AWARDSDIGEST

GSC13,370-I

As shown in the Figures and more particularly in Figures i, 5

and 6, a split rail gripper in accordance with this invention is

comprised of a pair of rails 32, 34, separated by triangularly

arranged ball bearings 38, 40, 42, 44, which are driven in mutually

opposite directions by a rack and pinion gear mechanism, shown in

Figures 7, 8 and 9. There each rail includes a set of rack gear

teeth 106, 120, which engage respective pinion gears ii0, 116, and

where the top rail 32 engaging the gear Ii0 is driven by a drive

unit including a harmonic drive 60 and stepper motor 62 coupled to

an acme screw 58. The other pinion gear 116 is driven by the top

pinion gear ii0 and engages the set of rack gear teeth 120 included

in the bottom rail 34. As the top rail 32 is moved in or out by

the drive unit, the upper pinion gear ii0 is rotated, causing the

other pinion gear 116, in turn, to rotate in the opposite

direction. This causes the bottom rail 34 to move in an opposite

linear direction relative to the top rail 32. An outwardly

extending finger assembly 46, 48 is attached to respective ends of

the rails, with each finger comprising an arrangement of vertically

and horizontally mounted roller members 68-74, 80 and 84-90, 92

which operate to automatically center and engage an H plate type

interface 50 as shown in Figure 3 secured to the object 52 being

grasped (Figure 4). The gripper assembly also includes a base

plate 26 to which is attached a motor mount 64 including a load

cell as shown in Figure i0 for measuring the gripping force exerted

by the fingers 46, 48. A telescoping section type retractable

rotary tool driver 54 and tool 56 is also centrally mounted on the

base plate 26.

Novelty is believed to reside primarily in the concept of the

two rails being separated by four crossed-roller bearings which

form two stable triangles and gripper fingers each of which include

two pairs of horizontal rollers and an intermediate vertical roller

whose axes lie in a common plane.

Inventor:

Employer:

George M. Voellmer

National Aeronautics and Space Administration
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SPLIT RAIL GRIPPER ASSEMBLY AND TOOL DRIVER THEREFOR

Origin of the Invention

This invention was made by an employee of the U.S.

Government and accordingly may be manufactured and used

by or for the Government for governmental purposes

without the payment of any royalties thereon or therefor.

Field of the Invention

The present invention relates generally to robotics

and more particularly to apparatus for robotically

gripping objects, for example, in zero gravity

environments and to operate a retractable rotary tool.

Background of the Invention

Where there is a need to position and remotely

control machinery with a high degree of precision, robots

are often used to perform these tasks. Robots in the

past have been utilized to perform both simple and

complex tasks in various environments, particularly where

human activity is inaccessible, prohibited or relatively

hazardous. More recently robotic type apparatus has been

put to use in outer space. Typically where such robots

are used, either tools or end-effectors are attached to

movable robot arms so that objects can be grasped,

lifted, manipulated and otherwise operated upon.

One known type of end-effector comprises a gripper

comprised of two mutually opposing jaws that are movable

with respect to one another in opposite directions by a

rack and pinion gear mechanism. Such apparatus is shown,

for example, in U.S. Patent 4,707,013 issued to John

Vranish et al on November 17, 1987, and which is herein

meant to be incorporated by reference. There the gripper

includes a pair of rails having parallel thrown motion

with a gripper finger sub-assembly attached to opposite

ends of each rail. A pair of preloaded linear bearings

engage the outward surfaces of each rail and two roller
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bearings spaced on either side of a pinion, all mounted

on a common pinion shaft, engage the inward sidewalls of

each rail. The rails are typically driven by an electric

motor.

Further, the gripper design as disclosed in U.S.

Patent 4,707,013 incorporates separate racks that need to

be first fabricated and then bolted to the rails,

increasing the weight and bulk and making it relatively

complicated to assemble. In order to achieve sufficient

strength in the pinion shaft bearing, moreover, the back

of the gripper housing protrudes significantly, further

adding to the weight and bulk of the gripper. Also, when

the fingers experience a sideload, the rails tend to move

sideways relative to one another. Because the ends of

the other sides of the rails are fixed, it has the effect

of spreading the housing, eventually bending it open,

causing preload on the bearings to be lost.

