The race to increase productivity forces automated equipment and operators to work at near breakneck rates, with little margin for error.
The chance that a person will be distracted, or a machine might move unexpectedly, raises the possibility of injuries, prompting manufacturers to look at techniques to minimize accidents.
Light curtains have become the technology of choice in many fields. “Our metal stampers and punch presses run automatically once they’re loaded. If the operator sticks his hand in during the automatic cycle, he could get hurt. The light curtain will detect an intrusion and either stops the machine, or stops it and pulls the mechanical arm back,” says Steve Amaro, process engineer at Buhrke Industries, of Arlington Heights, Ill. He notes that Buhrke has 20 to 30 light curtains in its facility.
For the past several years, light curtains have been one of the key sensing technologies used to safeguard personnel and equipment. Competing technologies such as lasers and fiber optics are starting to see increasing usage, particularly in areas where explosions are possible.
Safety pays
While productivity is a key focus for today’s factories, protecting workers is just as critical. In most facilities, safety ranks with quality as a must-have. In addition to the emotional impact that ripples throughout a facility that suffers a major injury, there’s a huge impact on operations. It is also important to make sure accidents don’t damage equipment.
“Safeguarding personnel is primary, but you’ve also got to protect processes and machinery. If you have a $10,000 die that crashes because someone does something inappropriately, it can have a huge impact on your operation,” says Mike Carlson, manager for safety products at Banner Engineering Corp., in Minneapolis.
One of the most common safeguarding techniques is to use light to form a wall around dangerous areas. For years, light curtains have watched for intrusions, sending shutdown signals or other warnings when something comes between the light source and detector. Now, equipment manufacturers are starting to replace lamps and light-emitting diodes (LEDs) with lasers.
“Laser scanners are becoming more popular, more often as a perimeter guard. One market is in painting, where you might have a hazardous area with the possibility of explosions,” says Gil Guajardo, Safety Product Marketing Manager at Omron Electronics LLC, based in Schaumburg, Ill.
Markets in these hazardous environments often change slowly. “Any electronic device has to be evaluated for things like sparks, energy, temperature, anything that might ignite an explosive mixture,” Guajardo says.
Lasers are also starting to show up on robotic equipment from Motoman Inc., which has made light curtains a fairly standard work cell element over the past several years. The West Carrollton, Ohio, company is expanding its use of proximity laser sensors made mainly by Sick Inc., of Minneapolis.
“These laser scanners are very cost effective compared to light curtains,” says Chris Anderson, technology leader at Motoman.
Lasers can be added even in the applications that still use safety mats, which sense pressure to determine intrusions. “People can remove the mat, and put the laser detector in the upper corner of the area. When someone steps into the area the mat used to cover, the laser can shut the system down or sound an alert,” Anderson says.
While optical technologies have virtually eliminated safety mats on Motoman robots, lasers may not cause the demise of the light curtain. “We’re adopting lasers on a case-by-case basis,” says Carl Traynor, marketing director at Motoman. He adds that Motoman has been using lasers for around 18 months.
Curtain tips
Regardless of whether lasers or conventional light sources are used, setting the right parameters for safety is one of the essential aspects of the project. Making the safe zone too large may cause unwanted false positives, when people or equipment break the light beam but are in no real danger.
Designers must calculate where to place the light curtain, depending on the speed of the machine and the time it will take to trigger a response to an intrusion. “If the machine goes faster, the light curtain must be farther from the machine,” says Buhrke Industries’ Amaro. However, he notes that when light curtains are located farther away, there’s a chance that operators may reach in above or below the sensor, either intentionally or accidentally.
Making larger light curtains isn’t always a cost effective solution. “It can be cheaper to put a piece of sheet metal across the top or bottom,” Amaro says. Another cost-effective way to make a light curtain cover a larger area is to use mirrors, he adds.
The use of mirrors, as well as pricing declines for many light generating and detecting components, is letting engineers fine tune light curtains, putting them in smaller regions closer to dangerous moving parts. “We’re using tighter resolution to detect fingers or hands, using 14 to 30 mm resolution,” says Banner Engineering’s Carlson. Previously, larger resolution beams located farther away from the danger point were tuned to detect torsos.
Often, light curtain are focused by angling the beam of light. That expands or shrinks the coverage area, helping prevent injuries without covering too much area.
Regardless of the type of sensing technology being employed, these sensors are now being tied directly to networks. “We’re seeing more networking with safety devices and safety controllers. They’re tied to DeviceNet or another network,” says Guajardo, at Omron. That makes it possible to monitor for problems, and when they arise, to notify managers immediately.
Sensors are also being used in conjunction with the time-tested safety technique of putting a piece of metal or hard plastic around a dangerous area. In applications in which these so-called hard guards are deployed, contact switches or sensors have been used to determine whether the guard is replaced after it’s removed for maintenance or other reasons. But that’s changing. “There’s a trend away from mechanical contact switches to non-contact sensors like magnetics,” Carlson says. Banner is now using fiber optics to determine when these guards are not in place. By shining a light to a reflective point on the guard, the unit determines that the guard is missing when the light beam is broken. The benefits of fiber optics are similar to those of lasers. “One advantage is that there’s no electrical energy, so it works well in explosive environments,” Carlson says.
Sensitive robots
Sometimes, sophisticated products can monitor for collisions without the addition of special safety-oriented sensors. Robot manufacturers use the equipment itself to determine when an arm is encountering an object that should not be in the arm’s path.
“We can sense the torque of the servo motor. If we see an abnormally high torque, the robot will stop. The robots are sensitive enough that if you put your arm in the path, it will stop, though not always immediately for a small change in torque,” says Motoman’s Anderson.
Though safety mats are fading in some applications such as robots, they remain viable in other roles. Bircher America Inc., of Elk Grove Village, Ill., and Larco Manufacturing, of Brainerd, Minn., both continue to upgrade their safety mat systems, adding functions such as remote monitoring and improved sensitivity.
For more information search keywords “safety” and “sensors” at www.automationworld.com.
See sidebar to this article: Sensors, Not Fingers, Stop Saw Blade
