1939-1412 (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TOH.2019.2912570, IEEE Transactions on Haptics Combining Haptic and Bang-Bang Braking Actions for Passive Robotic Walker Path Following Marco Andreetto, Stefano Divan, Francesco Ferrari, Daniele Fontanelli, Luigi Palopoli and Domenico Prattichizzo Abstract—Robotic walkers are a promising solution for physical and cognitive support to older adults. This paper proposes a low cost path following strategy combining the advantages of a simple mechanical braking guidance, such as safety, passivity and a low cost, and the ones of a vibrotactile haptic guidance, such as comfort and portability. The user is guided by providing indications on the directions of motion using the haptic interface so that he/she can autonomously and comfortably follow the planned path. However, whenever the user significantly departs from the path (for instance s/he gets too close to obstacles), the braking system kicks in to safely steer the user back along the proper direction. The formal correctnesses of the hybrid strategy ruling the combination of the two guidance systems is proved theoretically. Moreover, a comprehensive experimental study with users aged 64 to 100, including also psychological evaluations, has been performed. The hybrid combination of the braking and the haptic guidance systems is shown to outperform the two individual approaches in isolation. The combination of the two retains the same level of the users’ perceived comfort typical of the haptic-only guidance while ensuring the adequate path following performance typical of the braking-only guidance. In particular, the combined approach produces a mean path following error equal to 41% of the mean path following error ensured by the haptic- only approach. Conversely, thanks to the haptic feedback, the combined approach halves the activation time and the number of interventions needed in the braking-only approach. Index Terms—Assistive technology, haptics applications, haptic I/O, human performance, perception and psychophysics, dynamic systems and control I. I NTRODUCTION Service robots are becoming nowadays relevant as supports for seniors to increase their quality of life and help them in their daily duties. Indeed, the interest in inexpensive and easy to use robotic mobility aids is motivated by their supposed efficacy in helping users remain active beyond the walls of their houses [1] and hence reduce physical problems and cognitive decline [2], [3]. The robotic walker FriWalk, which is a standard commercial walking aid endowed with sensing abilities to understand the This project has received funding from the European Union’s Horizon 2020 Research and Innovation Programme - Societal Challenge 1 (DG CONNECT/H) under grant agreement n ◦ 643644 “ACANTO - A CyberphysicAl social NeTwOrk using robot friends”. D. Fontanelli is with the Department of Industrial Engineering (DII), University of Trento, Via Sommarive 5, Trento, Italy daniele.fontanelli@unitn.it. M. Andreetto, S. Divan, F. Ferrari and L. Palopoli are with the Department of Information Engineering and Computer Science (DISI), University of Trento, Via Sommarive 5, Trento, Italy {luigi.palopoli,stefano.divan,marco.andreetto} @unitn.it. D. Prattichizzo is with the Dept. of Information Engineering and Mathematical Sciences, University of Siena, Via Roma 56, Siena, Italy prattichizzo@dii.unisi.it. surroundings [4], with communication abilities to connect to cloud services and with planning abilities to produce safe paths in the environment [5], is an interesting solution developed in the context of the European project ACANTO [6]. The main purpose of the FriWalk is to act as a navigation aid guiding the users without sacrificing their perceived freedom of movement. One standard approach to satisfy this goal and generate “comfortable” manoeuvres is to resort to motors on the back wheels of a standard rollator. Commonly adopted interfaces to control such system are joysticks [7], force sensors [8], [9], laser scanned shin positions [10], depth cameras tracking the lower limb [11]. Unfortunately, the presence of actuation usually generates potential safety problems, which can instead be removed using passive robots [12], [13]. The rationale of a passive robot is to leave the responsibility of the locomotion to the user. The safety comes directly from the absence of thrusting motors in a passive robot (where instead the propulsion is generated by the user): if a system malfunctioning takes place in an active assistive walker, accidental motion may occur and the user may be pushed or pulled (because of the presence of the active motors) and loose his/her balance. Although active walkers have the potentiality to help the users missing the necessary strength to push the device [14], in this work we studied the FriWalk as a passive walker, hence preferring safety to the possibility of supplying additional thrust. Straightforward ways to build a passive robot is either to use actuated steering wheels [15], to simulate passivity through actuation [16], [14], to resort to modulated electromagnetic brakes for differential drive [17], [18]. As aforementioned, modulating the braking force imply the adoption of an appropriate sensing system to estimate the torques applied by the user. In order to reduce the system final cost and its complexity, elementary passive solutions should be implemented. To this end, a bang-bang passive walker solution, in which the vehicle is turned on the left by blocking the left rear wheel or on the right by blocking the right rear wheel, has been proposed in [19]. The solution is simple (on/off control action) and inexpensive since additional hardware is not required for braking modulation or human force sensing as in [17], [18], [20]. Moreover, an automated intelligent braking system is a required feature (we could say the bare minimum) to guarantee safety for all robotic walkers. A dual use of the passive braking system (using the brakes also for guidance) clearly contributes to reduce the cost. Despite the inherent safety, the small set of manoeuvres it produces enables a relatively accurate tracking of the path, although with a questionable user comfort [21].