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02

How does a wearable motion capture system work?

PIVOT sensori turingsense

IMU

​In a motion analysis or capture system, it is easy to explain what the heart of the system consists of: it is the inertial measurement unit (IMU).

PIVOT sensori in mano

PIVOT

IMU represents the sensitive element of the system, in this case it is the component that is able to sense movement in inertial terms. In practice, in a very similar way to how our smartphone is able to understand (within certain limits!) that we are rotating the display to be able to better see our favorite video, similarly IMU, if positioned on the body, takes the measurements primaries necessary to subsequently describe the movement of the underlying body part.

Fascia torace PIVOT

What does a single IMU provide?

Basic quantities

Derived quantities

  • acceleration felt by the IMU

  • rotation speed sensed by the IMU

  • IMU linear speed *

  • IMU position in 3D space *

  • orientation changes of IMU in 3D space *

Thanks to these quantities, there are many possible applications using a single IMU. To give a very practical example, thanks to a single IMU positioned integrally on the upper part of the torso, it would already be possible to measure how many times a person sitting at a desk is able to maintain a more or less correct posture during the day .

Unfortunately, even this simple scenario is not easy to implement for the following reasons:

  • The quantities highlighted above with a * are quantities whose reliability depends not only on the algorithm used for their calculation, but also on the quality of the individual components internal to the IMU. This is where the chasm opens up between high-end IMU systems (to be clear, those present in vehicles such as airplanes) and miniaturized IMU systems using MEMS (Micro-Electro-Mechanical-Systems) technology.

  • Even when using high-end IMUs, some of the measurements indicated above with a * are affected by errors coming from sources of noise intrinsic to the electronic components and, therefore, cannot be eliminated. The accuracy in these cases cannot even be determined a priori or predicted/eliminated, but only compensated within certain terms

PIVOT fascia polso e mano

Why a single sensor is not enough?

in applications, it is important to distinguish between the use of a single unit and the use of a "chain" of nodes arranged on different parts of the body.

Logically, the presence of multiple IMUs allows different parts of the body to be measured simultaneously. Therefore this represents the very first advantage.

However, to understand how a chain of IMUs can help describe the movements of the human body it is necessary to address some basic concepts which for some could be a simple review, but for others a new knowledge.

PIVOT kit ricerca

DID YOU KNOW?

​the first generation of Turingsense wearable system was PIVOT. It is still available, complete with bands for interfacing with the body and many software features to access. For more information, visit our Roadmap

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