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.


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

Continued on page 62... 61

Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80