Grippers are generally custom-designed for specific applications. If a robot is to be used flexibly for various tasks, it may be necessary to change the gripper. This is done using change systems, which consist of an upper and lower part that must be coupled in a reproducible manner. The change systems should be lightweight, have a low overall height, and feature a modular design. A self-locking mechanism and an emergency unlocking device should also be included.
Since grippers are often designed for use with only one or a few objects, their applications range from large items in the industry, such as engine blocks or vehicle parts, to small components like SMD parts in printed circuit board assembly. A wide variety of grippers are available from several companies, including PTM (Gröbenzell, Germany), Festo (Esslingen, Germany), IPR (Eppingen, Germany), Schunk (Lauffen am Neckar, Germany), Robotiq (Levis, Canada), and SMC (Tokyo, Japan).
Mechanical Grippers
Mechanical grippers are available in various configurations, such as rigid, rigid-articulated, or elastic designs. They can be designed as two-finger or multi-finger grippers. Mechanical grippers can be powered mechanically, pneumatically, or electrically. Pneumatic drives are widespread due to their ease of use but are increasingly being replaced by electric drives, particularly in the field of laboratory automation.
The simplest way to grip an object is by using a two-finger gripper. There are two types: two-finger parallel grippers and two-finger angular parallel grippers. In the case of two-finger parallel grippers, where both gripper jaws move parallel to each other, the range of movement varies from tiny strokes of just a few millimeters and compact designs a few centimeters in size, to larger grippers capable of handling heavy objects like engine blocks with high precision. The key factor in all these versions is always a firm grip with high force, ensuring that objects do not slip during transport. One common feature of all two-finger parallel grippers, however, is that the stroke is always small to the overall size of the gripper.
Two-finger-angled parallel grippers, on the other hand, are designed for gripping cylindrical objects. In addition to typical two-finger grippers, three-finger and four-finger grippers are also available. The advantage of using these types of grippers is their ability to securely grip and center round objects during the gripping and placing process.
Pneumatic Grippers
Pneumatic grippers operate based on either the suction or pressure principle. With the pressure principle, the workpiece is clamped. For the reliable application of the suction principle, a smooth, clean, dry, and airtight surface is required. Both methods are primarily used for flat parts. In laboratory automation, this principle is particularly suitable for tasks such as picking and placing the lids of microtiter plates. Classic vacuum grippers function with a vacuum connection. To achieve a secure grip with a larger contact surface, bellows grippers are often used for cylindrical objects. Bellows grippers are available for both external and internal gripping. Hole grippers are a type of gripper designed for internal gripping. There are also pneumatic grippers suitable for external gripping. The gripper is guided around the object to be gripped, and when air is supplied, a bellows inflates and wraps around the object in a form-fitting manner. This allows even objects with a diameter much smaller than the inner diameter of the outer gripper to be gripped securely.
The category of clamp grippers is based on the principle of clamping an object using a mechanism and releasing it after transport.
Magnetic Grippers
Magnetic grippers can be divided into permanent and electromagnet grippers. Permanent grippers are simple in construction but require additional equipment to release the gripped objects. Electromagnetic grippers pick up and release objects by generating or switching off a magnetic field using electrical energy. The challenge with this gripping method is that multiple objects can be picked up simultaneously, as it is difficult to achieve a highly directed magnetic field. A prerequisite for the use of magnetic grippers is that the objects must be made of ferromagnetic material.
Adaptive Grippers
Adaptive grippers are those that can adapt to different object shapes or textures. Grippers are typically designed for a single process. However, there are applications in which a gripper must be capable of gripping different objects. Adaptive grippers are available from companies such as Festo (Esslingen am Neckar, Germany) and Robotiq (Levis, QC, Canada), among others.
Laboratory automation systems often require the transport of microplate laboratory supplies as well as tubes. In these scenarios, tubes are frequently racked—either in microplate footprints or in proprietary formats. In some cases, tubes must be handled individually, for example, for sealing, uncapping, or decanting. As long as the form factors of the various labware types in a system are similar, robot transports can be set up using commercially available grippers. A challenge for robotic handling arises when laboratory devices vary greatly in size and shape and must be handled by a robot within the system. The limiting factors here are the gripper finger path and the lack of flexibility in independent gripper finger control. At the Center for Life Science Automation (celisca, Rostock, Germany), a highly flexible gripper was developed to handle a wide variety of labware geometries. The adaptive 4-finger rotary lever gripper, installed on an articulated arm robot, enables the transport of a broad range of labware.
Fig. 1: Grippers for laboratory automation. A: 2-finger gripper vertical (classical gripper in liquid handling systems); B: 2-finger gripper horizontal (classical gripper for robots); C: 3-finger gripper; D: 4-finger flexible gripper for different labware dimensions. © celisca
Sensors and safety systems for grippers
Gripping systems use various sensory principles to detect the correct position and presence of an object to be gripped and to ensure a proper gripping process. Strain gauges, incremental encoders, acceleration sensors, proximity switches, and image processing systems are commonly used for this purpose.
Additional protective devices can be implemented to protect the grippers from damage, such as from collisions or overloads. These include predetermined breaking points, snap-in couplings, or switch-off fuses. These safety components are typically hardware solutions connected to the robot controller’s safety circuitry. The ensure safe operation and an immediate stop of the robot in case of a collision, overload, or unexpected conditions. At the same time, they protect both the robot and gripper hardware, as well as the surrounding environment.
Numerous companies offer sensors and systems for grippers, including PTM (Hattgenstein, Germany), Zimmer Group (Rheinau-Freistett, Germany), Nordbo Robotics (Odense, Denmark), Schunk (Lauffen am Neckar, Germany), OnRobot (Odense, Denmark), and Bota Systems (Zurich, Switzerland).
The portfolio of robot and gripper sensors is a rapidly growing segment. In addition to sensors integrated into robots and grippers or end effectors, developments are underway for tactile sensor coatings for collision detection. This also includes tactile sensor arrays that can be placed on the surface of gripper finger pads, enabling enhanced gripping, scanning, or workpiece detection capabilities.
Contact
Prof. Dr.-Ing. habil. Kerstin Thurow
Center for Life Science Automation
University of Rostock, Germany
www.celisca.de
[email protected]
Read the articles of the series “Automation in the Laboratory”:
Part 1: Definition, applications, and potential of laboratory automation.
Part 2: The LUO concept in laboratory automation.
Part 3: Liquid handling in laboratory automation.
Part 4: Low-volume liquid handling in laboratory automation.
Part 5 : Laboratory automation – solid dispensing.
Part 6: Automated heating, shaking and mixing.
Part 7: Automation of polymerase chain reaction (PCR).
Part 8 : Automated centrifugation.
Part 9 : Automated filtration.
Part 10: Laboratory automation – Automated sonication and evaporation.
Part 11: Laboratory automation – Automated incubation.
Part 12: Laboratory automation – automated sample purification.
Part 13: Laboratory automation – automated sample identification.
Part 14: Laboratory automation – robots in laboratory automation.
