Tuning PID Controllers in PCS 7 APL and CEMAT Systems

To calculate and tune PID parameters for industrial loops (such as coal mill fan dampers, raw mill dampers, and furnace draft controllers) in SIMATIC PCS 7, utilize the integrated Siemens PCS 7 PID-Tuner software tool. In Continuous Function Chart (CFC), the PID-Tuner injects controlled step changes into the process manipulated variable to identify the plant's dynamic response (process gain, delay time, time constants) and automatically calculates optimal proportional gain (Gain), integral action time (TI), and derivative time (TD). For CEMAT installations (e.g., PCS 7 V7.0 with CEMAT), continuous closed-loop control is executed using either the standard CTRL_PID block or APL PIDConL block, both of which are fully supported by the PID-Tuner.

Industrial Loop Characteristics in Heavy Material Processing

In cement manufacturing, power generation, and bulk processing plants, damper controls for coal mills, raw mills, and primary combustion air fans represent demanding control loops. These processes exhibit significant non-linearities, transport delays (dead time), and strong aerodynamic coupling: opening a damper affects mill draft pressure, air velocity, and temperature simultaneously.

Operating with poorly tuned PID parameters leads to damper 'hunting'—where the mechanical actuator continuously oscillates, causing rapid mechanical wear on linkage gears, thermal fluctuations in burner flame profiles, and excessive electrical power draw. Properly tuning PID parameters establishes smooth, critically damped response without overshoot.

The Role of the Integrated PCS 7 PID-Tuner Tool

SIMATIC PCS 7 includes the PCS 7 PID-Tuner, an automated mathematical tuning package integrated directly into the CFC editor:

Identification Phase: In manual mode or closed-loop mode, the tool applies a controlled step change to the manipulated variable (MV / P_Output) and records the process value (PV) curve over time.
Process Modeling: The software fits the measured curve to an empirical transfer function (First Order Plus Dead Time - FOPDT or higher-order PTn models), extracting process gain ($K_p$), time constant ($T_1$), and dead time ($T_t$).
Parameter Optimization: The tool applies recognized tuning criteria (such as Chien-Hrones-Reswick or Lambda tuning) to calculate recommended controller parameters: proportional gain (Gain), integral reset time (TI), and derivative time (TD).

PID Parameter Tuning in CEMAT vs APL Environments

In installations running the specialized CEMAT library (such as PCS 7 V7.0 SP1 or V8.2 with CEMAT Minerals):

Actuator vs Closed-Loop Split: CEMAT provides drive and positioning blocks (C_DRV_1D, C_DAMPER) to handle mechanical motor commands, run feedback, torque switches, and pulse-width positioning. However, the continuous analog control loop calculating the damper target position is handled by standard PID blocks.
Supported PID Blocks: In PCS 7 V7.0, continuous control utilizes block CTRL_PID (FB61) from the PCS 7 Standard Library. In modern PCS 7 V8.x/V9.x, it utilizes block PIDConL (FB1874) from the Advanced Process Library. The PCS 7 PID-Tuner supports both block architectures seamlessly.

Comparison of Controller Tuning Methods in PCS 7

The table below compares tuning methodologies for industrial fan damper and flow control loops.

Tuning MethodologyMathematical AlgorithmTuning Procedure in PlantEngineering EffortSuitability for Mill DampersPCS 7 PID-Tuner (Auto)Chien-Hrones-Reswick (CHR) / Ziegler-NicholsInjected Step Response via CFC ToolLow (Automated curve fitting)Optimal: Highly reliable, prevents damper huntingEmpirical Trial-and-ErrorManual adjustment by operatorOnline tweaking of Gain and TI in faceplateHigh (Risk of process oscillation)Poor (Can trigger mechanical damper oscillation)Lambda TuningZero-overshoot pole placementMeasured step test calculated in spreadsheetMediumExcellent for coordinated mill air balancingZiegler-Nichols Ultimate GainContinuous cycling at stability limitDrive process to sustained oscillationHigh (Risky in active production)Unacceptable for live operating mills

Step-by-Step Procedure to Tune PID Loops with PID-Tuner

Follow this procedure in CFC to tune a damper control loop using the PCS 7 PID-Tuner:

Prepare the Control Loop:
- In CFC, ensure the PID block (PIDConL or CTRL_PID) is placed in a cyclic organization block (typically OB35, 100ms).
- Confirm that the process value (PV_In) and feedback position signals are noise-free and filter parameters configured.
Launch PID-Tuner from CFC:
- In the CFC editor, select the PID block.
- Select menu Options -> PID-Tuner (or click the PID-Tuner icon).
- The PID-Tuner interface launches, connecting directly to the online CPU.
Execute Identification Step Test:
- Switch the controller mode to Manual in the PID-Tuner.
- Establish a steady-state process condition (e.g., damper steady at 40% open).
- Click Start Identification: inject a small, safe step change (e.g., step output to 45%).
- Allow the process value to settle completely to its new operating equilibrium.
Calculate and Apply Parameters:
- Click Calculate Parameters: review the identified model quality (ensure fit is > 90%).
- Select the desired control behavior: choose Aperiodic (No overshoot) for mechanical dampers to prevent wear.
- Click Download to Controller: the tool transfers updated Gain, TI, and TD values directly into the online block instance DB.

Verification

Verify loop tuning performance through the following commissioning checks:
1. Switch the controller to Automatic mode in the WinCC faceplate.
2. Introduce a 5% setpoint step change: monitor the trend curve in WinCC TrendControl.
3. Confirm that the process value smoothly reaches setpoint within the expected settling time without persistent ringing or mechanical chatter.
4. In the CFC chart, execute 'Read Back Parameters' to record the optimized PID values into your offline engineering project archive.

Limitations and Operational Precautions

Never execute open-loop step identification tests on mill fan dampers when the furnace, boiler, or mill is operating at maximum thermal load. Sudden airflow changes can trip fuel safety interlocks, induce burner flameouts, or cause baghouse pressure spikes. Always coordinate step tests with plant operations personnel, and ensure physical safety limits (MV_HiLim, MV_LoLim) restrict actuator movement during testing.

Engineer FAQ

Does the PCS 7 PID-Tuner require a separate software license?

No. The PID-Tuner is an integrated engineering feature bundled with the SIMATIC PCS 7 Engineering System license.

Can I use PID-Tuner on a running plant without stopping production?

Yes, provided the step change applied during identification is small enough (e.g., 2% to 5%) that it does not disrupt process equilibrium or trip safety boundaries.

What is the difference between Chien-Hrones-Reswick (CHR) 20% overshoot and aperiodic tuning?

CHR 20% overshoot provides faster response times but induces dynamic ringing. Aperiodic tuning provides smooth, asymptotic approach to setpoint with zero overshoot, which is ideal for preserving mechanical damper linkages.