Four-bar linkage example (demo_MBS_fourbar.cpp)

This tutorial is similar to the demo_crank example, but here a small user-interface is created, so that the user can play with a slider to modify the speed of the motor.

The user interface is created with simple GUI features of Irrlicht, but other kind of interfaces/visualization systems could be used.

Here you can also learn:

  • how to make a four-bar linkage
  • an example of how to plot/fetch data during the simulation.
  • use a direct solver instead of iterative solver, for highest precision.
// =============================================================================
// Copyright (c) 2014
// All rights reserved.
// Use of this source code is governed by a BSD-style license that can be found
// in the LICENSE file at the top level of the distribution and at
// =============================================================================
// Authors: Alessandro Tasora
// =============================================================================
// Demo code about
// - using IRRLICHT as a realtime 3D viewer of a four-bar mechanism
// - using the IRRLICHT graphical user interface (GUI) to handle user-input
// via mouse.
// =============================================================================
#include "chrono/physics/ChLinkMotorRotationSpeed.h"
#include "chrono/physics/ChSystemNSC.h"
#include "chrono_irrlicht/ChVisualSystemIrrlicht.h"
// Use the namespaces of Chrono
using namespace chrono;
using namespace chrono::irrlicht;
// Use the main namespaces of Irrlicht
using namespace irr;
using namespace irr::core;
using namespace irr::scene;
using namespace irr::video;
using namespace irr::io;
using namespace irr::gui;
// Some static data (this is a simple application, we can do this ;) just to allow easy GUI manipulation
IGUIStaticText* text_motorspeed = 0;
// The MyEventReceiver class will be used to manage input
// from the GUI graphical user interface (the interface will
// be created with the basic -yet flexible- platform
// independent toolset of Irrlicht).
class MyEventReceiver : public IEventReceiver {
MyEventReceiver(ChSystemNSC* system, IrrlichtDevice* device, std::shared_ptr<ChLinkMotorRotationSpeed> motor) {
// store pointer to physical system & other stuff so we can tweak them by user keyboard
msystem = system;
mdevice = device;
mmotor = motor;
bool OnEvent(const SEvent& event) {
// check if user moved the sliders with mouse..
if (event.EventType == EET_GUI_EVENT) {
s32 id = event.GUIEvent.Caller->getID();
switch (event.GUIEvent.EventType) {
if (id == 101) // id of 'motor speed' slider..
s32 pos = ((IGUIScrollBar*)event.GUIEvent.Caller)->getPos();
double newspeed = 10 * (double)pos / 100.0;
// set the speed into motor object
if (auto mfun = std::dynamic_pointer_cast<ChFunction_Const>(mmotor->GetSpeedFunction()))
// show speed as formatted text in interface screen
char message[50];
sprintf(message, "Motor speed: %g [rad/s]", newspeed);
return false;
ChSystemNSC* msystem;
IrrlichtDevice* mdevice;
std::shared_ptr<ChLinkMotorRotationSpeed> mmotor;
int main(int argc, char* argv[]) {
GetLog() << "Copyright (c) 2017\nChrono version: " << CHRONO_VERSION << "\n\n";
// 1- Create a Chrono physical system
// 2- Create the rigid bodies of the four-bar mechanical system
// (a flywheel, a rod, a rocker, a truss), maybe setting
// position/mass/inertias of their center of mass (COG) etc.
// ..the truss
auto my_body_A = chrono_types::make_shared<ChBody>();
my_body_A->SetBodyFixed(true); // truss does not move!
// ..the flywheel
auto my_body_B = chrono_types::make_shared<ChBody>();
my_body_B->SetPos(ChVector<>(0, 0, 0)); // position of COG of flywheel
// ..the rod
auto my_body_C = chrono_types::make_shared<ChBody>();
my_body_C->SetPos(ChVector<>(4, 0, 0)); // position of COG of rod
// ..the rocker
auto my_body_D = chrono_types::make_shared<ChBody>();
my_body_D->SetPos(ChVector<>(8, -4, 0)); // position of COG of rod
// 3- Create constraints: the mechanical joints between the
// rigid bodies. Doesn't matter if some constraints are redundant.
