## 2 Dual Piezostack-Driven Jetting Dispenser  
The structural configuration of the proposed piezoelectric jetting dispenser is shown in Figure 1. Two piezo stack actuators are installed in parallel and generate the required force and displacement for fluid jetting. Adjusting bolts are prepared to control the preload of the piezostack actuators. In order to protect the actuators from external shock, upper and lower caps made of stainless steel are attached to the top and bottom of the piezostack actuators. The displacement that is generated by the piezostack actuators, which is normally less than 0.1 mm, is not sufficient for direct fluid jetting. Thus, a displacement amplification device is adopted, based on a lever-hinge mechanism. For amplifying the displacement, the lever is fixed to the hinge and kept in contact with the piezostack actuators by the return springs. When the left-side piezostack is elongated and the right-side piezostack is contracted, the displacement generated by the piezostack actuator makes the lever mechanism rotates counterclockwise, and hence the needle moves upward. In contrast, when the left-side piezostack is contracted and the right-side piezostack is elongated, the lever rotates clockwise, and hence the needle moves downward. The lower caps of the piezostack actuators are in contact with the hinge portion of the displacement amplification device, and disk-type return springs are installed underneath the hinge to recover the initial position of the hinge. The needle of the dispenser is directly connected to the end of the lever part of the displacement amplification device. The design parameters for the dispenser components, such as the lever, hinge, and return spring are given in Table 1[15]. The proposed piezoelectric jetting dispenser performs fluid jetting through the following four steps:  
1. Rising Stage: The lever rotates in a counterclockwise direction and the needle moves upward when the left- and right-side piezostack actuators are elongated and contracted, respectively. Subsequently, the fluid in the syringe begins to fill the empty space in the dispenser head. 2. Open Stage: The position of the needle is maintained by applying the proper control input voltage to obtain perfect filling of the dispensing fluid. 3. Falling Stage: The lever rotates in a clockwise direction and the needle moves downward when the left- and right-side piezostack actuators are contracted and elongated, respectively. The needle penetrates the fluid and dispenses it through the nozzle of the head by using the impact energy. 4. Closed Stage (delay): The position of the needle remains lowered until the next jetting operation. Figure 1: Structural configuration of the piezostack-driven jetting dispenser. The schematics of the needle position and input voltage profiles are shown in Figure 2a,b, respectively. As shown in Figure 2b, since unidirectional piezoelectric actuators are used in this study, the voltage from 0 to maximum voltage can be applied as the input, and hence it is not necessary to apply offset voltage. The implemented controller controls the time for the rising, open, falling, and closed stages as well as the operating frequency and the stroke of the needle by controlling the input voltage to the piezostack actuators. A sudden voltage application can cause an overshoot, and an excessive overshoot can degrade the accurate performance of the dispenser system. This should be carefully considered from the stage of an appropriate selection of the piezoelectric actuator. The overshoot can be reduced by reducing the magnitude of the input voltage per unit time [17; 18]. In this work, to prevent overshoot, the loading rate of the input voltage is reduced by dividing the input voltage into small steps (digitized), and the time per step is 5.8 \(\upmu\)s. In order to obtain a rise of time of 2 ms, 340 steps are required, and the voltage is increased by 0.4 V per step, so that a maximum of 135 V can be applied. This control strategy is programmed in the controller and experimentally realized.