In Part 1 of this article, I looked at the pressure-sensitive devices (safety edges) themselves. This part explores the design requirements that engineers and technologists need to consider when applying these devices.
Full disclosure: I use examples from both Rockwell Automation and Pepperl + Fuchs in this article. Neither firm has any relationship with me, and no financial or other considerations were offered or solicited in relation to this article or any other work on this blog.
The information you need to design this kind of application includes:
- The closing speed of the power-operated guard
- Stopping time of the power-operated guard
- Force exerted by the power-operated guard when closing
- Your choice of a pressure-sensitive edge device
- Initial deflection distance of the pressure-sensitive device
- Intended reaction to contact with an object – stop and hold or stop and retract?
Testing is the best way to find the closing speed and stopping time of a power-operated guard. Whether you use a stopping performance test set or use video to determine the stopping time, you can readily obtain the required information. DON’T GUESS as this is critical information for the design.
The closing force of a fluidically operated (hydraulic or pneumatic) guard can be easily calculated based on the applied pressure and the bore of the cylinder. If you are using an electrical actuator of some kind, you may need to devise a means to measure the applied force.
Your choice of pressure-sensitive edge device is a design decision that may be driven by purchasing agreements or by the preference of the end user.
The initial deflection distance can be found in the device manual.
Intended reaction to contact with an object is a design decision that needs to be made when developing the safety requirements specification.
Depending on the outcome of the risk assessment, there are two separate safety functions that need to be considered:
- Prevention of unexpected starting of the hazard BEHIND the guard; and
- Prevention of injury from the motion of the guard as it closes.
Unexpected starting is prevented based on the interlocking system used with the movable section of guarding. Just because you have a pressure-sensitive device on the leading edge of the guard doesn’t mean that you can get away without a guard interlock. The interlock’s PL should be determined based on the risk presented by the machine hazards behind the guard.
The second function, prevention of injury from the closing of the guard, is relevant to the design of the safety-related controls for the motion of the guard itself, which is the hazard in the case. The opening motion is rarely a consideration, it’s the closing motion that matters. The risk assessment should consider the failure modes of the mechanical aspects of the guard mechanics, like the coupling between the prime mover and the guard itself, the connections between the prime mover and its supports, etc. The speed and mass of the moving section of the guard must also be considered, as well as any shearing hazards created at the point of closing.
Electrical Installation Requirements
Pressure-sensitive device manufacturers specify safety modules that are designed to work with the specific characteristics of the sensing device. One of the critical considerations are the failure modes inherent in the sensing device. The failure modes in the A-B product are clearly different from those of the P & F device. As part of the functional safety design process, an FMEA or an FTA may be useful for the development of a list of failure modes. This list is used to determine the required mitigation measures, and may also be needed to determine Diagnostic Coverage.
The design constraints on the electrical side start with the PL or SIL. If you are not sure about what I’m talking about here, I’d suggest reading my series on How to Do an ISO 13849 Analysis. Once you’ve got the architecture selected, you can then look at what you need to do with the output signals from the pressure-sensitive device’s safety module.
At the end of the electrical design chain, you will come to the interface with the mechanical portion of the system. This interface could be a reversing contactor arrangement for an electrically actuated guard or a fluidic valve for a hydraulic or pneumatically actuated guard.
Mechanical Design Considerations
Remember that conventional stopping time calculations won’t work for applications where the edge is being used to stop a power actuated guard. To determine the stopping performance requirements, you need to consider how much deflection the edge requires before detection occurs, and use that as the stopping distance. The stop time is then the time it takes the guard to traverse the detection distance at the closing speed. More on this later.
How should the pressure-sensing mechanism react when an obstacle is detected? If the guard should stop and hold position, then there is the possibility of trapping a person between the guard and the surrounding structure. A more common approach is to have the guard stop and then reverse to an open state. If the guard is fluidically actuated, then a 5/2 solenoid valve, that has a spring return to the raised position is probably the best choice. (See figure below.) This selection will cause the guard to return to the raised position in the event that electrical power is lost.
The downside? If the return spring should fail, then the guard will continue down until it closes or traps the person. At that point, the full available force will be applied to the trapped body part.
Alternatively, a 5/3 centre-blocked solenoid valve like that shown below, can be used.
Using this type of valve, with spring return to centre and dual solenoids, the control system can select RAISE, LOWER or STOP. If electrical power is lost, the valve spool will return to the centre blocked position, stopping the motion of the guard. The downside to this approach is that you can still end up with a trapped person; however, you can design a parallel manual actuator that can be used for rescue purposes or, if the valve is selected with manual overrides, you can manually override the control system to select the raise condition for rescue. If this is the design decision, then the valves need to be located in a protected location, and the RAISE override needs to be clearly marked.
Thanks to one of our readers, Mr Philip G Horton, for asking the questions that inspired this article, and for being patient with me while I carved out the time to write it.
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