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In each motor, there are two potential sources of error in the current requested. Using a testboard and an ammeter, we observed the characeteristics of these errors on a sample of boards.
First, the current the user requests, the commanded current, could be mismatched with the current the board actually supplies because of imprecision in the amplifiers. This is currently calibrated using the program sawRobotIO1394CurrentCalibration and stored in the xml file for each arm. The commanded current offset is linearly correlated with the supply voltage. In sample boards, we found the following characteristics.
| Max | Average | |
|---|---|---|
| Offset in commanded current | 63.3 mA | 21 mA |
| Correlation to supply voltage | 0.9 mA/V | 0.34mA/V |
The calibration procedure for the error in commanded current is based on the current feedback from the board; however, this presents a second source of error. The board could be reading the wrong current because of imprecision in board components such as the resistors. Characeteristics of this error are as follows.
| Min | Max | Average | |
|---|---|---|---|
| Feedback error | -20 mA | 38 mA | 2 mA |
This can be observed using sawRobot1394IOQtConsole. Enable all boards, but turn off each actuator. Since the actuators are off, the current is necessarily zero, and the current feedback it displays is the measurement error.