Experimentally Driven Design of a Palpating Gripper with Minimally Invasive Surgery Considerations

Adam Spiers 1, Sarah 8aillie2, Raj Persad3

1Bristol Robotics Laboratory, University of the West of England

2School of Veterinary Science, University of Bristol

3Bristol Urological Institute, South mead Hospital Bristol

ABSTRACT

A natural extension of current robotic minimally invasive surgery (MIS) is the addition of tactile and kinesthetic feedback, which would give an operating surgeon valuable haptic infonnation about tissue under observation. Identification of haptic properties requires active exploration [1][2]. It is believed that such exploration may be facilitated by a tool capable of replicating the major exploratory procedures (EPs; haptic information gleaning maneuvers) of a surgeon. Such a tool must also fit within the fabrication and other constraints of similar MIS tools.

In this paper the requirements of such a gripper are highlighted via experiments with surgeons. The results have been used to develop a 2-finger, 3dof (degree of freedom) gripper based on an inverted closed chain serial manipulator. A primary advantage of this system is its dexterity and compactness, compared to other manipulators. The gripper is shown to be capable of replicating the most frequently observed two-finger exploratory procedures. It is argued that a single tactile sensor is sufficiently useful for a two fingered system and a scaled prototype of the system has been designed to accommodate such a sensor. This system has been designed with later laparoscopic grade manufacture in mind.

KEYWORDS: Robot Manipulator Design, Exploratory Procedures, Palpation Minimally Invasive Surgery, Haptic Sensing

INDEX TERMS: H.I.2 - [User/Machine Systems] - Human Factors 1.2.9. [Robotics] - Manipulators. H.5.2: [User Interfaces] -Interaction Styles

1 INTRODUCTION

Tele-operated medical robotic devices such as da Vinci surgical system [3] offer great advantages of precision and dexterity to laparoscopic surgeons. However, a major drawback of the many da Vinci systems in operation is the lack of true tactile feedback during tissue manipulation, which can provide the operator with a great deal of relevant and intuitive infonnation.

A manual or tele-operating surgeon may use a number of laparoscopic tools during a single operation, with each tool suited to a particular set of tasks. It is an argument of this paper that tele­haptic tissue analysis may also require a specialist tool, which should reflect the requirements of the surgeons using it.

From discussions with urological surgeons, it was highlighted that a combination of exploratory motions is what gives a surgeon

Email: adam.spiers@brl.ac.uk sarah.baillie@bristol.ac.uk tony.pipe@brl.ac.uk rajpersad@bristolurology.com

IEEE Haptics Symposium 2012 4-7 March, Vancouver, BC, Canada 978-1-4673-0809-0/12/$31.00 ©2012 IEEE

their appreciation of tissue. In terms of EPs that would enhance radical prostatectomy, it was suggested that contour following would permit definition of key areas during surgery, such as the bladder neck, or specific landmarks of the prostate, which would aid identification, and therefore preservation, of structures that need to be left intact after the procedure. Detennination of deformability and compliance was also important in tissue appreciation, in particular, for the identification of embedded regions of hardness and lumps. In addition the identification of pulsatility during MIS was also of interest.

Figure 1. Current da Vinci 3dof laparoscopic tools, remotely

actuated through cables (3)

In order to ensure the usefulness, and technical efficiency of a dedicated palpation tool, a study has been undertaken on the techniques and abilities of the human haptic system under some of the constraints of MIS. Even in a non-robotic sense, these constraints severely limit the sensory capabilities and dexterity of a surgeon. Likewise, the design of a robotic MIS manipulator is constrained by size limitations and other requirements, such as hygiene [4]. Though it may be theoretically ideal to fabricate a miniature force-reflecting anthropomorphic hand with sensor enabled fingertips, this is unlikely to be feasible given the scale and budget constraints. Hence, our experiments also aimed to identify areas that may be simplified in the design of the gripper. The presented study involved two experiments. The first looked at the finger and thumb movements employed by surgeons when identifying the surface and material properties of different objects by touch alone. This experiment used the exploratory procedures (EPs) framework of Lederman and Klatzky [1][2]. The goal of this experiment was to detennine the most beneficial functions of the proposed palpating manipulator to provide the operator with useful and flexible interaction.

The second experiment investigated subjects' ability to locate objects in a compliant medium with reduced tactile sensation. This experiment was conducted to examine the necessity for

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tactile sensors (such as those from [5][6][7]) in the gripper design. A haptic-sensing gripper without tactile sensors is certainly feasible, in terms of kinesthetic sensing, and would be easier to integrate into an MIS scenario.

