| | 76 | == Optional Strain Gage Joint-Torque Sensor == |
| | 77 | |
| | 78 | The 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 | |
| | 80 | The full SG curve is here: |
| | 81 | http://web.barrett.com/support/BarrettHand_Documentation/BH8-280_StrainGageCalibration.pdf |
| | 82 | |
| | 83 | The 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 | |
| | 85 | The 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 | |
| 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 | | |
