Proceed With Caution

ControlDesign.com

Keywords: Safety, safety design, machines, operator, maintenance and Dan Hebert

Good and Thoughtful Safety Design Yields Productive Machines That Are Easy to Operate and Maintain

By Dan Hebert, PE, Senior Technical Editor

A machine can be designed to be safe in all instances but virtually unusable in day-to-day plant operations. If it’s at all possible, such a machine inevitably will have some or all of its safety features disabled by plant operations and maintenance personnel. A machine also can be designed to leave all shutdown decisions in the hands of operators, unfairly and unwisely burdening them with split-second, life-or-death decisions.

Machine builders can avoid these situations by making sure that their machines are not only safe, but also designed for ease-of-use and maintenance. This means making realistic assumptions about how an operator will respond to potential hazardous situations.

You’ll hear experienced builders say the best way to make a machine safe is to identify and understand applicable safety standards prior to machine design and incorporate these standards into the machine design. Testing the completed machine in various failure modes and modifying the design as required is the final and necessary step.

Failure to follow these procedures certainly will produce an unsafe machine, either initially or eventually, as plant personnel redesign the machine for more-convenient, but often unsafe, operations and maintenance.
Abstract discussions of safety are important, but nothing brings safety to life like a recounting of actual unsafe practices or machine accidents. Reviewing these incidents can illuminate hazardous issues and show a path toward improvement.

Is it Safe? What’s the worst machine accident or near-accident you’ve experienced? What went wrong? Was it the safety theory or the actual application and practice? Add your perspective at ControlDesign.com/safe.

I, Robot

“The most serious accident I know of occurred when one of my fellow programmers was pinned against a wall by a robot that he programmed at the customer site,” says Shahvar Pirouznia, engineering manager and founder of Balance Automation Solutions, Longmont, Colo.

Balance Automation makes automated material handling systems for semiconductor and general-industry applications. Its systems are divided equally between fully automated robotics-based systems and semi-automated, pneumatic pick-and-place mechanisms. This incident didn’t involve Balance Automation, says Pirouznia, but he doesn’t identify the company in question.

Injury was minimal in this case, but the situation is frightening to contemplate and could have been avoided easily. “The incident would have been prevented had the programmer not defeated the safety circuit and entered the safety enclosure while the robots were running,” recounts Pirouznia. “This was completely his fault as he was attempting to fix a problem while the system was running. It was the wrong decision.”

As a result, it was mandated that everyone in the company be trained to prevent similar incidents. “Unfortunately, this effort was weak and infractions continued without additional injuries to personnel,” chides Pirouznia.

Why Machine Accidents Happen 1. Machines are not built and tested with proper      safety systems, interlocks and alarms 2. Safety features are too cumbersome and are      removed by end users 3. End users are not properly trained 4. Plant production needs takes precedence      over safety 5. Plant operators are unfairly expected to      make split-second shutdown decisions

Injury to personnel is the most serious outcome, but damage to equipment can be costly. “Years ago I worked at a company where we designed automated hard-disk-certification work cells,” continues Pirouznia. “One of our robots hit a spinning disk spindle and destroyed it. The problem was a bug in the robot’s path that caused it to swing too wide and hit the customer’s equipment. The bug was determined to be a problem with the vendor’s motion servo algorithm. The robot’s paths were calculated dynamically so it would have been very time-consuming to test all possible paths prior to shipping the work cell.”

Testing all possible robot paths or machine operating modes is not practical in many situations, so perhaps the best we can hope for is testing of all instances for which injury is possible. These types of safety risk-reward decisions are an inevitable and difficult part of the machine designer’s job.

Incidents Presage Accidents

Unsafe practices don’t always lead to accidents, but it’s easy to see how they could. “In one instance, I found two oven gas-pressure switches that were bypassed by the end user,” says Niels Bogh, owner of Bogh Industries, Puyallup, Wash. Bogh Industries is a consulting company for the heat-treat industry, and a large part of its business is combustion-hazard assessment and combustion safety checks on gas-fired equipment.