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PID tutorial materials from http://support.automationdirect.com.

PID tutorial materials from http://support.automationdirect.com.

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Pid Trainer Pid Trainer Presentation Transcript

  • DirectLOGIC PID
    • Setup and troubleshooting of a PID loop.
    BEGIN SETUP TROUBLE- SHOOTING
  • Initial Setup …
    • The following steps will guide you through the initial steps for setting up a PID loop in the PLC.
      • For best results, connect to your CPU and have it in Program mode.
        • Select P LC
        • Select S etup
        • Select PID
  • … initial setup
      • You should now see the ‘Set PID Table Address’ window.
        • Set the ‘Table Start Address’. Allow yourself 32 words for each complete PID table.
          • See table below for valid V-Memory ranges.
        • Assign the total ‘Number of Loops’. This will vary according to the CPU you are using.
          • See table below for max loops per CPU.
        • Once these are assigned you should select ‘Update and Exit’.
    V-Memory Ranges Max Loops Per CPU
  • Configure Details Minimum Requirements …
    • These next steps will walk you through the setup for items that are ‘required’ for a basic loop to operate.
      • We will cover the ‘Setup PID’ window, tab by tab.
      • ‘ Doc’ tab is optional.
      • ‘ Configure’ tab: see ‘Details’ for more in-depth information on each.
      • Algorithm: select the following
        • Position
          • Sample rate: 0.05 (sec.)
          • Forward acting
        • Transfer Mode:
          • Bumpless I or II is OK
        • SP / PV & Output format:
          • Common format
        • Common Data Format:
          • Unipolar data format
          • 12-bit data format
        • Loop Mode:
          • Unselected
  • Basic Setup … SP/PV
    • Basic setup requires nothing to be changed in this tab.
      • ‘ SP/PV’ tab: see ‘Details’ for more in-depth information on each.
        • Setpoint Variable: No changes required for the loop to work
          • Address: Is fixed and determined by the beginning Loop table address
          • Remote SP: Only used w/ Cascading Loops
          • Enable Limiting: This is optional
        • Process Variable: No changes required for the loop to work
          • Address: Is fixed and determined by the beginning Loop table address
          • Sq. root: Not selected. Used only in certain specific apps.
          • Auto transfer: Not selected. (good option) see ‘Details’ for more information
        • SP/PV Data Format:
          • Only selectable if ‘Independent format’
          • was selected in the Configure tab.
    SP/PV Details
  • Basic Setup … Output
    • Basic setup does require changes in this tab.
      • ‘ Output’ tab: see ‘Details’ for more in-depth information on each.
        • Output: Upper limit must be set
          • Address: Is fixed and determined by the beginning Loop table address
          • Upper Limit: Can vary, but should typically be set to match the maximum range value
          • Lower Limit: Can vary, but should typically be set to match the minimum range value
          • Auto transfer: Not selected. (good option) see ‘Details’ for more information
        • Output Data Format:
          • Only selectable if ‘Independent format’
          • was selected in the Configure tab.
    Output Details
  • Basic Setup … Tuning
    • Basic setup requires nothing to be changed in this tab. (Values are required, but can be set later in the PID view)
      • ‘ Tuning’ tab: see ‘Details’ for more in-depth information on each.
        • At least one of these variables must be set in order for the loop to calculate an output.
        • It is possible to run with only one set and there are specific applications that work better with only specific P-I-D values active.
        • A very high percentage of all processes that we see, will work well with only P-I values active.
        • Gain/Bias: Values can be set now or later. You can also Autotune from the PID window and let the process calculate the values.
          • Gain (Proportional): Will vary per process. This is a multiplier for the error.
          • Reset (Integral): Will vary per process. This is a time constant that determines the frequency that the error is added to the bias.
          • Freeze Bias: Selected. (good option)
          • see ‘Details’ for more information
          • Rate (Derivative): Will vary per process. This
          • is a compensation that reacts to sudden change.
          • Derivative gain limiting: Not selected.
        • Error: No selection required .
          • Error Squared: Specific applications only.
          • Enable Deadband: Specific applications only.
    Tuning Details
  • Basic Setup … Alarms
    • Basic setup requires nothing to be changed in this tab.
