Figure 4: Experimental setup for controller experiments. Above: Initial
configuration of robot as seen in the left and right cameras. A
visual goal near the floor is given to the algorithm. Below: Goal
configuration achieved by the controller. The image space trajectory
of the tracked features is overlaid.
The following experiments use a common setup. Two cameras are positioned to view the desired portion of the robot workspace. We have found that the exact placement is not particularly important, and that as long as the angular separation of the cameras is greater than about 30 degrees we have a sufficiently well conditioned problem. On the robot end effector we have mounted a 30 by 40 cm T-shaped foam piece with three small light bulbs, one at each end. A fourth light bulb is connected to a rod extending 40 cm above, and 40 cm out of the plane of the T. We use the same general feature trackers that we use later in the visual specification experiments. The light bulbs simply make the tracking more reliable and accurate.
The main purpose of these experiments is to study kinematic accuracy. To make sure the trackers keep up with the features, and that the test rig does not vibrate very much, we run the robot at low speed. At the start of an experiment series a Jacobian approximation is obtained by executing a sequence of test moves near the middle of the (visually defined) workspace.
A typical experiment is performed as follows: The robot is moved to a pre-defined pose specified by joint angles. Readings are taken from the trackers to define a visual goal. This ensures that the visual goal is actually attainable, something which is typically not true of a set of randomly selected feature values. After registering the goal, the robot is deflected to a random position in the selected workspace. During the deflection all control algorithm input is shut out, disabling model updating. At the end of the deflection additional noise is injected into the Jacobian model, and other adjustments are made to the control algorithm depending on the purpose of the experiment.
The controller is then turned back on, and relevant measuring processes are started, acquiring, for example, timing and visual trajectory information of the sort plotted in fig. 4. The controller is turned off when the visual goal is achieved within a preset accuracy, or when it is no longer moving any closer to the goal. Endpoint accuracy measurements are taken at this point. Then the whole procedure is repeated a number of times (between 50 and 200 times) until results are statistically significant.