Control Algorithm Architecture of PCS 7 APL PIDConL: Position vs Velocity Type

The standard continuous PID controller in SIMATIC PCS 7 Advanced Process Library (APL), PIDConL (FB1874), implements an ideal/practical position-type (absolute) control algorithm. The block calculates the absolute value of the manipulated variable (MV, scaled 0.0% to 100.0%) as a direct output to the actuator based on continuous proportional, integral, and derivative equations. The mathematical formulation of PIDConL is fixed and cannot be parameterized to a velocity algorithm (delta MV). For applications requiring velocity-type or pulse-width modulated stepping outputs (such as motorized valves without analog feedback), engineers must use the companion step-controller block PIDStepL (FB1875) or VlvMotL.

Mathematical Foundation of the APL PIDConL Controller

In SIMATIC PCS 7 Advanced Process Library (APL V8.1, V8.2, V9.0, V9.1), closed-loop process control is anchored by the PIDConL block (FB1874). In classical control theory, digital PID controllers are divided into two fundamental classes:

Position-Type (Absolute) Algorithm: The algorithm calculates the absolute output value $MV(t)$ directly:
$$MV = Gain \cdot \left(1 + \frac{1}{TI \cdot s} + \frac{TD \cdot s}{1 + \frac{TD}{DiffGain} \cdot s}\right) \cdot ER$$
where $ER$ is the system error ($ER = SP - PV$ or $PV - SP$), $s$ is the Laplace complex frequency variable, and $DiffGain$ is the derivative damping factor (typically 5 to 10).
Velocity-Type (Incremental / Incremental Output) Algorithm: The algorithm calculates only the change in output during each execution cycle: $\Delta MV(t) = MV(t) - MV(t-1)$. In physical implementations, the mechanical integrator is located inside the actuator (such as a stepping motor drive that accumulates pulses).

In PIDConL, the mathematical formulation is strictly a position algorithm. The block outputs MV (0.0 to 100.0%) representing the exact physical positioning command transmitted to control valves or variable frequency drives.

Position-Type Algorithm Architecture in Process Control

The position-type architecture is the worldwide standard in continuous chemical, oil and gas, and pharmaceutical processing because modern control valves utilize electro-pneumatic positioners (e.g. SIPART PS2) that expect an absolute 4-20mA or HART/PROFIBUS positioning setpoint.

A continuous position algorithm guarantees:
- Bumpless Transfer: When switching from Manual to Automatic mode, or when tracking external setpoints, PIDConL initializes its internal integrator state so that the calculated output exactly matches the current manual output, preventing valve bumping.
- Anti-Windup Protection: When the manipulated variable reaches physical saturation limits (MV_HiLim or MV_LoLim), the integral component freezes automatically, eliminating integral overshoot.

Because the equation is hardcoded into the compiled SCL engine of FB1874, there is no configuration bit or parameter that switches PIDConL into a velocity algorithm.

Comparison of APL PID Controller Blocks: PIDConL vs PIDConR vs PIDStepL

The following table compares the algorithms, outputs, and target use cases of PID controller blocks in PCS 7 APL:

Controller BlockBlock NumberAlgorithm TypeOutput TypePrimary ApplicationPIDConLFB1874Position (Continuous Absolute)Analog MV (0.0 to 100.0%)Standard control loops with analog positioners (SIPART PS2, VFDs)PIDConRFB1875 (Restart-aware)Position with Run-Up LogicAnalog MV (0.0 to 100.0%)Complex loops requiring specialized startup routinesPIDStepLFB1878Velocity / Stepping AlgorithmDigital pulses (QOpen, QClose)Motorized valves without continuous feedback (actuator integrates)FmContFB1833Hardware Position AlgorithmHardware-managed outputHigh-speed loops executed on FM 355 controller modulesVlvAnLFB1896Valve Positioning ProxyAnalog position demandSplits MV into split-range valves or adds travel feedback supervision

Implementing Velocity-Type and Stepping Control via PIDStepL

If a plant application utilizes an electric actuator without position feedback—where the valve position is incremented or decremented by pulsing two digital output relays (Open contact and Close contact)—engineers must not attempt to alter PIDConL. Instead, replace PIDConL with PIDStepL (FB1878):

Stepping Algorithm Mechanics:
- PIDStepL implements an incremental velocity algorithm combined with an internal software integrator or pulse generator.
- Parameter MotorMHigh defines the total travel time (in seconds) required for the motor to stroke from 0% to 100%.
- Parameter PulseTime and BreakTime define minimum contact closure durations to prevent relay chattering.
CFC Interconnections:
- Connect process setpoint to SP and process transmitter to PV.
- Connect output QOpen to the digital output driver for the opening contactor, and QClose to the closing contactor.
- The internal algorithm pulses QOpen and QClose in direct proportion to $\Delta MV$, relying on the mechanical motor to integrate the position.

Verification

Verify PID controller algorithm response in CFC Online Test Mode:
1. Open the CFC chart containing PIDConL and activate Online test mode (Test -> Watch on).
2. Place PIDConL in Manual mode and set MV = 50.0%.
3. Set TI = 10.0s, TD = 0.0s, and Gain = 1.0.
4. Switch to Automatic mode with SP = 50.0 and PV = 50.0 (Error = 0). Confirm MV remains exactly at 50.0%.
5. Step SP to 60.0. Confirm that MV immediately steps upwards by 10.0% (proportional kick: $Gain \cdot \Delta ER$) and continues ramping upwards at 1.0%/sec (integral action), confirming absolute position-type execution.

Limitations and Operational Precautions

Derivative on PV vs Error: To prevent derivative spikes upon setpoint changes, configure Feature.Bit2 = 1 (Derivative action on process value PV rather than error ER).
Sampling Time Sizing: PIDConL must be placed in a periodic cyclic interrupt Organization Block (e.g. OB35 - 100ms, OB34 - 200ms). The block dynamically determines its execution period from the OB call time (SampleTime). Never place PIDConL in OB1.
Actuator Deadband: Ensure parameter Deadband is configured on noisy loops (e.g. flow measurement) to prevent continuous micrometric hunting that wears out mechanical control valve packings.

Engineer FAQ

Is PIDConL a velocity-type or position-type controller?

PIDConL is strictly a position-type (absolute) controller. It calculates the absolute manipulated variable (MV) directly as an analog percentage (0 to 100%) for the final control element.

Can I switch PIDConL to velocity type by setting a parameter?

No. The mathematical algorithm of PIDConL is fixed in compiled SCL code. If you require a velocity or pulse-stepping algorithm, you must use block PIDStepL (FB1878).

What is the difference between PIDConL and PIDStepL?

PIDConL outputs an analog continuous 4-20mA position signal (MV) for valves with positioners. PIDStepL outputs digital pulses (QOpen, QClose) for motorized valves that integrate position mechanically.