Siemens FB41 PID Controller: OB1 vs OB35 Execution Analysis

Overview

The Siemens FB41 "CONT_C" continuous PID block from the Standard PID Control library is the most widely deployed regulatory controller on SIMATIC S7-300/400 systems programmed with STEP 7 V5.x. Where the block is called from — the free-cyclic OB1 or a time-triggered cyclic interrupt such as OB35 — has direct consequences for loop accuracy, derivative noise, integral wind-up behavior, scan-time jitter, and CPU utilization. This reference consolidates the architectural rules, the FB41 input/output contract, the sampling-time math, and the tuning workflow required to commission a deterministic PID loop in production.

Organization Block Architecture in S7-300/400

An S7 CPU executes organization blocks (OBs) according to a fixed priority ladder. The two OBs relevant to a PID application are OB1 (priority 1, free-cyclic) and the OB30–OB38 range (priority 2–24, hardware- and time-triggered cyclic interrupts). The STEP 7 help on "Organization Blocks" and the manual "S7-300 Programmable Controller, Hardware and Installation" list the priority class for each OB.

Two architectural facts follow directly from the priority structure:

1. OB35 preempts OB1. A configured OB35 interval elapses during OB1 execution, the CPU suspends OB1 mid-scan, copies the entire stack, executes OB35 to completion, then resumes OB1 where it left off. The technical term "cyclic interrupt" therefore refers to the fact that OB3x interrupts the OB1 cycle, not that it interrupts a hardware line.
2. OB1 has no guaranteed period. The next OB1 run begins only after the previous OB1 plus any pending higher-priority OBs finish. Anything that increases OB1 scan time — adding a new block, expanding a string operation — silently changes the effective PID sampling rate.

OB1: Main Cyclic Program

OB1 is the free-cyclic main program. With no other OBs loaded, OB1 executes once per cycle and the next OB1 run starts immediately after the current run finishes. A typical mid-range S7-300 station (CPU 315-2 PN/DP) reports OB1 scan times of 10–30 ms; a heavy CPU 417 reports 5–15 ms. The relevant consequence for FB41 is that the actual PID sampling time equals the variable OB1 scan time, not a configured constant.

Pros of FB41 in OB1

• Simplest possible project structure — only OB1 exists, no hardware configuration of OB35 required.
• Process image is naturally coherent: all inputs are read once at the top of OB1, FB41 computes on the same image, outputs are written once at the bottom.
• No preemption, so debug breakpoints behave intuitively.

Cons of FB41 in OB1

• The CYCLE input of FB41 cannot be tied to a real-time clock value; it is whatever the user types in, and the actual sampling period silently drifts with OB1 scan time. The integrator inside FB41 still adds the discrete increment (GAIN * error / TI) * dt, but the dt is whatever the integrator remembers from the previous call, not what the wall clock has actually elapsed.
• Differentiator gain TD / CYCLE becomes wrong when the OB1 scan doubles because someone added a recipe loader to OB1.
• CPU utilization on a CPU 315-2 with two FB41 loops plus HMI traffic can climb above 60% in OB1, leaving little headroom.

OB35: Cyclic Interrupt

OB35 is the cyclic interrupt OB with the Siemens default interval of 100 ms. The interval is set per slot in HW Config → CPU Properties → Cyclic Interrupts; allowable values are 1 ms to 60 000 ms in 1 ms increments, with phase offset in the same dialog. Once configured and downloaded, OB35 fires every 100 ms (± the OB1 jitter of typically < 1 ms) regardless of OB1 length. OB35 is the recommended host for any FB41 / FB42 / FB43 call when:

• The process has a dominant time constant that is at least 10× the configured interval (Nyquist–Shannon sampling theorem applied to control, see Section 5).
• Multiple loops share the CPU and OB1 scan time becomes variable under recipe/HMI load.
• The loop drives a final-control element (valve, VFD, heater) where deterministic sampling is part of the safety case.

Pros of FB41 in OB35

• Constant sampling rate. Set CYCLE := T#100ms on FB41 and it stays 100 ms forever. Derivative and integral gain stop drifting.
• Reduced OB1 load. Heavy computation moves out of OB1, which becomes shorter and more deterministic. On a CPU 314 with three FB41 loops, moving them from OB1 to OB35 typically drops OB1 from ~45 ms to ~12 ms.
• Predictable PID output. The LMN_PER / LMN values are computed at a known cadence, making trend recordings and acceptance tests reproducible.

