## 2 Related Work
### Multi-Mode Haptic Feedback Using Multiple Actuators  
The most common way to realize multi-mode tactile feedback has been combining multiple actuators. For instance, a finger wearable device was designed by [PERSON] et al., which produced three types of feedback simultaneously [14]. The device rendered normal force, lateral force, and vibrotactile force feedback for the interaction of the finger and the touch screen through three internally integrated actuators. An MR foam actuator was integrated to provide normal force feedback, while the piezoelectric actuator was integrated to generate vibrotactile feedback. The lateral force feedback was provided by a DC motor and a friction wheel. As another example, a fingertip haptic display was developed to provide integrated force, tactile, and thermal feedback [15]. They used two DC motors and a Peltier module to generate the sensations. [PERSON] et al. designed a hybrid device that rendered multi-modal stimuli. They utilized a haptuator to render vibrotactile feedback, while a 1-DoF impact actuator to render impact feedback [16]. On the other hand, an underground augmented reality system was designed to alter the roughness and friction of a rigid 3D object [17]. A vibration waveform was generated to model the texture roughness. However, it was generated by using a voice coil actuator. To render friction, a solenoid inside the stylus applies force to the ball.  
Several studies have focused on combining pneumatic feedback with other kinds of technologies. To this end, [PERSON] et al. presented a soft tactile display that produces force and vibration feedback using a pneumatic balloon and electromagnetic coil [18]. A haptic device was also designed to show the impact of simultaneously applying normal stress and vibrotactile stimulation [19]. They used a piston with a DC motor to apply normal force to the skin and two shaftless cylindrical vibrator motors to generate vibrotactile stimulus. Another example is a hybrid electromagnetic-pneumatic actuation system that was used to apply the tactile stimulus and a Peltier element attached to the display to render thermal stimulus [20]. [PERSON] et al. [21] introduced a portable haptic glove that provided both force and tactile feedback. A double-acting pneumatic cylinder was used to produce force feedback. It was operated by using solenoid DC valves through the pulse-width modulation technique. [PERSON] et al. developed a haptic interface to show the virtual interactive information through multi-mode haptic feedback [22]. A piston-type magnetorheological actuator and a voice coil actuator were combined to form a hybrid actuator that rendered force feedback. In contrast, a linear resonant actuator was utilized to generate vibrotactile feedback. On the other hand, a touch screen was developed that allowed the user to simultaneously feel both large bumps and small textures [23]. They employed lateral force to feel the shape and mechanical vibration to feel the texture.  
A common shortcoming of combining multiple actuators for delivering different modalities of haptic feedback is that the whole haptic system becomes bulky and impractical, thus hindering fine haptic experience.