By Loren Shaum, contributing editor
In its origin, machine safety for the most hazardous machines, like punch presses and press brakes, focused on control redundancy. Clearly, equipment failure that could lead to something catastrophic for an operator or maintenance person required the statistical reassurance of redundancy. Mean-time-between-failure (MTBF) of components, first initiated by military standards, became a measuring stick for failure probability. Of course, the contradictory position is that the more redundancy, the more components you have that could fail.
Safety standards first were introduced to Machine Builder Nation in 1970 to ensure that safety-rated applications comply with minimum control requirements. The Williams-Steiger Occupational Safety and Health Act established an early measure for control reliability on mechanical power presses. It read: “The control system shall be constructed so that a failure within the system does not prevent the normal stopping action from being applied to the press when required, but does prevent initiation of a successive stroke until the failure is corrected. The failure shall be detectable by a simple test, or indicated by the control system.”
Machine safety now is driven by reasonably mature global safety standards, and the reliability of components on such equipment appears closely matched with military, aerospace and other governmental reliability requirements. So, with much more electronic reliability available, safety equipment suppliers are focusing on the value proposition of not only safer machines, but also machines, because they are safer, producing faster and cheaper than ever before.
New or Retro?
The question is when to use which type of safety device. And that decision typically is based on whether the machine is new or a retrofit and the subsequent configuration. A perfect example of this can be seen at Centerline, a builder of automated assembly, tube processing, metalworking and welding systems in Windsor, Ontario (Figure 1).
Figure 1: One programmable safety relay replaced eight and was retrofitted to a welding cell at Centerline. The relay connects to the existing machine PLC via DeviceNet.
However, when do numerous relays give way to a controller? Is there a middle ground? And, what do you do with existing machines?
For installed machines, ensuring safe operation can become a sticky issue and often is compromised in favor of sustaining production. Many safety-solution providers offer safety surveys that inform users of machines that do not meet standards and offer safety options that will place the machine in compliance. The most common solutions resulting from these surveys are machine perimeter guarding and point-of-entry guarding. However, to monitor and interlock this equipment to the existing machine control, a safe interface is required. Enter the notion of a safety relay.
Presented by Pilz Automation Safety several years ago as a redundant relay in one package instead of two electromechanical relays in parallel, safety relays not only provide Category 4 protection by monitoring emergency stops, safety doors, light curtains and two-hand control installations, but some also allow configuration to the specific application.
J.B. Titus, manager, business development and industry standards, Siemens Energy & Automation says a safety relay should incorporate:
- stop category 0 according to EN 60204-1
- two electronic enabling circuits
- two floating enabling circuits
- one electronic signaling output
- 24 Vdc power
Figure 2: The costs and sophistication of safety devices will vary with the needed functionality.
When the emergency stop control switch is activated, outputs switch off. The outputs are switched back on again when the e-stop control switch releases, the protective door locks, a feedback circuit is closed and a start button is activated.
When the protective door enable is activated, outputs are switched off and solenoid control outputs are switched on with a time delay. Then the protective door is released. When the protective door enable is activated once again, the solenoid outputs are switched off and the protective door is locked.
The more safety devices that are required, the higher the likelihood of incorporating a safety controller instead of continually adding more safety relays. “The choice is based primarily on the number of safety points,” says Titus (Figure 2). “Invariably, more than eight to 10 safety points favor a safety controller.” Because relays are still mainly electromechanical, they can fail. So using fewer relays makes sense.