7.7. Custom instruments

7.7.1. Introduction

There are two main reasons why someone would want to define their own instrument parameters:

• New mechanical design

• Using an existing instrument but change it’s behavior (e.g. change control point, depth limit)

Caution

Make sure you know what you’re doing when modifying instrument definition files. This can lead to unexpected motions on the manipulators and physical damage.

Caution

Starting with the dVRK 2.0 and firmware 6, it is possible to automatically detect the instrument using the Dallas chip. If the instrument you’re using is not a standard instrument (either mechanical or parameter changes), you won’t be able to use the automatic instrument detection and you will have to revert to either fixed or manual instrument specification.

7.7.2. Main files

Some examples are provided under dvrk_config_jhu/jhu-dVRK/custom-tool. You should open https://github.com/dvrk-config/dvrk_config_jhu/tree/main/jhu-dVRK/custom-tool in a separate tab to browse through the example files.

The main files are:

• System file: See comments in system-PSM1-Custom-Dummy.json. By default the system application will infer the arm configuration file name from the name and serial fields by concatenating them (<name>-<serial>.json). In the example for a custom instrument, we added two fields.

  1. system is used to help locate configuration files for the arm, namely the sawRobotIO1394-<name>-<serial>.json by adding share/<system> to the search path.

  2. arm to overwrite the default rule to locate the arm configuration file. We recommend to follow the convention <name>-<serial>-<instrument-name>.json to name your new arm file (e.g. PSM1-28007-Custom-Dummy.json).

• Arm file: See comments in PSM1-28007-Custom-Dummy.json. There are three fields used for custom instruments:

  1. custom-tool-index is used to point to your list of custom instruments. This list will be added to the default list of instruments loaded by the system (i.e. share/tool/index.json).

  2. kinematic can be used to overwrite the default behavior of the PSM itself (first 3 joints). This can be used to change the joint limits on the first 3 joints based on the design of your custom instrument. This is a very specific case and we expect most users should use the default, i.e. kinematic/psm.json.

  3. tool-detection should be set to either MANUAL or FIXED`. If set to MANUAL, you will need to provide the tool name in the configuration file using the convention <name>:<model>. The custom name and model are defined in the instrument index file (see below).

• Instruments index file: See comments in custom-tool-index.json. This file contains a list of custom instruments with:

  1. model: 6 digits number, usually start with 40 for Classic instruments and 42 for S instruments. We recommend to preserve this naming convention for your custom instruments and use the last 4 digits to identify your instrument. See also generation field below.

  2. names: All caps names with underscores. These names are used by the dVRK software/firmware to identify the instruments with a somewhat human readable string. The software allows multiple names (aliases) to support different spellings from the Dallas chip. Since automatic detection is not compatible with custom instruments, you should only provide one name.

  3. description: Name that will appear on the Qt GUI and in logs.

  4. generation: Generation can be either Classic or S. This is used to determine at which depth the instrument should be engaged or not (estimating if the instrument is inside the cannula or not) and how close to the RCM point the instrument can go in cartesian mode. If you are designing your own instrument based on a Classic (or S) instrument and the shaft’s length is unchanged, use the generation from the donor instrument.

  5. file: File with path relative to the share directory containing the actual instrument’s definition.

• Instrument definition file: See comments in CUSTOM_DUMMY_400900.json. File containing the DH parameters for the instrument, actuator to joint coupling matrix, position and torque joint limits… See examples in share/tool.

• Manipulator definition file: See comments in psm-Custom-Dummy.json. File containing the DH parameters for the PSM itself, i.e. first 3 joints. This file should be based on the default PSM kinematics found in share/kinematic/psm.json. This might be useful if you need to change the joint limits to avoid collisions with your custom instruments. Most users probably won’t need to create their own version of this file.

7.7.3. Examples

7.7.3.1. Change the control point

In your custom instrument definition file, change the last column of the tooltip-offset:

"tooltip-offset" : [[ 0.0, -1.0,  0.0,  0.0 ],
                    [ 0.0,  0.0,  1.0,  0.01],  // yaw to tip in meters, about 10 mm
                    [-1.0,  0.0,  0.0,  0.0 ],
                    [ 0.0,  0.0,  0.0,  1.0 ]]

7.7.3.2. Change jaw’s angles (grasping torque)

When control in position, the jaws maximum grasping torque is based on the PID output. To increase the PID output without changing the PID gains and get a stronger grasp, the trick is to decrease the minimum joint limit. This way, when the jaws stop moving because they’re in contact with the object to grasp, the tracking error will increase and so will the grasping force. The jaws’ joint limit (qmin) is defined in the instrument definition file:

"jaw" : {
     // for last joint, manual says [0, 30] but we need -40 to allow stronger torque, 80 to open wide
     "qmin": -0.698132, // -40 degrees - overriding ISI values
     "qmax":  1.39626,  //  80 degrees - overriding ISI values
     "ftmax": 0.16
 }

We suggest lowering the limit by small increments until you reach the grasping force you need. This will work up to a point since the torque itself is capped using the ftmax value defined for the jaw in the instrument definition file. The following is not recommended but you can also increase the value for ftmax. Keep in mind than the IO level will then apply another cap defined in the sawRobotIO1394-PSMx-xxxxx.json file (defined in amps).

7.7.3.3. Disable the engage procedure

If you’ve designed a very delicate instrument or added delicate apparatus on your instrument, you might want to make sure the instrument doesn’t get engaged when inserted (sequence of motion applied to the last 4 actuators to mate the sterile adapter disks with the instrument). To do so, you can modify the tool-engage-position in the instrument definition file:

"tool-engage-position" : {
     "lower" : [-0.0, -0.0, -0.0, 0.0],
     "upper" : [ 0.0,  0.0,  0.0, 0.0]
}

In a normal sequence of events, the user should add the sterile adapter first. At that point the software will rotate the last 4 actuators back and forth to engage the adapter’s disks. At the end of that sequence, all 4 actuators will move to 0. Then the user will add the instrument. Since the actuators are all at 0 and the tool-engage-position defines a range from 0 to 0, the last 4 actuators will not move.

7.7.3.4. New mechanical design

When you design your new instrument, there might be a few different issues to keep in mind.

• Can the kinematic be described using DH parameters for a serial manipulator?

  • If so, the built-in numerical solver might be usable and so will the cartesian control modes. This will allow you to use the built-in tele-operation component.

  • If the kinematic is very specific (e.g. snake like, parallel platform…), integration will be trickier. To get started, we suggest you ignore the cartesian API and only use the joint API. You can then implement your own forward/inverse kinematic in a separate ROS node (C++/Python/Matlab) and send/receive joint states to/from the dVRK system. If you need a tighter level of integration, you will need to derive the mtsIntuitiveResearchKitPSM class. Reach out to the dVRK developers if you get to that point.

• Is your instrument a full 6 dof instrument?

  • If it is, just make sure the DH parameters match your instrument.

  • If your instrument is not a full 6 dof instrument, not all cartesian position will be reachable (not event taking into account joint limits). Since the internal numerical solver is not able to handle this case, we suggest creating some virtual links at the end of your kinematic chain.

    • Define virtual links at end of kinematic chain with position offsets set to 0.

    • Coupling matrix must be invertible so keep some ones on the diagonal for the virtual joints.

• Is there a jaw on your instrument?

  • If there is, just make sure the jaw parameters match your instrument.

  • If there isn’t, we’re going to pretend there is one:

    • Set all the jaw parameters to 0.

    • Make sure the last element on the diagonal of the actuator to joint coupling is set to 1.