The conflict between keeping machines running and maintaining a safe working environment alwayswill exist. Fortunately, advances in machine automation and safety systems are like having a guardian angel on your shoulder, minimizing tradeoffs between uptime and safety.
Safety can be defined as a state where risks and hazards are judged to be acceptable or in control. Although this classic definition doesnt posit control to be the same as automation, better control through automation can reduce risk to an acceptable level.
Old-school versus new
Traditional safety systems are simple, effective and proven during decades of use. Thats the good news. The bad news is that these systems can be devilishly difficult to troubleshoot and repair.
An old-school system from way back in the past century might consist of a number of e-stop pushbuttons and electromechanical interlocks connected to a fail-safe relay system. Such a system reacts quickly and reliably to shut down affected equipment.
Maintaining and repairing machines protected by an old-school system is easy if the work can be done with no time constraints and if no power needs to be applied to any part of the machine. Also, lots of time is required to find root causes, as diagnostic information is minimal or nonexistent. Another issue is that no part of the machine or line can be started up until all work is finished.
But in the real world, woe comes to those who must quickly repair old-school systems, or need to maintain these systems during short time windows. Unfortunately, these systems shut down entire production lines with no thought given to the time and effort required to get machines and lines back up and running.
However, new-school systems recognize the need to get machines back up quickly, and also make accommodations for effective maintenance. Safety controllers, digital safety networks and smart sensors provide a host of benefits to plant personnel who need to quickly and safely fix and maintain equipment.
What would you say to a vendor who tried to sell you a complex machine controlled by panels full of relays and timers? Youd probably throw the salesman out without a second thought.
What if the vendor knew nothing of digital networks, and chose to connect sensors, motor drives and other devices to the controller via hard wiring? Chances are youd look for a supplier that knew when and where to deploy digital communications to speed startup, reduce wiring errors and ease troubleshooting.
You probably also would want a technically proficient source that could provide smart sensors connected to the digital network to speed diagnostics and troubleshooting.
Move to integrated systems
Most plant personnel know enough about automation to require machine control via PLCs or other solid-state controllers, and many also want to see digital networks connected to smart sensors and drives. But, many of these same plant personnel dont know they can and should demand at least some of these automation features in their safety systems. Some dont realize the benefits, and others are reluctant to abandon the tried and true for a new solution, especially in a critical area like safety.
Lets face it, theres something reassuring about pushing a heavy-duty e-stop pushbutton or engaging an electromechanical switch, immediately hearing the satisfying clunk of a safety relay, and seeing an entire line safely and quickly shut down. Giving up this reassurance for a more advanced safety controller connected to smart sensors by digital communications is quite a leap, even when the new system meets or exceeds required safety codes and standards.
Machine builders and other suppliers recognize that getting some plants to accept newer safety systems can be difficult. The addition of modern safety devices has caused a great deal of confusion when installing new machines in a plant with lots of older equipment, reports Todd Evans, electrical engineering supervisor at Fosber America (www.fosber.com). Fosber is an OEM supplier of equipment for the corrugated paper industry (Figure 1).
The older equipment isnt usually guarded to current requirements, and its sometimes difficult for our customers to understand why all of the newer safety devices are needed, Evans says. Knowledge of device networks, how the devices work and how the devices interlock may not be ingrained into the maintenance department. This is changing, however, as every year our customers engineering departments are upgrading safety requirements for their machines.
But dealing with the newer technology has its rewards. Using safety networks instead of hard-wiring is a change culturally and in practice, says Mark Harned, vice president of controls for Astec (www.astecinc.com), which produces continuous and batch-process hot-mix asphalt facilities and soil remediation equipment. Our customers quickly found out that they didnt have to use their voltmeters to troubleshoot everything. They could actually monitor from a computer and pinpoint problems by looking at the operator interface. We programmed the operator interface so our customers could see all the functions, specifically down to which node was causing the problem down to the individual e-stop or sensor level.
As Evans and Harned say, it isnt always easy to incorporate advances in safety systems. But as with many advances in technology, the benefits are real and must eventually be adopted to avoid obsolescence. Lets take a look at how these automated safety systems, digital safety networks and smart sensors can work together to ease repairs and maintenance.
It starts with the machine
Many of the potential safety hazards in your plant can be found on or around machines purchased from OEM suppliers. Fortunately, these suppliers realize how important it is to integrate safety as part and parcel of their machines.
Our Extrol computer control systems monitor critical parameters in each machine, such as motor loads, pressure and temperatures, says Paul Brancaleone, engineering manager of software/controls at Battenfeld Gloucester (www.bge.battenfeld.com), a maker of plastic processing equipment for film and sheet extrusion. For something like pressure in an extruder, we would warn the operator and halt any speed increase before reaching a critical point. The Extrol also monitors its own health and triggers alarms for things like memory failure and reference voltage problems, adds Brancaleone. This is a good example of how advanced control systems can predict problems before they stop production, allowing maintenance to be scheduled proactively.
