PID Loop Operation (DL450 only) In This Chapter. . . .
8--2 PID Loop Operation (DL450 only) DL450 PID Control DL450 PID Control Features Along with control functions discussed in this manual, the DL450 CPU features PID process control.
PID Loop Operation (DL450 only) Number of loops Selectable, 16 maximum CPU V-memory needed 32 words (V locations) per loop selected, 64 words if using ramp/soak PID algorithm Position or Velocity form of the PID equation Control Output polarity Selectable direct-acting or reverse-acting Error term curves Selectable as linear, square root of error, and error squared Loop update rate (time between PID calculation) 0.05 to 99.99 seconds, user programmable Minimum loop update rate 0.
8--4 PID Loop Operation (DL450 only) Introduction to PID Control Maintenance and Troubleshooting PID Loop Operation (DL450 Only) What is PID Control? In this discussion, we will explain why PID control is used in process control instead of trying to provide control by simply using an analog input and a discrete output. There are many types of analog controllers available, and the proper selection will depend upon the particular application.
PID Loop Operation (DL450 only) 8--5 Maintenance DL405 User Manual, 4th Edition, Rev. A PID Loop Operation (DL450 Only) The PID controller controls a continuous feedback loop that keeps the process output (control variable) flowing normally by taking corrective action whenever there is a deviation from the desired value (setpoint) of the process variable (PV) such as, rate of flow, temperature, voltage, etc.
8--6 PID Loop Operation (DL450 only) Maintenance and Troubleshooting PID Loop Operation (DL450 Only) Introducing DL450 PID Control The DL450 is capable of controlling a process variable such as those already mentioned. As previously mentioned, the control of a variable, such as temperature, at a given level (setpoint) as long as there are no disturbances (cold water) in the process.
PID Loop Operation (DL450 only) 8--7 Standard DL405 analog input modules are used to interface to field transmitters to obtain the PV. These transmitters normally provide a 4--20mA current or an analog voltage of various ranges for the control loop. For temperature control, thermocouple or RTD can be connected directly to the appropriate module. The PID control algorithm, residing in the CPU memory, receives information from the user program, primarily control parameters and setpoints.
--8 PID Loop Operation (DL450 only) Maintenance and Troubleshooting PID Loop Operation (DL450 Only) Process Control Definitions Manufacturing Process -- the set of actions that adds value to raw materials. The process can involve physical changes and/or chemical changes to the material. The changes render the material more useful for a particular purpose, ultimately used in a final product. Process Variable -- a measurement of some physical property of the raw materials.
PID Loop Operation (DL450 only) 8--9 PID Loop Operation PID Position Algorithm PID Loop Operation (DL450 Only) The Proportional--Integral--Derivative (PID) algorithm is widely used in process control. The PID method of control adapts well to electronic solutions, whether implemented in analog or digital (CPU) components. The DL450 CPU implements the PID equations digitally by solving the basic equations in software. I/O modules serve only to convert electronic signals into digital form (or vice versa).
8--10 PID Loop Operation (DL450 only) The DL450 also combines the integral sum and the initial output into a single term called the bias (Mx). This results in the following set of equations: Mxo = Mo Mx = Ki * en + Mxn--1 Mn = Kc * en -- Kr(PVn -- PVn--1) + Mxn The DL450 by default will keep the normalized output M in the range of 0.0 to 1.0. This is done by clamping M to the nearer of 0.0 or 1.0 whenever the calculated output falls outside this range.
PID Loop Operation (DL450 only) Freeze Bias Adjusting the Bias 8--11 Maintenance DL405 User Manual, 4th Edition, Rev. A PID Loop Operation (DL450 Only) If the “Freeze Bias” option is selected when setting up the PID loop (discussed later) then the CPU simply stops changing the bias (Mx) whenever the computed normalized output (M) goes outside the interval 0.0 to 1.0.
8--12 PID Loop Operation (DL450 only) Step Bias Proportional to Step Change SP Maintenance and Troubleshooting PID Loop Operation (DL450 Only) Eliminating Proportional, Integral or Derivative Action Velocity Form of the PID Equation This feature reduces oscillation caused by a step change in setpoint when the adjusting bias feature is used.
