How Much Safety Is Enough?

What Level of Safety Is Appropriate for Low-Level Robotics on Metal-Cutting Machines?

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September’s Problem

Our metal-cutting machines include some low-level robotics functionality, making safety a huge issue. The more we look into safety components, the more confused we get. We can provide a low level of protection quite easily, but making our machines completely safe is an expensive and complex proposition. How do we decide exactly what level of safety is appropriate for our custom machinery?

—from July ’08 Control Design

Answers

Safety Standards and Risk Assessment

The safeguarding requirements for the robotic portion of an integrated machining cell can be found in the following applicable safety standards—B11.TR3 Machine tool risk assessment; R15.06 Robot safety standard; and B11.20 Safety requirements for integrated Machining cells.

Also, the particular type of machine you are building may have a specific related standard, e.g., mechanical power press. Other safety standards may be applicable; this is where a risk assessment is helpful.

When trying to decide the safeguarding requirements for hazards contained in the robotic portion of the integrated machining cell, the risk assessment is the primary tool to decide which safeguards are needed to reduce the risk to tolerable levels. In addition, the circuit performance—e.g., ISO13849 categories B, 1, 2, 3, 4 and the new PL/SIL ratings—necessary are also derived from the needed risk reduction. A risk assessment must identify the hazards and the risk associated with them. The standard also provides guidance on how to correctly implement the safeguards to be effective.

Most of the safeguarding in any robotic guarding falls into one of these categories:

  • Perimeter safeguarding and interlocked access gates
  • Alternative protective measures for minor servicing
  • Control of hazardous energy
  • Point of operation/operator interfaces
  • Safety guarding of entry points for material flow in and out of the cell.

Pilz has experienced personnel participating in most of the machinery safe standards subcommittees such as the RIA R15.06 Robot Safety Standard, of which our general manager, Roberta Nelson-Shea is chairperson. This provides our people with insight to the intent of the standards and helps them to stay current with new developments.

Gil Domininguez, consultant, Pilz Automation Safety

Safety and Productivity

Traditional hardwired safety systems have a major shortcoming in handling emergency situations. Typically they simply shut the entire equipment down. In the case of a metal-cutting machine, this may cause the parts within the machine to be lost and resuming operations can be a time-consuming process. To keep machine output going, safety systems are being manipulated at times to achieve higher machine availability.

But one shouldn't have to compromise safety for productivity. Programmable safety systems allow smart safe reactions to emergency situations. Depending on its state, machine reaction after emergency situations can be programmed. For instance using Safe Motion Control could allow for safely limited speed of the machine in a manual mode with the spindle off, but the door open to allow for easier machine setup. Also in emergency-stop situations the drives can come to a coordinated stop and safely hold their position, allowing the machine to resume a cutting program once the emergency stop has been cleared without ever losing motion precision.

Safety Training
Safety Training
An Omron STI technician trains a maintenance engineer on how a new light curtain and muting package operates on a press brake.
Source: Omron STI

Utilizing programmable safety systems allow an OEM total design flexibility on how to react on safety-critical situations while still being able to achieve the highest safety categories at a cost comparable to traditional hardwired safety solutions.

Robert Muehlfellner, director of automation technology, B&R Industrial Automation

Free Standards

Since you mention robots, a good starting place is to review ANSI/RIA R15.06 Industrial Robots and Robot Systems—Safety Requirements. The standard is laid out such that there are two paths the reader can follow in safeguarding a robot system. The first is the Prescribed Method, in which the requirements are very prescriptive and are at a high level of safety. The other path, the Risk Assessment Method, allows for justification of other safeguarding solutions, which is what it sounds like you are looking for. If you do not have a copy of this standard, you can purchase one at IHS Standards Store, or you can get a copy free from your local Banner Engineering representative. Your local Banner Engineering representative can also help and make suggestions as to appropriate safeguarding solutions.

Mike Carlson, safety products marketing manager, Banner Engineering

 

Danger
It’s Curtains for Danger
Safety light curtains and perimeter guards protect access to a robot cell and monitor the interior of this assembly and welding robot station.

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