SINAMICS S120 Torque Control: Telegram and Parameter Setup

Torque-controlled operation on a SINAMICS S120 drive is fundamentally different from speed-controlled operation and cannot be approached as a "drop-in" replacement for a VFD that was previously running a tightening cycle in a different vendor's product line. The S120 exposes torque as the primary closed-loop variable through specific telegrams (notably 136, 138, and the supplementary telegram 750) and through a dedicated set of closed-loop parameters (p1300, p1500, p1501, p1513, p1520-p1525). For a tightening process that must ramp up at high speed, hit a defined torque window of 90-140 Nm, and stop within a small angular window (for example 25° referenced to the output shaft of an i=16 gearbox), the cleanest implementation is not a pure torque-controlled axis. It is speed control with torque limiting (telegram 2 or 4) combined with a downstream position-handling logic on the S7-1500 controller.

Application Overview: Tightening with Torque Window

The reference application is a part-tightening station with the following constraints:

• Torque acceptance window: 90 Nm to 140 Nm (a part tightened below 90 Nm is rejected, above 140 Nm is a reworked/failed condition).
• Initial approach speed: high (clamped to a safe maximum, e.g. 1500 rpm at the motor shaft, which after a 1:16 gearbox equals 93.75 rpm at the output).
• Final stop position: an angular reference of 25° measured at the output shaft, with a search routine that drives to the closest 25° position once the torque window is satisfied.
• Mechanical load: external encoder + 1.1 kW induction motor + planetary gearbox i=16.

Replacing an Altivar 31 plus a third-party axis controller with a Siemens stack (CU320-2 PN + PM240-2 + CPU 1515) is feasible, but the application logic must be re-implemented because the Altivar's analog torque-reach logic does not map 1:1 to S120 status words.

Hardware Configuration and Firmware Prerequisite

The hardware stack referenced in this configuration is:

The PM240-2 catalog number ends with -0UL1 (3 kW blocksize); the 1.1 kW motor is undersized relative to the PM240-2, which is normal because the Power Module must be sized to the peak motor current, not the continuous rating. Verify motor module current rating in r0207 against the motor nameplate p0305 value during commissioning.

Telegram Selection Strategy

The S120 standard telegrams relevant to torque-handling applications are summarized below. Telegrams are configured in p0922 on the drive and mirrored in the IO device configuration in TIA Portal (or STEP 7 Classic with the GSD file GSDML-V2.3x-Siemens-SINAMICS_S120-...).

Recommendation for the tightening application: telegram 2 (or 4 if the encoder is wired to SMC30 and the controller needs position actual). Telegram 136 is designed for winder/unwinder applications where torque is the controlled variable and web tension is the feedback. Using telegram 136 for a tightening process that must ramp up at a defined speed and then collapse to a stop at a known angle will require additional outer control logic to clamp speed and to handle the position stop, which is what telegram 2 + the PLC logic already does naturally.

Control Modes: p1300 Configuration

The closed-loop control mode is selected via p1300. The torque-relevant values are:

For the recommended telegram 2 path, set p1300 = 21 (speed control with encoder). The encoder on the motor shaft (read by SMC30) closes the speed loop inside the drive; the torque limits delivered over PZD3/PZD4 of telegram 2 then cap the current reference, which produces the natural speed collapse when the joint closes.

If the design truly requires p1300 = 23 (torque control with speed limiting), the speed clamp must be implemented via:

• p1082 — maximum speed reference for normal operation
• p1532 — torque limit for forward speed clamp (motoring side)
• p1533 — torque limit for reverse speed clamp (regenerative side)

Torque Setpoint Path: p1500, p1501, p1513, p1520-p1525

The torque setpoint source chain on the S120 is:

p1500 (Torque setpoint source selection)
  ↓
p1501 (BI: torque setpoint selection, e.g. switch between setpoints)
  ↓
p1503 (CI: torque setpoint main) / p1513 (CI: additional torque setpoint)
  ↓
p1520/p1521 (CO/BI: torque limit upper, motoring)
p1522/p1523 (CI: torque limit upper, motoring)
p1524/p1525 (CI/CO: torque limit lower, regenerative)
  ↓
Current controller (torque produced by motor)