PID Loop Operation (DL450 only) 8--13 Maintenance DL405 User Manual, 4th Edition, Rev. A PID Loop Operation (DL450 Only) Bumpless Transfer The DL450 loop controller provides for bumpless mode changes. A bumpless transfer from manual mode to automatic mode is achieved by preventing the control output from changing immediately after the mode change.
8--14 PID Loop Operation (DL450 only) Loop Operating Modes Maintenance and Troubleshooting PID Loop Operation (DL450 Only) Special Loop Calculations The DL450 loop controller operates in one of two modes, either Manual or Automatic. Manual In manual mode, the control output is determined by the operator, not the loop controller.
PID Loop Operation (DL450 only) 8--15 Error Deadband Control With error deadband control, no control action is taken if the PV is within the specified deadband area around the setpoint. The error deadband is the same above and below the setpoint. Once the PV is outside of the error deadband around the setpoint, the entire error is used in the loop calculation.
8--16 PID Loop Operation (DL450 only) Maintenance and Troubleshooting PID Loop Operation (DL450 Only) Ten Steps to Successful Process Control Modern electronic controllers such as the DL450 CPU provide sophisticated process control features. Automated control systems can be very difficult to debug, because a given symptom can have many possible causes. We recommend a careful, step-by-step approach to bringing new control loops online: The most important knowledge is -- how to produce your product.
PID Loop Operation (DL450 only) DL450 CPU V-memory Input Module Loop 1 Data Channel 1 PV Channel 2 PV SP OUT Loop 2 Data SP OUT 8--17 Output Module Channel 1 Channel 2 Channel 3 Channel 3 Channel 4 Channel 4 Process 1 Process 2 After selection and procurement of all loop components and I/O modules, we can perform the wiring and installation. Refer to the wiring guidelines in Chapter 2 of this Manual, and to the DL405 Analog I/O Module manual as needed.
8--18 PID Loop Operation (DL450 only) PID Loop Setup Some Things to Do Have your analog module installed and operational before beginning the loop setup (refer to the DL405 Analog I/O Modules Manual, D4--ANLG--M). The DL450 CPU and Know Before gets its PID loop processing instructions from V--memory tables. There isn’t a PID Starting instruction that can be used in RLL, such as a block, to setup the PID loop control. Instead, the CPU reads the setup parameters from system V--memory locations.
PID Loop Operation (DL450 only) Establishing the Loop Table Size and Location 8--19 On a PROGRAM-to-RUN mode transition, the CPU reads the loop setup parameters as pictured below. At that moment, the CPU learns the location of the loop table and the number of loops it configures. Then during the ladder program scan, the PID Loop task uses the loop data to perform calculations, generate alarms, and so on. There are some loop table parameters the CPU will read or write on every loop calculation.
8--20 PID Loop Operation (DL450 only) First type the beginning address in the PID Table Address dialog. After the address has been entered, the memory range will appear. Also, entering the number of PID loops (1 to 16) will set the total V--memory range for the number of loops entered. After the V--memory address has been entered, the necessary PID parameters for a basic loop operation for each loop can be setup with the dialogs made available.
8--21 PID Loop Operation (DL450 only) Loop Table Word Definitions Word # The parameters associated with each loop are listed in the following table. The address offset is in octal, to help you locate specific parameters in a loop table. For example, if a table begins at V2000, then the location of the reset (integral) term is Addr+11, or V2011. Do not use the word# to calculate addresses.
8--22 PID Loop Operation (DL450 only) PID Mode Setting 1 The individual bit definitions of PID Mode Setting 1 word (Addr+00) are listed in the following table.
8--23 PID Loop Operation (DL450 only) PID Mode Setting 2 The bit definitions for PID Mode Setting 2 word (Addr+01) are listed in the following table. More information about the use of this word is available later in this chapter.
8--24 PID Loop Operation (DL450 only) Mode/Alarm Monitoring Word (Addr + 06) The individual bit definitions of the Mode/Alarm monitoring word (Addr+06) are listed in the following table.
