10 The Dynamics EngineTop8 Grasp Force Optimization9 Grasp Planning


9 Grasp Planning

9.1 General concepts and planner types

Grasp planning is one the most useful (and most widely used) tools in GraspIt!. The core of this process is the ability of the system to evaluate many hand postures quickly, and from a functional point of view (i.e. through grasp quality measures). Using a simulated environment allows us to test grasps much much faster than in real life, and also at a lower cost. The quality metrics give us feedback on the grasps, often more than just a binary success / fail outcome. This is therefore the general concept: try out lots of grasps really fast and see which work. Of course, there is an infinite number of possible implementations, optimizations, refinements, etc. that can be played starting from this simplified idea. GraspIt! comes with a couple of grasp planners, each of them different in its own way, but all have roots in the same concept presented above.

The grasp planners within GraspIt! are grouped in three families:

This section only presents the Primitive-based Planner; the other two families have their own sections in this manual.

All the types of grasp planners have been extensively described in various publications. If you are interested in the machinery behind the scenes and the theory of the planners, the Publications section has many more details than presented here.

9.2 The Primitive-based Grasp Planner

The Primitive-based Planner is accessible via the Grasp -> Planner menu. It has a couple of restrictions: it only works on the Barrett hand, and only if the user also supplies a primitive approximation of the object to be grasped. When the Planner dialog is opened, GraspIt! will attempt to load the primitive version of the current object from the $GRASPIT/models/objects/primitives directory. In order to create a primitive file, see the examples in the primitives directory that are included with the distribution. Note that a primitive file may only include spheres, cubes, cylinders and cones. For more details, see the relevant publication.

The Planner itself goes through 2 stages. The Planner dialog window has two groups, one for each stage. The first stage (accessed through the button group on the left) is to generate many pre-grasps for your object. Pre-grasps are generated based on the primitive version of your object. You can generate as many pre-grasps as your computational resources and allocated time will allow you to test. The number of pre-grasps generated is controlled by the density factors. You can either choose a master density factor (Automated sampling) and allow GraspIt! to do the rest, or choose sampling densities along different dimensions separately. Alternatively, you can pre-specify the exact pre-grasps to be tested by loading them from a file, which is useful for debug purposes. Once you have set the desired parameters, click the Generate button to generate your pre-grasps.

The second stage is to compute the grasps that result from the chosen pre-grasps. Note that grasp execution is done on the actual object (even though pre-grasps are generated on the simplified primitive version). You can also choose which Quality Measure should be used to rank these grasps. If a usable QM exists already, you can select it from the drop-down list. If not, use the New button to fire up the QM creation dialog and create a new one. Once you have set the desired metric, you are ready to test all the pre-grasps by clicking the Test button.

After testing is finished, the hand will be set back to its initial position and the Show button will become enabled. Use the Show button to cycle through the list of found grasps, sorted in order of the Quality Metric.

IMPORTANT: you can choose to visualize the testing process by checking the Visualize process box. This means that the process of executing all the grasps will be rendered, and you can see the hand trying out each of them. You must check the Visualize box before clicking the Generate button for this to work. Visualization makes for a more compelling demo, but rendering slows down the planning process considerably. For time-sensitive applications, we recommend disabling the visualization.

When rendering is disabled, we have found that the computational bottleneck for the Primitive-based planner is collision and contact detection.

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10 The Dynamics EngineTop8 Grasp Force Optimization9 Grasp PlanningContents