Elsevier

Mechatronics

Volume 50, April 2018, Pages 87-103
Mechatronics

Implementation of a revolute-joint-based asymmetric Schönflies motion haptic device with redundant actuation

https://doi.org/10.1016/j.mechatronics.2017.12.004Get rights and content

Abstract

In this work, a revolute joint-based asymmetric Schönflies motion (SM) haptic device with 4-Degree-of-Freedom (DOF) force feedback capability is developed. The SM haptic device is composed of a redundantly actuated parallel sub-module which has translational 3-DOF output motion, a pantograph limb which takes the role of providing 1-DOF rotational output motion, and a revolute joint allowing the relative motion between them. All five DC motors without gearhead are placed on the ground by employing proper parallel transmission linkages for power transmission. The large singularity-free workspace and the improved kinematic characteristics are secured by redundantly actuating the 3-DOF sub-module. Thus, the SM haptic device has excellent features such as the unlimited 1-DOF rotational output motion, minimal friction, minimal inertia, and large dexterous workspace. Mobility analysis, kinematic modeling, singularity analysis, optimal design, and linear inertia modeling of the SM haptic device are conducted. Then a prototype with two operational modes such as gravity compensation and linear inertia compensation modes is implemented. Through friction measurements, motion tests for gravity and/or linear inertia compensation modes, and virtual wall experiments, it is confirmed that the prototype possesses minimal friction as well as good gravity and linear inertia compensation performances sufficient for the high-quality haptic device applications such as medical training, robot-assisted surgery, etc.

Introduction

Haptic technology has broadened its applications areas and increased its intervention level significantly in various fields such as surgical operation and training, teleoperation, manufacturing and design, rehabilitation, etc. [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17] Depending on tasks, various forms of haptic mediums such as tactile, kinesthetic, visual, aural, etc., could be targeted. In any case, the main functions of the haptic devices are to provide the information to the user and to assist him/her to effectively and safely conduct the given tasks. “Telepresence” providing realistic information on actual operation site environment is the important factor to the haptic devices and also “virtual information and virtual environment” such as virtual fixtures could be helpful in enhancing the role of the haptic devices. Numerous forms of haptic devices have been introduced so far and some of those devices have been successfully adopted to verify their effectiveness in their application fields.

The haptic devices, which are developed focusing on providing the kinesthetic information, could be categorized into several types by their structure, or by power transmission medium, or by input variable, or by motion degree-of-freedom (DOF). By structure, they could be classified as serial, parallel, and hybrid types. By Power transmission medium they could be classified as linkage, cable, and levitation types [6], [7], [8], [9], [10], [11], [12]. By input variable, they are divided as impedance type or admittance type. By the DOF, they could be distinguished as a mTnR type or simple a n-DOF devices, where mT and nR denotes the translational m-DOF and the rotational n-DOF, respectively. Depending on task requirements, various forms of haptic devices could be formed by properly combining those different features. For example, the haptic device could be designed combining both cables and linkages, in which the linkages could provide the rigidity whereas the cables could undertake the role of force transmission.

In aspects of DOFs, most of commercialized haptic devices are designed to have either 3T motion with force feedback capability or full 3T3R(6-DOF) motion with either translational 3-DOF or 6-DOF force-reflection capability such as Phantom Omni/Premium haptic devices by 3D Systems Inc. [13], Falcon by Novint Inc. [14], Omega haptic devices by Force dimension Inc. [15], and W3D and W6D haptic devices by Entactrobotics Inc. [16]. Phantom Omni/Premium haptic devices adopt either the serial or hybrid structure. Both Omega and Falcon haptic devices adopt the parallel structure, i.e., a 3T sub-module which is composed of three RRPaR type limbs (or a 3RRPaR type structure), where R and Pa denote the revolute joint and the planar parallelogram linkage, respectively. To provide additional rotational DOFs to those 3T sub-modules (or haptic devices), either a passive or an active serial type spherical linkage can be mounted on their moving plates [14], [15].

