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What about keep tracking of /joint_states or /l_arm_controller/state? /l_arm_controller/state does give you the error between desired and actual, for example:

---
header: 
  seq: 422241
  stamp: 
    secs: 4226
    nsecs: 27000000
  frame_id: ''
joint_names: ['l_shoulder_pan_joint', 'l_shoulder_lift_joint', 'l_upper_arm_roll_joint', 'l_elbow_flex_joint', 'l_forearm_roll_joint', 'l_wrist_flex_joint', 'l_wrist_roll_joint']
desired: 
  positions: [-0.45063500000000001, -0.453629, 2.9684300000000001, -1.73685, 2.6124100000000001, -1.3944000000000001, 0.28170399999999973]
  velocities: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
  accelerations: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
  time_from_start: 
    secs: 0
    nsecs: 0
actual: 
  positions: [-0.4506183813835607, -0.3535970257554899, 2.9684618999661581, -1.7368655244971523, 2.6124013457752877, -1.3943968838867686, 0.28170416664188558]
  velocities: [-0.0028369264248514835, 0.0021461982864228284, 0.0030925275806860746, 0.0022533051752136402, -0.00019810275617700331, -0.0011666050558722749, 0.00023889403521337726]
  accelerations: []
  time_from_start: 
    secs: 0
    nsecs: 0
error: 
  positions: [1.6618616439312284e-05, 0.1000319742445101, 3.1899966157933335e-05, -1.5524497152252792e-05, -8.6542247124121729e-06, 3.1161132314494466e-06, 1.6664188584591955e-07]
  velocities: [-0.0028369264248514835, 0.0021461982864228284, 0.0030925275806860746, 0.0022533051752136402, -0.00019810275617700331, -0.0011666050558722749, 0.00023889403521337726]
  accelerations: []
  time_from_start: 
    secs: 0
    nsecs: 0
---