Measuring and testing – Instrument proving or calibrating – Dynamometer
Reexamination Certificate
1999-02-22
2002-05-07
Williams, Hezron (Department: 2856)
Measuring and testing
Instrument proving or calibrating
Dynamometer
Reexamination Certificate
active
06382012
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the calibration of a force sensor mounted on an industrial robot.
2. Description of the Related Art
The calibration of a force sensor mounted on an industrial robot is conducted as a part of the manufacturing process after the force sensor is assembled. This precisely calibrated force sensor is mounted on the tip area of a band, such as a wrist flange, of a robot, and the robot is shipped from a factory in a form mounted with the force sensor.
Here, a conventional calibration of the force sensor is described with reference to FIG.
5
.
A calibration stand
60
comprises a base
64
, a firm
62
stood on the base
64
, and a beam
66
one end of which is fixed to the upper end of the fulcrum
62
. A force sensor
40
is mounted on an upper face
66
a
of the beam
66
, and a calibration jig
68
is mounted on the force sensor
40
. Further, a weight
70
is hung on the calibration jig
68
via a hanger
72
.
The calibration of the force sensor
40
is operated by varying the magnitude of force and the moment applied to it by varying the posture of the force sensor
40
in which it is fitted to the calibration stand
60
and the total weight of the weight
70
in many ways.
The principle of the calibration will be described below with reference to the force sensor, which detects six axial forces comprising translational forces Fx, Fy and Fz in mutually orthogonal directions of Y and Z axes and axial moments Mx, My and Mz about these axes.
This force sensor for detecting the six axial forces has eight strain gages attached to the mechanistic section of the force sensor as a force detecting section, and these eight strain gages output voltages as detection signals from the force detecting section correspondingly to loads working on the force sensor.
The voltage output from the strain gages is denoted by v
1
, v
2
, . . . v
8
and the output of the force sensor is denoted by the translational forces Fx, Fy and Fz together with the axial moments Mx, My and Mz as described above, and, further, a calibration matrix of conversion parameters for obtaining the power for force detection to be calculated from output signals from the force detecting section is denoted by C, and then the relationship among them is represented by equation 1.
[
C11
C12
⋯
C18
C21
C22
⋯
C28
⋮
⋮
⋮
C61
C62
⋯
C68
]
[
v1
v2
⋮
v8
]
=
[
Fx
Fy
Fz
M
x
My
Mz
]
(
1
)
When a known force Fx is applied to the force sensor
40
, the relationship of equation 2 below is established using an unknown calibration matrix C.
Fx
=
[
c11
c12
⋯
c18
]
[
v1
v2
⋮
v8
]
(
2
)
If this force Fx is varied in many ways and added m times (i.e. forces Fx
1
, Fx
2
. . . Fxm are added) and the output of the strain gages (v
11
−v
1
m, . . . , v
81
−v
8
m) is measured each time, the relationship of equation 3 below will hold.
[
Fx1
Fx2
⋯
Fxm
]
=
[
c11
c12
⋯
c18
]
[
v11
v12
⋯
v1m
⋮
⋮
⋮
v81
v82
⋯
v8m
]
(
3
)
Equation 3 above can be rewritten into the following equation 3′.
FX
T
=c
1
T
V (3)
Incidentally, in the above-mentioned equation 3′, matrices Fx, and c
1
are:
Fx=[Fx
1
Fx
2
. . . FxM]
T
c
1
=[c
11
c
12
. . . c
18
]
T
And matrix V is represented by the following equation 4.
V
=
[
v11
v12
⋯
v1m
v21
v22
⋯
v2m
⋮
⋮
⋮
v81
v82
⋯
v8m
]
(
4
)
The minimum approximate square solution of matrix c
1
is determined by equation 6 as the c
1
that minimizes the value of the following equation 5.
E
2
=(Fx
T
−c
1
T
V)
T
(−c
1
T
V) (5)
c
1
T
=FX
T
V
T
(VV-
T
)
−1
(6)
In the above-described equation 6, V
T
(VV
T
)
−1
is a pseudo inverse matrix of a matrix V. Similarly, when the forces of the other elements than force Ex are applied at the same time and the output of the force sensor is recorded, the relationship of the equation 7 below is obtained.
