Wie funktioniert SLS in der Praxis?

SLS (Safely Limited Speed) monitors whether the drive speed remains below a configurable limit value. Two independent limit values are available. If the limit value is exceeded, the drive automatically triggers STO. Typical application: reduced speed for collaborative robots or in setup mode.

Was ist der Unterschied zwischen SLS und SMS?

SLS limits the speed to a configurable value below the nominal speed—e.g., 250 mm/s in collaborative mode. SMS monitors the maximum mechanical speed of the system as an absolute upper limit. SLS can be flexibly parameterized, while SMS protects against mechanical overload.

Unterstützt SOMANET mehrere SLS-Grenzwerte?

Yes, SOMANET supports two independent SLS limits per axis. These are switched by the safety PLC via FSoE—e.g., a lower value for collaborative operation and a higher value for automatic operation behind a protective fence. Both are SIL3 PLe certified.

Safe Limited Speed (SLS) and Safe Maximum Speed (SMS)

What are SLS and SMS?

Safely Limited Speed (SLS) and Safe Maximum Speed (SMS) are speed monitoring safety functions in accordance with IEC 61800-5-2. SLS limits the motor speed to a configurable value when the function is activated. SMS continuously monitors an absolute speed limit in both directions of rotation.

Why is this important?

In collaborative robotics and machines with operator interaction, stopping alone is not enough—many applications require safe continued operation at reduced speed:

Human-robot collaboration (HRC) — ISO/TS 15066 defines "speed and separation monitoring" as a collaborative operating mode. SLS ensures the drive-side speed limit when a person enters the work area.
Speed-dependent safety zones — with four independent SLS instances, graduated speed limits can be implemented for different protection zones (e.g., 500 mm/s in the outer zone, 250 mm/s in the inner zone, standstill at closest approach).
Setup mode — when the safety door is open, SLS limits the speed to a safe level for the operator.
Overspeed protection — SMS intercepts uncontrolled speed violations, even if the application control sends incorrect setpoints.

How does it work?

SLS — Demand-driven speed limiting: SLS is activated via an FSoE command or a secure digital input and goes through several phases:

Activation — Triggered by FSoE or Safe I/O
Start delay (t_D_SLS) — gives the application controller time to initiate deceleration
Ramp monitoring — the safety monitoring system checks that the speed decreases at least at the configured rate a_SLS.
Stabilization time (t_L_SLS) — the speed must remain below the limit value n_SLS for this duration.
Active monitoring — any exceedance of n_SLS triggers the configured response

Four independent SLS instances (SLS1–SLS4) are available, each with its own speed limit and time parameters.

SMS — Continuous speed monitoring: SMS is not a demand-driven signal, but monitors continuously. Two parameters define a speed window: n_pos_max (upper limit) and n_neg_max (lower limit). Any exceedance of this window immediately triggers the configured error response.

Configurable error responses in the event of limit violations: Both SLS and SMS support multiple response options: STO (immediate torque removal), SS1-t or SS1-r (controlled stop, then STO), SS2 (controlled stop, then SOS), or SOS (standstill monitoring).

How does SOMANET implement this?

SLS and SMS are implemented on the SOMANET Safe Motion Module (SMM) and require encoder feedback for safe speed calculation. The achievable safety level depends on the encoder configuration (up to SIL 3 / PLe / Category 3).

Configuration is performed via the Object Dictionary: SLS via 0x6690–0x6698 (four instances, each with command, speed limit, times, delay rate, and error response), SMS via 0x66A9–0x66AD. The internal cycle time of the SMM is 1 ms, the minimum FSoE cycle time is 6 ms.

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