Tuesday, July 15, 2008

Safety in Automation (Part 1 of 3) Creating opportunities for safe technologies

Motion Control and Automation

Safety in Automation

The spotlight on safety is creating opportunities for safe technologies

Safety technologies are benefiting from the surge of interest in machine safety, even though it is being propelled in large part by external forces, marketplace issues and regulations. But achieving higher levels of safe motion is also a good thing for OEM machinery builders and users because the end result extends beyond human safety, to material assets and reduced probability of machine failures.

This special section takes a look at solutions for safety in automation. The focus on safety is effectively providing an impetus for the adoption of technologies and products that ultimately will be the basis of new generations of safer machines.

Focus on Feedback Redundancy
Safety-related position measuring systems rely on pure digital, dual feedback

Redundant systems are critical to the goal of functional safety which minimizes and reduces the risks that can occur during normal or impaired operation of machinery. So, a fundamental requirement for axes of motion in safety-oriented applications is redundant position information to perform corresponding safety functions.

“A key requirement for safety is built-in redundancy,” says Tom Wyatt, national sales and product manager for Heidenhain. “To achieve safer machines, it has always been a requirement for feedback suppliers to come up with effective dual feedback systems for redundancy.”

Heidenhain has made a major ongoing commitment to safety-related position measuring systems using its EnDat 2.2 pure serial data interface. Drive encoders have achieved safety approvals with control category SIL-2 (in accordance with IEC 61508 or performance level “d” of ISO 13849). A linear scale product already available with the interface is in the process of gaining safety approvals from the various governing bodies and their goal is to get all products switched over.

“What we have done is taken a rotary encoder, along with a linear scale perspective as well, and built two encoders into one,” says Wyatt. He says, on the rotary encoder side, there are two independent scanning methods and pure serial communications, as well. The requirement on the driver control side is a dual processor system, so when a data packet is sent there are two independent data packets one right after the other on the same line from two independent measuring systems built into the encoder.

“You can imagine with a machine tool, that if something happens with a drive axis, there is always a runaway situation where damage can happen,” says Wyatt. “The combination of sensors and feedback systems prohibits runaway situations and puts the system in safety stop if something is missing. If the two data packets that come back from the encoder suddenly don't match or one is missing, immediately within microseconds the system is shut down.”

In the past two years since the initial approval of this position measuring technology by BGIA in Europe, the equivalent of the Occupational Safety and Health Administration in the U.S., the focus has been on safe machines to meet European machine tool standards. He says the concept is now being looked at for all types of automation applications.

A key area for growth is the robotics industry because of the need for the work envelope for robots to be safe and the ability it provides for manufacturers to save space on the manufacturing floor by placing robots into tighter spots. But there is also interest in packaging for applications such as palletizing, all driven by the spotlight on European machine tool standards and the increased general focus on the need for safety approvals.

Wyatt says the technology has established itself but there are still certain levels to fulfill. The technology already achieved SIL-2 which is probability of failures and Category 3 which pertains to machine tool safety. But the whole idea is to reduce the probability of failure and along with that comes both higher levels of material safety and, of course, human safety.

“We continue to see safety as a rising issue down the road and safety is a big issue with high-end interfaces,” says Wyatt. “The probability of failure is really the only place you can improve and we are currently at 10-8 and 10-7 for SIL-2. The probability of failure is already next to nothing, but the next step is improving on that even though you can only go so far.”

Part 2 continues...

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