Presence Sensing Device Initiation
Presence Sensing Device Initiation (PSDI) is a machine operating mode where a presence-sensing safety device, such as a light curtain, both stops hazardous motion when a person enters the danger zone and automatically initiates a single machine cycle once the sensing field is clear and all operator body parts are safely away. This differs from standard presence-sensing devices that only stop motion and require separate manual initiation.
On a shop floor, PSDI is typically applied to part-revolution mechanical presses or similar automated equipment. The operator feeds or removes parts with hands inside the light curtain field, which prevents any stroke while intruded. Upon withdrawing hands, the PSDI logic confirms the field is clear, safety distance is met, and all conditions (e.g., brake monitor, interlocks) are satisfied, then automatically initiates one machine stroke without manual controls. This mode is used for frequent operator interaction to boost productivity. Implementation requires safety distance calculation per OSHA 1910.217(h)(5), time delay logic to prevent immediate restart, and brake monitoring. The control system must meet high integrity levels (e.g., SIL 2-3, PL d/e) with dual-channel inputs and safety-rated outputs. For compliance, PSDI must be logged as a distinct operating mode with machine type, applicable standards, validation status, and functional safety parameters.
- Using PSDI on full-revolution mechanical presses, which is explicitly prohibited by OSHA 1910.217(h) and can lead to uncontrolled strokes while hands are near the hazard.
- Failure to meet validation and documentation requirements, such as lacking OSHA-recognized third-party validation for presses, resulting in non-compliance even if hardware is capable.
- Incorrect safety distance or timing setup, such as not measuring actual stopping time or placing the light curtain too close, allowing operators to reach the hazard before motion stops.
How does PSDI differ from a standard light curtain safety function?
A standard light curtain stops or prevents hazardous motion when the field is breached and requires a separate deliberate start (e.g., two-hand controls) for the next cycle. PSDI uses the same sensing field to initiate the machine cycle when the field becomes clear and all safety criteria are satisfied, without requiring a separate start command from the operator once PSDI mode is enabled.
What control system architecture is typically required for PSDI?
PSDI requires dual-channel safety inputs from the PSD (e.g., OSSD outputs) feeding a safety PLC or safety relay, with Category 3 or 4 / PL d or e / SIL 2-3 for inputs and output control elements. It integrates a brake monitor for mechanical presses, a time delay device to block cycle initiation until the danger zone is verified clear, and a set-up/reset command per operator position to intentionally enable PSDI mode.
What are the key OSHA PSDI requirements for part-revolution presses?
Per 29 CFR 1910.217(h), PSDI is permitted only on part-revolution mechanical power presses. Requirements include minimum safety distance based on measured stopping time and PSD response time, automatic restart prohibition (only single strokes), brake monitor with specific performance criteria, and third-party validation by an OSHA-recognized organization with initial certification, annual recertification, recordkeeping, and labeling.