Siemens S7-1200 PID Control in SCL: Tuning and Scaling Guide

Siemens S7-1200 PID Control in SCL: Tuning and Scaling Guide

This reference covers the implementation of a discrete PID controller in SCL for an S7-1200 or S7-1500 CPU, including scaling of the 0-27648 analog I/O range to engineering units (psi) and the use of the Astrom-Hogglund relay auto-tuning method to derive Kp, Ki, Kd for pneumatic valve control. It also documents the NORM_X and SCALE_X conversion instructions, the cyclic interrupt OB configuration, the use of the built-in PID_Compact instruction as an alternative, and a troubleshooting matrix for the most common drift and oscillation symptoms reported on field commissioning.

1. System Overview and Signal Chain

A typical pressure-control loop on an S7-1200 consists of the following signal path:

1. Process variable transducer — 4-20 mA pressure transmitter, output mapped to the S7 analog-input word 0-27648.
2. Analog input module — SM 1231 (e.g. 6ES7231-4HD32) configured for 4-wire current, 0-20 mA or 4-20 mA. The module returns INT 0-27648 with 0 = 4 mA (or 0 mA depending on configuration) and 27648 = 20 mA.
3. PID algorithm — runs in a cyclic interrupt OB (OB30-OB38) so that the sample time Ts is deterministic. The CPU 1214C supports OB30-OB38 with phase offset; the CPU 1215C/1217C extend the available OBs.
4. Analog output module — SM 1232 (e.g. 6ES7232-4HB32) or onboard AQ of the CPU, driving 4-20 mA to a current-to-pressure (I/P) transducer.
5. I/P transducer — converts 4-20 mA to 3-15 psi pneumatic output that positions the control valve actuator.

The mapping from the S7-1200 analog word to engineering units follows the Siemens convention used by every SM 1231/1232 and by the signal boards:

2. Prerequisites

• STEP 7 (TIA Portal) V16 or later; PID_Compact V2.3 or later requires TIA V16+. Earlier TIA versions ship PID_Compact V2.0/2.1/2.2.
• S7-1200 CPU firmware V4.2 or later (for the cyclic interrupt OB30+ blocks to be available) — see Siemens Industry Online Support for the firmware update matrix.
• SM 1231 AI module, 4-20 mA, configured in the device view.
• SM 1232 AQ module or onboard AQ of the CPU, 4-20 mA.
• I/P transducer with 3-15 psi output and 4-20 mA input.
• Pneumatic actuator and a regulating valve with a known Cv characteristic.

3. Scaling with NORM_X and SCALE_X

The NORM_X and SCALE_X instructions are the canonical Siemens way to linearize an integer analog word to a REAL engineering value. They are documented in the TIA Portal help under Basic Instructions > Conversion Operations.

• NORM_X(MIN, MAX, VALUE) returns a REAL normalized to the closed interval [0.0, 1.0]. VALUE is any integer or REAL.
• SCALE_X(MIN, MAX, VALUE) takes a normalized REAL in [0.0, 1.0] and scales it to the closed interval [MIN, MAX].

The two functions can be chained in either order:

• AI to engineering unit: NORM_X then multiply by span, or SCALE_X from [0.0, 1.0] into [eng_min, eng_max].
• Engineering unit to AQ: NORM_X into [0.0, 1.0], then SCALE_X into [0, 27648].

The Siemens FAQ "How do you scale integer values in real numbers and vice versa for analog inputs and outputs in STEP 7 (TIA Portal)?" covers both directions with worked examples — see Siemens Industry Online Support, entry ID 39334504.

3.1 Read the pressure transmitter (AI to psi)

For a 4-20 mA pressure transmitter with 0-10 psi range, the integer word 0-27648 must become 0.0-10.0 psi:

// SCL — convert AI word to engineering unit
#PV_norm := NORM_X(MIN := 0,    MAX := 27648.0, VALUE := "AI_Pressure_RAW");
#PV_psi  := SCALE_X(MIN := 0.0, MAX := 10.0,    VALUE := #PV_norm);

Equivalently, since the inner NORM_X result is already in [0.0, 1.0], a single multiply works:

#PV_psi := NORM_X(MIN := 0, MAX := 27648.0, VALUE := "AI_Pressure_RAW") * 10.0;

3.2 Drive the I/P transducer (control % to AQ word)

For a controller output of 0.0-100.0 percent mapped to 4-20 mA:

// SCL — convert PID output percent to AQ integer
"AQ_Valve_INT" := REAL_TO_INT(
    SCALE_X(MIN := 0.0, MAX := 27648.0,
            VALUE := NORM_X(MIN := 0.0, MAX := 100.0,
                             VALUE := "DB_PID".CV_Percent))
);

Note that SCALE_X returns a REAL; cast to INT or DINT before assigning to the analog-output address. The SM 1232 ignores the bottom three bits, so values 0-27648 are valid even when the LSBs are zero.

