Proceeding of the 6th International Symposium on Artificial Intelligence and Robotics & Automation in Space: i-SAIRAS 2001, Canadian Space Agency, St-Hubert, Quebec, Canada, June 18-22, 2001.

Space Robot Hand for the General Purpose On-orbit Task ---for Robot Un-friendly Work by Space Robot---

Kohtaro Matsumoto, Sachiko Wakabayashi(NAL),

Ichiro Kawabuchi (TechExperts), Takefumi Terashita(Hosei Univ.)

National Aerospace Laboratory 7-44-1, Jindaiji-Higashi machi, Chofu, Tokyo, Japan 182-8522

matumoto@nal.go.jp

Key Word: Robot Hand, Human Hand, Finger, Robot Un-friendly

Abstract In this paper, we will describe a preliminary result of the small robot hand, developed as a mock of human hand and for the general robot un-friendly task capability in the orbit. Using the commercial motors and planetary gear system, we build a small finger. The size is slightly larger than human finger, but can hold and carry small objects even on the ground. Every joint can be controlled independently to realize the dexterous human hand action. A simple graphic simulation for the teleoperation is proposed to decide the finger joint angle by the collision detection.

1. Introduction

From Space Shuttle, ETS-VII, to International Space Station, the hand of the practical space robot arms had been developed under the assumption that the work objects shall have the very specific holding tool, the exclusive grapple fixture (GPF). The shuttle arm GPF is about 50cm diameter and even the JEM small fine arm GPF is about 14cm diameter. Both GPF are extremely large, and limit the possible works for the space robot, because of the very limited parts and/or components, where those GPF can be attached.

On the other hand, the human space systems, such as Space Shuttle, assume service works and maintenance tasks by astronauts, as extra vehicular activities (EVA). As the EVA effects for the space station, the space suit glove of the five fingers enable the various and flexible design of the outboard equipment with various human maintenance work, although the space glove works

are difficult to say to be excellent. Not only from the ARAMIS[1] report of the

early Space Station Project, the expectations for the space robot on-orbit servicing system are emphasized widely as the supplements and/or alternatives of the dangerous human EVA. It is exactly possible for the space robots to go and work even at the polar orbit, the geostationary orbit, or the Lagrange points, where human being could not go.

However, almost all on-orbit advanced servicing systems, such as the OMV (Orbital Maneuvering Vehicle) and FTS (Flying Telerobot Servicer), except the ETS-7 and SPDM (Special Purpose Dexterous Manipulator system-hus, tasks of current space robots are limited only, so to speak, simple crane work, such as transportation and the assembly of a large module on the orbit, until the SPDM. It is no exaggeration to say that the manned EVA work is the only practical maintenance function for the on-orbit Hubble telescope repair, in the present space system, although it is dangerous.

Although robots are used very actively in a various commercial product line on the ground, the space robots are used only for the crane work level, on the orbit. One of the main reasons for this gap might be the exclusive GPF demand. Robot works, widely active on the ground, are focused to the works of a small kind, in large quantities, and repeatedly. As a matter of course, “robot-friendly” has been assumed for the work objects, those shall have the specific tools and/or shapes for the robot work.

However for the EVA tasks on-orbit, various and rare (small amount) work, etc. are essential, and such "non robot-friendly" work is the weakest field for the robot even on the ground. The exclusive GPF/tool demand for the space robot work is to

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demand the environment of various on-orbits service as essentially same “robot-friendly” as the ground. As the result, the space robots have shut various work possibilities by themselves.

For the various on-orbit service possibilities, it is necessary to design the future space robot based on the assumption of non robot-friendly work environment. To adopt the future robot to the on-orbit human oriented and various work environment, the next generation end effector shall be essential, that is very similar in its size and function to the flexible human hand for the dangerous human EVA. This next generation hand, with multi fingers, can have the various work possibilities and remarkably improve the adaptability to the diverse work object as the entrance.

2. General Purpose Hand A lot of hands, like human hand, are reported

until now. Most of those are the one that so-called robot end-effector with the touch sensor etc., but with less flexibility and larger size than a human hand. A few hands were developed aiming at the mock of human hand operations and its size. [2][3][4]

In the following, we set the following two points as our basic targets of our hand for the general-purpose EVA tasks. (Fig. 1)

・ How is the size close to human hand? ・ How flexible can it imitate human hand’s

movement?