// .. a motor between flywheel and truss
auto my_link_AB = chrono_types::make_shared<ChLinkMotorRotationSpeed>();
my_link_AB->Initialize(my_body_A, my_body_B, ChFrame<>(ChVector<>(0, 0, 0)));
auto my_speed_function = chrono_types::make_shared<ChFunction_Const>(CH_C_PI); // speed w=3.145 rad/sec
// .. a revolute joint between flywheel and rod
auto my_link_BC = chrono_types::make_shared<ChLinkLockRevolute>();
my_link_BC->Initialize(my_body_B, my_body_C, ChCoordsys<>(ChVector<>(2, 0, 0)));
// .. a revolute joint between rod and rocker
auto my_link_CD = chrono_types::make_shared<ChLinkLockRevolute>();
my_link_CD->Initialize(my_body_C, my_body_D, ChCoordsys<>(ChVector<>(8, 0, 0)));
// .. a revolute joint between rocker and truss
auto my_link_DA = chrono_types::make_shared<ChLinkLockRevolute>();
my_link_DA->Initialize(my_body_D, my_body_A, ChCoordsys<>(ChVector<>(8, -8, 0)));
// Create the Irrlicht visualization system
auto vis = chrono_types::make_shared<ChVisualSystemIrrlicht>();
vis->SetWindowSize(800, 600);
vis->SetWindowTitle("Four-bar mechanism");
vis->AddCamera(ChVector<>(0, 0, -8));
// ..add a GUI text and GUI slider to control motor of mechanism via mouse
text_motorspeed = vis->GetGUIEnvironment()->addStaticText(L"Motor speed:", rect<s32>(300, 85, 400, 100), false);
IGUIScrollBar* scrollbar = vis->GetGUIEnvironment()->addScrollBar(true, rect<s32>(300, 105, 450, 120), 0, 101);
// ..Finally create the custom event receiver
MyEventReceiver receiver(&sys, vis->GetDevice(), my_link_AB);
// Configure the solver with non-default settings
// By default, the solver uses the EULER_IMPLICIT_LINEARIZED stepper, that is very fast,
// but may allow some geometric error in constraints (because it is based on constraint
// stabilization). Alternatively, the timestepper EULER_IMPLICIT_PROJECTED is slower,
// but it is based on constraint projection, so gaps in constraints are less noticeable
// (hence avoids the 'spongy' behaviour of the default stepper, which operates only
// on speed-impulse level and keeps constraints'closed' by a continuous stabilization).
// Simulation loop
// use this array of points to store trajectory of a rod-point
std::vector<chrono::ChVector<> > mtrajectory;
while (vis->Run()) {
// .. draw a grid
tools::drawGrid(vis.get(), 0.5, 0.5);
// .. draw a circle representing flywheel
tools::drawCircle(vis.get(), 2.1, ChCoordsys<>(ChVector<>(0, 0, 0), QUNIT));
// .. draw a small circle representing joint BC
tools::drawCircle(vis.get(), 0.06, ChCoordsys<>(my_link_BC->GetMarker1()->GetAbsCoord().pos, QUNIT));
// .. draw a small circle representing joint CD
tools::drawCircle(vis.get(), 0.06, ChCoordsys<>(my_link_CD->GetMarker1()->GetAbsCoord().pos, QUNIT));
// .. draw a small circle representing joint DA
tools::drawCircle(vis.get(), 0.06, ChCoordsys<>(my_link_DA->GetMarker1()->GetAbsCoord().pos, QUNIT));
// .. draw the rod (from joint BC to joint CD)
tools::drawSegment(vis.get(), my_link_BC->GetMarker1()->GetAbsCoord().pos,
my_link_CD->GetMarker1()->GetAbsCoord().pos, ChColor(0, 1, 0));
// .. draw the rocker (from joint CD to joint DA)
tools::drawSegment(vis.get(), my_link_CD->GetMarker1()->GetAbsCoord().pos,
my_link_DA->GetMarker1()->GetAbsCoord().pos, ChColor(1, 0, 0));
// .. draw the trajectory of the rod-point
tools::drawPolyline(vis.get(), mtrajectory, ChColor(0, 0.5f, 0));
// We need to add another point to the array of 3d points describing the trajectory to be drawn..
mtrajectory.push_back(my_body_C->Point_Body2World(ChVector<>(1, 1, 0)));
// keep only last 150 points..
if (mtrajectory.size() > 150)
// Perform time integration
return 0;
void AddTypicalLights()
Simple shortcut to set two point lights in the scene.
Definition: ChVisualSystemIrrlicht.cpp:286
void drawPolyline(ChVisualSystemIrrlicht *vis, std::vector< ChVector<> > &points, chrono::ChColor col, bool use_Zbuffer)
Draw a polyline in 3D space, given the array of points.