As haptic devices having other types of motion DOFs, the 5-DOF to 7-DOF high definition haptic devices (HD2) are available by Quanser Inc. [17] and a 3R type haptic device employing the spherical structure with force feedback capability is suggested [6]. Note that both haptic devices, W6D and HD2, have 6-DOF motion but has an axis which allows unlimited rotation. This unlimited rotation capability is beneficial in tasks requiring the large or the continuous rotations.

Table 1 shows the summary on the mechanical specifications of the commercialized haptic devices such as back-drive friction, tip inertia, magnitude of continuous force feedback, and workspace size. Some of those haptic devices such as Touch X, Premium, Felcon, and Omega have been successfully attempted to various haptic applications [1], [18]. It can be noted that the haptic devices such as Falcon and Omega adopt Delta-structure which has 3T output motion even though it employs only resolute joints [19]. Thus, they take advantages of the parallel structure such as high precision and rigidity and also take advantage of the minimal friction coming from the resolute-joint-based structure. Also it can be noted from the Table 1 that Touch X and W6D haptic devices have fairly small backdrive friction which is about 3.43% and 0.35% of its continuous output forces, respectively. In aspects of gravity, Premium 1.5, W6D, and HD2 employ counter balancing while Omega 6 provides active gravity compensation. No commercialized haptic device providing inertia compensation is available up to now. Workspace size and magnitude of force feedback of haptic devices vary depending on their design targets.

In abreast with these commercialized haptic devices, there has been continuous efforts to develop the enhanced haptic devices. A 6-DOF haptic device is suggested, which is obtained by connecting two 3T type Delta modules to a steering handle [8]. In their design, the device successfully secures both low inertia and friction by locating all direct-drive actuators on the ground. Also, by employing the counter mass for the gravity compensation the device could generate the high stiffness environment requiring in teleoperation tasks. Another innovative 3T type haptic device, which consists of two limbs of the Delta structure but is redundantly actuated by four actuators to secure large workspace and better kinematic characteristics, is suggested and a 6-DOF haptic device is also suggested by attaching the serial type 3R module on the moving plate of the 3T module [20]. Later, by appropriately connecting two 3T modules in [20] through serial 3R type linkages, another 6-DOF haptic device which has 5 active DOFs and one passive rotational DOF is suggested [21].

In particular, Schönflies motion generators (SMGs) which has translational 3-DOF motion and rotational 1-DOF motion about the fixed axis are effective, particularly in robot applications requiring high-speed operations, such as pick-and-place, loading and unloading, parts assembly, screwing, and riveting tasks [22], [23], [24], [25], [26], [27], [28], [29]. As efforts to employ the SMG structure for haptic devices, several diverse versions of SMGs [30], [31], [32], which employ a revolute-joint based limb, are investigated based on the workspace size, singularity, and kinematic characteristics. Note that the structure of some SMGs intrinsically allows the unlimited rotation about a fixed axis in addition to the 3T motion. Thus, those types of SMG-structured haptic devices would be highly effective in tasks requiring SM motions as well as 3T motion. In addition, it could also be employed as a 3T1R type base module for the 6-DOF haptic. The structure of the 6-DOF haptic devices would be similar to the one in [21] but it will be simpler since the additional 2-DOF rotational sub-module is attached instead of 3-DOF sub-module.

As addressed, the desired features for a haptic device could differ depending on specific task requirements. However, the following could be the commonly desired features for most haptic devices: (i) low friction, (ii) simple geometric structure, which possesses the closed-form forward/inverse position solution or which secures less computation in real time, (iii) singularity-free workspace enough for its application, (iv) force-reflection capability sufficient for its application specification, (v) gravity balancing or gravity compensation capability to provide the fatigue-free characteristics to the operator, (vi) dynamic compensation capability to provide the fatigue-free and low impedance characteristics to the operator, and (vii) compactness and light weight.

