12.5.2 Nonprehensile Manipulation

Manipulation by grasping is very restrictive. People manipulate objects in many interesting ways that do not involve grasping. Objects may be pushed, flipped, thrown, squeezed, twirled, smacked, blown, and so on. A classic example from the kitchen is flipping a pancake over by a flick of the wrist while holding the skillet. These are all examples of nonprehensile manipulation, which means manipulation without grasping.

The temptation to make robots grasp objects arises from the obsession with estimating and controlling the state. This task is more daunting for nonprehensile manipulation because there are times at which the object appears to be out of direct control. This leads to greater uncertainty in predictability and a larger sensing burden. By planning in the I-space, however, it may be possible to avoid all of these problems. Several works have emerged which show that manipulation goals can be achieved with little or no sensing at all. This leads to a form of minimalism [175,321,681], in which the sensors are designed in a way that simplifies the I-space, as opposed to worrying about accurate estimation. The search for minimalist robotic systems is completely aligned with trying to find derived I-spaces that are as small as possible, as mentioned in Section 11.2.1. Sensing systems should be simple, yet still able to achieve the task. Preferably, completeness should not be lost. Most work in this area is concerned primarily with finding feasible solutions, as opposed to optimal solutions. This enables further simplifications of the I-space.

This section gives an example that represents an extreme version of this minimalism. A sensorless manipulation system is developed. At first this may seem absurd. From the forward projections in Section 10.1.2, it may seem that uncertainty can only grow if nature causes uncertainty in the configuration transitions and there are no sensors. To counter the intuition, compliant motions have the ability to reduce uncertainty. This is consistent with the discussion in Section 11.5.4. Simply knowing that some motion commands have been successfully applied may reduce the amount of uncertainty. In an early demonstration of sensorless manipulation, it was shown that an Allen wrench (L-shaped wrench) resting in a tray can be placed into a known orientation by simply tilting the tray in a few directions [321]. The same orientation is achieved in the end, regardless of the initial wrench configuration. Also, no sensors are needed. This can be considered as a more complicated extension of the ball rolling in a tray that was shown in Figure 11.29. This is also an example of compliant motions, as shown in Figure 12.44; however, in the present setting $F$ is caused by gravity.