Maybe You Should Think About Integrating Safety with Machine Control
Safety shouldn’t be an imposition, an afterthought imposed on a machine after it’s designed. Because safety should be designed into the machine from the outset, more manufacturers are asking builders like the Eagle Technologies Group (www.eagletechnologies.com) in Bridgman, Mich. to integrate safety into their machine controls. Not only does the strategy protect both people and equipment, but it also streamlines production and enhances the bottom line.
A case in point is a friction welder that the builder of assembly, testing and material handling machinery developed for suppliers to the appliance industry. The welder joins the two halves of a doughnut-shaped plastic ring designed to be filled with fluid to balance the load in a washing machine. While holding one half of the ring stationary, the machine spins the other half while pressing it into its mate. This melts the plastic along the seam and creates a leak-proof weld in several seconds. The spinning must stop within 1 degree of rotation to align slots and holes.
Eagle developed the welder at the request of a plastic parts fabricator that was dissatisfied with the performance of its existing machines. Inadequate control had been resulting in problems—leaky welds, misaligned slots, collapsed walls—that drove the scrap rate as high as 15 percent. And any design changes that an appliance manufacturer would make to its balance rings required tedious reprogramming to the machine’s motion controller.
Moreover, strain on the motors required rewiring them about once a year, on average. Not only does the welding operation require high torque at low speeds, but power to the motor also had to be cut as the operator reached into the welding zone four or five times per minute to make adjustments or remove parts. “This abrupt stoppage of current is very hard on the motor,” notes Earle Cooper, project manager. “All that power has to be dissipated somewhere.”
Cooper and his colleagues solved these problems by working with Morrell Inc. (www.morrellinc.com), an automation distributor based in Auburn Hills, Mich. The team specified an IndraMotion MLD servo control platform from Bosch Rexroth Corp. (www.boschrexroth-us.com) of Charlotte, N.C. The Sercos III Ethernet-based system integrates the machine’s motion logic control directly into the drive, eliminating the need for a separate programmable logic controller (PLC).
The drives also contain Bosch Rexroth’s Safety on Board technology, which is embedded, programmable, safety-based logic functions. “Once in a safety mode, the IndraDrive monitors position or motion,” says Joaquin Ocampo, a Bosch Rexroth product manager. “If the position, motion or speed goes out of the set safety limits, then there is a shutdown.” The embedded logic eliminates the need for lockout-tagout stoppages and external safety hardware, thereby reducing strain on the motors and the complexity of the control system. Simplicity, in turn, shortens response time to little as 2 msec, which can also boost both productivity and safety.
In Eagle’s new friction welders, the servodrive responds to signals from the light curtain separating the operator from the welding area, receiving the signals through dual, redundant safety channels. In conformance to the IEC 60204-1 safety standard, the machine goes into one of two “pause” modes that stop motion in a controlled manner; the two modes differ in whether the motor remains under any torque. Because both modes maintain power to the machine, welding can resume without a reset procedure once the operator withdraws from the protected zone.
Besides saving time and enhancing safety, the more responsive machine has reduced the scrap rate to less than 1 percent. “We still don’t know exactly how much longer the motors are lasting than before,” says Cooper, “because none of them have required any repairs.”
Using today’s safety programmable automation controllers (PACs) is another way to integrate safety with machine control. A benefit of these controllers is that users can access various diagnostic tools developed by automation vendors. “Diagnostics on your safety system will help you isolate where a door might be open or where an e-stop might be pushed,” says Tim Roback, marketing manager for GuardLogix safety systems at Milwaukee-based Rockwell Automation (www.rockwellautomation.com/safety). These tools can minimize the downtime necessary for troubleshooting why a safety system put a machine, especially a large one, into safe state.
Besides enhancing production efficiency, the integration inside a safety PAC can also streamline the design and configuration of machinery. Consider a modular line of automated core recycling systems built by Automatic Handling International Inc. (AHI) (www.automatichandling.com), a builder of roll handling and wrapping systems based in Erie, Mich. The line processes the cores inside the huge rolls of paper that manufacturers of bathroom tissue and paper towels use to feed the converting machines that produce their products.
These “parent” rolls can have diameters as large as 140 inches. Because the rolls are heavy, the paper is typically wound around a 16-in diameter core made of dense, industrial-grade cardboard that can weigh more than 100 lbs. Tissue manufacturers can buy millions of dollars worth of these cores in a year, so a recycling system can pay for itself in six months.
