CIRP Encyclopedia of Production Engineering

Living Edition
| Editors: The International Academy for Production Engineering, Sami Chatti, Tullio Tolio

Grasping

  • Gualtiero FantoniEmail author
  • Marco Santochi
Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-35950-7_16832-1

Synonyms

Definition

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).

Extended Definition

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

Gripper Selection

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).

Several grasping principles based on different physical effects (Fig. 1) have been successfully used in different applications. Some principles can be applied only at the microscale (e.g., acoustic levitation or laser tweezers), while others proposed for microhandling are now expanding beyond that field (e.g., van der Waals forces) (van Brussel et al. 2000).
Fig. 1

Grasping principles (Figure a reproduced with permission of IFIP)

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.

The sensors are chosen to comply with gripper characteristics and grasping task. These sensors may be integrated into the gripper or might be mounted on an external fixture. Different kinds of sensing principles for the three main parameters presence, force/torque and position/orientation have been proposed and are used in commercial grippers (Fantoni et al. 2014) (Fig. 2).
Fig. 2

Sensing principles: (a) mechanical switch; (b) electrical sensor; (c) photoelectric sensor; (d) vision based; (e) force/torque sensor; (f) tactile sensor; (g) strain gauges; (h) optical based; (i) capacitive or electrostatic; (j) led-photodiode (often IR); and (k) vision-based monitoring

The sensing principles most used within grippers are the following:
  • 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.

Cross-References

References

  1. Fantoni G, Santochi M, Dini G, Tracht K, Scholz-Reiter B, Fleischer J, Kristoffer Lien T, Seliger G, Reinhart G, Franke J, Nørgaard Hansen H, Verl A (2014) Grasping devices and methods in automated production processes. CIRP Ann Manuf Technol 63(2):679–701CrossRefGoogle Scholar
  2. Krüger J, Lien TK, Verl A (2009) Cooperation of human and machines in assembly lines. CIRP Ann Manuf Technol 58(2):628–646CrossRefGoogle Scholar
  3. Monkman GJ, Hesse S, Steinmann R, Schunk H (2007) Robot grippers. Weinheim, Wiley-VCHGoogle Scholar
  4. 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
  5. van Brussel H, Peirs J, Reynaerts D et al (2000) Assembly of microsystems, keynote paper. CIRP Ann 49(2):451–472CrossRefGoogle Scholar

Copyright information

© CIRP 2017

Authors and Affiliations

  1. 1.Department of Civil and Industrial EngineeringUniversity of PisaLargo Lucio LazzarinoPisaItaly

Section editors and affiliations

  • Joerg Krueger
    • 1
  1. 1.IWFTechnische Universität BerlinBerlinGermany