## 5 Discussion  
We have demonstrated the proposed approach and compared the results with those from the conventional approach. However, there are several topics worth mentioning. Recently, [20] highlighted the difficulty in a wrist flexion task. Although the authors suggested a joystick-like mechanism to increase the flexibility, we proposed a new movement pattern to free up the wrist while moving. While [12] presented a movement velocity analysis for a similar task, namely teleoperation for welding, we focused on a problem in the positioning of the end-effector. Hopefully, these two studies can be combined to improve the performance of the operator.  
The use of the Kalman filter shown in this paper provided results as impressive as those from other Kalman applications [34; 35]. Our results confirm that applying a linear estimator, specifically the Kalman filter, incorporated with the proposed arm movement pattern is viable for tele-abrasive and other similar applications. However, in this experiment, the traveling path was set at 10 mm, which is relatively small compared to other applications, such as aircraft navigation [36] and vehicle position estimation [37]. This is because the path width we selected in this experiment was set based on an empirical study to find the participant preference, where the result came out to be 10 mm.  
However, the system can be adjusted to work with larger-scale experiments. The suggestions are as follows:  
1. If the real path is larger than the size used in the experiment, the movement between the master controller and the slave device can be scaled so that the operator can maintain the movement distance at which he or she best performs.
2. If there are some errors remaining in the filtered output, they would be amplified during the scaling process. However, we can adjust the parameters in the Kalman filter, i.e., process covariance and measurement covariance, to minimize the error so that the setting is suitable with the scaled moving distance of the nozzle.  
In addition, we have demonstrated how the proposed approach helps to reduce variation due to human arm movement error in the output of the human operator stage. The period needed to train each participant on the proposed arm movement pattern was relatively short (approximately 15 min per person).  
After the experiment, we asked the participants the following two questions:  
1. Which type of movement did you perform with less stress and tension?
2. Do you have a problem with the adjustment to the new movement pattern?For the first question, all of the participants agreed that they worked with the proposed movement pattern more comfortably. Furthermore, seven of the participants did not have a problem adjusting to the new movement pattern. They thought that 15 min of training is sufficient to perform the task, and they believed that the longer they work, the better they can perform. However, three of the participants thought that they need more time for training. They suggested that the visual perception did not follow the habituation because their hands moved up and down vertically while the nozzle moved straight horizontally.  
From the study, we can conclude that the operators feel more comfortable and relaxed with the proposed arm movement pattern as opposed to that in the standard system, and our pattern yielded a better system output as a result.