      • ‘ Alarms’ tab: see ‘Details’ for more in-depth information on each.
        • The alarms are monitor function ONLY. They have no bearing on the loop functions, unless you act upon them in the ladder.
        • Limit Alarms: No values required
          • High-High: Maximum or critical High alarm.
          • High: Minimum or non-critical High alarm.
          • Low: Minimum or non-critical Low alarm.
          • Low-Low: Maximum or critical Low alarm.
        • Monitor Rate of Change: No values required
        • Enable PV Deviation: No values required
        • Alarm hysteresis: No value required
    Alarm Details
  • Basic Setup … R/S
    • Basic setup requires nothing to be changed in this tab.
      • ‘ R/S’ tab: Ramp / Soak will be covered in a separate trainer. Please consult your manual for detailed information about the setup and implementation of the Ramp / Soak table.
        • Table Location: Determined by the user. Completely separate from the PID loop table address.
        • Ramp/Soak Enable: Not selected. Enables the setup of the ramp / Soak table.
        • Ramp: Acceleration to a specific setpoint.
          • SP: Target Setpoint
          • Slope: The time it will take to achieve the target SP.
        • Soak: Hold time.
          • Time: The duration it remains at the target SP.
          • Deviation: Error, set by you, that triggers if the PV
          • varies from the SP by more than this amount.
  • Basic Setup … Complete
    • Once you have been through all of the tabs, you are ready to save your configuration.
      • The Icons on the Setup PID window allow you to write you current configuration to the PLC and to your disk. You also have the options to read from PLC or disk if you want to load an existing configuration into a new loop.
      • As mentioned before, the ‘Doc’ tab allows you to place a description w/ your process. This is useful if there is more than one loop in your application.
      • You are now ready to run your PID loop.
  • Starting the PID ...
    • You will now want to switch the CPU to ‘RUN’ mode.
      • It is not necessary to have any ladder code at this point. The CPU does require an END statement before it will switch to RUN. We will discussed data transfer with ladder logic later in the trainer.
      • It is recommended that you open the ‘PID view’ and ‘Data View’ windows.
  • Viewing the PID ...
    • This is one option.
    • By viewing both windows, you can
    • compare your variables in the PID View with
    • the actual values in the V-memory location in
    • a Data View window.
    • Make sure you reference page 8-8 of
    • the USER manual for details on what each
    • word of the PID loop table represents.
    • With this view, you can see where the
    • P-I-D values have been set and the process is
    • controlling.
    • In the data view window, you can see
    • the current SP, PV& Output values, along with
    • the rest of the PID table addresses.
    • Make sure you reference page 8-8 of
    • the USER manual for details on what each
    • word of the PID loop table represents.
    P I D System in Operation …
    • In the Ladder you want to make sure that the
    • SP and PV are in BIN data format before they are
    • moved to the PID table address.
    • Then verify that the Output from the PID is
    • converted from BIN to BCD as needed.
    • Make sure you reference page 8-8 of the
    • USER manual for details on what each word of the
    • PID loop table represents. This table is also a good
    • reference for the data type of each word.
    Ladder Code ...
  • Position -vs- Velocity … Configure Details
    • Select from the following data sheets for specific details on the area you are looking for.
    Algorithm: Transfer Modes: SP/PV & Output Format: Loop Mode: Sample Rate Forward -vs- Reverse Common -vs- Independ. Unipolar -vs- Bipolar 12-bit 15-bit 16-bit BMP I -vs- BMP II Independ. of CPU Configure Tab Alarms Details SP/PV Details Output Details Tuning Details
  • … SP/PV Details
    • Select from the following data sheets for specific details on the area you are looking for.
    SP/PV Format: Common -vs- Independ. Unipolar -vs- Bipolar 12-bit 15-bit 16-bit Set Point: Address Process Variable: Address Remote SP Pointer Enable Limiting Square root Auto Transfer Configure Details Alarms Details SP/PV Tab Output Details Tuning Details
  • Address … Output Details
    • Select from the following data sheets for specific details on the area you are looking for.