8--25 PID Loop Operation (DL450 only) Ramp/Soak Table Location (Addr + 34) V2000 V2037 Each loop that you configure has the option of using a built-in Ramp/Soak generator dedicated to that loop. This feature generates SP values in a continuous stream, called a profile. To use the Ramp/Soak feature, you must program a separate table of 32 words with appropriate values. A DirectSOFT dialog box makes this easy to do.
8--26 PID Loop Operation (DL450 only) Maintenance and Troubleshooting PID Loop Operation (DL450 Only) Configure the PID Loop Once the PID table is established in V--memory, configuring the PID loop continues with the DirectSOFT PID setup configuration dialog. You will need to check and fill in the data required to control the PID loop. Select Configure and the following dialog will appear for this process. Select the Algorithm Type Chose either Position or Velocity. The default algorithm is Position.
PID Loop Operation (DL450 only) 8--27 Select Forward/Reverse It is important to know which direction the control output will respond to the error (SP--PV), either forward or reverse. A forward (direct) acting control loop means that whenever the control output increases, the process variable will also increase. The control output of most PID loops are forward acting, such as a heating control loop. An increase in heat applied will increase the PV (temperature).
8--28 PID Loop Operation (DL450 only) Setpoint V+02 + Σ -- Control Output V+05 Loop Calculation Process Variable V+03 PID Mode 2 Setting V+01 Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Data formats 00 Select data format using bits 0 and 1.
PID Loop Operation (DL450 only) 8--29 In Cascade Mode, the loop operates just as in Automatic Mode, with one important change. The data source for the SP changes from its normal location at V+02, using the control output value from another loop (the purpose of cascading loops is covered later in this chapter). So in Auto or Manual modes, the loop calculation uses the data at V+02. In Cascade Mode, the loop calculation reads the control output from another loop’s parameter table.
8--30 PID Loop Operation (DL450 only) Maintenance and Troubleshooting PID Loop Operation (DL450 Only) SP/PV Addresses An SP/PV dialog will be made available to setup how the setpoint (SP) and the process variable (PV) will be used in the loop. If this loop is the minor loop of a cascaded pair, enter that control output address in the Remote SP from Cascaded Loop Output area. It is sometimes desirable to limit the range of setpoint values allowed to be entered.
PID Loop Operation (DL450 only) 8--31 Set Control Output Limits Another dialog that will be available in the PID setup will be the Output dialog. The control output address, V+05, (determined by the PID loop table beginning address) will be in view. Enter the output range limits, Upper Limit and Lower Limit, that will meet the requirement of the process and which will agree with the data format that has been selected.
8--32 PID Loop Operation (DL450 only) PID Loop Operation (DL450 Only) Enter PID Tuning Parameters Another PID setup dialog, Tuning, is for entering the PID parameters shown as: Gain (Proportional Gain), Reset (Integral Gain) and Rate (Derivative Gain) Recall the position and velocity forms of the PID loop equations which were introduced earlier. The equations basically show the three components of the PID calculation: Proportional Gain (P), Integral Gain (I) and Derivative Gain (D).
PID Loop Operation (DL450 only) 8--33 NOTE: You may elect to leave the tuning dialog blank and enter the tuning parameters in the DirectSOFT PID View. Derivative Gain Limiting The derivative gain (rate) has an optional gain--limiting feature. This is provided because the derivative gain reacts badly to PV signal noise or other causes of sudden PV fluctuations. The function of the gain--limiting is shown in the diagram below.
8--34 PID Loop Operation (DL450 only) Enable Deadband y When selected, the enable deadband function takes a range of small error values near zero, and simply substitutes zero as the value of the error. If the error is larger than the deadband range, then the error value is used normally. Freeze Bias The term reset windup refers to an undesirable characteristic of integrator behavior which occurs naturally under certain conditions. Refer to the figure below.
