The Shrinking Footprint of Robot Safety
by Hugh Shah, Contributing Editor
Robots are in the zone. They are are cleverly focusing their movements to finely tuned angles and reshaping the robot cell, reducing its footprint, and providing robot users more creativity and flexibility in safely automating new processes. The primary enabler is safety-rated soft axis and space limiting technology.
Robots are not alone in this endeavor. In conjunction with new safety standards and advanced safety peripheral devices, robots and humans are now able to go where few dared before. They’re venturing into new corners of high-density, labor-intensive production and down new avenues for collaboration.
“The advantage of the new standard and the new, different types of technologies is that it does allow for a degree of creativity,” says Pat Davison, Director of Standards Development at the Robotic Industries Association in Ann Arbor, Michigan. “Manufacturers have more options in terms of technology, and through risk assessment, the end users have a variety of options to utilize and still achieve safety. Compare that to a prescriptive standard that essentially says put a cage around everything.”
ANSI/RIA R15.06-2012, Part 1
5.12.3 Safety-Rated Soft Axis and Space Limiting
Soft limits are software-defined limits to robot motion. Space limiting is used to define any geometric shape which may be used as an inclusionary or exclusionary zone, either limiting robot motion within the defined space, or preventing the robot from entering the defined space.
In the standard, the “software-defined limits to robot motion” are realized through innovative software functions embedded in the latest robot controllers. The major robot OEMs provide this feature offered under various brand names. This replaces hardware-controlled limits and represents a significant leap in safe motion control. The benefits are many.
“It’s faster, it’s cheaper, and it takes up less floor space,” says Davison. “It’s more productive, it has more uptime. You’re going to spend less on hardware and have fewer mechanical devices to tinker with, a reduction in capital investment.”
Cage or Control the Hazard
A risk assessment is now mandatory to comply with the safety standard. Although this requirement is no longer optional, the measure allows integrators and end users to take advantage of a variety of advanced technologies in the newer robots and safety peripherals to spare valuable factory real estate and increase process efficiencies.
“With the robot standard also embracing risk assessment and trying to really understand the hazards and the exposures, it means that you can craft solutions that are beyond cookie cutter,” says Roberta Nelson Shea, Global Marketing Manager of Safety Components for Rockwell Automation Inc. in Milwaukee, Wisconsin. “Have a hazard, put a guard. Instead, now it’s have a hazard, control the hazard.”
Ann Arbor, Michigan
The Leading Event for Industrial
Robot Safety and Compliance
Nelson Shea has served as chair of the R15.06 Subcommittee for Industrial Robot Safety for more than 20 years and was involved with the robot safety standard since the first committee meeting in 1982. She says the ANSI/RIA standard actually preceded the ISO and EN robot standards. The R15.06-2012 and ISO 10218:2011 robot safety standards are now harmonized.
“That particular standard really tried to look at how people use the equipment and how you craft the restrictions around the design and implementation, so the robot systems can be productive, reliable, and safe for intended use,” says Nelson Shea.
On a very basic level, explains RIA’s Davison, the safety-rated soft axis and space limiting technology replaces hard stops. Think of the doorstops around your house or apartment. It’s the same idea with the robot axes.
“Traditionally, to limit the robot movement or limit those axes, you would have to put a physical block or obstruction in place,” says Davison. “Generally, that was on axes 1, 2 and 3, or the base, the shoulder and the elbow.”
“The advantage is instead of just limiting three axes, you can limit every axis and then you also impose dynamic limits,” explains Davison. “Imagine a building post or some sort of obstruction in the middle of the restricted space. You can tell the robot to always go around that obstruction, so it doesn’t collide with it. You have a lot more flexibility in creating an allowable space or a restricted space. It can be any subset of the maximum space that you want it to be.”
Part 1, Section 5.12.4, covering “dynamic limiting devices” is new to the R15.06 safety standard.
