - Gains
® Chapter 2 PID settings 10 CNT-APG002-EN The throttling range determines the responsiveness of a control system to disturbances. The smaller the throttling range, the more responsive the control. You cannot directly program the throttling range in Tracer con-trollers; rather, the throttling range i...
Page 16 - Calculating the gains; Example
Calculating the gains CNT-APG002-EN 11 ® Calculating the gains Table 1 shows recommended initial values for the proportional and inte-gral gains for several applications. Most applications do not require a derivative contribution, so the derivative gain is not shown. We recom-mend using a ratio of 4...
Page 17 - Sampling frequency
® Chapter 2 PID settings 12 CNT-APG002-EN Sampling frequency The sampling frequency is the rate at which the input signal is sampled and the PID calculations are performed. Using the right sampling fre-quency is vital to achieving a responsive and stable system. Problems can arise when the sampling ...
Page 19 - Calculating the sampling frequency; To calculate the sampling frequency:
® Chapter 2 PID settings 14 CNT-APG002-EN Calculating the sampling frequency PID loops are carried out by programs, such as process control language (PCL) programs and Tracer graphical programming (TGP) programs. Since the PID calculation occurs when the program executes, the sam-pling frequency and...
Page 22 - Action; Direct action; Reverse action
Action CNT-APG002-EN 17 ® Action The action of a PID loop determines how it reacts to a change in the mea-sured variable (such as a room temperature). A controller using direct action increases the output when the measured variable increases. A con-troller using reverse action decreases the output w...
Page 23 - Determining the action; Example 1
® Chapter 2 PID settings 18 CNT-APG002-EN Determining the action Table 3 shows the action settings for several applications. These settings are a good starting place for most applications. To find the action for other applications, determine whether the actuator and measured variable move in the sam...
Page 24 - Error deadband; Typical applications
Error deadband CNT-APG002-EN 19 ® Error deadband Error deadband is typically used to minimize actuator activity. It can also be used to allow for some slack in system sensors and actuator mechanics. Error deadband prevents the PID output from changing when the abso-lute value of the error is less th...
Page 25 - Adjusting error deadband for modulating outputs; IMPORTANT; Adjusting error deadband for staged outputs
® Chapter 2 PID settings 20 CNT-APG002-EN Adjusting error deadband for modulating outputs In most applications, start with an error deadband of five or ten times the sensor resolution. For example, thermistors have a resolution of approxi-mately 0.1°F (0.06°C), so 0.5°F (0.3°C) is an appropriate err...
Page 26 - To adjust the error deadband for staged outputs:; Other PID settings
Other PID settings CNT-APG002-EN 21 ® With the preceding guidelines in mind, use the following procedure to determine error deadband. To adjust the error deadband for staged outputs: 1. Run the system manually. If possible, do so under worst case conditions for the site. Although it is not always po...
Page 28 - Chapter 3; Programming PID loops; Programming in PCL
CNT-APG002-EN 23 ® Chapter 3 Programming PID loops This chapter presents programs written in process control language (PCL) and the Tracer graphical programming (TGP) editor. This chapter does not discuss how to use the PCL or TGP editors. For information on using these editors, refer to Universal P...
Page 29 - Follow these steps to program PID loops in PCL:
® Chapter 3 Programming PID loops 24 CNT-APG002-EN Follow these steps to program PID loops in PCL: 1. Make sure that the setpoint is within reasonable limits. Use the MIN and MAX operators to set a ceiling and floor for the set- point, as shown in lines 1 and 2 of Table 7 on page 25. 2. Run the PID ...
Page 31 - Programming in TGP
® Chapter 3 Programming PID loops 26 CNT-APG002-EN Programming in TGP Figure 17 shows the PID block used to program PID loops in TGP editor. The PID block is more flexible than the DDC function in PCL. The enable/disable and failure inputs can accept any binary value, regardless of source. The setpo...
Page 32 - Follow these steps to program PID loops in TGP:
Programming in TGP CNT-APG002-EN 27 ® Follow these steps to program PID loops in TGP: 1. Use the Limit block to make sure that the setpoint is within reason-able limits. 2. Run the PID calculation. 3. Define failure and other operation-dependent conditions. Check for fan-status and measured-variable...
Page 34 - Applications; Discharge-air temperature control
CNT-APG002-EN 29 ® Chapter 4 Applications This chapter describes several HVAC applications that use PID control. It includes specific settings and recommendations for each application. Discharge-air temperature control When controlling hot/chilled-water valves in discharge-air applications, a PID lo...
Page 37 - Building pressure control
® Chapter 4 Applications 32 CNT-APG002-EN Building pressure control Space pressure is typically controlled by opening and closing relief damp-ers. A PID loop controls these dampers based on a space pressure setpoint and the measured space pressure. The space pressure in the building should remain sl...
