10.4.3. SUJ

10.4.3.1. Overview

When setup joints are present, the following positions are reported:

• ECM frames:

  • ECM-SUJ-Local:

    • Defined with respect to patient cart origin

    • Uses DH * tooltip-offset from suj.json

    • Defines ECM-RCM

  • ECM-SUJ:

    • Same as ECM-SUJ-Local

  • ECM-Local:

    • Defined with respect to the ECM RCM

    • Uses DH * tooltip-offset from share/kinematics/ecm.json

    • tooltip-offset depends on type of scope (looking straight, up or down)

  • ECM:

    • Camera frame defined with respect to patient cart

    • Uses ECM-SUJ * ECM-Local (ECM-SUJ is the base frame)

• PSM frames

  • PSM-SUJ-Local:

    • See ECM-SUJ-Local

  • PSM-SUJ:

    • Defined with respect to camera frame

    • Uses inverse(ECM) * PSM-SUJ-Local (inverse(ECM) is the base frame)

  • PSM-Local:

    • See ECM-Local

    • DH, tooltip-offset… depend on type of instrument (found in share/tool)

  • PSM:

    • PSM tooltip frame defined with respect to camera frame

    • Uses PSM-SUJ * PSM-Local (PSM-SUJ is the base frame)

The following assumes the ECM is the reference arm for all the PSMs. Versions greater than 2.1, one can change the reference arm in the SUJ configuration file. ⊙ represents the reference frame and the arrows show the transformations.

dVRK transformation compositions and topics

Command	PSM tip	PSM RCM	Cart	ECM RCM	ECM tip
ECM/measured_cp			⊙	➡	➡
ECM/local/measured_cp				⊙	➡
SUJ/ECM/measured_cp			⊙	➡
SUJ/ECM/local/measured_cp			⊙	➡
PSM/measured_cp	⬅	⬅	⬅	⬅	⊙
PSM/local/measured_cp	⬅	⊙
SUJ/PSM/measured_cp		⬅	⬅	⬅	⊙
SUJ/PSM/local/measured_cp		⬅	⊙

10.4.3.2. Intuitive SUJ

The following is the graph of transformations for a system that uses a dVRK SUJ controller. These controllers (on Classic or Si) can read the potentiometers of the SUJ and therefore allow us to compute the forward kinematics of each SUJ arm.

Patient’s side with SUJ dVRK controllers

10.4.3.3. SUJ Fixed

For groups that don’t have access to a SUJ controller but have an ECM, we recommend using the SUJ Fixed class. This class maintains the transformation graph one would get with a real SUJ but the SUJ transformations have to be provided by the user. These transformations are usually the result of a registration. Using the SUJ Fixed class allows user to control the PSMs with respect to the ECM even when the ECM moves (e.g. ECM teleoperation).

Patient’s side with SUJ without dVRK controllers (SUJ Fixed)

The class SUJ Fixed doesn’t make any assumption regarding the main reference frame (aka “World”). Users could use an optical tracking system and find the respective base frame of the PSMs and ECM with respect to the tracking system. In this case, the result of the registration represent /SUJ/PSM{1,2,3}/local/measured_cp and /SUJ/ECM/local/measured_cp.

If the registration method is a hand-eye registration with respect to the ECM base (aka ECM RCM), the transformation for /SUJ/ECM/local/measured_cp is identity and ECM RCM is the same as World (see https://github.com/jhu-dvrk/dvrk_camera_registration).

Patient’s side with SUJ Fixed, hand-eye registration