Elsevier

Mechatronics

Volume 17, Issue 7, September 2007, Pages 345-356
Mechatronics

The concept of “Haptic Tweezer”, a non-contact object handling system using levitation techniques and haptics

https://doi.org/10.1016/j.mechatronics.2007.04.008Get rights and content

Abstract

This paper proposes a new tool concept for object handling. The new tool, named “Haptic Tweezer”, is a combination of a non-contact levitation system, such as magnetic- and electrostatic levitation, and a haptic device to realize intuitive object handling. Without mechanical contact, effects such as stiction, contamination or damage to fragile parts can be avoided. In this collaborative system, the haptic device assists the operator in real time to perform an object handling task, such as Pick and Place. By linking a haptic force to the levitation position error, instability of the levitation system during the Pick Up and Place task can be reduced. It realizes an additional stiffness between the tool and the object. Applications of the Haptic Tweezer concept could be the handling of micro-objects or handling fragile objects such as silicon wafers or thin glass plates of flat panel displays (FPD), where non-contact object handling is beneficial. This paper describes the concept of Haptic Tweezer and two realized prototypes, based on magnetic levitation, are shown. Experimental results showed unique characteristics in picking up and placing, which contribute to dexterous and intuitive object handling.

Introduction

As the size of consumer electronic products steadily reduces while the number of components inside is increasing, the average size of these micro-components also becomes smaller. For example, the size of electric parts such as diodes and capacitors or even mechanical machined parts can now be much smaller than 1 mm. In research and development laboratories or even at a production line, where components are manufactured in a wide variety, but in small quantities, these small components are mostly handled and assembled by hand. Due to the scaling laws, techniques used for object handling and manipulation in the macro world, can be ineffective in the micro-world [1], [2]. One of the major problems in micro-object handling and manipulation is stiction, an effect in which the object sticks to the tool because of the usually neglected forces, such as Van der Waals’, electrostatic, and surface tension forces [3], [4]. They are more dominant at the micro-scale over gravitational or inertial forces. Stiction disturbs precise object handling and complicates releasing the object. This effect can occur if the size of object becomes smaller than approximately 1 mm in diameter [5] and an example is shown in Fig. 1.

Different strategies have been proposed to solve problems related to stiction. It can be reduced by using special coatings on the materials [6] or by reducing the effective contact area of the handling tool [7]. Overcoming stiction is also widely applied by using techniques such as dynamic release [8] or by using a second supporting tool with a smaller contact area to release the handled objects [9]. However, the most promising way of dealing with stiction, is by avoiding it completely through handling the object without contact [10].

Many of the principles currently used in the micro-world for object manipulation are no longer based on the conventional gripper [11], but exploit various physical phenomena. For example, optical levitation is used to trap objects in a highly convergent laser beam, which is more commonly referred to as optical tweezers [12], [13]. By using dielectrophoresis, small particles can be manipulated in liquids [14], [15]. Also ultrasonics is used to trap particles in pressure nodes of an acoustic wave [16], [17]. However, most of these techniques require specific conditions and have a limitation on the maximum particle size they can handle.

The handling of (sub)-millimeter sized objects has not been researched extensively, although this size-range is more commonly used in research and development, and manufacturing. In this field it is generally thought that no special tool or technology is needed and handling small objects can be achieved by using existing tools, such as a pair of tweezers, as long as the necessary skill for object handling is adequate.

Besides the problem of stiction, the physical contact between the tool and object can cause damage to fragile components or contamination. This is also evident for micro-sized components or structures deposited on a larger area such as silicon wafers in the semiconductor industry or thin glass plates in flat panel display (FPD) manufacturing. These fragile objects also have to be handled very careful to avoid damage as a result of contact. They are typically handled by wands (manual or with vacuum technology), forks or tweezers and there is mechanical contact in all cases.

It is for this field that it is important to develop a new object handling tool that is based on a non-contact principle. By employing mechatronic technology, such as levitation techniques, haptics and collaborative systems, the efficiency of non-contact handling should be enhanced such that it allows also non-skilled operators to perform these handling tasks in a natural and easy way.