Strain gauges are also mounted on the fingers to

sense gripper pressure, with service loops connecting to

the gripper base. The service loops being subject to

many cycles of flexing, have a tendency to snag on other

parts of the gripper. Moreover, finger mounted strain

gauges are easily damaged, if the part being gripped is

inadvertently gripped on the finger shanks. Furthermore,

strain gauges require individual calibration for each set

of fingers to obtain reliable readings since each

mounting situation is relatively different and the

fingers vary slightly from element to element. Also,

finger mounted strain gauges are sensitive to loads other

than the finger grip force such as sideloads.

It should also be noted that rotary tools,

particularly those mounted on robot end-effectors, are

generally not retractable and fully protrude into the
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work space and as a result, generally require an actuator

to release them.

Summary

Accordingly, it is an object of the present

invention to provide an improvement in robotic apparatus.

It is another object of the invention to provide an

improvement in a robotic gripper which is operable in a

zero gravity environment.

It is a further object of the invention to provide

an improvement in split rail robotic gripper apparatus

for operation in outer space.

And yet another object of the invention is to

provide an improved split rail robotic gripper which is

fitted with a retractable rotary tool for operating on a

workpiece being gripped.

Briefly, the foregoing and other objects are

achieved by a split rail gripper comprised of a pair of

rails which are driven in mutually opposite directions by

a rack and pinion gear mechanism, wherein each rail

includes a set of rack gear teeth which engage respective

pinion gears and where the top rail engaging one of the

pinion gears is driven by a harmonic drive and motor unit

coupled to an acme screw. The other pinion gear is

driven by the top pinion gear and engages the set of rack

gear teeth included in the bottom rail. As the top rail

is moved in or out by the drive unit, the upper pinion

gear is rotated, causing the other pinion gear, in turn,

to rotate in the opposite direction. This causes the

bottom rail to move in an opposite linear direction

relative to the top rail. An outwardly extending finger

assembly is attached to respective ends of the rails,

with each finger comprising an arrangement of vertically

and horizontally mounted roller members which operate to
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automatically center and engage an H plate type interface

secured to the object being grasped. The gripper

assembly also includes a base plate attached to an

interface plate of a robotic tool changer mechanism. A

retractable rotary tool driver and tool is centrally

mounted on the base plate.

Brief Description of the Drawinqs

Other features, advantages, and objectives of the

invention will be more readily apparent when the

following detailed description of the invention is

considered in conjunction with the accompanying drawings,

wherein:

Figure 1 is a perspective view generally

illustrative of the split rail gripper in accordance with

the preferred embodiment of the invention;

Figure 2 is a partial perspective view being

illustrative of the roller members located at the upper

end of the gripper fingers;

Figure 3 is a perspective view illustrative of an Hplate type interface member which is engageable with the

rollers of the gripper fingers shown in Figure i;

Figure 4 is a partial front plan view illustrating

the gripper fingers engaging a workpiece and with the

retractable tool holder and tool shown in Figure 1

extended into the workpiece;

Figure 5 is an end plan view of the embodiment shown

in Figure i;

Figure 6 is a top plan view further illustrative of

the embodiment shown in Figure I;

Figure 7 is a sectional view taken along the lines

7-7 of Figure 6 and being illustrative of the bearing

configuration between the top and bottom rails;
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Figure 8 is a partial schematic view illustrative of

the rack and pinion gear assembly included in the

embodiment shown in Figure i;

Figure 9 is a sectional view taken along the lines

9-9 of Figure 6 and being further illustrative of the

rack and gear pinion gear assembly for moving the top and

bottom rails;

Figure i0 is a partial cross-sectional view

illustrative of the details of the strain gauge motor

mount for measuring the force exerted by the gripper

fingers shown in Figure I; and

Figure II is a partial end planar view illustrative

of the means for coupling the acme screw shown in Figure

1 to the top rail gripper finger.

