A new torque sensor system for cobots – integrated into a precision strain wave gear – enhances the performance of cobots and lightweight robots, as well as shortening cycle times. This represents a major advancement in breaking down the barriers to the widespread use of robots in industrial automation, says Ralf Moseberg, Head of the Industrial Automation Business Unit at Schae er. Strain wave gears have been used for decades as precision servo gearboxes in industrial automation. Today, they are also the preferred gear types for small articulated robots with payloads of up to around 20 kg. The outstanding characteristics of strain wave gears include high positioning accuracy, high repeatability, zero backlash, compact design and low mass. The number of strain wave gear manufacturers today is modest, primarily due to the numerous niche applications and challenging gear tooth technology. Sales of collaborative robots (cobots) could skyrocket if only a few technical problems were solved: how to achieve higher speeds/accelerations and therefore greater productivity of cobots in autonomous operation; another demand is sensitivity in collaborative mode – and all this at an attractive cost. Cobots with both of these properties could be used exibly, for example, by working collaboratively during the day and autonomously at night. This would result in signicant increases in productivity and a faster payback. For cobots that must execute processes with sensitivity, force control and for smooth teaching processes, there is no alternative other than to use sensor systems for determining external forces and moments. The most popular products are sensor modules, i.e. sensors that are oered as separate components for cobots. However, they require additional installation space and must be integrated into the cobot design. External torque sensor concept With this type of sensor, the forces and moments are detected using an elastic structure on which the actual sensor system is typically glued. The design of such sensors inherently involves a compromise between high resolution and low deformation. For applications in which the end eector or gripper must precisely follow a trajectory or hold a position under load, the position deviation caused by the sensor is a major disadvantage. Internal torque sensor concept The approach taken by Schaeer uses an existing component of the drivetrain in the joint for measuring torque instead of an additional elastic element. This means that no additional elasticity is introduced into the cobot structure and the cobot rigidity remains at 100% even with the sensor system included. Cobots are clearly disadvantaged due to their slender design, which results in a higher elasticity compared with that of industrial robots. The thin cobot structure vibrates noticeably with high accelerations – particularly when positioning with maximum braking. What is gained in terms of a shorter cycle time through high speeds and accelerations is lost through the longer time it takes for the vibrations to settle down during positioning. The natural frequency of a cobot is mainly in uenced by the tilting rigidity of the joint bearings / gear unit main bearings, as well as the torsional stiness of the gears (and the torsional stiness of the torque sensors). With external torque sensors, the torsional stiness of a joint can drop to 25% due to the additional elasticity introduced. With the concept developed by Schaeer, the torsional stiness of the joint is completely retained. Characteristics of internal torque sensors Schaeer is highly experienced in the development, application and mass production of integrated torque sensors. The functionality is realised through a submicrometer-thick, strain-sensitive metal PVD coating. The component itself becomes the sensor and the sensor becomes the component. For application in robotics, the ex spline of the precision strain wave gear is used because it lies directly in the path of power ow. No additional installation space is required. No adhesives or transfer polymers are required. nCORPORATEPROFILE To explore more visit us here: www.schaeffler.co.uk Cutting cobot cycle times
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