1. Introduction

A number of kinematics modules support the switching of kinematics calculations. These modules support a default kinematics method (type0), a second built-in method (type1), and (optionally) a user-provided kinematics method (type2). Identity kinematics are typically used for the type1 method.

The switchkins functionality can be used for machines where post-homing joint control is needed during setup or to avoid movement near singularities from G-code. Such machines use specific kinematics calculations for most operations but can be switched to identity kinematics for control of individual joints after homing.

The kinematics type is selected by a motion module HAL pin that can be updated from a G-code program or by interactive MDI commands. The halui provisions for activating MDI commands can be used to allow buttons to select the kinematics type using hardware controls or a virtual panel (PyVCP, GladeVCP, etc.).

When a kinematics type is changed, the G-code must also issue commands to force synchronization of the interpreter and motion parts of LinuxCNC. Typically, a HAL pin read command (M66 E0 L0) is used immediately after altering the controlling HAL pin to force synchronization.

2. Switchable Kinematic Modules

The following kinematics modules support switchable kinematics:

  1. xyzac-trt-kins (type0:xyzac-trt-kins type1:identity)

  2. xyzbc-trt-kins (type0:xyzbc-trt-kins type1:identity)

  3. genhexkins (type0:genhexkins type1:identity)

  4. genserkins (type0:genserkins type1:identity) (puma560 example)

  5. pumakins (type0:pumakins type1:identity)

  6. scarakins (type0:scarakins type1:identity)

  7. 5axiskins (type0:5axiskins type1:identity) (bridgemill)

The xyz[ab]c-trt-kins modules by default use type0==xyz[ab]c-trt-kins for backwards compatibility. The provided sim configs alter the type0/type1 convention by forcing type0==identity kinematics using the module string parameter sparm with an INI file setting like:

KINEMATICS = xyzac-trt-kins sparm=identityfirst

2.1. Identity letter assignments

When using an identity kinematics type, the module parameter coordinates can be used to assign letters to joints in arbitrary order from the set of allowed coordinate letters. Examples:


# conventional identity ordering: joint0==x, joint1==y, ...
KINEMATICS = genhexkins coordinates=xyzabc

# custom identity ordering: joint0==c, joint1==b, ...
# KINEMATICS = genhexkins coordinates=cbazyx
If the coordinates= parameter is omitted, the default joint-letter identity assignments are joint0==x,joint1=y,…

The joint assignments provided for identity kinematics when using the coordinates parameter are identical to those provided for the trivkins module. However, duplication of axis letters to assign multiple joints for a coordinate letter is not generally applicable for serial or parallel kinematics (like genserkins, pumakins, genhexkins, etc.) where there is no simple relationship between joints and coordinates.

Duplication of axis coordinate letters is supported in the kinematics modules xyzac-trt-kins, xyzbc-trt-kins, and 5axiskins (bridgemill). Typical applications for duplicate coordinates are gantry machines where two motors (joints) are used for the transverse axis.

2.2. Backwards compatibility

Switchable kinematics initialize with motion.switchkins-type==0 implementing their eponymous kinematics method. If the the motion.switchkins-type pin is not connected — as in legacy configurations — only the default kinematics type is available.

3. HAL Pins

Kinematics switching is controlled by the motion module input HAL pin motion.switchkins-type. The floating point pin value is truncated to integer and used to select one of the provided kinematics types. The zero startup value selects the type0 default kinematics type.

The motion.switchkins-type input pin is floating point in order to facilitate connections to motion module output pins like motion.analog-out-0n that are controllable by standard M-codes (typically M68EnL0).

Output HAL pins are provided to inform GUIs of the current kinematics type. These pins can also be connected to digital inputs that are read by G-code programs to enable or disable program behavior in accordance with the active kinematics type.

3.1. HAL Pin Summary

  1. motion.switchkins-type Input (float)

  2. kinstype.is-0 Output (bit)

  3. kinstype.is-1 Output (bit)

  4. kinstype.is-2 Output (bit)

4. Usage

4.1. HAL Connections

Switchkins functionality is enabled by the pin motion.switchkins-type. Typically, this pin is sourced by an analog output pin like motion.analog-out-03 so that it can be set by M68 commands. Example:

net :kinstype-select <= motion.analog-out-03
net :kinstype-select => motion.switchkins-type

4.2. G-/M-code commands

Kinstype selection is managed using G-code sequences like:

M68 E3 Q1 ;update analog-out-03 to select kinstype 1
M66 E0 L0 ;sync interp-motion
...       ;user G-code
M68 E3 Q0 ;update analog-out-03 to select kinstype 0
M66 E0 L0 ;sync interp-motion
An M66 wait-on-input command updates the #5399 variable. If the current value of this variable is needed for subsequent purposes, it should be copied to an additional variable before invoking M66.

These G-code command sequences are typically implemented in G-code subroutines as remapped M-codes or with conventional M-code scripts.

Suggested codes (as used in sim configs) are:

Conventional User M-codes:

  1. M128 Select kinstype 0 (startup default kinematics)

  2. M129 Select kinstype 1 (typically identity kinematics)

  3. M130 Select kinstype 2 (user-provided kinematics)

Remapped M-codes:

  1. M428 Select kinstype 0 (startup default kinematics)

  2. M429 Select kinstype 1 (typically identity kinematics)

  3. M430 Select kinstype 2 (user-provided kinematics)

Conventional user M-codes (in the range M100-M199) are in modal group 10. Remapped M-codes (in the range M200 to M999) can specify a modalgroup. See the remap documentation for additional information.

