Pneumatic, tactile gripper generating a gripping force controlled by the weight of the handled object

The invention relates to a pneumatic gripper comprising at least one movable finger of which the clamping force is controlled by the weight of the object handled. Each movable finger includes a pneumatic actuator (1) of which the movable member (2) is solidly joined to a flexible blade (4). A stop member (4) is fastened to the free portion of this blade (4) and will touch the object to be seized. Two potentiometers (9, 11) are located on the sides of this blade (4) and their respective outlets (9b, 11b) are connected to an actuation chamber of the actuator so as to achieve a pressure difference between these chambers (1a, 1b) and hence a clamping force directly related to the blade bending. Each finger furthermore includes proximity sensors (7) for the object to be seized, whereby this object can be approached rapidly without danger of tipping it over when making contact with it.

The invention conceived at the INSTITUT DE MECANIQUE DES FLUIDES of the 
Toulouse Institut National Polytechnique, which is a laboratory associated 
with CNRS 040005, relates to a pneumatic gripper comprising at least one 
movable finger with the sense of touch and of which the gripping force is 
controlled by the weight of the object handled. In particular it applies 
to a multi-digital clamp mounted to the end of a manipulator arm or to an 
industrial robot and capable of seizing and carrying arbitrarily shaped 
objects of very diverse weights. 
The rapid growth of robotics and the importance assumed by robots in many 
industries for purposes of automating the various production stages have 
led to creating a great many gripping devices. 
One of these devices in particular is described in the Belgian Patent 
document No. 0.117.679. This patent describes an industrial robot hand 
using a double-acting jack or actuator as the drive means for the gripper 
fingers. These fingers consist of several phalanges hinging on each other 
and are mounted by means of articulations on one hand on the body of the 
actuator and on the other hand on its movable part. The displacement of 
this movable part therefore drives these fingers in a direction 
perpendicular to the longitudinal axis of the actuator. The main drawback 
of such devices is the absence of a control on the gripping force which, 
depending only on the pressure of the fluid feeding the actuator, remains 
constant. Therefore this device makes it impossible to handle fragile 
objects likely to break. Moreover the use of such a hand requires perfect 
initial positioning of the object to be carried lest one of the fingers 
touches this object before the others and tips it over. 
Other devices such as described in the European Pat. No. 0.112.729 have 
been suggested to remedy these drawbacks. This patent relates to a 
manipulator with a gripper of which the gripping force is selected on the 
basis of data provided by detection means which check whether the object 
has been raised or not. These detection means are connected to a 
regulating electric circuit which can stepwise increase the gripping force 
from a pre-selected minimum, force to permit lifting the object. 
Consequently this device allows applying to the gripping fingers the 
minimum force needed to lift the handled object. However the data 
transmitted by the detection means are processed by regulation means 
requiring that a robot be used which includes a relatively complex central 
processing unit capable of managing these data and to pass on these 
commands to the gripper fingers. Moreover, this device does not resolve 
the problem of perfectly positioning at the outset the object which must 
be seized: the "open" and "close" commands do drive the hand assembly, 
none of the fingers operating independently. 
Other devices such as described in the PCT application published under No. 
WO 82/01156 make use of a hydraulic actuator as the drive mechanism for 
the hand. This actuator is of the multi-cylinder or multi-stage type, the 
various cylinders or stages being supplied or not in relation to data on 
the weight of the object being lifted that are transmitted to the robot 
central processing unit (CPU). These data allow controlling the switch of 
solenoid valves mounted on the various supply mains for the actuator's 
cylinders or stages. As the previous devices, these also require using a 
robot with a complex CPU and demand accurate initial positioning of the 
objects to be seized due to the lack of independence of the various 
gripper fingers. 
A device described in the PCT application #WO 82/01155 is based on similar 
principles and also uses hydraulic actuators as the drive mechanisms for 
the hand. The opening and the closing of the fingers of the clamp are 
controlled from solenoid valves mounted on the supply mains of the 
actuators driving these fingers. These solenoid valves are controlled from 
instructions delivered by the robot CPU. This CPU also controls another 
solenoid valve located upstream of the previous ones and designed to 
supply the actuators by means of two circuits causing different pressure 
drops. The robot CPU determines the order of switching between these two 
circuits on the basis of weight data of the object being lifted that are 
provided to this CPU. 