[
Fx1
Fx2
⋯
Fxm
Fy1
Fy2
⋯
Fym
⋮
⋮
⋮
Mz1
Mz2
⋯
Mzm
]
=
&AutoLeftMatch;
[
C11
C12
⋯
C18
C21
C22
⋯
C28
⋮
⋮
⋮
C61
C62
⋯
C68
]
[
v11
v12
⋯
v1m
v21
v22
⋯
v2m
⋮
⋮
⋮
v81
v82
⋯
v6m
]
(
7
)
or
F=CV (7′)
Incidentally, in equation 7′, F is a matrix represented by the following equation 8, comprising the output of the force sensor measured m times.
[
Fx1
Fx2
⋯
Fxm
Fy1
Fy2
⋯
Fym
⋮
⋮
⋮
Fz1
Fz2
⋯
Fzm
]
(
8
)
Calibration matrix C given by the above-stated equation 7 or 7′ is to be found out by the following equation 9.
C=FV
T
(VV
T
)
−1
(9)
In order to obtain the pseudo inverse matrix of matrix V, it is necessary and sufficient to apply to the force sensor such forces and moments as will give eight sets or more of linear independent strain gage outputs.
Calibration matrix C obtained in this way is stored and, when the robot is operated and forces are detected by the force sensor, the output from the strain gages of the force sensor (v
1
, v
2
. . . v
8
) and this calibration matrix C are put into arithmetic operation of equation 1 to determine the translational forces Fx, Fy and Fz, and the moments Mx, My and Mz.
On the other hand, if an excessive load is applied to the force sensor and plastic deformation or the like occurs, the measuring accuracy will decrease, and according to the prior art it is necessary to dismount the force sensor from the robot temporarily and perform the above-described calibration again with the stand
60
, calibration jig
68
and other members described above to find out a new calibration matrix C and store it. This not only entails much trouble but also may entail a slight mounting shift in the dismounting/remounting procedure because the force sensor fitted to a tip of the robot is dismounted and remounted. In particular, where the offset between the robot face plate and the tip of the tool is great, the shift of the tool center point (TCP) may often be too significant to ignore, necessitating fine adjustment in the teaching of the robot.
OBJECTS AND SUMMARY OF THE INVENTION
It is the object of the present invention to provide a force sensor permitting ready re-calibration while remaining mounted on the tip of the hand of the robot, so that, even if an accident such as a clash occurs to the force sensor while in use mounted on the robot, and the force sensor is overloaded as a result, with its mechanistic section plastic-deformed and measuring accuracy deteriorated, it can be subjected to simplified calibration entailing only minimal man-hours without requiring replacement.
To attain this object, according to the present invention, reference data for carrying out simplified calibration needing no dismounting of the force sensor from the robot are acquired and stored beforehand, and when re-calibration is needed because of a drop in measuring accuracy by any reason, the simplified calibration can be operated by utilizing the reference data.
The present invention makes it possible to carry out the re-calibration of the force sensor by utilizing tools for conventional use or other members without having to dismount the force sensor from the robot and without utilizing a weight or the like, whose weight and position of the center of gravity are precisely known, so that the force sensor can be easily restored to its normal state in a short period of time.
REFERENCES:
patent: 4620436 (1986-11-01), Hirabayashi et al.
patent: 5092154 (1992-03-01), Eldridge et al.
patent: 5230672 (1993-07-01), Brown et al.
patent: 5261266 (1993-11-01), Lorenz et al.
patent: 0 361 663 (1990-04-01), None
patent: 62-237335 (1987-10-01), None
patent: 3-55189 (1991-03-01), None
patent: 42029 (1992-02-01), None
patent: 7-19982 (1995-01-01)
Ban Kazunori
Hara Ryuichi
Fanuc Ltd.
Fayyaz Nashmiya
Staas & Halsey , LLP
Williams Hezron
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