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TinyG Homing
The following describes the function of homing cycles and related Gcode for version 0.95 and later versions.
TinyG's homing behaviors are adapted from Peter Smid's CNC Programming Handbook, version 2 and LinuxCNC. G28 and G30 homing functions are similar to LinuxCNC and grbl:
Gcode | Parameters | Command | Description
------|------------|---------|-------------
G28 | _axes_ | Go to G28.1 position | Goes through intermediate point if _axes_ are present
G28.1 | | Set position for G28 | Axes are not used and are ignored if present
G30 | _axes_ | Go to G30.1 position | Goes through intermediate point if _axes_ are present
G30.1 | _axes_ | Set position for G30 | Axes are not used and are ignored if present
These additional "non standard" homing functions are also provided:
Gcode | Parameters | Command | Description
------|------------|---------|-------------
G28.2 | _axes_ | Initiate homing sequence | Homes all axes present in command. At least one axis must be present. The axis value must be present but is ignored.
G28.3 | _axes_ | Set machine origin | Set axes specified. Useful for zeroing and setting origins in cases where the axis cannot otherwise be homed (e.g. no switches, infinite axis, etc.)
Some limitations / constraints in TinyG homing as currently implemented:
- The homing sequence is fixed and always starts with the Z axis (if requested). The sequence runs ZXYABC (but skipping all axes that are not specified in the G28.2 command)
- Supports a single home position. I.e. it does not support multiple-homes such as used by dual pallet machines and other complex machining centers
TinyG has 8 switch pin pairs and a 3.3v take-off pair located on the J13 jumper next to the reset button. The switch pairs are labeled:
Pair | Notes
-----|-------------
3.3v | This pair is located on the J13 connector closest to the corner of the board
Xmin | Corresponding switch is typically positioned at left-most travel of the machine
Xmax | Switch typically at right-most travel
Ymin | Switch typically at front of machine
Ymax | Switch typically at rear of machine
Zmin | Switch typically at maximum height of Z travel
Zmax | Switch typically at minimum height of Z travel or omitted
Amin | Most of the time A is infinite and not homed. This position can be used for a machine kill
Amax |
For each switch pair the pin closest to the board edge is the ground, the pin next to it is the switch input as labeled on the silkscreen. The inputs are 3.3v logic inputs and must not have 5v applied to them or you will burn out the inputs. The inputs are tied high - with strong pullups for v7 boards and on-chip weak pullups for earlier boards.
Two 3.3v output pins are made available for opto-coupled and other powered switch options, but TinyG does not currently support this. It should work but you will need to be careful not to damage the inputs. If you draw the 3.3v do not pull more than 30 ma.
To connect a switch to an input pin simply wire the switch across the ground and the input. This applies to both normally open (NO) and normally closed (NC) switches. Either NO or NC switches may be used, but all switches must be of the same type. We recommend using NC switches for better noise immunity.
Wire a single switch to each axis that will be part of homing. The following configuration is typical for most milling machines and 3D printers:
Pin | Function | Position on machine
-----|-------------|-------------------------
Xmin | X homing switch | on the left of the machine
Ymin | Y homing switch | at the front of the machine
Zmax | Z homing switch | at the top of the Z axis travel
The unused inputs may be wired as axis limit switches (kill) or left unused. If you wire limits you should connect the switch to its proper axis, and not connect multiple switches to an input. Adding limit switches would add these three switches to the example above:
Pin | Function | Position on machine
-----|-------------|-------------------------
Xmax | X limit switch | on the right of the machine
Ymax | Y limit switch | at the back of the machine
Zmin | Z limit switch | at the bottom of the Z axis travel
The A inputs (if otherwise unused) can also be used as a machine kill.
It is mandatory that the switch configuration settings match the physical switch configuration otherwise homing simply won't work. In the case of NC switches the entire machine may be rendered inoperative if these settings are not in alignment.
The following settings are used for switch configuration.
Setting | Description | Notes / Setting Example
--------|-------------|--------------
**$ST** | Switch Type | sets the type of switch used by the entire machine - 0=NO, 1=NC.
**$XSN** | X Minimum Switch Mode | 3=limit-and-homing
**$XSX** | X Maximum Switch Mode | 2=limit-only
**$YSN** | Y Minimum Switch Mode | 3=limit-and-homing
**$YSX** | Y Maximum Switch Mode | 2=limit-only
**$ZSN** | Z Minimum Switch Mode | 0=disabled
**$ZSX** | Z Maximum Switch Mode | 3=limit-and-homing
**$ASN** | A Minimum Switch Mode | 0=disabled
**$ASX** | A Maximum Switch Mode | 0=disabled
Modes:
- 0=Disabled - Switch closures will have no effect. Unused switch pins must be disabled.
- 1=Homing-only - Switch is active during homing but has no effect otherwise
- 2=Limits-only - Switch is not active in homing but will act as a kill switch during normal operation.
- 3=Homing-and-limits - Switch is active during homing and acts as kill switch during normal operation.
It is important to configure all switch pins (all 8) even if you are not using them. Configure all unused switches as Disabled. Otherwise NC configurations will not work.
An axis should only be configured for one homing switch - it should not have two. The homing switch position (min or max) may be configured as homing-only or homing-and-limit. If there is a second switch on the opposite end it should be either disabled or configured as limit-only.