This device as the previous one entails the drawback of requiring a unit 
centralizing complex commands, with all the instructions, opening and 
closing the clamp, detection of the object to be seized, analysis of the 
gripping force, issuing from this CPU. Moreover this device also fails to 
solve the problem of dependency of the gripper fingers. 
The object of the present invention is to overcome the drawbacks of those 
known devices and to create an improved gripper capable of determining the 
proximity of an object, of seizing this object and of carrying it. 
One object of the present invention is to create a multi-digital gripper 
wherein each finger is independent. 
Another object of the invention is to create a gripper device capable of 
carrying an object while applying to it a clamping force which is a 
function of its transportation conditions. 
Yet another object is to provide a gripper which does not require accurate 
centering of the object to be seized within the gripper axis. 
Still another object is to create a pneumatic gripper of which the 
performance is insensitive to any slight variations in the supply sources. 
To that end, the multi-digital gripper for seizing and maneuvering objects 
comprises at least one movable finger driven by a pneumatic actuator or 
jack of which the two inner chambers are connected by means of lines to at 
least one supply source, the gripper having each movable finger comprises: 
a flexible blade solidly joined to the movable member of the actuator so 
that a free portion of the blade can bend in a direction perpendicular to 
its longitudinal axis, a stop member fastened to the free portion of the 
blade and provided with a stop surface which shall come into contact with 
an object to be seized, proximity sensors for the object to be seized and 
means for distributing the fluid flow to each actuator chamber, the 
distribution means being controlled by the sensors and being designed to: 
(a) control a relative pressure difference between the two actuator 
chambers to allow the movable member to move in the absence of contact 
between the stop member and the object to be seized, 
(b) control an approximate re-balancing of the relative pressures of the 
two actuator chambers when there is contact between the stop member and 
the object to be seized, at least one pneumatic potentiometer consisting 
of a fluid container with a supply intake connected to a fluid source, a 
nozzle with a leakage aperture and with a fluid outlet connected to an 
actuator chamber, the leakage aperture and with a fluid outlet connected 
to an acutator chamber, the leakage aperture of the potentiometer being 
arranged to be opposite the free end of the flexible blade so as to obtain 
a pneumatic value at the outlet of this potentiometer which depends 
directly on the blade bending. 
Accordingly the implementation of the gripper of the invention requires 
only as commands from the robot CPU the opening and closing of the fingers 
and the lifting of the object. These are "go, no-go" commands and hence 
independent of the weight or shape of the object to be maneuvered, whence 
the robot CPU now may be simplified. 
Once the command to close the fingers has been transmitted, each movable 
finger will move until touching the object to be seized. As shall be more 
clearly understood below, the sensors then allow stopping the motion of 
each actuator rod. It should be noted that because each movable finger is 
provided with its own sensors, it no longer is necessary to precisely 
center the object into the clamp axis. After the object has been placed 
between the fingers, the grip command may be issued and the lifting may 
take place. The gripper then is autonomous and reacts in relation to the 
weight of the object and to any accelerations this object may be subjected 
to. All these factors cause a more or less pronounced deformation of the 
flexible blade resulting in a variation of the value obtained at the 
potentiometer outlet and hence in an increasing or decreasing imbalance 
between the relative pressures of the actuator chambers. Accordingly the 
clamping force is directly proportional to the weight of the object to be 
seized and to the carrying conditions of this object. 
In a preferred embodiment: the distribution means consist of a pneumatic 
valve body integrated into the stop member and having at least one intake 
connected to the supply source and at least one outlet connected to that 
actuator chamber of which the expansion displaces the movable member 
toward the object to be seized, the sensors consist of a valve shutter 
moving between two end positions, namely: (a) an extended position wherein 
it projects relative to the contact side of the stop member and wherein it 
allows the fluid to flow between intake and outlet of the valve unit, (b) 
a retracted position wherein it is within the contact side and where it 
prevents the flow of fluid between the intake and the outlet of the valve 
unit. 
Accordingly the gripper finger can be operated using a single supply fluid. 
Furthermore, these pneumatic sensors are easily connected to the pneumatic 
supply system of the actuator. This connection, as shall be seen further 
below, offers the advantage of requiring no auxiliary dynamic device. 
