External axis offsets are supported during teleop (world) jogs and coordinated (gcode) motion. External axis offsets are enabled on a per-axis basis by ini file settings and controlled dynamically by hal input pins. The hal interface is similar to that used for wheel jogging. This type of interface is typically implemented with a manual-pulse-generator (mpg) connected to an encoder hal component that counts pulses.

1. Ini file Settings

For each axis letter (L in xyzabcuvw):

[AXIS_L]OFFSET_AV_RATIO = value (controls accel/vel for external offsets)

  1. Allowed values: 0 <= value <= 0.9

  2. Disallowed values are replaced with 0.1 with message to stdout

  3. Default value: 0 (disables external offset). Consequence: omitted [AXIS_L]OFFSET_AV_RATIO disables external offset for the axis.

  4. If nonzero, the OFFSET_AV_RATIO (r), adjusts the conventional (planning) max velocity and acceleration to preserve [AXIS_L] constraints: 

    planning max velocity        = (1-r) * MAX_VELOCITY
external offset velocity     = (  r) * MAX_VELOCITY
    planning max acceleration    = (1-r) * MAX_ACCELERATIOIN
external offset acceleration = (  r) * MAX_ACCELERATION

2. Hal Pins

2.1. Per-Axis Motion Hal Pins

For each axis letter (L in xyzabcuvw)

  1. axis.L.eoffset-enable Input(bit): enable

  2. axis.L.eoffset-scale Input(float): scale factor

  3. axis.L.eoffset-counts Input(s32): input to counts register

  4. axis.L.eoffset-clear Input(bit): clear requested offset

  5. axis.L.eoffset Output(float): current external offset

  6. axis.L.eoffset-request Output(float): requested external offset

2.2. Other Motion Hal Pins

  1. motion.eoffset-active Output(bit): non-zero external offsets applied

  2. motion.eoffset-limited Output(bit): motion inhibited due to soft limit

3. Usage

The axis input hal pins (enable,scale,counts) are similar to the pins used for wheel jogging.

3.1. Offset Computation

At each servo period, the axis.L.eoffset-counts pin is compared to its value in the prior period. The increase or decrease (positive or negative delta) of the axis.L.eoffset-counts pin is multiplied by the current axis.L.eoffset-scale pin value. This product is accumulated in an internal register and exported to the axis.L.eoffset-request hal pin. The accumulation register is reset to zero at each machine-on.

The requested offset value is used to plan the movement for the offset that is applied to the L coordinate and represented by the axis.L.eoffset hal pin. The planned motion respects the allocated velocity and acceleration constraints and may be limited if the net motion (offset plus teleop jogging or coordinated motion) reaches a soft limit for the L coordinate.

For many applications, the axis.L.eoffset-scale pin is constant and the net axis.L.eoffset-request response to axis.L.eoffset-counts is equivalent to the product of the accumulated value of axis.L.eoffset-counts and the (constant) axis.L.eoffset-scale pin values.

3.2. Machine-off/Machine-on

When the machine is turned off, the current position with external offsets is maintained so that there is no unexpected motion at turn off or turn on.

At each startup (machine-on), the internal counts register for each hal pin axis.L.eoffset-counts is zeroed and the corresponding hal output pin axis.L.eoffset is reset to zero.

In other words, external offsets are defined as ZERO at each startup (machine-on) regardless of the value of the axis.L.eoffset-counts pins. To avoid confusion, it is recommended that all axis.L.eoffset-counts pins are set to zero when the machine is off.

3.3. Soft Limits

External axis offset movements are independently planned with velocity and acceleration settings specified by the [AXIS_L]OFFSET_AV_RATIO. The offsetting motion is not coordinated with teleop jogs nor with coordinated (gcode) motion. During teleop jogging and coordinated (gcode) motion, axis soft limits ([AXIS_L]MIN_LIMIT,MAX_LIMIT) restrict movement of the axis.

When external offsets are applied and motion reaches a soft limit (by external offset increases or teleop jogging or coordinated motion), the hal pin motion.eoffset-limited is asserted and the axis value is held nominally to the soft limit. This hal pin can be used by associated hal logic to truncate additional eoffset counts or to stop the machine (connect to halui.machine.off for instance). If the axis is moved within the soft limit, the motion.eoffset-limited pin is reset.

When operating at a soft limit during coordinated motion that continues to change the planned axis value, the hal output pin axis.L.eoffset will indicate the current offset — the distance needed to reach the limit instead of the computed offset request. This indicated value will change as the planned axis value changes.

The hal pin axis.L.eoffset-request indicates the current requested offset that is the product of the internal counts register and the eoffset-scale. In general, the axis.L.eoffset pin value lags the axis.L.eoffset-request value since the external offset is subject to an acceleration limit. When operating at a soft limit, additional updates to the axis.L.eoffset-counts will continue to affect the requested external offset as reflected in the axis.L.eoffset-request hal pin.

When teleop jogging with external offsets enabled and non-zero values applied, encountering a soft limit will stop motion in the offending axis without a deacceleration interval. Similarly, during coordinated motion with external offsets enabled, reaching a soft limit will stop motion with no deacceleration phase. For this case, it does not matter if the offsets are zero.

When motion is stopped with no deacceleration phase, system acceleration limits may be violated and lead to: 1) a following error (and/or a thump) for a servo motor system, 2) a loss of steps for a stepper motor system. In general, it is recommeneded that external offsets are applied in a manner to avoid approaching soft limits.

