Load cells SIL / PL

Load cells and force transducers for SIL and PL applications

When a force measurement takes on a protective function — overload protection on a crane, rope force monitoring above the audience area, closing force limitation on a press, force monitoring in human-robot collaboration — the transducer becomes the first link in the safety chain. It then has to do more than measure accurately: it has to make its own failure detectable.

One measuring circuit is not enough for that

If a single strain gauge bridge fails, it often keeps delivering a plausible signal — and the controller notices nothing. Safety-related transducers therefore work two-channel in a 1oo2 architecture: two completely independent full bridges sit on the same spring element, galvanically isolated, with physically separated wiring and separate cable outlets or isolated connectors. Both measure the same force. If they diverge, the safety system detects the discrepancy and moves into the safe state.

Two ways to make a fault visible

• Inverted analogue signals. Channel A outputs 4…20 mA, channel B runs the other way with 20…4 mA. The sum has to be 24 mA at all times, and any deviation is a fault — an addition in the controller is enough to check it.
• Digital safety protocols. Integrated electronics with CANopen Safety or PROFIsafe monitor timing, data integrity and lost telegrams cyclically.

The evidence is part of the transducer

SIL to EN IEC 61508 (SIL 1 to SIL 4) and PL to EN ISO 13849-1 (PL a to PL e) call for figures, not just hardware. For the Sistema calculation of your installation we supply MTTFd, B10d for electromechanical components, the diagnostic coverage DC of the built-in electronics and the FMEA.

Versions

• LREA load washer — low-profile, fitted under the bolt head or nut, for preload and clamping forces. 10 to 300 kN, bolt sizes M6 to M48, 17-4 PH stainless steel, protection class IP66; on request in a version with SIL 2 to IEC 61508-2.

With the other designs the two-channel version is an order option and therefore not listed as a separate item. If you need a particular design for a safety function — load pin, tension link, load cell — just ask: we design the spring element with two channels and supply the reliability figures with it.

What goes with it

You will find the evaluation under Measurement Electronics SIL / PL — amplifiers with two galvanically isolated outputs and the programmable safety controller to go with them. Architecture, reliability figures and typical applications are explained on the page Functional Safety SIL | PL. The complete range is in the load cells & force transducers overview.