In another characteristic of the invention, each movable finger comprises 
two potentiometers of which each outlet is connected to a different 
actuator chamber, the potentiometers being arranged on each side of the 
flexible blade so that their leakage rates will vary inversely when the 
blade is being bent. 
In another preferred embodiment, the fluid system feeding the intakes of 
these potentiometers is designed to introduce different pressure drops so 
as to achieve a slight pressure offset between the actuator chambers when 
the blade is at rest. 
The presence of a potentiometer on each side of the flexible blade allows 
to precisely control the respective relative pressures of each cylinder 
chamber and consequently the gripper clamping force even in the presence 
of any slight fluctuations in the supply-fluid source. The arrangement of 
these potentiometers in association with the pressure drops introduced by 
their respective supply systems in particular makes it possible to apply 
an initially defined clamping force in accurate manner to the object to be 
seized, where this force allows the movable finger to detect object 
slippage during lifting. Moreover, the use of two potentiometers allows 
doubling the sensitivity of the gripper. 
In another feature of the invention, where a quicker response of the 
flexible blade to object slippage is sought while nevertheless using a 
sufficiently stiff blade to assure sufficient clamping to hold the object 
without saturating the potentiometers, each blade advantageously may be 
provided with at least one intermediate rest arranged in such a way as to 
modify its flexural constant. 
The gripper described above can be fastened to the end of a manipulator arm 
or to an industrial robot arm, where the robot is equipped with a control 
CPU in order to achieve a multi-digital gripper. This gripper comprises at 
least one movable finger and at least one fixed finger arranged in such a 
manner that they make possible seizing and lifting an object, the 
manipulator CPU or that of the industrial robot being connected to the 
gripper in order to drive the fingers.

The simplified device illustratively shown by FIGS. 1 and 2 constitutes one 
of the fingers of a multi-digital pneumatic gripper, this finger being 
meant to be fastened to the end of the arm of a manipulator or an 
industrial robot. Each finger is provided with a proximity sensor for the 
object to be seized and also with a vertical-force sensor. 
Each movable finger includes a pneumatic jack or actuator 1 of which the 
two inner chambers (1a 1b) are directly connected to a pneumatic supply 
system. This actuator is designed so as to reduce the friction between the 
piston and the body of the actuator so as not to degrade the gripper 
sensitivity. This can be accomplished by suitably machining the inside 
wall of the actuator body and the periphery of the piston so as to reduce 
friction while minimizing the leakage between the inner actuator chambers. 
Illustratively this machining may be in the form of flutings on the piston 
periphery. 
A support 3 is fastened to the end 2a of the rod 2 of this actuator. In 
turn this support 3 holds the end of a flexible blade 4 made of a rigid 
material and of which the other end is free so as to bend in a direction 
(.DELTA.) perpendicular to its longitudinal axis. 
A stop member 5 to come into contact with an object to be seized is mounted 
on this free end 4a of the flexible blade 4. This stop member is mounted 
on the end of the blade in such a manner that it allows a slight relative 
play between these two components to avert any flexure of the blade 4 when 
it makes contact with the object to be seized. Inside this stop member 
there is an integrated body of a pneumatic valve 6 provided with a supply 
intake 6a and two outlets 6b and 6c. The intake and outlets issue into two 
opposite sides of the stop member which are located in planes parallel to 
the longitudinal axis of the flexible blade 4. The body of this valve 6 is 
arranged so as to issue into the side 5a which is called the contact side 
of the stop member 5 and which is opposite the flexible blade 4. Its 
shutter 7 is designed to move perpendicularly to the side 5a, between two 
end positions: an extended position, wherein its end 7a projects relative 
to the side 5a, and a retracted position where this end is approximately 
in the same plane as the side 5a. Also, the contact side 5a of the stop 
member is covered with a flexible lining 8 for instance made of rubber to 
increase the adhesion of this side. 
Each finger further includes a pneumatic potentiometer 9 consisting of a 
fluid container conventionally provided with an feed intake 9a, a nozzle 
having a leakage aperture 9c and an outlet 9b connected to a chamber 1a or 
1b of the actuator 1. 
This potentiometer 9 is located near one side of the blade 4 and 
perpendicularly to the direction of bending of the blade so that the plane 
of the leakage aperture 9c be substantially parallel to the side. 