    Output: Limits Auto Transfer Output Format: Common -vs- Independ. Unipolar -vs- Bipolar 12-bit 15-bit 16-bit Configure Details Alarms Details SP/PV Details Output Tab Tuning Details
  • -P- Proportional Gain … Tuning Details
    • Select from the following data sheets for specific details on the area you are looking for.
    Gains & Bias: Limits & Errors: -I- Integral Gain -D- Derivative Gain Freeze Bias Error Sq. Enable Deadband Derivative Gain Limiting Configure Details Alarms Details SP/PV Details Tuning Tab Output Details Bias
  • … Alarm Details
    • Select from the following data sheets for specific details on the area you are looking for.
    Limit Alarms: Alarm Hysteresis: Alarm Hysteresis Rate of Change: Monitor Rate of Change Configure Details Tuning Details SP/PV Details Alarms Tab Output Details PV Deviation: Enable PV Deviation Limit Alarms
  • Algorithm :
    • Position -vs- Velocity Profile
    • A high percentage of all applications are Position.
    • This will include your standard heating and cooling
    • loops and most position or level controls.
    • A typical Velocity control would consist of a
    • process variable like a flow totalizer. As the definition
    • explains, it is working off of a “rate of change.”
  • Sample Rate:
    • Typically you will find that a faster sample rate
    • will give you better results in your process. One of the
    • only reasons for extending your sample time is if your
    • PID process is not that critical and your CPU scan is a
    • higher priority.
  • Forward/Reverse Acting:
    • You will find that everyone defines this part of the control differently. One systems Forward (direct) acting may be completely opposite from another. So it is very important to read this section to determine how you intend on your process to react.
    • Forward or Direct acting is also known as a heating loop. The greater the error (SP-PV), the greater the output will be.
    • If you manually increase the output, the PV will increase. (forward)
    • Reverse acting is, of course, right the opposite.
    • The greater the error the less the output.
    • If you manually increase the output, the PV will
    • decrease. (reverse)
  • Bumpless Transfer Modes:
    • This is intended to keep your system from slamming the
    • output in one direction upon initial startup. If a system has
    • been off and the error (SP-PV) is great, it can cause your
    • output to jump full on or full off, in turn shocking the system
    • or damaging the output device. This is designed to keep
    • that from happening.
    • The different modes allow you to select how much
    • suppression you want to place on your process.
    • Note: Bumpless Transfer I will always set the SP equal to the
    • PV on a Manual to Auto mode change. If you do not want this
    • feature, select Bumpless Transfer II.
  • Common -vs- independent:
    • This allows you to configure the PID more closely with your actual process. You may have a 16-bit resolution input module while your output module is only 12-bit resolution.
      • By selecting Independent Format, you can set the SP/PV separately from the Output. This selection grays out the format selection in the Configure Tab and enables the selection in the SP/PV tab as well as the Output Tab
      • The Common Format will allow you to select one format for the entire process, SP/PV & Output. This selections grays out the format selection in the SP/PV tab as well as the Output Tab and enables the selection in the Configure Tab.
  • Unipolar -vs- Bipolar:
    • Unipolar and Bipolar selection is pretty straight forward.
      • Unpolar is values above zero. (ex. 0 to 4095)
      • Bipolar is positive & negative values. (ex. -4095 to +4095)
  • 12-15-16-Bit Resolution:
    • The resolution you select will be determined by the
    • modules that are supplying the SP/PV and the module
    • receiving the Control Output or the data value range that
    • your process needs to control.
    • Most of our Analog modules are 12-bit resolution.
    • The exception would be the THM & RTD temperature
    • modules and isolated output modules which are 16-bit.
  • Common -vs- independent:
    • This allows you to configure the PID more closely with your actual process. You may have a 16-bit resolution input module while your output module is only 12-bit resolution.
      • By selecting Independent Format, you can set the SP/PV separately from the Output. This selections grays out the format selection in the Configure. Tab and enables the selection in the SP/PV tab as well as the Output Tab
      • The Common Format will allow you to select one format for the entire process, SP/PV & Output. This selections grays out the format selection in the SP/PV tab as well as the Output Tab and enables the selection in the Configure Tab.
  • Unipolar -vs- Bipolar:
    • Unipolar and Bipolar selection is pretty straight forward.
