Position control apparatus using servomotor

Details Position control apparatus using servomotor Position control apparatus using servomotor

1999-05-17

2002-06-18

Black, Thomas (Department: 2121)

Data processing: generic control systems or specific application

Generic control system, apparatus or process

Digital positioning

C318S560000, C318S592000, C700S056000

Reexamination Certificate

active

06408216

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a position control apparatus to be controlled using a servomotor, and more particularly, to a position control apparatus which can minimize a positioning time by optimizing an accelerating command for the position control apparatus.
2. Description of the Related Art
FIG. 5
is a block diagram showing an example of conventional position control apparatus. This position control apparatus comprises a numerical control divider
10
, a servo control divider
20
, a motor
30
, and a position detector
40
. In the numerical control divider
10
, the program interpreter
12
generates a desired value data MD according to the content of the program input to part program storage section
11
. A function generating section
15
a
calculates a speed unit quantity N (n) on the basis of the maximum speed unit quantity Nmax set as a parameter into the maximum speed storage section
13
, the accelerating unit quantity &Dgr;Na and the decelerating unit quantity &Dgr;Nb set as parameters into the acceleration storage section
14
, and the desired value data MD, and then outputs the calculated N (n) to the servo control divider
20
. The servo control divider
20
generates a position command CON by integrating the output speed unit quantity N (n) with respect to time by means of an integrator
21
. Next, a position and speed control section
22
generates a torque command MT on the basis of the position command CON and the detected position APA detected by a position detector
40
, and supplies the torque command MT to the motor
30
via an inverter
23
to drive the motor
30
. In this case, because the position detector
40
is connected to the motor
30
by means of a coupling, the motor
30
is controlled by a position feedback control through feeding back the detected position APA detected thereby to the position and speed control section
22
.
FIG. 6
is a flow chart showing the operation of the function generating section
15
a
shown in
FIG. 5. A
program interpreter
12
generates the desired value data MD according to the content of the program input to the part program storage section
11
, and supplies the desired value data MD to the function generating section
15
a.
The function generating section
15
a
calculates the difference between the position command CON of the desired value data MD and the detected position APA to obtain a residual distance DR (S
1
).
Next, the difference between the residual distance DR and a deceleratable distance DD is calculated, and then mode discrimination is carried out to discriminate whether a speed unit quantity N (n+1) at the next step should be set to an accelerating mode or a decelerating mode according to the polarity of the calculated difference. In other words, if DR>DD, then the discrimination shows an accelerating mode. On the contrary, if DR≦DD, then the discrimination shows a decelerating mode. In this case, the deceleratable distance DD is calculated by executing an integral computation on the basis of the present speed unit quantity N (n) and the decelerating unit quantity &Dgr;Nb set as a parameter beforehand, to determine the decelerating time (S
2
).
When an accelerating mode has been discriminated at S
2
, the difference between the present speed unit quantity N (n) and the maximum speed unit quantity Nmax set as a parameter into the maximum speed storage section
13
is calculated. Then, the mode discrimination is executed to discriminate whether the speed unit quantity N (n+1) at the next step should be set to an accelerating mode or a constant speed mode according to the polarity of the calculated difference (S
3
).
When an accelerating mode has been discriminated at S
3
, N′ (n+1) is calculated by adding the accelerating unit quantity &Dgr;Na, set as a parameter into the acceleration storage section
14
, to the present speed unit quantity N (n) (S
4
a
).
Next, the difference between the N′ (n+1) calculated at S
4
a
and the maximum speed unit quantity Nmax set as a parameter into the maximum speed storage section
13
is calculated. Then, the polarity of the calculated difference is discriminated (S
5
).
When the result of the calculation at S
5
is larger than zero, that is, N′ (n+1)−Nmax>0, the speed unit quantity N (n+1) at the next step is decided as N (n+1)=Nmax (S
6
).
When the result of the calculation at S
5
is smaller than or equal to zero, that is, N′ (n+1)−Nmax≦0, the speed unit quantity N (n+1) at the next step is decided as N (n+1)=N′ (n+1) (S
7
).
On the other hand, when a constant speed mode has been discriminated at S
3
, because the present speed unit quantity N (n) is equal to Nmax, the speed unit quantity at the next step is decided as N (n+1)=Nmax (S
8
).
When a decelerating mode has been discriminated at S
2
, the speed unit quantity N (n+1) at the next step is calculated by subtracting the decelerating unit quantity &Dgr;Nb, set as a parameter into the acceleration storage section
14
, from the present speed unit quantity N (n) (S
9
).
The upper figure of
FIG. 7
shows a change of the speed unit quantity N (n) and a waveform of a motor speed when a desired position has been given. In this waveform of a motor speed, trz shows an accelerating period, tfz a decelerating period, and tz a positioning period necessary for arriving at the desired position. The speed unit quantity N (n) used as a position command is generated in the function generating section
15
a
for every calculation period T.
A period from (
1
) to (
2
) shows a period for an accelerating mode. During this period, a position command is generated by adding the accelerating unit quantity &Dgr;Na, set as a parameter into the acceleration storage section
14
, to the present speed unit quantity N (n). Therefore, the waveform of the motor speed shows constant acceleration having a positive inclination. Time t
3
shows the end of the accelerating period where the N (n) is equal to Nmax.
A period (
3
) shows a period for a constant speed mode. During this period, a position command is equal to the maximum speed unit quantity Nmax.
A period from (
4
) to (
5
) shows a period for a decelerating mode. During this period, a position command is generated by subtracting the decelerating unit quantity &Dgr;Nb, set as a parameter into the acceleration storage section
14
, from the present speed unit quantity N (n). Therefore, the waveform of the motor speed shows constant acceleration having a negative inclination.
The lower figure of
FIG. 7
shows a waveform of motor torque. In this figure, Tq
1
is accelerating torque during the period from (
1
) to (
2
), Td frictional torque during the period (
3
), and Ts decelerating torque during the period from (
4
) to (
5
).
FIG. 8
is a diagram showing an output torque characteristic of a motor. The maximum output torque of the motor varies with a change of the motor speed as follows:
In the range where (0≦motor speed<Nc), the maximum output torque shows a constant value of Tqmax (constant torque region). This is caused by the fact that the electric current to be supplied to motor
30
is restricted by the servo control divider
20
.
In the range where (Nc≦motor speed<Nmax), the maximum output torque shows a curve connecting a point [Nc, Tqmax] and a point [Nmax, Tq
1
] in a coordinate system of [motor speed, motor torque]. Relationships between the coordinates of these two points are (Nc<Nmax) and (Tqmax>Tq
1
). In other words, the motor torque decreases with increase of the motor speed (power supply saturation region).
This is caused by the fact that an induced voltage in a motor increases in proportion to the motor speed, and the voltage difference between the induced voltage and the DC voltage supplied to the inverter
23
decreases, and consequently the motor current decreases below the lower limit in the servo control divid

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