# Open Loop Position Control of Soft Hydraulic Actuators for Minimally Invasive Surgery
## 5 Discussion
The speed was measured in response to a step input in desired contraction from 0 mm to 17 mm. The force exertion results compare well with [19; 20; 25], and their actuators were of a larger size, which increases force exertion. A single sub chamber in our design had dimensions \(L_{0}\) and \(D\) of 22.67 mm and 30 mm, whereas these were 25 mm and 85 mm in [19], 40 mm and 160 mm in [25], and 20 mm and 40 mm in [26]. Due to the surface tension of the hydraulic actuation fluid and the associated pressure rise due to [PERSON]'s law, the measured forces exerted by the actuator do not follow the same relationship as the pneumatic actuators, and the Laplace pressure must be compensated for. A liquid with low boiling point was used to fill an actuator in [29] that was later sealed such that, when heated, the liquid evaporates and contracts the pouch with no need for inlet tubes. The slow actuation speed and controllability make this less effective but would be interesting for untethered or wireless actuation. In [30], wireless actuation was achieved by heating the low boiling point liquid with a laser, which evaporates the liquid and induces contraction. Future work will include finding an expression for the force exertion in terms of the membrane curvature \(\theta\) and the internal pressure. This could be used to sense the forces exerted on the actuators based on the hydrostatic pressure. The hydraulic supply tubes used were 0.5 m in length, but for endoscopic applications, the supply tubes would measure approximately 2 m, which would increase the resistance to flow in dynamic situations. [PERSON]'s law could be used to compensate for transient pressure changes due to flow, but this would represent a more complex problem. However, in static situations the pressure could be used to estimate forces as in [31], for example. Sensorising the actuators using approaches from electrical impedance tomography (EIT) as in [32] to achieve closed-loop position control is another area of investigation. Adding this type of sensor would also facilitate detection of external disturbances. A simpler technique is to use a conductive working fluid and monitor changes in the resistance as in [33], which is also a viable option for these hydraulic actuators. However, one of the advantages of this type of actuator is the low-profile, low-cost construction; therefore, a simple open loop control strategy was implemented that required neither bulky sensors nor intensive computation or modelling. Conceptualisation and Investigation M.R., J.A. and G.M; Methodology and Analysis M.R. and J.A. ;Writing M.R. ; Review and Editing M.R., J.A. and G.M. ; Supervision A.D. and G.M. All authors have read and agreed to the published version of the manuscript. This work was funded by the NIHR Imperial BRC. [PERSON] is an Imperial College Research Fellow. The data presented in this study are openly available inmrunciman/482 MDPL_SoftActuatorData: MDPL_Data_Archive at doi: [[https://zenodo.org/record/5175850](https://zenodo.org/record/5175850)]([https://zenodo.org/record/5175850](https://zenodo.org/record/5175850)), 483 reference number 10.5281/zenodo.5175850. The authors declare no conflict of interest.