## 3 Experiments
### Adjustable Balancing Experiment  
We conducted experiments for two scenarios, one using only a counterweight for balancing and the other combining one spring and two counterweights.  
To control the extended link length of the counterweight, we designed a linear motion actuator, as shown in Figure 6. This motor is grounded at the left end of the link, and it extends the length of the link to the right. The variable range of motion is \(50\) mm. The variable length is also calculated via an encoder sensor mounted on the linear motion actuator.  
Figure 5: Ear surgical device (**a**) balancing by counterweight only. (**b**) Balancing by both a counterweight and spring.  
In scenario 1, only the counterweight is used. A weight of 1.28 kg was set at the distal end to balance the mechanism in the initial state. In this state, the length of the counterweight was 157 mm. To break the balancing state, we put an additional mass of 31 g at the distal end. As a result, the mechanism fell because balancing of the mechanism was no longer maintained. The encoder installed at the joints senses the movement of the mechanism when it falls. To establish gravity balancing of the mechanism, the length of the counterweight should be adjusted again. From the encoder sensor, the angular velocities of the links can be calculated. The length of the counterweight is controlled such that the angular velocities become zero. In this scenario, the length of the counterweight is increased by 45 mm within three seconds, as shown in Figure 7. In short, the added mass by 31 g requires variation of the extended link length by 45 mm, which is close to the motion range of the extended link. This implies that the allowable variation of the payload is too limited in the case of adjustable balancing by means of the counterweight alone.  
In the second scenario, both the counterweight and spring are used. A weight of 1.32 kg was set at the distal end to balance the mechanism in the initial state. In the initial state, the length of the counterweight was 154 mm. The spring only contributes to design the adjustable balancing part with minimum mass by replacing a large counterweight. Two counterweights are used to rearrange balancing by adjusting the extended link length of the counterweight when the mass at the distal end is changed.  
To break the balancing state, we put a mass of 62 g at the distal end of the mechanism. To rebalance in the changed state, in the same way as in the first scenario, the link length of the counterweight was increased by 20 mm within 3 s, as shown in Figure 8. Therefore, there was still more room for extension of the link. In other words, there is still more room for the added mass at the distal end. This implies  
Figure 6: Variable extended link.  
Figure 7: Balancing by counterweight only.  
that the allowable variation of the payload is relatively large in the case of adjustable balancing by means of both a spring and a counterweight.  
From these two experiments, we could confirm that the method of combining a spring and a counterweight is much more effective for adjustable balancing in a wider range of payload variations. The added external mass in the second scenario is heavier than the added external mass in the first scenario, while the increased link length of the counterweight is shorter. The video clip provided with this paper demonstrates more experimental cases.