Spiral lead platen robotic end effector

A robotic end effector is disclosed which makes use of a rotating platen (1) with spiral leads (2,3) used to impact lateral motion to gripping fingers (46,48). Actuation is provided by the contact of rolling pins (54,56) with the walls of the leads. The use of the disclosed method of actuation avoids jamming and provides excellent mechanical advantage while remaining light in weight and durable. The entire end effector is compact and easily adapted for attachment to robotic arms currently in use.

TECHNICAL FIELD OF THE INVENTON 
This invention relates generally to the field of robotics, and more 
specifically to the mechanism of a general purpose robotic end effector. 
BACKGROUND OF THE INVENTION 
Robotic end effectors are used to handle a variety of materials in the 
performance of repetitive tasks and to act as remote manipulators in 
hazardous or isolated environments. Although the tasks required of these 
mechanisms are diverse, to varying degrees they share common requirements 
for precision positioning, computerized control, resistance to jamming, 
and durability. Additionally, many tasks require a combination of compact 
size and high gripping forces. Finally, in specialized areas such as space 
construction, there are high premiums on high strength for weight and high 
penalties for mechanical failure. 
Prior art has attempted to answer these competing demands, but sometimes 
with unacceptable compromises. For instance, when high gripping forces are 
obtained, the tendency of the mechanism to jam also rises. Additionally, 
high forces applied to complicated gear trains or transmission linkages 
promote wear and ultimately result in positioning errors. 
Space applications require designers to coordinate exceptional demands for 
small size and light weight with those for power and high reliability. The 
goals for robotic manipulators include having the smallest motor possible 
coupled with a very strong and simple linkage which has a high mechanical 
advantage but will not jam under load. It is also desirable to have an end 
effector which, by the simple expedient of replacing a part or two, can be 
customized as to rate of motion and mechanical advantage without complete 
re-design of the mechanism. 
Remote control of the gripping function requires the ability to determine 
the path necessary for the grippers to follow, as well as the ability to 
precisely position the grippers on the work piece. Both of these tasks, 
whether pre-programmed or performed interactively, are facilitated by true 
parallel gripper motion in a single axis of freedom. Many end effectors in 
present use have grippers which move both axially and radially when 
actuated. Parallel gripper motion minimizes both programming difficulties 
and operator errors. 
Therefore, an object of the present invention is to provide an end effector 
mechanism which provides high gripping force through high mechanical 
advantage, while at the same time being resistant to jamming while under 
load. 
A further object of the present invention is to provide an end effector 
with true parallel opposed jaw movement so as to provide advantages in 
programming and positioning over those prior art designs in which the jaws 
move both radially and axially upon actuation. 
A further object of the present invention is to provide an end effector 
with as small a profile as possible so as to facilitate maneuvering in 
confined areas. 
A further object of the present invention is to provide an end effector 
with a low probability of mechanical failure for applications such as 
radiological waste handling and space flight where failure costs are 
particularly high. 
A further object of the present invention is to provide an end effector 
with a high ratio of grip strength to weight for applications such as 
space construction which place a premium on weight. 
A further object of the present invention is to provide an end effector 
with highly accurate and repeatable gripper positioning by designing for 
optimally small clearances and very low operating friction. 
A further object of the present invention is to provide an end effector 
with easily modifiable rate of motion and mechanical advantage 
characteristics so that it can be easily customized for specific 
applications. 
BRIEF SUMMARY OF THE INVENTION 
According to the invention, the foregoing and additional objects are 
attained by an end effector mechanism utilizing a moving inclined plane in 
the form of a rotating platen with a spiral lead in the form of a groove 
in the upper surface thereof to convert rotary motion into linear motion. 
Gripping fingers are actuated by contact with the sides of the spiral lead 
as the platen turns. Because these fingers are mechanically constrained so 
as to be free to move in only one axis, the rotation of the platen results 
in linear movement by the grippers. 
A means of rotating the platen is included which comprises a motor along 
with appropriate gearing to modify the rate of rotation and torque applied 
to the platen by the motor. 
All moving parts of the end effector are mounted in a rigid frame, which 
includes mounting surfaces for attaching the end effector to any standard 
robotic arm by customary methods. 
In the preferred embodiment, the generalized form of the invention is 
realized as follows: 
A pair of spiral grooves which serve as leads to guide the movement of the 
robotic grippers are machined into the surface of a circular, rotatable 
platen. These spiral leads are centered around the axis of rotation of the 
platen and diverge from the center at a constant linear rate. The spirals 
originate at positions 180 degrees apart, but are substantially identical 
in form. 
A rigid track consisting of a pair of metal rails is positioned over the 
center of the platen and parallel to the surface thereof. These rails 
carry two mechanical fingers which are mounted to the rails by precision 
linear bearings, and these fingers have pins mounted in roller bearings 
which extend down into the spiral leads. 
A high precision harmonic drive gear is mounted to the back of the platen, 
and by means of a shaft connecting it to an electric motor, rotational 
motion is imparted to the gears, and then to the platen. 
