Source: Max Planck Institute for Biological Cybernetics (MPI), Fraunhofer IPA

To date, cable-driven parallel robots have been used in production environments, where they meet high requirements. The systems surpass conventional industrial robots in size and payload by between one and two orders of magnitude. The end effector can be freely moved with high accuracy by up to eight cables and winches. Based on this technology and in a world first, the idea of a cable-driven motion simulator has now been realized under the lead of Professor Heinrich Bülthoff from MPI for Biological Cybernetics.

Technical innovations

In the cable-driven simulator, the motion of the simulator cabin is controlled by eight unsupported steel cables attached to winches. In contrast to conventional motion simulators, the use of cables makes it possible to reduce the moving mass and to scale the workspace to any required size. A total drive power of 348 kW allows the cabin to accelerate at 1.5 times gravitational acceleration along freely programmable paths inside a 5 x 8 x 4 m³ workspace. In addition, the cables can be reattached in under an hour to enable the simulator to be adapted to different cabins and thus used for a range of scenarios.

During the two-year collaboration between both Institutes, Philipp Miermeister, a member of Fraunhofer IPA’s Cable-driven Robotics working group headed by Junior Professor Andreas Pott, has contributed much know-how to driving forward the design and realization of the simulator. The scientists have not only implemented the control algorithms, but also developed a lightweight yet rugged carbon fibre cabin capable of withstanding the high dynamic loads during operation. Made entirely from carbon fibre tubes, the cabin frame maximizes the usable cabin volume with a diameter of 260 cm for projection surfaces and cockpit instrumentation. This allows it to be used for high-quality video projections and realistic operator interfaces. At the same time, the light 80 kg frame is capable of accelerating at high speed while also withstanding high forces, because, in operation, the cables pull on the outer structure with up to 1.5 tonnes.

The CableRobot Simulator as VR platform

Its large workspace and dynamic capabilities make the simulator suitable for a wide spectrum of VR (virtual reality) applications, including driving/flight simulation as well as investigation of basic perception processes in humans. “This simulator offers us entirely new possibilities for studying motion perception with possible applications in neurological research into balance disorders,” says Professor Bülthoff, who is a long-time perception researcher.

The CableRobot Simulator can be used in combination with different VR solutions such as Oculus Rift or HTC Vive. Depending on the simulation scenario and available space, the smallest version of the robot fits in a room with 5 x 5 x 5 m³. Optimal results will be achieved with a system of 10 to 15 m frame length. There are as good as no limits regarding the size and application range of our simulator. With the current system it is already possible to set up a system for rooms with more than 20 m edge length”, says Miermeister.

Source: Max Planck Institute for Biological Cybernetics (MPI), Fraunhofer IPA

The cabin uses a very flexible structure which provides a spherical internal space which currently is used to mount a race seat, computer systems for visualization, and an additional motion tracking system. The additional tracking system is used to separate the head acceleration signal of the Oculus Rift from the acceleration signal of the cabin.  “Currently we are implementing an additional rotation unit inside the cabin which allows us to rotate the seat independently and gives us more flexibility with regards to possible simulation scenarios” explains Philipp Miermeister. The first use case for the extended cabin design will be a realistic helicopter hover trainer, which allows 1:1 mapping of real flight data to the simulator workspace.

Specialized Software

The CableRobot simulator is controlled by a motion cueing software developed at the Max-Planck-Institute, which allows to replicate the motion from a simulation most accurately. It is also possible to use motion cueing and simulation software form third party providers, or to use the system for motion based cinematic experience. “Recently we started to work on motion reconstruction for 360° 3D footage using the optical flow information from the video stream”, mentions Miermeister. This technique allows reconstructing the camera trajectories for movies whose motion data is not available.

Through the linkage with computer games, such as race and flight simulations as well as 360° 3D videos, it is possible to use the CableRobot simulator for the presentation of highly diverse content. Currently a portable solution is developed which allows to set up the modular and lightweight motion simulator at different locations.

The developed robot system is capable of simulating thousands of typical usage scenarios on any kind of device with a human-machine interface, such as a touchscreen. (Source: Fraunhofer IPA/Photo: Rainer Bez)

Thousands of repetitions in the space of just a few days: while manually impossible, this can be accomplished by a newly developed robot system from Fraunhofer IPA. Whether it’s a matter of pressing keys, inputting data or using fingers or a stylus, e.g. for a signature pad – the robot system is capable of realistically simulating typical applications of human-machine interfaces. The system can test how long the materials human-machine-interfaces are made of and the associated functions remain fully functional. This provides device manufacturers with accurate knowledge about the quality of their products, which is of importance for possible warranty claims. In addition, customers can gain accurate information on durability.

Robot imitates typical usage with its forces and path

“For a device manufacturer, automated testing offers the advantage that the performed conditions are always identical and the test scenarios are reproducible and therefore comparable. The motion of the robot system can be precisely controlled to guarantee that the testing is completely traceable and verifiable,” says Geravand.

For each device, the researchers collaborate with the manufacturer and draw up the typical usage scenarios and loading profiles to set up the robot system. This includes, in particular, setting up the end effector, which is capable of holding a variety of tools – similar, for example, to a finger or stylus – and simulating various loading scenarios.

“We’ve succeeded in setting up the robot system so that it is capable of accurately simulating an application such as the use of a cash machine. Force sensors in a robot-guided finger-like end effector accurately measure and evaluate the force exerted by the user in a multiplicity of interaction cycles,” explains Milad Geravand, research assistant at Fraunhofer IPA. The accuracy of the tests is guaranteed by measuring systems as well as by regular checks by the employees.

The programming of the robot system depends on features that describe the interaction performed by the user on the devices with human-machine interfaces. The researchers measure and process these features in authentic situations with test subjects. That could include the duration and force registered during the interaction. If the device under test is a touchscreen, it might also be relevant where the action is typically performed. This data is used by the scientists to suitably configure the forces and path of the robot.

Uniform quality of testing

By the end of each intense test, device manufacturers are provided with a detailed report on the tests and results as well as with a brief summary, which can also be used as a reference for customers. The scientists are ready to employ this technology for life-cycle evaluation of human-machine-interfaces either in the labs at Fraunhofer IPA or on the manufacturer’s site by setting up the testing systems.