Controlling Gravity Loads with SINAMICS P1513 Torque Compensation

Vertical axes driven by a permanent-magnet servo motor present a fundamentally asymmetric dynamic load: lifting a hanging mass requires active motor torque, while lowering the same mass is assisted by gravity. A single, direction-independent gain set for the speed and position controller cannot deliver equal dynamic performance in both directions, and the uncompensated load produces a step disturbance every time the holding brake releases. This reference describes a field-proven compensation method using the SINAMICS supplementary torque setpoint connector input P1513, and shows how to combine it with a single speed/position controller parameter set so the drive behaves as if it were driving a horizontal axis.

1. Problem Definition: Asymmetric Gravity Load Dynamics

A gravity axis (elevator cabin, hoisting winch, press ram, vertical pick-and-place, theatre fly-bar, telescope counterweight) is mechanically a second-order system with a constant disturbance force F_g = m * g acting in the downward direction. The servo motor converts this to a torque disturbance T_g = F_g * r * η, where r is the effective lead or pulley radius and η is the gear/train efficiency (≈0.85–0.95 for helical gearheads, ≈0.95 for planetary, ≈0.98 for direct-drive torque motors).

The consequences in the closed loop are:

• Up direction: Motor torque must overcome T_g plus acceleration. Required torque T_up = J*α + T_g + T_friction.
• Down direction: Gravity assists the motion. Required torque T_down = J*α − T_g + T_friction. If T_g > J*α + T_friction the load overruns the motor (runaway) and the speed controller saturates against the negative torque limit.
• At standstill: The motor must continuously deliver T_g just to hold position. Any controller error produces a position sag/overshoot proportional to the gravity torque.

Directional differences show up as:

2. Solution Strategy: Decouple the Speed Controller from Gravity

The clean engineering solution is to pre-inject the gravitational torque into the torque setpoint chain so the speed controller never sees a constant disturbance. In SINAMICS S120, S210, and V90 the parameter that exposes this injection point is P1513 (supplementary torque setpoint, CI of speed controller). With P1513 the closed-loop motor + load system appears to the speed controller as a pure inertia, and a single gain set (P gain P1460, integral time P1462) provides symmetric behavior in both directions.

3. SINAMICS Parameter Chain for Torque Feedforward

The supplementary torque setpoint enters the closed loop between the speed controller output and the torque/current setpoint channel:

Relevant parameters (SINAMICS S120/S150, firmware V5.x):

Complete parameter list is documented in the SINAMICS S120/S150 List Manual (firmware V5.2 SP3).

4. Calculating the Gravity Compensation Torque

For a fixed mass the gravity torque is a constant that can be derived from mechanics, but in practice the effective load often varies (unwinding cable, tool changer, multi-product machine). Three methods are used in order of accuracy:

4.1 Mechanical calculation

T_g = (m_payload + m_carrier) * g * r * η_gear / i_gear

where i_gear is the gear ratio motor-to-load. With m = 50 kg, r = 0.05 m (lead-screw pitch 0.05 m, treated as radius-equivalent for force-to-torque), η = 0.9, i = 5:

T_g = 50 * 9.81 * 0.05 * 0.9 / 5 = 4.41 Nm

4.2 Empirical "lift-off" method

1. Close the holding brake, lock the motor against the load (use p0850 = 0 then back to 1 with brake open inhibit).
2. Enable the drive with p1545 = 1 (travelling load compensation) and p1513 = 0 initially.
3. Command a very small positive velocity (e.g. 0.5 rpm) while monitoring r0079 (torque setpoint total) and r0080 (torque actual value).
4. The torque required to break static friction and begin motion is T_static. The torque needed to maintain 0.5 rpm is T_steady (≈ T_g + friction).

4.3 Automatic measurement with SINAMICS

SINAMICS S120 with the Technology Extension "Flying Saw / Torque Estimator" or the standard function p2195 / p2196 (moment of inertia and load torque estimator) can derive the gravitational torque automatically when the drive executes a controlled motion. The estimator is enabled with p2195 = 6 and reports the result in r2197 (load torque). This r-value can be interconnected to p1513 for self-adapting compensation.

5. Step-by-Step Commissioning Procedure

Prerequisites

• Drive commissioned per SINAMICS Startup Wizard (motor identification done, current controller optimized).
• Holding brake wired to SINAMICS BR+/BR- and configured (p1215 = 1 for active brake control).
• Mechanical safety: gravity axis must be equipped with a second, mechanically independent holding brake or a backstop before any drive enable.
• STARTER or Startdrive project on a PG/PC with online connection.

