S7-1200 Closed-Loop Stepper Speed Control via PTO and Encoder

Overview

A stepper motor is by definition an open-loop actuator: the drive issues a fixed number of pulses and the rotor is expected to advance by one electrical step per pulse. When pulse rate is stable, speed is stable; the only failure mode is a stall, which an incremental encoder cannot "rescue" because the rotor has already lost synchronism with the magnetic field. Despite that fundamental limit, an encoder on the shaft is still useful in three practical situations: detecting stall, holding the average speed constant against varying mechanical load, and replacing the stepper with a servo-class profile once torque margins are exhausted. All three are feasible on a SIMATIC S7-1200 CPU by combining the on-board Pulse Train Output (PTO) with a High Speed Counter (HSC) in frequency mode and a software PI corrector, or by offloading the loop to a dedicated closed-loop stepper drive or a real servo drive.

This reference consolidates the engineering decisions for the S7-1200 path: which CPU and signal-board to pick, how to wire an incremental encoder into the HSC, how to configure the PTO technology object in TIA Portal, how to scale and tune the velocity PI loop, and what verification tests prove the loop is closed. The official S7-1200 Motion Control documentation describes the PTO mechanism in detail at Stepper motor on the PTO (S7-1200).

Why Closed-Loop Control on a Stepper?

The open-loop assumption is only valid while the rotor can keep up with the commanded step rate. Once load torque approaches the available motor torque — typically above 70–80 % of the motor's pull-out curve — the rotor starts to lag and eventually stalls. An encoder alone cannot prevent stall in a pure stepper drive; once a step is missed the magnetic field has moved on. What an encoder can do is:

• Detect stall quickly. A frequency mismatch between commanded pulse rate and measured pulse rate from the encoder is the cleanest stall indicator.
• Hold average speed against friction or windage changes. A PI controller that compares setpoint frequency against HSC-measured frequency can trim the PTO velocity command by 5–15 % to compensate for load drift, without ever changing the underlying step resolution.
• Detect missed steps. A position counter fed by the encoder Z pulse and by the PTO pulse counter diverges by the missing-step count; the controller can then trigger a controlled stop.

System Architecture

Hardware Prerequisites

The S7-1200 platform supports the required I/O on most current CPU models. Selection is driven by how many high-speed channels you need and whether the CPU already has PTO channels on-board or requires a Signal Board.

Encoder Selection and Scaling

The HSC samples the encoder pulses over a configurable integration window. For a speed loop with cycle time T_c of 10 ms, the resolution of the frequency measurement is:

f_resolution = 1 / T_window

At T_window = 100 ms you get 10 Hz resolution. Choose the encoder PPR so that the worst-case operating speed produces at least 1000 counts per window. For example, a 1000 PPR encoder on a 600 RPM shaft produces 6000 Hz at top speed, comfortably above the threshold. The measured frequency in Hz from HSC is:

f_hz = encoder_counts / T_window

Convert to mechanical RPM:

n_rpm = 60 × f_hz / PPR

And to stepper-step equivalents (so the loop operates in the same units as PTO):

f_step_eq = f_hz × (PPR_stepper / PPR_encoder)

Keep this scaling factor k as a tag constant; it is the same multiplier referenced in the field-proven algorithm ERROR = k * A − B used to align commanded velocity with measured velocity.

PTO Output Configuration in TIA Portal

Configure the pulse generator as a Technology Object of type "PTO". TIA Portal handles the underlying PWM generator and the hardware-specific routing automatically.

1. Add a new technology object under Technology objects > Add new object > Motion Control > TO_PulseGenerator (or use the axis-based "TO_SpeedAxis" if you want pre-built ramping).
2. Select the signal source: Onboard CPU or Signal Board. For CPU outputs choose Q0.0 (pulse) and Q0.1 (direction).
3. Set the maximum output frequency to the drive's input limit. Typical stepper drives accept 25 kHz, 50 kHz, 100 kHz, or 200 kHz; do not exceed the drive rating.
4. Set the pulse/direction output mode to PTO (pulse A, direction B); the S7-1200 outputs a 50 % duty-cycle pulse train.
5. Enable the configured output with MC_Power (axis variant) or CTRL_PTO (legacy variant), and start the train with MC_MoveVelocity or by writing to <PTO>.Frequency.
6. Wire the pulse output to the drive PUL+ terminal and direction output to DIR+; tie PUL− and DIR− to the CPU M terminal (24 V common).

High Speed Counter in Frequency Mode

The HSC hardware on the S7-1200 supports four operating modes. Frequency mode is the one you want.

Add a Technology Object of type TO_HighSpeedCounter, set operating mode to Frequency, and configure the integration window to 100 ms (10 Hz update, 10 Hz control bandwidth). For a tighter loop, drop to 50 ms but expect more quantisation noise on the encoder signal. Configure the encoder input to be one of the supported HSC channels (I0.0–I0.5 on most CPUs).

