Robots: there's one for every job — Part 1

The proliferation of robot technologies has enabled businesses of all sizes to access the benefits of automation — but which type is best?

The first industrial robot was installed at a General Motors factory in New Jersey in 1961. In 1969, the Stanford Arm with six degrees of freedom was developed, making robots suitable for an extraordinary range of tasks.

For a half century now, robots have been the centrepiece of Industry 3.0, which is typically defined as the age of pre-digital automation. They will be just as critical if not more so as the world transitions to Industry 4.0, the digital automation age. Robots have been changing the industrial landscape since their introduction in the 1970s; they represented a quantum leap in productivity, flexibility and reliability. Almost any repetitive motion involving the movement of an object — be it tooling like a welding gun or sheet metal being welded — can be done faster and with greater precision and repeatability by a robot.

Early on, the image of the large, six-axis articulated robot welding car and truck bodies became fixed in the popular imagination. Articulated robots have spread throughout and beyond heavy industry with many improvements to the robot itself as well as the development of more end effectors to address the wider need for flexible automation. Articulated robots are used in sectors as diverse as health care, food and beverage, steelmaking, warehousing — wherever there are repetitive or environmentally or ergonomically challenging tasks that can be accomplished faster, more reliably or more cost-effectively. Robots are even assembling new robots.

Today, robots are easier than ever to integrate into any manufacturing or processing environment, including food zones, cleanrooms and warehouses. All that said, given this plethora of options, it’s more important than ever to know which type of robot best fits a company’s needs from both a capabilities and cost standpoint. The major types of robots — articulated, Cartesian, SCARA, delta and cobot — each have strengths and limitations. Understanding these pros and cons is a necessary starting point for making an initial investment in robotics.

New concepts in robotics

Initially, the robot revolution provided major manufacturers like automakers with even greater economies of scale, but offered nothing to most small and medium-sized businesses. Developments in Cartesian robotics (linear), SCARA and delta robots to name the most widely used — as well the newest commercial innovation, collaborative robots — have made automation accessible to businesses of almost any size.

Each type of robot comes with benefits and limitations. For would-be new adopters of robotics, it’s important to understand those possibilities and pitfalls.

Robots come with 1–7 axes, each axis providing a degree of freedom. A two-axis Cartesian gantry typically plots on the X-Y or Y-Z axes. A three-axis robot has three degrees of freedom and performs its functions through the X-Y-Z axes. These small robots are rigid in form and cannot tilt or rotate themselves, although they can have attached tooling that can swivel or rotate or adapt to the shape of a small payload. Four- and five-axis robots have additional flexibility to rotate and tilt. A six-axis articulated robot has six degrees of freedom — the flexibility to move objects in any direction or rotate them in any orientation. The latter type is generally chosen when an application requires complex manipulation of objects. The seventh axis allows extended reach in one axis; in other words, it allows displacement of the six-axis articulated robots.

Seven-axis robots are likewise fully free; the seventh axis can allow additional orientations to manoeuvre tooling in tight spaces. For example, such a robot can weld a car body frame from the inside of the cabin by inserting the end effector through the window opening and rotating it backwards 180 degrees. Seven-axis robots can operate up closer to the work piece than other articulated robots for potential space savings.

Articulated robots: benefits and limitations

The popularity of six- and seven-axis articulated robots reflects the great flexibility that six degrees of freedom permit. They are easy to program, come with their own controller, and movement sequences and I/O activation can be programmed via a user-friendly teach pendant. In most applications, only a basic knowledge of programming is required to activate the robot. On the hardware side, industrial articulated robots can be relatively small or massive (capable of handling loads like locomotive wheelsets weighing over a ton). They can have substantial reach, over three metres with certain models. This range of sizes makes articulated robots suitable for a great number of industries and applications involving making or moving of materials or finished goods.

