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Configuring Dual Independent Y-Value Axes in WinCC Online Trend Control for Process Values and Manipulated Variables

Configuring Dual Independent Y-Value Axes in WinCC Online Trend Control for Process Values and Manipulated Variables

To configure two independent Y-value axes with distinct measurement ranges (e.g., 0–150 bar for Process Value PV and 0–100% for Manipulated Variable MV) in WinCC Online Trend Control, open the control properties in Graphics Designer, navigate to the 'Value Axis' tab, and click 'New' to create a second value axis (e.g., ValueAxis_2). Next, navigate to the 'Trends' tab, assign Trend 1 (PV) to ValueAxis_1, and assign Trend 2 (MV) to ValueAxis_2. Finally, in the dynamic properties, bind the scale limits of ValueAxis_1 to .PV_OpScale (or .PV_LR / .PV_HR) and bind ValueAxis_2 to .MV_OpScale to enable automatic independent range scaling.

Table of Contents

Executive Summary

In continuous process monitoring on WinCC Operator Stations, operators frequently need to evaluate closed-loop controller dynamics (such as PIDConL) by inspecting Process Value (PV), Setpoint (SP), and Manipulated Variable (MV) simultaneously on a single trend display. A recurring configuration roadblock occurs when engineers attempt to scale trends using block range tags (.PV_LR / .PV_HR): the scaling applies uniformly to the primary Y-axis, forcing a 0–100% control valve output to share the scale of a 0–250 bar pressure transmitter or 0–1000 °C furnace temperature. The valve curve becomes flattened and unreadable. Achieving legible, multi-variable trend visualization requires configuring dual independent value axes within the WinCC Online Trend Control and binding dedicated scale properties to each axis.

Architecture and Fundamental Mechanics

The multi-axis architecture of the WinCC Online Trend Control operates through modular graphic object assignments:

  1. Trend Control Object Hierarchy: WinCC Online Trend Control decouples Trend Curves (the actual process data series) from Value Axes (the numerical scales rendered on the left or right margins). By default, a newly inserted trend control contains only one Value Axis (ValueAxis_1). Multiple trend curves assigned to the control default to sharing this single axis.

  2. Dual-Axis Decoupling: The control supports multiple distinct Value Axes. Engineers can configure:
    - ValueAxis_1 (Left margin): Assigned to physical process variables (PV and SP), formatted in engineering units (e.g., bar, °C, m³/h).
    - ValueAxis_2 (Right margin): Assigned to actuator manipulated variables (MV), formatted as a percentage (0.0% to 100.0%).

  3. Dynamic Scaling Property Binding: In WinCC Graphics Designer, the properties of each Value Axis are exposed for dynamic tag linking:
    - ValueAxis_1 limits bind to TrendControl.ValueAxis.BeginValue $\leftarrow$ Tag.PV_OpScale#BeginValue (.PV_LR) and TrendControl.ValueAxis.EndValue $\leftarrow$ Tag.PV_OpScale#EndValue (.PV_HR).
    - ValueAxis_2 limits bind to TrendControl.ValueAxis_2.BeginValue $\leftarrow$ 0.0 (or .MV_OpScale#BeginValue) and TrendControl.ValueAxis_2.EndValue $\leftarrow$ 100.0 (or .MV_OpScale#EndValue).
    If an engineer links tags to the general trend control without specifying the second axis object, WinCC applies the scaling only to the primary axis.

Step-by-Step Implementation Procedure

Configure dual independent Y-axes in WinCC Graphics Designer following these steps:

  1. Open the Trend Picture in Graphics Designer:
    - In WinCC Explorer, open Graphics Designer.
    - Open the target process graphic or trend window (e.g., @pg_apl_trend.pdl or a custom PDL).
    - Double-click the WinCC Online Trend Control object to open its Properties dialog.

  2. Create the Second Value Axis:
    - Select the Value Axis tab.
    - Under the 'Value axes' list, click the New button.
    - Name the new axis ValueAxis_2 (or MV_Axis).
    - Set its Alignment property to Right (renders the scale on the right-hand border of the trend area).
    - Enter static default range values: Start value = 0.0, End value = 100.0.
    - Set decimal places to 1 and enter unit %.

  3. Assign Trends to Dedicated Axes:
    - Select the Trends tab in the same dialog.
    - Select Trend 1 (PV) -> in the Value axis drop-down, select ValueAxis_1.
    - Select Trend 2 (SP) -> in the Value axis drop-down, select ValueAxis_1.
    - Select Trend 3 (MV) -> in the Value axis drop-down, select ValueAxis_2.
    - Click Apply and OK.

  4. Configure Dynamic Range Property Linking:
    - In the graphic object properties window for the Trend Control, navigate to Properties -> Value Axis.
    - Locate ValueAxis_1 -> BeginValue -> configure dynamic tag binding to [Tag_Prefix].PV_OpScale#BeginValue.
    - Locate ValueAxis_1 -> EndValue -> configure dynamic tag binding to [Tag_Prefix].PV_OpScale#EndValue.
    - Locate ValueAxis_2 -> BeginValue -> configure dynamic tag binding to 0.0 (or [Tag_Prefix].MV_OpScale#BeginValue).
    - Locate ValueAxis_2 -> EndValue -> configure dynamic tag binding to 100.0 (or [Tag_Prefix].MV_OpScale#EndValue).
    - Save the PDL file.

Verification

Verify dual-axis scaling and trend alignment in WinCC Runtime:

  1. Visual Axis Inspection: Activate WinCC Runtime. Open the trend display. Confirm that the left Y-axis renders the engineering scale (e.g., 0 to 150 bar) and the right Y-axis renders the percentage scale (0 to 100%).

  2. Curve Separation: Modulate the PID loop setpoint. Confirm that the MV curve utilizes the full 0–100% vertical height of the right axis while the PV curve tracks within its 0–150 bar range on the left axis, eliminating vertical compression.

  3. Dynamic Scale Adaptation: Change the scale limits of PV_OpScale in the AS. Confirm that the left Y-axis automatically rescales in runtime while the right MV axis remains locked at 0–100%.

Limitations and Operational Precautions

Do not configure more than 3 distinct value axes on a single trend window, as multiple overlapping vertical scales severely clutter operator screens and impede rapid situational awareness. If more than 3 variables must be observed, utilize stacked trend windows or configure user-selectable trend profiles.

Engineer FAQ

Why did my scale tags only affect one axis?

The tags were bound to the default properties of ValueAxis_1. In WinCC Online Trend Control, each value axis maintains independent BeginValue and EndValue properties that must be configured individually.

Can I place the second value axis on the right side of the screen?

Yes. In the Value Axis properties tab, set the Alignment property to Right. The scale will render on the right edge of the chart canvas.

Can value axes share the same color as the corresponding trend curve?

Yes. In Value Axis properties, set the axis line and text color to match the line color of the assigned trend (e.g., green for PV, yellow for MV).

What tag holds the measurement range in APL blocks?

APL blocks store engineering measurement ranges in the structure tag .PV_OpScale (containing BeginValue and EndValue).

Can an operator toggle between shared and independent axes in runtime?

Yes. Enabling the WinCC Trend Control toolbar allows operators to click the 'Configure axes' button and switch curves between axes at runtime.

Technical References

Related Articles

Related Tags

  • Trend Control
  • WinCC
  • Value Axis
  • PIDConL
  • PV_OpScale
  • Graphics Designer
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