4. Cyclic Interrupt OB Configuration

A discrete PID must run at a deterministic sample time. Use a cyclic interrupt OB (OB30-OB38) rather than the main OB1. The OB period sets the controller sample time Ts.

Configure the OB30 properties in the device view of the CPU (Properties > Cyclic Interrupts). Set Phase Offset = 0 ms initially. Confirm that the OB30 is not overwritten by the same-numbered OB elsewhere in the program; S7 CPUs error out on duplicate OB numbers.

5. Discrete PID Implementation in SCL

The following SCL function block implements a positional PID with anti-windup and derivative-on-error. It runs once per OB30 tick and exposes both the engineering-unit output and the percent output.

FUNCTION_BLOCK "FB_DiscretePID"
{ S7_Optimized_Access := 'TRUE' }
VERSION : 0.1
   VAR_INPUT
      PV_psi        : REAL;   // Process variable in engineering units
      SP_psi        : REAL;   // Setpoint in engineering units
      Kp            : REAL;   // Proportional gain
      Ti_s          : REAL;   // Integral time (seconds), 0 = no I action
      Td_s          : REAL;   // Derivative time (seconds), 0 = no D action
      Ts_s          : REAL;   // Sample time (seconds)
      OutMin_Pct    : REAL;   // Output lower limit in %
      OutMax_Pct    : REAL;   // Output upper limit in %
      Enable        : BOOL;
   END_VAR

   VAR_OUTPUT
      CV_Percent    : REAL;   // 0-100 % controller output
      Error_psi     : REAL;   // SP - PV in engineering units
      TrackingActive: BOOL;
   END_VAR

   VAR
      IState        : REAL;   // Integrator state
      LastError     : REAL;   // For derivative-on-error
      LastPV        : REAL;   // For derivative-on-PV (alternative)
      DerivOnPV     : BOOL := FALSE;
   END_VAR

BEGIN
    IF NOT #Enable THEN
        #CV_Percent := 0.0;
        #TrackingActive := TRUE;
        RETURN;
    END_IF;

    #TrackingActive := FALSE;
    #Error_psi := #SP_psi - #PV_psi;

    // Proportional
    #CV_Percent := #Kp * #Error_psi;

    // Integral with conditional integration (anti-windup)
    IF #Ti_s > 0.0 THEN
        #IState := #IState + (#Kp / #Ti_s) * #Error_psi * #Ts_s;

        // Clamp integrator when output saturates AND error pushes further into saturation
        IF (#CV_Percent + #IState) > #OutMax_Pct
           AND #Error_psi > 0.0 THEN
            #IState := #OutMax_Pct - #CV_Percent;
        END_IF;
        IF (#CV_Percent + #IState) < #OutMin_Pct
           AND #Error_psi < 0.0 THEN
            #IState := #OutMin_Pct - #CV_Percent;
        END_IF;
        #CV_Percent := #CV_Percent + #IState;
    END_IF;

    // Derivative on PV (avoids derivative kick on setpoint change)
    IF #Td_s > 0.0 THEN
        IF #DerivOnPV THEN
            #CV_Percent := #CV_Percent
                          - (#Kp * #Td_s / #Ts_s) * (#PV_psi - #LastPV);
            #LastPV := #PV_psi;
        ELSE
            #CV_Percent := #CV_Percent
                          + (#Kp * #Td_s / #Ts_s) * (#Error_psi - #LastError);
            #LastError := #Error_psi;
        END_IF;
    END_IF;

    // Output clamp
    IF #CV_Percent > #OutMax_Pct THEN #CV_Percent := #OutMax_Pct; END_IF;
    IF #CV_Percent < #OutMin_Pct THEN #CV_Percent := #OutMin_Pct; END_IF;
END_FUNCTION_BLOCK

Key implementation notes drawn from the TIA Portal help and from Eurotherm's PID tuning primer:

• Always use derivative-on-PV for valve control. Derivative-on-error causes a spike (derivative kick) on every setpoint change.
• Use conditional integration instead of plain clamping of the integrator. Clamping alone freezes the integrator during saturation but does not discharge it, which produces windup when the setpoint finally drops.
• Run the FB exactly once per OB30 tick; do not call it from OB1 as well.

6. Relay / Astrom-Hogglund Auto-Tuning Method

The relay feedback method (Astrom and Hogglund, 1984) identifies the ultimate gain Ku and ultimate period Tu of the process without driving it into sustained oscillation with a P-only controller. The process is forced to oscillate by switching the controller output between two levels (+d and -d) around the setpoint, and the resulting limit cycle is measured.