“It used to be that everybody would geometrically center the robot in the cell and put equipment around it,” says Rockwell’s Nelson Shea. “When in reality, for the application, it only needs to move through 60 degrees of it, so why not put the robot into a space and safely control it with safety-rated soft axis and space limiting. Lo and behold, you have a footprint that’s much smaller.”
“As engineers we need to break out of our old habits of making everything neat, tidy and centered,” she adds.
Space and Speed Limiting
With the new standard, one of the advantages is that you’re able to embed more of the safety-related functions in the robot controller and eliminate the need for external fixtures or mechanical devices to supply this functionality. Some see this as the biggest enabler, the catalyst for a perfect storm, fueled first by safety-rated robot controllers. Then strengthened by the robot safety standards and advanced safety peripherals that fully engage this space-saving paradigm.
“It evolved over a number of years to where now we can have an independent, control computer system that will monitor the robot’s speed and location,” says Gil Dominguez, Safety Consultant for Pilz Automation Safety, L.P., in Canton, Michigan. “Robots are now commonly purchased with a safety system that monitors where it is and compares it to a defined space, so we can restrict their movement and make the cells smaller. If the robot can’t move there, then I don’t have to guard people in that area.”
Dominguez also serves on the R15.06 Subcommittee with Davison and Nelson Shea. They all agree that the latest trend in safety peripherals is to use sophisticated safeguarding devices in more clever ways. The traditional way was to simply shut off the machinery. Now, the configurations are endless.
“Now we use safeguarding devices for risk reduction that is more than just a stop,” explains Nelson Shea. “For example, as you get to one particular distance from the hazard area, the robot system slows. You get closer, it slows more. Then you get to a certain point and it stops.”
“That in itself can reduce the total footprint because we decreased the stopping time and the distance in which the robot system operates as we decrease the speed,” she says. “But really what it means is that we’re starting to use safeguarding devices in a more creative way and that has been enabled by having more safety embedded in the controllers and drives.”
She cites an example of this technological coupling recently on display at IMTS in a fenceless FANUC collaborative robot system using Rockwell safety sensors.
“The system uses our SafeZone™ Mini Safety Laser Scanners,” says Nelson Shea. “The scanners are installed close to the floor, scanning horizontally for any approach to the system. As an intrusion gets closer, these scanners tell the robot system to operate slower. There are additional SafeZone Mini scanners hidden under the robot system mounting. Their fields are like an invisible box surrounding the robot system area. Any intrusion into this invisible box of light initiates a protective stop to the FANUC robot system. Operation can resume once the intrusion is removed.”
FANUC’s new Kermit green collaborative prototype, along with several other major robot manufacturers’ safety-embedded controller technologies, were profiled in this recent article on robot OEMs fast-tracking cobots.
Safe Stopping Distance
Historically, when robots were put in emergency stop mode, power would be shut off to the servo motors. Under the new safety standard and with new functionality embedded in the newer robot controllers, there are certain cases where the robot is in a safe stop mode but power to the servo motors remains.
“In the past, robots were not designed to stop as quickly as possible,” says Dominguez. “That wasn’t an issue when it was behind a fence.”
Because of the imprecise way robots used to coast to a stopped position, presence-sensing safety devices such as light curtains could consume a lot of floor space.
“Based on the requirements of the standards for safe distance, a light curtain had to be placed far enough away, so when you approach the robot at what is considered the normal approach speed, the robot would stop before you reach it,” explains Dominguez. “Since we had no way to guarantee how fast the robot was going from a safe, reliable standpoint, we had to figure worse case and sometimes that meant placing the light curtain 6, 8, or even 10 feet away. For some applications, that negated the benefit of the light curtain.”
“With this new technology, if we can sense someone coming, then we can command the robot to slow down and reduce the limits that it can move with speed, such that we can push the light curtain in much closer,” says Dominguez.
“Now it’s possible to take measures to reduce the robot stop time to less than a tenth of a second,” adds Dominguez. “That not only reduces the footprint, but also allows people to actually penetrate into where the robot operates.”