Detailed Description of the Invention

Referring now to the drawings wherein like

references numerals refer to like components throughout,

reference is first made to Figure 1 where there is shown

what is considered to be the preferred embodiment of the

invention and wherein reference numeral 20 denotes a

gripper and tool driving assembly which is adapted to be

grasped by a robotic arm, not shown, via an interface

plate 22 of a robotic tool changer mechanism, not shown.

Such a tool changer mechanism, however, is disclosed in

U.S. Serial No. 07/608,657 (GSC 13,239-1), entitled,

"Robotic Tool Change Mechanism", which was filed on

behalf of the present inventor on November 2, 1990, and

which is also intended to be incorporated herein by

reference. On the rear edge of the interface plate 22

there is an electrical connector 24 which is also adapted

to be connected to the robotic arm to which the gripper

assembly 20 is attached.
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The gripper assembly is primarily comprised of a

baseplate 26 which mounted on a tool motor drive assembly

28 secured to the interface plate 22 and including

electrical drive motor 30 extending out to one side. The

gripper itself is comprised of two elongated rails,

namely a top rail 32 and a bottom rail 34, which are

operable to extend partially out of a housing 36 and

being mutually separated for crossed roller bearings,

which is shown by reference numerals 38, 40, 42 and 44 as

shown in Figures 5 and 7 and which will be discussed in

further detail subsequently.

Attached to opposite ends of the top and bottom

rails 32 and 34 are a pair of outwardly projecting finger

members 46 and 48 which are adapted to move toward and

away from each other to grip an H plate interface member

50 shown in Figure 3 and to which is attached a workpiece

as shown in phantom view by reference numeral 52 of

Figure 4 and which is to be operated on by a retractable

tool mechanism 54 containing a rotatable tool 56.

Movement of the gripper fingers 46 and 48 is controlled

by an acme screw member 58 connected between the lower

end of the_finger member 46 attached to the upper rail 32

and a harmonic drive gear reduction mechanism 60 and

electric motor 62 which are secured to a motor mount and

base structure 64 affixed to the baseplate 26 by a

housing 61. The motor mount 64 also includes a load cell

66 as shown in Figure i0 for measuring the gripping

reaction forces on the finger elements 46 and 48.

Referring now to Figure 2, shown thereat are the

details of the gripper elements located at the outer end

of the fingers 46 and 48. As shown, the end of the

finger 46 includes first and second pairs of rollers 68,

70 and 72, 74 which are respectively aligned on a pair of
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parallel horizontal axes 76 and 78. A fifth roller 80 is

located between the rollers 68, 70 and 72, 74 at mutually

right angles thereto and is aligned along a vertical axis

82 which intersects the parallel axes 76 and 78. In a

like manner, the opposite finger 48 includes the same

arrangement which is shown in Figure i; however, only

rollers 84, 88 and 92 are shown consisting of an upper

pair of horizontal rollers 84, 86, a lower pair of

rollers 88 and 90, and an intermediate orthogonal

vertical roller 92. The upper pair of rollers 84 and 86

are depicted in the top planar view of Figure 6.

The configuration of the roller members located at

the ends of the two fingers 46 and 48 are operative to

automatically position and grip the opposite ends of the

H plate type interface member 50 shown in Figure 3.

Referring now briefly to Figure 3, the interface plate 50

is comprised of beveled bifurcated ends of reduced with

including pairs of end portions 94, 96 and 98, i00,

between which are located two generally circular recesses

102 and 103.

In operation, as the two finger elements 46 and 48

are moved inwardly toward one another, the roller members

at the ends thereof will contact the end portions 94, 96,

and 98, I00, and automatically guide the plate 50 into

position such that the recesses 102 and 103 straddle the

two intermediate vertical rollers 80 and 92 as shown in

Figure 4. This permits the retractable tool driver 54,

which includes a plurality of telescoping sections 104,

to be extended and position the tool 56, which may be,

for example, a screw driver or a nut runner, at the

workpiece 52 which is attached to the plate 50. As

shown, the tool 56 comprises an allen wrench or key.
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This now leads to consideration of the principal

feature of the invention which is directed at the details

of the way in which the two rails, the top rail 32 and

the bottom rail 34, are supported and driven to control

the position the two fingers 46 and 48.

