Method for controlling a gripper
A method for controlling grasping of an object by a gripper provides for computing a reference position of a grasped object with respect to a reference system integral with the gripper, and monitoring a position of the grasped object. If a displacement of the grasped object with respect to the reference position is detected, the method provides for commanding the gripper to increase grasping force.
1 . A method for controlling grasping of an object by a gripper, the method comprising steps of:
a) commanding the gripper to grasp the object with a predetermined grasping force;
b) measuring an effective grasping force;
c) verifying that a stable grasp of the object is achieved by determining that a difference between the predetermined grasping force and the effective grasping force is below a predetermined threshold value;
d) subsequent to verifying that the stable grasp is achieved, computing a stable reference position of the grasped object, wherein the stable reference position is computed as an average of a set of measurements obtained over a predetermined time interval by at least one position sensor and/or by at least one center of pressure sensor;
e) storing the computed stable reference position in a memory;
f) monitoring a position of the grasped object; and
g) if a displacement of the grasped object with respect to the stored stable reference position is detected, commanding the gripper to increase grasping force.
2 . The method of claim 1 , wherein, following step g), the method is repeated from step f).
3 . The method of claim 1 , wherein the displacement of the grasped object with respect to the stored stable reference position is detected if a difference between a position datum obtained by the at least one position sensor and the stored stable reference position is above a predetermined threshold value and/or if a difference between a position datum obtained by the at least one center of pressure sensor and the stored stable reference position is above a predetermined threshold value.
4 . The method of claim 1 , wherein, if the stable reference position is computed using the at least one center of pressure sensor, a grasping force sensor for detecting the effective grasping force is used as the center of pressure sensor, the grasping force sensor being suitable to carry out torque measurements, CoPx and CoPy coordinates of a center of pressure (CoP) being computed as:
{
CoP
X
=
M
Y
❘
"\[LeftBracketingBar]"
F
Z
❘
"\[RightBracketingBar]"
CoP
Y
=
M
X
❘
"\[LeftBracketingBar]"
F
Z
❘
"\[RightBracketingBar]"
Mx and My being measured moments of the effective grasping force according to X and Y axes, respectively, and |Fz| being a modulus of the effective grasping force along a Z axis.
5 . The method of claim 1 , wherein step a) is preceded by steps of: monitoring a grasping area to detect a presence of the object to be grasped; comparing a position of the object to be grasped with a predetermined grasping position, the predetermined grasping position being a relative position or pose between the object and the gripper appropriate to initiate grasping; and wherein step a) is carried out when a distance between the position of the object to be grasped and the predetermined grasping position is below a predetermined threshold value.
6 . The method of claim 1 , wherein, during step g), an increase of the grasping force is carried out continuously.
7 . A gripper, comprising:
a gripper body;
at least two grasping jaws movable with respect to the gripper body between an idle open position and a closed position for grasping an object;
at least one proximity sensor suitable to detect a presence of the object to be grasped within a field of vision of the at least one proximity sensor;
a grasping force sensor, suitable to measure a grasping force exerted by the grasping jaws on the object;
at least one center of pressure sensor, suitable to detect coordinates of a center of pressure between the grasping jaws of the gripper when the grasping jaws exert the grasping force on the object;
and a processing unit comprising a memory and programmed to carry out a method for controlling the gripper, the method comprising steps of:
a) commanding the gripper to grasp the object with a predetermined grasping force;
b) measuring an effective grasping force by the grasping force sensor;
c) verifying that a stable grasp of the object is achieved by determining that a difference between the predetermined grasping force and the effective grasping force is below a predetermined threshold value;
d) subsequent to verifying that the stable grasp is achieved, computing a stable reference position of the grasped object by computing an average of a set of measurements obtained over a predetermined time interval from the at least one position sensor and/or the at least one center of pressure sensor;
e) storing the computed stable reference position in the memory;
f) monitoring a position of the grasped object; and
g) if a displacement of the grasped object with respect to the stored stable reference position is detected, commanding the gripper to increase the grasping force.
8 . The gripper of claim 7 , wherein the processing unit is programmed to repeat the method for controlling the gripper from step f) after having carried out step g).
9 . The gripper of claim 7 , wherein the at least one center of pressure sensor coincides with the grasping force sensor, the grasping force sensor being suitable to carry out torque measurements, the processing unit being programmed to compute CoPx and CoPy coordinates of the center of pressure (CoP) as:
{
CoP
X
=
M
Y
❘
"\[LeftBracketingBar]"
F
Z
❘
"\[RightBracketingBar]"
CoP
Y
=
M
X
❘
"\[LeftBracketingBar]"
F
Z
❘
"\[RightBracketingBar]"
Mx and My being measured moments of the effective grasping force according to X and Y axes, respectively, and |Fz| being a modulus of the effective grasping force along a Z axis.
10 . The gripper of claim 7 , wherein the at least one proximity sensor is an ultrasonic or infrared sensor.
11 . The gripper of claim 7 , wherein the processing unit is also programmed to: monitor a grasping area, by the at least one proximity sensor, to detect the presence of the object to be grasped; compare a position of the object to be grasped with a predetermined grasping position, the predetermined grasping position being a relative position or pose between the object and the gripper appropriate to initiate grasping; and command the gripper to grasp the object with the predetermined grasping force when a distance between the position of the object to be grasped and the predetermined grasping position is below a predetermined threshold value.
12 . The gripper of claim 7 , further comprising proportional control means commandable by the processing unit to continuously control the grasping force.
13 . The gripper of claim 7 , wherein the grasping force sensor and/or the center of pressure sensor are made with a sensor unit array so as to provide a tactile skin, and/or with a force/torque sensor.
14 . The gripper of claim 13 , wherein the sensor unit array is of the capacitive type.