Development of a new “6-axis” force connected sensor.
Authors: LEFORT Romain , DECATOIRE Arnaud , ABDI Malek , LACOUTURE Patrick , BUISSON Raymond 
Published: September 2019

This new sensor project has been initiated mainly in order to take measurements in the field of biomechanics during motions of human bodies. For that, it’s necessary to detect the efforts at the contacts with these human bodies in real situation, such as during working, walking, running, biking and so on.
Up to now, most of 6 components force sensors which are used, for instance are sensors with each component measuring device as perfectly as possible decoupled from each other’s. This leads to expansive or very expansive sophisticated sensors.
The present sensor is a stand-alone wireless, small sized 6-axis force sensor with a powerful and precise conditioning and acquisition system. The sensitive cell is a raw Stewart mechanical structure (strain-gages based) with, conversely to usual multicomponent sensors, force and moment components not decoupled at all, but optimally coupled.
Owing to the powerful numerical capabilities of the sensor, the 6 effective components of a given mechanical action are instantaneously computed. Thanks to that, even for small quantity production, the sensor cost price is significantly reduced. This reduction is bigger for larger quantity productions like for: robotics, machine tools, hoisting machines…
Added to the sensor design, the project include also a theoretical mechanical research in order to find an accurate calibration method, as easy as possible to be performed. This results in calibration tests needing only a standard traction-compression test machine running with mechanical effects decoupling tools dimensioned so that the calibration relative uncertainty is kept below 1‰. With that, only 6 elementary loading tests have to be applied to the sensor. The whole sequence of calibration is done automatically, completely governed by a powerful calculation and acquisition software.
All the raw tests results (strain in µm/m) are automatically collected, converted and analyzed. At the end of the numerical treatment of each set of measurements, all the calibration data attesting the traceability to the International System of units (SI) of the sensor, including :

raw calibration results,
sensitivities coefficients matrix needed for later data reduction and conversion in solicitation components (force and moment),
sensor performances characteristic curves (non-linearity, hysteresis error curve, zero shift error, etc.),
calibrations uncertainties,

are stored in the computer memory.
The calibration matrix is then uploaded on the sensor. So, the measurement results (values of solicitations components) are directly expressed in mechanical units traceable to SI.
This sensor is able to perform high data rate wireless streaming with time-synchronization protocol or low data rate transmissions compatible with IOT connectivity.
The following paper describes and comments most important engineering job sequences and calibration results.
It’s also an example of future connected sensors structures able to gather, not only the staff needed to give accurate high levels measurement results, but also all the key pieces of information’s relative to the measurement traceability proof and quality management, all of them being instantaneously available on the net (IOT).
This research and development job got the funding of FEDER-FSE-2014-2020 Nouvelle-Aquitaine program and of CRITT-Sport et Loisirs.