## 5 Discussion  
Combining findings from the two previously discussed experiments, it can be shown that the vibrotactile feedback along the beam can be controlled, and a freely positionable vibrotactile feedback system can be achieved. Figure 7 shows the potential vibration profiles that can be achieved by changing excitation frequency. If a lower vibration magnitude is desired for a sustained period, then the excitation magnitude can be adjusted accordingly. If a higher or lower vibration magnitude is desired for a short period, then a pulse excitation signal can be used. Figure 7a shows the vibration response when exciting one actuator at its relevant natural frequencies. By changing the frequency, the desired mode shape can be achieved to account for the nodes and provide at least 29.42 m/s\({}^{2}\) (3 g) throughout most of the beam.  
Figure 6: Acceleration along the beam, exciting both actuators at: (**a**) 171 Hz, varying one actuator’s pulse excitation duration; (**b**) 181 Hz, varying one actuator’s pulse excitation duration.  
Figure 7: Unique vibration mode shapes that were experimentally measured when exciting: (**a**) a single actuator at 171 or 196 Hz; (**b**) both actuators at 181 and at 215 Hz.  
Figure 7b shows the vibration response when exciting both actuators simultaneously, either at 181 or 215 Hz. By switching between these two similar frequencies, the location of the node can be accounted for and controlled. This results in a minimum vibration of 24.5 m/s\({}^{2}\) (2.5 g) in the beam, with the majority of the beam able to exceed 39.22 m/s\({}^{2}\) (4 g). Note that the system's output response frequencies are all close to the Pacinian corpuscles' most sensitive range and well above the minimum detection threshold [23; 24].  
Figure 8 demonstrates the potential of the touch bar actuation system. The solid black line represents the maximum achievable vibration for each point along the touch bar beam. Using each unique vibration mode shape, a freely positionable and fully controllable point of vibrotactile stimulation can be created throughout the touch bar display. To adjust the magnitude of a sustained response in the beam, the actuator's voltage magnitude can be adjusted as previously shown in the actuator magnitude experiment. To create a pulse increasing or decreasing response in the beam, the number of actuators excited can be adjusted as shown in the pulse experiment. By varying the number of actuators excited, the magnitude, excitation frequency, and signal duration, the desired vibrotactile stimulation can be generated at any point along the touch bar.  
Although the direct actuator-to-display mounting method provides greater control over the display's mode shape, this can only be achieved by varying the frequency. In this system, vibration at a specific location is linked to actuation frequency. A change in frequency is required to generate vibrations at every location along the display. It is currently unclear if a user would be able to feel the different stimulation frequency because all four of the system's natural frequencies used in this study are close in frequency. Human trials are needed to identify the touch bar's feel and effectiveness.