Meeting emerging challenges in manufacturing with lightweight robotics: Part 1

Advances in mechanics and controls are enabling the practical and economical application of lightweight robotics for manufacturing on an increasingly broad scale.

Today’s global marketplace has changed, and continues to change, the dynamics of manufacturing. The speed of business is accelerating, competition has increased dramatically and competitors are as likely to emerge from across the globe as around the corner. Consumer expectations for product consistency and quality have reached unprecedented levels. No quarter is given for where and how goods are manufactured; quality is today’s de facto universal standard, regardless of place of product origin or consumption.

For those manufacturers who have moved facilities abroad to leverage lower labour costs, the realisation is rapidly dawning that maintaining high-quality products using manual production methods is not a sustainable, long-term strategy. Further, the need for high levels of productivity is escalating as greater opportunities and surprising demand emerge from new markets, while the volatility of demand across all markets makes it difficult to predict and plan for. Labour itself is problematic, less from a cost perspective than from demographics and capability. Indeed, increasingly sophisticated manufacturing processes need a skilled workforce that simply doesn’t exist in the numbers employers need to fill new positions, particularly as older skilled workers retire.

Figure 1: The Barrett WAM arm.

Add to this mix the powerful movement towards mass customisation. The once prevalent high volume, low mix model of manufacturing is rapidly giving way to lower volumes and higher product mix. Companies involved in the production of specialised components and products for industrial customers are challenged with producing small lot sizes efficiently and meeting high quality standards consistently while being cost competitive.

To operate effectively in this environment, manufacturing agility is key. The large, centralised production plant is becoming a dinosaur. The factory of the future will be small, flexible, movable and local — one of the ironies of rampant globalisation is that it ultimately leads to a return of local production.

In this competitive landscape, manufacturing equipment must meet certain essential requirements:

• Easy to set up and implement into production operations (ideally portable).
• Flexible.
• Cost effective.
• Highly reliable.
• Fast.
• As compact and lightweight as possible.

Figure 2: The Mitsubishi PA10 arm in action.

Pursuit of these requirements has helped drive the development of the lightweight robotics and desktop automaton solutions increasingly prevalent in today’s manufacturing automation. By providing scalable and modular functionality in increasingly agile and compact packages, these solutions are dramatically changing manufacturing by enabling automation on a smaller and more flexible scale, and helping achieve the responsiveness necessary to compete in today’s rapidly changing global markets. Among the tasks and processes for which lightweight robotics technologies are now employed are:

• Feeding, screwing, and mounting small components.
• Setting adhesive points.
• Electronic testing, such as approach to contact points, and resistance tests.
• Flexible positioning of workpieces and components.
• Logistics and storing operations.
• Sample preparation, dispensing, transport and distribution in medical diagnostics.

What is a lightweight robot?

Lightweight robots are particularly designed for transportability, and interaction with previously unknown environments and humans. Robot mobility combines the requirements of a lightweight design with high load-to-weight ratio (close to the 1:1 ratio) and high motion velocity (tip velocity of 6 m/s). Moreover, collaborative robots that interact with humans and in unknown environments require sensing and control capabilities to enable skilful, compliant interaction.

Figure 3: The KuKa LBR iiwa (intelligent industrial work assistant).

Structural and control considerations

Lightweight metals or composite materials are used for the robot links. In fact, the design of the entire system is optimised for weight reduction in order to enable the mobile application of the robotic system.

In order to increase performance and safety of the arms, additional and sometimes variable mechanical compliance is introduced into the joints of some lightweight collaborative robots. Within the lightweight robot concept, a strong emphasis is set on robust performance as well as active safety for the human and the robot during their interaction.

Compared to standard industrial robot control, the following aspects are of particular importance:

• Extensive use of sensor feedback from the environment, including vision, force-torque sensing at the end-effector and in the joints, tactile sensing, as well as distance and proximity sensors.
• The control implementation is not limited to position control, but also includes the interaction forces in the constrained directions using methods such as impedance control. In this way, instead of prescribing a position or a force, the dynamic relation between the two is prescribed, while the actual force and position resulting during interaction also depend on the environmental properties.
• Position control has to compensate for the effects of the inherent robot elasticity (such as vibrations or steady state position error) to ensure the performance of positioning and trajectory tracking.
• The robot needs control strategies that allow detection of unexpected collisions with the environment or humans, and to be able to react in a safe manner. In some lightweight robots, torque sensors in each joint play a key role for so-called ‘soft robotics control’. These sensors allow the implementation of most of the aspects described above with high accuracy and performance.

