A tactile micro-gripper is disclosed having a combined lever and spring linkage. The lever flexes at predetermined points and is coupled to the spring and to an actuator. The spring is of a parallelogram type, so that the ends remain parallel to one another when the spring is bent. When the actuator causes the lever to move transversely, the end of the spring is caused to move laterally in a parallel manner. A pair of lever and spring linkages are arranged together so that they cooperatively act together to manipulate a pair of fingers, which are attached to the ends of the springs, to grip and release objects. Tactile sensing is accomplished by means of a cantilever beam and an optical detection system disposed in the tip of the fingers.

DESCRIPTION 
1. Technical Field 
The present invention relates to robotic grippers and more particularly to 
grippers used to manipulate small objects. 
2. Background Art 
Robotic manipulators are becoming increasingly important in the fields of 
manufacturing and material transport in extreme environments. In 
particular, robotics are being used increasingly for very delicate 
procedures involving manipulation of small objects. Uses include 
micro-electronic component testing and manufacture, biomedical procedures, 
and micro-surgery. These uses require sensitive, yet highly reliable 
manipulators. 
Various devices and mechanisms have been designed for gripping objects for 
use with robotic arms and the like. Generally, these devices fall within 
several types of designs. Of particular interest are manipulators for 
small objects where robotic fingers handle the object. One type has a pair 
of fingers which are caused to move in parallel paths or in arcuate paths. 
Designs of this type, however, suffer from a loss of tactile sensitivity 
because of static friction. A second type of design is based on a tweezers 
type mechanism. Designs of this type eliminate some of the problems 
associated with static friction, but require long arms to provide near 
parallel movement of the tips. Precise control of the long arms and tips 
can prove difficult. 
A gripper of the first type may be found in U.S. Pat. No. 4,808,898, to 
Pearson, wherein a gripper system is disclosed in which a pair of fingers 
are operatively controlled by input signals. The fingers move opposite one 
another in parallel paths from an open position to a closed position. A 
motor and rack and pinion linkage are used to cause motion of the fingers. 
The drive shaft of the motor provides servo information. This type of 
gripper is best suited for large objects. 
Grippers of the second type are disclosed in related U.S. Pat. Nos. 
4,610,475 and 4,666,198, to Heiserman. Essentially, the grippers disclosed 
here are either based on a tweezers or pliers design. The grippers are 
actuated by piezo strips which cause either bending or pivoting of the 
mechanisms. 
Given the above limitations with prior gripper designs, it is an object of 
the present invention to provide a compact micro-gripper with improved 
tactile sensitivity and parallel movement of its fingers. 
It is another object of the present invention to provide a gripper with 
means for sensing a gripping force. 
SUMMARY OF THE INVENTION 
The above objects have been achieved by a gripper design which utilizes in 
combination a parallelogram spring attached to a lever. The lever has 
points about which the lever may flex, thereby eliminating problems with 
static friction. The lever is connected to an actuator means and to a free 
end of the parallelogram spring, so that transverse movement of the lever 
is translated into lateral movement of the free end of the spring. 
Opposite the free end of the spring is a fixed end which is attached to a 
support frame. A gripper finger may be attached to the free end of the 
spring. Typically a pair of springs and levers act in unison to manipulate 
a pair of fingers to grip or release an object, but only one combination 
of spring and lever is needed so long as there is an opposing support to 
work against. 
The lever and spring are combined to overlay one another. That is, each 
lies in separate but parallel planes. Therefore, to balance the forces 
between the lever and spring, and to eliminate a tendency to twist, a 
sandwich-type design is used. Either two springs with a lever in between 
or two levers with a spring in between are used. This allows for a 
balanced application of force between the members. A sandwich-type 
arrangement is not necessary if there is sufficient lateral stiffness to 
the members. 
Kinematically the arrangement of the lever and spring is constrained such 
that the lever has a flex point that generally coincides with an axis of 
rotation of the free end of the spring. Movement of a point on the free 
end is described by an arc having an axis of rotation. For the above 
arrangement of spring and lever to operate, the lever needs to have a flex 
point close to the axis of rotation of the free end of the spring. 
Since the present invention is also concerned with providing micro-force 
control, the tips of the fingers used in the gripper may be provided with 
force sensing means. This is accomplished by forming in the tip of a 
finger a cantilever beam section which has an optical sensing means. The 
optical sensing means comprises a mirror attached to the beam section and 
which is positioned opposite a fiber optic cable. Light emitted from the 
fiber optic cable strikes the mirror and is fed back into the cable. As 
the cantilever beam bends, more or less of the mirror is presented to the 
fiber optic cable, so that more or less light is reflected back into the 
cable depending on the amount of force causing the beam to bend. In this 
way the gripping force may be determined over a range. The bending of the 
cantilever beam may also be detected visually by an operator, thereby 
giving the operator an indication of the amount of force applied. A 
microscope may aid in observing the deflections of the cantilever beam. 
An advantage of the present invention is that the parallelogram springs 
provide parallel movement of the fingers. This gives the gripper the 
ability to grab objects having a range of sizes. Another advantage is that 
because the lever and spring flex rather than pivot, problems with 
sensitive tactile control due to static friction are virtually eliminated. 
Therefore, the force applied to an object can be varied infinitely over a 
range of values. Thus, very delicate procedures may be accomplished. 