Thus far, dynamic compensation is not incorporated to most of the developed haptic devices. However, the dynamic compensation is an important function in controlling the haptic device since it could be significantly effective either when the inertia of the haptic device may not be negligibly small or when the haptic device is employed in tasks requiring very high acceleration/deceleration.

In this study, a new asymmetric and revolute-joint based 4-DOF SMG type haptic device is investigated. The main target of the haptic device is to develop the one which is highly suitable for the applications in robot-assisted surgery or medical training. The desired features of the new haptic device can be summarized as follows:

  • (i)

    The mechanism is an asymmetric type composed of three limbs; two limbs are identical and the third one is different.

  • (ii)

    All joints are revolute type to reduce friction.

  • (iii)

    All actuators are mounted on the ground to minimize gravity and inertial effects.

  • (iv)

    Both the gravity-load and the dynamic inertial load compensation are supported.

  • (v)

    Redundant actuation is adopted to secure large singularity-free workspace.

  • (vi)

    The mechanism could be used as a base module to construct the higher DOF haptic devices.

In the following sections, the mobility analysis, kinematic modeling using the screw theory, singularity analysis, and optimal kinematic design to secure the sufficient workspace size and the enhanced kinematic isotropy property of the SM haptic device are conducted after its brief descriptions. The prototype is implemented and its dynamic analysis is performed along with the development of two supporting functions such as gravity and linear inertia compensations. Then, to verify its motion capability as a haptic device, various experiments are conducted. Lastly, after brief discussion on results, conclusions are drawn.

Section snippets

Descriptions

The suggested SM haptic device is formed using two RRPaR type limbs, a RR(nR)pR type limb, and a revolute joint connecting them as shown in Fig. 1. Here, (nR)p denotes a pantograph composed of n revolute joints whose axes are parallel. Note that in Fig. 1, the total number of revolute joints of the pantograph is selected as n=15. The kinematic chain structure of this device is denoted as (2RRPaR)+R+(RR(15R)pR), where (2-RRPaR) denotes a 3-DOF translational parallel sub-module (it will be

Kinematic performance index

Without loss of generality, the kinematic characteristic of the (2RRPaR)+R+(RURU) type SMG which has a kinematically equivalent structure with the (2RRPaR)+R+(RR(nR)pR) type SM haptic device is investigated. Two design aspects are considered: workspace shape/size and global kinematic isotropy index. The local kinematic isotropy index is defined as σKI=σmin/σmax, where σmin  and σmax  represent the minimum and maximum singular values of the Jacobian matrix of the manipulator, respectively. The

Hardware design

For the compact design of the SM haptic device, all heavy actuators are placed on the ground as shown in Fig. 1. Specifically, the first joint θi1(fori=1,2) of each of the two RRPaR type limbs is directly driven by motors #1 and #3 mounted on the ground, whereas the second joint θi2(fori=1,2) of each of the two RRPaR type limbs is driven by motors #2 and #4 mounted on the ground through both a parallelogram and a five-bar linkage as shown in Fig. 7. Note that when the RURU type limb is directly

Results and discussions

As addressed in the introduction, most of the parallel-structure haptic devices employ 3-DOF Delta-structure. However, in this work, a new asymmetric SM structure is adopted to develop the haptic device. Output motion of the structure has the translational 3-DOF and the rotational 1-DOF about the fixed axis. The additional DOF would be useful to develop higher DOF haptic devices, simply adding lower DOF module on the top of the developed haptic device. Note, in particular, that the prototype

Conclusions

In this study, a revolute joint-based SM haptic device with low friction, low gravity, and low inertia is developed. To secure the enhanced performance of the developed SM haptic device, a complete analysis including its mobility analysis, singularity analysis, kinematic modeling, dynamic modeling, and optimal kinematic design are conducted. The prototype is then developed based on these analysis results, and its performance as a haptic device application is tested through friction measurements

Acknowledgment

This research was supported by the Korea University Research Fund.

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