Safety controllers: a primer

Maurizio Lauria -- Control Engineering, 9/1/2002

For an expanded version of this article, click here.

Clearly, interest in providing safer workplaces is growing among machinery manufacturers and end-users. This interest has been stimulated by new and emerging safety standards and guidelines (such as the European Machinery Directive, ANSI B11.19, ANSI B11.20, ANSI/RIA 15.06, OSHA National Emphasis Programs), and by increased awareness of the benefits of a safer work environment.

The level of interest is marked by a growing use of 'safety-approved' machine guarding components and by a greater awareness of the importance of risk assessment by the equipment designer and end-user.

Those involved in workplace protection and machinery guarding have been influenced by a growing body of standards and regulations. In particular, decision-tree models (such as EN 954), and more recently publication of ANSI's risk assessment guidelines (ANSI Technical Report B11.TR3), have prompted OEMs and end-users to recognize the importance of ensuring reliable operation of safety circuits through fault monitoring and detection by using 'safety controllers.'

Safety controllers are electromechanical or microprocessor-based monitoring devices installed between machine guarding input devices and the machine's primary stop controls, such as motor contactors or control relays. These controllers typically contain redundant, self-checking safety system monitoring circuits and positive-guided output relays, commonly called 'safety relays'.

Each controller is designed to detect faults in the safety circuit's components and interconnection wiring, as well as in its own internal monitoring circuits and output relays. In addition, it senses the actuation of a machine guard interlock/E-stop switch.

This basic safety contorl system simply and inexpensively provides fault detection plus cross-monitoring and self-checking using a safety controller ('black box'). The safety controller achieves the same function as a hard-wired circuit consisting of three positive-guided relays and more thatn 40 wiring points. Source: Schmersal

If the controller detects a fault or open machine guard, it disables the output signals, stops the machine, and/or keeps the machine from restarting until the fault has been corrected. Units are available to satisfy a broad range of Stop Category 0/1 application requirements.

Depending on their design, safety controllers can detect various types of faults. These include:

• Welded or stuck interlock/E-stop switch contacts;
• Misaligned guards;
• Open circuit in interconnection wiring;
• Short-to-ground faults in interconnection wiring;
• Fault in the controller's monitoring circuits;
• Welded or stuck contacts in the controller's safety relays;
• Insufficient operating voltage to the controller;
• Capacitive/inductive interference on the controller's inputs; and
• Welded or stuck contacts in the controlled primary machine stop element.

Some microprocessor-based safety controllers also feature integrated system diagnostics with LED displays, which indicate fault type and location, speeding troubleshooting and minimizing machine downtime.

Ensuring that a safety system will perform requires ability to detect safety circuit/component faults, then shut down the machine until the fault has been corrected. Safety controllers heighten safety system reliability and reduce the possibility of worker injury.

For more information, visit www.schmersalusa.com

Numerous applications exist in which the use of safety controllers is recommended. These include, but are not limited to applications where:

• Assessed risk of injury is relatively high. For example, applications assessed as Safety Category 3 or 4 using EN954, or medium or high assessed risk levels using ANSI B11.TR3 guidelines;
• Safety system inputs are from coded-magnet (reed switch-based) sensors;
• The designer wishes to satisfy ANSI 'control reliability' requirements; and
• Relatively low level of assessed risk exists for which the user wishes to heighten the overall reliability of the system.