TVM802 conversion - do I need homing switches? Or just go sensorless?

[jbasia]

My Marlin controller finally works. Starting with the PnP machine wiring. This will probably take quite a while. Quite fiddly work.

One starting question: Do I need the homing switches? TVM802 has them on X and Y (Z has shared axis) - but the TMC2209 drivers can manage sensorless homing.

The (4 optical) switched are there anyway. I wonder if one has any advantage over the other one. 

[dc42]

The Trinamic drivers only check for a stall once every full step or once every 4 full steps depending on setting. So you must expect the homing position to vary by at least */- 1 full step, which if your axes have 80 microsteps/mm at x16 microstepping is +/- 0.2mm. If you need better reproducibility than that, you need homing switches.

[jbasia]

For the TMC sensorless - does homing accuracy really matter? I mean the PCB fiducials give high accuracy, and the homing accuracy should be good enough so the PnP can find the fiducials. 

But anyway, I will wire up the homing switches in case I want to swap drivers or use external drivers. They are there already.

[Jan]

The homing error translates - to my understanding - directly to every
position on the machine, like feeders (the ones, that use static
positions, not the ones, that can update their position automatically
using vision like blinds feeder or push pull feeder) and up-looking
camera. And from the up-looking camera the error translates into
placement position errors. I'm not sure, mechanical limit switches are
better then stall detection of stepper motor drivers. However, visual
homing was introduced to reduce the homing position error. So you might
stick with homing using the stepper drivers and add visual homing. 0.2mm
is sufficient for visual homing.

Jan

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[mark maker]

Always use visual homing, it solves so many issues and problems:

https://github.com/openpnp/openpnp/wiki/Visual-Homing

Some users do not even have homing switches, they just jog the camera roughly to the fiducial, then do pure visual homing (the HOME_COMMAND just resets the coordinates to match those of the fiducial, likely with G92).

_Mark

[Ravi Ganesh]

Well, I can claim to be a proud use of this technique. Just a reminder of the best practices:

1. While closing OpenPnP remember to park it at home position. (a script may come handy I hope)

2. While powering up the machine, the controller and openPnP's coordinates should be in sync. So the option "Sync Initial Location" in the Driver Settings should be turned on. Else the next jog would certainly crash the head. 

3. Have at least soft limits in OpenPnP active to prevent collision after homing.

4. Before homing (and if unfortunately the machine in not parked at home at power-on as mentioned in #1 above) the machine is vulnerable to collision and negligent operation could easily collide the axes. Hence extreme care is required during this vulnerable stage.

It requires some learning, but I can vouch that it works even with novice operators.

Ravi

PS: TMC2209 stall detection is very interesting and I will definitely use this driver next time.