Tion of IMUs’ positions relative for the decrease limb joints. In accordance with the international society of biomechanics (ISB) common  plus the joints coordinate method defined in human anatomy , 3D rotation for the decrease limbs joints could be defined as: (1) hip: flexion/extension (HFE ), abduction/adduction (HAA ), internal/external rotation (HIE ), (two) knee: flexion/ extension (KFE ), abduction/adduction (KAA ), internal/external rotation (KIE ), (3) ankle: flexion/extension (AFE ), abduction/adduction (AAA ), internal/external rotation (AIE ). For the IMU YB-0158 Description position estimation, human reduced limbs may be simplified as rigid segments connected by joints. PACOCF3 manufacturer Figure 1 presents a model of human left reduce limbs; as the human physique is symmetrical, it could also be applied for the proper reduced limbs. The 4 IMUs are denoted as S, S A, B, C, D getting mounted on the waist, upper leg, reduced leg, and foot. The hip, knee, and ankle joints are denoted as Ji , Ji JH , JK , JA . r JK could be the rotation axis of your knee joint. Og will be the global coordinate technique, which represents the coordinate technique in the 3D space object. Os , S A, B, C, D is definitely the sensor coordinate system, which requires IMUs’ center because the coordinate origin.OgA VJH, A JH VJH, B B VJK, B JK VJK, C C VJA, C VJA, D JA DOsrJKFigure 1. A model of a human left reduce limb connected by joints.The human reduce limb joints could be classified into a spherical joint and hinge joint, where the hip and ankle are spherical joints and also the knee joint may be the hinge joint. At time step t (t = 1 . . . n), the accelerations measured by the accelerometers are denoted as aS (t). The angular velocities measured by the gyroscope are denoted as wS (t), S A, B, C, D. (1) The spherical joint The spherical joint is analyzed around the hip joint as an example and is applicable for the ankle joint. Assuming A and B are connected via the joint JH , the position of theSensors 2021, 21,four oftwo IMUs relative towards the joint could be determined from the sequence of measurements of A and B. Let V JH , S = [ x JH , S , y JH , S , z JH , S ] T , S A, B denote the vector pointing from the joints center for the origin of your two IMUs coordinate systems in Figure 1. By means of the IMU position estimation algorithm proposed in , the spherical joint model is defined by Equation (1). a A (t) – A (t) – a B (t) – B (t) = 0, S (t) = wS (t) (wS (t) V JH , S ) + S (t) V JH , S , S A, B, (1) (two)exactly where || | may be the norm for vectors, can be a cross solution. The angular acceleration calculated by the angular velocity is denoted as S , that is defined by . The IMU position displacement will result in the errors of Equation (1), and also the equation isn’t equal around the left and ideal. The errors are defined by Equation (3). e JH (t) = a A (t) – A (t) – a B (t) – B (t) (three)(2) The hinge joint The knee joint is often a hinge joint, and the model in the knee joint rotation axis estimation proposed in  is defined by Equation (four). w B (t) r JK , B – wC (t) r JK , C = 0, (4)where r JK , B , r JK , C are the coordinates with the unit vector parallel towards the knee joint axis in the OB and OC . Figure two show the coordinates of r JK , B and r JK , C in spherical coordinates, and converts it for the rectangular coordinate system in Equation (five) r JK , S = [cos( S ) cos(S ), cos( S ) sin(S ), sin(S )], S B, C , (five)exactly where [0, ] is definitely the pitch angle, [0, 2 ] will be the yaw angle. The IMU position displacement will lead to Equation (4) to become unequal on the left and r.