Mix-and-match machine safety

In our May cover story, Senior Technical Editor Dan Hebert, PE, takes a practical look at the convergence of safety-related devices and networks with their machine control counterparts.

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By Dan Hebert, Senior Technical Editor

Implementing machine safety with tried-and-true hardwired components is expensive, but it is simple and effective. Machine safety with safety controllers, safety networks, and safety I/O is more complex in the design phase and can be more expensive in component cost. However, the resulting systems are cheaper to build, easier to operate, far-more flexible, and incorporate all the advantages of programmable controllers, digital networks, and smart I/O.

As the regulations governing the use of programmable controllers in safety systems in North America changed over these past five years, machine builders began to migrate from hardwired safety to some level of digital safety systems as their customers recognized the benefits (detailed in Table I below), and began pushing the machine builders to get with it.

TABLE I: BENEFITS OF SAFETY CONTROLLERS, SAFETY NETWORKS, AND SMART SENSORSProvide extensive diagnostic information to help pinpoint problems
  • Allow part of a machine or line to run while making other areas available for repairs or maintenance
  • Eliminate wiring problems by greatly reducing the number and length of wiresAutomatically alert appropriate personnel as needed via e-mail and/or the Internet
  • Fewer moving parts require less maintenance than electromechanical safety systems
  • Can be reprogrammed easily to accommodate production changes and needs
  • Often less expensive than hardwired systemsMuch harder to bypass or disable than electromechanical safety systems

The challenge is to provide these benefits at a reasonable cost by optimally implementing and integrating safety components with standard automation components.

Separate Safety
Designing a safety system that’s physically separate from the automation system is expensive. A typical separated system has an automation controller, an automation network, automation I/O, a safety controller, a safety network, and safety I/O. An HMI of some type usually interfaces to the automation controller and to the safety controller, so there’s at least one level of integration between the automation system and the safety system. This is typical of how digital safety systems were installed in their initial stages of deployment in Europe, and still is considered by many as the most-desirable design scheme.

This type of system is relatively easy to design, very capable, and provides extensive benefits over a hardwired safety system, but it’s also expensive to maintain because there are two controllers, two sets of I/O, and two digital networks.

Battenfeld Gloucester Engineering, Gloucester, Mass., makes plastic processing equipment for film/sheet extrusion. “To guard against being pulled into harms way by a dangerous nip point or roll, we use a variety of electrical components throughout our lines,” says Paul Brancaleone, engineering manager of software/controls at Battenfeld Gloucester. “Cords, photo-eyes, light curtains and crash buttons are used where they make the most sense. These often are connected to a safety relay from Pilz Automation Safety. Visible as well as audible alarming will be used to warn of any roll closures or machine movements. Our control systems monitor critical parameters in each machine such as motor loads, pressure and temperatures. For something like pressure in an extruder, we would warn the operator and halt any speed increase before reaching a critical point.”

The Simple Way
The simplest method is to use a safety controller, safety I/O, and a safety network for all machine safety and automation control. This actually could turn out to be more expensive than two separate systems, depending on the number of safety I/O points and the total number of I/O. For example, if your machine has 30 I/O points and 10 of these points are safety related, the cheapest option might be to use all safety-rated components.

Problems arise with more typical machine control applications where the bulk of the I/O is related to automation and only a few I/O points are safety-related. Again, it’s best to look at the simplest methods first, and this often indicates using a hardwired safety system.

A hardwired system generally consists of emergency stop pushbuttons, guard switches, and other safety sensors wired to one or more safety relays. An output from the safety relay is hardwired to an input on the automation controller to provide indication that the safety relay was tripped.

This is the approach used by FMC Technologies Citrus Systems, Houston, an OEM producer of citrus fruit extraction and processing equipment. “We use a hardwired emergency stop system controlling a safety relay,” says Keith Bunce, controls manager at FMC. “The safety relay feeds power to the PLC outputs, so any e-stop condition removes power to the PLC outputs.”

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