In Part 1, we got our system set up for homing.
I can’t emphasize enough the importance of verifying that the positive direction
moves towards the positive overtravel limit switch.
If you haven’t seen that video then please go do that before starting this one.
I intentionally have the positive direction going to the left which is the opposite direction
from the homing mode images you’ll see in the sample graphics.
Again, that’s just to highlight that no matter which way you define as positive, the
positive limit switch has to be in that direction.
I’m picking up where that last video left off.
We are connected, the USB light is blinking, we’re online with the drive and we had
just clicked on this guy to bring up the homing configuration and control pane.
Notice that when we switched away from the jog to this homing pane, the software automatically
disabled the drive's output for us.
I’ll re-enable the drive's output by clicking the drive-on button.
So this isn’t really a “drive-on” button, it’s really a drive output enable or motor
enable.
This dialog only gives us homing mode numbers, so I’ll bring up that screenshot of the
homing configuration options we captured in Part 1.
Mode 1 says move in the negative direction – that’s this way for our system – using
these ramps and velocity - and when it reaches the negative over travel limit, reverse the
motor until the over travel limit goes away and then find the first encoder’s index pulse.
That’s a single pulse the encoder generates each revolution.
The drive will find the index, and then circle back at this slower marker search speed or
some people call it “creep speed” to stop right at the index.
I’m going to use these values for this first example to keep things slow enough for you
to see them in the video but increase the ramps so we can clearly see when there are
direction changes.
The reason I’m not using the homing screen in the setup wizard to do this is because
this has to be written to the drive on another screen.
Instead, I’ll enter Mode 1 here which takes effect immediately.
We see the little diagram here to remind us what we selected.
Double-check that the drive is enabled.
Normally your controller will toggle a digital input that you assigned as the homing start signal
to start homing, but we can override that by clicking this guy.
Before I do that though, notice the value in the position register isn’t zero.
Ok, I’ll hit the homing start button.
The motor ramps up to speed, and looks for the negative over travel limit.
When it finds it, it reverses direction until it‘s back off the limit and then it creeps
back to the motor’s index at this slower marker search speed.
And it reset the position register.
I put this black triangle on here so each time we do an index pulse search we should
see this black triangle land in the exact same place.
Of course, you can adjust all of these accelerations and speeds to suit your needs.
In general you will want to keep homing slow – especially the part where it creeps up
on the final location - because it makes homing more accurate.
That’s the number 1 issue we see with homing – folks trying to home too quickly.
Mode 2 is the same thing but in the other direction.
The position register currently says this is the zero position.
I’ll hit the start homing button.
It moves in the positive direction this time - I’ll speed up the video to get us there
quicker - finds the positive over travel limit, reverses out of the limit to the next encoder’s
index pulse. We see the black triangle is right back where we expected it
and the position register has been reset so now, this is the zero position.
Homing on the encoder’s index pulse is way more accurate than homing relative to
the limit switches so use it whenever you can.
But what if the index pulse doesn’t align with your application?
Easy.
Just add in an offset in Parameter 607C.
If you do want to use mechanical switches, then take a look at the Metrol precision limit
switches. They are designed specifically to give you super-accurate mechanical positioning.
These all use the home sensor – this guy - to locate home instead of the over travel
limits.
That still gets us to the encoder’s index pulse, but it allows us to do it anywhere
on the slide, not just relative to the limit switches.
These are the positive move version.
We just did a positive direction homing move, so let’s use one of the negative direction
moves just so we can send the carriage in the other direction.
How about Mode 11.
It says, move in the negative direction – that’s this way on our slide - to the positive edge
of home sensor, reverse direction to back off the home sensor and then creep to the
encoder’s index pulse.
You can also see that on this little diagram.
Start in the negative direction – which is backward from our slide because it uses
the other definition for positive motion – find the home sensor, reverse direction, look for
the next index pulse.
I’ll hit the homing button.
Yep, the carriage moved in the negative direction, found the home sensor, backed off the home
sensor and stopped at the first index pulse it found. And we see our black triangle.
And it also resets the position register, so now this is the zero position.
That was easy because the home sensor was ahead of us when we started homing.
What happens if the home sensor is behind us when we start homing?
Well, this diagram tells if we start moving in this direction with the home sensor behind
us, then when it hits the over travel limit, it will reverse direction, and then go find
the positive edge of the homing sensor and then creep to the next index pulse.
Wait a minute, it went right by this rising edge of the home signal and used the falling
edge to decide when to stop.
What’s going on?
Weren’t we supposed to stop on the positive edge?
That’s an important subtly to keep in mind.
The “positive” edge this description is referring to isn’t necessarily the rising
edge.
It’s only the rising edge of the home signal when the carriage is traveling in the
homing routine’s initial direction.
This was the initial direction, so this is defined to be the positive edge, regardless
of which way the carriage is moving.