PID Loop Operation (DL450 only) 8--35 Setup the PID Alarms Although the setup of the PID alarms is optional, you surely would not want to operate a process without monitoring it. The performance of a process control loop may generally be measured by how closely the process variable matches the setpoint. Most process control loops in industry operate continuously, and will eventually lose control of the PV due to an error condition.
8--36 PID Loop Operation (DL450 only) If the process remains out of control for some time, the PV will eventually cross one of the outer alarm thresholds, named High-high alarm and Low-low alarm. Their threshold values are programmed using the loop table registers listed above. A High-high or Low-low alarm indicates a serious condition exists, and needs the immediate attention of the operator.
PID Loop Operation (DL450 only) 8--37 PV Rate--of--Change Alarm An excellent way to get an early warning of a process fault is to monitor the rate-of-change of the PV. Most batch processes have large masses and slowly-changing PV values. A relatively fast-changing PV will result from a broken signal wire for either the PV or control output, a SP value error, or other causes.
8--38 PID Loop Operation (DL450 only) PV Alarm Hysteresis The PV Absolute Value Alarm and PV Deviation Alarm are programmed using threshold values. When the absolute value or deviation exceeds the threshold, the alarm status becomes true. Real-world PV signals have some noise on them, which can cause some fluctuation in the PV value in the CPU. As the PV value crosses an alarm threshold, its fluctuations cause the alarm to be intermittent and annoy process operators.
PID Loop Operation (DL450 only) 8--39 Ramp/Soak R/S (Ramp/Soak) is the last dialog available in the PID setup. The basic PID does not require any entries to be made in order to operate the PID loop. Ramp/Soak will be discussed in another section in this chapter. PID Loop Operation (DL450 Only) Complete the PID Setup Once you have filled in the necessary information for the basic PID setup, the configuration should be saved.
8--40 PID Loop Operation (DL450 only) PID Loop Tuning Maintenance and Troubleshooting PID Loop Operation (DL450 Only) Once you have set up a PID loop, it must be tuned in order for it to work. The goal of loop tuning is to adjust the loop gains so the loop has optimal performance in dynamic conditions. The quality of a loop’s performance may generally be judged by how well the PV follows the SP after a SP step change.
PID Loop Operation (DL450 only) Manual Tuning Procedure 8--41 It is not necessary to try to obtain the best values for the P, I and D parameters in the PID loop by trial and error. Following is a typical procedure for tuning a temperature control loop which you may use to tune your loop. Monitor the values of SP, PV and CV with a loop trending instrument or use the PID View feature in DirectSOFT (see page 8--49).
8--42 PID Loop Operation (DL450 only) S S PID Loop Operation (DL450 Only) S Maintenance and Troubleshooting S S Now, return the Proportional gain to the stable response, for example, 9.7. The error, SP--PV, should be small, but not at zero. Next, add a small amount of Integral gain (reset) in order for the error to reach zero. Begin by using 80 seconds (adjust in minutes if necessary). The error should get smaller. Set the Integral gain to a lower value, such as 50 for a different response.
PID Loop Operation (DL450 only) 8--43 The foregone method is the most common method used to tune a PID loop. Derivative gain is almost never used in a temperature control loop. This method can also be used for other control loops, but other parameters may need to be added for a stable control output. Test your loop for a high PV of 80% and again for a low PV of 20%, and correct the values if necessary. Small adjustments of the parameters can make the control output more precise or more unstable.
8--44 PID Loop Operation (DL450 only) Auto Tuning Procedure The auto tuning feature for the DL450 loop controller will only run once each time it is enabled in the PID table. Therefore, auto tuning does not run continuously during operation (this would be adaptive control). Whenever there is a substantial change in loop dynamics, such as mass of process, size of actuator, etc., the tuning process will need to be repeated in order to derive new gains required for optimal control.
PID Loop Operation (DL450 only) 8--45 Open--Loop Auto Tuning During an open--loop auto tuning cycle, the loop controller operates as shown in the diagram below. Before starting this procedure, place the loop in Manual Mode and ensure the PV and control output values are in the middle of their ranges (away from the end points). NOTE: In theory, the SP value does not matter in this case, because the loop is not closed.