Light curtains, laser scanners, safety mats, and other safety peripherals can be used in various ways to reduce or eliminate safety fencing. Rather than serving as hard and fast barriers to the robot cell, now with advanced features, they are providing greater freedom in cell design and access to automated processes.
Presence-sensing devices such as the Pilz SafetyEYE® system work in conjunction with the safety-rated soft axis and space limiting capabilities embedded in the latest robot controllers to define protected zones. Dominguez explains how the three-part system works.
“The SafetyEYE system consists of a sensing device, which is three cameras with stereotype vision. That allows it to determine distance and therefore depth and volume. The sensor can be mounted anywhere from 3 to 7 meters high. The higher you go, the bigger the area you see. You can set different geometric or volumetric shapes of the hazard and warning zones. You can switch those off and on, allowing someone to walk into the space, and at other times, restricting the space. Then you have the computer that reads all these cameras and compares them to the defined zones. Then you have the interface to the outside world, the safety control, which receives safety-rated inputs and sends safety-rated outputs to implement the safety functions that you designed.”
He says a big driver behind this technology is the ability to robotize areas not previously conducive to automation due to floor space limitations or robot cell placement.
“The biggest potential is where you have highly labor-intensive applications, high-density people-per-square-foot areas,” says Dominguez. “If I can reduce the footprint, I can open up applications I couldn’t do before. Where I could put a robot in for a simple application, but I couldn’t afford to lose the two or three stations that were adjacent to it. On the first go around, most of the robots will be co-located to the people. Later, maybe interaction.”
Human-robot collaborative interaction will require a higher level of technological sophistication.
“In the future, you’ll want to look at the speed of the person approaching and compare that to the actual location of the robot arm and its speed (in real time), and then come up with algorithms to decide when the robot should stop or perform an alternative task. So the level of communication between SafetyEYE and external control will have to grow.”
The human and robot collaborative workspace is growing by leaps and bounds. To ensure safety, the technology will have to stay ahead of the curve.
“Traditionally, everyone that thought of safety devices only looked at the front end,” says Chris Soranno, Safety Application Specialist for SICK, Inc., in Minneapolis, Minnesota. “Do I have an interlock switch? Do I have a light curtain? For a long time, that was good enough.”
“The traditional mindset was that you have a machine and if there’s a hazard, you then isolate the person from the hazard and the only time the person can interact in that area is when the hazard is completely locked out or controlled through safeguards,” says Soranno, who also participates on the R15.06 Subcommittee. “With the new robot standard, it’s not black and white. Now’s there’s this new grey area, which is the collaborative workspace.”
“As long as you know where the robot is in space, and the speed of the force in which it is operating, and you know where the people are in that same space, now you can increase the logic and the functionality,” explains Soranno. “Now you can allow certain functions to occur with both the robot and the person in the same area.”
He says sophisticated control technology is driving this new approach to how people interact with machines.
“Now there’s a focus on the back end,” says Soranno. “We have the light curtains, scanner, or whatever on the front end detecting whether it’s safe to run. Then we have these really cool logic devices, which are microprocessor-based, programmable, drag-and-drop type of logic where you can control a lot of complex systems within a single device.”
“Our S3000 Expert Safety Laser Scanner has a feature that we call our Sim-4-Safety that allows an area scanner to establish four separate protected zones with a single device,” explains Soranno. “Typically with an area scanner, you can have two protected fields and two or four warning fields. With this, we actually get four protected fields and each protected field has two warning fields.”
Soranno says the system can be used to adjust presence-detection distances based on the robot’s speed. “Our S3000 technology is driven by where the industry is going, that new grey area, allowing people and moving machinery to share the same space under certain conditions.”
Built-in logic and safe motion are also making the systems easier to use, maintain, and troubleshoot.
