Motion Control Resources
Collaborative Robots Offer New Automation Options
by Kristin Lewotsky, Contributing Editor
Motion Control & Motor Association Posted 06/25/2018
For decades, robots have been a fixture in the industrial landscape, performing tasks that are tedious or arduous or unsafe for humans. Robots could function at a higher speed than humans and sustain it over an entire shift without getting tired or losing focus. The mass and speed of typical industrial robots presented significant risk to human operators, however. As a result, industrial robots were traditionally placed inside of fixed enclosures. This presented limitations accessing the robot for any means required a complete shutdown and time-consuming restart.
Although the old-school approach to robotics continues to thrive, a new type of robot makes it possible for humans to work closely and safely together with the robot without the need for safety barriers. Different levels of collaboration exist. At the most sophisticated, a collaborative robot is capable of collaborative operation, which means cooperation with a human being within a defined workspace. They tend to be built to handle lower forces and move more slowly to enable them to respond in a safe manner around human operators.
Classes of collaboration
ISO-10218, the safety standard that laid the groundwork for much of collaborative robotics, defines four collaborative features:
Safety-rated Monitored Stop
Safety-rated Monitored Stop is for applications in which the human operator needs to occasionally enter the workspace of the robot. With Safety-rated Monitored Stop functionality, the robot ceases motion when it detects that a human operator has entered the workspace. The robot stops by using brakes rather than by cutting power to the drive. That feature speeds recovery while maintaining a safe environment.
Speed and separation monitoring
In speed and separation monitoring, the robot monitors the position of the human operator within a defined space to invoke software-defined rules. The greater the separation between human operator and robot, the faster the robot is allowed to move. As the human operator moves closer to the robot, the robot slows down or may even stop in order to maintain a safe condition for the circumstances.
Programming robots can be complex and time-consuming, even when function libraries are used. Hand guiding provides an alternative. The human operator can touch and move the robot arm, teaching it a position or path. These units can be conventional industrial robots but require additional sensors such as a force-torque sensor to give the robot an ability to “feel” contact.
This is the model for what is widely referred to as a collaborative robot. Through sensors, the robot is able to detect force applied by contact with a human being and will stop immediately. ISO-15066 defines the maximum amount of force that can be applied to various human body parts without causing pain or damage. This information is used in programming the robot. Unlike the other three categories, these types of robots are not standard industrial robots repurposed. Instead, they are specifically designed to dissipate force, being smoother and rounder with no sharp edges, pinch points, or exposed motors.
Building collaborative robots
Collaborative robots are equipped with multiple sensors, enabling them to perceive their environments. They generally handle lower payloads and move more slowly than conventional industrial robots. They involve advanced control systems that enable them to analyze and operate within a more complex environment.
In collaboration, the human operator might stand on the opposite side of the table from the robot, allowing the robot to pick up a load, after which the human operator could install a smaller part that requires more manual dexterity. The collaborative concept moves beyond simply industrial applications and manufacturing to encompass service robots ranging from wheelchair fixtures to smart exoskeletons.
From the motion control standpoint, servo motors, drives, and gearing play an essential role in robotics in general and collaborative robots, in particular. The most common approach is to use a frameless motor built directly into the joint. The approach saves space and reduces weight. Frameless designs often have boreholes in the center, making it easy to route signal and power Again, this applies equally to an exoskeleton as to industrial robot arm.
“We see a big trend toward miniaturization,” says Christian Fritz, director of sales for Motion Controls and Electronics for Maxon Precision Motors (Foster City, California). “There is a need for small, powerful motors that become part of the mechanical structure. When we work with robotics or with companies building, commercializing robotics systems, customization is a topic that comes up every time – customization of windings, flat motors that better fit in the envelope of the mechanical system of the robot.“
Integrated components are another trend that Fritz identifies. Gear motors with integrated sensors are one example, or smart motors with integrated controllers for drives. The latter approach minimizes cabling, which reduces cost, complexity, and points of failure.
