Adding programmable controller safety functionality is not a huge job. If you are currently using AS-i, the majority of the work is already done
Each day, industrial machine OEMs and plant engineers are bombarded with new technology with the potential to help improve both the speed and efficiency of their equipment. As technology grows and speeds increase, so do the safety liabilities to the equipment operators.
The safety aspect of the job has become one of today's more complex issues when machines are designed and production lines are installed. The time spent in the design and selection of safety devices has become a major part of new designs, as well as retrofits. So why shouldn't we use newer technology in the safety area, too?
The key to the design of any system is simplicity and ease of implementation and operation. In the manufacturing environment, the integration of multiple machines achieves the highest quality and quantities possible. As many of us know, this has been achieved by using networks. Networks provide the means for better control and monitoring of new and existing lines and a reduction in installation cost.
After the network connections are established, changes to the system can be made without the assistance of plant personnel or outside contractors hired to run conduit and wiring.
This leads me to the concept of using technology for safety. There are numerous fieldbus systems on the market and several have developed the technology into their systems to have both process-level control and safety in one network. The AS-i (Actuator-Sensor Interface) bus system has a safety bus that operates in conjunction with the process-level control. This allows the engineer to develop safety and non-safety data on a single bus.
Siemens Energy & Automation brought this to my attention about a year ago when I decided to use AS-i as a process-level control. The company has been instrumental in the development and the approval of this system, and its technical help and support team kept me informed of the latest innovations.
One of our concerns as a company was the approval of this type of system for use in the U.S. by NEC committees. Now, with the recent acceptance of this method in the latest release of NFPA 79, which will allow the use of "Control Systems Incorporating Software and Firmware Based Controllers," we OEMs have options in designing integral systems.
After the successful implementation of AS-i, I decided to further the use of the network and implement AS-Interface Safe on a family of casepackers we've built for a near-future brewery application.
The key element in this system is the "monitor" and its role in managing the integrity of the safety circuit. The monitor sends out a defined telegram, which is continuously changing based on a specific algorithm, to each of the safety slaves. If the monitor sends a telegram and doesn't receive a correct reply as a result of a fault or alarm, then the system switches off. The worst-case-scenario response time is 35 msec. The monitor has the same functionality as the safety relay currently used in most safety circuits.
Adding this programmable controller safety functionality is not a huge job. If you are currently using AS-i, the majority of the work is already done. That's a big advantage. The complex wiring and documentation are not needed. Simply install the monitor and the safety modules on the existing wiring, program the monitor, and your system is ready.
Another advantage to a bus system is built-in diagnostics. The majority of the current safety-relay-style systems do not have diagnostics that allow the operator to know where a problem is located. If feedback is needed, as for door-status annunciation to an HMI, it requires the use of another set of contacts separate from the safety circuit. If there is a failure of one of the contacts in the safety circuit and not in the annunciator circuit, the operator may not be made aware of the problem. This then requires calling in a line technician to diagnose the problem--and that leads to production losses. With network safety, the monitor will tell the PLC the location of the error and, in turn, the error can be transmitted across the network to the line-management processor.
As I said earlier, the key to all processes is to make it functional and simple. With the technology that is available and always improving, bus safety is possibly the next significant step in the automation process.
Bill Elrod works in the Electrical Design Group at Hartness Intl., a Greenville, S.C.-based manufacturer and global supplier of a wide variety of packaging machinery. Learn more about Hartness Intl. at www.hartness.com.
