Using a common wire: Safety PLCs with safety Ethernet
Specifying a safety PLC upfront in planning makes it possible to save significant time and money when designing the wiring and planning functionality of a project. Using Ethernet allows projects to do start/stop, speed references, and safety all over the same wire. One design used safety PLCs and safe I/O to include the guarding zones between presses.
Jim Neufeldt, Advanced Engineering
Safety systems today are growing bigger and more intricate. As they do, so, too, grows the importance of keeping costs down while working faster. Specifying a safety PLC upfront in planning makes it possible to save significant time and money when designing the wiring and planning functionality of a project.
A systems integrator in Franklin, TN, and a Siemens Solutions Partner, Advanced Engineering and its technicians have designed and installed many systems with more than 100 safety I/O points. Using Ethernet allows them to do start/stop, speed references, and safety all over the same wire.
In the estimation of Advanced Engineering, the day is coming when everything will have safety Ethernet. Simply plug up to a device to obtain control, diagnostics, and safety from it. In addition, zones can be set up, and the device reset. In smart devices, such as a drive, other functions are available as well.
The integrator works extensively in the automotive industry and has installed safety systems with more than 200 I/O safety points. Typically, that number of I/O points requires a great deal of wiring. However, by distributing the I/O over Profinet using a Siemens Simatic S7-300F processer, wiring is reduced and operator safety enhanced significantly.
In one case, an automaker was setting up zones for robots and stamping presses. The original specification to safeguard the zones called for many safety relays. However, the automaker was concerned with the cost for a relay based system. The suggestion was made for a design using safety PLCs and safe I/O up front that included the guarding zones between the presses. The design also determined which functions to shut down for each zone to make it safe for the operators to enter a zone.
In this instance, the quote for using safety relays reached $100,000. Done with a safety PLC, the cost was reduced to about $60,000. In addition, the automaker has since modified the system, which would have been nearly impossible with relays and which would require costly downtime. With the safety PLC system, the automaker simply added new zones into the safety PLC logic, similar to programming a normal PLC.
Ultimately, using a safety PLC is more productive than relays. For example, one company installed a safety relay system on a line of machines. It estimated that unplanned downtime caused a 20% drop in productivity. In fact, the company’s production charts revealed that as each machine was fitted with safety relays, productivity dropped 20% on average. On the other hand, safety PLCs are flexible, offer diagnostics, require few wires, and can accommodate distributed I/O points without the downtime.
If safety is approached as an afterthought, if becomes very difficult to retrofit it into an existing machine. However, incorporating safety into the design at the beginning of the process has shown time and again to be cost effective. Whether an integrator or a plant, it is possible to start small. For example, Siemens offers a small PLC with distributed ET200S I/O; the heads of the I/O have safety capabilities. As the system grows, it can accommodate a central processor that can control these remote heads and allow zones to be set up.
- Jim Neufeldt is president, Advanced Engineering, Franklin, TN; Edited by Jeanine Katzel, Control Engineering consulting editor. www.controleng.com.