Velocity control device using servo motor

A velocity control device controls a velocity of an object via a servo motor so that a damping phenomenon during acceleration or deceleration and a swelling phenomenon of a specified frequency arising at a specified velocity are relieved, while disturbance suppression ability is improved under a wide range of low accelerations. The device comprises an integral gain variable calculation section including an acceleration variable section for calculating an acceleration variable integral gain from an acceleration factor according to an acceleration command value and a reference integral gain and a weighting calculation section for calculating an integral gain from a weighting factor according to a velocity command value, a reference integral gain, and an acceleration variable integral gain. The device may therefore variably set an integral gain according to an acceleration command value and a velocity command value.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a velocity control device applied to a 
velocity control system, for example, a numeral control machine. 
2. Description of the Related Art 
FIG. 6 is a block diagram of a conventional velocity control device which 
controls a velocity of an object mechanically connected to a motor, by 
controlling a velocity of a servo motor. To a velocity control device 1, a 
velocity command value V is supplied by a precedent command device (not 
shown in the figure). A subtracter 100 subtracts a motor velocity vm from 
the velocity command value V and calculates a velocity deviation V-vm. 
Here, the velocity deviation is amplified by an operation of PI 
(proportional integration) amplification described below, and becomes a 
torque command value .tau.c. The velocity deviation V-vm is amplified by a 
factor of a proportional gain Kp in an amplifier 101 and produces a 
proportional component .tau.p. Furthermore, the velocity deviation is 
amplified by a factor of an integral gain Ki in an amplifier 102 and 
becomes d .tau.i/dt, is integrated in an integrator 103, and produces an 
integral component .tau.i. The .tau.p and the .tau.i are added together in 
an adder 104 to become a torque command value .tau.C. 
A power amplifying section 105 is comprises an electric power amplifier 
(not shown in the figure) and a servo motor (also not shown), and is a 
section to amplify a torque command value .tau.c to a motor output torque 
.tau., and the amplification factor thereof is expressed by a torque 
conversion constant Ct. An object system 107 comprises a motor and a 
controlled object (not shown in the figure) mechanically connected to the 
motor. A disturbance torque .tau.d is a disturbance torque acting on the 
object system from outside, and is added to the motor output torque .tau. 
in an adder 106 shown equivalently, and finally, a torque acted on the 
object system becomes .tau.+.tau.d. A position detector (not shown in the 
figure) or a velocity detector (not shown in the figure) is connected to 
the motor and, on the basis of the detected information thereof, a motor 
velocity vm showing a velocity of a controlled object can be obtained. 
Here, provided that the controlled object is a rigid body and the motor and 
the controlled object are rigidly connected, by using a total inertia 
moment J of the motor and the controlled object, a velocity response of a 
conventional velocity control device 1 shown in FIG. 6 can be expressed by 
the following expression. 
Expression 1 
EQU {(V-vm)Kp+.intg.Ki(V-vm)dt+.tau.i(0)}Ct+.tau.d=J(dvm/dt) 
when .tau.i(0)=vm(0)=0 are initial conditions, when Laplace transformation 
is carried out, the following expression can be obtained from Expression 1 
(wherein S shows an operator of Laplace transformation indicating a 
derivative action, and 2 shows square). 
Expression 2 
##EQU1## 
Here, a damping factor .zeta. of a generally used normal quadratic form 
and a natural frequency .omega.n of the system are defined as follows 
(wherein { } (1/2) shows the one-half power of { }). 
Expression 3 
EQU .zeta.=(Kp/2){Ct/(Ki.multidot.J)} (1/2) 
EQU .omega.n={KiCt/J} (1/2) 
If Expression 2 is expressed, being divided into a command response 
characteristic when letting .tau.d=0, and a disturbance suppression 
characteristic when letting V=0, it becomes the following expressions. 
Expression 4 
EQU vm(S)=[{2.zeta..omega.nS+.omega.n 2}/{S 2+2.zeta..omega.nS+.omega.n 2}]V(S) 
Expression 5 
EQU vm(S)=[(S/J) {S 2+2.zeta..omega.nS+.omega.n 2}].tau.d(S) 
That is, as for a conventional velocity control device shown in FIG. 6, 
Expression 4 expresses the command response characteristic and Expression 
5 expresses the disturbance suppression characteristic. 
