Making the Transition from Manual to Robotic Assembly
There's nothing like watching an assembly robot at work. The tireless reliability and efficiency are a sight to behold. However, effective robotics systems don't just happen spontaneously. They take a lot of planning. The devil is in the details. Anticipating and resolving problems early on can mean avoiding headaches down the road.
In particular, when transitioning from manual assembly to robotics, it's important to discern exactly what workers are doing when they assemble a product's various parts. Human beings can often do what it takes to get a part to fit, even if it is slightly out of spec-which can ultimately lower quality and increase scrap.
A robot, on the other hand, is much more consistent and less tolerant of parts differences. As a result, engineers need to clearly define a range of part variances so the assembly system can identify acceptable parts. They may ultimately have to tighten parts specifications, or redesign the product and its constituent parts so it will be easier for a robot to assemble. This is an often underappreciated aspect to robotics-that it forces assemblers to clean up their act.
In some ways, this process is similar to that used in streamlining manual assembly. For example, reducing the number of parts in a product will cut costs and increase process reliability, regardless of the type of equipment used to assemble it. However, other guidelines are more specific to robots.
For example, unlike human assemblers, a single robot cannot hold a part in one hand and a tool in the other. Therefore, products should be designed so they can be assembled in layers from the bottom up, without having to be reoriented midway through their assembly-wasted motion that serves only to increase cycle times.
Similarly, because robots still cannot move with quite the same repeatability as dedicated, "hard" automation, it is a good idea if parts have self-aligning features, such as lips or chamfers, to help the robot insert them.
Throughout this process, operators should be consulted on the nuances of their work: how they identify good parts; what techniques they may use to facilitate assembly. Encouraging input from employees can provide the insight necessary to create an effective, total application.
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Along these same lines, engineers need to anticipate exactly how parts will be presented to an assembly robot. Recent advances in machine vision have provided systems integrators with powerful new tools in this area. For example, using vision, a robot can now both inspect and pick parts randomly oriented on a conveyor, or even jumbled together in a bin. However, this kind of technology, although increasingly affordable and easy to use, can still be both expensive and tricky to implement in the real world.
For example, lighting has been described by some as being almost an art, due to the many variables involved. Cameras and image-processing software can be impeded by things like sunshine or light given off by other equipment. It may also be necessary to modify parts so they will provide the machine vision system with recognizable visual cues.
Bear in mind that parts that don't naturally lie flat may not be suitable for a conveyor, even with an advanced machine vision system; the same goes if parts are piled on top of one another. These are not insurmountable problems-for example, overlapping parts can simply be allowed to travel to the end of the conveyor and then returned to the beginning for another pass. Assemblers can also use cascading conveyors or other separating systems. The point here is that anticipating these kinds of issues will make implementing a robot much easier.
Although it's generally easier to present fixtured parts on a rack, tray, or indexed table or conveyor, it is important to make sure that all parts will rest in their respective pockets in a consistent, stable manner. The pockets should provide clearance for the gripper's fingers, and the robot should be able to pick up a part and insert it without any further manipulation. Equipping the robot with a self-centering gripper or even machine vision will allow engineers to use less-precise trays or fixtures.
Similarly, when using a vibratory bowl feeder, engineers need to ensure that the parts being presented will not tangle or overlap, so they can picked by the robot without interference.
Robotic grippers are not as nimble as human hands, and some parts are easier for robots to grip than others. A part with two parallel surfaces can be handled by a two-fingered gripper. A circular part can be handled by either its outside edges or its inside edges, if it has a hole in the middle. Adding a small lip to a part can help a gripper reliably manipulate the part and increase the efficiency of the system. If the robot will handle more than one type of part, those parts need to be designed so they can all be manipulated with the same gripper. Tool changers and multifunction end effectors are another option.
Finally, part material can be a factor in robot assembly, especially when using grippers. Flexible parts can pose a challenge and may require special fixturing. Parts with cosmetic surfaces or parts made from fragile materials may require grippers or some other kind of fixturing that will handle them more gently.