8--46 PID Loop Operation (DL450 only) When the loop tuning observations are complete, the loop controller computes Rr (maximum slope in %/sec.) and Lr (dead time in sec). The auto tune function computes the gains according to the Zeigler--Nichols equations, shown below: PID Tuning SP Range P = 1.2* nm/LrRr P = 0.9* nm/LrRr I = 2.0* Lr I = 3.33* Lr D = 0.5* Lr D=0 Sample Rate = 0.056* Lr Sample Rate = 0.
PID Loop Operation (DL450 only) 8--47 The following timing diagram shows the events which occur in the closed--loop auto tuning cycle. The auto tune function examines the direction of the offset of the PV from the SP. The auto tune function then takes control of the control output and induces a full--span step change in the opposite direction. Each time the sign of the error (SP y PV) changes, the output changes full--span in the opposite direction. This proceeds through three full cycles.
8--48 PID Loop Operation (DL450 only) Maintenance and Troubleshooting PID Loop Operation (DL450 Only) Use DirectSOFT 5 Data View with PID View Open a New Data View Window Open PID View The Data View window is a very useful tool which can be used to help tune your PID loop. You can compare the variables in the PID View with the actual values in the V--memory location with Data View.
PID Loop Operation (DL450 only) 8--49 The PID View will open and appear over the Ladder View which can be brought into view by clicking on it’s tab. When using the Data View and the PID View together, each view can be sized for better use as shown on the facing page. PID Loop Operation (DL450 Only) The two views are now ready to be used to tune your loop. You will be able to see where the PID values have been set and see the process that it is controlling.
8--50 PID Loop Operation (DL450 only) The diagram below illustrates how the to use the views to see the current SP, PV and Output values, along with the other PID addresses. Refer to the Loop Table Definitions page 8--21 for details of each word in the table. This is also a good data type reference for each word in the table. Scale the time axis of the viewing The trend can be cleared and window by using this input box. restarted from the left at anytime.
PID Loop Operation (DL450 only) 8--51 Using Other PID Features How to Change Loop Modes The first three bits of the PID Mode 1 word V+00 requests the operating mode of the corresponding loop. Note: These bits are mode change requests, not commands (certain conditions can prohibit a particular mode change -- see next page). PID Mode 1 Setting V+00 Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Manual Cascade Automatic The normal state of these mode request bits is “000”.
8--52 PID Loop Operation (DL450 only) Operator Panel Control of PID Modes Since the modes Manual, Auto, and Cascade are the most fundamental and important PID loop controls, you may want to “hard-wire” mode control switches to an operator’s panel. Most applications will need only Manual and Auto selections (Cascade is used in a few advanced applications). Remember that mode controls are really mode request bits, and the actual loop mode is indicated elsewhere.
PID Loop Operation (DL450 only) 8--53 Creating an Analog The DL450 does not support a built in filter, however, your analog inputs can be filtered effectively using the following RLL program example. Be sure to change the Filter in Ladder example memory locations to those that fit your application. Logic Filtering can induce a 1 part in 1000 error in your output because of “rounding”. Because of the rounding error, you should not use zero or full scale as alarm points.
8--54 PID Loop Operation (DL450 only) Use the DirectSOFT 5 Filter Intelligent Box Instruction For those who are using DirectSOFT 5, you have the opportunity to use the Analog Helper Intelligent Boxes (IBox) instructions. Following is one example which is available. IBox instruction IB--402, Filter Over Time in Binary (decimal) will perform a first--order filter on the Raw Data on a defined time interval.
PID Loop Operation (DL450 only) 8--55 Ramp/Soak Generator Introduction Our discussion of basic loop operation noted the setpoint for a loop will be generated in various ways, depending on the loop operating mode and programming preferences. In the figure below, the ramp/soak generator is one of the ways the SP may be generated. It is the responsibility of your ladder program to ensure only one source attempts to write the SP value at V+02 at any particular time.