From the above description, it is clear that in a velocity control device, 
a command response characteristic and a disturbance suppression 
characteristic can be variable by operating a proportional gain Kp and an 
integral gain Ki, and it is further clear from Expression 5 that setting a 
large integral gain Ki is effective for improving disturbance suppression 
ability. However, if an integral gain Ki is simply set large, a damping 
factor .zeta. is simultaneously lowered and, therefore, damping 
characteristics of the command response characteristic shown by Expression 
4 are worsened. Moreover, since a reduced damping factor arises under 
certain frequencies, swelling phenomena of a specified frequency at a 
specified velocity have an arisen under the influence of rotational errors 
or the like of a position detector having a frequency proportional to the 
velocity. 
SUMMARY OF THE INVENTION 
The present invention is made to address these problems, and an object of 
the present invention is to provide a velocity control device, by which a 
damping phenomenon at the time of acceleration or deceleration and a 
swelling phenomenon of a specified frequency arising at a specified 
velocity, produced as harmful effects because of setting an integral gain 
Ki to be large, are relieved, and disturbance suppression ability may be 
improved under the condition of low acceleration excluding said specified 
velocity and including a wide range of constant velocities. 
The present invention relates to a velocity control device which controls a 
velocity of a controlled object via a servo motor, and the object of the 
present invention is achieved by having an integral gain variable 
calculation section comprise of an acceleration variable section which 
determines an acceleration factor according to an acceleration command 
value and which calculates an acceleration variable integral gain from a 
reference integral gain and the acceleration factor, and a weighting 
calculation section which determines a weighting factor according to a 
velocity command value and finally determines an integral gain at the time 
of velocity control calculation from the reference integral gain and said 
acceleration variable integral gain and weighting factor.

DESCRIPTION OF PREFERRED EMBODIMENT 
FIG. 1 is one example of a block diagram of a velocity control device 1 in 
which the present invention is practiced. FIG. 1 corresponds to FIG. 6 of 
the conventional example and corresponding parts will be given the same 
names and reference numerals and their description will not be repeated. 
An integral gain variable calculation section 2 is composed of an 
acceleration variable section 3 and a weighting calculation section 4. 
First, the action of the acceleration variable section 3 will be 
described. A velocity command value V is differentiated in a 
differentiator 5, becomes an acceleration command value A, and is input 
into an acceleration factor determining section 6. The acceleration factor 
determining section 6 outputs an acceleration factor Ga after a 
calculation described later. A reference integral gain Kio is an integral 
gain which has initially been set into the velocity control device after 
being decreased with a margin so that a sufficiently stable action may be 
possible in all operational states of the velocity control device. The 
reference integral gain Kio is multiplied by the acceleration factor Ga in 
a multiplier 7 and becomes an acceleration variable integral gain Kia. 
FIG. 2 is a graph explaining an example of the action of the above 
mentioned acceleration factor determining section 6. The horizontal axis 
shows an absolute value .vertline.A.vertline. of an acceleration command 
value A, while the vertical axis shows an acceleration factor Ga as an 
output of the acceleration factor determining section 6. A variable limit 
acceleration Aa on the high velocity when letting acceleration factor 
Ga=1, a variable limit acceleration Ab on the low velocity (wherein Aa&gt;Ab 
holds), and a maximum acceleration factor Gamax employed at an 
acceleration of not more than Ab, are initially set in the acceleration 
factor determining section 6. The acceleration factor determining section 
6 determines an acceleration factor Ga as an output by using these 
parameters, employing an absolute value .vertline.A.vertline. of an 
acceleration command value A as an input, from the following relational 
expression. 
