Design and control of a haptic knob
Introduction
Our everyday life is rich of electronic devices which are controlled by the human finger. Touch interaction is now a standard user interface, and easily replaces a physical button or a knob. In many MP3 players for example, a knob is replaced by a wheel on which users move their thumb. This solution is easy to manufacture since it requires no moving part. However, users no longer receive any affordance cues, like the ‘click’ of a button for example, which reduces their performances [3]. To cope with this problem, the solution consisting in producing a haptic feedback proved to be useful in this context [4].
The haptic feedback can be achieved by several ways. Vibrotactile stimulation [6] can be produced by using rotating motors with an eccentric mass [7], or a linear actuator [10]. However, with these technics, the actuator takes time to start and stop, and it is difficult to calibrate the stimulation. Moreover, they produce a vibration of the whole object, so the hand holding the device perceives a stimulation as well as the touching finger. A piezoelectric bender attached to the case of the touched device is an other way to enhance interaction with vibrotactile stimulation [16], [15]. Dynamic stimulation can be produced, but is still limited by the low resonating frequency of the actuator.
Friction reduction based tactile devices are a second way to achieve the tactile feedback. With this operating principle, we use an active surface in order to control the friction with user's finger. By modulating it according to the fingertip's displacement, it is possible to produce the illusion of touching various surfaces, like smooth or rough surfaces [18] or gratings [1]. In the paper, we used squeeze film air bearing to produce the friction reduction [17]. It consists in producing a high frequency vibration (above 25 kHz) at low vibration amplitude (~1 μm), on a plate. The air trapped between the fingertip and the vibrating plate is subject to non-linear expansion and contraction. The squeezed film makes the friction decreasing as the vibration amplitude increases.
In this paper, we present a haptic knob which is able to produce programmable haptic effects. There exist several designs in the literature of such device. For example, [12] uses a DC motor attached to a dial. In [5] a same principle is applied, but with a more sophisticated actuator. For both examples, users turn the dial and the response torque of the device can be controlled according to the knob's position which is measured. These solutions require several moving parts, while the user's hand is fixed on the knob. Our approach is different, and consists in designing a wheel on which user's thumb slides. Friction reduction obtained from the squeeze film effect will produce a haptic feedback in order to reproduce the illusion of manipulating a rotating knob.
The paper is organized as follows. In the following section, we present the design requirements. Then, the procedure to design and control such a knob is detailed, to finally present a user study in the last section.
Section snippets
Design of the haptic knob
The device has been designed in order to fit into user's palm, and to be controlled by the thumb; a CAD view of the prototype is presented in Fig. 1.
The haptic knob is made of a ring shape active area which vibrates in order to produce the squeeze film air bearing. It is built up with a copper ring attached by its center to the plastic case of the knob. The ring itself is not allowed to rotate. The size of the ring, the internal and outside radii, are determined to allow a free sliding motion
Principle
The position of the fingertip is used to modulate the vibration amplitude, and thus create the tactile pattern. Position should be detected with both accuracy and speed. In previous work [1], we found that it was possible to simulate fine gratings with spatial period as small as 2 mm with 25% of spatial resolution. Thus, the required resolution of the sensor can be estimated to be 0.5 mm. On the 36 mm mean circle of the touched area, this leads to a resolution of 1.6°.
For this purpose, there exist
Description of electronic control circuit
A specific electronic board has been designed for controlling the haptic knob, and is described in Fig. 12, with an actual view in Fig. 13. It is composed of an electronic DC/AC power converter which energizes the electrodes of the piezoelectric device with two 15 V/40 kHz voltages approximately. A DSP Piccolo F28027 is used to produce the pulse signals to the power converter thanks to its Pulse Width Modulation module. Moreover, an FTDI chip allows communication with a conventional PC through an
Haptic rendering
It's for purpose of illustration that the following test was conducted. It consists of a selection task, with or without haptic feedback. Without haptic feedback, user's thumb slides on the wheel until a requested target is selected. In addition to this, and with haptic feedback, users feel the virtual sectors of the wheel as programmed on the interface. By this way, we are able to introduce more physicality in the touch interaction [13], and make users have an experience closer to their
Conclusion
We presented the design and the control of a haptic knob. It uses the squeeze film effect to produce friction reduction on a fixed wheel. Size selection and manufacturing process are presented step by step. It uses a position sensor, based on force measurement. The architecture allows a resolution of 3.6° as well as the measurement of the force exerted by the user. Moreover, since the user's fingertip damps vibration, a control has been achieved in order to maintain the tactile feedback at any
Acknowledgements
This work has been carried out within the IRCICA Stimtac Project, and the INRIA Mint Project. The authors thank M. Messaoudi, A. Caroulle and J. Chlebicki for their work in the design of the haptic knob.
Frédéric Giraud (BS’95 Paris-XI University, MS’97 Institut National Polytechnique de Toulouse) graduated from the Ecole Normale Supérieure de Cachan, France in 1996 in electrical engineering, and received his PhD from the University Lille1, in 2002. He is a member of the electrical engineering and power electronics laboratory of Lille where he works as an Associate Professeur. His research deals with the modelling and the control of piezo-electric actuators.
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Frédéric Giraud (BS’95 Paris-XI University, MS’97 Institut National Polytechnique de Toulouse) graduated from the Ecole Normale Supérieure de Cachan, France in 1996 in electrical engineering, and received his PhD from the University Lille1, in 2002. He is a member of the electrical engineering and power electronics laboratory of Lille where he works as an Associate Professeur. His research deals with the modelling and the control of piezo-electric actuators.
Michel Amberg has been teaching electronics at the University of Lille, France. He graduated from Ecole Normale Supérieure de Cachan, France in 1981. He has tutored more than a hundred Bachelor Students during their projects in the field of telecommunications, computer science and electronics. He is now research engineer at IRCICA, and works on the electronic design of tactile devices.
Betty Lemaire-Semail (PhD’90, University of Paris XI, Orsay). From 1990 to 1998, she was assistant professor in the Ecole Centrale of Lille and she is now professor at the University Lille1. She is a member of the electrical engineering and power electronics laboratory of Lille and head of the research axis on the control of electrical systems. She has studied electromagnetic motors and her main field of interest now deals with the modelling and control of piezoelectric actuators, for positioning and force feedback applications.