Simulation

Simulation functionality in Klamp’t is built on top of the Open Dynamics Engine (ODE) rigid body simulation package, but adds emulators for robot sensors and actuators, and features a robust contact handling mechanism. When designing new robots and scenarios, it is important to understand a few details about how Klamp’t works in order to achieve realistic simulations.

Boundary-layer contact detection.

Other rigid body simulators tend to suffer from significant collision handling artifacts during mesh-mesh collision: objects will jitter rapidly, interpenetrate, or react to “phantom” collisions. The primary cause is that contact points, normals, and penetration depths are estimated incorrectly or inconsistently from step-to-step. Klamp’t uses a new boundary layer contact detection procedure that leads to accurate and consistent estimation of contact regions. Moreover, the boundary layer can simulate some limited compliance in the contact interface, such as soft rubber coatings or soft ground.

In Klamp’t, contact is detected along the boundary layers rather than the underlying mesh. The thickness of the boundary layer is a simulation parameter called padding. Padding for each body can be set in XML world files via the padding attribute in the <simulation>{<robot>,<object>,<terrain>}<geometry> element, or using the SimBody.setCollisionPadding() / SimBody.getCollisionPadding() functions.

All bodies padded with 2.5mm by default. This allows it to handle thin-shell meshes as illustrated in the following figures.

Simulation boundary layer 1
Simulation boundary layer 2
Simulation boundary layer 3
Simulation boundary layer 4

The first step of Klamp’t’s collision handling routine is to compute all contacts between all pairs of geometric primitives within the padding range. This is somewhat slow when fine meshes are in contact. In order to reduce the number of contacts that must be handled by ODE, Klamp’t then performs a clustering step to reduce the number of contacts to a manageable number. The maximum number of contacts between two pairs of bodies is given by the maxContacts global parameter, which can be set as an attribute in the XML <simulation> tag.

For more details, please see: K. Hauser. Robust Contact Generation for Robot Simulation with Unstructured Meshes. In proceedings of International Symposium of Robotics Research, 2013.

Collision response

In addition to padding, each body also has coefficients of restitution, friction, stiffness, and damping. The stiffness and damping coefficients can be set to non-infinite values to simulate softness in the boundary layer. Please note, however, that soft boundary layers are more prone to being fully penetrated through, and may lead the simulation to be less robust, unless the collision padding is quite large.

These can be set for each simulation body in world XML files (kRestitution, kFriction, kStiffness, and kDamping attributes in <simulation>{<robot>,<object>,<terrain>}<geometry> XML elements). They can also be modified after loading a world using the SurfaceParameters object used by SimBody.getSurface() and SimBody.setSurface().

When two bodies come into contact, their coefficients are blended using arithmetic mean for kRestitution, and harmonic means for kFriction, kStiffness, and kDamping. The blending mechanism is convenient because only one set of parameters needs to be set for each body, rather than each pair of bodies, and is a reasonable approximation of most material types. Currently there is no functionality to specify custom properties between pairs of bodies.

Actuator simulation

Klamp’t handles actuators in one of two modes: PID control and torque control modes. It also simulates dry friction (stiction) and viscous friction (velocity-dependent friction) in joints using the dryFriction and viscousFriction parameters in .rob files. Actuator commands are converted to torques (if in PID mode), capped to torque limits, and then applied directly to the links. ODE then handles the friction terms.

In PID mode, the torque applied by the actuator is

\[\tau=k_P(\theta_D - \theta_A)+k_D(\dot{\theta}_D - \dot{\theta_A})+k_I I\]

where

  • \(k_P\), \(k_I\), and \(k_D\) are the PID constants,

  • \(\theta_D\) and \(\dot{\theta}_D\) are the desired position and velocity,

  • \(\theta_A\) and \(\dot{\theta}_A\) are the actual position and velocity,

  • and I is an integral error term.

The friction forces resist the motion of the joint, and Klamp’t uses a simple stick-slip friction model where the sticking mode breaking force is equal to \(\mu_D\) and the sliding mode friction force is

\[-sgn(\dot{\theta}_A)(\mu_D+\mu_V|\dot{\theta}_A|)\]

where \(\mu_V\) is the viscous friction force. Note: passive damping should be handled via the friction terms rather than the PID gain kD.

Like all simulators, Klamp’t does not perfectly simulate all of the physical phenomena affecting real robots. Some common phenomena include:

  • Backlash in the gears.

  • Back EMF.

  • Angle-dependent torques in cable drives.

  • Motor-induced inertial effects, which are significant particularly for highly geared motors. Can be approximated by adding a new motor link connected by an affine driver to its respective link.

  • Velocity-dependent torque limits (e.g. power limits). Can be approximated in a controller by editing the robot’s driver torque limits depending on velocity.

