servo motor home positioning

hi folks,
m workin on Mitsubishi Servo amplifier MR J2 S20A & Servo motor of 200 w (mitsu make)....I have configured the whole application & it's workin fine but if power fails i lost the reference position of the servo motor shaft...i can regain the home position by mounting a sensor & using ZRN istruction (FX 1N plc) but i don't want to use the sensor coz of the space constraints.....
i've gone through the ABS (absolute data read) instruction & even fitted a battery for encoder but when power is cut the encoder can't keep the track of the motor shaft motion (if applied manually) during that time....so on regaing power i lost the home position
can anybody tell me a solution for this problem without using sensor & if not possible here how same thing is implemented in other industrial grade servo systems? gimmee make of some good servo systems
also pls tell me how encoder in the industrail grade servos work
(i hope it's disc type optocoupler & not pot type but some more
details)
thanx for goin through such a lenthy mail.....

cheers,
joe.

I have used a drive called Quicksilver, and can do a home by "Homing against a hard stop".

i.e. If I have a lead screw, I set the torque limit quite low so I don't smash the end bearing, rewind the drive to the end of travel so it hits the end mount, then advance the drive to the internal index marker.
Voila - homed without adding a sensor!

RT

I've only worked a little with servo motors and the others on this site may be of more help. But generally the position is held within the servo itself and, if power is totally lost, that position is lost. The encoder by itself does not give position information, it gives movement information which is then used by the servo to update the current position. Some servos provide for a 'logic' power seperate from the main motor power. This logic power, if uninterrupted, should hold the current position and, if the encoder is also powered, will update the internal position on manual movement when the motor power is disconnected.

I've been using same J2S servo couple of times but never needed
absolute positioning. If you do, you could and should use
battery (as you already tried).
I don't have time to check the manual but I don't think that
just connecting the battery would do the trick. There is
parameter ("basic parameters") that you must set in order to
make it work in absolute mode.

hi there,
RT's solution won't work in my case coz my system involve a disc mounted on a servo shft....the disc carries 40 characters on engraved on periphery which need to be punched on chassi.....so it's a cont revolving motor system with gear box .......
hey bernie_carlton, can u name that logic backed up servo u were talkin bout?
anyways thanx guys for acknolowdging

Rob T,

I must say.. your approch is very novel....

try a resolver

Lucifer,

"the resolver is a passive transducer. It is a brushless rotary transformer with one rotor and two stator windings......"
Does not use pulses, so is immune to vibration and is aware of position after power down/up.

Check out Autotech Controls Rotary Position Transducers. I have used this product and it worked flawlessly with its own decoder that can give you position,speed,loss of motion detection etc etc..

Hope this helps,

Brian.

to panic mode....
i've set the necessry parameters......& i hope i've gone thr' manul very carefully......even the mitsubishi system house people are doubtful about this thing & they advised me to use the sensor
but i wonder if this is the case then how large systems like Welding Robots work which needs lot of axes & in turn lots of servos.....do the use sensors for reference position?

hi silver
can u give me more details ......
how can i use it in existing system?...whos the manufacturer?....any links?.......pls

Ok, you made me look it up.
For absolute positioning you have to set Param #1 to be 1.
(or use param#101 if you use AD75M1 motion card).
I have only SH(NA)30007C manual at hand which is for J2S-B.
Yours should be SH(NA)30006E...
It can be downloaded from www.meau.com (size is just under 4mb).
Check out chapter 15 of the SH(NA)30006E
.

Resolver

Lucifer,

I have never worked with servos so I can't tell you how to use resolver technology in your application.
I was using a Modicon PLC and Autotech (brand) supplied a PLC input card that had the decoder that is used to convert the resolver's analog signal to digital values that I could read in the PLC.
It was a few years back but I think Autotech is still in business,
otherwise there must be other resolver manufactures,
The technology is very solid.

Brian.

elevmike said:

Rob T,

I must say.. your approch is very novel....

Click to expand...

Acutally it is very common, especially in high precision machine tool applications. Sometimes it is the homing routine, or other times it is at the other end. Then it is called feed to positive stop instead of home. Indramat cans the routine in their controllers.

I am not familiar with mitsubishi, but do they have an option for a multi-turn absolute encoder? I use them with indramat and they never lose their position, even if you turn the shaft with the power off.

hi panic mode,
m using FX 1N PLC......outputs Y0 & Y2.......i had tried earl;ier setting para 1 setting 1xxx but it doesn't work...in absolute positioning as per this setting the motor doesn't respond when i fire those o/p pulses....there's nothing wrong in ckt......i can read pulses in D8140 ......the motor is not even servo locked in this motor
now what?
it's your turn......

Rick,

To me its novel. In the case of elevators we do not have the option of driving a device aganst an fixed object to stall the elevator, then reset the poision; as code prohibits it.

My experience is largely limited to my field, so this is new to me. I have done a few other type projects, and never considered homing using this approch. To me everything is an elevator, (regarding my approch).

A few years ago I did a positioning system for a large peice of equipment. The enviroment is very dirty, and the limits cause problems if not well maintained. This approch may resolve the issue. I now have to re-think it based on this approch.

So now I learned something new.

Mike.

lucifer,

Can you post your wiring diagram? It is obvious that
you use high speed outputs to send pulse trains to the
servo but I don't know what else you have connected.
Are you using CN1A-8 and CN1A-9 (ABSM and ABSR)?