8--56 PID Loop Operation (DL450 only) Maintenance and Troubleshooting PID Loop Operation (DL450 Only) Now that we have described the general ramp/soak generator operation, we list its specific features: S Each loop has its own ramp/soak generator (use is optional). S You may specify up to eight ramp/soak steps (16 segments). S The ramp soak generator can run anytime the PLC is in Run mode. Its operation is independent of the loop mode (Manual or Auto).
PID Loop Operation (DL450 only) 8--57 The parameters in the ramp/soak table must be user--defined. the most convenient way is to use DirectSOFT, which features a special editor for this table. Four parameters are required to define a ramp and soak segment pair, as pictured below. S Ramp End Value y specifies the destination SP value for the end of the ramp. Use the same data format for this number as you use for the SP.
8--58 PID Loop Operation (DL450 only) Ramp/Soak Table Flags Many applications do not require all 16 R/S steps. Use all zeros in the table for unused steps. The R/S generator ends the profile when it finds ramp slope = 0. The individual bit definitions of the Ramp/Soak Table Flag (Addr+33) word is listed in the following table.
PID Loop Operation (DL450 only) 8--59 The normal state for the ramp/soak control bits is all zeros. Ladder logic must set only one control bit at a time. S Start y a 0 to 1 transition will start the ramp soak profile. The CPU must be in Run Mode, and the loop can be in Manual or Auto Mode. If the profile is not interrupted by a Hold or Jog command, it finishes normally. S Hold y a 0 to 1 transition will stop the ramp/soak profile in its current state, and the SP value will be frozen.
8--60 PID Loop Operation (DL450 only) DirectSOFT Ramp/Soak Example The following example will step you through the Ramp/Soak setup. Maintenance and Troubleshooting PID Loop Operation (DL450 Only) Setup the Profile in The first step is to use Setup in DirectSOFT PID to set the profile of your process. Open the Setup PID window and select the R/S tab, and then enter the Ramp/Soak PID Setup data.
PID Loop Operation (DL450 only) 8--61 Program the Refer to the Ramp/Soak Flag Bit Description table on page 8--59 when adding the Ramp/Soak Control control rungs to your program similar to the ladder rungs below. For the example below, the PID parameters begin at V7000. The Ramp/Soak bit flags are located at in Relay Ladder V7033. PID Loop Operation (DL450 Only) Maintenance DL405 User Manual, 4th Edition, Rev.
8--62 PID Loop Operation (DL450 only) After the Ramp/Soak program has been developed in RLL, test the program. Check your profile by using PID View. If there are any changes to be made in the profile, they are made in the PID Setup R/S profile. Make the changes in Program mode then start the Ramp/Soak process again. Maintenance and Troubleshooting PID Loop Operation (DL450 Only) Test the Profile DL405 User Manual, 4th Edition, Rev.
PID Loop Operation (DL450 only) 8--63 Cascade Control Introduction Cascaded loops are an advanced control technique that is superior to individual loop control in certain situations. As the name implies, cascade means that one loop is connected to another loop. In addition to Manual (open loop) and Auto (closed loop) Modes, the DL450 also provides Cascaded Mode. NOTE: Cascaded loops are an advanced process control technique.
8--64 PID Loop Operation (DL450 only) Cascaded Loops in In the use of the term “cascaded loops”, we must make an important distinction. Only the minor loop will actually be in the Cascade Mode. In normal operation, the major the DL450 CPU loop must be in Auto Mode. If you have more than two loops cascaded together, the outer-most (major) loop must be in Auto Mode during normal operation, and all inner loops in Cascade Mode.
PID Loop Operation (DL450 only) Tuning Cascaded Loops 8--65 When tuning cascaded loops, you will need to de--couple the cascade relationship and tune the minor loop, using one of the loop tuning procedures previously covered. Once this has been done, have the minor loop in cascade mode and auto tune the major loop (see Step 4). 1. If you are not using auto tuning, then find the loop sample rate for the minor loop, using the method discussed earlier in this chapter.
8--66 PID Loop Operation (DL450 only) Time-Proportioning Control Maintenance and Troubleshooting PID Loop Operation (DL450 Only) The PID loop controller in the DL450 CPU generates a smooth control output signal across a numerical range. The control output value is suitable to drive an analog output module, which connects to the process. In the process control field, this is called continuous control, because the output is on (at some level) continuously.