Definition: ChIrrTools.cpp:680
void drawSegment(ChVisualSystemIrrlicht *vis, ChVector<> start, ChVector<> end, chrono::ChColor col, bool use_Zbuffer)
Draw line segments in 3D space with given color.
Definition: ChIrrTools.cpp:667
Definition: ChCoordsys.h:38
void AddSkyBox(const std::string &texture_dir=GetChronoDataFile("skybox/"))
Add a sky box in a 3D scene.
Definition: ChVisualSystemIrrlicht.cpp:299
virtual void Initialize()
Initialize the visualization system.
Definition: ChVisualSystemIrrlicht.cpp:160
void drawCircle(ChVisualSystemIrrlicht *vis, double radius, ChCoordsys<> pos, chrono::ChColor col, int resolution, bool use_Zbuffer)
Draw a circle line in 3D space with given color.
Definition: ChIrrTools.cpp:695
ChLog & GetLog()
Global function to get the current ChLog object.
Definition: ChLog.cpp:39
bool Run()
Run the Irrlicht device.
Definition: ChVisualSystemIrrlicht.cpp:217
virtual void AddLink(std::shared_ptr< ChLinkBase > link)
Attach a link to the underlying assembly.
Definition: ChSystem.cpp:164
void AddCamera(const ChVector<> &pos, ChVector<> targ=VNULL)
Add a camera in an Irrlicht 3D scene.
Definition: ChVisualSystemIrrlicht.cpp:268
Representation of a 3D transform.
Definition: ChFrame.h:34
virtual void BeginScene(bool backBuffer=true, bool zBuffer=true, ChColor color=ChColor(0, 0, 0))
Clean the canvas at the beginning of each animation frame.
Definition: ChVisualSystemIrrlicht.cpp:501
Namespace with classes for the Irrlicht module.
Definition: ChApiIrr.h:48
Definition of a visual color.
Definition: ChColor.h:26
const ChQuaternion< double > QUNIT(1., 0., 0., 0.)
Constant unit quaternion: {1, 0, 0, 0} , corresponds to no rotation (diagonal rotation matrix)
Definition: ChQuaternion.h:451
Definition of general purpose 3d vector variables, such as points in 3D.
Definition: ChVector.h:35
void drawGrid(ChVisualSystemIrrlicht *vis, double ustep, double vstep, int nu, int nv, ChCoordsys<> pos, chrono::ChColor col, bool use_Zbuffer)
Draw grids in 3D space with given orientation, color, and spacing.
Definition: ChIrrTools.cpp:794
void AddUserEventReceiver(irr::IEventReceiver *receiver)
Attach a custom event receiver to the application.
Definition: ChVisualSystemIrrlicht.cpp:380
int DoStepDynamics(double step_size)
Advances the dynamical simulation for a single step, of length step_size.
Definition: ChSystem.cpp:1422
virtual void EndScene()
End the scene draw at the end of each animation frame.
Definition: ChVisualSystemIrrlicht.cpp:546
void SetWindowTitle(const std::string &win_title)
Set the windoiw title (default "").
Definition: ChVisualSystemIrrlicht.cpp:124
virtual void AttachSystem(ChSystem *sys) override
Attach another Chrono system to the run-time visualization system.
Definition: ChVisualSystemIrrlicht.cpp:144
virtual void Render()
Draw all 3D shapes and GUI elements at the current frame.
Definition: ChVisualSystemIrrlicht.cpp:556
Main namespace for the Chrono package.
Definition: ChBarrelShape.cpp:17
void SetTimestepperType(ChTimestepper::Type type)
Set the method for time integration (time stepper type).
Definition: ChSystem.cpp:455
void AddLogo(const std::string &logo_filename=GetChronoDataFile("logo_chronoengine_alpha.png"))
Add a logo in a 3D scene.
Definition: ChVisualSystemIrrlicht.cpp:260
virtual void AddBody(std::shared_ptr< ChBody > body)
Attach a body to the underlying assembly.
Definition: ChSystem.cpp:154
Class for a physical system in which contact is modeled using a non-smooth (complementarity-based) me...
Definition: ChSystemNSC.h:29
void SetWindowSize(unsigned int width, unsigned int height)
Set the window size (default 640x480).
Definition: ChVisualSystemIrrlicht.cpp:120