Scaling Non-Standard Analog Output Ranges (2-10V from 0-10V Module) in PCS 7 APL (Pcs7AnOu and VlvAnL)

In SIMATIC PCS 7 with the Advanced Process Library (APL), controlling a 2-10V actuator connected to a standard 0-10V analog output module without distorting WinCC faceplate percentage displays requires proper channel driver scaling. Rather than clamping VlvAnL minimum limits to 20% (which incorrectly forces the faceplate to read 20% when the valve is fully seated), configure the Pcs7AnOu channel driver with ScaleOff = 1 and set its input scaling range Scale from -25.0% to 100.0%, or mathematically rescale the raw 5530 to 27648 peripheral count span directly using an auxiliary calculation block.

The Engineering Challenge of Offset Analog Control Signals

In industrial process control, standard electrical signal ranges include 4-20 mA current loops and 0-10V DC voltage loops. However, specialized HVAC actuators, pneumatic positioners, and legacy field control valves frequently operate on an offset voltage range of 2 to 10V DC:
* $0.0 ext{V to }2.0 ext{V}$: Reserved for live-zero wire break detection or off-limits.
* $2.0 ext{V}$: Represents 0.0% (Fully Closed).
* $10.0 ext{V}$: Represents 100.0% (Fully Open).

When interfacing these devices with a standard Siemens analog output module (such as an SM 332 AO 8x12 Bit configured in HW Config for $0 ext{ to }10 ext{V}$):
* $0.0 ext{V} = 0 ext{ raw peripheral counts}$.
* $2.0 ext{V} = 5530 ext{ raw peripheral counts}$ ($20\% ext{ of }27648$).
* $10.0 ext{V} = 27648 ext{ raw peripheral counts}$ ($100\%$).

Engineers must configure the PCS 7 control loop so that an operator commanding $0.0\%$ on the WinCC faceplate outputs exactly $2.0 ext{V}$ ($5530$ counts) to the field, while displaying true $0.0\%$ to $100.0\%$ on all HMI indicators.

Why Clamping VlvAnL Minimum Output is an Antipattern

A common initial attempt to solve this is adjusting the minimum manipulation limit on the analog valve block (VlvAnL FB 1896) or PID controller (PidConL FB 1874):
* Setting MV_Min = 20.0% prevents the block from commanding less than $20\%$.

Why This Fails in Operation

Misleading Operator Interface: On the WinCC faceplate and process graphic, when the operator commands the valve closed or when an interlock trips, the position indicator rests at 20.0%. Operators will report that the valve is stuck open and fails to close.
Controller Windup: PID controllers with a minimum limit at 20% experience integral windup and distorted anti-reset windup clamping during low-load operation.
Broken Safe State Logic: In fail-safe or de-energized conditions, the block cannot naturally distinguish between a true 0% physical closure and an offset command.

Comparison Table - Methods for Handling 2-10V Actuators

The following table compares different engineering solutions for controlling 2-10V actuators using 0-10V hardware.

MethodWinCC Faceplate DisplaysRaw Peripheral Counts at 0%Implementation MechanismRecommended StatusDriver Scaling Offset (ScaleOff = 1)0.0% to 100.0%5530 counts (2.0V)Configure Scale on Pcs7AnOu from -25% to 100%Certified Clean SolutionMathematical Rescaling (Polygon / SCL)0.0% to 100.0%5530 counts (2.0V)Linear formula: $ ext{Out} = 20.0 + (0.8 imes ext{In})$Fully FunctionalLimiting VlvAnL.MV_Min = 20.0%20.0% to 100.0%5530 counts (2.0V)Restrict manipulated variable limitStrictly Prohibited (Misleading HMI)Hardware Range Converter0.0% to 100.0%Handled in hardwareExternal signal conditioner moduleAdds hardware cost

Step-by-Step Channel Driver Scaling Configuration

The cleanest, most elegant solution within the PCS 7 APL architecture is leveraging the internal scaling parameters of the analog output driver block Pcs7AnOu (FB 1867):

Method 1: The Linear Range Offset via `ScaleOff`

In linear mathematics, if 0% to 100% of actuator stroke must map across the 20% to 100% span of the DAC converter:
$$ ext{Span Ratio} = 100\% - 20\% = 80\% = 0.8$$
$$ ext{Zero Offset} = -20\% \div 0.8 = -25.0\%$$

