Resolving Sinamics S120 Encoderless Spindle Servo Control Limits

Overview

Operating a spindle without an encoder on a Sinamics S120 drive is a recurring question for machine builders retrofitting older spindles, designing low-cost turning centers, or migrating from analog AC drives to digital servo platforms. The hard engineering fact, documented in the SINAMICS S120 Function Manual section 4.11 "Encoderless operation," is that SERVO control mode cannot run without an encoder. The control structure depends on the encoder for actual-value acquisition, rotor-position identification, and the high-dynamic current/speed loops that define the servo performance class.

For encoderless operation, the S120 must be configured in Sensorless Vector Control (SLVC), not in SERVO mode. The trade-off is well documented: SLVC delivers acceptable steady-state speed accuracy but exhibits large speed dips on sudden impact loads, which is precisely the loading profile a spindle experiences during a heavy cut. This article consolidates the engineering constraints, the parameter set that governs the SLVC behavior, the firmware version that introduced new options, and the commissioning workflow that lets an integrator select the right mode the first time.

Sinamics S120 Control Architecture

The S120 supports two fundamentally different closed-loop control structures selected via p1300 (open-loop/closed-loop control mode) and the drive-object configuration in STARTER or SINAMICS Startdrive (TIA Portal):

Reference: SINAMICS S120/S150 List Manual (Entry ID 109740020), function block diagram 6710 and parameter description for p1300.

Why SERVO Mode Cannot Run Encoderless

The SERVO control structure of the Sinamics S120 relies on a real-time model of the motor that is updated at the current-controller cycle (typically 31.25 µs or 62.5 µs on a CU320-2). Three signals are required and cannot be reconstructed from terminal voltage and current alone at this update rate:

1. Rotor position (electrical angle): Required for the d-q transformation of the current vector. In SERVO mode this comes from the encoder evaluation (SMCxx, CUAxx, or direct DRIVE-CLiQ encoder). Without a position signal, the field-orientation collapses within the first electrical cycle.
2. Actual speed value: Derived from the incremental position. Encoderless observers exist, but their bandwidth is too low for the servo speed loop (typically 2-4 kHz bandwidth).
3. Pole-position identification verification: The initial rotor angle after a "pole-position identification" routine (p1980 = 1 or 2) is only valid for the standstill position. After any movement, the encoder tracks the change; without an encoder, the angle is lost as soon as the motor turns more than a few electrical degrees.

Per the S120 Function Manual, section 4.11: "The servo control can only be operated with an encoder as no model calculation of the actual values is used in the servo control due to the computing speed required."

Sensorless Vector Control (SLVC) for Spindle Applications

SLVC is the only native encoderless closed-loop mode on the S120 platform. The drive reconstructs the rotor flux angle and speed from the motor model using measured stator voltages and currents. The model accuracy is good at medium-to-high speed but degrades near zero speed and during fast load transients.

Reference: SINAMICS S120/S150 Parameter List Manual (Entry ID 109739308).

Behavior Bands of SLVC

The SLVC operating range is divided into three speed bands, each with distinct dynamic behavior:

1. 0 - p1755 (typically 0 - 10% n_rated): Open-loop current injection with slip compensation. No speed feedback. Speed accuracy is roughly +/- 5% of setpoint. Torque is limited to p1610 to keep the model stable.
2. p1755 to p1755 * 1.5: Hysteresis band where the model is being armed but speed feedback is still filtered. Avoid dwell times in this band for high-precision work.
3. > 1.5 * p1755: Closed-loop sensorless vector control. Speed accuracy +/- 0.5% of setpoint, dynamic response suitable for steady-state turning and light milling cuts.

Spindle-Specific Engineering Constraints

A machine spindle is one of the most demanding encoderless applications because the load profile includes:

• High starting torque with the chuck loaded.
• Impact loads on tool engagement (entry shock on a cut).
• Wide constant-power range (typically 1:4 to 1:8 for turning spindles).
• Field weakening above base speed for high-rpm operations.
• Orientation (M19) for tool changes in many turning centers.

SLVC satisfies items 3 and 4 reasonably well. Item 5 (orientation) is not feasible in SLVC - the spindle cannot stop at a defined mechanical angle without an encoder. If your process requires oriented spindle stop, an encoder (typically a hollow-shaft TTL or sin/cos encoder on the spindle housing) is mandatory. Item 2 - impact loads - is the most problematic. On a heavy cut, the speed dip in SLVC can be 10-15% of setpoint for 200-500 ms, compared to < 1% dip for a closed-loop vector or servo with encoder. For a finishing cut, this is acceptable; for a roughing cut with a variable chip load, surface finish suffers.

When SLVC Is the Right Choice

Despite the impact-load limitation, SLVC is the correct mode for several common spindle applications:

1. Wood-working and plastics spindles - Cutting forces are small and gradual.
2. Quartz and stone cutting spindles - Constant feed, no impact transients, water-protected housings make encoder installation difficult.
3. Grinding spindles - The wheel is dressed, not impact-loaded. Speed accuracy in the steady state is more important than dynamic response.
4. Spindles with integrated motor encoders (pre-fitted) - Some spindle motor vendors (e.g., HSD, Kessler, Fischer) deliver the motor with a built-in sin/cos encoder. The S120 reads this directly via DRIVE-CLiQ, eliminating the field wiring.

