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HaptiStylus: A Novel Stylus Capable of Displaying Movement and Rotational Torque Effects

With the emergence of pen-enabled tablets and mobile devices, stylus-based interaction has been receiving increasing attention. Unfortunately, styluses available in the market today are all passive instruments that are primarily used for writing and pointing. In this paper, we describe a novel stylus capable of displaying certain vibro-tactile and inertial haptic effects to the user. Our stylus is equipped with two vibration actuators at the ends, which are used to create a tactile sensation of up and down movement along the stylus. The stylus is also embedded with a DC motor, which is used to create a sense of bidirectional rotational torque about the long axis of the pen. Through two psychophysical experiments, we show that, when driven with carefully selected timing and actuation patterns, our haptic stylus can convey movement and rotational torque information to the user. Results from a further psychophysical experiment provide insight on how the shape of the actuation patterns effects the perception of rotational torque effect. Finally, experimental results from our interactive pen-based game show that our haptic stylus is effective in practical settings


INTRODUCTION
There is a large and growing set of mobile applications, and in particular games, that use stylus-based input for manipulating and interacting with virtual objects. Despite the popularity of stylus as an input device, stylus-based interaction has not reached its potential, partly because styluses available in the market are passive instruments. These instruments were primarily designed for writing and pointing on mobile devices, and they lack the ability to create supportive feedback during interaction. To this end, we designed a novel stylus that is capable of creating two tactile effects in an effort to enrich stylus-based interaction. In particular, we built an active stylus that is capable of displaying movement and rotational torque effects.
Our design, which we named HaptiStylus, is equipped with two vibration motors positioned at the ends of its casing. Asynchronous actuation of these motors results in a sensory illusion known as "apparent tactile motion" [1]. With appropriate combination of stimulus duration and inter-stimulus onset interval (ISOI) of the vibration actuators, we show that a movement effect can be displayed along the body of the stylus.
The other effect that HaptiStylus is capable of displaying is rotational torque effect about the long axis of the stylus. A high torque rated DC motor, positioned underneath the fingers enables us to create a sense of clockwise and counter clockwise rotation. Powering up the DC motor creates a torque on the casing of the stylus. When the voltage pulse is cut off, the motor displays a reaction torque in the opposite direction. In our previous work, we showed that the torque display force feedback to a user without the use of mechanical linkages [6]. Wintergerst et al. proposed a stylus, with a magnetically operated brake at the tip, to display haptic effects by controlling the friction between the tip and a display surface [7]. Withana et al. developed a haptic stylus that dynamically changes its effective length to create an illusion of penetrating into a display surface [8]. Poupyrev et al. explored the benefits of using vibrotactile haptic feedback displayed through a touch screen in pen-based interactions [9]. They placed four piezo actuators at the corners of pen-based touch display in between the LCD and the protective glass panel and displayed vibrotactile haptic feedback to a user in varying amplitude and frequency. The results of the user study showed that the subjects preferred haptic feedback when it was combined with an active gesture as in dragging a slider or highlighting a text using a pen interface. Our work complements this body of work by the virtue of focusing on different set of haptic effects (movement and rotational effects).
The haptic movement effect, tactile illusion of creating a continuous motion, was investigated first by Sherrick and Rogers via two vibration motors separated by a distance and placed on the skin of thigh [1]. They adjusted the stimulus duration and the delay between the actuation times 4 to create an effect of travelling haptic stimulus. Tan and Pentland developed a wearable tactile chair equipped with vibration motors to display static lines and two-dimensional geometric patterns to the back of a user [10]. Israr and Poupyrev proposed an algorithm to display twodimensional dynamic haptic effects to the back of a user sitting on a chair through a matrix of vibration actuators [11]. In another line of work, Israr and Poupyrev investigated the control parameter space for producing reliable continuous moving patterns on the forearm and back via haptic movement effect [12]. The results of the study showed that the ISOI space for forearm was influenced by both the motion direction and spacing of the actuators, while the ISOI space for back was only affected by the direction of actuation. Kim et al. created a sensation of travelling wave between two vibration actuators embedded in a cell phone by adjusting the magnitude and timing of the actuators [13]. Lim et al. used frequency modulation to display haptic movement effect between two hands [14]. Our work explores the use of haptic movement effect (apparent tactile motion illusion) on a pen-based interface that can be used with a tablet or a mobile device.
In our stylus, the actuators do not come in direct contact with the skin, and the vibrations are transmitted indirectly through the stylus casing.
Compared to the haptic movement effect, the haptic rotational torque effect has been investigated less in the literature. Winfree et al. developed iTorqU, an ungrounded handheld device, to create directional torque feedback by means of a flywheel inside of a two-axis actuated gimbal [15]. Porquis et al. created a torque illusion via suction by using a vacuum driven interface embedded in a stylus [16]. Using a rotating flywheel, Amemiya and Gomi displayed directional torque feedback through a mobile device [17]. Unlike these studies, we simply utilize 5 consecutive torque pulses displayed through a DC motor attached to the casing of our pen-based stylus to generate a perceptual sense of bidirectional rotation.