The results of the experiments have been used to influence the design of the manipulator prototype, which is also presented.

2 RELATED WORK

Haptically equipped MIS tools have been of interest since the advent of robotic surgery. Approaches have mainly centered on adding force sensing capabilities to existing tool structures, where one or two rigid jaws pivot around a central point (as in Figure 1). Such force sensing is commonly performed on some area of tool shaft [8][9] and / or the gripping jaws [10].

Alternatively, [11] and [7] both performed remote tactile object exploration using force-sensing industrial scale robot manipulators to position a tactile array about a compliant object. Though active exploration was achieved, the extraction of object properties is significantly limited by the single point of interaction and restricted array orientation.

According to the EP framework of [1][2], the identification of particular object properties is linked to specific movements. For example, the pressure EP gleans hardness data, while contour following describes shape. To permit a full range of haptic object properties to be identified in a tele-operation scenario, a slave system should be capable of performing a full range of EPs. This is not achievable with the 'forceps' arrangement of typical MIS tools, or at least not without involving mUltiple tools.

Though EPs were first determined in a two handed human scenario attempts have been made to apply the techniques to less dexterous robotic systems.

[12] used an anthropomorphic 5 finger hand with autonomous finger EPs of 'tapping and squeezing', which relate to the pressure EP. Here, objects were supported in the palm which, like each finger, was instrumented with tactile sensors. In a more reduced version, [13] attempted to automate object manipulation and sensing via the sequencing of EPs. Here, a two fingered robot with a fixed 'palm' switched between phases of 'rolling and sliding' to make effective use of a single tactile array mounted on one finger. This permitted extraction of some surface properties.

[6] and [14] also used two finger robot manipulators with one or two sensor enabled finger-tips, respectively. These robots were implemented in the tele-operated (user controlled) sense.

In [13][14][6] the fingers employed have identical kinematic structure and size. This leads to large overall mechanisms, and working envelopes for these robots, in comparison to workspace. Though this is not a problem in the intended applications of such systems, these mechanisms are not directly suited to the confines of MIS, where manipulators must be passed through a port.

Recently, [4] virtually prototyped two versions of parallel MIS grippers and determined that independent jaw controlled to improved lump detection. However, the parallel constraint meant that shape detection could not be performed by this system.

3 EXPERIMENT ONE

As has been highlighted in [1][2], the haptic system is composed of an interaction between sensing and manipulation. As such, both of these aspects have been subjects of consideration. The first experiment is aimed at determining the most prominent exploratory movements used during unseen object exploration and feature extraction. This experiment was inspired by the method of [1] but with a two finger constraint.

3.1 Method During the experiment, participants (two urological surgeons and two heart surgeons) were requested to insert a hand into a box though a hole. The box contained three suspended objects, which were hidden fTom view. Using only their index finger and thumb, the participants were requested to verbally describe as many properties of each object as possible. The two finger constraint mimicked the two points of contact used in both typical and robotic laparoscopic instruments. Participants were asked to wear surgical gloves, to create a typical level of tactile stimulation and surface interaction during medical procedures.

Figure 2. Setup for experiment 1

The objects in the box consisted of a hard wooden rectangular block, a glass marble wrapped in soft and compressible packing foam and another rectangular block made from the same foam (all illustrated in Figure 2). The objects were designed to provide varying surface and structural properties, which were expected to elicit a variation in EPs. In addition, it was hoped that the marble in foam object would provide clues about techniques used for investigation of compliant tissues containing lumps. Conversely, the wooden block was deliberately designed to be a very artificial object that would not be encountered inside the body. This was included so that any EPs observed specifically for this object could be rated as having low necessity for surgical use.

There was no time constraint, which allowed participants to explore freely until satisfied that all the discernable properties of each object had been identified.

A video of each trial was recorded (from the viewpoint of Figure 2) for later identification of EPs. The analysis technique involved splitting each video into 1 second intervals. For each interval a note was made of which EPs were observed. In the case of several EPs being observed simultaneously, notes were made of all EPs. Finally, for each object, the total number of occurrences for each EP was then divided by the number of seconds that the object was being handled by the experimenter. This provided a measure of the prevalence of each EP, independent of the interaction time with each object.

3.2 Results Object interaction times varied between 12 and 92 seconds. The result of this analysis is illustrated in Figure 3. The horizontal axis of this figure refers to the various EPs which were under scrutiny during the trials. Note that RF refers to the case where the object is not rolled for re-orientation (i.e. it is rolled back and forth or and / or returns to its original orientation).