The measured value is exposed at <HSC>.MeasuredValue in Hz. Read it in OB1 every cycle.

Closing the Loop with a PI Controller

The fundamental control law, as field-tested on stepper PTO systems, is:

ERROR = Setpoint_speed − Measured_speed

Output_freq = Setpoint_freq + (Kp × ERROR + Ki × ∫ERROR dt)

Implementation in Structured Text using the S7-1200 PID_Compact function block is overkill for a simple velocity trim; a hand-rolled PI in OB1 is more transparent.

// Velocity PI for stepper PTO — runs in OB1 (cyclic) every 10 ms
VAR
    fSetpointHz   : REAL; // commanded speed in Hz from HMI/tag
    fMeasuredHz   : REAL; // HSC frequency in Hz
    fError        : REAL;
    fIntegral     : REAL;
    fOutputHz     : REAL;
    fKp           : REAL := 0.8;
    fKi           : REAL := 0.05;
    fLastUpdate   : TIME;
END_VAR

fMeasuredHz := HSC1.MeasuredValue;
fError      := fSetpointHz - fMeasuredHz;

fIntegral   := fIntegral + (fError * fKi);
// Anti-windup clamp
IF fIntegral >  500.0 THEN fIntegral :=  500.0; END_IF;
IF fIntegral < -500.0 THEN fIntegral := -500.0; END_IF;

fOutputHz   := fSetpointHz + (fError * fKp) + fIntegral;

// Clamp to drive limits
IF fOutputHz > 100000.0 THEN fOutputHz := 100000.0; END_IF;
IF fOutputHz <     10.0 THEN fOutputHz :=     10.0; END_IF;

// Hand the corrected frequency to the PTO
PTO1.FrequencySetpoint := REAL_TO_UDINT(fOutputHz);

Tuning procedure for the loop:

1. Disable the integrator (Ki = 0). With Kp = 0.5, command a step from 0 to 30 % of max speed and observe the encoder-derived frequency on a trace.
2. Increase Kp until you see a small overshoot (5–10 %) on a step change; back off 30 %.
3. Enable the integrator at 10 % of Kp and increase slowly until steady-state error is removed in < 500 ms.
4. Apply a load step (e.g., a brake applied to the driven shaft) and verify recovery in < 1 s with no oscillation.

Alternative 1: Closed-Loop Stepper Drive (Recommended for Most Applications)

A dedicated closed-loop stepper drive (also called "step-servo" or "closed-loop stepper") accepts step/direction pulses exactly like a normal stepper drive, but adds an onboard encoder on the motor and an internal torque/velocity loop. From the S7-1200's perspective, nothing changes — the PTO still issues pulse/direction, and the drive guarantees that the rotor follows. The drive exposes an alarm output (open-collector or relay) that the PLC can read on a standard DI for fault handling.

This is the recommended architecture whenever the load is variable or unpredictable. The S7-1200 code reduces to open-loop control plus alarm handling, which dramatically simplifies commissioning and eliminates PI tuning.

Alternative 2: Real Servo Drive

If the mechanical requirements exceed what a stepper — closed-loop or otherwise — can deliver (high acceleration, high peak torque, short settling time), step up to a SINAMICS V90 or similar servo drive. The S7-1200 PTO can still drive the V90's pulse input, and the V90 has its own internal position/velocity/torque cascade with an encoder on the motor. The PTO bandwidth on the S7-1200 (200 kHz with SB 1222) supports up to 6000 RPM on a 2500-line encoder in pulse/direction mode, which covers the vast majority of micro-servo applications.

For applications above ~6000 RPM or with extensive electronic gearing, move to PROFINET-based control with a SINAMICS S210 or V90 PN, which exposes the standard TO_SpeedAxis objects directly to TIA Portal.

Commissioning and Verification

1. Open-loop sanity check. Disconnect the PI output and command a fixed frequency. Verify with a handheld tachometer or the encoder that motor RPM matches the expected value: RPM = f_pulse × 60 / (PPR_stepper × microstep_factor).
2. Encoder sign check. Run the motor in positive direction, trace HSC frequency, confirm positive sign. Reverse A/B wires if negative.
3. PTO update check. Trace PTO1.FrequencySetpoint and PTO1.ActualFrequency on a TIA trace. The actual should follow the setpoint within one cycle.
4. HSC update check. Trace HSC1.MeasuredValue. The value should update every integration window (100 ms by default).
5. Closed-loop step response. Apply a 10 % step to the setpoint with the PI enabled, trace fMeasuredHz and fOutputHz. Overshoot should be < 10 %, settling time < 500 ms.
6. Load disturbance rejection. Apply a mechanical load step (e.g., a brake clamp). The PI should recover speed within 1 s with no sustained oscillation.
7. Stall detection. Manually stall the shaft by holding it. The HSC frequency should drop to 0 while the PTO continues pulsing; this is the diagnostic condition for raising a fault.

Wiring Reference

Troubleshooting Matrix

Frequently Asked Questions