Applying programmable safety PLCs
During the design phase of a project, it is sometimes impossible to cover all scenarios from a safety integration standpoint, but it is still best to cover as many bases as possible before beginning further work, according to D&D Automation.
Before any device can be operable, says D&D Automation, Stratford, Ontario, one would expect that all safety circuits would be in place. Logically, then, safety is the place to start when on site. During the design phase of a project, it is sometimes impossible to cover all scenarios from a safety integration standpoint, but it is still best to cover as many bases as possible before beginning further work. Once all safeties are in place and all devices are operational, other safety concerns may come to light.
However, these should be easy to remedy once the majority of safety integration is already in place During a body shop start-up project for a car assembly plant, the first step D&D took on all framing lines involved establishing communication and network connections using a Pilz PSS3000 safety PLC, a programmable safety and control system. Any SafetyBus or device issues were resolved before moving on. Then once the safety communications and networks were up, the integrator began flagging the safety I/O.
In most cases, a safety PLC is a luxury allowing safety logic to be tailored and modifications made according to needs without having to rewire relays. However, this is not to say that hardwired safety circuits are commissioned any differently; the commissioning steps still apply. In addition, the ability to modify safety logic in a safety PLC can also be a disadvantage. To prevent improper edits or tampering, all work should be password protected.
For this project, D&D found it needed to get as much of safety as possible running as soon as possible because of the heavy traffic moving in and out of the cell and the timeframe required for tooling adjustments. To open and close fixtures, E-stops, gates, and light curtains needed to be functional to protect those that were working within or around the cell. One advantage of using programmable safety PLCs is the ability to tailor the safety functionality almost entirely to individual needs—and in some cases to do so on the fly.
The controls department in the customer’s facility was able to create its own modular function blocks for the PSS3000, the products that were primarily used on this project. D&D integrators worked with the controls department to develop and implement a combination light curtain/safety gate block. In this arrangement, a violation of the light curtains coupled with open gates generates a safety stop in the adjacent work cell.
Another customized function was to establish safety synchronization feedback from the work robots via their respective SafetyBus nodes. These dual channel outputs from the robot nodes are activated after a robot completes its own safety routine and is used within the safety program for motor interlocks. From a safety perspective, this function indicates that the robot is within its safe working envelope. Among the greatest advantages of programmable safety PLCs is the reduction of troubleshooting time.
With the ability to monitor safety programs, missing input conditions can be found quickly without using a multimeter or having to sort through drawing sets
Also see the Control Engineering Machine Safety blog and the System Integration channel.
Read more about Pilz safety.
- D&D Automation, Stratford, ON, Canada – www.ddauto.com
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