Levitation techniques are mostly used in bearings and suspension systems to avoid friction in for instance pumps or motors. In non-contact object handling it is important to realize a temporarily suspension of an object in order to manipulate or move it. For ferro-magnetic objects, magnetic levitation [18], [19] can be used and it has been researched extensively. For other materials, such as dielectrics, electrostatic levitation [20], [21] is feasible, although its suspension force is relatively weak. Without physical contact between the tool and the object, problems related to contact handling can be avoided. However, levitation systems are typically more sensitive to disturbances and thus more difficult to handle, which means that still experienced skillful operators are necessary to compensate for stability problems. This can be improved by combining a machine with the human operator to work together in a so-called Human-machine collaborative systems (HMCS) to get better performance.

The field of HMCS has greatly improved in the last decade. It combines the human superior decision making, versatility, creativity, skill and experience with robotic strength, endurance, precision and possibilities. HMCS were used in the beginning as collaborative robots or cobots [22] in which they are rather passive devices to support humans in terms of power [23], [24]. Variants, more integrated with humans are found in wearable exoskeletons aiming to increase human performance [25], [26]. However, with recent advances in the field of haptics, HMCS become more active in their interaction with humans as they can provide force sensations and force scaling actively in for instance surgery [27], [28]. These systems are capable of augmenting reality through the sense of touch [29], [30]. The SmartTool [31] is a good example of such a system as it “haptizes” sensory information to increase the efficiency of a task. The haptic sensation is a more natural way to perceive information over visual and auditory signals, reducing the stress of an operator in performing a task. Furthermore, by using virtual fixtures [32], the operator’s motion can be confined or guided, increasing performance of manipulation and medical tasks [33], [34].

In our research, a new tool is being developed under the name “Haptic Tweezer” and it combines a non-contact handling device with a haptic interface to handle small, fragile or contact-sensitive objects. This concept is illustrated in Fig. 2, where a small object can be picked up, transported and placed on another location, e.g. transporting a micro part to a microscope for analysis. The haptic device will assist in performing the task and prevent unwanted instability in levitation. Previous research [35] describes the initial prototype of a system using magnetic levitation and a haptic device. These results are expanded in this paper by defining the framework of the “Haptic Tweezer” concept and by also describing a new miniaturized prototype.

In this paper, two prototypes based on magnetic levitation will be presented as a feasibility of such a system, which is proposed by the concept. The first prototype is a macro model that can handle objects in the order of about 1 cm. The second prototype is a micro model in which millimeter sized objects are handled. Experimental data of both models will show the pick and place behavior and the effectiveness of these prototypes.

Section snippets

Basic concept

Handling objects without mechanical contact by means of levitation techniques, has the advantages of no stiction, no high contact force, and no contamination from the handling tool. However, non-contact handling tools have disadvantages as they are in general more complex and less stable (more sensitive to external disturbances). A human operator, directly operating the levitation tool, will be the main source of transmitting disturbances to the levitation system. Especially in tasks such as

Macro-scale prototype: system design

The first prototype is realized to show how a combination of levitation system and haptic interface will perform. This macro-scale prototype combines a magnetic levitation system with a commercially available haptic interface, the PHANToM Omni (Sensable Technologies). The structure and control method of each system is described successively.

Macro-scale prototype: experiment

For the object handling task of Pick and Place, the details are given for Pick Up task and the Place task separately. The performance of the macro-scale prototype on these tasks is evaluated by a test with ten subjects. The handled object is an iron ball, 12.7 mm in diameter and it weighs 8.3 g. The ball should be picked and placed on a raised placing platform with a height of about 63 mm. The control settings and other characteristics are given in Table 2.

Micro-scale prototype

Based on the previous results, a new prototype has been developed, capable of handling much smaller objects in the millimeter order. Both the magnetic levitation device and the haptic device will be discussed below. In this setup, the motion is restricted to only one degree of freedom, mainly because of a limitation in sensing capabilities.

Conclusion

In this research, the concept of a new tool named Haptic Tweezer is proposed that combines a haptic device with non-contact levitation techniques to handle minute, fragile or precise objects by human operators. A first macro-scale prototype is shown that can handle objects of about 1 cm. It combines the haptic interface PHANToM Omni with a Zero-Power magnetic levitation device to realize optimum non-contact object handling behavior. Experiments show how the performance of a Pick and Place task

Acknowledgements

This work was supported in part by a Grant-in-Aid for Scientific Research, No. 18360117, from MEXT of Japan.

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