# Design and Experimental Evaluation of an Electrorheological
## 3 Evaluation of the Haptic Module
### Dynamic Testing: Experimental Results  
Our first test examined the sensor's ability to accurately track the indenter during the press. Figure 9 shows the force generated by the actuator overlayed with the depth tracked by the sensor over time. Depth was determined by linearly mapping the sensor's maximum and minimum readings to 0 and 1 mm, respectively, as enabled by the sensor's linear response to bending. Three excitation signals were used: (a) 0 kV, i.e., off-state; (b) 4 kVDC; and (c) \(\pm\) 4 kV sine wave at 3 Hz. As shown, two indentations were performed (i.e., the actuator was compressed and released two times); these results indicate that the sensor is effective at following indentation depth across multiple loading cycles and excitation conditions. Furthermore, adding the sensor does not significantly impede upon the generated kinesthetic and tactile sensations, as evidenced by the increased peak forces in the 4 kVDC condition and the oscillations present in the force curve for the sinusoidal condition. Having found the sensor to be dynamically accurate, we look now to use it to inform our controller for actuator output. The added sensor can be used to programmatically switch the actuator's response on and off, as shown in Figure 10. Here, we program turning on the high voltage (2, 3, and 4 kV) at 0.6 mm into the stroke (see Figure 10a). As shown in Figure 10b, when the voltage was switched on, the force curve's slope increased according to the magnitude of the applied voltage, where increased voltage resulted in increased force. To best examine the electrorheological effect, Figure 10c isolates the force due to the fluid's viscosity change from the total force by subtracting the actuator's passive resistance, i.e., the actuator's force-displacement curve when no voltage is applied. As shown, the force due to the ER effect rises rapidly at 0.6 mm when the high voltage is switched on. For the 2 kV applied voltage, the applied voltage triggered about 0.4 N maximum due to the ER effect. For 3 kV, about 0.7 N was generated. The greatest effect was observed for 4 kV, which produced about 1.1 N at maximum depth. This type of switching response can be used to emulate a button that has \"bottomed out\" or contacted a spring, causing the button's stiffness to increase significantly. To determine whether these changes in force are perceivable to end users, the performance must be compared to a perception threshold known as the just-noticeable-difference (JND). The JND denotes the degree to which a stimulus must change to be noticed by a user. In the case of a change of stiffness, the metric used for the JND is the force rate. The force rate is defined as the ratio of the difference between the maximum (the force output with voltages applied) and the minimum (i.e., the force output with 0 kV applied) force to the maximum force. The threshold for which humans can reliably feel changes in force is known to be about 7-10% for forces between 0.5 and 200 N [61]. Figure 10d shows the force rates across the actuator's stroke for each voltage step. As shown, when the step-on voltage switches on at 0.6 mm, the force rate increases for all voltages, where the 4 kV excitation resulted in the highest peak force rate of about 25%. All curves peak above the 7-10% threshold, indicating that the stiffness change because of the voltage being stepped on is noticeable to end users. Figure 9: Actuator and sensor response subjected to (**a**) 0 kV, (**b**) 4 kV DC, and (**c**) 4 kV sine wave at 3 Hz. Furthermore, the sensor can be used to switch the actuator response on and off rapidly, as shown in Figure 11. Here, we prescribe pulsing high voltage (2, 3, 4 kV) on at 0.6 mm and off shortly after (~25 ms, illustrated in Figure 11a; ER fluid has a response rate of ~5 ms [65], so this is sufficient to get a complete change in pressure). Figure 11b shows the actuator's response to the step input. As shown, when the voltage pulse is applied at 0.6 mm depth, the slope of the force curves increases.