      • Unpolar is values above zero. (ex. 0 to 4095)
      • Bipolar is positive & negative values. (ex. -2047 to +2047)
  • 12-15-16-Bit Resolution:
    • The resolution you select will be determined by the
    • modules that are supplying the SP/PV and the module
    • receiving the Control Output or the data value range that
    • your process needs to control.
    • Most of our Analog modules are 12-bit resolution.
    • The exception would be the THM & RTD temperature
    • modules and isolated output modules which are 16-bit.
  • Common -vs- independent:
    • This allows you to configure the PID more closely with your actual process. You may have a 16-bit resolution input module while your output module is only 12-bit resolution.
      • By selecting Independent Format, you can set the SP/PV separately from the Output. This selections grays out the format selection in the Configure. Tab and enables the selection in the SP/PV tab as well as the Output Tab
      • The Common Format will allow you to select one format for the entire process, SP/PV & Output. This selections grays out the format selection in the SP/PV tab as well as the Output Tab and enables the selection in the Configure Tab.
  • Unipolar -vs- Bipolar:
    • Unipolar and Bipolar selection is pretty straight forward.
      • Unpolar is values above zero. (ex. 0 to 4095)
      • Bipolar is positive & negative values. (ex. -2047 to +2047)
  • 12-15-16-Bit Resolution:
    • The resolution you select will be determined by the
    • modules that are supplying the SP/PV and the module
    • receiving the Control Output or the data value range that
    • your process needs to control.
    • Most of our Analog modules are 12-bit resolution.
    • The exception would be the THM & RTD temperature
    • modules and isolated output modules which are 16-bit.
  • Limit Alarms:
    • The limit alarms give you a good way to monitor your system. If the process gets out of control, you can have the PLC let you know. You can have a visual alarm in the PID view as well as using the PID mode word to trigger bits and react upon those in the ladder.
  • Monitor Rate of Change:
    • This calculation works off of the amount that the
    • PV has changed in one sample time period. You set the
    • ‘ maximum’ rate of change you would expect in your
    • process and if it exceeds this value you will get an alarm
    • notification.
  • PV Deviation:
    • The PV deviation alarms are a good way
    • to monitor a stable process. With this you can
    • determine if the process is going out of control.
  • Alarm Hysteresis:
    • This works in conjunction with other
    • alarm settings to give you a way to omit
    • nuisance alarms generated by erratic fluctuations
    • in the process.
  • Proportional Gain:
    • The Proportional Gain is the most widely used factor of
    • the three values in the algorithm. It is basically a multiplier.
    • Take the error (SP-PV) and multiply it times the ‘P’ value and
    • that is your output.
  • Integral Gain:
    • The Integral Gain is calculated by the error (SP-PV). The
    • value of time in this setting determines how often the current
    • error is added to the Bias.
  • Derivative Gain:
    • The Derivative Gain is the least likely to be used of all the
    • gains. This is basically a compensator. It compares the current
    • error (SP-PV) with the previous error and adjust accordingly.
    • The majority of all processes we find can be controlled
    • with a P-I loop to required standards.
  • Freeze Bias:
    • Freeze Bias is an excellent option in processes that are slow to
    • respond. In slower applications, the the Output will climb to the max
    • value before the Process Variable reaches the Set Point. During this
    • time the Bias term will continue to increment. Once the PV crosses
    • the SP, the Bias must ‘unwind’ before the output will start to drop.
  • Bias:
    • The Bias term, like the Control Output, is a calculated value that
    • is determined by the algorithm. The Bias begins with the initial Output
    • value and builds from there. On each Manual to Auto transition, you will
    • see the Bias is set equal to the current output. The result of the integral
    • term is then added to the Bias.
  • Derivative Gain Limiting:
    • Because the Derivative Gain’s main purpose is to compensate
    • for a drastic change in the process, this makes it vulnerable to signal
    • noise in the PV. This limit allows you to place a ‘clamp’ on the
    • Derivative gain so the process does not get out of control.
  • Error Squared:
    • You will find that this is typically not used. It has specific applications
    • where it comes into play and that is only when you will see it used.
  • Error Deadband:
    • The Error Deadband is an area that is equally above and below the
    • Set point. While the PV is within this area the error is equal to 0.
