KE-750_MAINTE.pdf - 第130页

page), both the speed contro l and the positioning cont rol are controlled with the rotary encoder of the motor. T herefore, if there is a mechanical positioning offset in the dr ive system, it is difficult to accurately…

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(5) When the XY axes move, the sensor head performs ③ magnet

detection (detects the magnetic material embedded every 40 µm) on

the magnescale, and moves. The ④ detection signal from the sensor

head is then multiplied by 4 at the magnescale detector (1 pulse = 10

µm), and the multiplied signal is fed back as ⑤ positioning pulse to

both the deviation counter of the AC servo driver and the counter of the

AC servo control board.

(6) The counter reading on the AC servo control board increases.

When the positioning pulse is input to the deviation counter of the AC

servo driver, the deviation counter performs a computation (Command

pulse minus positioning pulse). According to the counted value on the

deviation counter, the control circuit controls (positioning control) the

position of the XY axes. Therefore, when the XY axes approach the

target position which is 100 mm, the motor decelerates. When the XY

axes reach the position of 100 mm, the deviation counter value is set 0,

and the motor stops. Stop operation and acceleration/deceleration

operation differ depending on the gain setting of the AC servo driver.

So, an appropriate gain setting for the system shall be found and set.

When the (7) motor stops, the counter on the AC servo control board reads

100 mm/20 µm= 5000.

As described above, in the fully closed loop system, the speed control and

the positioning control are performed independently, and the position of the

XY axes is always detected with the magnescale, the axis can be stopped

accurately even if there is an over load or the belt becomes lose.

Using the fully closed loop system, a twin servo system of the Y axis can be

easily realized. In the twin servo system, two motors are simultaneously

driven. However, the characteristics of the servo driver and of the servo

motor are not the same between the YL axis and the YR axis. In addition,

the load conditions between the YL axis and the YR axis cannot be made

the same because of the difference in the position and installation of the

heads. Accordingly, there is a difference between the positions at the left

and the right even when the YL axis motor and the YR axis motor are

rotating.

In this case, in the fully closed loop system, the positions of the YL axis and

the YR axis are detected with the dedicated magnescale to each axis so

that an accurate positioning can be made.

On the other hand, in the semi-closed loop system (described on the next

page), both the speed control and the positioning control are controlled with

the rotary encoder of the motor. Therefore, if there is a mechanical

positioning offset in the drive system, it is difficult to accurately control the

left and right positioning.

To assure a high positioning accuracy (high resolution and accuracy), the

fully closed loop system is better. When a long ball screw is used in the

semi-closed loop system, the error among the lead pitches of the ball

screws shall be corrected for every machine. This correction value must

be stored as a parameter in the program, and this makes adjustment

troublesome.

(2) Control system of Z/θ axes

For KE series, a semi-closed loop system is used to control the motor of the

Z/θ axes. The following explains the theory of operation.

Figure 6.2 shows the fully closed system for the Z axis. And, the example

shows that the Z axis is moved by +5 mm from the origin.

① + Command

voltage

Z/θ servo control board AC servo driver

Power for the U, V, W phase driver

Control

circuit

Control circuit

Speed control

Deviation

counter

Motor

LM628

② Speed detection and e position detection

Rotary encoder

Counter

(Stores the

current position.)

Figure 6.1 Semi-closed loop system

(1) The SUB CPU1 board of the control unit sets the data such as moving

distance, acceleration/deceleration, and maximum speed of the Z axis

to the Z/θ control board, and issues a start command.

(2) The motor controller IC, LM628, a dedicated IC for positioning control

and motor drive, and it has the deviation counter and the positioning

counter for the current position. However, the AC servo driver

dedicated to the speed control does not have the deviation counter for

positioning control. Therefore, the AC servo driver controls the speed,

and LM628 controls the position. The current position counter

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memorizes the stop position at the start time but the deviation counter

(3) When LM628 analyzes the data above, and profile of the motor drive is

complete,① command voltage is gradually applied to the AC servo

driver. The AC servo driver then controls the rotation speed of the

motor according to the voltage being applied. (The range of the

command voltage is ±10 V.)

(4) The AC servo driver counts the number of pulses from the rotary

encoder (1 pulse = 10 µm), and detects (② speed detection) the

rotating speed of the motor. On the other hand, the counted pulse is

input to both the current position counter and the deviation counter

(DOWN side) of LM628. As a result, the reading of the current

position counter increases, and that of the deviation counter decreases.

(③ position detection)

(5) The deviation counter performs a computation, (command pulse) -

(position pulse), and when the Z axis approaches the target position

plus 5 mm, the command voltage goes near 0 V to decelerate the

motor. As a result, when the Z axis reaches at +5 mm position, the

deviation counter reads 0, and the command voltage is set 0 V to stop

the motor. The operation of stop and acceleration/deceleration may

largely change depending on the gain setting of the AC servo driver.

Therefore, an appropriate gain setting value must be found and set for

the system.

(6) When the motor stops, the absolute counter of LM628 reads the position

at the start plus 500.

As described above, in the semi-closed loop system, both the speed control

and the positioning control are performed together with the rotary encoder

directly coupled to the motor. Therefore, if there is a defect in the

mechanical system before the motor shaft (overload, loose screw, etc.), the

stop position of the ball screw may be offset even when the rotating position

of the motor is normal.