LM - Lateral Motion

RF - Rolling between fingers RR - Rolling for Re-orientation Pr - Pressure CF - Contour Following En - Enclosure SC - Surface Contact US - Unsupported Holding

3.3 Observations It was noted early in the analysis of the videos that the two finger constraint meant that the 'standard' exploratory procedures of [1][2] were no longer entirely valid. One reason for this appeared to be the lack of support provided by an additional hand or

fingers. This was in part compensated for by suspending the objects, though this is not equivalent to a fixed 'ground' that is sometimes experienced in minimal gripping scenarios [13].

0.7 -,--------=------------(;" � 0.6 +------::::--I---U---------=>

l 0.5 +-----e----e----1 ] 0.4 � 03 " 0.2 z E 0.1 ;::

o LM RF RR Pr CF En SC US

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Wood

• Surgeon 1 • Surgeon 2 • Surgeon 3 • Surgeon 4

Marble in Foam

• Surgeon 1

• Surgeon 2 • Surgeon 3 • Surgeon 4

Foam

• Surgeon 1 • Surgeon 2 • Surgeon 3

• Surgeon 4

Figure 3. Time normalized exploratory procedure occurrence

during investigation of the three objects

In such a non-redundant gripping scenario, such actions as lateral motion and contour following are frequently forced to differ fTom that which may be observed in the two handed scenario, where a reliable ground is present. Lateral motion, for example, was rarely manifested as a repetitive back and forth stroking motion (as described in [1][2]), as with only a single point of support, this would lead to object re-orientation. Considering that lateral motion has the primary function of extracting texture data, it appeared, from coinciding verbal recordings of the surgeons, that the rubbing motion had been replaced with a 2-fingered stroking.

The most prominent modifications of EPs have been illustrated in Figure 4, with the two fingered stroking shown on the left.

Pressure was also replaced with a pinching motion. In particular it should be noted that several EPs were occasionally combined into a single motion, as in the case of contour following, with pinching and two fingered stroking, which serves the dual purpose of providing support and extracting texture information.

From Figure 3, it may be deduced that the most fTequently observed exploratory procedure was pressure, which manifested itself in the squeezing of the object under observation. It must be stated however that it was often unclear whether the object was merely being held in a pinch grip or being actively squeezed. Also, there were scenarios where the action of applying pressure appeared to be combined with other EPs (as in the 'Combined' EP of Figure 4). Lateral motion, contour following and rolling the object between fingers provided the next most observed EPs.

.c (;) , Q.,

.g � o \

:::E ,

Lateral Motion

Lateral Motion

\ \

Pressure Contour Following

Pressure Combined Figure 4. EP variation and combination in the 2 finger scenario.

The three topmost EPs (from [2)) are simultaneously achieved in

the 'combined' motion.

4 EXPERIMENT TWO

In the second experiment the importance of tactile over kinesthetic sensing was investigated, in the context of compliant material palaption for lump identification. The motivation of this experiment was to investigate the necessity of mounting tactile sensors on a hypothetical tele-haptic slave gripper.

4.1 Methods Ten subjects were given several attempts to locate the centre of five different objects that had been embedded in an opaque and compliant rubber block. The subjects consisted of two urological surgeons (one of whom had been involved in experiment one), four clinical staff and four people from non-medical professions.

The block (dimensions O.3xO.07xO.OSm) was created by a fabricator of surgical training devices. The compliance properties of the rubber approximated abdominal tissue. A 30cm plastic ruler was fixed beside the block, this allowed participants to quantifiably define where they believed the centre of each object to be.

The objects embedded in the block consisted of a knotted piece of string, a flat and thin abstract resin shape, a ISmm3 mdf block, a vertical dowel rod (16mm diameter, 40mm length) and a ball bearing (II mm diameter). The surface and material properties of these objects varied, which affected how easy it was to locate and

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pinpoint their centre. The subjects were not informed of the nature or quantity of the objects.

Once again subjects were requested to use only the thumb and forefinger in the experiment. In the initial attempt both digits were covered with flat rigid pads (measuring 45mm x 15mm), held in place with elastic bands (Figure 6). These pads made fine tactile sensing impossible. The experiment was performed 3 times in total, first with both pads worn, then with a pad only on the thumb and finally with no pads. A time limit of 60 seconds was enforced for each trial, with warnings given at 30 and 45 seconds.

Rubber Block Embedded Objects

Plastic Ruler Rigid Mount

Figure 5. Setup of experiment 2. The finger pads for obscuring

tactile sensation have not been shown in the figure.

Figure 6. The rubber black and finger pads used in experiment 2.

During each trial subjects were requested to describe the centre point of each object, in relation to the ruler. Previously identified objects were requested to be re-announced in subsequent trials, permitting changes in accuracy to be observed.