For a speed-controlled drive with torque limits arriving from the PLC over telegram 2 PZD3 and PZD4, the typical wiring is:

• p1500 = 0 — main torque setpoint = 0 (the drive is speed-controlled; torque setpoint is generated internally from the speed error).
• p1522[D] = r2050[2] — torque limit upper sourced from PZD3 of telegram 2 (the 3rd receive word).
• p1524[D] = r2050[3] — torque limit lower sourced from PZD4 of telegram 2.
• p1520, p1523 — left at defaults (typically +400 % / -400 % rated torque) to act as ceiling clamps in case the bus value is missing.

For pure torque control on telegram 136:

• p1300 = 23
• p1500 = 1 (analog setpoint) is replaced with p1500 = 1000 (PROFINET/PROFIdrive) or wired via p1503 = r2050[1] on telegram 136.
• p1501 = 0 (no selector switch).
• p1532 and p1533 set to the rated motor torque for the speed clamp.

SINA_SPEED Block Integration on the S7-1500

FB 285 (SINA_SPEED) is the standard TIA Portal block for closed-loop speed control of an S120 from an S7-1500 over PROFINET. It assumes telegram 1, 2, 3, or 5 (the standard speed-control telegrams) and exposes a structured input/output that maps the relevant PZDs. Key inputs for the tightening application:

The torque limit is normalized to percent of rated motor torque (r0333), not the gearbox output torque. To set a gearbox-output limit of 140 Nm with a 16:1 gearbox on a motor with r0333 = 7.0 Nm:

T_motor_limit_Nm = T_load_limit_Nm / gear_ratio
                = 140 Nm / 16
                = 8.75 Nm

T_motor_limit_pct = (T_motor_limit_Nm / r0333) * 100
                  = (8.75 / 7.0) * 100
                  = 125 %

Therefore the input TorqueLimitUpper on FB 285 must be set to 125.0 %. If the calculation yields a value above 400 %, the motor is undersized relative to the demanded load torque and the application must be reviewed with the motor datasheet (overload capability per p0640 and thermal model p0611).

SINA_SPEED must be called in a cyclic OB (OB1 or OB30 at the configured send clock, typically 1-4 ms on PROFINET IRT). The associated hardware identifier for the S120 drive must be linked to the HardwareId input. For SINA_SPEED to expose torque limits, the drive's p0922 must be telegram 2, 4, 5, or 6.

Encoder Configuration for Torque Control

The external encoder mounted on the Lenze motor (typically an incremental encoder, HTL or TTL) must be configured on the SMC30 module attached to the CUA32 / CU320-2. The essential parameters are:

For the answer to the specific question “do I need an encoder value if I use torque control?”: Yes. Even in torque control mode (p1300 = 22 / 23) the speed limiter and the speed actual reading require an encoder for accurate control at low speed. Without an encoder, the drive derives speed from the current/voltage model, which loses accuracy below ~10 % rated speed. For a tightening process that ends at near-zero output speed (after the gearbox), encoder feedback is mandatory. With telegram 4 the encoder position is also visible to the PLC as G1_XIST1 / G1_XIST2, which can be used directly for the 25° position handling.

Position Stop Strategy at 25°

The original requirement — stop the axis at 25° regardless of where the torque window is satisfied — cannot be implemented by the drive alone when running in speed-control-with-torque-limit mode. The drive does not natively know that the axis must rest at a specific angle. The position handling is therefore a PLC task.

