Grasping is defined as a series of handling operations which provide forces and torques necessary to get and maintain the part in a relative position and orientation with respect to the grasping device (e.g., tweezers for small parts or vacuum cups for flat and nonporous objects). The end effector that exerts the grasping is called “gripper” and it is also used in cases of holding rather than actual grasping (Monkman et al. 2007).
Nowadays several factors such as the increasing cost of human labor, the spread of automation and the decreasing cost of robotic systems have pushed both industry to the adoption of grasping systems to automate many production processes in different fields. While in the past robot hands and industrial grippers were oriented to achieve different goals, nowadays the gap is reduced and it is often difficult to distinguish a simplified robotic human-like hand from a complex industrial gripper (Krüger et al. 2009).
Theory and Application
The design of a gripper depends on the object characteristics (porosity, roughness, water sensitiveness, stiffness, and electrical conductivity) but it is also affected by the task characteristics as the position to be reached and the orientation of the target releasing point (derived from previous phase as feeding) and by handling performances (accelerations, precision in positioning and releasing difficulties).
Mechanical grippers are based on friction or on form closure, but also intrusive grippers belong to that class. They are the most widespread. Suction based grippers and magnetic grippers dominate metal sheet handling in the automotive field. While Bernoulli grippers, even if also they exploit negative pressure, are now receiving more attention since they handle parts in a contactless way. Electrostatic grippers are based on charge difference (sometimes induced by the gripper itself) between the gripper and the part, while van der Waals grippers are based on the low force (electrostatic forces) due to the atomic attraction between the molecules of the gripper and those of the object.
Capillary grippers use the surface tension of a liquid meniscus between the gripper and the part, such a small liquid quantity is frozen in cryogenic grippers: the required force is due to the resulting ice. Other grippers are even more complex and adopted mainly at the micro or nanoscale: the ultrasonic based grippers generate standing pressure waves used to lift up a part, while laser sources can produce an optical pressure able to trap and move microparts in a liquid medium. (Tichem et al. 2003).
Monitoring the Grasping Process
The process of monitoring the effectiveness of grasping is one of the key aspects to pay attention during the design and selection of a gripper.
Mechanical switches (a) and electrical sensors (b) are used to detect the presence of the part but, due to their characteristics, imply a physical contact.
In case of a photoelectric sensor (c) or a vision based system (d), the presence of the part can be monitored in a contactless way.
Force/torque sensors (often at robot wrist level as in (e)), tactile sensors (f), strain gauges (g) are implemented to assess the grasping forces, fundamental in case of grasping of delicate or fragile parts; indirect force measurement systems based on optical methods also exist (h).
Capacitive or electrostatic sensors (i); led-photodiode (j) or vision-based monitoring (k) are used to gather further information on the grasped part as for example its orientation.
- Monkman GJ, Hesse S, Steinmann R, Schunk H (2007) Robot grippers. Weinheim, Wiley-VCHGoogle Scholar
- Tichem M, Lang D, Karpuschewski B (2003) A classification scheme for quantitative analysis of micro-grip principles. In: Proceedings of the 1st international precision assembly seminar (IPAS’2003), Bad Hofgastein, 17–19 Mar 2003Google Scholar