"Collaborative" Is Not Just About the Robot
With all the excitement about the new “Collaborative Robot” (cobot) offerings being introduced, it would be a natural conclusion to think that being "Collaborative" is all about the hardware. Since many of the new robots being released are significantly different in their design or appearance than their traditional industrial cousins, it’s an easy leap to make. In the end though, all robots have one thing in common: they all need to pass a Safety Risk Assessment.
One of the greatest determining factors in a robot solution passing a Risk Assessment is the actual application it is performing and/or the process it is involved in. To paraphrase the ISO (ISO 10218) and RIA (R15.06) Safety standards:
“A risk assessment shall be done on the overall process comprising a risk analysis and a risk evaluation. This means identifying all risks and reducing them to an appropriate level”
The rub is that regardless of the type of robot being used, some applications and processes just aren’t suited for collaborative or unguarded automation. Take Plasma Cutting: having an operator rubbing shoulders with a robot while it’s wielding a plasma cutter just isn’t safe. It doesn’t matter if the robot is force limited, the torch is still dangerous. It doesn’t matter if you limit the speed of the robot, the torch is still dangerous.
Another situation might be using a robot to tend multiple stamping machines. The end of arm tooling or the part the robot is placing into the stamping machines may not pose a safety hazard, but due to the layout of the stamping machines, you may need to guard the entire process because of the hazards inherent with stamping machines. In the end, the robot is just one piece of the puzzle that needs to be safe. There is still significant work to be done to ensure the rest of the system and the process is safe for the operators.
There Are Different Ways to Collaborate
When people talk about Collaborative Robots, many people default to what is called Force Limited Robots (FLR’s). Although this is one of the fastest growing and most talked about segments of industrial automation, it is only one way to have a robot used in a collaborative fashion. Per the ISO and RIA Safety Standards, there are actually four ways to collaborate with a robot:
Safety Monitored Stop
This is certainly one of the most basic, but it may be just the trick for applications that are non-cycle intensive or where there may be a significant dwell between cycles for an operator to replenish or retrieve parts. In essence, this requires that you use a safety rated presence-sensing device such as a Safety Mat or Area Laser Scanner to detect the presence of someone in a the work area of the robot.
Once the safety device is triggered, the robot comes to a complete stop instead of entering an E-Stop condition. While it is stopped, it’s position is constantly monitored in a Category 3 safety rated manner. Since this is reliant on external safety devices and the robot is stopped, most recent industrial robots can be used in this fashion. With the robot being stopped vs. E-Stopped, once the work area is cleared the robot will pick up where it left off without having to wait for the E-Stop to be cleared or the system to be powered back up. This not only keeps the interruption to the robot’s cycle time to a minimum, but it decreases the wear and tear on the hardware.
This is where an external device is used to translate the pressure and direction exerted by an operator's hand into motion of the robot at a limited safe speed. This is most commonly used for "teaching" the robot waypoints and paths for applications like welding and dispensing. Although less common, it can also be used in situations where a robot is being used to precisely move a heavy or awkward load with visual guidance from an operator. This is another scenario where most current industrial robots would be suitable.
Speed and Separation Monitoring
This is a recent development and requires a fairly sophisticated combination of safety rated sensing technology, a very capable robot safety interface, and strong understanding of the risks involved in the application. In it’s simplest form, this is where the robot work area is broken up in smaller areas based on the amount of risk an operator is exposed to while being there. These areas are monitored by external safety devices and are tied into the safety interface of the robot. Based on which area an operator may be in, the speed, acceleration, and/or reach of the robot will altered in a safety monitored fashion. This is particularly useful when the robot will be working in a given area of the cell for a significant portion of it’s cycle, leaving other monitored areas open for operators to work in.
Force and Power Limited
These are the robots that are getting the lion’s share of the attention these days. The short version is that these robots are designed in such a way that they are inherently limited to where they aren’t able to produce enough force to hurt someone. The challenge with this is the definition of ‘hurt’ has been in the eye of the beholder. Later this year, ISO and RIA will be releasing an addendum (TS 15066) to their safety standards that will eliminate some of the guess work on what force is reasonable based on the part of the body impacted. As you can imagine, this type of collaboration requires is a unique type of robot and controller. When used for the right applications this type of solution can provide significant advantages over traditional robots.
Not all Force Limited Robots Are Created Equal
Since most people think of Collaborative Robots as Force Limited Robots, it is important to understand that even though the robot OEMs insure that they conform to the ISO and RIA directives for Force and Power Limited Robots, each manufacturer gets there in a different fashion.
- Rethink’s ‘Baxter’ uses elastic actuators vs standard servos and gearboxes. The essentially gives it a ‘breakaway’ effect in the event of a collision.
- Universal Robots uses a proprietary combination of both hardware and software for limiting the force being produced at the tool plate regardless of its speed or load.
- Kuka and ABB use a series of transducers and sensors.
- Some others even add a padded force sensitive ‘skin’ to the arm.
With all these variations you may ask yourself which one is best for your requirements. The good news is, at least for the time being, each of the offerings from the major players are filling a particular need and/or price point. Whether that need is an extremely low cost two arm unit to handle slower speed, low repeatability applications or to precisely monitor tool plate forces and the sky is the limit, these robots are filling automation niches that traditional robots have never been able to go before.
If you have questions about Collaborative Robots or the applications that they may be suitable for their, please feel free to contact us.