Three implementation paths are valid on the S120 + S7-1500 stack:

Path A: PLC Position Correction via SINA_SPEED with Reference Search (Recommended)

1. Use telegram 4 so the drive delivers the encoder actual position G1_XIST in PZD6.
2. Reset the encoder to zero at the start of each tightening cycle (rising edge of Enable) via control word bit 11 (set reference point) or via p0480.
3. On the PLC, integrate the incremental position into a load-side angle by dividing by the gear ratio and scaling to degrees: load_angle_deg = G1_XIST / p0408 * 360 / 16.
4. When the torque-window-reached condition is detected (actual torque between 90 Nm and 140 Nm, monitored via MIST in PZD4 or via r0080), read the current load angle.
5. Compute the delta to 25° (mod 360°) and the shortest direction (forward if delta ≤ 180°, reverse otherwise).
6. Drive the axis to the target at a slow speed (e.g. 30 rpm) using SINA_SPEED with torque limits at the upper bound of the tightening window.
7. On reaching the target, remove Enable to stop the drive; the drive enters standstill via the OFF1 ramp.

Path B: EPOS Fixed Endstop

Switch the drive to telegram 7 (or 9) and activate the basic positioner. Use the travel to fixed endstop function (activation via control word bit assignment on p2631/p2632). Configure:

• p2614 = torque limit for fixed endstop detection (corresponds to the 140 Nm upper bound).
• p2620 = positioning target (in LU).
• p2621 = traversing profile (creep speed, approach distance).

This path is cleaner from the drive's perspective but requires the PLC to switch telegrams between approach and final position, which is awkward when the speed-control path is already running.

Path C: Drive-internal Stop on 25° via SINA_POS with Telegram 7

Use telegram 7 throughout, configure the basic positioner, and let the drive execute the positioning to a fixed modulo position. This requires careful startup of the EPOS function block (FB 286 SINA_POS), including the mechanical referencing / absolute-encoder setup. For an incremental encoder without an absolute reference, a homing run is required at first power-up.

Step-by-Step Commissioning Procedure

1. Hardware check. Verify CUA32 ribbon cable connection to CU320-2 PN; verify SMC30 wiring of the external encoder (HTL/TTL levels, A/B tracks, optional Z). LED status on SMC30 must show green READY.
2. Drive first commissioning (Starter / Startdrive). Use TIA Portal V17+ with the Startdrive add-in. Drive wizard detects PM240-2 via CUA32, prompts for encoder type, motor data (Lenze 1.1 kW), and fieldbus configuration.
3. Configure telegram. Set p0922 = 2 on the drive object. In the TIA Portal device configuration for the S120 IO device, set the assigned telegram to Standard Telegram 2 (4 PZD in / 4 PZD out). Compile hardware.
4. Configure encoder. Set p0400, p0408, p0420, p0430, p0437 per the table above. Acknowledge faults and verify r0061[0] shows the encoder speed matching the actual motor speed (with r0061 ~ 0 rpm at standstill).
5. Configure control mode. p1300 = 21. p1500 = 0. p1522[D] = r2050[2], p1524[D] = r2050[3]. Save with RAM to ROM and Copy RAM to startup.
6. Configure SINA_SPEED instance on the S7-1500. Drop FB285 from the SINAMICS Blocks library onto a cyclic OB. Wire HardwareId, Mode = 2, SpeedSp, and the two torque-limit inputs to data block values.
7. No-load test. Disable the application mechanics (decouple gearbox if possible). Run a slow jog (e.g. 50 rpm) and verify direction, encoder feedback, and torque-zero reading. r0031 must be near 0.
8. Load test. Engage the application, jog in forward direction, monitor r0031 (actual torque) and the torque-limit status bits in ZSW1. Increase torque limits from 0 in 10 % steps up to the calculated 125 % to verify the drive can sustain the load.
9. Tightening sequence test. Implement the PLC logic per Path A above. Run 10-20 cycles and verify torque-window detection and the 25° final position.
10. Fault injection test. Block the output shaft mechanically. Verify the drive enters torque limit, draws the configured current, reports F07902 only if p2177 time-out is exceeded, and trips cleanly via the OFF2 / OFF3 path from the PLC.

Verification and Diagnostics

After commissioning, verify the following trace points during a single tightening cycle:

For long-term diagnostics, configure the trace recorder with a 4-second window around the torque-window-reached edge and capture r0021, r0031, r0052, and the digital input from the contact sensor. Save as a re-usable trace configuration for production-floor diagnostics.

Troubleshooting Matrix

FAQ