## 2 System Prototype and Controller Fabrication  
To further study the dual-electrode ERA and work towards applying it to narrow touch displays, a preliminary touch-bar-style display was designed and fabricated. Figure 1a demonstrates the conceptual design of the haptic touch bar interface based on the dual-electrode ERA. The system is a proof-of-concept design, made primarily to study the effect of different ERA actuation parameters on the vibration response in the system. The system primarily consists of three main components: the left side dual-electrode ERA, the right side dual-electrode ERA, and the long, narrow touch display. As previously discussed, most large touch displays with vibrotactile feedback utilize a two-degree of freedom mounting method with clamped touchscreen ends, which can limit both the vibration amplitude and vibration profiles. To resolve this problem, this study proposes a mounting method that directly connects the actuator's oscillating mass to the display. To allow for this connection, the spring-mass actuator component was redesigned, as shown in Figure 1b. The modified actuator design features a central hole and welded M4 hex nut on the upper surface of the plate spring component. The radial beam springs of the central component have a stiffness of 16.18 kN/m. To achieve the desired total mass of 50 g per actuator, this design relies on the mass from a touch display as well as the nut and screw used to attach the actuator to the beam. Electrodes with holes in the center were used so as to not significantly interfere with the performance of the actuator and allow it to be directly connected to the beam. To amplify the performance of the actuator, a previously studied dual-electrode structure composed of an upper electrode and a lower electrode was applied. Figure 1: Haptic touch bar interface based on dual-electrode ERA: (**a**) An exploded perspective view of the touch bar interface design consisting of two dual-electrode ERAs and a long, narrow touch display; (**b**) assembled dual-electrode ERA and its components. To control each ERA's actuation parameters simultaneously, a custom high voltage controller was fabricated. The controller can adjust one actuator's frequency, magnitude, phase delay, and signal duration independently from the other. This allows any combination of these parameters to be controlled simultaneously. The controller is also necessary to power the upper and lower electrodes within each actuator in an alternating fashion so as to never simultaneously create opposite attractive forces on the central vibrating component. The timing between each electrode activation must be accurate and consistent, making the controller highly important for proper function of the dual-electrode ERA. In addition to a controller, a high voltage amplifier must be used to amplify the control signal inputs. The amplifier takes the controller's low voltage input and significantly increases it, outputting the high voltage inputs required to properly power the electrodes within the actuator. Figure 2a shows the schematic structure of a circuit diagram for the controller and high voltage amplifier. This controller can power both actuators simultaneously with different parameters to produce separate signals. A total of 2 kV is required to power each electrode of the ERA module. A portable rechargeable battery is used as a power source to generate input signals of 2 V to the high voltage amplifier. Next, the high voltage amplifier increases the signal and outputs 2 kV to the electrodes. To produce high-voltage cosine signals with the desired frequency, high-voltage opto-diodes (OZ150 SG, Voltage Multipliers Inc., Visalia, CA, USA) were adopted. The opto-diodes can adjust the high-voltage leakage according to the amount of infrared radiation. To produce a high-voltage cosine signal for one channel, two high-voltage opto-diodes are required for charging and discharging.