Step-by-step

1. Capture motor data and encoder calibration. Run motor identification (p1910 = 1) and speed controller optimization (p1960 = 1) with the load mechanically locked (use a mechanical clamp or backstop). Do not run auto-tuning with an uncompensated gravity load — the drive will either run away or lift off violently.
2. Set the safety torque limits. p1520 = +0.8 * p0338 and p1521 = -0.8 * p0338 initially. This limits the runaway current in the down direction during commissioning.
3. Configure P1513 source. In the expert list, set p1513[0] = r1515[0] (output of the speed controller is the default, so temporarily this looks like a no-op). For an external constant source from the PLC, set p1513[0] = r2050[0] on the appropriate PZD word. For a fixed load you can also drive P1513 with a fixed analog value via p1513[0] = r0755[0] and a potentiometer.
4. Inject a starting value. Set p1516[0] = 1.0. Bypass the speed controller integrator by enabling the feedforward in BICO and pre-charge the integrator state with the same value: set p1496 = 100 % and use the acceleration pre-control on the position controller. A simple test: enter p1513[0] = 4.4 Nm as a fixed analog value equivalent. In STARTER, the value can be set on the speed controller configuration screen under "Supplementary torque".
5. Mechanical back-up brake test. With the load suspended, command the holding brake closed (p0858 = 0). The load must not move. The motor holding brake plus the external safety brake together must hold 1.25 * m * g in the worst case (per ISO 13849-1 PLd considerations for suspended loads).
6. Soft enable. Open the brake (p1215 will manage this), set the speed setpoint to 0 rpm. The drive should hold the load with no audible movement. If the load drifts, increase P1513 magnitude in 5 % steps. If the load rises, decrease.
7. Tune the speed controller with the load present. Now that the load is decoupled, perform a manual speed step response. Apply a 5 % step on the speed setpoint and record r0060 (actual speed) and r0079 (torque setpoint). Adjust p1460 until overshoot is 5–10 % and rise time matches the application. Adjust p1462 for zero steady-state error in 50–200 ms.
8. Tune the position controller. With DSC enabled (for high-dynamic S120 with PROFIdrive) or without (for V90 / S210) set p2538 (Kp) and p2540 (Tn). The position controller Kv should be set to roughly 16–33 1/s for high-dynamics hoisting and 8–16 1/s for comfort elevators.
9. Verify in both directions. Execute a symmetric motion profile (trapezoidal or sinusoidal) and log r0061 (speed actual), r0062 (position actual), and r0080 (torque actual). The current profile should be mirror-symmetric about the speed axis; the torque baseline should be near zero with peaks only during acceleration/deceleration.

6. Auto-Tuning on a Gravity Load

SINAMICS auto-tuning (p1960 = 1 standard, p1960 = 2 with inertia pre-set, p1960 = 3 with vertical axis / suspended load profile since firmware V4.7) executes a controlled motion sequence with sinusoidal current injection to identify J_total and load torque. With a suspended gravity load, the controller must be told about the load, otherwise the auto-tuner assumes a horizontal axis and the integrator will wind up.

Procedure for safe auto-tuning:

1. Set p0573 = 1 (auto-tuning enabled).
2. Set p1965 = 1000 (speed positive, in rpm) and p1966 = 50 (dwell) so the motion stays in a small range.
3. Set p1967 = 0 (positive direction only) initially; the tuner requires the load to be re-supported at end.
4. Critical: set p1545 = 1 (friction compensation / load torque compensation enabled) and inject an estimated T_g via P1513 before enable. Without this, the auto-tuner will fail with F07902 (motor stalled) or F07905 (mechanics).
5. Run p1960 = 1. Monitor faults via r0947. The tuner writes new p1460, p1462 and p0342 (ratio of total to motor inertia).

Fault code reference for the tuning procedure (SINAMICS S120 V5.x):

7. Acceleration Pre-Control for Symmetric Settling

For best dynamic performance the speed controller should not be the only authority producing acceleration torque. Enable the acceleration pre-control (also called "Kv feedforward") so the position controller directly pre-loads the torque setpoint from the planned trajectory acceleration:

T_accel = J_total * α_profile (where α_profile = dv/dt of the planned motion).

In SINAMICS this is implemented as:

• p1496[0] = 100 % (pre-control enabled, scaling 1:1)
• The pre-control value is sourced from the position controller output, e.g. p2532[0] = r1482[0] (speed pre-control from position controller).
• The pre-control bypasses the I-component, eliminating the following error trapezoid during constant-acceleration phases.

Total torque setpoint viewed by the drive is then:

r0079 = T_speed_controller_out + T_accel_pre + T_P1513_feedforward + T_accel_pre_filter

Where T_P1513_feedforward = T_g for a constant gravity load.

8. Position Controller Setup

With the speed loop and torque feedforward optimized, the position controller reduces to a single gain design rule. The classical formula for a critically-damped / symmetric response:

Kv = 0.25 / Tn_speed (in 1/s)

Where Tn_speed = p1462 in seconds. For p1462 = 30 ms, this gives Kv ≈ 8.3 1/s. In SINAMICS this corresponds to:

p2538 = Kv * p2541 / 1000 with p2541 being the position controller sampling time scaling. Practical values for high-dynamics vertical axes: p2538 = 1.0 – 5.0 depending on units (LU/rev).

Position controller tuning procedure:

1. Start with p2538 = 0.5 * calculated Kv and verify with a 10 mm step.
2. Increase p2538 in 20 % steps until overshoot exceeds 5 %.
3. Reduce to the last stable value; this is the operating point.
4. Verify symmetric performance by repeating the step in the up and down direction. Difference should be < 10 % in rise time.

