How to find center using software limit switches

[romulus]

Hi,

I'm using 3 stepper motors w/ incremental encoders (by Agilent technologies), NI 7344 board and NI MID-7604 power controller.

I don't have hardware limit switches and I don't have a need for them. I found the software limit switch option under MAX and tried configuring them. I've been testing with LabView's One Axis Find Reference with Status Monitor.vi and I'm having problems.

Under max my motors are configured as closed loop/absolute position/2000 steps per cycle. I set my software limits to 2000 forward and -2000 reverse. In the vi i'm specified Find Center under search type and the motor just seems to keep turning in clockwise direction and then abruptly stops with a modal error: -70166 saying that the axis tripped on the following error.

I would greatly appreciate detailed information on what I may be wrongly configuring in MAX and how to do it properly. Thanks in advance.

-Roman

[romulus]

..bump..

[Jochen]

There is a 99% chance that there is something wrong with your settings for the "encoder counts per revolution" and motor "steps per revolution" values.
There is a good document for troubleshooting this type of problem in the NI knowledge base.

Please post again to this thread if you need more help with this issue.

Best regards,

Jochen Klier
National Instruments Germany

[romulus]

Hello Jochen,

I have a correct setting for my encoder counts and my steps per revolution. I have manually checked the rotation and the encoder outputs 1600. That is the number I have set.
I'm using the load software limits vi and I'm not having any luck with this. I'm including my vi for your review. Your help is greatly appreciated.

Roman

[romulus]

Oh one more thing.

What I'm trying to accomplish is actually to simply find the stepper encoder's reference position. I have a setup where my motors rotate 3 wheels with 8 filters in each. In case of power outage or somebody manually rotating the motors, upon reboot I would like to be able to automatically go back to a preset default position. I don't need to know where I was stopped (thus no need for absolute encoders).

Perhaps these details are helpful.

Thanks.

[Jochen]

Ah, now I understand. Find Reference only works with hardware limits. In fact as you don't know where the axis is at the beginning software limits are meaningless. Thus a find referecne (center) doesn't make sense.

You have mentioned an index signal of your encoder. Find reference can search for this index pulse. I think this would make more sense.

Best regards,

Jochen Klier
National Instruments Germany

[romulus]

Precisely. I was a little confused by a vast amount of information on this topic but very little specific details. Stepper encoders always have an index pulse. So I'm going to try that in a few minutes. I'll post back with results.

Thanks for your help.

[AnalogKid2DigitalMan]

Why not drill a hole in your filter wheels and use an opto to act as a home indicator.

[romulus]

That is a good suggestion. The only problem is that we're using PMTs and the way the filters are positioned, there might be problems with light interference.

I have actually found what the problem is and why it couldn't locate the index pulse. Our electronics person made a mistake and ordered a 2-channel encoder without index pulse. So i'm ordering the correct encoder now.

Thanks.