# Open Loop Position Control of Soft Hydraulic Actuators for Minimally Invasive Surgery
## 5 Discussion  
Many technical challenges have to be faced in the field of MIS and new approaches are necessary to meet them. Soft robotics is a growing field of study that shows good potential to deliver benefits in comparison to conventional rigid robots in areas such as MIS where interaction with humans and human tissues is the focus. The aim of achieving open loop  
Figure 10: Deployable robot, test setup, and result. (**A–C**) Deployable robot at different angles. (**D**) Result of path tracing task for a 30 mm diameter circle. (**E**) Test setup image taken during test. position control with a soft actuator was achieved. Furthermore, the actuators proved to be highly repeatable: in each of the positioning tests, the paths were traced either 30 or 10 times, and, as can be seen in the figures, almost exactly the same path was taken each repetition. Therefore, the actuators show potential for use in a surgical context, and, in addition, they are soft and compliant. To further show the viability of using the actuators in a surgical robot, a deployable robot was manufactured and tested. The robot showed excellent responsiveness, and tracing the 25 mm diameter circle in the test was an easy task, even without a camera system focused on the target to aid the user, or any other guidance. Pulleys were used to increase the stroke of the actuators and reduce their size, which has benefits in MIS where space is limited. The large workspace of the robot is adequate for ESD, where tracing paths around cancer lesions more than 20 mm across is an important step. Success in this test serves as an important proof-of-concept for robotic systems that use inflatable, deployable structures and actuators. Further research is needed to increase the accuracy of the actuators to a level adequate for MIS. Both the structures and actuators were manufactured by laser welding thermoplastic sheets together in specific patterns. The result of this is that, when not inflated or filled with actuation fluid, the constituent parts are very flexible and low-profile. In addition, these parts are very economical to produce and could be made into single-use, customised robots. The design of the robot could be improved, as bending in the structure was observed when the parallel mechanism neared the boundary. In fact, prototyping of deployable robots of different types would be valuable: for example, serial or hyper-redundant robots manufactured with the same method. Regarding the actuators, the positioning results could also be improved by improving the calibration method or increasing the volume resolution of the pump, especially at higher contractions when sensitivity to volume change is high. The operating range could also be widened, as the limits were chosen heuristically in this work. The hysteresis results were all under 1 mm even after adding a pulley and were lowest for the antagonistic setup. The antagonistic actuators were tested in a horizontal configuration, so there was no restoring force in the opposite direction to the contraction of either actuator. This may explain why there is a flat hysteresis profile throughout the stroke in comparison to the vertical configuration where the hysteresis varies with the stroke. Given that the positioning was highly consistent, it is expected that the hysteresis could be compensated for successfully. A difference in the method to position the actuator anchor points may explain the difference in offset between the systems with and without pulleys. The approach used for the pulley mechanism appears to be superior. The slightly triangular paths traced may have been caused by friction in the rotary shaft coupling, or high tension in actuators as is common in parallel mechanisms as they approach their singular configurations at the boundaries. In ongoing work, the minimum and maximum actuator speeds recorded were approximately 4 mm/s and 26 mm/s at the limits of the 2 mm-17 mm contraction operating range, respectively.