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Simple Gripper Simulation

This simple gripper demo is used to exercise MultibodyPlant's contact modeling in a gripping scenario. SceneGraph is used for both visualization and contact handling.

This example models a gripper with two fingers. The right finger is fixed to the main body of the gripper, while the left finger has a prismatic joint to allow opening and closing the grip. The grip force can be specified with the command line option --gripper_force, which defaults to 10 Newtons of force. If we provide a value of exactly zero, the fingers of the gripper will be set to be at --grip_width distance (in meters) from each other. When --gripper_force > 0, --grip_width specifies the initial condition for the fingers position.

The demo also permits specifying a vertical harmonic motion of the gripper to evaluate the performance of the grip when the gripper is undergoing forced motions. The parameters controlling this motion are --amplitude, to specify the amplitude of the oscillations in meters and, --frequency to specify the frequency of the oscillations in Hertz. Additionally, the user might like to change the direction of the forced oscillations to either be in the horizontal or vertical direction. This can be accomplished by changing the <axis> specified for the <joint> named "translate_joint" in the SDF for the gripper model, simple_gripper.sdf. Only vertical or horizontal forced motions are supported in this example and thus the axis must either be the x-axis or the z-axis. Otherwise an exception is thrown communicating this fact. There is no gravity for the forced vertical oscillations case.

Fixed Gripper with Gravity

To run a case with gravity but no forced oscillations, set the <joint> axis for the joint named "translate_joint" to be the x-axis in the SDF file. The parameter --gripper_force specifies the grip force while --grip_width specifies the initial condition for the position of the fingers. If the gripper force is set to zero, the fingers will be placed at a fixed --grip_width distance apart. The latter is an important test case since it eliminates the additional complexity introduced by having moving fingers.

Prerequisites

From your drake workspace directory you first need to build this example and the visualizer.

Open a visualizer window

bazel run //tools:meldis -- --open-window &

Build this example with

bazel build //examples/simple_gripper

Running the Example

Launch the simulation with

./bazel-bin/examples/simple_gripper/simple_gripper --simulation_time=10.0 \
  --simulator_target_realtime_rate=1.0

where for this particular invocation example we are specifying the simulation time in seconds as a command line option. To get a list of command line options for this example run

./bazel-bin/examples/simple_gripper/simple_gripper --help

which will also provide a description for each option.