Mitsubishi Electric Melsec FX Series Beginners Manual page 34

Components of Positioning Control and their Roles
Absolute type servo motor encoder (absolute position detection system)
The absolute position detection system requires an absolute position motor encoder, a backup
battery on the servo amplifier, and a parameter specification setting. It is constructed so that the
current value stored in the positioning controller is always assured, regardless of power outages or
movement while the power is turned off. The advantage to using this method is that after executing
the zero point return function once, zero point return it is not needed again.
NOTE The zero point return function does not actuate movement to a physical zero address. Instead, the
zero point return function causes movement in a specified direction (positive or negative) in order
to define the physical zero address after contact with a DOG switch.
Example Example of DOG type zero return
In the example in Fig. 3-5, the DOG (which is attached to the workpiece) comes in contact with the
DOG switch to turn the DOG signal ON, which then initiates deceleration to creep speed. After the
backward end of the DOG passes the DOG switch, turning the DOG signal OFF, the first detected zero
point signal stops the motion, turns the CLEAR signal on, and sets the zero point address.
The zero point address (specified in the controller's parameters) is typically zero. When the zero return
function finishes, the zero point address is written to the current value register of the positioning
controller to overwrite the current address. Since the zero point address is not always zero, the zero
return function should be thought of as a homing function instead of a return-to-zero function.
The zero point return direction, zero point address, zero signal count, return speed, deceleration time
and creep speed are all set by parameters in the positioning controller.
The location of the DOG switch should be adjusted so that the backward end of the DOG is released
between two consecutive zero point signals (1 pulse per rotation of the motor).
In this example, the DOG length should not be less than the deceleration distance of the machine.
3 - 6
Zero point
return speed
Deceleration time
DOG switch
Creep
activated
speed
Zero point
DOG
Zero point return
switch direction
DOG DOG
Forward end Backward end
CLEAR signal
Fig. 3-5:
Initial position
Positioning controller
Positioning pattern of
DOG type zero return
MITSUBISHI ELECTRIC
313010da.eps