Robots: there's one for every job — Part 2

In approaching an investment in robotics, one should consider all aspects of an application prior to making a final selection.

In Part 1 of this article some history of industrial robots was presented, and the various pros and cons of articulated and Cartesian robots. In this second part SCARA and delta robots (and the newest type: cobots) will be discussed, before looking at the considerations necessary in choosing the right type.

SCARA robots: benefits and limitations

SCARA robots have been designed and optimised for light applications. (The SCARA acronym stands for Selective Compliance Articulated Robot Arm, although some use Assembly in place of Articulated.) They are a streamlined version of articulated robots. Their simplicity and small size make them easy to integrate on assembly lines and they can achieve quite impressive cycle times, with high accuracy. They are very adept at functions like inserting components in spaces with tight tolerances while maintaining their rigidity in such movements, which makes them a cost-effective choice in a lot of pick-and-place applications as well as small parts handling. (In performing such tasks, there are accessories like feeding systems to maintain a constant supply of parts.) Programming and commissioning is relatively easy and fast, using the manufacturer-supplied teach pendant.

But with low cost comes limitations. The SCARA robot requires a dedicated robot controller and is limited to planar applications. It is generally restricted to three axes. It may be the optimal solution where its full capability — three or four degrees of freedom — can be used, but if the job only requires two (horizontal and vertical movement, for example), the SCARA robot cannot be reduced to a two-axis system, making it less attractive than a Cartesian gantry-style robot from both a cost and performance standpoint. Like the articulated robot, the SCARA robot footprint also extends further than the working zone, resulting in a loss of functional space in and around the unit.

Delta robots: benefits and limitations

The delta robot is mainly renowned for its speed, with pick rates up to 300/min. Its mounting type puts it above its working zone, limiting the loss of footprint. It is often paired with a vision system to pick pieces randomly placed in more complex sorting and packing applications. Just like the articulated and SCARA robot, it will generally be provided with a teach pendant for easy programming.

On the other hand, the delta robot’s payload capacity is generally much lower than alternative technologies. Its wiry inverse tripod design makes it less robust than the other robot options, which reduces its maximum payload weight. That performance limitation is the price for achieving such a high dynamic capability. The delta robot has a quite limited working envelope. Its design does not allow long reaches. Like Cartesian and SCARA robots, delta robots are generally limited to four axes and cannot provide the flexibility of an articulated robot. Its complex assembly makes it more difficult to maintain and repair. And if used as a top mount on a large machine, there may be a need to reinforce the machine frame to bear the added weight.

Collaborative robots: benefits and limitations

Collaborative robots, or cobots, are a relatively recent development with a promising future in making possible safe human–machine interaction. By allowing a direct collaboration between a worker and robot, they are adding a dimension to our understanding of how automation can be integrated into industry. A cobot can be an articulated, Cartesian, SCARA or delta robot, although to date, most would be categorised as articulated. They come with payload capacity of 4–35 kg, scaling up in size and reach (also price) accordingly. There are models with up to seven axes; the latter can perform tasks that are particularly challengingly ergonomically, and are even being used as independent production line robots. The difference between cobots and other robots is their built-in safety features that allow direct interaction with humans, without protective shielding, safety curtains or other safety features. Since they don’t need fixed external safety barriers, some cobots can be mounted on mobile platforms to go wherever they are needed. It is important to note that a safety evaluation of the application has to be done and, while the cobot itself might be safe, if the tool being used on the end of its arm is sharp, an external safety barrier likely would be needed.

Cobots are limited in speed and payload, which disappoints some users looking for a conventional robot that doesn’t require expensive safety protection. The greatest value of cobots is where they can free a skilled employee from the menial aspects of their job to concentrate exclusively on the high-value aspects. For example, in complex device assembly requiring a deft human touch, a cobot can perform simple handling or manufacturing tasks in support of the worker, who can then focus exclusively on the part of the job that makes full use of their skills or knowledge. It’s a valid approach, providing the productivity boost provided by the collaboration delivers a reasonably quick payback of the investment. Otherwise, it can turn into what amounts to an expensive vanity project.

Considerations in making your choice

In approaching an investment in robotics, one should consider all aspects of an application prior to making a final selection. Below are some factors to consider.

Reach and payload

The reach and payload should be the first criteria, as this may immediately shorten the list of suitable options. On technical grounds alone, a large heavy load would rule out any consideration of lightweight handling technologies. On the other hand, if the reach is long but the payload weight is low, a lower-cost Cartesian robot might suffice.

Flexibility

If an application requires five or six degrees of freedom, an articulated robot may be the only viable solution. If it is, one option for the price-sensitive business requiring one or two robots might be repurposed (used) units. However, for simpler applications, like small parts positioning and loading, electronic parts insertion, and box and machine tool loading — any application where two or three axes are sufficient — why pay for more axes than the application calls for?

Speed

Does the application require a high pick rate, like that of a delta robot (up to 300/min is fairly common), or would a lesser pick rate of a Cartesian gantry or SCARA robot suffice? Here’s a real-world example: a machine builder constructed two virtually identical custom box loading and palletising machines for a dairy to load yoghurt containers into boxes. Each machine was intended for a different container format. For the first machine boxing plastic tubs, a high pick rate was required, so a top-mounted delta robot was installed for maximum throughput. For the second machine boxing four-packs (2x2) of single-serve cups, a lower pick rate was sufficient, and the machine builder was able to reduce the cost to the end user by substituting a two-axis Cartesian gantry as the box loader.

Space and footprint

More and more, machine and production line footprints are key planning concerns. Floor space is expensive, and companies want to optimise their shop floor layout. Cartesian and delta robots provide a clear advantage over the other technologies, since only vertical space is lost, which is generally less critical.

Engineering and project development

Design, assembly, installation and commissioning time and expense should be factored into comparative costing, especially incorporating a robot into a larger machine or system. Delays in receiving and assembling the robot could hold up the entire project. There are online and software tools that can minimise engineering and procurement risks.

Maintainability, repairability and availability

Unscheduled downtime is every production manager’s nightmare. Robots should be relatively easy to maintain and repair. This is a particularly important issue for businesses that don’t have a large robotic fleet with in-house technical know-how. Who is going to fix the robot? How long will it take? What is the lead time for delivery of critical replacement parts like motors and controllers? During planning stages, these issues are often overlooked at one’s future peril.

Standardisation: when good is good enough

Standardisation within a company or industry could be a valid consideration on business grounds, even if the robot selected is not the most ideally adapted or even the cheapest but capable of doing the job. Sometimes, the well-travelled path will prove to be the one of least resistance (and risk).

Conclusions

The proliferation of robot technologies has enabled businesses of all sizes to access the benefits of automation. Which one is best? It’s usually the one that is the best fit — not just for achieving the productivity gains from the investment and satisfying the technical requirements of the application, but also from the standpoint of related issues like plant safety, space utilisation and, of course, the going-in cost and after-sales support.

Image: iStock.com/anon-tae