﻿In Part 1 we got our system set up for homing. I can’t emphasize enough the importance of
verifying the 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 the last video left off. Were 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 switch away from 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 this 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 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 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 of the limit
and then it creeps back to the encoder’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 and reverses out of the limit to the next encoder’s index pulse
– we see that black triangle is right back where we expected it. And the position register has
been reset so this is now the zero position. Homing on the encoder’s index pulse is a 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 the 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
backwards 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 from 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 this is now 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 always 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 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 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'll 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 drives 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 moves 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 this is now 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 you are already at home and will look for the next one. You might use this with a rotary
table or a conveyor or 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 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 of the index pulse-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 had 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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