Which is good – right?
The positive edge never changes.
You always know exactly where it is.
The “rising” edge, on the other hand, changes depending on which way the carriage is moving
which just confuses things.
So on our slide, the initial direction is this way which means this is the positive
edge.
So after the carriage reverses direction, it will be looking for this edge of the home
sensor's signal.
Let’s try it.
Let’s go to jog and enable the drive's output and move the carriage in the negative direction
past this sensor.
There are two detection points on this carriage so we need to make sure both of those are past the
home sensor.
There we go.
Go back to homing, re-enable the motor – we’re still in Mode 11 - and when we hit the homing
start button we see the carriage move in the initial negative direction, hit the limit,
reverse direction.
We see the homing sensor light up but the carriage kept going until it found the positive
edge of the sensor which is actually the falling edge of the signal because the carriage was
moving in this direction when it found it.
Then it went to the next index pulse and stopped. There’s our black triangle.
And it also reset the position register to be the new zero position.
Perfect.
I know this positive edge stuff is subtle, but if you think about it, it’s really great
because it removes all ambiguity.
You always know exactly where the carriage is going to stop regardless of which way the
carriage is moving.
The third case is this guy.
That is if the homing move starts while the homing signal is active, then it will move
in the opposite direction until it sees the positive edge of the homing signal and then it
will creep to the next encoder pulse.
Again, just because homing started in the opposite direction doesn’t change which
edge is the positive signal.
The normal starting direction for this move is this way, so this is always the positive
edge.
These are all permutations of that same move, so we won’t dwell on those.
You can simply move to the nearest encoder index pulse, in either direction.
Again, using the encoder’s index pulse is a great way to home things like rotary tables.
Let’s select Mode 34 so we move in the positive direction.
The drive output is still enabled, so I’ll hit the home start button.
Hmmm… nothing.
What happened?
Well, this mode finds the nearest index pulse.
We were already on an index pulse from the last move so the drive says we’re done!.
Let’s go to jog, enable the drive's output.
We see the triangle marking the index pulse location so I’ll just jog us away from that.
Back to homing, enable the drive’s output, and hit home.
And yep – it moved to the next index pulse.
Perfect.
Current position just says use the current position as the home position.
Nothing actually moves when you execute this homing method – it just resets the position
register to zero.
I’ll disable the drive's output, and manually move the carriage.
Change to Mode 35, enable the drive’s output, and if you watch the position register, when
I hit home, it gets reset to zero.
This current position is now the new zero and all future moves will be relative to that.
These four all use a hard stop to home.
That is they literally wait for the carriage to smash into something and then either stop
or back up to the nearest index pulse.
And they ignore the limit switches.
If I select one of these, how about “-1,” then this torque limit and duration become
available so you can specify how hard of a hit the system has to take before declaring
homing done.
I would recommend starting with something really small like a 25 or 30% torque limit when you first
start out, so you don’t damage anything with the impact.
Let’s try it.
I’ll set the torque limit to something gentle like 20% - there’s an implied decimal here
so we have to type 200 to get 20.
A 50-millisecond duration is fine.
Make sure the drive is enabled and hit home.
We see the carriage ignore the limit switch, hit the end of the slide, reverse and go find
the next index pulse and it resets the position register so now this is the new zero position.
This is the same thing except it just stops when the torque limit is exceeded.
It doesn’t go search for that index pulse.
And finally, you can go to the home signals positive edge and just decelerate to a stop
without reversing.
If you activate home again, it goes to the next home sensor activation.
That’s unique – it’s the only mode that will ignore the fact that you are already at home
and will look for the next one.
You might use this with a rotary table or a conveyor belt application where you know
the home sensor is going to come back around, or on something like we have with this linear
slide with the two detection points or some folks call those flags.
For example, I’ll go to Jog and move the carriage past the limit switch but keep both
of these detection points to the right of the home sensor.
Go back to homing.
We want to home in the positive direction so I’ll choose Method -6.
Enable the drive’s output and hit home.
Sure enough the carriage goes to the positive edge of the home signal.
If I hit home again, it goes to the next positive edge of the home signal.
In all the index pules based methods, if you activate home after it’s homed, nothing
happens because you are already at the index pulse.
Notice that neither of these reset the position register at the end of homing.
If you do want it to reset to zero after homing, then just click this guy before executing
the move.
Finally, once you have chosen a homing method, it’s really important to thoroughly test
all possible scenarios.
Start homing with the flag activating each over travel limit.
Start homing with the flag on the home sensor.
Start with the flag to the left of the home sensor and to the right of the home sensor.
If there are multiple flags like we have here, you’ll have even more permutations.
And always ask yourself, is it possible for the flag to end up outside of the over travel
limits?
If so, what will the system do?
It’s better to test all of these now so you don’t have any surprises later.
Well, that ought to be more than enough to get you started with homing.
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