Tag Archives: CODESYS

PLC Programming Patterns- The Debounced Threshold/Limit

CODESYS, PAC, PLC, Programming, ,

Patterns in PLC Programming: The threshold + timer function – Debounced Threshold

PLC programs are frequently made up of the same sets of functions or patterns of logic. These patterns perform commonly required control routines. Examples include threshold detection for alarm purposes, input/output mapping and scaling functions, to name a few. These combination of functions make up re-usable patterns or function sets that form the building blocks of a PLC program.

Repeated usage of some functions in a PLC programming environment holds true to the universal 80/20 rule ;20% of the functions available in a programming environment( such as timers and logic functions) would probably be used to build 80% of PLC programs.

The debounced alarm/threshold pattern is one of these commonly required PLC programming patterns. This pattern is probably better written in ladder diagram   (instead of any of the other 61131-3 languages), for two reasons:

  • Maintenance personnel will be able to view the status of the process/monitored variable, associated timer and output all in the same place
  • It can be programmed in 2 rungs of ladder compared to more than 10 lines of code.

The ladder diagram version could look something like this:

Alarm ACTIVE
DebouncedThreshold_LD_ALARMAutoManualReset

Note: An Auto/Manual Reset Provision is built into the Alarm Reset Timer rung.

Alarm NOT ACTIVE

DebouncedThreshold_LD

The following pattern is the debounce pattern for detecting limit breaches along with a timer, written in Structured Text for the CODESYS environment. As described above, the structured text version takes more space and possibly looks more intimidating specifically to someone who is used to the ladder environment.

CODESYS-LimitThresholdScreenCap

The plain text version of the structured text version is attached below:

PROGRAM DeBounced_Limit

VAR

irProcessValue : REAL; (*Process value scaled from analog input signal*)

rProcessLowLimit : REAL; (*User defined process value low limit alarm threshold*)

qxLowLimitAlarm : BOOL;(*Low limit alarm bit*)

LowLimitAlarmTimer : TON; (*Process value low limit alarm trigger debounce timer*)

LowLimitAlarmResetTimer : TON; (*Process value low limit alarm reset debounce timer*)

LowLimitDebounceT : TIME; (*Process value low limit alarm trigger debounce time*)

LowLimitResetDebounceT : TIME; (*Process value low limit alarm reset debounce time*)

xAutoReset : BOOL;(*User enabled alarm auto reset function*)

ixUserReset : BOOL;(*User triggered alarm reset function for manual reset*)

END_VAR

(*If value is lower than low limit, start timer, else reset timer*)

IF irProcessValue < rProcessLowLimit THEN

LowLimitAlarmTimer(in:=TRUE, PT:=LowLimitDebounceT);

ELSE

LowLimitAlarmTimer(in:=FALSE);

END_IF

(*If timer lapses, trigger alarm bit*)

IF LowLimitAlarmTimer.Q THEN

qxLowLimitAlarm:=TRUE;

END_IF

(*If value is higher than low limit, start alarm reset timer, else reset debounce timer*)

IF irProcessValue > rProcessLowLimit THEN

LowLimitAlarmResetTimer(in:=TRUE, PT:=LowLimitResetDebounceT);

ELSE

LowLimitAlarmResetTimer(in:=FALSE);

END_IF

(*If timer lapses, and system permits auto-reset, reset alarm bit*)

(*If the system does not permit auto-reset, add user invoked reset bit here*)

IF LowLimitAlarmResetTimer.Q AND (xAutoReset OR ixUserReset) THEN

qxLowLimitAlarm:=FALSE;

END_IF

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CODESYS Pointers and Dereferencing

CODESYS, PAC, PLC, Programming, ,

Pointers and Arrays in CODESYS

The CODESYS language supports the usage of pointers which is essentially a variable that stores the location of another variable or data point instead of its value. The actual value of a variable that a pointer points to can be retrieved by dereferencing the pointer.

 

What could it be used for?

       1.Storing the location of the variable saves space specifically when pointing to large                 data points or structures.

  1. Pointers allow for sorting and re-arrangement of data sequences without actually changing the location of the original data points. To characterize in a diagram:

Pointers and Arrays Diagram

  1. Going down the path of object oriented programming in PLC’s/PAC’s, this supports abstraction by separating the usage, manipulation and parts of an object from the original object itself. This helps reduce the PLC code required to handle every scenario and also helps in situation where the original object can not be manipulated because its hardcoded to an input ( for example).

 

Example:

 

Let’s say the array of data on the left side of the diagram above is read (as inputs) from a set of 4 drives controlling 4 pumps. From top of the array to the bottom Array location 0 would be for Pump 1 , location 1 for Pump 2 …..location 4 for Pump 4.

 

If the data is each of the pumps runtime, the pointers to the runtime allow for it to be sorted and stored in the pointer of arrays without changing the order of the original array which is important, as the original array of runtime data may be hardcoded to the actual input or read values from the drives.

 

Secondly, when the decision is made to call a pump to run, the same mechanism of pointers and arrays and de-referencing can be used to point to the command word of each of the pumps on a communication bus.

The end result is the amount of code required in determining which of the pumps to run is reduced.

Pump Example Pointers and Arrays

What does the syntax look like in CODESYS?

To create a pointer:

The address operator or ADR is used.

Let’s say the variables are:

 

PROGRAM PLC_PRG

VAR

j: INT;

k:INT;

pointToJ: POINTER TO INT;

END_VAR

The pointer assignment would be:

 Pointer

in plain text:

pointToJ:=ADR( j);

Note: ADR is a CODESYS operator and not an operator prescribed in IEC61131-3.

 

To de-reference a pointer:

The ^ operator is used to de-reference pointers.

Example:

in plain text:

k:=pointToJ^;

This would yield the value of integer j.

SimulatedPointerDeReferencing

 

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