Homing with index

[ftec]

Where can I find enough information to understand the concept of homing with index and implementing it? Drives are VSD-A. Thanks.

[Xerxes]

There are only two parameters that sets homing with index:
-Use index pulse for homing: disabled, positive edge or negative edge
-Index search direction: positive or negative

So first define wich polarity pulse the encoder is outputting. In 99% cases its positive. Then just choose rotating direction where drive starts looking for index. If you choose positive, then drive will rotate motor to positive direction and stops at index pulse.

You might also want to use index with home switch or hard stop homing feature to make drive stop on correct revolution. You may also adjust homing speed from Trajectory planner page.

Its safe to experiment with settings if you set torque limits so low that hitting machine ends won't damage anything.

[ftec]

Ok, so one index pulse per rev. Can you tell how accurate this homing is? With rotational axis I don't feel like trusting external homing switches to get the angles right.

So, if I don't want to use homing switches at all I must power the machine off so that all motors are within the same revolution where I want them to be homed when power is turned on. Perhaps run them, say, a half rev away from the index home before turning the power off?

When power is turned on the homing obviously will start for all axis simultaneously. Can I control the starting of the homing with the Disable input (we have a separate Disable/Enable switch for each axis)?

If one axis dies is it possible to home only this one axis?

Thanks

PS. Is the hard stop homing feature you mentioned a feature in VSD-A? If so, how does it work?

[mactec54]

ftec
It is as accurate as your machine is Ballscrew Etc, & the same every time you home the machine

[ftec]

ftec
It is as accurate as your machine is Ballscrew Etc, & the same every time you home the machine

I meant accuracy within appr. how many encoder pulses. When homing with external switches you can expect the 0 point to be a little different each time. Definitely must be a lot better when homing with the motor axis itself.

What I'm after here is that once a rotational axis table has been aligned parallel to xy plane how accurately it will be parallel when homing with index or is a mechanical locking pin & adjusting still needed each time. (You can't answer this question for me though).

[mactec54]

ftec
I did answer your question,It will be the same every time, The encoder only has one Z pulse/index pulse, if using this your machine will be 0.0 everytime

[ftec]

ftec
I did answer your question,It will be the same every time, The encoder only has one Z pulse/index pulse, if using this your machine will be 0.0 everytime

Ok, I'll take your word for it then, sounds very promising. A short article of these extra homing features and how to use them would be good. They are mentioned on one line only in the manual.

[Xerxes]

For overview, there are two videos demonstrating drive homing in action:
http://www.youtube.com/user/granitedevices
Please read also video descriptions.

Ok, so one index pulse per rev. Can you tell how accurate this homing is? With rotational axis I don't feel like trusting external homing switches to get the angles right.

Repeatability is within one encoder count.

So, if I don't want to use homing switches at all I must power the machine off so that all motors are within the same revolution where I want them to be homed when power is turned on. Perhaps run them, say, a half rev away from the index home before turning the power off?

Yes, or use home switch/hard stop homing to do that for you.

When power is turned on the homing obviously will start for all axis simultaneously. Can I control the starting of the homing with the Disable input (we have a separate Disable/Enable switch for each axis)?

Yes, disable does that.

If one axis dies is it possible to home only this one axis?

What you mean by dying? Faulting? No need to re-home it, just enable drive and it goes back to position.

[ftec]

OK, that's it.

The hard stop doesn't seem like an option in our case though it looked pretty cool. Instead, I can see that defining the machine zeros just a few pulses off from the index home positions will work for this machine: I'll just run all the axis to machine zero when quitting for the day. Then, after the homing to index, the homing sequences will offset the axis back to the machine zeros. If I also mark the zeros approximately to be on the right rev it's easy to recover if power fails.

One last question at this point: The index wires are marked index+ and index-. Do both wires need to be connected to VSD-A or is the pulse measured against GND and only the positive or negative index wire is needed according to if it's negative or positive edge which is used? (I'm using BL servos).

[Xerxes]

The index input is identical to A and B channel inputs, so connect it same way than A & B.

[ftec]

OK, thanks.

I'll try to remember to give feedback of how it went but it'll take some time to make the new wirings and changes.

[ftec]

All four axes now have homing with Index and they seem to work perfectly and accurately. The positive Index Search Direction accepts negative values for Homing Offset so it was pretty easy to set the Homing directions the way I wanted.

The only negative thing I notice after playing with it a while was that if - and when - re-referencing with Mach position is needed the power must be turned off to start the Homing sequence. Currently I have no switches connected to the VSD-A Home inputs. Is it so that with NO panel switches connected to the Home inputs each drive could be homed when ever necessary and not only on power up?