“When something does go down, customers can’t afford to be down long, so they need a quick way to troubleshoot it,” says Tina Hull, Product Engineer for Safety Products at Omron Automation and Safety in Hoffman Estates, Illinois. She says Omron area scanners have individual sector indicators designed just for this purpose.
“We had a case where somebody emptied a trash can during a production break. When everybody else came back to start up production, the system would not start, because the safety said there was something in the zone. They spent hours trying to figure it out.”
She says the trash can was placed too close to the edge of the detection zone. With an Omron area scanner, lights on the scanner itself would have illuminated automatically, indicating which area caused the shutdown.
“That would reduce what they have to investigate,” says Hull. “Most factories put all the scanners and the indicators on an HMI, so it can blink or indicate which safety device caused the shutdown.”
Hull thinks the biggest trend in safety is to have everything integrated into one system. Integrated controllers with safe motion, and new safety laser scanners nearly half the size of previous models that can be configured for up to 70 zones, are all contributing to a smaller cell footprint.
“A lot of times now the safety is integrated into the robot system, and then we can also have things such as a conveyor line or a press controlled by one PLC and then integrated into the safety PLC to have one system,” says Hull. “Then you don’t need to have multiple controllers or expand it out further.”
Hull references an integrated pick-and-place system (pictured). “For servos and drives having safe motion, or built-in safety, the safety connection can be made directly into the safety PLC. This reduces the stopping time needed for the safe distance calculation since it no longer needs the force-guided contact relays.”
According to Hull, the system comprises three delta-style robots, two conveyors, and a vision system all controlled by the Omron NJ Machine Automation Controller. The safety system, consisting of five emergency stops and a light curtain, is monitored by the Omron NX Safety PLC. The entire system is networked together with EtherCAT and monitored through the HMI.
“Before safety was built in, they would have to connect those robots through force-guided relay contacts, which are two separate devices, and then from there, that would be the final on/off switch between the safety controller and the robot,” explains Hull. “There’s also a cycle time required to turn those switches on and off, which you’ve now reduced that time tremendously because now you don’t have that extra step.”
Integrated controls not only make the process faster, they take up less cabinet space.
“Floor space is a hot commodity,” says Hull. “So in the past, people have put these controllers above the systems in the safeguarded space. I’ve actually been in factories where they’ve built a mezzanine above their equipment just to hold their controllers. When you do that you induce another risk, which is the risk of falling. It’s one of OSHA’s top-cited violations.”
Hull also notes a move toward reducing the wiring and cabling as much as possible. She says issues with wiring account for approximately 95 percent of start-up failures. Less wiring also takes up less cabinet space and means a smaller footprint.
Opening the Door
On a mission to reduce the robot cell footprint, safety peripherals are teaming up to provide protection from cell hazards. Physical barriers are keeping hazards in and unintentional contact at bay, while presence-sensing safety devices are keeping tabs on cell occupants.
“If you’re using a safeguarding device in a vertical application, whether it’s a light curtain or a door, you can typically get much closer to the hazard than if you apply a sensor that is horizontal like a mat or typical area scanner,” says SICK’s Soranno.
“The benefit of an area scanner is that you can mount it in a corner and you can protect a very large area, and then you can program it,” explains Soranno. “It doesn’t have to be exactly square or round. With an area scanner, you can mold the sensing field to the shape that’s needed for the cell, not necessarily force the cell to fit the shape of your safeguard.”
“A lot of times, especially in robot applications, you see a vertical and a horizontal device,” says Soranno. “You use a vertical light curtain, for instance, so you can reduce the footprint. But if somebody can walk past the light curtain and be between the light curtain and the robot, and not be detected, then that’s a new hazard.”
He says that is when a light curtain or door is combined with an area scanner to detect the presence of a person between the light curtain or door and the hazard.
“One of the big advantages of doors versus light curtains is that a person might be able to move faster than the control system is able to react with a light curtain,” explains RIA’s Davison. “The way we mitigate that is to create a safety distance depending on reaction times associated with the safety-related parts of the control system.”