To implement this directly on Pcs7AnOu:
1. Open the CFC chart containing Pcs7AnOu.
2. Locate the input parameter ScaleOff (Scale Offset active). Set ScaleOff = 1.
3. Locate the composite input structure Scale:
* Set Scale.Low = -25.0.
* Set Scale.High = 100.0.
4. Connect the standard $0.0 ext{ to }100.0\%$ output of VlvAnL (MV or Out) directly to Pcs7AnOu.PV_In.
5. Resulting Calculation:
* When VlvAnL commands $0.0\%$, Pcs7AnOu calculates:
$$ ext{Percent of Span} = rac{0.0 - (-25.0)}{100.0 - (-25.0)} = rac{25.0}{125.0} = 20.0\%$$
$$ ext{Raw Counts} = 20.0\% imes 27648 = \mathbf{5530 ext{ counts}}\ (2.0 ext{V})$$
* When VlvAnL commands $100.0\%$, the calculation yields $100.0\% = \mathbf{27648 ext{ counts}}\ (10.0 ext{V})$.

Method 2: Upstream Linear Function Block (`Polygon` / `Mul04`)

If you prefer explicit chart logic:
1. Insert a standard math block or Polygon (FB 1883) between VlvAnL and Pcs7AnOu.
2. Configure the linear transfer equation:
$$ ext{Driver_In} = 20.0 + (0.8 imes ext{Valve_MV})$$
3. Keep Pcs7AnOu.Scale at standard $0.0 ext{ to }100.0\%$.
4. Both methods deliver exact physical voltages while preserving full $0-100\%$ display on operator faceplates.

Verification

Confirm proper electrical output and HMI display across the operating range:

Verify Electrical Zero (0.0% Command):
* In WinCC Runtime, command the valve to $0.0\%$ in Manual mode.
* Confirm that the WinCC faceplate indicates 0.0%.
* Measure the physical voltage at the analog output module terminals. Confirm that the multimeter reads exactly 2.00V DC ($\pm 0.02 ext{V}$).
Verify Mid-Span (50.0% Command):
* Command the valve to $50.0\%$.
* Confirm that output voltage measures exactly 6.00V DC ($2.0 ext{V} + (50\% imes 8.0 ext{V})$).
Verify Full Scale (100.0% Command):
* Command the valve to $100.0\%$.
* Confirm that output voltage measures 10.00V DC.

Limitations and Operational Precautions

Wire Break Detection on Voltage Modules: Standard 0-10V voltage output modules do not natively detect open circuits (wire breaks) as reliably as 4-20 mA current loops. If wire break surveillance is required, use 4-20 mA with a precision $500\,\Omega$ resistor at the actuator.
Module Resolution Considerations: An analog output card with 12-bit resolution provides 4096 discrete quantization steps. Over an 8V effective span, resolution is approximately 2.4 mV per count, which is more than sufficient for control valves.
Feedback Tracking: If the valve has an analog position feedback transmitter returning 2-10V, configure the input channel driver Pcs7AnIn with identical offset scaling so that feedback aligns perfectly with the manipulated variable.

Engineer FAQ

Can I configure the module in HW Config for 2-10V directly?

No. Standard Siemens analog output modules (SM 332 / ET 200M / ET 200SP) support hardware ranges for 0-10V, 1-5V, $\pm 10 ext{V}$, 0-20 mA, and 4-20 mA. The 2-10V range is not a standard hardware setting and must be handled via software scaling.

Why not use a 1-5V module range and double the voltage with an amplifier?

Adding external operational amplifiers introduces component failure points, temperature drift, and calibration overhead. Software scaling on a standard 0-10V card is far more reliable and requires zero additional hardware.

What happens to the output if the CPU enters STOP mode?

In HW Config under module properties, you can configure 'Reaction to CPU STOP'. You can set the module to output a substitute value of 5530 counts (2.0V) to hold the actuator at zero stroke during a CPU halt.

Does ScaleOff affect NAMUR NE43 quality status codes?

No. As long as the calculated peripheral counts remain within 0 to 27648, Pcs7AnOu generates healthy quality codes (ST = 16#80) without triggering diagnostic over-range alarms.

Technical References

Related Tags

Pcs7AnOu
VlvAnL
Scaling
Analog-Output
2-10V
APL

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Scaling Non-Standard Analog Output Ranges (2-10V from 0-10V Module) in PCS 7 APL (Pcs7AnOu and VlvAnL)