Commissioning Procedure (STARTER / Startdrive)

1. Select the drive object type in STARTER: Configure Drive Unit > Insert Drive > Vector (VECTOR). Do not insert a SERVO drive object for an encoderless spindle - the wizard will require an encoder selection.
2. Enter motor data from the motor nameplate. For third-party induction motors, fill p0300, p0301, p0304-rated voltage, p0305-rated current, p0307-rated power, p0308-power factor, p0310-rated frequency, p0311-rated speed, p0335-motor cooling method.
3. Set p1300 = 20 (Vector, encoderless SLVC) or use the dropdown "Control structure: Encoderless vector control".
4. Set p1980 = 0 to disable pole-position identification (encoderless) or p1980 = 4 for motion-based identification (motor must be unloaded).
5. Run motor identification: p1910 = 1 (motor data identification, standstill) then p1960 = 1 (rotating identification, motor must be free to rotate). The rotating identification tunes p1780, p1781 (motor model Kp, Tn) for the specific motor.
6. Set the SLVC changeover speeds per the parameter table above. For a 1PH8228-1HF10 spindle at 12,000 rpm rated: p1755 ~ 600 rpm, p1756 ~ 30 rpm, p1610 ~ 80 Nm (50% of rated torque).
7. Check the speed controller bandwidth by recording the response to a step in speed setpoint at 50% of rated speed. If the response is sluggish, raise p1460 (Kp) and p1470 (Tn). Do not exceed p1460 = 1.5 * p1460_default without consulting Siemens support.
8. Verify with the load on the spindle: command a 1000-rpm ramp, then a 5000-rpm step. Observe r0021 (actual speed), r0060 (DC link voltage), and r0078 (torque utilization). Speed ripple in steady state should be < 0.5% of setpoint.

Reference: SINAMICS S120 Commissioning Manual (Entry ID 109751325), chapter 5 "Vector control commissioning."

Fault, Warning, and Diagnostic Codes

Reference: SINAMICS S120/S150 Diagnostics Manual (Entry ID 109972842).

Troubleshooting Matrix

Firmware and Compatibility Notes

• SINAMICS S120 firmware V4.x: Supports VECTOR SLVC for asynchronous motors. Synchronous (SIMOTICS S) motors in SLVC are technically possible below firmware V4.7 but with very limited torque capability.
• V5.1 and later: Improved SLVC motor model (p1780 series), better low-speed behavior for SIMOTICS S induction-only equivalent motors. Synchronous sensorless control was enhanced but is still subject to the SERVO-vs-VECTOR architecture distinction.
• V5.2 SP3 / V5.3 (current at time of writing): Adds the new SLVC-2 model (p1750 bit pattern = 2) with adaptive flux model and better impact-load handling. Recommended for new encoderless spindle commissions.
• CU320-2 DP/PN Control Unit: Required for VECTOR drive objects with SLVC on a spindle. Older CU320 (without -2) has reduced compute headroom for the SLVC motor model and is not recommended for spindle duty.
• TM120 Terminal Module: Recommended for dual-winding 1PH8 spindle motors to monitor the temperature sensor in both windings. Required when the drive runs in field-weakening for long periods.

Reference: SINAMICS S120 Firmware V5.2 SP3 Release Notes (Entry ID 109763417).

Verification Checklist Before Production Release

1. STARTER trace recording of speed setpoint vs actual during a 0-100% speed ramp: overshoot < 2%, settling time < 200 ms, no oscillation.
2. STARTER trace during a representative full-power cut for 30 seconds: actual speed deviation < 1% of setpoint at steady state, < 5% transient dip on entry.
3. Verify oriented stop (M19) - document explicitly that this is not supported in SLVC if your machine HMI requests it.
4. Verify all safety functions: SLS (Safely Limited Speed), SS1, SS2 - these work independently of the motor encoder when used with a SINUMERIK Safety Integrated or PROFIsafe F-CPU.
5. Confirm p1910 / p1960 identification values have been saved (copy RAM to ROM, "Copy from RAM to ROM" button in STARTER or use p0977 = 1).
6. Run the drive for 4 hours at 80% rated speed with rated load: bearing temperature < 80°C (class B insulation), motor housing < 110°C (per motor nameplate).

Safety Considerations

Encoderless operation does not relax the safety requirements on a spindle drive. STO (Safe Torque Off) is independent of the motor encoder and must remain active. If the machine uses SINUMERIK 840D sl with Safety Integrated, the SLS limits apply through PROFIsafe and the drive's safe speed monitoring (SSM) operates from the model-based speed estimate, which Siemens qualifies for SLS1 and SLS2 use cases. SLS3 (with safe position) requires an encoder.

FAQ