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Delta Home
Delta machines differ from Cartesian machines in their handling of limit switches that establish the home position of the machine. For Cartesian machines, home is usually "zero at the limit switch". For delta machines, however, home is usually "arms way below limit switch, with arms horizontal." Delta machines trip the limit switch with "elbows up and away", which mechanically is a very awkward place to be since motion is highly restricted up near the homing limit switches. With arms horizontal, delta machines can move freely, and the horizontal arm position is a naturally place for the home and origin of a delta XYZ coordinate system. It does require introduction of a new concept, the home angle.
The delta home angle is the angle from the horizontal arm position up to the limit switch. Adopting the "angles increase downward" convention, the home angle is always negative. For FirePick Delta (FPD), the default home angle is -67.2 degrees.
Since we are dealing with a digital machine and limit switches, the angle will never be exactly -67.2 degrees. We do need to know the actual home angle to perform delta calculations properly. The delta kinematic model is non-linear, and home angle errors lead to Z Bowl Error, which is quite nasty. The home angle is only important in that it gives us the "arms level at home" angular distance from the limit switch. With an assurance that arms are level at home, delta kinematics will predict actual machine location with great accuracy.
Before calibrating the home angle, you will need to ensure that all arms will be at the same angle after homing. In particular, you must have completed:
- Stepper Calibration
- [Latch Backoff Calibration](Latch Backoff)
If [auto-calibration](Z-Bowl Error) is not possible, you can still calibrate your FPD manually. The basic approach is simple:
Most manual methods require attaching a measurement device to the pulley arm ball studs, so you'll need to remove the effector arms.
Position arm #1 to zero degrees:
{"mova1":0}Then move arm #1 up or down till it is level. Here we move it up to 1.5 degrees above the nominal zero position:
{"mova1":-1.5}The exact angle you specify doesn't matter--all you are doing is leveling the arm.
There are several ways to determine levelness. Pick one of the following or make up your own:
- Loose Canon 3DLC1043 FPD Calibration Tool.
- Dental floss between arm #1 and arm #2 ball studs
- etc.
Run the following command to accept the position of arm #1 as level:
{"calho":""}This will automatically set arms #2 and #3 to share the same home angle (this works because of the completed latch backoff calibration)
That's it!
If you are setting up many identical machines that all have exactly the same home angle, you can just do the following if you know the exact pulse offset. Let's assume that home angle pulse offset is -5000:
{"calho":5000}