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/* -------------------------------------------------------------------------- *
* Simbody(tm) Example: Sample and Hold *
* -------------------------------------------------------------------------- *
* This is part of the SimTK biosimulation toolkit originating from *
* Simbios, the NIH National Center for Physics-Based Simulation of *
* Biological Structures at Stanford, funded under the NIH Roadmap for *
* Medical Research, grant U54 GM072970. See https://simtk.org/home/simbody. *
* *
* Portions copyright (c) 2011-12 Stanford University and the Authors. *
* Authors: Michael Sherman *
* Contributors: *
* *
* Licensed under the Apache License, Version 2.0 (the "License"); you may *
* not use this file except in compliance with the License. You may obtain a *
* copy of the License at http://www.apache.org/licenses/LICENSE-2.0. *
* *
* Unless required by applicable law or agreed to in writing, software *
* distributed under the License is distributed on an "AS IS" BASIS, *
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. *
* See the License for the specific language governing permissions and *
* limitations under the License. *
* -------------------------------------------------------------------------- */
/* This example demonstrates one way to implement sample and hold behavior
in Simbody. The outline is:
1 define a discrete variable to hold the sampled signal
2 define a periodic event handler that reads the signal and updates
the variable
3 define a system that has some dependence on the variable
Here the system is just a simple pendulum swinging in gravity. We'll
sample the position of its end point periodically, and stretch a rubber
band line from the sampled location to the current location.
*/
#include "Simbody.h"
using namespace SimTK;
// This is a periodic event handler that interrupts the simulation on a regular
// basis to record the ground-frame location of a given station on a given
// mobilized body and record the result in a given variable.
class StationSampler : public PeriodicEventHandler {
public:
StationSampler(const MultibodySystem& system,
const MobilizedBody& mobod,
const Vec3& station,
const Measure_<Vec3>::Variable& sample,
Real interval)
: PeriodicEventHandler(interval), m_system(system),
m_mobod(mobod), m_station(station),
m_sample(sample) {}
// This method is called whenever this event occurs.
virtual void handleEvent
(State& state, Real, bool&) const override
{
m_system.realize(state, Stage::Position);
Vec3 location = m_mobod.findStationLocationInGround(state,m_station);
m_sample.setValue(state, location);
}
private:
const MultibodySystem& m_system;
MobilizedBody m_mobod;
Vec3 m_station;
Measure_<Vec3>::Variable m_sample;
};
// The only effect our example sample is going to have on this system is
// to change what's drawn. For that purpose we're defining a
// DecorationGenerator here that we can register with the Visualizer which then
// calls it each time it generates a frame to allow dynamically-generated
// geometry. Here we'll add a mark at the last-sampled location, and draw a
// line from there to the current location.
class RubberBand : public DecorationGenerator {
public:
RubberBand(const MultibodySystem& system,
const MobilizedBody& mobod,
const Vec3& station,
const Measure_<Vec3>::Variable& sample)
: m_system(system), m_mobod(mobod), m_station(station),
m_sample(sample) {}
virtual void generateDecorations(const State& state,
Array_<DecorativeGeometry>& geometry) override
{
m_system.realize(state, Stage::Position);
Vec3 current = m_mobod.findStationLocationInGround(state,m_station);
Vec3 previous = m_sample.getValue(state);
// Draw a small black circle at the sampled location.
geometry.push_back(DecorativeCircle(0.05)
.setColor(Black)
.setTransform(previous));
// Draw an orange line from the sampled to the current location.
geometry.push_back(DecorativeLine(previous,current)
.setColor(Orange)
.setLineThickness(3));
}
private:
const MultibodySystem& m_system;
MobilizedBody m_mobod;
Vec3 m_station;
Measure_<Vec3>::Variable m_sample;
};
int main() {
try {
// Create the system, with subsystems for the bodies and some forces.
MultibodySystem system; system.setUseUniformBackground(true);
SimbodyMatterSubsystem matter(system);
GeneralForceSubsystem forces(system);
Measure_<Vec3>::Variable prevPos(matter, Stage::Position, Vec3(0));
// Add gravity as a force element.
Force::UniformGravity gravity(forces, matter, Vec3(0, -9.8, 0));
// Create the body and some artwork for it.
Body::Rigid pendulumBody(MassProperties(1.0, Vec3(0), Inertia(1)));
pendulumBody.addDecoration(Transform(), DecorativeSphere(0.1).setColor(Red));
// Add an instance of the body to the multibody system by connecting
// it to Ground via a pin mobilizer.
MobilizedBody::Pin pendulum(matter.updGround(), Transform(Vec3(0)),
pendulumBody, Transform(Vec3(0, 1, 0)));
// Once a second we'll record the pendulum's ground location.
const Real Interval = 1;
system.addEventHandler(new StationSampler(system,
pendulum, Vec3(0),
prevPos, Interval));
// Visualize with default options; ask for a report every 1/30 of a second
// to match the Visualizer's default 30 frames per second rate.
Visualizer viz(system);
viz.addDecorationGenerator(new RubberBand(system,
pendulum, Vec3(0),
prevPos));
system.addEventReporter(new Visualizer::Reporter(viz, 1./30));
// Initialize the system and state.
system.realizeTopology();
State state = system.getDefaultState();
pendulum.setOneU(state, 0, 2.0); // initial velocity 2 rad/sec
// Simulate it.
RungeKuttaMersonIntegrator integ(system);
TimeStepper ts(system, integ);
ts.initialize(state);
ts.stepTo(100.0);
} catch (const std::exception& e) {
std::printf("EXCEPTION THROWN: %s\n", e.what());
exit(1);
} catch (...) {
std::printf("UNKNOWN EXCEPTION THROWN\n");
exit(1);
}
return 0;
}
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