2.1. Mock of Human Hand Size Recently, the miniaturization of motors is

drastically advanced to support the development of electronic equipments, medical technology, or a micro magic hand, etc. If the working torque requirement is not so severe, the motor, which can be accommodated within the human phalanx, is marketed.

In this paper, we aimed to demonstrate the work force of the electric switches operation under 1G

environment, with storing all the mechanical parts within the human hand size level. The deficiency of the motor working torque will be subsided to twice by the motor custom design instead of the marketed motors, shown in the following.

The length of human phalanxes are not same, and has shortened to the tip of a finger. It is about 5cm length in the root part though 2.5cm-3cm length in shortest phalanx. The thickness is about 2cm. Thus we aimed at about 5cm in length with 2cm thickness, as the first mock specification mainly because of the commercial motor specification limit. (Fig. 2, Fig. 3)

2.2. Hand Joint

Some human joint has two DOF (degree of freedom), differs from the robot joint. Human hand and finger can be expressed as follows as the robot joint.

(1) Thumb : (R-P)-P-P (2) Forefinger to Littlefinger : Y-P-P-P

The movement of thumb root part can be expressed as the dual axis joint. The movement of other fingers shall be expressed as four joint

Fig. 1 Mock Concept of Human Hand

Fig. 2 Assembled Hand

Fig. 3 Comparison of Assembled Hand

and Male Hand

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movement. In this paper, the fourth joint at the finger root

part were set as the future subject, since the action of the fourth joints depends heavily on the finger position, and the most critical difficulty of the human size hand is the accommodation of the mechanical part of the finger into the phalanx size. Thus the realized fingers are composed as P-P-P DOF set.

3. Finger Design In this section, our finger joint mechanism

design will be described. 3.1. Target Tasks Requirement The work force requirement for the hand is assumed roughly as follows:

(1) Torque which can be put out at the finger tip position, like switch operation etc.

(2) Finger holding power of weight material of constant load.

It is comparatively not difficult to improve the holding power (2) by adding some simple brake mechanism to the driving motor and/or the gear of a joint drive part.

The work power (1) is more severe requirement to demand the direct motor driving ability improvement in finger joint mechanism.

In toggle switch for the manned operation,

variously used in space, 250-1500gf is necessary as the work power.

In case of 5cm phalanx, as our design target, the necessary torque in the finger root axis becomes 0.3-1.5Nm, and exceeds some specification limits of a micro motor and the final reduction gear.

3.2. Micro Motor Selection

For a commercial micro motor of φ20mm or less, the speed of rotation is around 7800-18600 rpm and the start torque can be expected about 0.4mNm-11.6mNm.

If the final reduction gear ratio is enlarged, it is theoretically possible to realize the necessary finger tip torque. But, even if we design a too high final reduction gear ratio, because of the gear internal efficiency problem, desired high output torque cannot be expected. Moreover, the high final reduction ratio gear also prevent the quick movement of the finger tip.

Our design target was set as about 1 Nm effective torque with about 1 sec movement for 90° finger open and close operation.

Table 1 shows specification examples of micro motors, used for our finger. To expect a necessary performance by the finger thickness 20mm and about 50mm finger length, 1524SR and 1516SR micro motor were selected. For the 1524SR, if we implement about 500 final reduction gear ratio, ideally 3.4 Nm can be expected as the maximum starting torque with 19 rpm, from specification of the no load rotation speed 9900 rpm and start torque 6.76mNm.

The maximum operation torque of a micro motor is about 50% of the start torque. Also gear internal efficiency and the load of the weight of the finger will decrease the total performance. Thus, using these motors, 1.5-2.5 sec for 90° finger open and close operation, with about 0.8 Nm finger torque, could be expected. 3.3. Joint Drive Mechanism

Since the motor length of 1524SR is 29.1mm with pinion gear, the diameter and length of the final reduction gear system shall be designed within the limit of 20mm. Also, since the motor is to be accommodated serially within the phalanx, a bevel gear or a crown gear shall be accommodated in the joint mechanism to rotate vertically to the phalanx. Thus we designed custom reduction gear system with small size and large reduction rate.

