The Safety Framework
Designers should be aware of applicable safety standards that might apply to the product requirements, and they can learn lessons from the experience gained by robots deployed in the field. Awareness, fault tolerance, and explicit communication are the three areas of the robotic safety framework that will replace the need for cages.
Awareness can be enhanced by the use of inexpensive computing power and sensors that monitor the work environment. Human proximity detection has been accomplished by using video cameras, sonar, weight sensing mats, lasers, infrared detectors, 3D sensors using structured light, and time-of-flight sensors. Robotic self-awareness of the task being performed, speed, angles, and forces on the moving parts enables it to know if something goes wrong such as bumping into the person next to it. Designers can use any combination of the aforementioned technologies and strategies, but the best mix has yet to be discovered. Cost, reliability, and the added complexity present tremendous challenges that will be dependent upon the application and customer. The 2007 Swedish factory worker might not have been hurt at all if the robot was aware that he was there.
Erik Nieves, technology director at Yaskawa Motoman, demonstrated a robotic arm slowing down when he entered a work zone defined by a laser measurement system (see Figure 6-5). It is an example of speed and separation monitoring for collaborative robots by which the robot slows down if a person enters the danger zone. In speed and separation monitoring, you might use a traditional robot connected to safety-rated controllers/sensors and essentially keep the human and robot separated during full-speed operation and then slow to a speed of 250 mm/s when a worker moves within the collaborative space. The installation can claim compliance with the safety standards if it is certified by an integrator.
Figure 6-5. Speed and separation monitoring
The approach employed by Rethink Robotics’ Baxter and Universal Robots’ UR5 is that of power and force limiting (see Figure 6-6), and both companies state compliance with the applicable sections of the safety standard. This approach is inherently safe because the robot does not move too fast and does not exert excessive force when bumping into an object or person; thus, the requirement for external sensors is negated. Users are free to approach the robot and only need to perform their own safety assessment to identify and mitigate any potential hazards present in the environment and application.
Figure 6-6. Power and force limiting
In the Motoman example, a laser scanner points vertically to create a virtual wall at the front edge of the robot’s work area. SICK is one of the more well-known manufacturers of laser scanners and other sensors. Specifications to consider are the range of detection, viewing angle, resolution, sensitivity, and robustness of the interference. Figure 6-7 presents SICK’s LMS400 measurement system, which has a range of 0.7 to 3.0 meters and 99 degrees of view.
Figure 6-7. LMS400 laser scanner (© 2013 SICK AG)