Electricity: motive power systems – Positional servo systems – Plural servomotors
Reexamination Certificate
2002-11-27
2004-10-26
Donels, Jeffrey W (Department: 2837)
Electricity: motive power systems
Positional servo systems
Plural servomotors
Reexamination Certificate
active
06809493
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a servo controller that controls driving of an arm of a robot or a feed shaft of a drive mechanism such as a machine tool, injection molding equipment, or a pressing machine, and in particular, to tandem control in which one movable member is controlled by a plurality of motors.
2. Description of the Related Art
Tandem control is known as a drive method used for drive mechanisms for various machines such as robots, machine tools, injection molding equipment, and pressing machines. With this method, if a movable member to be moved is too large to accelerate or decelerate by one motor that drives a movement shaft of the movable member, the same movement command is provided to a plurality of motors, which then drive this movable member. Thus, the movable member is stably driven while maintaining a proper position. With this method, a drive shaft of each motor must have its position controlled so that the movable member will not be twisted.
FIG. 13
is a block diagram showing an example of configuration of conventional position tandem control. This drawing shows an example of tandem control for a drive mechanism in which two motors, a first motor
15
and a second motor
25
, drive one movable member
4
.
A controller for the first motor
15
comprises a position control section
11
that carries out position loop control, a velocity control section
12
that carries out velocity loop control, a current control section
13
, and a current amplifier
14
. Furthermore, the first motor
15
is provided with a velocity detector
17
that detects a velocity feedback amount (velocity FB
1
). A movable member
4
located closer to the first motor is provided with a position detector
18
that detects a position feedback amount (position FB
1
).
Further, a controller for the second motor
25
comprises a position control section
21
, a velocity control section
22
, a current control section
23
, and a current amplifier
24
. Furthermore, the second motor
25
is provided with a velocity detector
27
that detects a velocity feedback amount (velocity FB
2
). A movable member
4
located closer to the second motor is provided with a position detector
28
that detects a position feedback amount (position FB
2
).
Depending on the conditions of the movable member
4
such as its rigidity, only one or neither of the position detectors
18
and
28
are attached to the movable member
4
. In the former case, the other position detector is installed on an output shaft of the motor. Further, only one of the position detectors
18
and
28
may be provided. That is, the plurality of motors used for tandem control may be provided with the respective position detectors or one common position detector. Furthermore, the position detector
18
or
28
may be attached to the output shaft of the motor or the like to detect the rotating position of the motor and thus the position of the movable member. Alternatively, the position detector may be attached directly to the movable member to directly detect its movement. Further, the position detector may be composed of a linear scale or a rotary encoder.
The position control sections
11
and
21
each receive, from a higher controller (not shown), the same position command distributed by a command distributor
3
, and subtract the position feedback amount (position FB
1
or position FB
2
, respectively; if only one position detector is provided, then position FB
1
=position FB
2
) from the command to obtain a position deviation. The position control section then processes the position deviation amount obtained to output a velocity command.
The velocity control sections
12
and
22
each receive the velocity command from the position control section
11
or
21
, respectively, and subtract, from the velocity command, the velocity feedback amount (velocity FB
1
or velocity FB
2
), respectively) detected by the velocity detector
17
or
27
attached to the motor, respectively, to obtain a velocity deviation amount. On the basis of the velocity deviation amount obtained, the velocity control section executes a velocity loop process including a proportion and integration to output a current command.
The current control sections
13
and
23
each receive the current command from the velocity control section
12
or
22
, respectively, and subtract, from the current command, a current feedback amount from a sensor (not shown) that detects a motor current. The current control section then processes the current deviation amount obtained (current FB
1
or current FB
2
) to output a voltage command.
The current amplifiers
14
and
24
each receive the voltage command from the current control section
13
or
23
, respectively, and form a drive current to drive the motor
15
or
25
, respectively, thereby driving the motor
15
or
25
. Then, the motors
15
and
25
drive ball screws
16
and
26
, respectively, screwed in ball nuts attached to the movable member
4
, thus moving the movable member
4
.
In this manner, on the basis of the same position command, loop processes for position, velocity, and current are executed for the two motors
15
and
25
, so that the movable member
4
is driven by the resultant force of output torque from the two motors.
With the position tandem control described above, repeated accelerations and decelerations cause integral values of integrators of the velocity control sections
12
and
22
to increase on the plus and minus sides, respectively, owing to a difference in loading timing between the velocity feedback amounts (velocity FB
1
and velocity FB
2
) as well as quantization. Thus, an excessive current command may be generated. In particular, if only one position detector is provided and common position feedback is provided to all motors (position FB
1
=position FB
2
), when there is only a small difference between a motor drive position and a position detected by the position detector (when the position detector is attached to a rotating shaft of the motor or to a position close to a drive mechanism for the motor), the integrator in the velocity control section of the control system for the motor generates a current command based on its own integral value. As a result, position feedback is provided so as to eliminate this bias.
However, for the other motor, whether the integral value of the integrator in the velocity control section of the control system for the motor increases on the plus or minus side, position feedback is not provided in a manner such that it eliminates this bias. Accordingly, the integrator continues having such a biased integral value, thereby degrading controllability of the motor. Consequently, the motor may be overheated.
For example, in the example in
FIG. 13
, if there is no or only a small difference between the rotating position of the first motor
15
and the position detected by the position detector, it will be impossible for the integral value of the integrator in the velocity control section
12
of the control system for the first motor
15
to significantly increase. However, for the second motor
25
, position feedback (position FB
2
) is not provided in a manner such that it eliminates the bias in the integral value of the integrator in the velocity control section
22
, with the result that the integral value of the integrators becomes biased, thereby degrading controllability of the motor. Consequently, the second motor
25
may be overheated.
SUMMARY OF THE INVENTION
It is an object of the present invention to solve the above described problems of the prior art to correct a bias in an integral value of an integration element of a velocity control section, thus preventing degradation of controllability of a motor and occurrence of overheat of the motor which may be caused by this bias.
For the purpose of attaining this object, according to a first aspect of the present invention, there is provided a servo controller that allows one driven body to be driven by a plurality of
Iwashita Yasusuke
Maeda Kazuomi
Donels Jeffrey W
Fanuc LTD
Staas & Halsey , LLP
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