6.1 Procedure

1. Place the loop in manual. Force the controller output to a value that brings the PV close to the desired operating point (typically 50 % of range).
2. Switch to automatic with a relay output of amplitude d (e.g. ±5 % of CV span). The hysteresis ε prevents chattering from noise and is typically 2-3 × the PV noise band.
3. Wait for a clean limit cycle (usually 3-5 cycles). Record the peak amplitude a of the PV oscillation around the setpoint, and the period Tu between zero crossings of the same sign.
4. Compute Ku:

Ku = (4 * d) / (PI * sqrt(a^2 - eps^2));
Tu = measured period of oscillation (seconds);

If hysteresis ε is small compared to a, the simplified form is Ku ≈ 4d / (π · a). See the PID controller article for the derivation.

6.2 Ziegler-Nichols Tuning Rules

For a pneumatic valve on a pressure loop, the conservative "no overshoot" row is normally the correct starting point. The Tyreus-Luyben rules (Kp = 0.45·Ku, Ti = 2.2·Tu, Td = Tu/6.3) give a more damped response that is usually preferred for process control.

7. Output Scaling to 0-27648

The relationship between the PID output CV_Percent (0-100 %) and the AQ integer word is linear in a 4-20 mA configuration:

AQ_word = CV_Percent / 100 * 27648

Use SCALE_X so that the result is properly clamped to the integer range even when CV_Percent drifts slightly outside 0-100 % due to floating-point rounding:

// In OB30, after FB_DiscretePID runs:
"DB_Valve".AQ_Valve_INT := REAL_TO_INT(
    SCALE_X(MIN := 0.0, MAX := 27648.0,
            VALUE := NORM_X(MIN := 0.0, MAX := 100.0,
                             VALUE := "DB_PID".CV_Percent))
);
"AQ_Valve" := "DB_Valve".AQ_Valve_INT;

8. PID_Compact as the Built-in Alternative

Siemens ships PID_Compact (instruction number 2711500) for S7-1200 and PID_Compact / PID_3Steps / PID_Temp for S7-1500. They are documented in the TIA Portal help under Technology > PID Control and in the function manual S7-1200 Programmable Controller — Function Manual (entry ID 109751826). The instruction provides:

• Automatic output scaling to the 0-27648 analog word.
• Anti-windup and setpoint ramp built in.
• Tuning modes Pretuning and Fine tuning that perform the relay identification automatically.
• Configuration DB with hundreds of parameters (Retain.Cfg.LoadBackUp, Retain.Cfg.SetpointLimitation, etc.).

For new code, prefer PID_Compact and use the built-in pretuning rather than writing a discrete PID by hand. The hand-written discrete PID is justified only when:

• The sample time must be below 10 ms (e.g. fast pneumatic servo valves that close at < 50 ms).
• Custom control structures are required (cascade, override, gain scheduling) that PID_Compact does not expose.
• Migration of legacy STL/SCL code.

9. Troubleshooting Matrix

10. Verification and Commissioning Procedure

1. Wire the pressure transmitter; force 0 %, 50 %, 100 % of range with a calibrator and verify AI_Pressure_RAW reads 0, 13824, 27648.
2. Force AQ_Valve_INT = 0, 13824, 27648 in a watch table; verify with a multimeter that the I/P input reads 4, 12, 20 mA.
3. Verify the pneumatic output of the I/P at those currents: 3, 9, 15 psi nominal.
4. Set Kp = 0, Ti = 9999, Td = 0; place the loop in manual; step CV_Percent to 25 %, 50 %, 75 % and record PV behavior. Confirm time constant and dead time by inspection.
5. Run relay auto-tuning (Section 6); record Ku, Tu, a, d.
6. Apply Ziegler-Nichols conservative row; switch to auto; trend PV, SP, CV.
7. Reduce Kp and increase Ti until overshoot is below 5 %; this is the operating point.
8. Disable test mode; switch to production setpoint; record final Kp, Ti, Td in the tuning logbook.

11. Field-Notes Summary

• Confirm the analog module range (0-20 mA vs 4-20 mA, vs -10 to +10 V) before writing any SCALE_X block. An offset of 1 mA on the AI looks like windup; an offset on the AQ looks like bias.
• For the Astrom-Hogglund relay method, start with d = 5 % of the CV span and ε = 2-3 × the PV noise band. If the process will not oscillate, increase d to 10 %.
• Derivative action on PV, not on error, for valve control.
• Conditional integration is preferable to clamp-only anti-windup.
• For loops that must run faster than 10 ms, hand-written SCL is justified. For everything else, PID_Compact is faster, safer, and shipped with pretuning.
• Always confirm the I/P transducer air supply pressure is at least 20 psi above the maximum required output signal pressure.

FAQ