PID Loop Operation (DL450 only) 8--67 The following ladder segment provides a time proportioned on/off control output. It converts the continuous output in V2005 to on/off control, using the ouptut coil, Y0. On/Off Control Program Example SP + Loop Calculation Σ -- Time Proportioning V2005 continuous PV Y0 Process P V on/off The example program uses two timers to generate on/off control.
8--68 PID Loop Operation (DL450 only) Feedforward Control Feedforward control is an enhancement to standard closed-loop control. It is most useful for diminishing the effects of a quantifiable and predictable loop disturbance or sudden change in setpoint. Use of this feature is an option available to you on the DL450. However, it’s best to implement and tune a loop without feedforward, and adding it only if better loop performance is still needed.
PID Loop Operation (DL450 only) 8--69 To change the bias (operating point), ladder logic only has to write the desired value to V+04. The PID loop calculation first reads the bias value from V+04 and modifies the value based on the current integrator calculation. Then it writes the result back to location V+04. This arrangement creates a sort of “transparent” bias term.
8--70 PID Loop Operation (DL450 only) PID Example Program After the PID loop(s) has been setup with DirectSOFT, you will need to edit your RLL program to include the rungs needed to setup the analog I/O module to be used by the PID loop(s). Maintenance and Troubleshooting PID Loop Operation (DL450 Only) Program Setup for the PID Loop DL405 User Manual, 4th Edition, Rev.
PID Loop Operation (DL450 only) 8--71 The example program shows how an analog input module, F4--08AD is used to setup a PID loop. This example assumes that the PID table for loop 1 has a beginning address of V3000. All of the analog I/O modules used with the DL450 is setup in a similar manner. Refer to the DL405 Analog I/O Manual for the setup information for the particular module that you will be using. Note that the modules used in the PID loop example program were set up for binary format.
8--72 PID Loop Operation (DL450 only) Troubleshooting Tips Q. The loop will not go into Automatic Mode. A. Check the following for possible causes: S The PLC is in Program Mode. It must be in Run Mode for loops to run. S A PV alarm exists, or a PV alarm programming error exists. S The loop is the major loop of a cascaded pair, and the minor loop is not in Cascade Mode. Maintenance and Troubleshooting PID Loop Operation (DL450 Only) Q.
PID Loop Operation (DL450 only) 8--73 Maintenance DL405 User Manual, 4th Edition, Rev. A PID Loop Operation (DL450 Only) Q. The loop Setpoint appears to be changing by itself. A. Check the following for possible causes: S The Ramp/Soak generator is enabled, and is generating setpoints. S If this symptom occurs on loop Manual-to-Auto Mode changes, the loop is in Bumpless Transfer Mode 1. S Check your ladder program to verify it is not writing to the SP location (V+02 in the loop table).
8--74 PID Loop Operation (DL450 only) Maintenance and Troubleshooting PID Loop Operation (DL450 Only) Glossary of PID Loop Terminology Automatic Mode An operational mode of a loop, in which it makes PID calculations and updates the loop’s control output. Bias Freeze A method of preserving the bias value (operating point) for a control output, by inhibiting the integrator when the output goes out-of-range. The benefit is a faster loop recovery.
PID Loop Operation (DL450 only) 8--75 PV Absolute Alarm A programmable alarm that compares the PV value to alarm threshold values. PV Deviation Alarm A programmable alarm that compares the difference between the SP and PV values to a deviation threshold value. Ramp / Soak Profile A set of SP values called a profile, which is generated in real time upon each loop calculation.
8--76 PID Loop Operation (DL450 only) Fundamentals of Process Control Theory, Second Edition Author: Paul W. Murrill Publisher: Instrument Society of America ISBN 1--55617--297--4 Application Concepts of Process Control Author: Paul W. Murrill Publisher: Instrument Society of America ISBN 1--55617--080--7 PID Controllers: Theory, Design, and Tuning, 2nd Edition Author: K.