Cons of FB41 in OB35

• Process image read by FB41 inside OB35 is the "last OB1" image unless a partial update with SFC 26 "UPDAT_PI" / SFC 27 "UPDAT_PO" is programmed. For PID this is usually fine (one analog input + one analog output per loop), but multiple loops sharing an input module must each perform their own update.
• OB35 execution time still counts toward the overall scan; exceeding the OB35 interval triggers OB80 (time error) and the CPU goes to STOP if not handled.
• Debug breakpoints inside OB35 are intrusive: pausing inside OB35 while the wall clock keeps ticking causes an OB80 immediately on the next trigger.

Sampling-Time Theory for Digital PID

The discrete-time equivalent of a continuous PID is the velocity form:

delta_u(k) = Kp * [ (e(k) - e(k-1)) + (Ts/Ti) * e(k) + (Td/Ts) * (e(k) - 2*e(k-1) + e(k-2)) ]

where Ts is the sampling period. Three engineering rules apply:

1. Rule of ten. Ts <= 0.1 * T_process, where T_process is the dominant time constant (63 % rise of the open-loop step response). For a temperature loop with T_process = 90 s the maximum acceptable Ts is 9 s; for a flow loop with T_process = 0.6 s, Ts must be ≤ 60 ms.
2. Rule of Ziegler–Nichols closed-loop. Ts ≈ 0.1 * Tu, where Tu is the ultimate period measured with P-only control at the critical gain K_u.
3. Nyquist bound. Disturbance rejection above 1/(2·Ts) cannot be improved by the loop. For Ts = 100 ms this is 5 Hz — well above the bandwidth of virtually every thermal, level, pressure, and slow-flow loop, but a hard ceiling for fast pressure or hydraulic loops that demand Ts = 10 ms (OB38) or hardware interrupts (OB40).

FB41 "CONT_C" Block Specification

FB41 resides in Standard Library → PID Control Blocks. The instance DB is created once per loop and stores the integrator state, derivative state, and output limiter across power cycles. Critical inputs:

Step-by-Step: Configure FB41 in OB35

Prerequisites

• STEP 7 V5.5 or V5.6 with Standard Library > PID Control Blocks visible.
• CPU 31x or 41x with enough free work memory (FB41 instance DB ≈ 80 bytes per loop).
• Analog input module wired to the PV sensor, analog output module wired to the final control element.
• Known dominant time constant of the process (open-loop step test result, or P&ID data sheet).

Procedure

1. Enable OB35. Open the S7 station in HW Config, double-click the CPU, select the Cyclic Interrupts tab. Set OB35 to 100 ms and leave phase offset at 0 ms. Compile and download the hardware.
2. Insert FB41. In the LAD/FBD/ST editor, open OB35 and call FB41 with a name (e.g. "PID_FURNACE"). STEP 7 prompts to create the instance DB; accept and assign DB number 50.
3. Wire setpoint and process value. Connect SP_INT to a data-block REAL or directly to a CFC tag. Convert the raw PIW to percent using FC105 "SCALE" or the PV_PER path of FB41. The PV_PER path is preferred because FB41 internally scales the integer to percent using the 0–27 648 Siemens convention.
4. Set CYCLE. Open DB50 and confirm CYCLE = T#100MS. If you use OB38 instead, change it to T#10MS.
5. Initial parameters. Start with GAIN = 1.0, TI = T#300S, TD = T#0S (P+I mode for first tuning).
6. Bumpless transfer. Always wire MAN_ON := FALSE and MAN := LMN from the previous scan (using the same instance-DB output), or accept the manufacturer's default auto-tracking by leaving MAN = 0.0.
7. Compile and download. Save the project, download blocks to the CPU, and place the CPU in RUN.

Verification

1. Open the instance DB online and confirm CYCLE is the value you set and not T#0MS.
2. Watch QLMHLM / QLMLLM — they should be FALSE in normal operation; persistent TRUE means the output is clamped.
3. Force a 5 % step on SP_INT and trace PV and LMN in the trend recorder. The response should be stable, no overshoot above 10 %, settling within 3 dominant time constants.
4. Read the OB35 execution time with the SFC 78 / SFC 79 diagnostics or the CPU diagnostic buffer. If OB35 ever exceeds 100 ms (OB80 event), increase the configured interval or reduce the loop count.

PID Tuning Methods

Tuning is the empirical selection of GAIN, TI, and TD that stabilizes the closed loop as quickly as possible after a setpoint change or disturbance with minimum overshoot. Five well-documented methods cover roughly 95 % of industrial practice.