Furthermore this potentiometer is located in such a manner relative to the 
flexible blade that its leakage rate increases or decreases with the 
bending of this blade, the pneumatic outlet being connected to the 
actuator chamber 1a containing its rod or to the other actuator chamber 
1b. 
This movable finger is connected by three pneumatic lines R1, R2 and R'2 to 
three supply sources S1, S2 and S'2 or respective constant pressures P1, 
P2 and P'2. 
The first line R1 connects on one hand the source S1 to the feed intake 6a 
of the stop member 5 and on the other hand one of the operative outlets 6c 
of this member to the actuator chamber 1a. The second line R2 connects the 
second source S2 to the feed intake 9a of the potentiometer 9, the 
potentiometer outlet 9b being connected to the actuator chamber 1a. The 
third line R'2 directly connects the source S'2 to the actuator chamber 
1b. 
(This configuration corresponds to a position of potentiometer 9 causing 
the leakage rate to decrease in relation to the deformation of blade 4. An 
inverse configuration would have been similarly and equally feasible). 
These different lines R1, R2 and R'2 are designed to introduce pressure 
drops to control the fluid flows and to provide fine control in each case. 
In particular the line R1 is designed to introduced pressure drops so as 
to avoid excessive fluid flow from the source S2 toward the stop member 5. 
Regulating these pressure drops in this line R1 furthermore allows 
controlling the flow rate and hence the displacement speed of the actuator 
rod. 
Also, the pressure P2 of the supply source S2 can be regulated as a 
function of the pressure from the fluid of source S'2 which feeds the 
actuator chamber lb. in order that, when the blade 4 is free from bending 
loads, a slight offset exists between the respective relative pressures of 
these chambers la and lb. 
The operation of such a gripper takes place in two stages, namely 
approaching the object on one hand and on the other seizing and lifting 
it. 
During the approach stage (FIG. 1), the shutter 7 of the pneumatic valve 6 
projects from the contact side 5a of the stop member. In this position the 
shutter 7 allows free fluid flow between the feed intake 6a and the 
operative outlet 6c of the valve 6 (FIG. 4a). Therefore the supply source 
S1 is connected to the chamber 1a of the actuator 1 and the difference 
between the relative pressures of the two chambers 1a and 1b hence is 
increased by the pressure of the fluid from the source S1, this additional 
pressure making it possible to rapidly approach the object to be seized. 
When contact is made with this object, the shutter 7 acts as a proximity 
sensor and effects the closure of valve 6: the pressure imparted to the 
end 7a of this shutter 7 due to the contact with the object to be seized 
causes the shutter to retract within the valve body 6. In that position, 
the shutter seals the flow duct connecting the intake 6a to the outlet 6c. 
This sensor therefore allows rapidly approaching the object while avoiding 
tipping it over at the time of contact. Furthermore, to allow evacuating 
the fluid contained in that segment of the line R1 which connects the 
actuator 1 to the stop member 5, the shutter is designed, when in its 
retracted position, to allow free flow of the fluid between the operative 
outlet 6c and the evacuation outlet 6b issuing into the atmosphere (FIG. 
4b). 
At the end of this approach stage, the contact side 5a of the stop member 5 
is therefore placed against the object to be seized and applies against it 
an initial clamping force of about several tens of grams due to the 
initial offset between the relative pressures of the two actuator 
chambers. 
This initial clamping force makes it possible at the beginning of the 
gripping and lifting stage to sense the relative slippage between the 
object and the contact side 5a of the stop member 5. The gripper 
sensitivity is increases by using an actuator 1 with minimal friction. 
Thereafter, when lifting the object, the deformation of the flexible blade 
4 will increase in relation to the friction between the object and the 
stop member until an equilibrium position is reached which is the function 
of the object and the stop member until an equilibrium position is reached 
which is the function of the object weight: this deformation entails a 
decrease (or, in another configuration, an increase) of the leakage rate 
of the potentiometer 9 and consequently an increase in the relative 
pressure of the actuator chamber 1a. Therefore the clamping force applied 
as a function of the offset between the relative internal pressures of the 
chambers 1a and 1b of the actuator increases until a force is reached 
which can balance the weight of this object. In a carefully dimensioned 
gripper, this clamping force therefore may be that which is merely 
required to maneuver the object. 