Expressions 6 
(in a case of .vertline.A.vertline..ltoreq.Ab) 
EQU Ga=Gamax 
(in a case of Ab&lt;.vertline.A.vertline..ltoreq.Aa) 
EQU Ga=1+(Gamax-1)(Aa-.vertline.A.vertline.)/(Aa-Ab) 
(in a case of .vertline.A.vertline.&gt;Aa) 
EQU Ga=1 
Thus, in the acceleration factor determining section 6, an acceleration 
factor Ga having a tendency to increase with the decrease of 
.vertline.A.vertline., is determined according to an absolute value 
.vertline.A.vertline. of an acceleration command value A. 
Next, the action of the weighting calculation section 4 will be described. 
The weighting factor determining section 8 outputs a weighting factor Gb 
by a calculation described later. A subtracter 9 is a subtracter which 
subtracts a reference integral gain Kio from an acceleration variable 
integral gain Kia, and this subtracter output is multiplied by the 
weighting factor Gb in a multiplier 10. The multiplier output is added to 
the reference integral gain Kio in an adder 11, and becomes an integral 
gain Ki finally employed at the time of velocity control calculation. The 
integral gain Ki is set in an amplifier 102 as an amplification factor of 
the amplifier 102. A series of above mentioned calculations to calculate 
an integral gain Ki are expressed by the following expression. 
Expression 7 
EQU Ki=Kio+Gb(Kia-Kio) 
Here, provided Gb is determined in the range of 0.ltoreq.Gb.ltoreq.1, 
Ki=Kio holds when letting Gb=0, and Ki=Kia holds when letting Gb=1, and Ki 
approaches Kia from Kio as Gb approaches 1 from 0. That is, Gb is a 
weighting factor showing the degree of effects given by an acceleration 
variable integral gain Kia to an integral gain Ki. 
FIG. 3 is a graph explaining an example of the action of the above 
mentioned weighting factor determining section 8. The horizontal axis 
shows an absolute value .vertline.V.vertline. of a velocity command value 
V, while the vertical axis shows a weighting factor Gb as an output of the 
weighting factor determining section 8. In the weighting factor 
determining section 8, 3 pieces of velocities Va, Vb, Vc (wherein Va&gt;Vb&gt;Vc 
holds) and weighting factors Gba, Gbb, Gbc respectively corresponding 
thereto are initially set. The weighting factor determining section 8 
determines a weighting factor Gb as an output by using these parameters, 
by employing an absolute value .vertline.V.vertline. of a velocity command 
value V as an input, from the following relational expression. 
Expression 8 
(in a case of .vertline.V.vertline..ltoreq.Vc) 
EQU Gb=Gbc 
(in a case of Vc&lt;.vertline.V.vertline..ltoreq.Vb) 
EQU Gb=Gbb+(Gbc-Gbb)(Vb-.vertline.V.vertline.)/(Vb-Vc) 
(in a case of Vb&lt;.vertline.V.vertline..ltoreq.Va) 
EQU Gb=Gba+(Gbb-Gba)(Va-.vertline.V.vertline.)/(Va-Vb) 
(in a case of .vertline.V.vertline.&gt;Va) 
EQU Gb=Gba 
For example, in a case when Pattern a [Vb=(Va+Vc)/2, Gba=1, Gbb=0.5, Gbc=0] 
is initially set, a weighting factor Gb approaches 1 and an integral gain 
Ki approaches an acceleration variable integral gain Kia, as an absolute 
value .vertline.V.vertline. of a velocity command value V becomes larger. 
This becomes a setting for preventing induction of a swelling phenomenon, 
especially in a case where to set an integral gain Ki to be large, induces 
a swelling phenomenon in the area of low velocity. Next, in a case when 
Pattern b [Vb=(Va+Vc)/2, Gba 1, Gbb=0, Gbc=1] is initially set, a 
weighting factor Gb approaches 0 and an integral gain Ki approaches a 
reference integral gain Kio, at a position near the position [absolute 
value .vertline.V.vertline. of a velocity command value V.apprxeq.Vb]. 
This is effective for the countermeasures in a case when a swelling 
phenomenon of a specified frequency arises at a specified velocity. 
FIG. 4 is a graph in which a comparison of real time response at the time 
of input of a velocity command value expressed by the following 
expression, is made between a velocity control device according to the 
present invention and a conventional velocity device. 
