Method for controlling a robotic gripper using friction estimation
A gripper which has a friction estimation module configured to estimate static and/or dynamic friction forces acting on gripping jaws is provided. The static friction force is calculated on the basis of constraining reactions whereto the gripping jaws are subjected, the constraining reactions being calculated at least as a function of an actuation force exerted on the gripping jaws, a coefficient of friction of gripper materials and/or lubricant used being known. The dynamic friction force is calculated on the basis of speed of the gripping jaws, width of sliding surfaces and distance between the sliding surfaces of the gripping jaws, the viscosity of the lubricant used being known.
1 . A gripper, comprising:
a gripper body;
at least two gripping jaws, at least one of which is movable with respect to another one between a jaw opening position and a jaw closing position;
an actuation group housed inside the gripper body;
a transmission group housed inside the gripper body, the actuation group being operatively coupled to the at least one movable jaw by the transmission group to move said at least one movable jaw between the jaw opening position and the jaw closing position;
a jaw position sensor, suitable to measure an absolute position of the gripping jaws;
a gripping force sensor suitable to measure a gripping force applied by the gripping jaws to an item;
a processing unit operatively connected to said sensors and to said actuation group and comprising a friction estimation module and a friction compensation module, wherein:
the friction estimation module is configured to estimate static and dynamic friction forces acting on components of the transmission group and on the gripping jaws, the static friction force being calculated by the friction estimation module based on constraining reactions to which the components of the transmission group and the gripping jaws are subjected, the constraining reactions being calculated at least as a function of an actuation force applied by the actuation group and/or the gripping force measured by the gripping force sensor, a friction coefficient (μ i ) of materials of the gripper and/or of lubricant used being known, the dynamic friction force being calculated based on speed of the gripping jaws, width of sliding surfaces of the gripping jaws, and distance between said sliding surfaces, viscosity of the lubricant used being known; and the friction compensation module is configured to control the actuation group based on signals from the sensors and on estimation of the friction forces from the friction estimation module.
2 . The gripper of claim 1 , further comprising an actuation force sensor suitable to measure the actuation force applied by the actuation group to the at least one movable jaw.
3 . The gripper of claim 1 , further comprising at least one center-of-pressure sensor, suitable to detect coordinates of a center of pressure (CoP) between the gripping jaws when the gripping jaws apply the gripping force to the item.
4 . The gripper of claim 3 , wherein, with i indicating a number of constraining surfaces that apply a constraining reaction, the i-th component of the static friction force (F fi ) is estimated by
F
fi
=
R
i
*
μ
i
*
β
,
where R i is the i-th component of a constraining reaction acting on the i-th surface, and β is a parameter indicating a direction of the static friction force.
5 . The gripper of claim 4 , wherein the static friction force (F) is estimated using a Coulomb model expressed by
F
=
{
F
C
sgn
(
z
.
)
if
z
.
≠
0
min
(
F
e
,
F
C
)
sgn
(
F
e
)
if
z
.
=
0
where ż is the speed of the gripping jaws,
Fc=R i *μc, where R i is the i-th component of a constraining reaction acting on the i-th surface, μc is a kinetic friction coefficient, and
F e is a resultant of external forces.
6 . The gripper of claim 4 , wherein at least some components of the constraining reaction R i are calculated as a function of the coordinates of the center of pressure (CoP).
7 . The gripper of claim 3 , wherein the center-of-pressure (CoP) sensor is based on the gripping force sensor, the gripping force sensor being suitable to carry out torque measurements, the processing unit being programmed to calculate the coordinates of the center of pressure (CoP X , CoP Y ) as:
{
CoP
X
=
M
Y
/
F
Z
/
C
o
P
Y
=
M
X
/
F
Z
/
where M X and M Y are measured moments of the gripping force sensor according to X and Y axes, respectively, and |F Z | is the absolute value of the actual measured gripping force along a Z axis.
8 . The gripper of claim 1 , wherein, with i indicating a number of sliding surfaces of the gripping jaws, the i-th component of the dynamic friction force (F vi ) is estimated by
F
vi
=
C
i
*
z
.
where
Ci
=
η
i
*
(
S
i
/
d
i
)
,
where ż is the speed of the gripping jaws, η i is the viscosity of the lubricant used, S i is a width of the i-th sliding surface, and d i is a distance between the i-th sliding surface and a surface against which the i-th sliding surface slides.
9 . The gripper of claim 1 , wherein the actuation group is a pneumatic actuation group and comprises a piston unit comprising at least one piston slidably housed in a respective piston chamber formed in the gripper body, said at least one piston being operatively connected to at least one jaw sliding in a jaw guide.
10 . A method for controlling a gripping force of a gripper, wherein the gripper comprises:
a gripper body;
at least two gripping jaws, at least one of which is movable with respect to another one between a jaw opening position and a jaw closing position;
an actuation group housed inside the gripper body;
a transmission group housed inside the gripper body, the actuation group being operatively coupled to the at least one movable jaw by the transmission group to move said movable jaw between the jaw opening position and the jaw closing position;
the method comprising:
estimating static and dynamic friction forces acting on components of the transmission group and on the gripping jaws,
adjusting an actuation force applied by the actuation group to the at least one movable jaw based on an estimate of the friction forces,
wherein the static friction force is calculated based on constraining reactions to which the components of the transmission group and the gripping jaws are subjected, the constraining reactions being calculated at least as a function of the actuation force applied by the actuation group and/or a gripping force applied by the gripping jaws to an item, a friction coefficient (μ i ) of materials of the gripper and/or lubricant used being known,
and wherein the dynamic friction force is calculated based on speed of the jaws, width of sliding surfaces of the gripping jaws, and a distance between said sliding surfaces of the gripping jaws, viscosity of the lubricant used being known.
11 . The method of claim 10 , wherein, with i indicating a number of constraining surfaces that apply a constraining reaction, the i-th component of the static friction force (F fi ) is estimated by
F
fi
=
R
i
*
μ
i
*
β
,
where R i is the i-th component of a constraining reaction acting on the i-th surface, and β is a parameter indicating a direction of the static friction force.
12 . The method of claim 11 , wherein the static friction force (F) is estimated using a Coulomb model expressed by
F
=
{
F
C
sgn
(
z
.
)
if
z
.
≠
0
min
(
F
e
,
F
C
)
sgn
(
F
e
)
if
z
.
=
0
where
ż is the speed of the gripping jaws,
Fc=R i *pc, where R i is the i-th component of the constraining reaction acting on the i-th surface, μc is a kinetic friction coefficient, and
F e is a resultant of external forces.
13 . The method of claim 12 , further comprising:
calculating the coordinates of a center of pressure (CoP) between the gripping jaws when the gripping jaws apply the gripping force to the item; and
calculating at least some components of the constraining reaction R i as a function of said coordinates of the center of pressure (CoP).
14 . The method of claim 10 , wherein, with i indicating a number of sliding surfaces of the gripping jaws, the i-th component of the dynamic friction force (F vi) is estimated by
F
vi
=
C
i
*
z
.
where
C
i
=
η
i
*
(
S
i
/
d
i
)
,
where ż is the speed of the jaws, η i is the viscosity of the lubricant used, S i is a width of the i-th sliding surface, and d i is a distance between the i-th sliding surface and a surface against which the i-th sliding surface slides.