PID Trainer

1
DirectLOGIC PID

• Setup and troubleshooting of a PID loop.

BEGIN SETUP
TROUBLE- SHOOTING
2
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 PLC
• Select Setup
• Select PID

3
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
4
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

Configure Details
5
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
6
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
7
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
8
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
9
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.

10
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.

11
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.

12
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.

13
System in Operation

• 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
14
Ladder Code ...

• 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.

15
Configure Details

• Select from the following data sheets for
  specific details on the area you are looking for.

Algorithm
Transfer Modes
Position -vs- Velocity
Sample Rate
Forward -vs- Reverse
BMP I -vs- BMP II
SP/PV Output Format
Loop Mode
Common -vs- Independ.
Unipolar -vs- Bipolar
12-bit 15-bit 16-bit
Independ. of CPU
Configure Tab
Alarms Details
SP/PV Details
Output Details
Tuning Details
16
SP/PV Details

• Select from the following data sheets for
  specific details on the area you are looking for.

Set Point
Process Variable
Address
Address
Remote SP Pointer
Enable Limiting
Square root
Auto Transfer
SP/PV Format
Common -vs- Independ.
Unipolar -vs- Bipolar
12-bit 15-bit 16-bit
Configure Details
Alarms Details
SP/PV Tab
Output Details
Tuning Details
17
Output Details

• Select from the following data sheets for
  specific details on the area you are looking for.

Output
Address
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
18
Tuning Details

• Select from the following data sheets for
  specific details on the area you are looking for.

Gains Bias
-P- Proportional Gain
-I- Integral Gain
-D- Derivative Gain
Bias
Limits Errors
Freeze Bias
Error Sq.
Enable Deadband
Derivative Gain Limiting
Configure Details
Alarms Details
SP/PV Details
Tuning Tab
Output Details
19
Alarm Details

• Select from the following data sheets for
  specific details on the area you are looking for.

Limit Alarms
Rate of Change
Monitor Rate of Change
Limit Alarms
Alarm Hysteresis
PV Deviation
Alarm Hysteresis
Enable PV Deviation
Configure Details
Tuning Details
SP/PV Details
Alarms Tab
Output Details
20
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.

21
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.

22
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)

23
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.

24
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.

25
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)

26
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.

27
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.

28
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)

29
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.

30
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.

31
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)

32
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.

33
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.

34
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.

35
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.

36
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.

37
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.

38
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.

39
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.

40
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.

41
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.

42
Derivative Gain Limiting

• Because the Derivative Gains 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.

43
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.

44
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 its
  current value.

45
Output Address

• The Output address is determined by the starting
  PID loop
• table address. The address is V05. (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.

46
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.

47
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

48
Set Point Address

• The Set Point address is determined by the
  starting PID loop
• table address. The address is V02. (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.

49
Process Variable Address

• The Process Variable address is determined by the
  starting PID
• loop table address. The address is V03. (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.

50
Remote SP Address Pointer

• The Remote SP address pointer is determined by
  the starting PID
• loop table address. The address is V32. (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 (V05) of the Major Loop.
• Refer to the Loop Table Word Definitions chart on
  page
• 8-8 of your USER manual.

51
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.

52
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.

53
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.

54
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.

55
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.

56
Troubleshooting

• Select from the following data sheets of commonly
  seen trouble areas in PID setup and operation.

Will not switch to AUTO
No Output
Ladder Code
CPU not in RUN
Remote SP Pointer
Sample Rate
Output Limits
P-I-D Values
Algorithm
CPU not in RUN
Time Proportion Control
Auto Manual
Ladder Code
Output Auto X-fer
Incorrect or No SP/PV values
BCD to BIN
SP Limits
PV Auto X-fer
Ramp Soak
BEGIN SETUP
57
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 its 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.
58
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.
59
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.
60
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.
61
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 its 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 V00 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 V00
bit 1
62
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, V05, is being written to the correct
output channels for your process.
63
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)
64
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 CPUs, 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.
65
Will not switch to AUTO

• Ladder Code

The ladder code will switch the PID mode by
setting the appropriate bit. V00 bit 0 for
Manual and V00 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)
66
Will not switch to AUTO

• CPU Mode

The ladder code will switch the PID mode by
setting the appropriate bit. V00 bit 0 for
Manual and V00 bit 1 for Automatic.
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.
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.
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.
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.
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.
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)
67
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.
68
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.
69
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.
70
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.
71
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 CPUs, 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.
72
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.
73
Finished

• 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_at_automationdirect.com or call our tech
  support line _at_ 770-844-4200.

BEGIN SETUP
TROUBLE- SHOOTING
74
Valid Ranges
75
Number of Loops