“If you’re using a light curtain, you may need to have a large empty space between the light curtain and the closest hazard to create that safety distance,” he continues. “Compare that to a door or some other type of physical barrier. You can have that door right up against the hazard, because there’s no way for somebody to reach over, under, around, or through the door to get to the hazard.”
In conjunction with opto-electronic presence-sensing devices, machine protection doors offered by suppliers such as Rite-Hite Machine Guarding (formerly Frommelt Safety Products) and ASSA ABLOY help safely reduce the robot cell footprint.
“If you were to look at a work cell that just used a light curtain at the entry point of the cell and didn’t use a door, you have a bigger footprint because that light curtain has to be placed out in front of the work cell such that it stops the machine in time to prevent injury to a person,” says Jeff Nedblake, Vertical Segment Manager - Machine Protection for entrance system in Lawrenceville, Georgia. “You can have a much smaller footprint if you don’t have to factor in the safe stopping distance requirements of optical devices. Even beyond that is the benefit of having an approved physical barrier.”
Proponents of machine protection doors are quick to point out that there are a lot of safety peripherals that keep things from going into the robot cell, but the advantage of doors is that they keep things from coming out. Doors contain noise, weld spatter, fumes, odors, heat, and any mist or projectiles that may be expelled by robotic processes.
“My job as a safety professional is to make sure that people on a production floor go home in the same condition at the end of the day that they came in,” says Nedblake. “That doesn’t just mean that they didn’t get hit by a robot, but that their hearing is just as good as when they came in this morning, their eyes didn’t experience any arc flash, and they didn’t breathe any unnecessary contaminants.”
Nedblake says his doors are typically used for robotic welding cells, but also material handling, material removal and robotic waterjet applications.
The doors come in two versions. A tear-resistant door curtain (pictured) called RapidProtect™ 300 and a double-walled aluminum curtain, RapidProtect 2000, which meets safety requirements for laser welding. According to Nedblake, the doors are capable of opening and closing five times a minute due to the use of a variable frequency drive for motor control.
“We equip our doors with pre-running photo cells and inline door cells that reverse the curtain if the plane is broken while the door is closing,” says Nedblake.
Outside the Box
With an array of advanced safety peripherals on the market, robot controllers with embedded safety functions, and robot standards that provide for some degree of flexibility, the experts encourage creativity. But they caution against complacency, and stress competency.
“Unfortunately, we can’t control people from doing dangerous things in proximity of the robot, says Pilz’s Dominguez. “When you’re designing a system, you also have to come up with scenarios that might occur when someone is now interacting with the robot both intentionally and unintentionally. You can‘t say they shouldn’t have been standing in that spot or shouldn’t have put their hand in there. You have to consider it and lower the risk. When you start designing cells without fences, a lot more questions start coming up.”
Whether we’re talking about a partially restricted robot cell for easier access between successive processes, or full-blown human-robot collaborative interaction, all agree, the bottom line is the same. Do a risk assessment.
RIA’s Davison suggests: “Work with somebody you trust. There’s enough at stake here and enough options and variance, and a sufficient degree of complexity, that in many of these cases, you probably can’t do it on your own unless you have that experience or expertise.”
“There’s always that trade-off of how much is that extra productivity or reduction in downtime worth to an organization,” says SICK’s Soranno. “There’s going to be an increased level of complexity and an increased level of competency that’s required to make sure it’s done correctly.”
“It’s safety. You can’t get it mostly right,” he adds. “The part that’s wrong is going to hurt somebody.”
While there’s a significant degree of complexity, the reward may be well worth it.
“Competency is the key,” says Rockwell’s Nelson Shea. “You can get somebody buying a brand-new robot with all the optional features, and all the great things that they can do with it to have a smaller footprint. But if they implement the robot system just like they implemented the last one, they are not going to get that benefit. If you do now what you did then, you’re not going to be any better off. New features and new capabilities mean you have to think outside the box.”