This section includes documentation of information gathered from my 6-month co-op while studying in the Humanoid Robot research Center at KAIST, South Korea. This section is currently under development, so keep checking back for updates!
The functionality of the force-torque sensors (located in each of the wrists and ankles) is to provide force-feedback for each joint. The sensors in the wrist are capable of measuring torques in the X and Y direction, while the sensors in the ankles are not only capable of torques in the X and Y direction, but also Fg, the force of gravity.
As of the writing of this description, the functionality of the wrist sensors provides a way for the Hubo hand (and arm) to follow an input force through the wrist, like a handshake. This joint sensor allows for the Hubo arm to have a small degree of compliance, in that, the Hubo arm can either counter of follow through with torques applied to the wrist.
The ankle force-torque sensors, however, have more functionality and control to the hubo platform than the wrist sensor. Similar to the wrist sensor, the ankle sensor allows for the measurement of toques in the X and Y direction, but these sensors have a third degree of measurement, the capability to measure the force of gravity (Fg) on the foot (at the ankle). This allows for the computer to not only check the existence of toques on the foot, but if the foot is also in contact with the ground (and weight distribution on the feet). The torques allow for measurement if the foot were to contact the ground unevenly, also to see which way the hubo is leaning when balancing on one foot (instead of trying to figure out the torque through the motor currents)
The electrical specifications of the force torque sensors are:
Metal Foil-type strain gauge
Temperature Compensation for aluminum
Gage factor of 2.1 +/-0.5%
Transverse Sensitivity (K): 3%
Resistance: 350 ohms +/-0.2%
The strain gauges are placed in a full-bridge, Wheatstone bridge configuration to measure the stress in the aluminum. Click here for a similar schematic used by DASL's mini-humanoid robot. The schematic was constructed using OrCAD Capture 10.5.
The inertial measurement unit (IMU) is the key feature to having Hubo stand, balance, and walk. This sensor is one of the most crucial sensors on the robot platform.
The Hubo IMU uses two DAS inclinomters to detemine tilt. These sensors can only detect changes within +/- 30 degrees of gravity (60 degree spread), however, these sensors are more accurate than normal accelerometers. No Z tilt sensor is needed, just X and Y axes. The rate gyro sensors are single axis MEMs angular rate gyros mounted in three orientations of Yaw, Pith, and Roll. They account for how fast the Hubo's center of mass is rotating. THe angular rate gyros have little data regarding the exact model number, exact specs (like detection acceleration, speed, etc.) are unknown.
The inclinometers and the angular rate gyro voltage signals feed into the ADC of the TMS320F2808 DSP Board. A breakout board is necessary to connect to the proper pins. The breakout board also supplies a regulated 5 volt supply to the sensors and DSP board (This power supply is independent from the Hubo main power supply). The breakout board has connectors on it to allow sensor hook-up. In the event that the DSP Board needs to be removed for one reason or another, it can be by unplugging the power, and CAN connectors, and simply removing it from the breakout board.
A more detailed picture (below) shows the placement of the inclinometers, gyros, and the DSP board, all assembled and ready to put onto the Hubo.
For a more detailed look at the IMU, click here for a video of me assembling the IMU.
- Netburner DSP datasheet
- Netburner Pin IO Class
- Netburner PWM Application Note
- MCF5213 Freescale Processor datasheet
* For more information regarding the Netburner MOD5213 DSP, click here for the main webpage.
The Propeller chip is a microcontroller with eight processors in a single package. The processors (cogs) are fully independent (the first processor loads the compiled code into the other processors). I have used this for several projects, where true parallel processing is necessary.
- Propeller Microcontroller
- Propeller Article - Why the Propeller Works - by Chip Gracey
Hubo KHR4 F/T Sensor
The MOD5213 DSP
The Propeller Chip
Hubo Humanoid robot documentation is finally here! Check out information regarding the Force-Torque Sensor and IMU.