a) b)
To our knowledge, the application of the proposed haptic effects (haptic movement and rotation) on a pen-based interface is new. While the concept of haptic movement effect (i.e. apparent tactile motion) is not new and has been applied to the other parts of body such as forearm and back as discussed above, the haptic rotation effect is completely new. Moreover, we should emphasize that the earlier pen-based interfaces with haptic feedback has focused on  6 displaying haptic effects for 2D GUI events such as button click, drag and drop, menu selection, etc. However, the proposed stylus and the haptic effects open new avenues for 3D haptic interaction (as discussed in the "Application" section) since the proposed stylus includes multiple actuators along its body.

Design of the HaptiStylus
Our haptic stylus consists of a plastic cylindrical casing and three physical actuators: one DC motor for creating a sense of rotation, and two vibration actuators for generating movement effect. Two physical embodiments of our design are shown in Figure 1. The pen in Figure 1.a was designed as a low-cost embodiment, which doesn't include any circuitry inside, whereas the embodiment in Figure 1.b was designed as a standalone device with an embedded digitizing stylus tip, control circuitry for actuators, and a Bluetooth module for wireless communication ( Table 1). The first design was used to evaluate the effectiveness of our approach for generating movement and rotational torque effects, which was further verified with the second design in a more practical computer game setting. The second design was also used to further investigate how the perception of rotational torque effect is influenced by the shape of the waveform that drives the actuators.
We utilize two coin-type eccentric rotating mass (ERM) actuators to display a movement effect through the stylus. The coin type ERM actuators are compact, lightweight and have a larger frequency and amplitude bandwidth than some other vibration actuators. Two coin type ERM actuators are placed close to the ends of the styluses (Figure 1). We used specialized haptic drivers to improve the performance of the vibration actuators. The drivers offer shorter start-up duration (0.1 ms) using only a single-ended pulse width modulation (PWM) input signal. The use of specialized driver circuits proved to be critical for achieving the targeted effects.
In order to display the rotational torque effect, we used a high-torque rated DC motor. The DC motor is positioned to coincide with the position of the fingers while holding the stylus. Our preliminary studies showed that using a specialized driver to control the DC motor is also critical for displaying the targeted haptic feedback. Hence, we used special purpose motor drivers for both designs (Table 1).

Generation of the Haptic Effects
The HaptiStylus can generate two haptic effects: (1) up-and-down movement effect along the stylus, (2) rotational torque effect about the long axis of the stylus.

Movement Effect
We achieve the movement effect by actuating the two vibration motors while preserving a carefully selected delay between their actuation times. Sherrick and Rogers observed that actuating two vibration motors placed in close proximity of the skin leads to one of three sensations depending on the delay between the start-up times of the actuators [1]. In particular, depending on this delay, defined as the inter-stimulus onset interval (ISOI), participants perceive either a single stationary target, two discrete stationary targets or a single moving target as shown in Figure 2. When ISOI is small, it leads to the perception of a The earlier studies on haptic movement effect have also focused on identifying optimal stimuli duration and ISOI values, but only on sensation through human forearm and back [12]. To our knowledge, our work is the first to investigate the perception of tactile movement on a stylus through human hand and fingers.

