## 1 Introduction
Users who interact with these objects want to haptically sense the presence of the target objects and their surface properties such as texture and roughness. For satisfying users' demand, two or more mechanoreceptors need to be stimulated at the same time when he/she interacts with various surfaces. Beat vibration can be a solution for stimulating two mechanoreceptors at the same time. The beat vibration is an interference pattern of two different vibrations having slightly different frequencies [16]. The beat vibration has two different frequencies so that it can stimulate two mechanoreceptors at the same time. Because of this feature, the beat vibration allows users to perceive special touch sensation unlike normal vibration. Let us consider the case where two haptic actuators, which are actuated at the frequency of \(f_{1}\) and \(f_{2}\), respectively, are attached to a mobile device. If two actuators have slightly different operating frequency, the mobile device vibrates at the frequency of (\(f_{1}+f_{2}\))/2. At this time, the envelope frequency of the vibration (we call it beat frequency) becomes \(f_{1}-f_{2}\). If \(f_{1}\) is 206 Hz and \(f_{2}\) is 200 Hz, the mobile device vibrates at 203 Hz and its beat frequency is 6 Hz. In this case, Meissner corpuscle responses to 6 Hz vibration and Pacinian corpuscle is stimulated to vibration at 203 Hz. Therefore, vibration signal with beat can stimulate two mechanoreceptors (Meissner corpuscle and Pacinian corpuscle) at the same time. There have been a few research works for creating beat vibration [17; 18; 19; 20; 21]. [PERSON] et al. developed a pin-array tactile device and generated haptic beats by providing two pins with slightly different sinusoidal operation frequencies [17]. [PERSON] et al. created haptic beats using two vibrotactile actuators and investigated users' ability to distinguish emergent haptic beats from pairs of pure vibration presentations as a function of body location and frequency differences [18]. [PERSON] et al. developed a tactile module for generating beat and they investigated beat synchronization to vibrotactile electronic dance music in hearing and deaf people [19]. [PERSON] et al. suggested a tactile synthesis method using multiple frequency vibrations [20]. [PERSON] and [PERSON] investigated the perception of haptic beats when a user touched a mobile screen by actuators [21]. To create the beat vibration in their work, two or more vibrotactile actuators have to be inserted into the mobile device. Due to the two or more vibration motors, the conventional beat vibration module needs additional power consumption and additional cost. Furthermore, the beat module requires additional space to mount the two or more actuators. Another problem in beat vibration modules using vibrotactile actuators is that a conventional vibration module has narrow frequency bandwidth. To simulate the various virtual objects with the beat vibration, a new way with no or small number of actuators is demanded. In this paper, we present a new electro tactile beat (ETB) module, which has wide operating frequency range and create beat vibration without any mechanical vibrotactile actuators. The proposed ETB module is composed of three parts: a lower part, a connection part and an upper part. The upper part is connected to the lower part via the connection part (four connectors). We design interdigitated pattern on the upper side of the lower part using two electrodes. These two electrodes are used for an anode and there is an electrode in the upper part for a cathode. When two voltage inputs with slightly different frequencies are applied to the interdigitated electrode pattern, two electric fields are generated between the interdigitated pattern and the cathode. At that time, the created electric fields influence each other and then finally they are transformed to a new electric field having beat pattern in the lower part.