ViFlex: A Compact Haptic 2D Interface with Force ... - CiteSeerX

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ViFlex: A Compact Haptic 2D Interface with Force Feedback for Mobile Devices Samuel Roselier1

M o u s ta p h a H a f e z 2

Commissariat à l’Energie Atomique / CEA – LIST 18 Route du Panorama 92265 Fontenay-aux-Roses, France

ABSTRACT This paper introduces a compact and low-cost haptic device designed for mobile applications. ViFlex is a haptic device with two degrees of freedom force feedback. The generated stimulation with this kind of interfaces is a compromise between haptic and tactile, when kinesthetic interaction is not sufficient and the high resolution of a tactile display is not required. The system is composed of a mobile platform on an articulated mechanical structure with actuators and position detector. The size of the mobile platform is 45x45 mm² and its deflection is 20° (+/- 10°) around a surface of reference. An average torque all over the stroke of 20 Nmm is achieved. Keywords: Haptics, Embedded Systems, Force Feedback, Shape Exploration, Orientation Perception.

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INTRODUCTION 1

The exploration of a virtual object that requires shape recognition is usually carried out with a haptic device. However it is sometimes necessary to generate more detailed curvatures or slopes of a shape. This motivates the development of intermediate devices for fingertip interaction to fill the gap between haptic and tactile devices. Several interfaces based on different principles aiming at creating such interaction have already been developed, in his work, V. Hayward described different interaction scales and associated devices [1]. The two degrees of freedom interfaces can be divided into two categories. The first one includes planar haptic displays using the lateral rolling deformation of the finger to give the illusion of a haptic shape. The Pantograph developed at McGill University, the Slipping display developed by Colgate [2] and the Tactile slip display from Johns Hopkins University [3] are finger-sized devices based on this principle. The second category includes two rotational degrees of freedom interfaces. They reproduce the normal exploration mode of the finger on different surfaces. A platform on which the finger is placed can rotate and provide to the user orientation information of 3D shapes. The Morpheotron [4] is a spherical five bar used to orientate a plate and describes the shape of an object. The main drawbacks of all those devices for mobile applications are the weight and the size. They have been designed as desktop interfaces and can not be embedded. Relatively unexplored in the field of pedestrian guidance, mobile haptics could play an important role in spatial orientation and wayfinding. Assistive technologies have to fit the users’ needs but also adapt to their physical, sensory and cognitive capabilities. Several tests have been carried out on wearable interfaces for orientation and wayfinding [5]. Comparison between three interfaces using different communication modalities (audio, speech and shoulder tapping system) has been 1 2

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made on a panel of visually impaired during a street crossing task. Remarks concerning audio and speech were oriented toward the modality itself, hearing is indeed often used by blinds and visually impaired to localize dangers like cars. The best guidance results were obtained with the shoulder tapping system. This study emphasizes the interest in haptics as assistive technology for blinds and visually impaired. A handheld device with two rotational degrees of freedom could provide an orientation to the user and a way to follow in the horizontal plane. Using the vibrating mode along one axis could for example provide an alert or at least operate as a tactile guidance system. Drawbacks of mobile haptics and handheld interfaces have been underlined in [6] and [7]. The first one is the use of a fixed ground to apply low-frequency forces; the chosen solution is to create on the device its own reference. An other issue is the integration of power supply and the size and weight of the device. A limitation in the number of DOF should be considered to reduce the power consumption of actuation and by the way will reduce the weight of the interface [7]. A reduced number of DOF of the final device will also make it more intuitive and easily learnable to support the applications targeted: shape exploration, mobile interaction, pedestrian guidance for blinds and visually impaired. In all cases, the handheld device should work on multiple platforms without modifying the material and its connections. The implementation of a bluetooth connection would considerably simplify this point. Bluetooth is now widespread and widely implemented in apparatus such as computers and PDAs. A PDA could be used in the guidance application to collect and process the necessary information, for example to acquire the position with a GPS and the user orientation via a compass and then send the information to the haptic interface. The 2D interface ViFlex presented in this work is a haptic device designed for fingertip interaction with two rotational degrees of freedom with force feedback. ViFlex has been designed with the objective of being a small, robust and lowcost device with powerful actuators and a precise positioning system. The design approach leads to the simplification of the assembling process. Different applications are explored. The interface can be used as an output device for shape exploration or orientation perception, or as an active input device to replace a mouse or a joystick with force feedback. User’s fingers are positioned on the moving part of a platform that exert a force on them along the two rotational degrees of freedom (DOF). The following picture (Figure 1) shows the mechanism with its casing. A surface of reference has been placed around the platform, the casing is also a protection against exterior world and light disturbance.

damage as the applied pressure of the finger is distributed over the protection and the reference surface. Concerning the position resolution, the threshold for detection depends on the amplitude of the stimuli and its speed [4]. A resolution of 0.1 degree (2 mrad) would be sufficient. 3

BEARING SYSTEM

Flexure hinges are widely used in robotics and in high precision mechanisms where very high resolution is required [9]. They have many advantages over classical bearings: • No rubbing • No stick slip phenomenon • No lubrication • High transverse rigidity Figure 1: ViFlex Haptic device

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• Monolothic structure • Restoring force

SPECIFICATIONS

Studies with 2D interfaces have been carried out to determine different parameters: Deflection angle, type of stimulus [4]. Once identified, these parameters allow realistic rendering of haptic shapes. The following set-up (Figure 2) was built to determine the different parameters required for the design of the ViFlex. The prototype consists of a platform with one degree of freedom in rotation. The deflection angle and the torque applied to the platform can be modified. 9 subjects participated at the qualitative experiment to validate the following parameters:

There are also some limitations for the use of these components; the two main drawbacks are the complex geometrical model, with the appearance of curvilinear transformation, and an angular displacement that is limited. 3.1

Flexure design

The flexible bearing used in the interface is based on a notch joint, analytical models for flexures can be found in [10].

• Dimension of the platform • Deflection angle • Required force amplitude • Geometrical considerations (Surface reference…)

Figure 3: Notch hinge

A notch hinge has a rectangular section and a variable thickness which profile describes two concave half-circles. To ensure a constraints concentration factor of 1, the following conditions have to be validated: r ≥ 5 ; 10−6 m ≤ e ≤ 10−3 m ; 10−4 m ≤ r ≤ 10−3 m e

Figure 2: Set-up for parameters identification

The different components of the system such as the actuators, the bearings and the positioning system defined the dimension of the active surface of the platform. The contact surface with the fingertips is an hexagon of 45x45 mm² (Figure 2). The limitations for the deflection amplitude are due to the bearing system, which is in this case a flexible bearing. Experiments revealed that a minimum tilt of 20° (+/- 10°) in the two directions and a torque of 20 Nmm are required. During the experiment, the subjects realized that the orientation information was improved with the presence of a reference surface. It also acts as a protection to prevent from any

The following equations can be derived from the theory of beams. Kα M is the angular stiffness, α M the maximum value of the deformation angle and σ adm the maximum tensile stress before plasticizing σ lim divided by a safety factor S 1 .

Kα M =

αM =

1

σ adm =

σ lim S

2 E b e 2.5 9π r

3πσ adm r 4E e

with 1.5