Configuring Continuous PID Controller PIDConL and Resolving Readback and CPI Alarms in SIMATIC PCS 7 APL

In SIMATIC PCS 7 Advanced Process Library (APL), the continuous PID controller block PIDConL (FB1870) supervises automated process loops by generating an analog manipulated variable (MV) while monitoring process variables (PV) and optional position readback (Rbk). The recurring alarm 'Low Rbk value limit violated' occurs when input parameter Rbk is connected to an uninitialized signal reading 0.0% while readback limit monitoring is active; to resolve this, disconnect Rbk if position feedback is not physically wired, disable readback alarms by setting RbkWL_MsgEn = 0, or set the warning limit (RbkWL_Lim) to -9999.0. The 'Invalid CPI' error originates from an interconnected Control Performance Monitoring block (ConPerMon, FB1805) and is normal during Manual mode because the loop is not operating under automated control; for standard PID loops, ConPerMon is optional and can be disconnected.

Operational Architecture of the PIDConL (FB1870) Function Block

Within the SIMATIC PCS 7 Advanced Process Library, PIDConL (FB1870) is the core function block used for continuous closed-loop control of process parameters (temperature, pressure, flow, level).

Operating as a standard PID controller with anti-reset windup, feedforward disturbance compensation, and gain scheduling, PIDConL manages three primary signal channels:
1. Process Variable (PV): The measured engineering parameter acquired from field transmitters via analog input driver Pcs7AnIn.
2. Setpoint (SP): The target operating value entered manually by operators (SP_Int) or supplied by automated recipe sequences (SP_Ext).
3. Manipulated Variable (MV): The calculated control output ($0.0 - 100.0\%$) transmitted to final control elements (control valves, variable speed drives, heating elements) via analog output driver Pcs7AnOu.

Understanding the Manipulated Variable Readback (Rbk) Interface

In addition to the forward output MV, PIDConL provides an input parameter designated as Rbk (Readback Value).

The Rbk interface serves a specific engineering function:
- When MV controls a modulating pneumatic control valve equipped with a smart positioner (e.g., SIPART PS2), the valve stem position may lag behind the controller command due to friction, air pressure, or actuator inertia.
- By connecting the actual valve stem position feedback (acquired via Pcs7AnIn) to PIDConL.Rbk, the controller can monitor the physical actuator response.
- PIDConL evaluates whether the physical actuator is tracking the commanded output and provides high/low limit monitoring on the readback signal itself (RbkWH_Lim, RbkWL_Lim).

Resolving 'Low Rbk Value Limit Violated' Alarms

When a control engineer connects the Rbk input in CFC during testing or connects a field transmitter that is currently de-energized, WinCC immediately raises an alarm:
$$\text{Alarm Banner: } \textbf{Low Rbk value limit violated } (RbkWL_Act = 1)$$

This occurs because connecting Rbk activates readback supervision. If the signal reads $0.0\%$ and the default warning limit RbkWL_Lim is set higher (e.g., $5.0\%$) or left unconfigured, the block asserts the warning flag RbkWL_Act.

Engineering Remedies:

If No Physical Readback Exists: If the control loop drives an electric heater or simple pump without stem position feedback, leave parameter Rbk completely unconnected. When unconnected, PIDConL disables readback limit monitoring automatically.
Disable Readback Alarm Messages:
- In CFC, set parameter RbkWL_MsgEn = 0 (disables low readback alarm messages).
- Set parameter RbkWH_MsgEn = 0 (disables high readback alarm messages).
Set Wide Engineering Boundaries:
- Set RbkWL_Lim = -9999.0 and RbkWH_Lim = +9999.0. This ensures normal operating feedback never breaches the warning thresholds.

The Role of Control Performance Monitoring (ConPerMon) and CPI

A frequent source of confusion during commissioning is the appearance of the message 'Invalid CPI' on the PID faceplate.

What is CPI?