3.4. Notes

External offsets apply to axis coordinate letters (xyzabcuvw). All joints must be homed before external axis offsets are honored.

If an axis.L.eoffset-enable hal pin is reset when its offset is non-zero, the offset is maintained. The offset may be cleared by:

  1. a Machine-off/Machine on toggle

  2. reactivating the enable pin and incrementing/decrementing the axis.L.eoffset-counts hal pin to return the offset to zero.

  3. pulsing the axis.L.eoffset-clear hal pin

External-offsets are intended for use with small offsets that are applied within the soft-limit bounds.

Soft limits are respected for both teleop jogging and coordinated motion when external offsets are applied. However, when a soft limit is reached during coordinated motion, reducing the offending external offset may not move away from the soft limit if planned motion continues in the same direction. This circumstance can occur since the rate of correcting offset removal (as set by [AXIS_L]OFFSET_AV_RATIO) may be less than the opposing planned rate of motion. In such cases, pausing (or stopping) the planned, coordinated motion will allow movement away from the soft limit when correcting changes are made in the offending external offset.

3.5. Warning

The use of external offsets can alter machine motion in a significant manner. The control of external offsets with hal components and connections and any associated user interfaces should be carefully designed and tested before deployment.

4. Related Hal Components

4.1. eoffset_per_angle.comp

Component to compute an external offset from a function based on a measured angle (rotary coordinate or spindle). See the man page for details ($ man eoffset_per_angle).

5. Testing

The external axis offset capability is enabled by adding an [AXIS_L] setting for each candidate axis. For example:

[AXIS_Z]
OFFSET_AV_RATIO = 0.2

For testing, it is convenient to simulate a jog wheel interface using the sim_pin gui. For example, in a terminal:

$ sim_pin axis.z.eoffset-enable axis.z.eoffset-scale axis.z.eoffset-counts

The use of external offsets is aided by displaying information related to the current offsets: the current eoffset value and the requested eoffset value, the axis pos-cmd, and (for an identity kinematics machine) the corresponding joint motor pos-cmd and motor-offset. The provided sim configuration (see below) demonstrates an example pyvcp panel for the axis gui.

In the absence of a custom display, halshow can be started as an auxiliary application with a custom watch list.

Example ini file settings to simulate the hal pin eoffset connections and display eoffset information for the z axis (for identity kinematics with z==joint2):

[APPLICATIONS]
APP = sim_pin \
      axis.z.eoffset-enable \
      axis.z.eoffset-scale \
      axis.z.eoffset-counts \
      axis.z.eoffset-clear

APP = halshow --fformat "%0.5f" ./z.halshow

Where the file z.halshow (in the configuration directory) is:

pin+joint.2.motor-pos-cmd
pin+joint.2.motor-offset
pin+axis.z.pos-cmd
pin+axis.z.eoffset
pin+axis.z.eoffset-request
pin+motion.eoffset-limited

6. Examples

Provided simulation configurations demonstrate the use of external offsets in order to provide a starting point for user customization for real hardware

The sim configurations utilize the ini setting [HAL]HALFILE = LIB:basic_sim.tcl to configure all routine hal connections for the axes specified in the ini file [TRAJ]COORDINATES= setting. The hal logic needed to demonstrate external offset functionality and the gui hal pin connections for a pyvcp panel are made in separate hal files. A non-simulation configuration should replace the LIB:basic_sim.tcl item HALFILEs appropriate to the machine. The provided pyvcp files (.hal and .xml) could be a starting point for application-specific gui interfaces.

6.1. eoffsets_demo.ini

The sim config sim/configs/axis/external_offsets/eoffsets_demo.ini demonstrates a cartesian XYZ machine with controls to enable external offsets on any axis.

Displays are provided to show all important position and offset values.

A sim_pin gui provides controls for the axis offset pins: eoffset-scale & eoffset-counts (via signal e:<L>counts), eoffset-clear (via signal e:clearall)

A script (eoffsets_monitor.tcl) is used to set axis.L.counts pins to zero at Machine-off

6.2. jwp_z.ini

The sim config sim/configs/axis/external_offsets/jwp_z.ini demonstrates a jog-while-pause capability for a single (Z) coordinate:

Panel LEDs are provided to show important status items.

Controls are provided to set the eoffset scale factor and to increment/decrement/clear the eoffset counts.

6.3. dynamic_offsets.ini

This sim config sim/configs/axis/external_offsets/dynamic_offsets.ini demonstrates dynamically applied offsets by connecting a sinusoidal waveform to the z coordinate external offset inputs.

Panel LEDs are provided to show important status items.

Controls are provided to alter ini file settings for the Z axis max velocity and max acceleration.

Controls are provided to set the waveform generator parameters

A halscope app is started to show the applied waveform, the offset response, and the motor cmd response.

Note: changes to the z coordinate max-acceleration and max-velocity are not acknowledged while a program is running.

6.4. opa.ini (eoffset_per_angle)

The opa.ini configuration uses the hal component eoffset_per_angle ($ man eoffset_per_angle) to demonstrate an XZC machine with functional offsets computed from the C coordinate (angle) and applied to the transvers (X) coordinate. Offset computations are based on a specified reference radius typically set by a program (or MDI) M68 command to control a motion.analog-out-NN pin.

Panel LEDs are provided to show important status items.

Functions are provided for inside and outside polygons (nsides >= 3), sine waves and square waves. The functions can be multiplied in frequency using the fmul pin and modified in amplitude using the rfrac pin (fraction of reference radius).

Controls are provided to start/stop offset waveforms and to set the function type and its parameters.