Configuring Negative Gain and Direct-Reverse Action on APL PIDConL Controllers

In SIMATIC PCS 7 Advanced Process Library (APL), the standard continuous PID controller PIDConL (FB1874) restricts the proportional gain (Gain) parameter to positive values (and zero) on the standard operator faceplate to prevent accidental control inversion that could cause process runaway. To implement negative effective gain for direct-acting loops (where controller output must increase when the process variable exceeds setpoint), engineers should configure the boolean parameter NegGain = 1 in the Continuous Function Chart (CFC). To allow operators to toggle control direction from the OS, NegGain can be connected to an operator dialog block (OpDi01) linked to the controller faceplate via SelFp1, or the faceplate low limit can be customized.

Proportional Gain Architecture in APL PIDConL

In process control engineering, closed-loop controllers must accommodate both reverse-acting processes (where increasing the manipulated variable increases the process variable, such as heating or filling) and direct-acting processes (where increasing the manipulated variable decreases the process variable, such as cooling or draining).

In the SIMATIC PCS 7 Advanced Process Library, the continuous PID controller PIDConL (FB1874) calculates its proportional control action using the standard algorithm:

$$ER = SP - PV$$
$$MV_P = Gain \cdot ER$$

While mathematical definitions allow Gain to be negative, Siemens deliberately clamps the operator input field Gain in the standard WinCC parameter faceplate (@PG_PIDConL_Parameter.pdl) to positive values (AnaLinkLoLim = 0.0). This safety restriction prevents plant operators from inadvertently entering a negative gain value, which would invert closed-loop negative feedback into positive feedback and drive control valves to hard saturation.

Direct vs Reverse Acting Control and NegGain

To support direct-acting control loops cleanly without compromising operator safety limits, APL provides the dedicated boolean configuration parameter NegGain:

Parameter NameData TypeDefault ValueFunctional Role in PIDConLGainREAL1.0Proportional gain magnitude (strictly positive on faceplate)NegGainBOOLFALSESign inversion: 0 = Reverse-acting ($Gain > 0$); 1 = Direct-acting ($Gain < 0$)Gain.LowLimitREAL0.0Internal parameter lower boundary applied by APL faceplateSelFp1 / SelFp2STRUCTUnconnectedAuxiliary faceplate call input; opens companion operator dialogsPropFacREAL1.0Proportional action weighting factor in setpoint path

When NegGain = 1 (TRUE), the internal calculation engine multiplies the positive Gain value by -1.0, effectively reversing controller action while keeping the numerical Gain display positive on operator screens.

Method 1: Routing NegGain via OpDi01 and SelFp Faceplate Link

The recommended, guardrail-compliant method to grant operators or instrument engineers controlled access to NegGain without modifying system @ faceplates is through a secondary operator block:

Insert OpDi01 in CFC: In the same chart as PIDConL, insert an OpDi01 (Digital Operator Value) block from the APL library.
Disable Independent Block Icon: In the block properties of OpDi01, uncheck Create block icon so an unnecessary icon is not created on process pictures.
Connect Control Signals:
- Interconnect OpDi01.Out.Value to PIDConL.NegGain.
- Interconnect OpDi01.Out (or instance reference) to PIDConL.SelFp1.
Configure Operator Permissions:
- In Process Object View, set the authorization level for OpDi01 to authorization level 6 (Higher Process Control) or level 11 (Process Tuning), ensuring ordinary operators cannot inadvertently invert the gain.
Runtime Operation: In WinCC Runtime, clicking the auxiliary button on the PIDConL faceplate opens the OpDi01 dialog, allowing authorized personnel to toggle between Direct and Reverse action.

Method 2: Customizing Faceplate Input Limits in Graphics Designer

If a facility's corporate standard requires directly entering negative values in the Gain field:

Locate the parameter faceplate file in the OS project: @PG_PIDConL_Parameter.pdl.
Open the file in WinCC Graphics Designer.
Select the Gain I/O input field object and inspect its Object Properties.
Locate the property link for the lower limit: change AnaLinkLoLim from 0.0 to -100.0 (or -3.402823e+38).
Critical Maintenance Step: In WinCC Explorer, open the OS Project Editor. Under the Basic Data tab, ensure that @PG_PIDConL_Parameter.pdl is deselected or marked for preservation; otherwise, future executions of the OS Project Editor will overwrite the customization with default system files.

Verification

To verify direct-reverse action and negative gain behavior:
1. Open the CFC chart online and monitor PIDConL.
2. Verify that when NegGain = 0, increasing SP above PV produces a positive error ($ER > 0$) and drives output MV upward.
3. Toggle NegGain = 1; confirm that the controller inverts its output, driving MV downward when SP exceeds PV.
4. Open the PIDConL faceplate in WinCC Runtime and confirm that operating limits prevent entry of invalid values unless explicitly permitted through configured authorization levels.

Limitations and Operational Precautions

Never invert controller gain on a live, unbypassed safety or critical process loop without putting the controller into Manual mode (ManModLi = 1). Toggling NegGain while in Automatic mode can cause an instantaneous step change in manipulated variable output (MV), inducing thermal or hydraulic shock to downstream equipment.

Engineer FAQ

Why does the faceplate reject negative numbers when the manual states Gain is REAL?

While the S7-400 CPU firmware supports negative IEEE 754 floating point numbers for Gain, the WinCC APL faceplate enforces a lower limit of 0.0 as an operational safety interlock against accidental control inversion.

Does toggling NegGain cause a bump in controller output?

If the controller is in Automatic mode with an active control deviation ($ER \neq 0$), toggling NegGain inverts the sign of the proportional error, causing an immediate step change in output. Always switch to Manual before changing NegGain.

Can NegGain be controlled dynamically by recipe parameters?

Yes. NegGain can be interconnected in CFC to batch phase outputs (IEPH / IEOP) or recipe state words for processes that switch between heating and cooling.