Force torque sensors set to bring industrial robotics to the next level

Klajd Lika, CEO, BOTA Systems further explores.

Robots are essential in modern manufacturing and over half a million new industrial robots were installed just last year. While the range of tasks they perform is growing, there is one fundamental capability where robots lag behind: the sense of touch.

Unlike human beings, who can adapt movements based on sensory feedback, robots still require certain movements to be predefined. That means that the robot’s controller ‘must’ know all geometrical data within its workspace with very high precision. If an item is not exactly where it should be or its shape is slightly different, a robot often fails to adapt or just breaks it.

New ways need to be found to enable physical interactions and dynamic movements in robots, i.e. to allow robots to be used in more unstructured environments, to perform more demanding tasks and to co-exist with human collaborators. This can all be achieved by utilising force sensing and force control methods, which are increasingly becoming an essential component of robotic solutions.

Force sensitivity can be achieved by means of joint torque sensors, tactile skins or end-of-arm tooling through wrist force torque (FT) sensors, and collaborative robots (or cobots) are usually equipped with at least one of these sensors. Besides cobots, FT sensors and force control can be implemented on industrial robots and enable improved methods of process automation.

What is force control?

Traditional industrial robots are optimised for repeatability and accuracy. They can perform the same motion over and over again with high speed and reliability, primarily due to their high stiffness. However, these robots are not designed to have good flexibility and adaptability, meaning that they require many repetitions to be cost-effective, and the motions can be quite complex to program.

Nevertheless, there is an option of making these robots adaptive by integrating force control technology. Force control means that the output of a force torque sensor (also known as load cell) is used in a feedback loop to alter the robot's motion. The objective is to achieve a certain behavior rather than a fixed trajectory. An example of such a behavior can be to follow a previously unknown shape while applying the desired force.

In essence, force control regulates the contact force of the physical interaction between the robot’s manipulator, i.e., robotic hand or tool, and the environment. A common force control approach is admittance control, which relates a force/torque measurement to a position or velocity control command of the robot. Most robot brands (optionally) integrate this in their controllers, which enables them to be easily fitted with force-torque sensors, both in the joint space and the end-effector task space.

By design, industrial manipulators are only able to perform a limited set of tasks that rely on position control and pre-defined trajectories. By implementing force control, it is possible to unlock new robotic skills and achieve previously impossible tasks, such as polishing on complex surfaces, assembling delicate parts with tight tolerances, or handling interactive tasks in open environments.

What are the benefits of multi-axis sensors in industrial automation?

A six-axis force torque sensor mounted to the robot wrist is an easy and affordable way to achieve force control within a short implementation time. Sensors with integrated electronics are simple to install and can be directly connected to the robot's communication bus or controller. Using a sensor that is robust, compact and stiff allows for various applications and makes it possible to keep it installed on the robot even while it is not directly in use.

The flexibility of use and simplicity of installation rapidly deliver a positive return on investment (ROI). Besides automating labor-intensive and unhealthy work like grinding, it is also possible to achieve a more consistent process.

Using a complete six-axis sensor increases flexibility, whilst not all axes may always be needed at the same time for a specific application. For example, when a robot is polishing a surface, the force perpendicular to the tool and its alignment to the surface is regulated by force control, however, in the other directions, the robot follows a programmed or recorded position trajectory. Therefore, in many applications, a hybrid force/position control system is implemented. Concurrently, the process forces can be monitored in all directions to avoid manufacturing errors and to create a consistent surface finish quality.

Not every robot can become a cobot that successfully collaborates with humans, but force sensing enables it to become more adaptive to a changing and complicated environment, as well as more reactive to disturbances and uncertainties. Force sensing enables the detection of contacts along the tool path and offers an intuitive way to ‘teach-in’ new and adapt pre-programmed waypoints.

What are the limitations of multi-axis force torque sensors in industrial automation?

Of course, when planning to automate a process through force control, there also exist certain limitations that need to be considered when employing force torque sensors to make robots more adaptive. Force control is a reactive control approach and thus the robot must be able to physically respond quickly enough in order to reduce the force error.

Particularly when working with a stiff media, force peaks can appear quickly, and a robot arm is only able to achieve a finite acceleration due to its mass and limited torque. In an industrial application, force spikes can be reduced by lowering the tool velocity to enable the robot to respond in a timely manner. Therefore, it is important to distinguish between motion sequences that need force control and are slowed down by it, and high-speed non-contact motion sequences that are optimised for application speed.

Force control is an extensive topic that is often considered to be quite complex, thus hampering the realisation of many projects. However, perhaps this presumption is not entirely substantiated, and it stands to reason that one day it will be just as easy to integrate as standard position control technologies.

Fortunately, there are already many force control packages utilised by robot brands that are available for simplifying integration. Bota Systems has experience in force control and provides solutions that are as simple as possible to integrate.

What are the applications of force torque sensors?

Most tasks performed by human beings rely on their ability to sense force, and since robots are expected to replicate these tasks, they also need a similar sense of touch. Often combined with other sensors, force control is utilised in tasks such as product testing, process monitoring, and object identification.

The following paragraphs summarise some common applications that benefit from force torque sensors and force control. 

Surface finishing

Finishing applications like material removal, polishing, deburring or deflashing are some of the dirtiest and most exhausting tasks in manufacturing. By using force torque sensors and direct force control, it is possible to easily automate these challenging processes. The sensor feedback provides the flexibility and adaptiveness required, even for complex shapes, and enables a consistently high surface quality.

Robotic finishing:

• Ensures better product quality with consistent and controlled force application
• Addresses shortages of highly skilled workers
• Reduces risk of injuries
• Enables faster finishing
• Generates less wear on consumables for finishing applications

Assembly

Robots with sensors can repeatably perform assembly processes even with tight tolerances. By using force feedback during an insertion task, it is possible to increase flexibility, reliability and robustness. The insertion can be done with just the right amount of force, preventing damage to parts and ensures less downtime on a production line.

Benefits of force torque sensing for assembly:

• No need for an expensive testbench or assembly table with high tolerances
• No damage to the surface of high tolerance stiff metal parts
• Quality controlled assembly

Hand guidance and teach-in

Programming a robot can be time-consuming and complex because of the required coding. Flexibility in production is increased by integrating an intuitive and easy way to record waypoints and teach positions. This allows for a reduction of programming effort and enables automation tasks with only a few repetition cycles. Adding a force torque sensor at the wrist makes a robot safe and collaborative with high accuracy and reliability, and achieves the following benefits:

• Reduction of programming time from hours to seconds
• Intuitive programming - no code
• Flexibility by re-programming and correcting any error quickly

Telemanipulation and rehabilitation devices

Telemanipulated medical robots and rehabilitation devices have the most stringent requirements in the field of force torque sensors. The sensors need a high signal bandwidth while being accurate and sensitive enough to enable high-fidelity force rendering at the robot itself or at a remote device. Compact and lightweight sensors are needed that can be directly integrated into the structure.

Benefits of force torque sensors in medical applications:

• Reduced medical risks
• Sense of security by patient
• Shorter recovery time of the patient
• Lower cost per procedure

Conclusion

Robots need to be smarter, more flexible and safer, which are exactly the challenges that force control is presently tackling. Force control plays a fundamental role in the achievement of human-like robust and versatile behavior of robotic systems in open-ended environments, providing intelligent responses in unforeseen situations and enhancing human-robot interaction.

Bota Systems force torque sensors are specifically designed for robotic applications and are accurate, robust, compatible and easy to integrate.