Here's another look back at content we've produced during these 15 years. First published here in May 2001, this article can serve to remind us how far we've come in our thinking about whether control and safety actually could, should or would share a network wire. Machine Builder Nation was just getting used to the idea of a programmable safety controller replacing hardware relay. A single network was still heresy for some folks.
Exceptional machine safety performance results from paying close attention to potential outcomes. So this is something of a heads up about a new networking trend that may impact the way you think about machine safety system design.
Safety in automation is hardly a new concept. But safety issues formerly received less attention because they did not impact the overall ability of OEMs to sell their machines.
In the 1970s, industrial equipment users began moving from relays to solid-state devices. Relays always were susceptible to operator manipulation, but programmable solid-state equivalents were not. Early adopters included General Motors, which wanted an alternative to the hundreds of electromechanical relays in the control systems of stamping presses and assembly processes.
In the '90s, solid-state safety controls grew in popularity, particularly in Europe. That began to change things significantly with international approaches to machine safety, most notably by the western Europeans through the 1997 Machine Directive legislation on safety.
The law requires specific safety measures on equipment that bears the CE mark. "The driving force as far as we were concerned in the U.K., and in Europe generally, was the need for free trade," says Brian Clark, a U.K.-based standards consultant for machinery control and electrical safety. "The directives set out safety requirements in general terms and, in many cases, they were absolute requirements," Clark says. "But the directors were quite clever, particularly in the machine directive, because it says the machine shall be safe. There's no such thing as a safe machine — there's always a hazard. But it says, in order to meet this requirement, you've got to use the latest advances available to improve safety as time goes on." Clark's point: Just because a machine was safe in 1990, it might not be safe in 2000. Machine builders have to use the latest technology if it provides additional protection.
As some North American component vendors doing business with Europe found themselves having to comply with these regulations, so too did OEMs shipping machines to Europe. Machinery entering the U.S. from Europe demonstrated noticeable safety improvements that end users began to expect from their American OEMs. The movement had begun.
Some of this is due to the way components are defined, since some components are safety classified in Europe, but not in the U.S. In Europe, an e-stop has a very specific definition. It has to have a latching function, among other features. In the U.S., it can be either a latching function or just a regular pushbutton with a separate dropout relay.
There may be other reasons, too. "I would say Europe in general, and for sure Germany, France and the U.K., have a long-standing tradition and well-evolved safety culture," says Thomas Pilz, president, Pilz Automation Safety. "In Germany, there is a large set of standards and regulations for building safe machines. People are used to the idea that when you build the machine, you build the safety system, too."
Building the safety system first makes dollar and cents sense. "In fact, it's a very economic way to do it. It's a lot more expensive to retrofit a machine," says Derek Jones, manager, safety business development, EJA. "That thinking has been embedded in Europe."
Show Them the Money
The thinking about the relationship between the control system and safety has changed. "A few years ago, the machine control system was not considered to be safety-related," Clark says. "All it did was start and stop the machine. Quite often, the operator was in control of the hazard. If he ran a tool at too high a rate, he effectively generated his own hazard. And so all the safety measures were add-on." The safer the system, the less productive it must be. Companies felt that safety was a necessary expense that could not be recovered.
An oft-mentioned example involves safety gates that seal off dangerous operations from employees. Gates can take up to 10 seconds to open, 10 seconds to close. If this activity normally happens 100 times a shift, as operators move parts in and out, the productivity losses can be significant. The risk of a poorly trained employee overriding the gates also is a distinct possibility.