Glossary · Motion control and drives
Safe Direction (SDI)
Also known as: SDI, Safe direction of motion
German: Sichere Bewegungsrichtung (SDI)
In IEC 61800-5-2, Safe Direction (SDI) is a drive-integrated safety function that prevents the motor shaft from moving more than a defined amount in the unintended direction and initiates a defined reaction if it does.
- Motion control
- Functional safety
- Standards
In one sentence
Safe Direction (SDI) is a drive safety function that stops motion in the unintended direction and triggers a defined reaction.
Example
During setup, SDI allows a roll to be jogged only in the direction that moves material away from the operator's hands.
How it applies
- Engineering: SDI needs safe position or speed feedback. Tolerances for movement in the forbidden direction and the stop reaction (often STO or SS1) are parameters. It is often combined with Safely-Limited Speed (SLS) in setup modes.
- Commissioning: The safety parameters are set with the drive's safety tool, verified, and protected by a checksum or password; the function is then validated as part of the machine's safety function.
- Operation: Crossing the limit triggers the reaction, which the operator sees as a safety stop.
- Documentation: Describe for which operating modes and directions SDI is active and what users see when it trips. State the achieved SIL or PL of the complete safety function as determined for the machine, not the drive's rating alone.
SDI vs. software direction limit
A standard software limit can block commands in one direction, but it is not a safety function. SDI monitors actual motion with safety-rated hardware and firmware.