I would appreciate a practical viewpoint in a problem
involved in turret servo repair of Femco HL15
CNC lathe.
Original Mitsubishi Melservo MR-J2-A-S24 stopped
working obviously because of failed encoder. (It runs perfectly
when the encoder is borrowed from another identical machine for
testing) According to Mitsubishi policy the entire servo unit is
meant to be replaced. This brings us to the idea of repairing this
machine in practical and independently sustainable way. And further...
in a way that serves CNC-lathe owners in general.
My idea is to rebuild this turret servo using GD servo
amplifier with suitable and readily available encoder. Within my knowledge
and documentation of interface between servo system and CNC controller
(FANUC Oi-T) I see two main problems to be solved.
#1. Absolute positioning system.
- Original system comprises complicated absolute positioning memory.
Battery back-up for SRAM in servo, battery line for encoder where there
is an additional ultracapasitor to allow it to be disconnected from
servo amplifier for on hour or two. By the battery going flat within
intervals of few years there is a procedure to regain absolute
positioning.
- According to my understanding this positioning memory is needed
even thought the encoder is of absolute type. This is because mechanical
transmission ratio (1:9) between turret and motor. So it retains the track
of motor revolutions that the encoder cannot tell after power has been
down.
#2. Lack of I/O:s available.
- Communication between servo system and CNC-controller is simple I/O.
No sophisticated protocol is needed because amplifier firmware is hacked
directly to make 12 positions. Four lines are used to command position,
another four indicates the same back to controller (as feedback?) when the
positioning is attained. There are several I/O:s more to accomplish the
whole thing. Simple and well documented but takes so many lines.... So the
question is how we could interface a dozen of I/O lines to be operated
with the servo amplifier (Argon...?).
Ideas for the solution...
-Absolute positioning could also be attained at start-up by driving the carrier to
safe position and rotating the turret to find an index. How to make an index pulse
system is another story, there are several, feasible, ways to do it anyway.
No positioning memory, no absolute encoder, simple and robust approach.
-If demand for numerous I/O:s cannot be met by servo amplifier itself so one
possibility could be to use some microcontroller board as a front-end system
for this. But would it then be natural to give all interfacing tasks (indexing
also) to it... so servo amplifier just takes step and direction pulses in and
returns signal to tell positioning being attained.
- If this is the way to make an option to turret drives, and if this ever be
realized... the next question is how general it is and how general it could be
to serve every CNC-lathe owner. So you people out there, if you have knowledge
of tool turret control systems, please comment.