## 5 Discussion and Conclusions
One of the typical advantages of the proposed torque sensor is its wide applicability and its relatively high cost effectiveness, which would be very welcome in many industries. The way to maximize this inexpensive advantage is quite the opposite of the performance improvements mentioned above. First, the CRB used to enable fine rotation of the motor can be replaced with a less expensive component. Common ball bearings or plain bearings can be used as alternatives. Furthermore, replacing the bearing with a compliant mechanism that allows small rotation will greatly simplify the sensor structure. Next, the rod end bearings applied to the connection of the load cell and the lever/base arms can be also removed to be joined integrally. Users can also consider applying a strain gauge on the integral component instead of the load cell; this method may unduly reduce the accuracy of the torque measurement, so it would be desirable to avoid this replacement. Lastly, it is also considerable to remove the motor itself to fix the rotary motor arm and shaft on the lever arm. The motor is not an essential component for economic application, and it can cause a limit to the measurement range. A fixed shaft connecting the base arm and the distal coupling can eliminate the motor housing as well. Currently, excluding the motor assembly, the manufacturing cost of the torque sensor prototype is about $670 ($300 for the load cell; $220 for the commercial components such as the rod end bearings and the CRB; $150 for the machined components such as the levers, the housings and the lids). Therefore, it is currently difficult to claim the cost-effectiveness of this sensor with this cost. However, the current cost could be significantly reduced by modifying the design and changing the utilized commercial components. Assuming that a user can apply an existing load cell, the cost of configuring this torque sensor is expected to be reduced to less than $100. Compared to existing commercial torque sensors, this price is unusually cheap. Even if the load cell is included in the price of the torque sensor, considering its adjustability of measurement range and sensitivity, it will have sufficient competitiveness. Other application methods of the single-axis torque sensor proposed in this study can be considered together. First of all, this torque sensor is limited in miniaturization due to its long arms, so it will not be easy to build this sensor itself into the rotary joints of a robot system. However, this torque sensor could be embedded in the robots during the development stage of prototypes without severe space constraints, or it can be effectively used as an external measuring device for evaluating the performance of developed robots without built-in joint torque sensors [32]. Next, by adopting the structure of this torque sensor, it will be possible to extend to multi-axial measurement. For example, a six-axis force/torque sensor in the form of a hexagonal column with six legs that are not parallel to each other, similar to a Stewart platform, would be configurable. A wide measurement range can be obtained by using wide top and bottom plates with long arms, while high sensitivity can be obtained by using the aforementioned oblique model or a near-singular configuration [33]. In addition, if combined with a position sensor that can monitor the exact position of the combined load cell in real time, the measurement errors caused by deformation for high torque can be offset. In particular, in the oblique state of the extremely short moment arm, the rotational stiffness of this sensor decreases, and the influence of the assembly clearance increases. Therefore, it is expected that the presence of additional position sensors can greatly improve the measurement accuracy. The sensor proposed in this study can be applied to various torque measurements. As in the example shown above, it is possible to maximize the balancing ability by measuring the torque applied to the joints in the balanced-arms.