Configure Complex Kinematic Chains

Many components have complex kinematic chains and require an additional set of intermediate reference frames to use Viam’s Motion service.

  • For example, an arm has a reference frame originating where the arm is attached to a surface, but it also has links and joints whose frames of reference matter when attempting to move the arm to a pose with MoveToPosition().

If you want to implement a component with a complex kinematic chain that is not already built into the RDK, you need to add a file to your driver that details the attachment of the intermediate reference frames on the component.

This file can be a .json file in the same format as Viam’s built-in arm drivers, or an .URDF file.

Kinematic parameters

Viam supports two formats for supplying kinematic parameters to configure intermediate reference frames for a kinematic chain, defined as "kinematic_param_type":

  1. Spatial Vector Algebra (SVA): supplying reference frame information for each link and each joint.
  2. Denavit-Hartenberg (DH) parameters.

This is an example .json configuration as used by Viam’s Universal Robots arm driver:

{
  "name": "UR5e",
  "kinematic_param_type": "SVA",
  "links": [
    {
      "id": "base_link",
      "parent": "world",
      "translation": {
        "x": 0,
        "y": 0,
        "z": 162.5
      },
      "geometry": {
        "x": 120,
        "y": 120,
        "z": 260,
        "translation": {
          "x": 0,
          "y": 0,
          "z": 130
        }
      }
    },
    {
      "id": "shoulder_link",
      "parent": "shoulder_pan_joint",
      "translation": {
        "x": 0,
        "y": 0,
        "z": 0
      }
    },
    {
      "id": "upper_arm_link",
      "parent": "shoulder_lift_joint",
      "translation": {
        "x": -425,
        "y": 0,
        "z": 0
      },
      "geometry": {
        "x": 550,
        "y": 150,
        "z": 120,
        "translation": {
          "x": -215,
          "y": -130,
          "z": 0
        }
      }
    },
    {
      "id": "forearm_link",
      "parent": "elbow_joint",
      "translation": {
        "x": -392.2,
        "y": 0,
        "z": 0
      },
      "geometry": {
        "x": 480,
        "y": 120,
        "z": 100,
        "translation": {
          "x": -190,
          "y": 0,
          "z": 0
        }
      }
    },
    {
      "id": "wrist_1_link",
      "parent": "wrist_1_joint",
      "translation": {
        "x": 0,
        "y": -133.3,
        "z": 0
      },
      "geometry": {
        "x": 90,
        "y": 130,
        "z": 130,
        "translation": {
          "x": 0,
          "y": -110,
          "z": 0
        }
      }
    },
    {
      "id": "wrist_2_link",
      "parent": "wrist_2_joint",
      "translation": {
        "x": 0,
        "y": 0,
        "z": -99.7
      },
      "geometry": {
        "x": 80,
        "y": 150,
        "z": 100,
        "translation": {
          "x": 0,
          "y": 0,
          "z": -100
        }
      }
    },
    {
      "id": "ee_link",
      "parent": "wrist_3_joint",
      "translation": {
        "x": 0,
        "y": -99.6,
        "z": 0
      },
      "orientation": {
        "type": "ov_degrees",
        "value": {
          "x": 0,
          "y": -1,
          "z": 0,
          "th": 90
        }
      }
    }
  ],
  "joints": [
    {
      "id": "shoulder_pan_joint",
      "type": "revolute",
      "parent": "base_link",
      "axis": {
        "x": 0,
        "y": 0,
        "z": 1
      },
      "max": 360,
      "min": -360
    },
    {
      "id": "shoulder_lift_joint",
      "type": "revolute",
      "parent": "shoulder_link",
      "axis": {
        "x": 0,
        "y": -1,
        "z": 0
      },
      "max": 360,
      "min": -360
    },
    {
      "id": "elbow_joint",
      "type": "revolute",
      "parent": "upper_arm_link",
      "axis": {
        "x": 0,
        "y": -1,
        "z": 0
      },
      "max": 180,
      "min": -180
    },
    {
      "id": "wrist_1_joint",
      "type": "revolute",
      "parent": "forearm_link",
      "axis": {
        "x": 0,
        "y": -1,
        "z": 0
      },
      "max": 360,
      "min": -360
    },
    {
      "id": "wrist_2_joint",
      "type": "revolute",
      "parent": "wrist_1_link",
      "axis": {
        "x": 0,
        "y": 0,
        "z": -1
      },
      "max": 360,
      "min": -360
    },
    {
      "id": "wrist_3_joint",
      "type": "revolute",
      "parent": "wrist_2_link",
      "axis": {
        "x": 0,
        "y": -1,
        "z": 0
      },
      "max": 360,
      "min": -360
    }
  ]
}

{
  "name": "UR5e",
  "kinematic_param_type": "DH",
  "dhParams": [
    {
      "id": "base",
      "parent": "world",
      "a": 0,
      "d": 162.5,
      "alpha": 1.57079632679,
      "max": 360,
      "min": -360
    },
    {
      "id": "shoulder",
      "parent": "base",
      "a": -425,
      "d": 0,
      "alpha": 0,
      "max": 360,
      "min": -360
    },
    {
      "id": "elbow",
      "parent": "shoulder",
      "a": -392.2,
      "d": 0,
      "alpha": 0,
      "max": 180,
      "min": -180
    },
    {
      "id": "wrist_1",
      "parent": "elbow",
      "a": 0,
      "d": 133.3,
      "alpha": 1.57079632679,
      "max": 360,
      "min": -360
    },
    {
      "id": "wrist_2",
      "parent": "wrist_1",
      "a": 0,
      "d": 99.7,
      "alpha": -1.57079632679,
      "max": 360,
      "min": -360
    },
    {
      "id": "wrist_3",
      "parent": "wrist_2",
      "a": 0,
      "d": 99.6,
      "alpha": 0,
      "max": 360,
      "min": -360
    }
  ]
}

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