A Perturbation / Correlation Approach to Force - Guided Robot Control

Force guided robot control is a control scheme based on the interpretation of measured force acting on the robot end effector. A functional map relating the correction of motion to force measurements is generated based on the geometry of the workpiece and its kinematic behavior in interacting with the environment. In the traditional force-guided control schemes, the contact force measured by a force sensor is directly fed back to a feedback controller to generate a motion correction signal. However, the force information obtained at one point does not always contain sufficient information to determine the direction of motion. The forces measured are often erratic and noisy due to the friction at the contacting surfaces and the existence of irregular burrs. This often leads to a misjudgment of contact configurations and erratic control actions. Also, the erratic force feedback may lead to excessive contact forces and large friction, which impede smooth motion and incur jamming and damage to the objects. Therefore, it is important to maintain the contact force at an appropriate level. The issue central to force guided robot control is how to obtain reliable, consistent and copious force signals and extract useful information in order to successfully guide the robot while keeping the contact force at a desired level. In this thesis, instead of simply measuring contact forces, we take positive actions by giving perturbation to the end effector and observing the reaction forces to the perturbation in order to obtain much richer and more reliable information. By taking the correlation between the input perturbation and the resultant reaction forces, we can determine the gradient of the force profile and guide the part correctly. By applying a type of direct adaptive control, the contact force is maintained at the lowest level. This algorithm is applied to a pipe insertion task, in which the insertion force is minimized during the insertion. Based on the process model and stability analysis using the Popov stability criterion, conditions for stable, successful insertion despite nonlinearities and uncertainties in the environment are obtained. The theoretical results are verified using the experimental data. To generate high frequency perturbation, a vibratory end effector using piezoelectric actuators is designed and built. Also, this perturbation/correlation method is applied to a box palletizing task, in which a rectangular box is to be located in one corner of the wall while maintaining constant contact with the wall. Through both simulations and experiments, the feasibility and usefulness of these methods are demonstrated. Thesis Committee : Haruhiko Asada Professor, Mechanical Engineering Kamal Youcef-Toumi Professor, Mechanical Engineering Frank Feng Professor, Mechanical Engineering