  • Motor overheating. Can be implemented manually by simulating heat production/dissipation as a differential equation dependent on actuator torques.

API summary

To create and manage a simulation:

  • sim = Simulator(world): creates a simulator for a given WorldModel (note: cannot modify the number of entities in the world at this point, undefined behavior will occur if you do!)

  • sim.getWorld(): retrieves the simulation’s WorldModel

  • sim.updateWorld(): updates the WorldModel to reflect the current state of the simulator

  • sim.simulate(dt): advances the simulation by time dt (in seconds)

  • sim.fakeSimulate(dt): fake-simulates. Useful for fast prototyping of controllers

  • sim.getTime(): returns the accumulated simulation time

  • sim.getState(): returns a string encoding the simulation state

  • sim.setState(state): sets the simulation state given the result from a previous getState() call

  • sim.reset(): reverts the simulation back to the initial state

  • sim.setGravity(g): sets the gravity to the 3-tuple g (default (0,0,-9.8))

  • sim.setSimStep(dt): sets the internal simulation time step to dt. If simulate() is called with a larger value dt’, then the simulation will integrate physics forward over several substeps of length at most dt

To modify the properties of simulated rigid bodies: [NOTE: reference frame is centered at center of mass]

  • body = sim.body([RobotLinkModel or RigidObjectModel]): retrieves the simulated body according to a link or rigid object.

  • body.getID(): retrieves integer ID of associated object in world

  • body.enable(enabled=True)/isEnabled(): pass False to disable simulation of the body

  • body.enableDynamics(enabled=True)/isDynamicsEnabled(): pass False to drive a body kinematically along a given path

  • body.getTransform()/setTransform(R,t): gets/sets SE(3) element representing transform of body coordinates w.r.t. world

  • body.getVelocity()/setVelocity(w,v): gets/sets the angular velocity w and translational velocity v of the body coordinates w.r.t. world

  • body.getSurface()/setSurface(SurfaceParameters): gets/sets the body’s surface parameters

  • body.getCollisionPadding()/setCollisionPadding(m): gets/sets the body’s collision margin (nonzero yields more robust collision handling)

  • body.applyForceAtPoint(fw,pw), applyForceAtLocalPoint(fw,pl): adds a world-space force fw to a point, either pw in world coordinates or pl in body coordinates. Applied over duration of next Simulator.simulate() call

  • body.applyWrench(f,t): adds a force f at COM and torque t over the duration of te next Simulator.simulate() call

To inspect the contact status of objects:

  • sim.enableContactFeedbackAll(): turns on contact feedback for all objects

  • sim.enableContactFeedback(id1,id2): turns on contact feedback for contacts between objects with ids id1 and id2

  • sim.inContact/hadContact(id1,id2): returns True if objects id1 and id2 are in contact at the end of the time step / had contact during the prior time step

  • sim.hadPenetration/hadSeparation(id1,id2): returns True if objects id1 and id2 penetrated / were separated at any point during the prior time step

  • sim.getContacts(id1,id2): returns a list of contacts between id1 and id2 on the current time step. Each contact is a 7-list [px,py,pz,nx,ny,nz,kFriction]

  • sim.getContactForces(id1,id2): returns a list of contact forces, one for each of the contacts in sim.getContacts(id1,id2)

  • sim.contactForce/contactTorque(id1,id2): returns the contact force / torque at the end of last time step

  • sim.meanContactForce(id1,id2): returns the mean contact force over the entire last time step

  • from model import contact; contact.simContactMap(sim): returns a map from (id1,id2) pairs to contact.ContactPoint objects.

Example

In this example we’ll work from the template in Klampt-examples/Python3/demos/gl_vis.py. First, copy Klampt/Python3/demos/gl_vis.py to your own folder and rename it, for example, myapp.py.

First, let’s change to using the ATHLETE robot on a fractal terrain. Change the file being read from “../../data/hubo_plane.xml” to “Klampt-examples/data/athlete_fractal_1.xml”. Now, if you run

python myapp.py

and press ‘s’, the simulation will proceed with the ATHLETE robot dropping onto the terrain, but not doing anything in particular.

Sending commands

Next, let’s try sending a leg lift and lower motion to the controller. We’ll replace the first two lines of the idle method with the following code.

sim = self.sim
if sim.getTime() >= 2.0 and sim.getTime()-self.dt < 2.0:
    q=sim.controller(0).getCommandedConfig()
    q[7]-=1.0
    sim.controller(0).setMilestone(q)
    q[7]+=1.5
    sim.controller(0).addMilestone(q)

Now run the simulation and see what happens.

Sending a trajectory to the controller

Here we’ll use a trajectory that’s been saved to disk, using the klampt.trajectory module. Unlike the prior example, which used the controller’s trajectory queue, we’ll send this motion at a high rate to the robot using PID commands. (These override the controller’s trajectory queue.)

