Changes between Version 27 and Version 28 of Hand/280/KinematicsJointRangesConversionFactors


Ignore:
Timestamp:
Jun 13, 2017, 3:34:11 PM (7 years ago)
Author:
cv
Comment:

Updated strain gauge section

Legend:

Unmodified
Added
Removed
Modified
  • Hand/280/KinematicsJointRangesConversionFactors

    v27 v28  
    7474}}}
    7575
     76== Optional Strain Gage Joint-Torque Sensor ==
     77
     78The strain gauges measure the torque about the outer link of each finger. If your BarrettHand includes this option, you will be able to query the SG property to get the present strain gauge value. This value is a number between 0 and 4095, corresponding roughly to a fingertip force of -2 to +2 kg. A finger without any force applied should report an SG value of around 2000 when the strain gauge option is installed.
     79
     80The full SG curve is here:
     81http://web.barrett.com/support/BarrettHand_Documentation/BH8-280_StrainGageCalibration.pdf
     82
     83The HSG and LSG properties set the High Strain Gauge and Low Strain Gauge limits. For example, if you set HSG to 3000 before you issue a movement command that closes the finger, that finger will stop moving at the moment the SG value reaches 3000 (~1 kg). Likewise, if you set LSG to 1000 before you issue a movement command that opens the finger, that finger will stop moving at the moment the SG value reaches 1000 (~1 kg against the back of the finger).
     84
     85The fingers also have a "self-preservation" feature. When the torque about the outer link exceeds 1.14 Nm (SG < 667 or SG > 3460), the finger will automatically drive itself to minimize the fingertip force. If you wish to disable this feature, you can set HSG to the special value of 10000. This will not change HSG, it will simply turn off self-preservation. If you want to re-enable the self-preservation feature, you can set HSG to the special value of 10001.
     86
    7687== Spread Motion ==
     88
    7789The spreading action of fingers F1 and F2 on the !BarrettHand™ increases the dexterity of the entire unit with only one additional actuator.  Optimal grasp configurations can be achieved "on-the-fly" without costly tool changes associated with traditional grippers.  In addition, the backdrivability built into this degree of freedom causes the !BarrettHand™’s grasp shape to change in mid-grasp, creating a more stable grasp of oddly shaped target objects.
    7890
     
    88100}}}
    89101
    90 == Optional Strain Gage Joint-Torque Sensor ==
    91 The !BarrettHand™ provides an optional Joint-Torque sensor for each finger.  The Joint-Torque sensor measures the torque about the outer joint on each finger, see Figure 34.  The Joint-Torque sensor is comprised of a flexible beam with four foil strain gages applied and wired in a Wheatstone Bridge configuration.  When a force is applied to the fingertip, Force A, the torque is measured by the amount of deflection in the beam.  The beam deflection is proportional to the difference in cable tension, which translates to a force on the pulley attached to the flexible beam, Force B.  The flexing in the beam creates a measurable voltage change in the Wheatstone Bridge.  This difference in voltage is conditioned, amplified, converted and available to you in digital form.
    92 
    93 {{{
    94 #!div class="center" align="center"
    95 [[Image(htdocs:bhand/280/figure34.png)]]
    96 
    97 '''Figure 34 - Strain Gage Joint-Torque Sensor'''
    98 }}}
    99 
    100 The gages are adjusted before leaving the factory and should exhibit a no-load SG value between 100 and 140 for 8-bit strain on earlier hands.  Newer hands, using Pucks will have 12-bit resolution and the expected no-load SG value should be between 1600 and 2240.  If the gage values do not fall within the specified range, see Section 7.4.  For improved accuracy, the user can measure the no-load value before taking readings of SG.  For example, issue a GO command and then a FGET SG command to open the fingers against their J2 stops.  J3 has no open stop, so its torque will measure only second order effects, such as residual friction, gravity, and dynamic inertia effects (on a moving robotic arm).
    101 
    102 {{{
    103 #!div class="center" align="center"
    104 [[Image(htdocs:bhand/280/figure35.png)]]
    105 
    106 '''Figure 35 - Strain Gage Torque Curves'''
    107 }}}
    108 
    109 
    110 Note:  In Figure 35, true SG values have been adjusted so that the no-load value corresponds to zero torque.  If the torque curve measured does not approximate the torque curve shown in Figure 35, see Section 8.  The torque curves for each finger will be different due to the variations in materials.
    111 
    112102== Forward Kinematics ==
     103
    113104The forward kinematics for the !BarrettHand™ were determined using the Denavit - Hartenberg notation described in "Introduction to Robotics, Mechanics and Control 2nd Edition", John J. Craig.  Each finger is considered its own manipulator and is referenced to a wrist coordinate frame in the center of the palm.  Use the forward kinematics calculated in this section to determine fingertip position and orientation with respect to the palm.
    114105