To accomodate the difference in one participant identitying all objects inaccurately, compared to another identitying some objects well, a method of scoring each subject and trial was developed. This essentially measured the distance error of each object identified, from the established true value. For all objects not identified, a penalty error of 20mm was recorded, based on the fact that the largest error value for an object identified during a trial was 18mm. The error values were then combined to give a mean for each trial.

It was hoped that this method of scoring would reduce the importance of re-locating the same object (which is a concern due to learning effects) by measuring improvements in localisation accuracy. As such the penalty for not finding an object is not prohibitively worse than locating an object with poor accuracy.

4.2 Results The mean avergage distance error, incorporating the penalty for unidentified objects, is illustrated in Figure 7 for medically specialised participants and Figure 8 for non-medical participants.

These figures illustrate that object localisation accuracy improved with the removal of sucessive finger-pads. Additionally, the degree of improvement between pad removal seemed to suggest that there was greater improvement in enabling tactile sensitivity between zero and one finger, compared to one and two fingers.

One limitation is that learning played a part in the experiment and that subjects who found an object in the first trial, would find the same object again. Clearly, randomisation of the object location between trials would have eliminated the learning process, however this was not possible with the available resources. The learning factor was somewhat negated by measuring accuracy of object localisation, which should hopefully have improved with sucessive trials. It should also be noted though that not all participants reached the minimal mean error of

0.2cm, though more medical staff ended in his region.

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Medical Group

_Surgeon A

_SurgeonB

--Specialist A

-Specialist B

- - Anethetist A

- - Anethetist B

Figure 7. Object identification accuracy for medical professionals in

experiment 2.

1.8

1.6 Non-Medical 1.4 Group

I1.2 - Engineer A

i 1 � 0.8 - EngineerB

� 0.6 Microbiologist

0.4

0.2 -Administrator

1 Number of finger pads worn

Figure 8. Object identification accuracy for non-medical

professionals in experiment 2.

4.3 Observations The medical participants generally out-performed the non-medical participants in this study, though this observation would benefit from a greater sample size. This indicates that the abilities (and therefore requirements) of a tactile device designed for use by medical professionals may require higher fidelity than one designed for use by non-medical persons.

The main implications of the second experiment is that tactile sensing greatly improves embedded object localisation ability. In addition, though it is beneficial to have tactile sensing capability on each finger, it may also be acceptable for only a single finger to be equipped with tactile sensing, providing the other finger (in the two finger scenario) can actively be used to collaborate with it.

S GRIPPER DESIGN

The results of the two experiments were used to inform the design of a 'haptically-enhanced' laparoscopic gripper capable of enabling active exploration of objects for identification and feature extraction. The design specifications are as follows:

• To utilise two fingers, fitting into the framework of current MIS tools.

• To enable operators to perform the most common exploratory procedures observed in Experiment 1. These are:

o Pressure o Lateral Motion o

o

o

Contour Following Object rolling Combinations of these EPs

• To accommodate a single tactile sensor, whose use may be enhanced via active exploration

• To fit into the existing framework of MIS robotics (specifically the da Vinci system. This implies:

o Compactness o Remote Actuation o No more than 3dof (current dof on present robot

surgical systems [3][9]) Based on these designs considerations, a gripper design was

proposed that incorporated a 1 dof 'finger' and a 2dof dexterous 'thumb' (Figure 9). The finger and thumb are driven remotely either via cables (as in the da Vinci system) or rigid push-rods, as in typical, manual laparoscopic tools. The da Vinci grippers (Figure 1) also utilise 3dof, though in the proposed gripper these are arranged in a single plane. This maintains a near-spherical workspace via tool shaft rotation but eliminates 'wrist' abduction.

B

Figure 9. CAD model of the gripper with example poses showing,

in S, re-orientation, folding and parallel grasp, C&D pinching and

sliding between different locations

In order to permit dexterity while maintaining compactness, a parallelogram structure was implemented for control of the thumb mechanism highlighted by a dotted line in Figure 9.A. This structure provides a method of creating motions that are similar to those of a serial chain manipulator, but with the added bonus of compactness and isolation of the actuators and drive mechanics away from subsequent links in the serial chain. Other dexterous 'tactile' gripper systems utilizing a non-closed serial chain structure may be found in [13][14]. The parallelogram structure has been implemented in user-driven haptic interfaces, such as the Phantom [15] and MasterFinger [16]. A major benefit of the mechanism in those systems is reduced inertia along the links of the device and increased stiffness.