When it comes to robots with any of the collaborative use cases, approaches to the platform vary. “What a lot of people are doing right now in collaborative robots is basically just taking traditional robots and adding a bunch of expensive sensors to them or running them very slowly," says Jim Shimano, program manager at Precise Automation (Fremont, California). ”You need to take a different approach and really look at what makes a robot a robot and then what can make it safe to operate collaboratively and run at reasonable speeds.” Traditional industrial robots tend to use gear boxes with high reduction ratios. The Precise team went the opposite direction by lowering the gear ratio and working to reduce friction in the robot. “Once you have a lower gear ratio, low friction system you can do a much better job of predicting the torques you need to run a robot since the torque signal to noise ratio is significantly improved. This permits the robot to detect collisions by monitoring the motor torques, without extra expensive sensors, and to quickly react without generating the large contact forces inherent in high gear ratio/high friction drives because now there's not as much less signal to noise. This allows a collaborative robot to run safely even at cycle times comparable to industrial robots,” says Shimano.
The approach also changes the controls techniques. “The important part is you're relying more on the feed forward side of the controller than on the feedback side,” says Shimano. The [friction] is so big in the lot of traditional robots that you rely on the feedback part. [In the new version], feed forward is doing most of the work and feedback is for collision detection.” The model has to be dynamic, not static, with minute feed-forward adjustments being made to torque. “We're relying more on the feed-forward side and because of that the model has to take into account all of the dynamic characteristics of the robot,” he adds.
Sense and safety
One of the key concerns in collaborative robots is sensor deployment. Comprehensive sensing of the surrounding environment is essential for safe operation. Sensors can be expensive and complex integrate. The partial solution is to use sensing technology inherent to the motion elements themselves, such as sensor functionality that already exists in the drive. “When it comes to deployment, people start to streamline and want to move as much of the functionality into components that are already in the system,” says Fritz. "A simple example is the current flowing through the motor windings. In a DC motor, the current is proportional to the torque output. Selecting a drive that provides access to the current data allows developers to gain insight into what’s going on in their application based on the electrical and mechanical principals of a motor. You can, for example, use that information to detect whether anything is blocking the movement of the robot without adding an additional sensor.”
Safety is an essential element of the collaborative design. Safety rated drives and controllers provide the basic functionality. As with all safety systems, redundancy and eliminating common mode failure are key.
One approach to redundancy is the use of safety-rated software. “That's kind of the underpinning of a lot of these collaborative applications areas is to be able to have the automation integrated and deployed with safe software so that you can monitor the presence and absence of personnel and then act accordingly,” says Rick Maxwell, director of engineering, General Industry and Automotive Segment, FANUC America. “The actual monitoring of speed and torque are done on an independent processor that is always checking the application.” If the safety software is set to limit a rotary joint to no more than 300° per second, for example, it would block an application calling for the joint to move at 500° per second.
The final advice to robotics manufacturers, OEMs, and end-users alike is that safety needs to be applied strategically. “That's probably my biggest take away for the article is don't be afraid of looking at the safety side of it,” says Russ Jones, manager, New Market Development, FANUC America. “We all want our customers to be as efficient and competitive as possible, so it’s important that they evaluate each situation and understand that there are many applications that allow human/machine interaction and some that do not. That understanding will keep production running without unnecessary shut downs.
The field of collaborative robots is in its early days with developments ongoing. Here, motion suppliers can help OEMs find the optimal approach for their system. “I think we will see the emerge of many, many different robot systems in the near future and the people who are working and who are integrating and realizing those systems, they will have to focus on their core competency,” says Fritz. “I think what people should walk away with is that motion vendors are ready to help them out with some of the tasks, provide more integrated systems, and work with them on specialty motors. There are technologies and components available to give them a lot more than just a little piece that can spin and provide torque.”