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So, below, we provide different values to setMassCenter()for the 2 bodies, despite the fact that they have same mass center (that is, the same distribution of mass). What does differ between the 2 bodies is the location of their origin. (Note: We've presented one perspective of the situation. An alternative perspective is that we, the modelers, are given a frame, and must place our body in this frame. We can choose where we place the body in this frame, and we do so via the values we give to setMassCenter() and setIinertia().)
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// By choosing the following as the mass center, we choose the origin of // the anteriorBody frame to be at the body's positive-X extent. That is, // the anterior body sits to the -X direction from its origin. anteriorBody->setMassCenter(Vec3(-0.5 * segmentalLength, 0, 0)); // Posterior body sits to the +X direction from its origin. posteriorBody->setMassCenter(Vec3(0.5 * segmentalLength, 0, 0)); |
A.2: Define joints between
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bodies using CustomJoint's
We now connect the bodies, and thus define how they can move with respect to one another. The cat is free to move about in space, and we represent this using a 6-degree-of-freedom joint between the ground (the inertial frame) and the cat's anterior body. We could use a FreeJoint, which is simpler to define, but we instead employ a CustomJoint. In our case we do so because we want to specify the type of Euler angles that describe the orientation of the cat's anterior segment. Although the FreeJoint also uses Euler angles, it does not allow the modeler to choose which type of Euler angles are used.
We choose YZX Euler angles, named yaw, pitch, and roll, respectively. We specify this via a SpatialTransform object, which we'll use to define our CustomJoint. A spatial transform has 6 transform axes. The first 3 are rotations, defined about the axes of our choosing. The remaining 3 are are translations, for which we can also set the axes. We choose the translation axes to be the X, Y, and Z directions of the ground's frame, which are also the default (YZX Euler angles are not the default, though; XYZ Euler angles is the default).
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Body & ground = cat.getGroundBody(); using OpenSim::CustomJoint; using OpenSim::Array; OpenSim::SpatialTransform groundAnteriorST; groundAnteriorST.updTransformAxis(0).setCoordinateNames( Array<std::string>("yaw", 1)); groundAnteriorST.updTransformAxis(0).setAxis(Vec3(0, 1, 0)); groundAnteriorST.updTransformAxis(1).setCoordinateNames( Array<std::string>("pitch", 1)); groundAnteriorST.updTransformAxis(1).setAxis(Vec3(0, 0, 1)); groundAnteriorST.updTransformAxis(2).setCoordinateNames( Array<std::string>("roll", 1)); groundAnteriorST.updTransformAxis(2).setAxis(Vec3(1, 0, 0)); groundAnteriorST.updTransformAxis(3).setCoordinateNames( Array<std::string>("tx", 1)); groundAnteriorST.updTransformAxis(3).setAxis(Vec3(1, 0, 0)); groundAnteriorST.updTransformAxis(4).setCoordinateNames( Array<std::string>("ty", 1)); groundAnteriorST.updTransformAxis(4).setAxis(Vec3(0, 1, 0)); groundAnteriorST.updTransformAxis(5).setCoordinateNames( Array<std::string>("tz", 1)); groundAnteriorST.updTransformAxis(5).setAxis(Vec3(0, 0, 1)); Vec3 locGAInGround(0); Vec3 orientGAInGround(0); Vec3 locGAInAnterior(0); Vec3 orientGAInAnterior(0); CustomJoint * groundAnterior = new CustomJoint("ground_anterior", ground, locGAInGround, orientGAInGround, *anteriorBody, locGAInAnterior, orientGAInAnterior, groundAnteriorST); // Edit the Coordinate's created by the CustomJoint. The 6 coordinates // correspond to the TransformAxis's we set above. using OpenSim::CoordinateSet; using SimTK::convertDegreesToRadians; using SimTK::Pi; CoordinateSet & groundAnteriorCS = groundAnterior->upd_CoordinateSet(); // yaw // As is, the range only affects how one can vary this coordinate in the // GUI. The range is not a joint limit, and does not affect dynamics. double groundAnteriorCS0range[2] = {-Pi, Pi}; groundAnteriorCS[0].setRange(groundAnteriorCS0range); groundAnteriorCS[0].setDefaultValue(0); groundAnteriorCS[0].setDefaultLocked(false); // pitch double groundAnteriorCS1range[2] = {-Pi, Pi}; groundAnteriorCS[1].setRange(groundAnteriorCS1range); groundAnteriorCS[1].setDefaultValue(convertDegreesToRadians(-15)); groundAnteriorCS[1].setDefaultLocked(false); // roll double groundAnteriorCS2range[2] = {-Pi, Pi}; groundAnteriorCS[2].setRange(groundAnteriorCS2range); groundAnteriorCS[2].setDefaultValue(0); groundAnteriorCS[2].setDefaultLocked(false); // tx double groundAnteriorCS3range[2] = {-1, 1}; groundAnteriorCS[3].setRange(groundAnteriorCS3range); groundAnteriorCS[3].setDefaultValue(0); groundAnteriorCS[3].setDefaultLocked(false); // ty double groundAnteriorCS4range[2] = {-1, 5}; groundAnteriorCS[4].setRange(groundAnteriorCS4range); groundAnteriorCS[4].setDefaultValue(0); groundAnteriorCS[4].setDefaultLocked(false); // tz double groundAnteriorCS5range[2] = {-1, 1}; groundAnteriorCS[5].setRange(groundAnteriorCS5range); groundAnteriorCS[5].setDefaultValue(0); groundAnteriorCS[5].setDefaultLocked(false); |
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