Rotational Torque Effect
Unlike torque feedback devices, a stylus is an ungrounded apparatus, thus it is highly difficult to generate torque feedback using conventional actuators. Instead, we attempt to induce a sensation of rotation by powering the DC motor through discrete pulses. This creates a short, but perceivable sensation of rotation about the axis of the pen by slightly stretching the finger pads ( Figure 3).
The phenomenon that leads to the rotational torque effect is closely related to what happens when a DC motor is powered up. When a DC motor is first powered up, the rotor experiences non-zero angular acceleration until it reaches a terminal angular velocity ω max (i.e. during the on-time state) . This non-zero acceleration causes a corresponding torque τ in the casing of the motor in the reverse direction, which approaches to zero as the angular velocity approaches ω max . When the motor is powered off, (i.e. during the off-time state), a reaction torque is generated in the opposite direction. Generating successive pulses with carefully selected frequency and duration creates a sense of rotation that is perceivable by the users. We report a comprehensive analysis of the effects of these parameters in the evaluation section.
Another parameter that affects the perception of rotation is the shape of the waveform that drives the DC motor. Based on this observation, which became evident in our preliminary experiments, we designed and conducted a controlled psychophysical experiment to compare three alternative waveforms. The results are reported in the evaluation section.  the top left region representing the "discrete", and the middle right region representing the "continuous" stimuli. 13 We investigated the effects of stimulus duration, ISOI, and the direction of tactile movement on the perception of three possible cases. The responses of the participants were analyzed by a suggested that the perception of the tactile movement effect is higher as the duration of stimulus increases (p<0.05). The ANOVA analysis also suggested that ISOI was a significant factor on the the direction of movement has no effect.
The results of the first experiment also revealed that the effective parameter values for obtaining apparent tactile motion in a hand-held stylus differ from the values reported in the earlier studies performed on the human forearm and the back [12]. This result can be explained by the differences in the sensitivities of the human hand, the fingertips, the forearm and the back.

Experiment 3: Waveform User Study
The haptic rotational torque effect is displayed by square shaped input pulses as described in the second experiment. We further investigate how input waveform patterns affect the perception of the rotational torque effect. Based on our preliminary studies [2], we selected three input voltage waveforms for this purpose (see Figure 6)  The increase from 90% to 95.5% can be explained by the observation that gradual offset of Figure 8 -Correct identification percentages for "Square Shape", "Increasing" and "Decreasing" waveform patterns.
voltage would spread the effects of torque in the reverse direction over time and result in smaller differences in torque per unit time. We anticipate that the torque perceived in the undesired direction becomes less noticeable as the differences fall below the JND, but this topic obviously requires further investigation.

PRACTICAL USE CASES
In this section, we present two mockup demonstrations and an interactive stylus-based game to illustrate the vast potential of our flow and rotation rendering technologies in practical applications. The mockup demonstrations are supported by a supplementary video, and the discussion of the stylus-based game is supported by a rigorous user study that measures the degree to which the effects in question can be successfully rendered and perceived by the users.

Mockup Demonstrations
Our mockup demonstrations can be found in the supplementary video provided with the manuscript. The videos illustrate how our technology could potentially be embedded into two real-world applications to build engaging interfaces. The first application concerns a serious information visualization problem, while the second one is an interactive mobile game with rich graphical and interactive elements.

Information Visualization
Today, we mostly use the visual channel to extract information from data.  Using the proposed stylus, some of the climate information can be displayed through the haptic channel in order to alleviate the perceptual and cognitive load of the user (see Figure 8). For example, vorticity, which represents the amount of circulation or rotation, is a force, defined as the curl of the wind velocity. The direction of this circulation can be displayed to the user via the haptic rotation effect. Moreover, the vorticity makes the wind rise or fall due to the circulation of the air, which can be displayed to the user via haptic movement effect. In this Figure 9 -Mobile Dentist Game Mockup: In this game, the player provides dental care through virtual tools. The experience of using the tools can be enhanced by haptic effects (e.g., downward flow for the water syringe, upward movement for the suction tip, and rotation for the drill). 22 fashion, haptic feedback may improve the discovery, learning, and retention of cause-and-effect relationships in a multidimensional climate data [18].