4.3. INI file limit settings

LinuxCNC trajectory planning uses limits for position (min,max), velocity, and acceleration for each applicable coordinate letter specified in the configuration INI file. Example for letter L (in the set XYZABCUVW):


The INI file limits specified apply to the type 0 default kinematics type that is activated at startup. These limits may not be applicable when switching to alternative kinematics. However, since an interpreter-motion synchronization is required when switching kinematics, INI-HAL pins can be used to setup limits for a pending kinematics type.

INI-HAL pins are typically not recognized during a G-code program unless a synchronization (queue-buster) command is issued. See the milltask manpage for more information ($ man milltask)

The relevant INI-HAL pins for a joint number (N) are:


The relevant INI-HAL pins for an axis coordinate (L) are:

In general, there are no fixed mappings between joint numbers and axis coordinate letters. There may be specific mappings for some kinematics modules especially those that implement identity kinematics (trivkins). See the kins man page for more information ($ man kins)

A user-provided M-code can alter any or all of the axis coordinate limits prior to changing the motion.switchkins-type pin and synchronizing the interpreter and motionparts of LinuxCNC. As an example, a bash script invoking halcmd can be hardcoded to set any number of HAL pins:

halcmd -f <<EOF
setp ini.x.min_limit -100
setp ini.x.max_limit  100
# ... repeat for other limit parameters

Scripts like this can be invoked as a user M-code and used prior to the kins switching M-code that updates the motion.switchkins-type HAL pin and forces an interp-motion sync. Typically, separate scripts would be used for each kinstype (0,1,2).

When identity kinematics are provided as a means to control individual joints, it may be convenient to set or restore limits as specified in the system INI file. For example, a robot starts with a complex (non-identity) kinematics (type0) after homing. The system is configured so that it can be switched to identity kinematics (type1) in order to manipulate individual joints using the conventional letters from the set XYZABCUVW. The INI file settings ([AXIS_L]) are not applicable when operating with identity (type1) kinematics. To address this use case, the user M-code scripts can be designed as follows:

M129 (Switch to identity type1)

  1. read and parse INI file

  2. HAL: setp the INI-HAL limit pins for each axis letter ([AXIS_L]) according to the identity-referenced joint number INI file setting ([JOINT_N])

  3. HAL: setp motion.switchkins-type 1

  4. MDI: execute a syncing G-code (M66E0L0)

M128 (restore robot default kinematics type 0)

  1. read and parse INI file

  2. HAL: setp the INI-HAL limit pins for each axis letter ([AXIS_L]) according to the appropriate INI file setting ([AXIS_L])

  3. HAL: setp motion.switchkins-type 0

  4. MDI: execute a syncing G-code (M66E0L0)

The vismach simulation configurations for a puma robot demonstrate M-code scripts (M128,M129,M130) for this example use case.

4.4. Offset considerations

Like INI file limit settings, coordinate system offsets (G92, G10L2, G10L20, G43, etc) are generally applicable only for the type 0 default startup kinematics type. When switching kinematics types, it may be important to either reset all offsets prior to switching or update offsets based on system-specific requirements.

5. Simulation configs

Simulation configs (requiring no hardware) are provided with illustrative vismach displays in subdirectories of configs/sim/axis/vismach/ .

  1. 5axis/table-rotary-tilting/xyzac-trt.ini (xyzac-trt-kins)

  2. 5axis/table-rotary-tilting/xyzbc-trt.ini (xyzac-trt-kins)

  3. 5axis/bridgemill/5axis.ini (5axiskins)

  4. scara/scara.ini (scarakins)

  5. puma/puma560.ini (genserkins)

  6. puma/puma.ini (pumakins)

  7. hexapod-sim/hexapod.ini (genhexkins)

6. User kinematics provisions

Custom kinematics can be coded and tested on Run-In-Place (RIP) builds. A template file src/emc/kinematics/userkfuncs.c is provided in the distribution. This file can be copied/renamed to a user directory and edited to supply custom kinematics with kinstype==2.

The user custom kinematics file can be compiled from out-of-tree source locations for rt-preempt implementations or by replacing the in-tree template file (src/emc/kinematics/userkfuncs.c) for rtai systems.

Preempt-rt make example:

$ userkfuncs=/home/myname/kins/mykins.c make && sudo make setuid

7. Warnings

Unexpected behavior can result if a G-code program is inadvertently started with an incompatible kinematics type. Unwanted behavior can be circumvented in G-code programs by:

  1. Connecting appropriate kinstype.is.N HAL pins to digital input pins (like motion.digital-in-0m).

  2. Reading the digital input pin (M66 E0 Pm) at the start of the G-code program

  3. Aborting (M2) the G-code program with a message (DEBUG, problem_message) if the kinstype is not suitable.

When using jogging facilities or MDI commands interactively, operator caution is required. Guis should include indicators to display the current kinematics type.

Switching kinematics can cause substantial operational changes requiring careful design, testing, and training for deployment. The management of coordinate offsets, tool compensation, and INI file limits may require complicated and non-standard operating protocols.

8. Code Notes

Kinematic modules providing switchkins functionality are linked to the switchkins.o object (switchkins.c) that provides the module main program (rtapi_app_main()) and related functions. This main program reads (optional) module command-line parameters (coordinates, sparm) and passes them to the module-provided function switchkinsSetup().

The switchkinsSetup() function identifies kinstype-specific setup routines and the functions for forward an inverse calculation for each kinstype (0,1,2) and sets a number of configuration settings.

After calling switchkinsSetup(), rtapi_app_main() checks the supplied parameters, creates a HAL component, and then invokes the setup routine identified for each kinstype (0,1,2).

Each kinstype (0,1,2) setup routine can (optionally) create HAL pins and set them to default values. When all setup routines finish, rtapi_app_main() issues hal_ready() for the component to complete creation of the module.