CPI stands for Control Performance Index. It is generated by the Advanced Process Library block ConPerMon (FB1805):
- ConPerMon continuously evaluates the quality of PID control by analyzing setpoint step responses, settling times, integral error integrals, and loop oscillations.
- It calculates a statistical performance index (CPI) indicating whether the PID loop is tightly tuned, sluggish, or oscillating.

Why 'Invalid CPI' Occurs in Manual Mode:

When an operator switches PIDConL to Manual Mode (ManMod), the automated PID algorithm is disconnected, and the output MV is manipulated directly by the operator.
Because the control loop is no longer operating under closed-loop mathematical feedback, ConPerMon cannot calculate valid performance metrics. It asserts CPI_Stat = 0 (Invalid CPI), and the faceplate suppresses the index. As soon as the loop returns to Automatic Mode (AutoMod), ConPerMon resumes calculation.

For standard industrial loops that do not require plant-wide performance analytics, the ConPerMon block is completely optional and can be omitted from the CFC chart.

PIDConL Parameter Configuration and Alarm Matrix

The following matrix details key parameters governing PIDConL readback and performance monitoring:

Parameter NameData TypeDefault SettingOperational PurposePVSTRUCT (Real+Byte)0.0Process variable input from Pcs7AnInSP_Ext / SP_IntSTRUCT / REAL0.0External / Internal target setpointMVSTRUCT (Real+Byte)0.0 %Manipulated variable driving actuator via Pcs7AnOuRbkSTRUCT (Real+Byte)0.0 %Actual physical actuator position readbackRbkWL_LimREAL0.0 %Readback low warning limit thresholdRbkWL_MsgEnBOOL1 (TRUE)Set to 0 to suppress 'Low Rbk value limit' alarmsRbkWL_ActSTRUCT (Bool+Byte)0 (FALSE)Asserted 1 when readback drops below RbkWL_LimConPerMonBlock Call (FB1805)OptionalAnalyzes loop dynamics and generates Control Performance Index

Verification

To confirm that PIDConL operates without spurious readback or CPI alarms:
1. In CFC, open PIDConL in online monitoring mode.
2. In WinCC Runtime, open the PID faceplate: Confirm that 'Low Rbk value limit violated' does not appear in the alarm bar.
3. Switch the loop between Manual and Automatic modes:
- In Manual mode: Verify that the operator can adjust MV cleanly from $0.0\%$ to $100.0\%$.
- In Automatic mode: Enter a setpoint change (e.g., from $40\%$ to $50\%$); verify that the PID algorithm drives MV smoothly toward the new target.
4. Verify that the faceplate trend view plots SP, PV, and MV curves concurrently without diagnostic fault flags.

Limitations and Operational Precautions

Observe the following engineering precautions:
- Sample Time Consistency: Ensure PIDConL.SampleTime matches the cyclic interrupt task OB execution time (e.g., $100\text{ ms}$ in OB35). A mismatch between configured sample time and actual execution rate causes severe PID mathematical errors.
- Derivative Filter Time: When utilizing Derivative action (TD > 0), always configure the derivative filter time (TD_Lag > 0, typically $TD / 5$) to prevent high-frequency measurement noise from destabilizing the control valve.
- Loopback Testing Caution: If testing on a simulator, you can interconnect MV to Rbk, but ensure this loopback is removed when connecting live positioners.

Engineer FAQ

What should I connect to the Rbk pin if my valve has no position transmitter?

Leave the Rbk parameter completely unconnected. PIDConL detects that Rbk is open and disables all readback limit monitoring and alarms automatically.

Why does 'Invalid CPI' disappear when I switch to Automatic mode?

The Control Performance Index (CPI) is calculated by ConPerMon only when the controller is in Automatic mode actively controlling the process. In Manual mode, the calculation is paused because operator manual control cannot be evaluated as algorithmic PID performance.

Can I tune PIDConL using an automated tuning tool?

Yes. SIMATIC PCS 7 provides the PID Tuner utility in CFC (accessible via 'Options > PID Tuning') to automatically compute optimal Gain, TI, and TD parameters using step response analysis.