    • Once beyond this area the error assumes it’s current value.
  • Output Address:
    • The Output address is determined by the starting PID loop
    • table address. The address is V+05. (V = starting loop address)
    • value is fixed and can not be changed.
    • Refer to the Loop Table Word Definitions chart on page
    • 8-8 of your USER manual.
  • Output Limits:
    • These limits are important in the PID function. The upper limit is mandatory
    • before an actual output will be generated. If there is an output upper limit of zero,
    • as soon as the PID algorithm begins to calculate it thinks it has reached its max
    • limit and shuts off.
    • The values are not read on-the-fly. Meaning that if you try to write to these
    • location it will not acknowledge the data until the next PLC mode change.
  • Auto Transfer to Output module:
    • Simple and easy to setup. Just select the slot of the CPU base the analog output
    • module is in and select the channel on the card that this loop will be controlling.
    • This is a very nice feature but should only be used if you do not plan
    • on using any of the channels of the analog card for any other reason except
    • for PID outputs.
    • This, like any other setup method for the analog card,
    • is telling the CPU how to write data to the module. If this is
    • selected in conjunction with another programming method
    • it will cause conflicts in the CPU memory.
    • Note: This option is not available in the D4-450 CPU
  • Set Point Address:
    • The Set Point address is determined by the starting PID loop
    • table address. The address is V+02. (V = starting loop address)
    • value is fixed and can not be changed.
    • Refer to the Loop Table Word Definitions chart on page
    • 8-8 of your USER manual.
  • Process Variable Address:
    • The Process Variable address is determined by the starting PID
    • loop table address. The address is V+03. (V = starting loop address)
    • value is fixed and can not be changed.
    • Refer to the Loop Table Word Definitions chart on page
    • 8-8 of your USER manual.
  • Remote SP Address Pointer:
    • The Remote SP address pointer is determined by the starting PID
    • loop table address. The address is V+32. (V = starting loop address)
    • This is ONLY used in Cascade loop mode. If this address is setup
    • for normal loop operation, you could experience trouble with this or an
    • adjacent loop not going into Auto mode.
    • In the Minor loop setup , the Remote SP Address Pointer is set to the
    • Control Output (V+05) of the Major Loop.
    • Refer to the Loop Table Word Definitions chart on page
    • 8-8 of your USER manual.
  • Enable Limiting:
    • This allows you to put a Min. & Max. limit on the set point
    • for applications that require this to remain within a certain spec.
    • Refer to the Loop Table Word Definitions chart on page
    • 8-8 of your USER manual.
  • Auto Transfer from Input module:
    • Simple and easy to setup. Just select the slot of the CPU base the analog input
    • module is in and select the channel on the card that this loop will be reading.
    • This is a very nice feature but should only be used if you do not plan
    • on using any of the channels of the analog card for any other reason except
    • for PID PV inputs.
    • This, like any other setup method for the analog card, is
    • telling the CPU how to read data from the module. If this is
    • selected in conjunction with another programming method
    • it will cause conflicts in the CPU memory.
    • Note: This option is not available for the D4-450 CPU.
  • Square Root:
    • You will find that this is typically not used. It has specific applications
    • where it comes into play and that is only when you will see it used.
  • Independent of CPU mode:
    • This is selected only in processes where the PID control needs to
    • continue to operate even if the CPU is not in RUN mode. This feature
    • requires the selection of Auto Transfer to/from I/O module.
    • Note: This option is not supported in the D4-450 CPU.
  • Terms:
    • What is PID control?
    • This will give you a brief overview of the PID operation
    • and some of the commonly used terms.
    • Basically PID control is the cruise control on your car. It is the method
    • of controlling a process around a specific point with the least amount of
    • fluctuation as possible.
    • A good example of what is not PID control is your household heating
    • and cooling thermostat system. This is on full or completely off, very little
    • precision control
    • When would I use PID control?
    • Use PID control when you have a process that needs to operate around a
    • specific value and you can pull a process variable from it to monitor.
  • Troubleshooting
    • Select from the following data sheets of commonly seen trouble areas in PID setup and operation.