The following per-axis settings are used by homing. Substitute any of XYZABC for the 'x' in the table. The use of the settings is described in G28.1, below. See Configuring Homing Settings for how to set these.
Setting | Description | Notes
--------|-------------|--------------
**$xTM** | Travel Maximum | This axis parameter is used to limit travel during the search phase
**$xSV** | Homing Search Velocity | Velocity for initially finding the homing switch
**$xLV** | Homing Latch Velocity | Velocity for latching phase
**$xLB** | Homing Latch Backoff | Distance to back off switch during latch and for clears
**$xZB** | Homing Zero Backoff | Distance to back off switch before setting machine coordinate system zero
G28 will move the machine to the absolute coordinates set in G28.1, optionally through an intermediate point. Movement will occur at the traverse rate (G0 rate). Format is:
G28 X0 Y0 Z0 A0 B0 C0
G28 will move to coordinates for any specified axis: axes that are not specified are ignored (not moved). The axis value is the intermediate point for that axis.
For example, G91 G28 Z10 will move to a pre-set point in the XY plane. The tool will initially lift z by 10 mm (or inches); G91 is used to set relative mode for this command.
G28.1 sets the G28 position to the current absolute coordinates. On reset this position is zeroed, so G28's will return to machine home.
Works identically to G28 / G28.1
G28.2 is used to home to physical home switches. G28.2 will find the home switch for an axis then set machine zero for that axis at an offset from the switch location. Format is:
G28.2 X0 Y0 Z0 A0 B0 C0
Axes not present are ignored and zero values are not changed.
For example. G28.2 X0 Y0 will home the X and Y axes only. The values provided for X and Y don't matter, but something must be present.
- G28.2 homes all axes present in the command
- The homing sequence progresses through each axis provided in the G28.2 block in turn - i.e. it does not home on multiple axes simultaneously.
- Axes are always executed in order of ZXYABC. The order the axis words occur in the G28.2 block has no effect.
- Homing begins by checking the pre-conditions for homing:
- $xSV homing-search-velocity must be non-zero for the axis to be processed
- Homing begins by testing homing and limit switches for the currently homing axis. If a switch is tripped the axis will back off a distance from the switch as defines in the Latch Backoff value ($xLB). Backoff will always be in the opposite direction of the search.
- Once the switch is clear a homing move of length travel max ($xTM) is executed in the negative direction of the search velocity ($xSV). The move executes until the homing switch is hit. For example: A homing move in X with X-axis velocity of -1200mm/min and 400mm travel limit would move G1 F1200 X-400 in relative coordinates. To move to the opposite end of the machine a positive velocity would be configured (and presumably the limit-max switch would be hit).
- Once the switch is hit a backoff is performed as described above.
- Once backoff occurs the latch cycle is initiated if the homing-latch-velocity is non-zero. The latch-velocity must have the same sign as the homing-search-velocity. The axis moves onto the switch at the homing-latch-rate and a backoff to $xHZ is performed at the latch rate. The homing cycle advances to the next axis to be processed.
- Once all axes are processed the affected axes are moved to the absolute home location (machine zero).
- At this point the homing state will indicate that the machine has been homed.
Dual gantry operation differs somewhat from single gantry homing as specified above. Dual gantry operation assumes each of the two motors in the gantry has its own limit switch that can be read independently. It also assumes that the axis is not racked so much that it cannot move. If this is the case the machine must be manually squared so that the axis can move before starting the homing operation.
- The homing sequence progresses normally through each axis but executes the following if a dual gantry axis is detected. Detection is performed by looking at the motor mapping configs: $1MA - $4MA
- The standard pre-conditions apply
- Dual gantry homing begins by testing the homing switches for the dual axis. if either switch is tripped the other motor is advanced at the latch rate until that switch is also hit. "Advance" is actually the negative of the homing-travel for that axis (i.e. movement will be in the opposite direction specified by the latch rate sign). Once the second switch is hit a backoff is executed to the machine zero offset value.
- A search move is initated for both motors. The search ends when one of the switches is hit.
- The second switch is closed as per above, and a backoff is executed.
- A latch operation similar to the single gantry case is performed.
- The dual axis is now homed. Control is returned to the standard homing operation, above
[NOT IMPLEMENTED - INCLUDED FOR FUTURE REFERENCE ONLY]
G27 is used to verify table position and perform a Reset if table position tolerances are beyond specification. G27 will Home to a switch, record the switch location, and calculate the difference from this measured Home Location to the set Home Position. G27 should only be used after the machine has been Homed using G30. No Home Offset is performed during the G27 call.
Syntax of G27 is... G27 X0.001 Y0.002 Z0.003 F10
Where the axis values define the acceptable tolerance for table position error. The example above will Home all 3 axes simultaneously to the Home Switches. G27 is only available when doing a Hard Home to physical switches. If an axis is not called out in the G27 command, then that axis is ignored during execution. For example, G27 X0.001 Z0.005 will only home/verify the XZ axes. The axis value defines the acceptable tolerance. In the example, the X axis tolerance is set to 0.001. If the table position is off be more than 0.001 then the G27 command will put the machine in Reset.