As shown in Figures 5 through 9 and more

particularly in Figure 7, the two rails 32 and 34 are

located between the two bearings 38 and 42, while the

other two bearings 40 and 44 are located one on top of

the upper rail 32 and the other below the lower rail 34.

Each rail 32 and 34 is accordingly supported by three

intermediate bearings forming two stable triangles within

the housing 36 which also acts as a housing to hold the

various parts together.

Further as shown, the upper rail 32 includes a set

of rack gear teeth 106 which engage the teeth 108 of an

adjoining pinion gear ii0 which is aligned along the axis

112, being mounted on a shaft 113 and supported by three

sets of roller bearings 114, 116 and 118 and which might

be referred to as a vertical axis parallel to the axis 82

shown in Figure 2 for the roller member 80. The pinion

gear 108 is thus driven by translational motion of the

upper rail 32 which is imparted by a linear motion of the

acme screw 58 (Fig. i) as will be shown.

The pinion gear ii0 engages the teeth 114 of a

second pinion gear 116 which is aligned along a second

vertical axis 118 adjacent the axis 112 as shown in

Figure 9. The gear teeth 114 extend downwardly to engage

the gear teeth 120 of a rack formed on the side of the

lower rail 34 as depicted in Figure 8. The elongated

pinion gear 116, moreover, is supported by two pairs of

bearings 122, 124 and 126, 128 (Fig. 9). When desirable,

the single elongated pinion gear 116 can be implemented
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by two smaller pinion gears mounted on the shaft 130 with

one of the gears engaging the pinion ii0, while the other

engages the rack teeth 120 of the lower rail 34.

As noted above, the movement of the fingers 46 and

48 as controlled by operation of the two rails 32 and 34

in mutually opposite directions is provided by the acme

screw 58. The acme screw 58 as shown in Figure 6, for

example, is operated by a motor driven harmonic drive

gear reduction mechanism 60 which rotates a nut 130,

shown schematically in Figure i0. A harmonic drive is a

well known device and typically consists of a wave

generator member coupled to a shaft, a flexible cup-type

spline and a circular spline which is attached to a

motion coupling element. In the instant invention, the

harmonic drive is preferably comprised of a pancake type

harmonic drive and includes a dynamic circular spline,

not shown, which is used to transfer its rotary output to

the nut 130. The pancake type harmonic drive 60 is

furthermore hollow so that the acme screw 58 can pass

therethrough. Rotation of the nut 130 causes the acme

screw to exert a pulling or pushing force on the rails by

virtue of its connection to the upper rail 32. As shown

in Figure i0, a motor 62 provides rotary input to the

harmonic drive 60. The motor 62 typically comprises a

hollow brushless DC motor so that the acme screw 58 can

also pass axially therethrough.

The far end of the acme screw 58 terminates in a

ball and socket joint including a flattened socket member

132 and a ball 133 which fits into an open ended

rectangular end piece 134 secured to a rib section 135

projecting from the lower end of the finger 46 as shown

in Figures 1 and 5. The ball 132 includes a through hole

so as to accommodate a shoulder bolt 136 which fastens
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the acme screw to the end piece 134. This type of

coupling prevents the acme screw 58 from being turned and

thus the rotating nut acts to linearly translate the acme

screw 58 in and out of the harmonic drive 60 and motor

62. The shoulder bolt 136 is threaded such that it can

be manually removed, when required, by inserting an allen

screw, for example, at the head end 137 for disconnecting

the acme screw from the rail 32 and thus releasing the

fingers 46 and 48.