9. Edge Cases and Field-Proven Caveats

9.1 Variable load (cable unwinding, tool change)

If the load mass varies by more than 10 % over the operating range, a fixed P1513 value will under- or over-compensate. Two options:

• Use the load torque estimator (p2195 = 6, result in r2197) and interconnect p1513[0] = r2197[0].
• Provide a multi-setpoint table indexed by the PLC based on known load cases (e.g. 4 mass values for 4 production recipes).

9.2 Backlash in gearbox or chain

Mechanical backlash produces a dead zone where the load is decoupled from the motor. The P1513 compensation cannot see the backlash and will over-torque. The fix is to pre-tension the drivetrain (anti-backlash nut, dual-chain spring tensioner) and to add a small p1495 torque setpoint filter (low-pass with 5–10 ms) to avoid impulse torques at direction reversal.

9.3 Brake timing

Sequence: brake close at n < p1226 (zero-speed threshold) for p1217 ms (brake closing time) before pulses are cancelled. SINAMICS handles this with the internal sequence control, but the user must set:

• p1216 = 100 ms (brake release time, depends on brake)
• p1217 = 100 ms (brake closing time)
• p1226 = 30 rpm (zero-speed threshold for brake close)

If the P1513 torque is present before the brake fully opens, the motor will jerk the load. Add a brake-release delay so the P1513 torque is ramped in over p1216. This can be done with a PLC ramp on the analog/PZD value feeding P1513.

9.4 Brake wear and torque decay

As the mechanical holding brake wears, the holding torque it provides in addition to the motor torque decreases. A worn brake can mask an under-tuned P1513 because the brake is providing the residual holding force. Periodic check: command 0 velocity with motor energized, measure if the load drifts. If yes, the P1513 compensation is insufficient.

9.5 Vertical axis on S210 / V90 (no PROFIdrive position interface)

On the SINAMICS V90 with PTO mode or S210 with PROFINET, the same P1513 parameter exists in the drive's expert list. The P1513 source can be wired to a fixed value using a BICO interconnect from a parameter that is set via a write from the PLC. The position control then runs in the controller (S7-1500 TO or PLCopen) and the gravity compensation is uploaded as a function of the active load set.

10. Verification Procedure

1. Static hold test: Enable drive, set velocity setpoint 0, run for 60 s. Position drift must be < encoder resolution (typically < 1 LU).
2. Step response symmetric test: Command 100 mm upward step, then 100 mm downward step, repeat 5 times. Record r0061 and r0062. Overshoot in both directions must be < 5 % and equal within 10 %.
3. Following error test: Run a sinusoidal profile of amplitude 1 rev at 0.5 Hz. Peak following error (r2561 if DSC) must be < 1 % of amplitude and identical in both polarities.
4. Brake-open drop test: Open the brake, measure position drop. Must be < 0.1 mm or as required by application.
5. Loss-of-feedforward test: Set P1513 = 0. Drive should drift downward with a constant speed (gravity dominates). Reset P1513 — drive should re-hold. This verifies the compensation is doing the work.
6. Thermal test: Run a duty cycle (e.g. 20 lifts per minute) for 30 min. Motor temperature (r0035) must stay below rated. Excess I²R loss is a sign P1513 is not compensating correctly or torque limits are clipping.

11. Sample Starter / Startdrive Trace Interpretation

A correct tuning trace (trapezoidal profile) shows:

• Speed (r0061): clean trapezoid, no plateau slope (constant speed reached).
• Torque (r0080): baseline ≈ 0 at constant speed, +J*α during accel, −J*α during decel. No persistent positive or negative bias.
• Following error (r2561 with DSC): flat during constant speed (≈ 0), small triangular pulse during accel/decel.

Incorrect signs and what they indicate:

12. Standards and Safety Considerations

Suspended load applications are covered by ISO 12100 for general machine safety and require a risk assessment that typically mandates:

• Two independent braking systems (motor holding brake + mechanical safety brake or backstop).
• Functional safety rated to at least SIL 2 / PLd for the brake control, often using the SINAMICS integrated safety functions (STO, SBC) or external safety relay.
• For personnel transport (elevators, lifting platforms), additional standards apply: EN 81-1, EN 81-20, ASME A17.1, or regional equivalents.

The SINAMICS safety functions required for suspended loads include:

• STO (Safe Torque Off) — cuts the drive pulses, motor brake closes. Default safe state.
• SBC (Safe Brake Control) — drives the holding brake to a known safe state with two-channel control.
• SS1 (Safe Stop 1) — ramp to zero under controlled decel, then STO. Required for category 1 stops.

When STO is triggered, the drive pulses are removed within p9650 (typically 20 ms), but the load is held by the brake. P1513 is no longer effective during STO. This is intentional — the brake must hold the load without motor assistance during the safety state.

13. Practical Tuning Numbers (Reference)

These values are starting points and must be refined using the procedure in section 5. The full SINAMICS S120 commissioning guide is in the SINAMICS S120/S150 List Manual, and the function description of the speed controller is in the SINAMICS S120 Function Manual.

14. Frequently Asked Questions