First, we’ll load the trajectory into a class variable by putting these lines at the end of the __init__ method:

self.traj = trajectory.RobotTrajectory(self.world.robot(0))
self.traj.load("Klampt-examples/data/motions/athlete_flex.path")

… and then we will put the following code in the idle function to replace the code outlined above:

sim = self.sim
traj = self.traj
starttime = 2.0
if sim.getTime() > starttime:
    (q,dq) = (traj.eval(self.sim.getTime()-starttime),traj.deriv(self.sim.getTime()-starttime))
    sim.controller(0).setPIDCommand(q,dq)

That’s it!

Playing God: applying forces and constraining velocities

The robot controller is not able to apply arbitrary forces to its body or the world. This encapsulation is deliberate, because a robot cannot “play God” – it can only affect its body or the world via its actuators. But it is often useful to generate simulation scenarios by “playing God,” and to do so, you must access the SimBody elements that give you direct access to the rigid bodies in the underlying simulator.

The first step in doing so is to access the SimBody out of the Simulator corresponding to the desired object in the WorldModel. To do so, you would call something like this:

body = sim.body(world.robotlink(my_robot_index,my_link_index));
#or...
body = sim.body(world.rigidObject(my_object_index));

To apply forces, you may use the SimBody.applyForceAtPoint function as follows:

body.applyForceAtPoint([fx,fy,fz],[px,py,pz]);

Where the force (fx,fy,fz) and point (px,py,pz) are in world coordinates. You may also call SimBody.applyWrench to apply a force/torque about the center of mass.

Directly controlling the movement of a body (e.g., to move along a predetermined path, or according to a joystick) is possible but takes a few extra steps, because Klamp’t by default gives control of the body to the simulator. First, you will need to know the translational and angular velocity along which the body should be moving at each time step. Let us assume you have determined these quantities as (vx,vy,vz) and (wx,wy,wz); both are in world coordinates. Then, you will need to disable dynamic simulation, and during your time step you will need to set the velocities directly as follows:

body.enableDynamics(False)
body.setVelocity((wx,wy,wz),(vx,vy,vz))

Note the angular velocity is provided as the first argument.

Extracting contacts and contact forces

It is often useful to examine and record the contact forces generated by the simulation, and Klamp’t provides several functions for doing so.

The first step in extracting contact feedback is to enable it. Contact feedback can be 1) enabled for everything, or 2) enabled on a per body pair basis. The first option is as simple as calling:

sim.enableContactFeedbackAll()

Option 2 can be chosen to save a little overhead in computation and memory. (This overhead is relatively minor, so enableContactFeedbackAll is usually the better choice.) To do this, we will need the IDs of the pairs of objects we want to get feedback from.

Each SimBody in the world, including environment objects and robot links, is given a unique ID, and this ID is used to identify the corresponding body in the simulator. To get the ID of an object in the world you call getID() on it:

terrainid = world.terrain(terrain_index).getID()
objectid = world.rigidObject(object_index).getID()
linkid = world.robot(robot_index).link(link_index).getID()
#equivalent to
linkid = world.robotlink(robot_index,link_index).getID()

IDs are constant throughout the life of the simulation.

Note

IDs will change if you add or remove elements from the world, but adding and removing objects from worlds is not yet supported in simulation.

We can then just do something like this to enable only collision feedback between the terrain and all links on the robot:

for i in range(world.robot(robot_index).numLinks())
  sim.enableContactFeedback(terrainid,world.robotlink(robot_index,i).getID())

IDs are assigned contiguously, and hence it is possible to just loop through integers ranging from 0 to world.numIDs()-1 to enable all contact pairs.

Now, once we have enabled contact feedback, during the simulation loop we can use the following code to see what objects are in contact, and examine the contact forces/torques:

contacted=False
for i in range(world.numIDs()):
  for j in range(i+1,world.numIDs()):
    #you could loop over a selective set of id pairs rather than i and j, if you wanted...
    if sim.inContact(i,j):
      if not contacted:
        print("Touching bodies:",i,j)
        contacted=True
      f = sim.contactForce(i,j)
      t = sim.contactTorque(i,j)
      print(" ",world.getName(i),"-",world.getName(j),"contact force",f,"and torque",t)

Even more detailed information about the latest contact points can be retrieved using the sim.getContacts() function. This returns a list of 7-lists, each of which contains the 3D contact point, 3D contact normal, and the friction coefficient. So the following code would print out all contacts between the given objects:

contactlist = sim.getContacts(objectid,linkid)
for c in contactlist:
  print("Contact point",c[0:3],"normal",c[3:6],"friction coefficient",c[6])

Batch simulation example

This example will do a very simple Monte Carlo example on a 1-link robot in the Klampt-examples/Python3/exercises/control example, just to cover the basics on how to run the batch simulation module.