Another important advantage is that the gripper of the present invention 
may be made very small.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to FIGS. 1 and 2, a micro-gripper assembly 10 is shown. The 
assembly has a support frame 12 and a bellows actuator 14. The bellows 
actuator includes a pressure vessel 15 and bellows 17. Air is pumped into 
and released from the pressure vessel through passageway 19. Other types 
of actuators may also be used, such as those involving piezoelectric 
material. The actuator 14 operates in con]unction with a lever 16 and a 
parallelogram spring 18 linkage, to open and close fingers 20 and 22. The 
bellows 17 is supported by the support frame 12, while the pressure vessel 
15 is coupled to the lever 16 such that the pressure vessel moves relative 
to the support frame when there is a change in pressure within the 
pressure vessel. 
The lever 16 may be manufactured from various metals using electrodischarge 
machining (EDM) techniques. EDM fabrication allows the parts of the 
present invention to have dimensions on the order of millimeters, with the 
overall length of the device less than one centimeter. Alternatively, 
molded plastics may be used. In addition, material and etching techniques 
known in the semiconductor industry may be employed to form the lever and 
other components of the assembly. This would enable very small assemblies, 
even smaller than a few millimeters, to be manufactured. FIGS. 1a and 2a 
show the lever 16 isolated from the other apparatus. The lever has three 
flex points 24, 26 and 28. Flex point 24 is fixed relative to the support 
frame 12. Flex points 26 and 28 act like pivot pins in a four-bar linkage. 
Thus, transverse motion of flex point 28 causes lateral movement of flex 
point 26. This movement is employed to cause the spring 18 to bend or 
straighten out. 
Returning to FIGS. 1 and 2, the parallelogram spring 18 comprises a fixed 
end 30 attached to the support frame 12. Opposite the fixed end 30 is a 
free end 32. In between the fixed and free ends are two metal ribbons 34. 
The springs 18 may be made by any of the known metal working methods. In 
addition the springs may be made by molding plastic around metallic wire, 
with the plastic forming the fixed and free ends of the spring. A unique 
feature of parallelogram springs is that while the ribbons 34 bend, the 
ends remain parallel to one another. Thus when the levers 16, which are 
coupled to the free ends of the springs 18, are caused to move by the 
actuator 14, the free ends remain parallel to the fixed ends and move 
parallel to one another. Screws 36 or other means for attachment may be 
used to attach the fingers 20 and 22 to the free ends 32. 
In assembling the lever and spring linkage the spring 18 is arranged so 
that it is in an unbent condition when the gripper is in a fully open 
position. The lever 16 on the other hand is in a flexed condition. 
However, when the gripper is fully closed, the lever 16 is in an unflexed 
condition, while the spring 18 is bent. Typically, the force of the spring 
will bias the gripper towards an open position. The actuator 14, when 
embodied as a bellows also acts as a spring. Therefore, it is possible to 
bias the assembly such that the neutral position of the gripper is open to 
a greater extent than possible with just the biasing force of the 
parallelogram spring. In fact the neutral position can be open to an 
extent past the neutral position of the parallelogram spring. 
Turning to FIG. 3, an embodiment of the present invention is shown, wherein 
a spring 18 is sandwiched between two levers 16. The levers 16 act in 
unison and are both coupled to the free end of the spring. This 
arrangement allows for a balanced application of force to the spring via 
the levers. The levers 16 are coupled to the pressure vessel 15 via arms 
21. 
As discussed previously, a sandwich-type design is not necessary if the 
members are sufficiently stiff. FIG. 3a illustrates an assembly that has a 
single spring 18 and lever 16. Assemblies of this type may be more compact 
than the sandwich type. 
Alternatively, as is shown in FIG. 4, a balanced force may also be applied 
by the arrangement of two springs 18 sandwiching a lever 16. This 
arrangement has the advantage that there are two fingers 20 which provide 
a wider area for gripping. Alternatively, the ends of the fingers 20 may 
come together to form a single tip. 
FIG. 5 shows a tip of a finger 20 having a force-sensing means. The tip is 
formed with a cantilever beam section 40 to which is attached a mirror 42. 
The mirror is disposed opposite a fiberoptic cable 44, such that light 
emitted from the cable is reflected back into the end 46 of the cable 44 
by the mirror 42. When no force is on the beam 40, the mirror 42 is 
presented to approximately one half of the area of the fiberoptic cable 
end 46. As force is applied to an object gripped by the finger, the beam 
is deflected such that more of the mirror is presented to the optical 
cable. This increases the amount of light reflected back to the cable. 
FIG. 6 shows a schematic of the sensing means wherein an LED light source 
50 is optically coupled to a fiberoptic cable 52 which directs the light 
to beam splitter 54. Part of the split beam is then carried by the 
fiberoptic cable 44 to the fingertip as described above in relation to 
FIG. 7. The light reflected by the mirror 42 is returned along cable 44 to 
beam splitter 54 wherein a part of the split beam is carried by fiberoptic 
cable 56 to a photodetector 58. The photodetector 58 converts the light 
energy received into a voltage that is fed into operational amp 60 which 
produces an output voltage, v.sub.out. The output signal, v.sub.out, can 
be calibrated to represent the amount of force applied to the cantilevered 
beam section 40 of the fingertip. In this way, the force applied to an 
object in the gripper can be accurately measured and controlled. 
The embodiments shown and described above are intended to be illustrative 
only. It will be appreciated by those skilled in the art that other 
embodiments are possible and are within the scope of the invention.