Mobile Gaming
With the introduction of mobile devices into the market, mobile gaming has become wide-spread and increasingly more sophisticated. With the integration of rich haptic feedback to the mobile devices, these games will become more immersive, more enjoyable, and more engaging. Here, to motivate the reader, we suggest the use of haptic feedback in a mock-up dentist game designed for kids.
The dentist games available in the market are virtual platforms for kids to learn oral health. They typically involve simulation of many dental procedures (cleaning, drilling, brushing, filling, suction, etc.) performed with different dental tools. They are also excellent examples demonstrating the importance of multisensory experience since it is more fun to play these games with audio is on.
Hence, we believe that the haptic feedback displayed through the proposed stylus can further augment this experience by stimulating the tactile channel. For example, the tactile difference between brushing front, middle, and back teeth, tooth filling, as well as dental suction from mouth can be displayed via haptic movement effect (See Figure 9). On the other hand, tactile feeling of drilling or polishing a tooth can be displayed through the haptic rotation effect. 23

Spinning Tops Game
To further demonstrate the practical utility of the proposed haptic effects in an application, we developed an interactive pen-based game, called "Spinning Tops", As shown in Figure 10, the game scene involves 3 tops, spinning in 3 different speeds, 2 directions, and 3 boxes on each side to drop the tops. The boxes on the left

FUTURE WORK
In our experiments, we explored a comprehensive set of values for parameters that create a sense of rotation about the long axis of the stylus. However, we did not attempt to build a mathematical model of the relationship between the parameters and perception ratings. The challenge in relating input values directly to the perceived sensations lies in the fact that both the input parameters and the sensation ratings are discrete labels; however they are connected through a time varying torque signal, which is continuous. Hence, a feasible strategy might involve building a mathematical model of the relationship between input parameters and output torque, as well as the relationship between the torque profiles and the perceived effects. In order to assess the feasibility of this idea, we collected torque data for combinations of input parameter values by attaching a torque sensor on the stylus casing [2]. Preliminary results from a set of system identification experiments show that it is possible to obtain a mathematical model of the relationship between the input parameters and torque response. This leaves the task of relating the perceived user sensations to the displayed torque profiles as an interesting piece of future work.
This may potentially lead to a framework for inferring optimal actuation parameters.
Another piece of future work involves improving the physical and ergonomic characteristics of the stylus. Although our second design is substantially more compact compared to our original stylus, more improvement is needed to achieve the form factor of a regular stylus. This is partly because all used parts (actuators and the batteries) were off-the-shelf items. Replacing these with custom-made versions with smaller form factors is likely to lead to an improvement. The improved form factor is likely to improve the ergonomic character of the stylus, which deserves further exploration. A possible direction might involve investigating parameters such as stylus length, center of gravity, casing texture, and pen-skin contact area.

CONCLUSIONS
We introduced a novel haptic stylus capable of displaying two effects to the user: the movement effect and the rotational torque effect. The movement effect was displayed by asynchronously activating two vibration motors positioned on two ends of the stylus. The rotational torque effect was displayed through a DC motor creating torque pulses in the intended direction. We explored the effective parameter spaces for both haptic effects through 29 psychophysical experiments. The results of the psychophysical experiments using both HaptiStylus designs proved that the proposed haptic effects are perceived by the participants.
Our design opens new avenues for exploration in the pen-based computing and haptics communities. We believe that HaptiStylus can be used in a variety of mobile applications including games, entertainment, and education. For example, injection/discharge of fluids (e.g.
simulating needle injection in a game), trajectory of a moving object towards/away to/from the scene (e.g. an approaching fire engine in a video scene), and penetration into a soft object (e.g. touching and feeling the softness of organs in the visual scene) can be displayed to a user using the movement effect. Similarly, opening and closing of valves and screws, sense of rotational inertia in spinning and rolling objects (e.g. flying an airplane in a game) can be conveyed to the user via the rotational torque effect. This is not an exhaustive list, but serves as evidence of the abundance of potential uses.