10.4.2. Single arm

For each individual arm, the cartesian positions are based on:

• DH parameters: defined in JSON files, used to compute the forward kinematics using joint positions

• Tool tip offset (optional):

  • Defined in the kinematic JSON file, tooltip-offset is a constant offset appended at the end of the kinematic chain (PSM and ECM only, not MTM). The tooltip offset for the ECM is determined by the type of endoscope (straight, up, down) and automatically managed by the ECM arm class. The PSM tooltip offset is determined by the type of instrument (found in share/tool).

  • Most users should never deal with these, they are predefined for given PSM tools and ECM endoscopes

• Base Frame (base_frame in your system JSON file):

  • Prepended to the whole kinematic chain

  • Can be:

    • defined in the system JSON configuration per arm using base_frame

    • set at runtime by command or ROS topic set_base_frame

For example, the MTM kinematic (DH parameters) start from the link 0, i.e. close to the attachment point/shoulder (see share/kinematics/mtmr.json in https://github.com/jhu-dVRK/sawIntuitiveResearchKit) and has Z pointing up, X to the left and Y towards the user. But the ISI convention expects that X points to the left when viewed from the stereo display, Y should point up and Z away from the user. Furthermore, the ISI convention places the origin in the middle of the stereo display (i.e. between the operator’s eyes). In practice MTMs are always mounted rigidly to a frame, so we need to apply a constant rotation and translation to match the ISI convention. The two masters are also mounted apart from each other, so there is also a positive X translation for MTML and a negative X translation for MTMR. The change of reference frame should be defined using the base_frame in the system configuration file (see jhu-dVRK/system-MTMR-PSM1-MTML-PSM2-Teleop.json in https://github.com/dvrk-config/dvrk_config_jhu).

For a change of reference frame at runtime one should use the set_base_frame ROS topic. For example, this is used to make sure the PSMs are always defined with respect to the endoscope tooltip (camera frame) if the camera can move (i.e. mounted on an ECM or other robot).

To summarize:

• When used with the setup joints, the system class propagates the different base frames to make sure the PSMs are defined with respect to the camera frame (see SUJ).

• If you have an ECM and don’t have access to the setup joints but have a way to register the PSMs to the camera/ECM, you should use the SUJ Fixed class. We offer a simple method to register the PSMs to the ECM: https://github.com/jhu-dvrk/dvrk_camera_registration

• Finally, if the camera is fixed, you can use base_frame in your system configuration (similar to MTMs, see jhu-dVRK/system-MTMR-PSM1-MTML-PSM2-Teleop.json in https://github.com/dvrk-config/dvrk_config_jhu). Note that the base_frame depends on where your PSMs are mounted with respect to your fixed camera.

There are two possible cartesian positions to query:

• measured_cp: base_frame * DH * tooltip-offset

• local/measured_cp: DH * tooltip-offset

10.4.2.1. Surgeon’s side

The base frames defined in the system.json are such that the MTM tips (grippers) are defined with respect to the stereo display. By default, we assume the stereo display is pointing down with a 60-degree angle from the horizontal. This is the orientation of the HRSV on the Standard (aka Classic) and S da Vinci systems. If your display is mounted at a different angle, adjust your system.json accordingly.

Surgeon’s side transformations

10.4.2.2. Patient’s side

The base frames defined in your system.json depend on how your PSMs are mounted. If you have a fixed camera, you can use or hand-eye calibration method: https://github.com/jhu-dvrk/dvrk_camera_registration. If you have an ECM, see SUJ.

Patient’s side without any type of SUJ