Expression 9 
EQU V(t)=50Vt(0.ltoreq.t.ltoreq.20 ms) 
EQU V(t)=V(t&gt;20 ms) 
(wherein vm(0)=.tau.i(0)=0 holds as initial conditions) [0] in FIG. 4 is a 
graph showing this velocity command value. 
[2] and [3] in FIG. 4 are graphs showing real time response of a motor 
velocity vm to a velocity command of Expression 9 of a conventional 
velocity control device, and such an integral gain Ki and such a 
proportional gain Kp that a damping factor stand a natural frequency 
.omega.n of the system in Expression 3 may fulfill the following 
conditions, are set, respectively. 
EQU Condition of [2]: .zeta.=0.8, .omega.n=31.25 
EQU Condition of [3]: .zeta.=0.2, .omega.n=125 
That is, [3] is the same as [2] in Kp, and is 16 times as much only in Ki. 
It is clear from this that if an integral gain Ki is set to be large so as 
to improve the disturbance suppression ability, as mentioned above, 
damping characteristics are worsened in command response characteristics. 
[1] in FIG. 4 is a graph showing real time response of a motor velocity to 
a velocity command of Expression 9 of a velocity control device according 
to the present invention. In the present example, a reference integral 
gain Kio and a proportional gain Kp are set so that a damping factor 
.zeta. and a natural frequency .omega.n of the system in Expression 3 may 
fulfill the condition of said [2]. Furthermore, control parameters are 
arranged such that a maximum acceleration factor Gamax=16, a variable 
limit acceleration on the high side Aa&lt;50 V, a variable limit acceleration 
on the low side Ab&gt;0, and a weighting factor Gba=Gbb=Gbc=1. When a 
velocity command shown by Expression 9 is input into a velocity control 
device according to the present invention designed like this, a damping 
factor .zeta. and a natural frequency .omega.n of the system corresponding 
to the real time become as follows. 
EQU Condition of [1]: .zeta.=0.8, .omega.n=31.25 (0.ltoreq.t.ltoreq.20 ms) 
.zeta.=0.2, .omega.n=125 (t&gt;20 ms) 
In [1], an integral gain Ki is 16 times as much as that in [2] similar to 
that in [3], but it is clear that the amount of damping is improved, 
compared with that in [3]. 
FIG. 5 is a graph expressing disturbance suppression characteristics shown 
by Expression 5 under the condition of a constant velocity command value, 
for a velocity control device according to the present invention and a 
conventional velocity control device, and the horizontal axis is an 
angular frequency .omega.[rad/s], and the vertical axis is a gain G=vm 
(.omega.)/.tau.d (.omega.), and both axes adopt logarithmic coordinates. 
[1], [2] agree with conditions of the same numerals in FIG. 4. It is clear 
from this that in a velocity control device according to the present 
invention, a disturbance suppression ability especially for a disturbance 
input of up to more than 10 Hz is approximately 16 times that of a 
conventional velocity control device. 
As described above, a velocity control device according to the present 
invention includes an integral gain variable calculation section composed 
of an acceleration variable section which determines an acceleration 
factor according to an acceleration command value and calculates an 
acceleration variable integral gain from a reference integral gain and the 
acceleration factor, and a weighting calculation section which determines 
a weighting factor according to a velocity command value and determines a 
final integral gain from the reference integral gain, and said 
acceleration variable integral gain and weighting factor. Therefore, since 
a high integral gain can be set at a time excluding the time of 
acceleration or deceleration and the time of a specified constant 
velocity, the disturbance suppression ability can be improved under the 
conditions of low acceleration excluding a specified velocity and 
including a wide range of constant velocities, while relieving a damping 
phenomenon arising at the time of acceleration or deceleration and a 
swelling phenomenon of a specified frequency arising at a specified 
velocity. 
While there has been described what is at present considered to be a 
preferred embodiment of the invention, it will be understood that various 
modifications may be made thereto, and it is intended that the appended 
claims cover all such modifications as fall within the true spirit and 
scope of the invention.