# Magnetorheological Fluid-Based Haptic Feedback Damper
## 4 Psychophysical Experiments
### Experiment 2  
From experiment 1, it can be understood that the damping force generated by the magnetorheological fluid under the action of a proper magnetic field (regulated by controlling the distance) could increase the degree of comfort of the damped keystrokes. The results of experiment 1 are highly subjective. In order to evaluate the device more objectively, this experiment analyzed the effect of this damping force on the speed and error rate of keystroke input. **Experimental setup:** The specific key configuration was based on the device in Figure 16, and the output of the corresponding characters of the four keys were defined as \"abcd\" (upper-left key output a, lower-left key output b, upper-right key output c, lower-right key output d). During the experiment, the volunteers input the given strings on these four keys. **Subjects:** 20 volunteers were invited to participate in this experiment, involving 12 males and 8 females aged between 20 and 23 years old. The subjects had normal finger skin tactile sensation and had never participated in this type of research or other related studies, and were not sure of the purpose of the experiment. **Process:** The experimental steps were as follows:  
1. A string (100 characters) containing only \"abcd\" characters was generated, where the number of each character was equal. 2. The distance between the magnet and the magnetorheological fluid layer in the device was set to one of four random distances (0 mm, 1.5 mm, 3 mm, Non; all four keys were the same), as shown in Table 3.
3. The volunteers entered the string as fast as they could. The results were not displayed by the host computer and were stored on a data card for statistical analysis. 4. Steps 1, 2, and 3 were repeated four times. All volunteers completed the above experiment. **Results:** The input time is shown in Figure 18 and the input error rate is shown in Figure 19. It can be seen from Figure 18 that the volunteers spent the least amount of time at a distance of 0 mm, the most amount of time in the non-magnet state, and the time spent  
\begin{table}
\begin{tabular}{c c} \hline \hline
**The Distance between Magnet and** & **Proportion (\%)** \\
**Magnetorheological Fluid** & **10 in the other two cases was moderate and close to each other. The ANOVA showed that there was a significant difference between the four cases (\(F=11.07\), \(p<0.01\)); furthermore, the t-test showed that there was no significant difference in the time spent at a distances of 1.5 mm and 3 mm (\(t=1.686\), \(p=0.374\)). Figure 19 shows the error rate of inputting characters at the four magnet distances. The occurrence of errors consisted of input character errors, missed inputs, repeated inputs, etc. The volunteers made the most input errors at the 0 mm distance, followed by the non-magnet condition, and the lowest error rates in the other two conditions. The ANOVA showed a significant difference between the four cases (\(F=3.357\), \(p<0.03\)); furthermore, the \(t\)-test showed that there was no significant difference in the error rates at 1.5 mm and 3 mm distances (\(t=1.686\), \(p=0.419\)). **Analysis:** The experimental results show that at a distance of 0 mm, the magnetic field strength was the largest, and the damping force output by the magnetorheological fluid was the largest, which made the key feel hard, the trigger stress was the largest, and the key stroke was the shortest, which led to the fastest input speed; however, the short stroke caused the rebound feedback to be small, which made the operator's uncertainty about the input result increase, and thus, led to a high error rate.