    No Output: Output Limits P-I-D Values Time Proportion Control Will not switch to AUTO: Ladder Code CPU not in RUN Remote SP Pointer Incorrect or No SP/PV values: BCD to BIN SP Limits PV Auto X-fer Algorithm Auto Manual Ladder Code CPU not in RUN Output Auto X-fer Ramp Soak Sample Rate BEGIN SETUP
  • No Output
    • Output Limits
    In the Output Tab of the PID setup, you will see an Upper and Lower Limit. Verify that the upper limit has been set to a value greater than zero. An Upper Limit of zero will cause the PID to think it has reached it’s maximum value before it gets started. In cases where the output value needs to be restricted, it may be best to leave these limits at the output full range (ex. 0-4095 for 12-bit) and scale them in the ladder code.
  • No Output
    • P-I-D Values
    In the Tuning Tab of the PID setup, you will see the Proportional, Integral & Derivative gains. To get an output from the PID algorithm, you must have a value in at least one of these variables. You can also set these values from the PID View window. From here you can set and adjust the values while your process is running. This will allow you to manually tune the loop. In many cases you may only need one or two of the variables set to obtain the loop control that your process requires. P-I loops are among the more common process control configurations seen. What combination is best for your application? This can ONLY be determined by running the process to see how it will react.
  • No Output
    • PID Algorithm
    In the Configure Tab of the PID setup, you have the option to select Position -vs- Velocity algorithm as well as Forward Acting -vs- Reverse Acting. 90% or more of the applications that we have encountered are Position Algorithm. If your process is designed for one and you have the other selected you can expect to get unpredictable results at best. Forward & Reverse Acting loops are defined differently by different people. Make sure you understand the meaning of each before selecting one for your process. Forward Acting is also referred to as a ‘heating loop’ where Reverse acting may be referred to as a ‘cooling loop’.
  • No Output
    • CPU Mode
    In the Configure Tab of the PID setup, you will see a selection for Independent of CPU mode. If this is not selected the CPU must be in RUN mode before the PID will function. Typically this is not selected because you will need the processor in run to adequately filter the inputs from the process into the PID and allow the PID output to effect the process. There are cases where the PID does need to operate even if the rest of the process is not active. This is really the only reason for the Independent mode to be selected.
  • No Output
    • Auto, Manual
    The mode of the PID loop can cause the Output not to function. You can set the PID mode through the PID View or in the Ladder code. The ladder code ALWAYS takes precedence over any manual changes you try to make. In Manual mode the Output will hold it’s last state. This is the only mode which will allow you to actually write a value to the Output V-memory location. Manual mode is set by V+00 bit 0. Auto mode is the common operating state of the PID loop. In this mode the Output is determined by the PID algorithm. Auto mode is set by V+00 bit 1
  • No Output
    • Ladder Code
    The ladder code ALWAYS takes precedence over any manual changes you try to make. Check the ladder code to verify that the PID Output, V+05, is being written to the correct output channels for your process.
  • No Output
    • Time-Proportioning Control
    If you are using Time-Proportioning Control, also known as on/off control, then you will need to verify that the programming code has been entered into the ladder. You can find an example of this on page 8-52 of your USER manual. (pages may vary depending on manual and manual revision)
  • No Output
    • Auto Transfer to output module
    In the Output tab of the PID set you will find the option to select the auto transfer to I/O module. If this has been selected you will need to removed any analog module setup in your current code AND CPU memory. Auto transfer is one of many options for telling the CPU how to write data to the analog output modules. You can only use one method at a time. If you had previously been using the pointer method for setting up analogs in the 05, 06, 205 or 350 CPU’s, the pointer setup may remain in the CPU memory even if you remove the setup code from the ladder. You will need to clear the corresponding V-memory ranges before the auto transfer will function. Take into consideration that if you select the auto transfer that you can NOT use these other programming methods for the analog cards and this may restrict the use of the remaining channels.