The additional motion of the thumb afforded by the 2dof mechanism permits a number of exploratory procedures to be carried out more effectively than a typical scissor-like gripper. For

example, a variable point of contact between the finger and thumb (Figure 9, C and D) permits pinching, sliding and rolling actions. The additional rotary motion of the finger, coupled with the 2dof parallelogram of the thumb permits the entire gripping assembly to be rotated about its central pivot. This greatly increases the overall workspace of the gripper and also permits complex interactions such as the combined exploratory procedures illustrated in Figure 4. In addition, this allows the gripper to be rotated and folded into a minimal volume for deployment through a laparoscopic port (Figure 9. B). A video which accompanies this paper demonstrates the gripper dexterity in simulation.

Figure 10. The 3:1 scale prototype. The finger dof has

been locked and certain links exaggerated for ease of testing.

5.1 Physical Prototype A 3:1 scale prototype of the gripper has been constructed (where finger length = 65mm) and is currently under evaluation in terms of motion control and kinaesthetic sensing. This prototype will also be used to test the effect of the gripper motion on compliant objects, This interaction is expected to change in relation to contact surface materials e.g. performing rolling motion with an abrasive material may damage tissue under observation.

To facilitate ease of testing and fabrication at this stage, an oversize linkage mechanism has been put in place for actuating the parallelogram. The distance between the actuators and gripper has also been reduced for convenience.

For simplicity the finger dof has been fixed in place. All proposed motions may still be achieved by virtually moving the world co-ordinate system to reflect a moving finger. Actuation of the thumb is currently achieved via two Dynamixel RX-28 'smart actuators' from Robotis, which have been interfaced with Matlab.

5.2 Sensing The gripper is intended to support active tactile exploration of

objects by thumb manipulation, while the finger provides support and a tactile sensing platform. In the physical prototype the finger has been designed to accommodate a rigid platform or tactile sensor. This may be a 'typical' tactile array or a device such as the TACTIP [5]. This has been illustrated in Figure 11.

With a tactile sensor is not in place, it may still be possible to measure kinaesthetic force data based on actuator torques and the manipulator Jacobian. Experiments with custom built torque

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sensors are currently underway to detennine the effectiveness of this approach at determining the force vector experienced on the thumb linkage. If these prove feasible then a method will be provided for permitting haptic sensing outside the body, which may be easier and more cost effective to implement in practice than instrumenting MIS end effectors. Due to scaling effects, these initial results may not be applicable to a MIS scale devices, though this feature may still be useful for alternative remote tactile exploration applications.

Figure 11. CAD Model of the prototype, showing the

removable sensor platform and actuator linkages.

6 FUTURE WORK AND CHALLENGES

Currently, the prototype gripper is undergoing evaluation in terms of remote force sensing via actuator torque. The ability for the gripper to manipulate compliant objects in terms of stand­alone and combined EPs is also under investigation. As the gripper has been designed to be used in a non-autonomous, user driven scenario, it requires a human interface mechanism to drive the fingers. This will also require further development.

The inclusion of a tactile element into the gripper is planned, such as the TACTlP [5]. A comparison may then be made of the ability of the actuated thumb to enhance object feature extraction, as opposed to using such a tactile array independently. Both devices may be incorporated into a robot tele-manipulation system under development as part of this research.

A major challenge will be scaling the gripper prototype to appropriate MIS dimensions, by 3: 1 reduction. However, comparison of the proposed gripper with Figure 1 illustrates a comparable level of engineering complexity.

7 DISCUSSION

This paper has proposed a new gripper design for tele-haptic object manipulation in robotic surgery scenarios. A human­centered approach was taken to determine characteristics that would lend such a fingered device specifically to haptic sensing and manipulation of objects. These experiments highlighted the most prominent set of exploratory procedures that should be emulated by the device and also identified the acceptability of using a single tactile sensor, as opposed to one on each finger.

From the experimental results a gripper was designed which used a parallelogram structure to actuate the thumb. This provides the dexterity to achieve the most prominent EPs in a compact structure. A physical prototype is currently under evaluation. The

calculated abilities of this gripper match experimentally observed EPs executed by surgeons while also aligning with proposed robot MIS enhancements that were suggested during discussions. By permitting naturalistic exploration on MIS scale, it may be possible to significantly benefit MIS practice. In addition, the human basis for this gripper should make it, and the results of the study, useful for other tele-haptic applications.

8 ACKNOWLEDGEMENTS

Johnathon Henninger and the Record-Journal (Meriden, Connecticut) are thanked for the use of the image in Figure 1. Prof. Andy Levy fabricated the block used in Experiment 2. Thanks are obviously due to all experiment participants.

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