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Sensors & transducers


Figure 1: With a force, torque and pressure sensor enabling haptic feedback to the hands of the surgeon, robotics minimally invasive surgery can be performed with higher dexterity while minimising trauma to the patient


instrument and demands further consideration during the design stage in order to enhance sensor reusability. Appropriate electronic components, strain measurement method and electrical connections have to withstand additional autoclavable cycles as well as survive a high PH washing. Coping with these special design requirements invariably increases the unitary cost per sensor. However, extended lifespan and number of cycles consequently reduces the cost per cycle and brings financial affordability to direct measurement method. Hermeticity of high precision sub-miniature


load sensing elements is equally challenging to intra- abdominal direct force measurement. The conventional approach to sealing electronic components is the adoption of conformal coatings, which are extensively utilised in submersible devices. As much as this solution provides protection in low-pressure water submersion environments for consumer electronics, coating protection is not sufficiently airtight and is not suitable for high-reliability medical, reusable and sterilisable solutions. Under extreme process controls conformal coatings have shown to be marginal and provide upwards of 20 to 30 autoclave cycles. The autoclave sterilisation process presents a harsher physicochemical environment utilising high pressure and high temperature saturated steam. Similar to helium leak detection technology,


Instrumentation Monthly April 2019


saturated steam particles are much smaller in size compared to water particles and are capable of penetrating and degrading the coating over time causing the device to fail in a hardly predictable manner. An alternative and conventional approach to achieving hermeticity is to weld on a header interface to the sensor. Again, welding faces obstacles in miniaturised sensors due to its size constraints. All in all, a novel and robust approach is a monolithic sensor using custom formulated, Ct matched, chemically neutral, high temperature fused isolator technology used to feed electrical conductors through the walls of the hermetically sealed active sensing element. The fused isolator technology has shown reliability in the hundreds to thousands of autoclave cycles.


OTHER DESIGN CONSIDERATIONS


As aforementioned, miniaturisation, biocompatibility, autoclavability and high reusability are some of the unique characteristics imposed to a haptic sensor by the surgical environment. In addition to that, it is imperative that designers also meet requirements that are inherent to any high- performance force measurement device. Extraneous loads (or crosstalk) compensation,


provides optimal resistance to off-axis loads to assure maximum operating life and minimise reading errors. Force and torque sensors are engineered to capture forces along the Cartesian axes, typically X, Y and Z. From these three


orthogonal axes, one to six measurement channels derives three force channels (Fx, Fy and Fz) and three torque or moment channels (Mx, My and Mz). Theoretically, a load applied along one of the axes should not produce a measurement in any of the other channels, but this is not always the case. For a majority of force sensors, this undesired cross-channel interference will be between one and five per cent and, considering that one channel can capture extraneous loads from five other channels, the total crosstalk could be as high as five to 25 per cent. In robotic surgery, the sensor must be designed


to negate the extraneous or cross-talk loads which include frictions between the end-effector instrument and trocar, reactionary forces from the abdominal wall and gravitational effect of mass along the instrument axis. In some occasions, miniaturised sensors are very limited in space and have to compensate side loads using alternate methods such as electronic or algorithmic compensation. Calibration of direct inline force sensor


imposes restrictions as well. The calibration fixtures are optimised with SR buttons to direct load precisely through the sensor of the part. If the calibration assembly is not equipped with such arrangements, the final calibration might be affected by parallel load paths. Thermal effect is also a major challenge in strain measurement. Temperature variations cause


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