Robot safety systems must be concerned with graceful degradation in the face of component failures, bad inputs and extreme operating conditions. With the increasing complexity and prevelance of robots, one can expect these requirements to grow.

This document “Robot System Safety” describes the following safety features of Kuka robots-
— Restricted envelope
— EMERGENCY STOP
— Enabling switches
— Guard interlock

An interlock system described here for general control systems, is a mechanism for guaranteeing that an undesired combination of states does not occur – for e.g. robot moving when the cell door is open, or an elevator moving when the door is open. The combination of the controlled stop with the guard interlock is described as follows:

“The robot controller features a two–channel safety input, to which the guard interlock can be connected. In the automatic modes, the opening of the guard connected to this input causes a controlled stop, with power to the drives being maintained in order to ensure this controlled stop. The power is only disconnected once the robot has come to a standstill. Motion in Automatic mode is prevented until the guard connected to this input is closed. This input has no effect in Test mode. The guard must be designed in such a way that it is only possible to acknowledge the stop from outside the safeguarded space.”

This Stanford Linear Accelerator (SLAC) paper describes the advantages of PLCs for interlock design as:

. flexible system configuration due to modular hardware and software
. regularly scheduled background tests of PLC system and sensitive I/O
. comprehensive system self-tests
. intelligent fault diagnostics simplify trouble-shooting
. easy reconfiguration of the interlock logic
. no mechanical wear and tear
. improved security due to logic encapsulation in firmware

The SLAC use case is to lock the doors unless (a) the power has been shutoff or (b) the power is on but there is an explicit bypass for hot maintenance. They implement it on a Siemens PLC using statement lists (vs ladder logic or control system flowchart programming).
GE has a number of industrial interlocks for various safety functions – http://www.clrwtr.com/PDF/GE-Security/Sentrol-Catalog.pdf

A very useful article on interlocking devices – http://machinerysafety101.com/2012/06/01/interlocking-devices-the-good-the-bad-and-the-ugly/