The blades used are characterized by the deformation they undergo in 
relation to the supported weight. As a rule these blades are designed to 
deform by a magnitude which corresponds to the conventional deviation 
between blade and potentiometer for supported weights of several kg. The 
minimum sensed weight depends on the initial clamping force and may be as 
low as several tens of grams. 
It should be noted that in the approach stage the command to close the 
fingers automatically entails the displacement of the shutter 7 from its 
retracted position toward it extended position: the pneumatic forces 
applied by the source S1 to the end of this shutter are sufficient to 
drive it without resort to a return spring or any other know means. 
However the gripper also may be built by using as the sensor a 
jet-interrupting system integrated into the stop member (FIGS. 5a, 5b: the 
references here are identical to those of FIGS. 4a, 4b for those 
components having similar functions). Just as the previously described 
sensor, this system is provided with a feed intake 6a and an operative 
outlet 6c, the evacuation outlet being replaced by a vent 6b. the motion 
of the shutter 7 from its retracted position (stopped flow) to its 
extended position (free flow) is controlled by a spring 15. 
FIG. 3 shows a preferred embodiment mode comprising several design 
adaptations relating to the movable finger of which the operation was 
explained above in principle. First of all this gripper comprises two 
potentiometers 9 and 11 of which the respective intakes 9a and 11a are fed 
from the same source S2. The outlets 9b and 11b of these potentiometers 
are each connected to a chamber 1a and 1b respective of the actuator 1. 
These potentiometers 9 and 11 are mounted on either side of the flexible 
blade 4, the planes of their leakage apertures being substantially 
mutually parallel and perpendicular to the bending direction .DELTA. of 
the blade 4. Furthermore the pressure drops of the segments of the line R2 
connecting the source S2 to the respective intakes of the potentiometers 
are fitted to provide a slight initial offset between the relative 
pressures of the two chambers 1a and 1b of the actuator 1 so as to 
increase the clamping force when the blade is at rest. 
Using two potentiometers 9 and 11 in lieu of a single one allows precise 
regulation of the relative pressures of the actuator chambers in relation 
to the deformation of the blade 4 even if there were slight pressure 
variations at the source S2. Using two potentiometers also allows doubling 
the gripper sensitivity. 
An intermediate rest 12 of the flexible blade 4 is fastened to the support 
3 and located in such a manner that the distance between its end opposite 
the blade 4 and this blade is less than the distance between this blade 
and the leakage aperture of the potentiometer 9 connected to the chamber 
1a. 
Accordingly this intermediate rest modifies the flexural constant of the 
blade when gripping takes place. This modification allows using a 
relatively flexible blade 4 to achieve a sensitive system allowing to 
increase the weight of the transported objects without saturation of the 
potentiometers, thanks to the "stiffness" imparted to this blade by the 
rest 12. 
Furthermore the gripper shown in FIG. 3 uses a pneumatic mechanism 13 
controlling the closing or opening of each gripper finger. 
This mechanism allows connecting the source S1 either by the line R1 to the 
chamber la as described above in order to close the gripper finger, or by 
a line R'1 to the chamber 1b in order to open the finger. 
These movable fingers can be used to achieve a pneumatic gripper mounted to 
the end 14 of an industrial robot (FIG. 6). In this example the gripper 
includes at least two movable fingers and a third one which is optionally 
fixed or mobile. It should be noted that the number and the shapes of the 
fingers, both movable and fixed, varies as desired depending on the 
application of the gripper. The movable fingers are mounted on the end of 
the robot arm so as to converge toward one point located approximately in 
the axis of symmetry of the clamp. 
The commands to open and to close illustratively may be obtained from a 
go/no-go electric valve controlled by the robot CPU and transmitting the 
data from a micro-camera for purposes of verifying the object position. 
Thus, the clamp having been positioned opposite the object, the CPU 
commands closing the fingers through the electric valve. Starting from 
that position and totally independently, the gripper then is fit to move 
the fingers against the object with a very low clamping force (several 
tens of grams). It should be noted that when approaching, the fingers are 
mutually independent and therefore the object need not be positioned 
precisely in the gripper axis. 
Thereupon the robot arm can vertically lift the object, preferably at a low 
initial speed, in order to let the various fingers adapt themselves to the 
object weight by increasing their clamping force until it equals this 
weight. Thereupon more rapid displacements are permissible as far as the 
object destination, the clamping force being a function of the weight and 
of the carrying conditions of this object.