Today, two principal types of lightweight robots are being produced: those with compliance and those without. Originally, manufacturing robots were caged: for humans to interact with them, parts were fed from outside the cage. Today’s compliant lightweight robots have no need for such barriers — humans can be side by side with them because of built-in sensors that detect human presence and ensure safety, so workers can interact with them even when the robot is active. Other robots have lightweight structures, but without the sophisticated sensing capabilities of compliant robots, workers cannot directly interact with them while the robot is active.

Figure 4: The DLR MIRO demonstrating robotic surgery.

Good examples of lightweight robotics commercially available today include the Barrett WAM arm, the Mitsubishi PA10 arm, the KUKA LBR iiwa, the DLR MIRO robot and the Festo EXCM planar surface gantry.

The Barrett WAM arm is a cable-driven, lightweight arm that has actuators placed at the base of the manipulator to reduce the total moved weight. The joints are back-drivable due to its low reduction ratio.

The Mitsubishi PA10 arm is a lightweight redundant arm that weighs 38 kg with a payload of 10 kg. The PA10 is ideal for precise manipulation tasks because of its back drivability, precise positioning capabilities and zero backlash afforded by its harmonic drive transmission (HDT).

The Kuka LBR iiwa is a trailblazer for totally new forms of cooperation between humans and machines. The robotic innovation with sensory capabilities for safety, fast teaching and simple operator control opens new areas of application in the vicinity of humans that were previously off-limits for robots.

The DLR MIRO is the second generation of versatile robot arms for surgical applications. With its low weight of 10 kg and dimensions similar to those of the human arm, the MIRO robot can assist the surgeon directly at the operating table where space is limited.

The Festo EXCM is a compact planar surface gantry that can approach any position within its working space. The robot’s recirculating toothed belt moves the slide within a two-dimensional area (x and y axes). Fixed motors are connected to the slide, and the moving mass remains low because of the parallel-kinematic drive principle. The ready-to-install system allows fast positioning at speeds of up to 500 mm/s and repetition accuracies on the order of ±0.05 mm. This makes a compact solution suited to applications such as sample handling in medical and research laboratories and small parts assembly, and emerging technologies such as printed electronics production and 3D printing.

Figure 5: The Festo EXCM planar surface gantry.

Challenges in manufacturing

Operations are generally categorised in two production models: high volume, low mix (long runs with relatively few part changes) and low volume, high mix (short runs with frequent part changes). Originally, it was only high-volume operations that were automated; however, as noted above, the trend in manufacturing is towards mass customisation, which means lower volume and higher mix. Therefore, successful manufacturing operations need to be leaner, more agile and operate at higher efficiencies than ever before. This is doubly true as the speed of product introductions accelerates. Furthermore, the medical research, laboratory and other industries are increasingly working with ever smaller parts and precise processes. Automation must be able to work on these small scales.

That being the case, significant challenges in automated manufacturing processes remain:

• Part presentation: Parts are often presented in bulk and need to be channelled so that individual components can be consistently presented and handled in the assembly process. Manual methods significantly affect throughput and therefore some type of automatic feeding method or robotic handling is required.
• Machine access: Access to the machine tool for set-up and tool changes is critical. Automating the machine tool adjustment reduces downtime and eliminates any safety and product consistency issues that could arise when making adjustments manually. In simple systems, automatic adjustments are accomplished with the use of an integrated motor; in more complex arrangements, flexible robotic handling systems are used.
• Process rates: In all machining operations, shorter load/unload time is important; in fact, in smaller part, shorter cycle operations, it is critical.
• Space and layout considerations: Most production equipment is positioned for manual operations and to maximise machine density. Creating operational space for a robot to load and unload parts can be difficult, especially if safety fencing is required.
• Cost: In many countries, robotic automation has primarily been justified based on labour reduction. This is typically coupled with a short-term view of return on investment.

In Part 2

With the trend towards adaptive, low volume, high mix manufacturing, the current automation technologies designed to work with high volume and minimal change, while efficient, are no longer flexible enough to meet the challenges of globalisation and the need to meet the rapidly changing needs of the market. In Part 2 of this article we will examine how lightweight robots help alleviate these challenges.