  • Will not switch to AUTO
    • Ladder Code
    The ladder code will switch the PID mode by setting the appropriate bit. V+00 bit 0 for Manual and V+00 bit 1 for Automatic. If you are trying to change the PID mode from the PID View and it will not, chances are that the code is telling it to do something else. The PID is running asynchronies to the CPU scan. Therefore, it is possible for the PID to not see a mode request change in one CPU scan. Page 8-20 of the USER manual shows an example of how we recommend you change the modes. By using a Store Positive Differential (one shot contact) to energize a SET coil of the appropriate mode bit, we guarantee the PID to see the bit go high and the PID will in turn reset the mode bit for you. (pages may vary depending on manual and revision)
  • Will not switch to AUTO The ladder code will switch the PID mode by setting the appropriate bit. V+00 bit 0 for Manual and V+00 bit 1 for Automatic. If you are trying to change the PID mode from the PID View and it will not, chances are that the code is telling it to do something else. The PID is running asynchronies to the CPU scan. Therefore, it is possible for the PID to not see a mode request change in one CPU scan. Page 8-20 of the USER manual shows an example of how we recommend you change the modes. By using a Store Positive Differential (one shot contact) to energize a SET coil of the appropriate mode bit, we guarantee the PID to see the bit go high and the PID will intern reset the mode bit for you. (pages may vary depending on manual and revision)
    • CPU Mode
    In the Configure Tab of the PID setup, you will see a selection for Independent of CPU mode. If this is not selected the CPU must be in RUN mode before the PID will function. Typically this is not selected because you will need the processor in RUN to adequately filter the inputs from the process into the PID and allow the PID output to effect the process. There are cases where the PID does need to operate even if the rest of the process is not active. This is really the only reason for the Independent mode to be selected.
  • Will not switch to AUTO
    • Remote Set Point Pointer
    The Remote Set Point Pointer is ONLY used for Cascade mode operation. If there is a Remote set point pointer selected, the loop is looking for the Cascaded loop to be in auto mode also. If it does not see the this it will not allow this loop to function. The Major loop of the cascaded pair will not go into Auto mode unless the Minor loop is in Cascade mode.
  • Will not switch to AUTO
    • Sample Rate
    The Sample Rate determines how often the PID loop is updated. If you have a long scan time, it may take the PID a while to recognize that you have requested a mode change.
  • Incorrect or no SP/PV values
    • BCD to BIN
    The data type of the values being written to the Set Point and the Process Variable is an area where oversights are commonly made. The data type of these variables is required to be in Binary format for the PID algorithm. The default data type for all of our processors is BCD. Therefore, it is required that you convert these values from BCD to BIN before writing the data to the PID loop. See page 8-8 of the USER manual for the Loop Table Word Definitions. This gives you a column that specifies the data type for each word.
  • Incorrect or no SP/PV values
    • Set Point Limits
    In the SP/PV tab of the PID setup, you will see a selection for ‘Enable Limiting’. This allows you to place upper & lower limits on the Set Point variable. If the limits are inadvertently set to values that are not within the boundaries of the possible set point entries, you will see the entered values being overwritten.
  • Incorrect or no SP/PV values
    • Auto Transfer from I/O module
    In the SP/PV tab of the PID Setup, you will find the option to select the auto transfer from I/O module. If this has been selected you will need to removed any analog module setup in your current code AND CPU memory. Auto transfer is one of many options for telling the CPU how to read data from the analog input modules. You can only use one method at a time. If you had previously been using the pointer method for setting up analogs in the 05, 06, 205 or 350 CPU’s, the pointer setup may remain in the CPU memory even if you remove the setup code from the ladder. You will need to clear the corresponding V-memory ranges before the auto transfer will function. Take into consideration that if you select the auto transfer that you can NOT use these other programming methods for the analog cards and this may restrict the use of the remaining channels.
  • Incorrect or no SP/PV values
    • Ramp / Soak
    If a Ramp / Soak table has been set up and enabled, it now has control of the set point. You would be able to write to this value between steps, but it will revert back to the presets on the next ramp step.
    • You have just successfully completed the setup of a DirectLOGIC PID loop. If you experience problems with the loop operation, see the Troubleshooting section of this trainer.
    • If you have further setup or troubleshooting questions, feel free to contact us by e-mail support@ automationdirect .com or call our tech support line @ 770-844-4200.
    ** Finished ** BEGIN SETUP TROUBLE- SHOOTING
  • Valid Ranges:
  • Number of Loops: