Sensors & transducers

technologies such as in- depth finite element analysis (FEA), 3D printing and rapid prototyping. The team should also include electrical engineers capable of designing instrumentation solutions to meet strict signal conditioning requirements. This multidisciplinary approach results in a sensor element that meets the specifications in terms of nonlinearity, hysteresis, repeatability and cross-talk, as well as an electronic instrument that delivers analog and digital output, high sampling rate and bandwidth, high noise-free resolution and low power consumption, both equally necessary for a reliable turnkey haptics measurement solution. When it comes to the production floor,

material expansion, gage factor coefficient variation and other undesirable effects on the measurement result. For this reason, temperature compensation is paramount to ensure accuracy and long-term stability even when exposed to severe ambient temperature oscillations. The measures to counteract temperature effects on the readings are (a) the use of high-quality, custom and self-compensated strain gauges compatible with the thermal expansion coefficient of the sensing element material; (b) use of half or full Wheatstone bridge circuit configuration installed in both load directions (tension and compression) to correct for temperature drift and (c) fully internally temperature compensation of zero balance and output range without the necessity of external conditioning circuitry. In some special cases, the use of custom strain

gauges with reduced solder connections helps reduce temperature impacts from solder joints. Usually, a regular force sensor with four individual strain gages has upwards of 16 solder joints, while custom strain elements can reduce this down to less than six. This design consideration improves reliability as the solder joint, as an

Figure 2: Potential benefits for the healthcare stakeholders of a MIRS powered by haptics feedback

opportunity for failure, is significantly reduced. During the design phase it is also imperative to

consider such sensors to meet high reliability along with high-volume manufacturability, taking into consideration the equipment and processes that will be required should a device be designated for high-volume manufacturing. After all, the automated, high-volume processes could be slightly or significantly different than the benchtop or prototype equipment used for producing lower volumes. The scalability must maintain focus on reducing failure points during the manufacturing process, along with failure points that could occur on the field. Testing for medical applications is more related

to the ability of a measurement device that can withstand a high number of cycles rather than resist to strenuous structural stress. In particular for medical sensors, the overload and fatigue testing must be performed in conjunction with the sterilisation testing in an intercalated process with several cycles of fatigue and sterilisation testing. The ability to survive hundreds of overload cycles while maintaining hermeticity translates into a failure-free, high-reliability sensor with lower MTBF and more competitive total cost of ownership.


Although understanding the inherent design challenges of the haptic autoclavable sensor is imperative, the sensor solution provider has to be equipped with a talented multidisciplinary engineering team, in-house manufacturing capabilities supported by fully developed quality processes and product/project management proficiency to handle the complex, resource-limited, and fast-paced new product development environment. A multi-discipline engineering team identifies

and translates customer needs into a cross- functional design as well as deploys novel approaches to old problems whose solutions are not easily found by a single, one-sided technical view. From this perspective, a design team must be compounded by mechanical engineers and able to navigate through the latest machine design


a comprehensive in-house manufacturing capability enables the sensor design company to control all aspects of the production process to better meet the demanding schedule and quality requirements. Moreover, strategic control of all manufacturing processes (machining, lamination, wiring, calibration), allows the design company to engineer sensors with a Design for Manufacturability (DFM) mentality. This strategic control of manufacturing boils down to methodically selecting the bill of material, defining the testing plans, complying with standards and protocols and ultimately strategising the manufacturing phase based on economic constraints. Quality is expensive, but lack of quality costs

even more. In medical applications where failure is not an option and tolerances are tight, the project stakeholders cannot afford to have quality as a meager department of testers whose job is to find defects before the customer does, but rather a rigorous and relentless commitment to a culture of excellence which permeates throughout all company processes, from design to shipping. Needless to say that the sensor solution provider must demonstrate its quality system is effectively implemented and maintained thru ISO 13485 certification as well ISO 17025 accredited testing and calibration laboratories. Time to market and cost control is paramount

when developing a medical device, and it also holds true for a haptic feedback sensor, considering the latter as part of the complex surgical robot supply chain. With that in mind, the company product managers must be fluent in agile/waterfall project management practices to integrate and streamline project activities, track controllable costs and, most importantly, ensure that information flows among multiple stakeholders promptly.


Surgical robotic companies are pursuing a marketable product that incorporates haptic feedback whereas few specialised sensor technology companies combine the competences to offer a measurement solution that meets all design requirements imposed by intra-abdominal force measurement in MIRS: miniaturisation, biocompatibility, autoclavability, high reusability and measurement accuracy.

FUTEK April 2019 Instrumentation Monthly

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