Ziegler–Nichols Open-Loop Step Response

Apply a small step (5–10 %) to MAN_ON while in manual and capture the PV ramp. Read off the process gain K_p (%/% rise over % step), the dead time L (delay until PV leaves its noise band), and the time constant T (63 % of total rise).

Ziegler–Nichols assumes a self-regulating process with a clear dead time. It tends to overshoot; reduce GAIN by 30 % if overshoot is unacceptable.

Ziegler–Nichols Closed-Loop Ultimate Gain

With TI = T#∞ (enter T#999999MS or the maximum TIME constant) and TD = T#0MS, raise GAIN in steps until the PV oscillates with constant amplitude. Record the gain K_u and the oscillation period T_u.

Cohen–Coon

Uses the same step-response parameters K_p, L, T but produces less overshoot for processes with L / T > 0.3 (large dead time). Coefficients are tabulated in "Process Control: A Practical Approach" and the Siemens PID Self-Tuner manual. For PI mode: GAIN = (1/K_p) · (T/L) · (0.9 + L/(12·T)), TI = L · (30 + 3·L/T) / (9 + 20·L/T).

IMC (Internal Model Control)

Best for setpoint tracking. For a first-order-plus-dead-time model G(s) = K_p · e^{-Ls} / (T·s + 1) choose GAIN = T / (K_p · (L + lambda)), TI = T, TD = 0, where the closed-loop time constant lambda is chosen as lambda >= 0.8 · L. Larger lambda means slower but more robust response.

Lambda Tuning

European variant of IMC with explicit lambda. The recommended starting point is lambda = T. On integrating processes use GAIN = T / (K_p · lambda) with TI = 2 · lambda.

Auto-Tuning with Siemens PID Self-Tuner (FB58 "TCONT_CP")

For new projects, prefer the PID Self-Tuner (FB58 TCONT_CP) over FB41. FB58 has a built-in warm-start phase that identifies the process parameters online and writes GAIN, TI, TD into the same instance DB automatically. It can be left running during commissioning without the engineer manually applying Ziegler–Nichols numbers.

Practical Tuning Procedure (P-I first, then D)

1. Set TD = T#0MS, DEADB_W = 0.0, TM_LAG = T#100MS. Switch to manual (MAN_ON = TRUE).
2. Bring MAN to a value that holds PV at the desired operating point. Confirm PV is stable for at least one time constant.
3. Switch to automatic (MAN_ON = FALSE). Apply the Ziegler–Nichols PI formulas from Section 8.1.
4. Step SP_INT by 5 %. Observe overshoot and settling. If overshoot > 15 %, reduce GAIN by 30 % and increase TI by 50 %.
5. If a residual steady-state error remains, reduce TI in steps of 20 % until the error vanishes.
6. If the process has measurable dead time but you want faster settling, add derivative: TD = 0.25 · TI, then reduce GAIN by 10 % to compensate for the increased high-frequency gain.
7. Tune the disturbance feed-forward: wire DISV to the measured disturbance (e.g. inlet flow for an outlet temperature loop) and scale so the disturbance feed-forward cancels 70 % of the expected excursion.

Anti Wind-up and Bumpless Transfer

FB41 has a built-in anti-windup that freezes the integrator while LMN is at LMN_HLM or LMN_LLM. The associated flag is QLMNDT. To get the best behaviour:

• Set LMN_HLM and LMN_LLM to the actual physical limits of the actuator (e.g. 0–100 % for a valve, or 4–20 mA-equivalent 0–100 % for a current loop). Never leave them at the default ±100 if the actuator is narrower.
• For bumpless transfer from manual to auto, set MAN to the actual actuator feedback (not to the last LMN_PER value) so the integrator starts at the right state.
• Use QLMNDT in the HMI faceplate to warn operators that the loop is in saturation.

Diagnostics and Verification

Troubleshooting Matrix

Migration Notes: FB41 → FB58 "TCONT_CP"

For new projects in STEP 7 V5.5 and TIA Portal V14+ Siemens recommends FB58 "TCONT_CP" (PID Self-Tuner) and the technology object "PID_Compact" in TIA Portal. FB58 keeps the same inputs/outputs as FB41 plus TUN_ON, RET_CPF, SAVE_PAR, and uses the same CYCLE semantics. Existing FB41 programs can be migrated 1:1 by re-importing the FB41 source into a new project and replacing the FB call; the instance DB schema is compatible except for the added self-tuning fields.

FAQ