AHI builds its Fusion recycling systems from a set of machinery that its engineers configure and put together like Lego pieces so the final system will fit into each customer’s facility and accommodate their production requirements. Although the number of machines varies by customer, the basic flow is such that a buffer crane moves the cores through two processes, one that cleans used cores and another that fuses used cores together to repair damaged ends.
>> Click here to read about integrating pneumatics into safety circuits.
Cleaning is necessary because tissue manufacturers like to stop short of using the whole roll. AHI’s automatic core cleaner wields proprietary tools to remove this remaining few inches of paper, which can weigh between 200 and 300 lb. The process relieves operators from having to cut away the remnant manually with knives and risk cutting themselves or scoring the surface of the core. Any scoring can cause the cores to delaminate and explode as they spin at high speed.
After the cleaning process, the automated system checks the ends for the “mushrooming” that is caused by the converting lines after a few uses. The crane transfers cores with damaged ends to a core joiner for repair. The joiner saws off a few inches from each end to produce clean edges and then mills a taper on either the outer or inner diameter of one end. After gluing the taper into its mate on the end of the next core in line to be processed, the machine cuts the joined cores to length. The crane transfers the restored core to a buffer for reuse, while the joiner continues processing the remnant.
To tie these processes together safely, yet provide flexibility for customization, AHI’s engineers collaborated with their counterparts at Rockwell Automation. The engineering team specified the automation vendor’s GuardLogix integrated safety system that combines safety and machine control on the same platform. Allen-Bradley Point Guard I/O modules also communicate with the GuardLogix controllers using the Common Industrial Protocol (CIP) safety protocol over EtherNet/IP. Also on the network are the drives, including the PowerFlex 755 AC drives, which have a safe-off function.
“Designing the safety system presented many challenges due to the size of the system,” notes Dan Pienta, president of Automatic Handling. “Altogether, it’s more than a thousand square feet in size and requires multiple entry points and zones. Traditional hardwired safety circuits would be too cumbersome and significantly complicate troubleshooting and future adjustments for end users.”
The work’s already done
The biggest benefit of the integrated safety system for Pienta’s company is the flexibility that programmability offers his engineers. Because safety and machine control are already integrated inside safety controllers, engineers use software to configure the gate switches, light curtains and other safety devices connected to them. In fact, they can connect the devices as safety inputs before safety protocols are fully defined.
“If we’re using a gate switch, for example, we may not have defined what it’s going to turn off,” says Pienta. “We can write the software once we have buy-in from the customer on the safety protocol.” And if the customer decides later to add a few more drives to the protocol, the engineers alter the program, rather than rewiring the safety circuit.
Making these changes is much more difficult with conventional hardwiring. “If you want to shut off drives with hardwired circuitry, you need to install contactors, run wire to them, and set all your safety relays to shut things off,” explains Pienta. “You can’t wire in those relays if you don’t know how your zones are going to work up front.”
Because safety PACs make waiting for these decisions unnecessary, they streamline the design phase and shorten delivery. “We’re able to go from the proposal phase to having complete drawings of a machine in half the time because we standardized on Rockwell Automation hardware,” reports Dave Pienta, Dan’s brother, vice president and director of sales and marketing. The platform reduces changes during factory acceptance testing to only hours, rather than days or weeks, he says.
Another advantage of programming occurs in zone control. Because hardwired circuits convey limited information, a safety event in one section of a large machine or system often forces a complete shutdown. By contrast, the programmability of safety PACs simplifies dividing the machinery into zones that can continue working while one zone stops or rests in a safe state. “If you have a system that has 40 or 50 drives in it, you can just shut down enough of them to give the operator safe access to a particular zone,” notes Dan Pienta.
Dan Pienta recalls a recent project that required dividing a core recycling system into several zones. The customer wanted its operators and maintenance staff to be able to work safely on the machines without stopping the entire system. So, to replace a blade in a core cleaner, the operator pushes a button on the gate switch to request entry. “When the system finishes its sequence, it comes to a stop, makes itself safe, and opens the door,” he says. When the operator leaves the zone and closes the door, the cleaner can continue where it left off.
Not only are these advantages significant, but they also are good reasons to think about integrating safety with machine control.