First, create a new file in Klampt-examples/Python3/exercises/control called batch_test.py and use text editor to open it. In this example, we want to test if the controller works from different initial joint angles within the range [-pi, pi]. After a preset simulation time, we want to see if the angle is successfully controlled to the desired value.

First we have to import necessary modules, and load the xml file which defines the world. The world contains a one-link robot with an actuator.

import klampt
from klampt.sim import batch
import random, math

world = klampt.WorldModel()
fn = "world1.xml"
res = world.readFile(fn)
if not res:
    raise RuntimeError("Unable to load world "+fn)

Each simulation is initialized from some initial conditions that will be sampled at random, but we need to specify which parts of the world are actually sampled. We use the access module which can set and get named values in a world or simulation. Please refer to the access module documentation to learn more about how to use it.

We begin by defining a zero-argument sampling function that will sample the first DOF position of the robot from -pi to pi, as follows:

item = 'robots[0].config[0]'
itemsampler = lambda: random.uniform(-math.pi, math.pi)
initialConditionSamplers = {item:itemsampler}

For each of the N Monte-Carlo runs, itemsampler() will be called and the returned value will be assigned to the item that map accesses using the ‘robots[0].config[0]’ path. Any number of items in the world can be sampled by adding them to the initialConditionSamplers dictionary. For example, to sample the initial velocity, you would define a sampler for the ‘robots[0].velocity[0]’ item.

From these sampled initial conditions, batch.monteCarloSim will create a new Simulator instance and run a simulation trace. To customize the behavior of the simulation trace we can define three callback functions:

  • simInit, which is called when the simulation begins,

  • simStep, which is called every step, and

  • simTerm, which is called to determine whether the simulation should stop.

Here we’ll just change the simInit function, which is a one-argument function taking in a Simulator. In it we define some parameters of the robot’s controller:

def simInitFun(sim):
    controller = sim.controller(0)
    controller.setPIDCommand([0],[0])
    kP = 20
    kI = 8
    kD = 5
    controller.setPIDGains([kP],[kI],[kD])

which sets the target (0,0) and PID constants. (See the controller tutorial for more details about what these parameters mean.)

Next, we define an array returnItems that defines what data we want to retreive after each simulation run. In this example it means both the joint configuration and the joint velocity of the robot. Then we define duration of simulation and number of simulations, and call the batch.monteCarloSim function to simulate. See the documentation of monteCarloSim() for other options. The return value is a list of (initial condition, return items) pairs.

returnItems = ['robots[0].config','robots[0].velocity']
duration = 5
N = 100
res = batch.monteCarloSim(world,duration,initialConditionSamplers,N,returnItems, simInit=simInitFun)

Finally, we print the start and end configuration at each run, and use a file to record the data for post processing.

f = open('result.txt', 'w')
for i in range(N):
    initialCond,results = res[i]
    startConfig = initialCond['robots[0].config[0]']
    endConfig = results[returnItems[0]]
    print(startConfig,"->",endConfig)
    f.write('%lf\n'%(endConfig[0]))
f.close()

Then run the example by calling

python batch_test.py

After plotting the resulting error of the 0 angle, we obtain the following distribution:

_images/ErrorHist.jpg

which is a histogram of the final joint angle. From this figure we can know how the controller performs in order to control the joint angle from arbitrary value to 0 within 5 seconds. This can provide information on how the controller works based on how the parameters are tuned.

More advanced usage could add random parameters to the controller, which are sent as arguments to the simInit, simStep, and simTerm functions. This is accomplished using the special initial condition named ‘args’, which is a tuple that gets passed to each of these functions. For example, if we wanted to sample the target angle of the controller, we can do so as follows:

item = 'robots[0].config[0]'
itemsampler = lambda: random.uniform(-math.pi, math.pi)
initialConditionSamplers = {item:itemsampler}
initialConditionSamplers['args'] = lambda:(random.uniform(-0.5, 0.5),)

def simInitFun(sim,targetAngle):
    controller = sim.controller(0)
    controller.setPIDCommand([targetAngle],[0])
    kP = 20
    kI = 8
    kD = 5
    controller.setPIDGains([kP],[kI],[kD])

returnItems = ['robots[0].config']
duration = 5
N = 100
res = batch.monteCarloSim(world,duration,initialConditionSamplers,N,returnItems, simInit=simInitFun)

f = open('result.txt', 'w')
for i in range(N):
    #print(res[i][1])
    initialCond,results = res[i]
    startConfig = initialCond['robots[0].config[0]']
    endConfig = results[returnItems[0]]
    print("from",startConfig,"to",initialCond["args"][0],"->",endConfig)
    f.write('%lf\n'%(endConfig[0]))
f.close()