The manner in which the acme screw 58 is attached to

the rail 32 via the end piece 134 insures sufficient

energy is being provided to the screw 58 to break it free

should it become frozen. Sufficient backlash is built

into the attachment point between the acme screw 58 and

the end piece 134 so that the socket end of the screw can

rotate a few degrees on the ball 132 before encountering

a stop. This allows the motor 62 on the input side of

the gear reduction harmonic drive 60 to make one and one

half free rotations before experiencing any load. In

this fashion, sufficient kinetic energy can be built up

to overcome large sticking forces between the screw 58

and the nut 130.

The gripping force exerted by the two fingers 46 and

48 is sensed by the load cell 66 as shown in Figure i0.

The motor mount 64 includes top and bottom wall sections

138 and 140 accompanied by a hollowed mid section 142

which accommodates the load cell 66 and a pair of

transverse relatively thin blade type connecting elements

144 and 146 between the wall sections 138 and 140. The

friction between the acme screw 58 and the drive nut 130

is all in rotation. Accordingly, any pulling force is

exerted on the blade elements 144 and 146 along the axis

of the acme screw. The blade elements 144 and 146 are
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stiff in all other axes and rotations. With the load

cell coupled between the upper and lower wall sections

138 and 140 of the motor mount 64, it can measure the

force that the drive motor 62 is exerting on the base and

which is a direct function of the gripping force exerted

by the fingers 46 and 48 through the harmonic drive gear

reducer 60 and the acme screw.

With respect to the rotary tool mechanism 54 as

shown in Figure i, as noted above, it includes one or

more telescoping sections 104 as well as the tool tip

itself 56 which may be, for example, an allen key. The

telescoping sections 104 are spring loaded and include

keyway grooves, not shown, which permit the tool tip 56

to move axially while transmitting torque. When the tool

tip 56 is inserted into an allen head, for example, on

the workpiece 52 as shown in Figure 4, the spring loaded

sections 104 provide compliance in the axial direction

and press the tool 56 into the head. The final keyway

where the tool tip is attached to the first section has

a loose fit to allow the tool tip a small amount of

lateral compliance, which is important for robotic

applications. This compliance is not necessary, however,

for operation of the tool 56 itself. When the rotary

tool is no longer required, the telescoping sections 104

are axially compressed causing all the sections to

collapse until the rear end of the tool tip shaft engages

a catch, not shown, at the base of the tool. The catch

keeps the tool 56 fully retracted until it is released.

A release is achieved by activating the tool drive motor

30, whereupon the spring loaded telescoping sections 104

then extend to its full length.
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Thus what has been shown is an improved workpiece

gripper mechanism for robotically grasping objects in a

zero gravity environment.

Having thus shown and described what is at present

considered to be the preferred embodiment of the

invention, it should be noted that the same has been made

by way of illustration and not limitation. Accordingly,

all modifications, alterations and changes coming within

the spirit and scope of the invention are herein meant to

be included.
GSC13,370-I

ABSTRACT

SPLIT RAIL GRIPPER ASSEMBLY AND TOOL DRIVER THEREFOR

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A split rail gripper for robotic apparatus and

including a pair of rails which are driven in mutually

opposite directions by a rack and pinion gear mechanism.

Each rail includes a set of rack gear teeth which engage

respective pinion gears and where the top rail engaging

one of the pinion gears is driven by a harmonic gear

reduction drive and motor unit coupled to an acme screw.

The other pinion gear is driven by the top pinion gear

engaging a set of rack gear teeth included in the bottom

rail. As the top rail is driven in or out, the upper

pinion gear is rotated, causing the other pinion gear, in

turn, to rotate in the opposite direction. This causes

the bottom rail to move in an opposite linear direction

relative to the top rail. An outwardly extending gripper

finger assembly is attached to respective ends of the

rails, with each gripper finger including an arrangement

of vertically and horizontally mounted roller members

which operate to automatically center and engage an Hplate type interface secured to the object being grasped.

The gripper assembly also includes a base plate attached

to an interface plate of a robotic tool changer

mechanism. A retractable rotary tool driver and tool is

also centrally mounted on the base plate.
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