Electricity: motive power systems – Limitation of motor load – current – torque or force
Reexamination Certificate
1998-10-05
2001-04-03
Masih, Karen (Department: 2837)
Electricity: motive power systems
Limitation of motor load, current, torque or force
C318S432000, C318S433000, C318S291000, C318S635000
Reexamination Certificate
active
06211636
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a pressing machine for conducting pressing by means of motor output torque and to a torque control method for a motor controlling this press.
DESCRIPTION OF THE RELATED ART
Pressing machines for conducting pressing by controlling pressing force by controlling the output torque of a motor, are commonly known. This conventional pressing force control obtains the required pressing force by applying a torque limit to the output torque of a motor, in other words, by restricting the output torque of the motor.
In this case, the torque limit value for the motor is derived from the intended pressing force and the static relationship between the torque generated by the motor and the pressing force. For example, if a simplified pressing machine as illustrated in
FIG. 8
is considered, then assuming that the effect of friction is ignored, the static relationship between the torque T generated by the motor and the pressing force fp is given by Equation (1) below.
fp=a·T
(1)
In this equation, a is a coefficient for converting rotational force to linear force.
Therefore, the torque Tm for obtaining the target pressing force F is given by the following equation, where fp=F and Tm=T in equation (1) above.
Tm=F/a
(2)
Therefore, if the output torque of the motor is limited to Tm, in other words, if the torque limit value is set to Tm, then the desired pressing force F can be obtained.
As described above, conventionally, a torque limit value Tm corresponding to the target pressing force F is derived from equation (2) above, and the required pressing force is obtained by driving the motor while restricting its output torque to this derived torque limit value Tm.
FIG. 9
is a control block diagram of a servo motor of a conventional pressing machine for conducting pressing by restricting the output torque of the servo motor where a servo motor is used as a motor. Velocity control means
1
implements velocity loop control, such as proportional plus integral control, or the like, in accordance with the instructed velocity command Vc and a velocity feedback value vf which is fed back from a position and velocity detector
6
for detecting the rotational position and velocity of a servo motor
5
, and determines a torque command Tc. Thereupon, a torque command Tc′ limited by torque limiting means
2
, in which torque limit value Tm determined in equation (2) is set, is obtained, current control is implemented by current control means
3
in accordance with the torque command Tc′, and the servo motor
5
is driven via an amplifierlifier
4
.
If no pressing load is applied, the servo motor
5
will follow the command velocity Vc, without a large load being applied thereto, there will be no large velocity deviation between the command velocity Vc and velocity feedback value vf, so the torque command Tc output by velocity control means
1
will be a small value, and this torque command value Tc can be output without being restricted by the torque limiting means
2
. In other words, Tc=Tc′.
If a metal pattern is placed on the work and a pressing load is applied, the velocity deviation will increase and the torque command Tc output by velocity command means
1
will increase and rise above the torque limit value Tm. However, since the torque command Tc having risen above the torque limit value Tm is restricted to the torque limit value Tm by torque limiting means
2
, the torque command Tc′ output by current control means
3
will be the torque limit value Tm. Thereby, the output torque of the servo motor will assume the toque limit value Tm, and the target pressing force F will be a Tm, according to equation (1) above (F=a·Tm).
However, in order to reduce noise during pressing, and the like, in some cases, pressing is conducted under deceleration. Furthermore, in some cases, pressing may be conducted under acceleration. In these cases, in the conventional motor control method described above, the required pressing force differs from the actual pressing force. Supposing that the acceleration of the motor is taken as a and the total mass of the moving body driven by the motor is taken as M, then if friction is ignored, the relationship between the pressing force fp (=pressing reaction) and the motor output torque T will be as shown in equation (3) below.
a·T−fp=M&agr;
(3)
hence
fp=a·T−M&agr;
(3′)
However, if the motor output torque T is restricted to a torque limit value Tm(=F/a) corresponding to the target pressing force F, (in other words, T=Tm), then equation (3′) above becomes
fp=a·Tm−M&agr;=F−M&agr;
(4)
In equation (4) above, if &agr;<0, in other words, when the motor is decelerating, the generated pressing force fp is greater than the target pressing force F(fp>F), and there is a possibility that the metal pattern will rupture. On the other hand, if &agr;>0, in other words, when the motor is accelerating, the generated pressing force fp is smaller than the target pressing force F(fp<F), so the required pressing force will not be obtained.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a pressing machine and a motor torque control method for a pressing machine, whereby the required pressing force can be obtained during acceleration or deceleration also.
The present invention is a motor torque control method for a pressing machine which applies pressing force by limiting the output torque of a motor by restricting a torque command through torque limiting means provided in a motor control circuit, wherein a torque limit value corresponding to a target pressing force is corrected by the torque required for acceleration or deceleration, and the motor is driven whilst the torque command value is restricted by this corrected torque limit value, such that the target pressing force is applied to a work during acceleration or deceleration.
The torque required for acceleration or deceleration is determined by the actual velocity detected by a velocity detector or the acceleration as calculated from the velocity command, and this is taken as a torque limit correction value. Alternatively, an observer for estimating acceleration from the torque command value and the actual velocity detected by a velocity detector is provided, and the acceleration estimated by the observer is taken as the aforementioned necessary torque for acceleration or deceleration, and this is taken as the torque limit correction value.
Since the present invention corrects the torque limit value corresponding to the target pressing force by the torque required for acceleration or deceleration and takes this as a torque limit value for restricting the torque command, it is possible to apply the target pressing force to a work at all times, during acceleration and deceleration also. Consequently, instances of the metal pattern rupturing due to application of excessive pressing force to the work, or of insufficient pressing force, do not occur.
REFERENCES:
patent: 4852434 (1989-08-01), Bald
patent: 5265590 (1993-11-01), Takagi
patent: 5509847 (1996-04-01), Jinno et al.
patent: 5555178 (1996-09-01), Hashimoto
patent: 5690577 (1997-11-01), Enzmann et al.
patent: 4-210899 (1992-07-01), None
patent: 6-31499 (1994-02-01), None
Hamatani Hidekatsu
Iwashita Yasusuke
Matsubara Shunsuke
Fanuc Limited
Masih Karen
Staas & Halsey , LLP
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