A rigid metal frame holds all parts in alignment, and provides mounting 
points for attachment to other pieces of robotic equipment.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIGS. 1 and 2, the preferred embodiment of the invention 
is shown to comprise a robotic end effector wherein the opposing gripping 
surfaces (50,52) derive their action by the engagement of two pins (54,56) 
with a rotating metal disc (1) hereinafter referred to as the platen. Two 
spiral grooves (2,3) have been machined in the upper surface of the 
platen, and form leads which are followed by the grippers as detailed 
below. The gripping surfaces (50,52) are on the inner faces of two 
moveable fingers (46,48), which attach by means of mounting brackets 
(42,44) to moveable carriages (38,40). Each carriage has two linear 
bearings (12,18) which are held in bearing brackets (32,35) by bearing 
bracket caps (28,30) and retaining rings (20,21). The linear bearings ride 
round rails (10,11) which are parallel to each other, parallel to the 
surface of the rotating platen and spaced laterally at equal distances 
from the center of rotation of the platen. In this way, the moveable 
fingers are constrained to move in opposition along straight lines that 
converge at a point above the center of the platen. 
The platen (1) is attached by means of a mounting plate (7) to a gear drive 
(8). In the preferred embodiment, this gear drive consists of an harmonic 
reduction gear providing an 80:1 reduction of input rotation to output 
rotation. The device portrayed is Model #HDUF-20-80, manufactured by the 
Emhart Machinery Group of Wakefield, Mass. It is a standard item of 
manufacture which is commercially available in a variety of sizes and 
reduction ratios. It provides a high reduction in rotational speed with 
concomitant increase in torque, and it is very strong and durable as well 
as being manufactured to close dimensional tolerances. Although any number 
of alternative gear configurations could be used in place of the harmonic 
drive and still provide a useful reduction in rotational speed, a 
particular benefit of using harmonic gearing is the impossibility of back 
driving. Therefore, the gripping surfaces maintain their grip even after 
the power is turned off to the drive motor. By varying the ratio of 
reduction in the installed gear drive, or by varying the angle of the 
spirals, the end effector can be readily customized for specific 
applications requiring different combinations of grip stength and rapidity 
of movement. 
The reduction gear (8) is driven by a drive shaft (78) which is supported 
in a bearing (66) and which is attached to a motor shaft (76). The motor 
shaft is supported by a bearing (64) that is fitted to the motor housing 
(62). The rotor (74), stator (70), commutator (72) and a brush (68) are 
shown for the motor. The motor is designed such that by controlling the 
input power, the number of rotations, and therefore gripper position, can 
be accurately controlled. 
All parts are held in alignment by a frame (61). The frame secures the ends 
of the parallel rails (10,11) by means of rail caps (58,60) at each end. 
The gear drive (8) is attached to the upper surface of the frame, and the 
drive shaft bearing (66) is fitted to the base. The motor housing (62) 
also attaches to the base of the frame (61), and the lower surface (63) of 
the motor housing provides a flat mounting surface for attaching the end 
effector to any sort of mechanical arm which would typically be used to 
position the end effector when in use. 
Referring to FIG. 3, note that the leads (2,3) originate opposite the 
center of rotation of the platen (1) from each other, and diverge at equal 
linear rates. When the end effector is in operation with the jaws open, 
the pins (54,56) would engage the platen at the locations marked X and Y. 
If the platen is rotated 360 degrees about the center in the direction of 
the arrow (Z), the pins would move inward to the positions marked X' and 
Y'. The distance moved inward by the gripping surfaces is measured by the 
distances between lines A & A' and between B & B', respectively. It can be 
seen that a large movement in rotation of the platen produces a small 
lateral movement of the grippers, thereby providing a large mechanical 
advantage and great gripping strength. To ensure accurate repeatability of 
gripper placement, the positioning and widths of the leads must be 
carefully controlled. 
FIG. 4 is a sectional view of the rotating platen (1) taken along line 
IV--IV of FIG. 3. The pins (54,56) are also shown in their engagement with 
the leads (2,3). As the platen rotates, the advancing vertical walls (5,6) 
of the two leads are each brought to bear on a pin (54,56), and are thus 
forced to move radially towards or away from the center of the platen 
depending on the direction of motion. 
FIG. 5 shows a sectional view taken along line V--V of FIG. 2. It shows one 
of the moveable grippers (46) with the mounting bracket (42) and carriage 
(38). The bearing (55) is pressed into a hole (57) in the base of the 
carriage, and the pin (56) is then pressed into the center of the bearing, 
allowing the pin to turn. Thus the pin has a low-friction, rolling contact 
with the wall (6) of the spiral lead (2). The pin is machined to be only 
very slightly smaller in diameter than the width of the groove in the 
platen, so that backlash and the resultant positioning errors are kept to 
a minimum. 
The invention can also be practiced in a robotic end effector which has a 
fixed center post against which a single moveable gripper acts to grasp 
the workpiece, or in a configuration of more than two moveable grippers 
moving radially from the center. The rotatable platen can have any number 
of spiral leads with any variation in depth or wall shape while still 
retaining the essential mechanical element of force being applied to 
actuate multiple robotic fingers by an inclined plane formed into the 
walls of a rotatable spiral groove. 
Although specific embodiments of the invention have been described herein, 
they are to be considered exemplary of the novel features thereof and are 
not exhaustive. There are obviously many variations and modifications of 
these specific examples that will be readily apparent to those skilled in 
the art in light of the above teachings without departing from the spirit 
or scope of the appended claims. It is, therefore, to be understood that 
the invention may be practiced otherwise than is specifically described.