How to Sample, Latch, and Report Transient Analog Process Values in PCS 7 and WinCC

To capture and latch an analog process value (such as vessel weight or ingredient temperature) at a specific process milestone and preserve it for WinCC batch PDF reports and operator comparison, engineers can utilize APL block freezing or an SCL sample-and-hold circuit. In CFC, toggling the block enable input (EN := 0) on an APL MonAnL block immediately halts cyclic calculation, freezing the output (PV_Out) at its last valid reading indefinitely until re-enabled. To ensure the value is captured during true steady state, the SFC sequencer should evaluate the rate of change (PV_Grad < 0.01) before asserting the latch trigger. The latched value can then be archived via an event-driven archive tag or bound directly to a WinCC Report Designer layout.

The Engineering Challenge of Transient Measurement Latching

In batch processing, chemical blending, and material transfer applications, certain physical measurements are meaningful only at specific, fleeting moments in time:

Final Dispensed Weight: Once a dosing valve closes, piping draining can take 5 to 10 seconds. The true batch weight is valid only when fluid movement ceases, but before downstream discharge pumps start.
Peak Reaction Temperature: The maximum or final holding temperature reached during an exothermic cooking cycle.
Pre-Dilution Concentration: Analytical density or conductivity before water addition.

If the raw transmitter signal continues updating dynamically, subsequent operations overwrite the reading. To include this value in Electronic Batch Records (EBR) or display 'Current Batch vs Previous Batch' values on SCADA, the value must be captured, latched, and preserved.

Method 1: Freezing MonAnL Output via Block Enable (EN)

The fastest and most robust method in the Advanced Process Library (APL) is utilizing the standard block enable input (EN) on a dedicated MonAnL (Measured Value Monitoring) block:

Insert a Dedicated Monitoring Block: Place MonAnL in the CFC chart (e.g., named Weight_Hold).
Expose the EN Parameter: In the block I/O properties, set parameter EN to visible. By default, EN = 1 (block processes normally).
Sequence Control from SFC:
* While dosing: Weight_Hold.EN := 1. Weight_Hold.PV_Out tracks the live sensor value.
* Milestone reached: In the SFC step, set Weight_Hold.EN := 0.
* Result: The CPU skips execution of Weight_Hold. Its output pin PV_Out remains permanently frozen at the exact value captured at the moment of de-assertion.
* Resetting: At the start of the next batch cycle, set EN := 1 to unfreeze.

Method 2: Steady-State Gradient Detection via PV_Grad

Capturing a measurement at a fixed time delay can result in erroneous data if fluid sloshing or mechanical settling is still underway. APL blocks provide native gradient evaluation:

In MonAnL, enable gradient calculation (GradEn = 1).
The block continuously calculates the rate-of-change per minute or second, outputting PV_Grad.
In the SFC chart, configure the transition condition:
ABS(Weight_Transmitter.PV_Grad) < 0.01 AND Step_Timer > T#5s
This guarantees that the measurement is latched only when the vessel is physically at steady state, preventing inaccurate reporting caused by sloshing dynamics.

Historical Comparison: Displaying Current vs Previous Batch Values

To display both the current batch value and the previous batch value simultaneously on the WinCC operator station:

Dual Latch Architecture:
* Instantiate two monitoring blocks: MonAnL_Current and MonAnL_Previous.
Sequence Execution in SFC:
* Batch Initialization Step (Start):
MonAnL_Previous.PV_In := MonAnL_Current.PV_Out; (Copies old value into historical buffer).
* Batch Dosing Step (Active):
MonAnL_Current.PV_In := Live_Transmitter.PV_Out; (Tracks current run).
* Batch Completion Step (End):
Freeze MonAnL_Current.EN := 0.
In WinCC Graphics Designer, place two I/O fields:
* Field 1 binds to MonAnL_Current.PV_Out (Current Batch).
* Field 2 binds to MonAnL_Previous.PV_Out (Previous Batch).

WinCC Report Designer and Acyclic Archive Integration

To include the latched value in automated PDF batch reports:

Event-Driven Archiving:
In WinCC Tag Logging, configure the archive tag for Weight_Hold.PV_Out with 'Acyclic / Event-driven' acquisition, triggered by the batch completion boolean flag.
WinCC Report Designer:
* Open Report Designer -> Layouts in WinCC Explorer.
* Edit the print layout (e.g., Batch_Report.rpl).
* Insert an archive table or dynamic tag object linked to the latched variable.
Automated Print Job:
In the SFC final step, trigger a WinCC print job via C-script (ReportPrint("Batch_Report")) to generate and save the timestamped PDF report automatically.

Transient Latching Method Comparison Matrix

The following table compares methods for capturing transient process values in PCS 7:

TechniqueCFC Block UsedTrigger MechanismSteady-State DetectionResource ImpactBlock Enable FreezingMonAnL (EN=0)SFC step booleanManual or timer-basedConsumes 1 Process ObjectSCL Sample-and-HoldCustom SCL FBClock pulse / FlagProgrammed in SCLLightweight; zero extra POsGradient LatchMonAnL (PV_Grad)Rate-of-change thresholdNative dynamic checkConsumes 1 Process ObjectAcyclic Tag LoggingWinCC Tag LoggingEvent tag triggerN/A (HMI level)OS Server disk storage only

Combining gradient verification with APL block enable guarantees certified measurement accuracy.

Verification

To verify value latching and reporting:
1. In CFC test mode, monitor Weight_Hold.PV_Out while varying the simulated raw weight input.
2. Set Weight_Hold.EN = 0; continue varying the input; confirm that PV_Out remains completely frozen.
3. In WinCC Runtime, verify that the 'Current Value' display holds steady.
4. Advance the SFC to trigger the report print job; open the generated PDF report and verify that the printed value matches the frozen value exactly.
5. Initiate a new batch; verify that the old value transfers to the 'Previous Batch' field cleanly.

Limitations and Operational Precautions

Toggling EN := 0 disables all block processing, including alarm evaluation; ensure high/low safety limits are monitored on a separate, permanently enabled channel block.
When using event-driven tag logging, ensure the trigger bit remains active for at least two acquisition cycles to prevent missed logging samples.
If the AS controller is restarted while EN = 0, verify that initialization logic (OB100) does not reset the frozen value to default zero.
Ensure report destination directories have adequate disk storage to prevent PDF generation failures.

Engineer FAQ

Does freezing MonAnL via EN = 0 consume an extra Process Object (PO)?

Yes. Each instantiated MonAnL block configured with OCM consumes one AS RT Process Object license. If licensing is constrained, a custom SCL block can hold multiple values in a single block.

Can I store multiple historical batch values (e.g., last 5 batches) in CFC?

Yes. Create an SCL FIFO buffer (array of REALs) that shifts values down the stack (Value[4] := Value[3]; ...) upon each batch completion trigger.

How do I trigger a WinCC PDF print job from a CFC chart?

Create an operator message or connect an SFC boolean output to an internal WinCC tag; configure a WinCC global action to monitor the tag and execute ReportPrint('LayoutName').