IEEE Robotics & Automation Magazine - December 2018 - 52

500
400
300
200
100
0
-100
-200

Current (mA)

Current (mA)

Grasp Force Estimation
Grasping a Soft Object, SoftHand

Reach
5

10

15
Time (s)
(a)

20

25

Grasp

Reach
0

5

10

15
Time (s)
(c)

Release
20

25

30

Measured Current Residual

Reach

Release
Grasp

0

Grasp Force Estimation
Grasping a Stiff Object, SoftHand

500
400
300
200
100
0
-100
-200

500
400
300
200
100
0
-100
-200

30

Current (mA)

Current (mA)

0

Release

Grasp

Grasp Force Estimation
Grasping a Soft Object, Soft Gripper

5

10

15
Time (s)
(b)

20

25

30

Grasp Force Estimation
Grasping a Stiff Object, Soft Gripper

500
400
300
200
100
0
-100
-200

Release

Reach
Grasp
0

5

10

15
Time (s)
(d)

20

25

30

Saturated Current Residual

(e)

(f)

Figure 7. (a)-(d) The current residuals obtained during underwater grasping tasks with the (e) soft object and (f) stiff object. Object
dimensions are 60 mm # 60 mm # 120 mm. (a)-(c) refer to the SoftHand terminal device; (b)-(d) refer to the soft gripper. Both the
measured (in blue) and saturated (for analysis simplification, in red) current residuals are reported. The soft gripper absorbs more
current than the SoftHand, by virtue of the former's longer phalanges.

and velocity profiles are used to estimate the parameters (their
numerical values are reported in Table 1) by means of conventional least-squares identification algorithm. This leads us
to the estimated disturbance torque xt model, and the following
equality holds:
xt model

- K model I cal,

(5)

where the calibration current I cal, which is the current the
motor needs to close the empty terminal device, can now
be determined.
52

*

IEEE ROBOTICS & AUTOMATION MAGAZINE

*

december 2018

Taking I cal as the current offset, experiments in which the
hand grasps various objects were carried over. Now, we can
establish the following relation:
xt int

- K int I res - J (q) T w ext,

(6)

where I res = I m - I cal is the current residual measured by the
calibrated motor and is recognized as the current the hand
needs to close and grasp an object (recall that I m is the total
absorbed motor current); so it is zero if the hand closes
empty and proportional to the resistance it encounters



IEEE Robotics & Automation Magazine - December 2018

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - December 2018

Contents
IEEE Robotics & Automation Magazine - December 2018 - Cover1
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