Hutchison Hayes (www.hutch-hayes.com)makes high-speed vertical and horizontal centrifuges (Figure 2), and like Battenfeld Gloucester, automated controls predict problems. We monitor the load related to process rate via Ethernet communications between the drive VFD and the PLC, says Lee Hilpert, Hutchison Hayes engineering manager. If the load exceeds a specific setpoint, the process rate is reduced to protect the safety of the operators as well as the equipment. This digital link between the VFD and the PLC also allows us to monitor for an over-speed condition as a result of operator error, and to prevent subsequent catastrophic failure of the equipment.
Goss International (www.gossinternational.com) uses safety PLCs on its web offset printing presses and post-press finishing equipment. E-stop and guard switches are wired to standard and redundant I/O, explains Chris Cote, Goss manager of R&D electrical engineering. The standard I/O is processed by a PLC that is also performing other machine control. The redundant I/O is processed by a separate safety PLC whose primary function is comparing the state of E-stop and guard switches against the state being reported by the standard I/O CPU., Cote says. A mismatch in state will signal an E-stop condition, resulting in machine shutdown and annunciation of the problematic device. The system requires redundant PLC CPUs, redundant I/O, and a redundant Profibus control network, but provides the versatility to deal with many configurations as well as zoned guarding requirements.
Because the Goss safety system is PLC-controlled, it is a snap to find out which input caused the shutdown, which can greatly expedite repairs. Smart safety sensors and switches connected to the safety PLC by the digital Profibus network provide diagnostic information so plant personnel can see if a switch or a sensor was activated or is faulty.
Safety PLCs also enable zoned guarding, which can be a great boon for repair and maintenance of large machines like Goss printing presses. Zoned guarding allows operators to keep parts of a machine up and running while ensuring that other parts of the machine are safe for adjustments, repairs and maintenance.
Know what youre getting into
The diagnostic information provided by automated safety systems not only helps plant personnel find the problem, it also can help when planning repairs. We use sensors and instrumentation to ascertain the state of the machine so the operators know the tasks to be performed before they enter into the hazard area, reports Mike Harrington, director of engineering at Alliance Machine Systems, a builder of automation and material handling equipment for corrugated box plants. Knowing what tasks need to be performed beforehand allows manuals and procedures to be reviewed before starting work.
Another good way to know what the problem is before de-energizing all or part of the machine is to plug into the controller. This can be problematic because the controller often is located in a live electrical panel, but theres a way around this issue (Figure 3). We install data entry ports and auxiliary power outlets through the control enclosure so a programmer can go online with the processor without having to open a live panel and be exposed to an arc-flash hazard, says Scott Bivens, P.E., electrical engineering manager at Packaging Technologies (www.packt.com). Bivens company makes packaging, processing and filling machinery.
Flexibility pays
Machine builders offer many innovative safety solutions by being creative when deploying products from leading automation vendors. Not surprisingly, many of these vendors have their own leading-edge opinions on safety systems.
Different plant personnel need different levels of machine operability, and a smart and flexible safety system can help. Flexible safety system functions include the ability to enable only a portion of machine or process, thus preventing unnecessary exposure to hazards unrelated to the task at hand, says Kelly Schachenman, marketing manager for safety systems, Rockwell Automation (www.ra.rockwell.com). If a robot needs to be taught, then maintenance personnel can be admitted to the cell with robot servo power enabled as long as a grip pendant is held in the proper state. If the pendant is released, then the safety system removes power from the robot servos.
If an operator wants to enter the cell and remove an ejected part, the safety system can remove power from the servos, allowing the operator to enter the cell without a grip pendant, says Schachenman. If maintenance needs to swap out a failed motor or perform mechanical maintenance, then a traditional lockout/tagout method can be employed.
Along with technologies that include personnel detection and motor/actuator safety control, Schachenman says there is increasing use of handheld three-position enable pendants. These pendants put control of the hazard in the hand of the person needing protection.
The urge to disable
Old-school safety systems often are disabled because theyre cumbersome to repair and maintain. Plant personnel want to avoid dealing with these systems, and its pretty easy to wire around a troublesome hard-wired electromechanical door safety switch connected in series to a safety relay.
A well-designed automated safety system is much easier to repair and maintain, so plant personnel have less reason to disable it. Its also much harder to disable all or part of an automated system.
Whether a safety system is old-school or thoroughly modern, it can be so poorly designed that maintenance and operations personnel are forced to make a choice between being safe and keeping the plant running. Instead, engineers and technicians should use a well thought out combination of safety controllers, digital networks and sensors to build a smart and flexible safety system. These guardian angel systems protect personnel and machinery without making troubleshooting, repair and maintenance harder, and in some cases can even help expedite these activities.