Page 39 - Cascade control—first stage
® 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 ver...
Page 42 - Staging cooling-tower fans; Setting up the PID loop
Staging cooling-tower fans CNT-APG002-EN 37 ® Staging cooling-tower fans Staging applications organize individual pieces of equipment into a group to accomplish a single task. For example, several fans might be used to maintain the supply water temperature in a cooling tower. Staging appli-cations c...
Page 47 - Determining the staging points; To determine the staging points:
® Chapter 4 Applications 42 CNT-APG002-EN Determining the staging points This section describes how to find the points at which stages are turned on and off. Start with these guidelines: • To avoid having a stage turn off at the lowest extreme, always have at least one stage on at 10% of the output ...
Page 50 - Chapter 5; Troubleshooting; Troubleshooting procedure; Follow these steps to troubleshoot a PID loop:
CNT-APG002-EN 45 ® Chapter 5 Troubleshooting This chapter offers a general troubleshooting procedure and tips for spe-cific problems. Troubleshooting procedure When following this troubleshooting procedure, change only one thing at a time, then wait to see the effect the change has on the system. Fo...
Page 51 - Changing the sampling frequency
® Chapter 5 Troubleshooting 46 CNT-APG002-EN Tips for specific problems Table 17 provides tips for troubleshooting specific problems. Changing the sampling frequency The major cause of actuator cycling is time lags in the system. If a 10% change in PID output requires two minutes to affect the measu...
Page 52 - Examples
Examples CNT-APG002-EN 47 ® Examples This section presents troubleshooting scenarios from a hot-water valve application. The three examples have the same symptom but different solutions to the problem. Example 1 A hot-water valve cycles closed every few minutes. Although the space temperature remain...
Page 53 - Example 2
® Chapter 5 Troubleshooting 48 CNT-APG002-EN The application is running in a cold climate during winter, so the chilled-water valve should not open at all (because chilled water is not being used). However, it might open in the following cases: • The building automation system has information that c...
Page 55 - Example 3
® Chapter 5 Troubleshooting 50 CNT-APG002-EN Example 3 The technician experiences the same problem as in the first two exam-ples: a hot-water valve cycles closed every few minutes, and the dis-charge-air temperature swings across a range of 10°F (5.6°C). The technician graphs the discharge-air tempe...
Page 56 - Chapter 6; Frequently asked questions; Why is the output of my PID loop always zero?
CNT-APG002-EN 51 ® Chapter 6 Frequently asked questions Why is the output of my PID loop always zero? • Maximum PID output may be set to zero. • PID action setting may need to be changed. • Setpoint may be zero or negative, driving the output to zero. Change the setpoint to a reasonable value manual...
Page 57 - Why is the derivative gain usually zero?
® Chapter 6 Frequently asked questions 52 CNT-APG002-EN I tried the 4:1 ratio for proportional and integral gains, but this did not optimize my system. Can I try another ratio? We recommend maintaining a 4:1 ratio between the proportional and integral gains. Changing the gains may slightly improve t...
Page 58 - What’s the best sampling frequency?
Frequently asked questions CNT-APG002-EN 53 ® What’s the best sampling frequency? The best sampling frequency depends on the application. See “Calculat-ing the sampling frequency” on page 14 for recommended sampling fre-quencies. You may need to adjust the sampling frequency (usually to slow it down...
Page 60 - Appendix A; The math behind PID loops; Velocity model formula; Proportional control formula
CNT-APG002-EN 55 ® Appendix A The math behind PID loops This appendix presents the mathematical formulas used for PID control in Tracer MP580/581 controllers, the programmable control module (PCM), and the universal programmable control module (UPCM). Velocity model formula The formula used to calcu...
Page 62 - Glossary
CNT-APG002-EN 57 ® Glossary action A PID setting that determines how the PID loop reacts to a change in the measured variable (such as a room temperature). A controller using direct action increases the output when the measured variable increases. A controller using reverse action decreases the outp...
Page 65 - system time constant; throttling range; velocity model; windup
® Glossary 60 CNT-APG002-EN system time constant The time it takes to reach 63.21% of the difference between the start point and the end point when controlling an output over a known range. Used to calculate the sampling frequency. Using 2 / 3 (66%) rather than 63.21% provides a good approximation o...
Page 66 - Index; Numerics
CNT-APG002-EN 61 ® Index Numerics 4 to 1 ratio for gains, 11, 52 A action, 17-18 determining, 18direct, 17, 52examples, 18recommended values, 18reverse, 17, 52 actuator and error deadband, 19and PID output, 2cycling, troubleshooting, 46 aliasing, 12 applications, 29-44 building pressure control, 18,...