Fig. 5 shows our joint mechanism. For the axis orthogonalization, a crown gear is used as the first stage. For the large reduction rate, 2 stages planetary gear train is adopted. (Fig. 4)

The output of planetary gear can be taken from the inside cogwheel or carrier gear. (Fig. 5)

Table 1 Specification of Micro Motors[5]

Diameter(mm)

Length (mm)

Initial Torque(mNm)

Unloaded Rev. (rpm)

Weight (g)

0816S 8.0 25.2 0.4 16900 3.5 1016G 10.0 25.1 0.9 18400 6.5 1212G 12.0 21.4 0.3 18600 6.5 1219G 12.0 27.9 1.2 16000 11.0 1319S 13.0 24.3 3.6 16100 11.0 1331S 13.0 36.5 8.9 11300 20.0 1336C 13.0 47.0 8.4 9000 23.0 1516S 15.0 21.1 1.0 15800 10.0 1516SR 15.0 21.1 1.6 12800 13.0 1524S 15.0 29.1 2.6 15000 18.0

1524SR 15.0 29.1 6.8 9900 21.0 1616S 16.0 21.1 1.1 16300 12.0 1624S 16.0 29.1 5.2 13800 21.0 1717S 17.0 28.1 4.9 13000 17.0 1724S 17.0 35.1 10.5 8000 26.0 1727C 17.0 38.1 11.6 7800 28.0 2017S 20.0 26.2 2.3 13200 21.0 2025S 20.0 33.7 7.5 8600 42.0

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The axis of the reduction gear is set as common with the joint axis compactly. In our finger, it is important to decrease the inclination of the load at the joint rotation axis, which works with relatively a large torque as much as possible. The outer case of inside cogwheel was used as the output element, and the output load was arranged in about central of the joint axis.

To miniaturize the mechanism and to facilitate the machining, two cogwheels for the two stages planetary gear mechanism are made as unified one. The unified cogwheel works as a fixed element at the 1st planetary gear stage, and receives the output from the second stage planetary gear system as a cogwheel. Since the cogwheel, which works as a fixed element at the 1st planetary gear stage, also rotates with the joint rotation at the 2nd planetary gear stage, integrated reduction ratio becomes as follows:

Zs : sun gear ratio, Zi : cogwheel ratio, Zp : planet gear ratio Wi : rotation speed of 1st sun gear Wc : rotation speed of 1st carrier

Wo : rotation speed of 2nd cogwheel Zp = (Zi – Zs) / 2 Zs*Wi = (Zs+Zi)*WC + Zi*Wo (1) Zs*Wc = Zi*Wo (2) Wo/Wi = Zs2/(2Zs*Zi + Zi2) (3)

Using this unified cogwheel, each planetary gear behavior is described in formula (1) & (2). The second term in (1) represent the effect of rotation of the fixed cogwheel. The final reduction ratio is Wo/Wi.

In our finger implementation, Zs, Zi, Zp are set as 14, 37, 88. The gear ratio of the pinion and the crown are 9 & 90. Thus the overall reduction ratio becomes about 1/520.

Fig. 6 Joint Mechanics

Fig. 7 Outside Image of a Joint

Fig. 5 Joint Mechanics

Fig. 4 Planetary Gear

Crown Gear & Sun Gear of 1st

Planetary Gear (Wi:Revolution)

Carrier & 2nd Sun Gear (Wc:Revolution)

Cogwheel (Wo:Output Revolution)

Axis of the Joint & Reduction Gear

1st Stage

2nd Stage

Motor

Carrior

Sun Gear (Ratio:Zs)

Cogwheel (Ratio:Zi)

Planetary Gear (Ratio:Zp)

50.7mm Motor Pitch

Crown Gear(R:90) 1st Sun Gear (R:14)

1st Planetary (R:37) Carrier & 2nd Sun Gear (R:14)

Motor on Phalanx-B

Motor on Phalanx-A

Cogwheel on B (R:88) Phalanx B

Planetary Gear on B (R:37)

Phalanx A

Upper Cross Section

Side Cross Section

Cogwheel Sun Gear

Planetary Gear

Side Outlook

Joint Axis

Motor Pinion (R:9)

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3.4. Finger Assembled Fig. 6 shows the overall joint mechanism with

the unified cogwheel two stage planetary gear. Fig. 7 and Fig. 8 show the assembled finger. The weight of the finger is about 200g. The length is about 16cm. The length of one phalanx is less than 50mm.