The articulated robot also has issues which can restrict its utility or boost its cost profile. A small-sized articulated robot is easy to install; its base only need be bolted to a frame or floor. But it can only lift so much or reach so far. Where the job requires a larger robot, civil engineering may be required to ensure the structure can handle the weight and torque caused by the load offset. An articulated robot grows in reach and payload simultaneously. The longer the reach, the greater the payload it can manage, the more space and engineering it requires, the more it costs. Where an application involves handling a small load over a long reach or a heavy load over short distances, an articulated robot may not be the most cost-effective solution.

By design, the articulated robot occupies space and footprint that can’t be utilised for other purposes. It also has singularities, ie, locations and orientations in the surrounding space it cannot access. These spatial limitations require more complex safety precautions since the robot will often be used in zones where workers are present, even just occasionally. Expensive devices such as zone scanners or safety mats are often necessary, and more advanced functionalities are then required, such as Safety-Limited Speed (SLS) or Safe Speed Monitor (SSM). The fact it requires its own controller to handle the inverse kinematics (the conversion of the multiple motor rotary positions to usable Cartesian coordinates and orientation in space) can also represent a double dip from a hardware perspective, since in certain cases the robot controller will need to communicate with a higher-level PLC on the production line. The bottom line is an issue as well. Where the full flexibility of a six-axis robot is not required, like many pick-and-place or packaging applications, other types of robots may do the job just as well if not better, and at a lower cost.

Cartesian robots: benefits and limitations

One of those lower-cost alternatives is the Cartesian or linear robot. Its design consists of an assembly of linear actuators and sometimes a rotary actuator at the end of an arm for 3D applications. It’s easy to install and maintain. The Cartesian robot is fully adaptable; strokes and sizes of each axis can be customised to the application. Its reach and payload are independent of each other, not intertwined. The linear axis comes in a number of designs which further adapt it to the function it performs. For example, toothed belt actuators allow high velocities while ball screw actuators permit high precision and high feed force, with pick rates up to 100/min fairly typical of this type.

The adaptability of these handling systems makes them price-optimised for a wide range of straightforward applications where the dexterity of an articulated robot isn’t required. That can involve extremely light to very heavy parts placement, sorting or box-loading, device inspection and much more. Another major advantage and differentiator of the Cartesian robot is its excellent space economy. It allows full access to the footprint it occupies. There is no dead space or singularities. Safety requirements are less stringent and hence less costly since the robot’s reach is limited to its small working zone. Fences, door switches or light curtains are often sufficient to ensure proper safety. Little space around the robot is wasted.

Programming the Cartesian robot doesn’t usually require a specific motion controller. Since the actuators are moving along the workspace coordinate system axis, interpolation of the motor’s position is not mandatory to determine the robot end-of-arm position in space. In other words, no calculation of inverse kinematics is needed. The system PLC can often be used to control each axis directly, without the addition of a second controller. Also, Cartesian robot designs are readily scalable, and are more often than not composed entirely of standard, catalogue components from servo drives/motors and controller to slides and grippers. That’s part of their affordability and assures replacement parts are readily available and quickly installed.

The Cartesian robot’s main limitation is comparative inflexibility. It will easily accommodate linear movement in three axes and a forth, rotative axis. However, one has to add a motion controller to perform more complex synchronised tasks, such as CNC. Cartesian robots are rarely used in washdown environments; they don’t provide sufficient protection against water ingress. Precision and thoroughness at installation is required. Each axis must be carefully aligned and surface flatness must be adequate, especially in larger systems. Cartesian robots are also configured uniquely for each application. While product or packaging format changes can be performed quickly via the PLC, mechanical modifications of the unit may be required for more extensive changes in the application. Finally, Cartesian robots, if used without a separate motion controller, may require more programming time than other robot types. Teach pendants are less common, so programming of sequences must be done in the PLC, with each axis addressed and commissioned individually.

In Part 2

In Part 2 of this article, SCARA and delta robots — and the newest type, cobots — will be discussed, before looking at the considerations necessary in choosing the right type for your application.

Image: iStock.com/anon-tae