4. Graphic Simulation of Grasping by Finger

Since the DOF of finger joints become 12 even at the three finger case, the analytical method is not practical to the joint value decision for object grasping, at present. Thus in most multi finger/ human like hand operation, direct teleoperation by the human operators hand posture shape are used. The hand posture is decided by operator hand joint angle sensors. However, for the long communication delay teleoperation of the hand operation, precise predictive simulation and display are essential requirements for delicate tasks for human like hand. In our simulation, we approximate,

Grasping operation => Bending fingers. Under this approximation, we made the following scenario for grasping operation;

Grasp => {Bending each finger joint one by one until

coming in contact with the object}. As a simple example, if the finger is

approximated by a square pillar, and the object by a sphere, the contact between those two is detected analytically as a problem of the interference between a plane and a sphere. When the holding object is a polyhedron, the contact (the angle of the joint) can be decided by the interference between a plane and a ridge line. Such contact detection is achieved as a collision detection function in the robot simulator etc. Those asymptotic contact detection are fast to be calculated, and easy to decide the finger angle at semi real time on a recent, high-speed computer.

Thus in our simulation, the finger posture is decided one by one, from the finger root phalanx contact to the finger tip phalanx contact. (Fig. 9)

5. Examination Results In this section, the preliminary examination

results will be described.

5.1. Finger Operation: Each finger can be set to an arbitrary posture

under 1G environments. Fig. 10 shows the entire three fingers posture.

The 90° open and close operation of a finger almost required about six seconds for each joint. This operation speed is about 1/3 of the designed speed. For the faster operation speed, adjusting the parameter of the control system will be examined in the future.

5.2. Static Pressure:

The static pressure of about 300g in stable and 600g as the peak value were demonstrated by the finger posture of Fig. 10, aiming at under vertical. This corresponds 0.3-0.6Nm joint torque at the finger root part joint, and is about half the

Fig. 8 Assembled Finger

Fig. 9 Graphic Simulation of Grasping by Finger

Fig. 10 Hand for Electric Switch Toggling

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performance of the designed value. The improvement of the momentary maximum

static pressure will be expected, if the control system will become able to accept the overload state. However, for the improvement of the stable static pressure, the sun gear of the 2nd stage planetary gear will be re-designed and/or re-machined, since the torque at this sun gear might be near the mechanical strength limit.

5.3. Switch Operation: As one of the hand design target work, the toggle

switch operation examination was done. For a small toggle switch, both a single axis operation of the finger tip and two axes operation in the finger middle were completed.

6. Concluding Remark For the all-purpose EVA tasks, the finger/hand is

designed to replicate the human finger in its size and freedom of motion. The finger joints are directly driven by the motor and planetary gear systems.

The graphical simulation for the automatic capturing of a work object for the teleoperation is introduced.

For the non robot-friendly EVA work, we will develop the hand palm mechanism to replicate the human hand action. The operation demonstration for the major EVA tasks, and its teleoperation will be done in the future.

Reference [1]D.L.Akin etal “Space Application of Automation, Robotics and Machine Intelligence Systems (ARAMIS)-Phase II”, NASA CR-3834, 1983 [2]H. Kawasaki and T. Komatsu. “Mechanism Design of Anthropomorphic Robot Hand: Gifu Hand I” J. of Robotics and Mechatronics, Vol. 11, No.4, pp.269-273, 1999 [3]http://vesuvius.jsc.nasa.gov/er_er/html/robonaut/robonaut.html [4]G. Hirzinger etal “A Mechatronics Approach to the design of light –weight arms and multifingered hand” Proc. IEEE-ICRA, April, 2000 [5]Main catalogue 2000 of minimotor SA of

Faulhaber, http://www.minimotor.ch/uk/

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