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Avionics

The goal of the Robonaut Avionics development has been to create tightly integrated electronics and mechanisms to reduce the volume for external electronics boxes, as well as the size and number of the cable harnesses needed to transmit signals throughout the system. The Robonaut avionics consists of four main subsystems: embedded motor control, data acquisition, power distribution and control, and brainstem data processing. Primary initial focus of internal development has been the embedded motor control, with subsystems listed in chronological priority of technical development.

Embedded Motor Control

The approach used to efficiently package the motor control for 14 degrees of freedom in each dexterous hand and wrist module has been to design 3-axis FPGA motor controllers coupled with hybrid 3-axis motor drivers. Using this approach the number of wires for the motor control of Robonaut were reduced from over 300 to just over 75. The hand motors are clustered in four triple-motor packs and each motor pack is interfaced to a 3-axis hybrid power driver and FPGA using flexible printed circuit boards (PCB) and nano- connectors. The two wrist motors, which control pitch and yaw, are integrated with two single axis motor drivers. The FPGA motor control PCB which measures 1.5" x 2.0" x 0.25" is populated on both sides with surface mount device (SMD) components. The hybrid motor driver is rated to deliver 2A continuously at 28 VDC from -55C to +125C. The 3-axis driver measures 2.88" x 0.8" x 0.175" and the single axis driver measures 1.6"x0.6"x.175". The flexible PCB serves as the interconnect between the three motor pack, the hybrid motor driver and the FPGA controller. Nano-connectors provide the 28VDC power and FPGA data interface, the hybrid motor driver is connected to the outside of the flex circuit for good thermal conductivity to the forearm structure. The flex circuit bends around the triple motor pack to provide connection of the motor wires. The flexible PCB is a highly irregular shape to meet the geometric constraints for packaging, but has outer dimensions of 3.25" x 4" x 0.025".

The FPGA motor controller provides the following functions:

Six step commutation of the motors using the hall sensor feedback
open loop pulse width modulation (PWM) control or
closed loop velocity control of the motor using the motor incremental encoder six PWM, high and low motor phase outputs to control the switching in the driver over temperature monitoring and shutdown using thermostat feedback motor position, over temperature, and communication status feedback bi-directional host CPU communication with address decoding and data integrity checksum using synchronous RS-485 drivers.

The functions that the hybrid motor driver performs are:

  • Level translation of logic level control signals
  • Gate drive of the high and low side MOSFETS of the three phase power bridge
  • Motor phase current supply using MOSFETS and ultra-fast recovery diodes

The first generation of embedded motor controller and hybrid motor driver are complete and operational in Robonaut. Work has begun on a next generation set of hardware to expand the operational capabilities of these components as well as reducing their size and power consumption. The hybrid driver is being redesigned with radiation hardened components to provide a more robust capability for a flight system. While the embedded motor controller has been fabricated as a radiation tolerant application specific integrated circuit (ASIC), the latest generation of radiation tolerant and radiation hardened FPGA’s hold much promise as a timely and cost effective solution for a future flight system. Ongoing and future upgrades to the embedded motor controller will include adding current control and feedback capability, and more axes of control per device.

Data Acquisition and Sensory Input

data acquisition board

The two Robonaut hand/wrist modules contain 84 sensors for feedback and control, 60 of which are analog and require signal conditioning and digitization. Each degree of freedom has a motor position sensor, a joint force sensor, and a joint absolute position sensor. The two arm modules contain 90 sensors, 80 of which are analog. Each actuator contains a motor incremental position sensor, redundant joint torque sensors, redundant joint absolute position sensors, and four temperature sensors distributed throughout the joint. To acquire the analog sensor data a ruggedized COTS data acquisition system (DAS) was selected to best meet the environmental operating requirements. Several vendors produce a DAS certified to MIL-STD-810B for military and aerospace applications, which meet many of the Robonaut requirements. The current Robonaut DAS was made by Acra Controls. The DAS has been integrated with the analog sensors and the brainstem computers. The sensor data is transmitted to the brainstem in an IRIG-106-93 PCM format. The DAS has the capability to accept 48 channels of strain gage input, 32 channels of programmable 0-5V analog input, 96 channels of fixed 0-5V analog input, and 16 channels of thermocouple input.

Additionally Robonaut has 5 six-axis force/moment sensors (FMS) to enable endpoint and localized contact force sensing. The FMS are located in the forearms, shoulders, and upper torso. The FMS interface directly to the brainstem computer, having internal signal processing separate from the DAS. The FMS are a combination of commercially available and custom designs made by JR3.

Development on a next generation of DAS has begun with the design and testing of custom designed mixed signal ASIC’s to perform the majority of the sensor signal conditioning. These ASIC’s make up the beginning of a more distributed DAS.

Power Distribution, Control and Safing

The power distribution and control subsystem includes power to the arm and waist brakes, and thus doubles as a system-wide safing capability. This is implemented with both computer and manual override shutdown controls. A manual enable switch is also included for each brake to facilitate partial element testing and reconfiguration. As Robonaut is a fairly slow moving robot, it does not currently have any redundant computer safing capability in the case of primary computer malfunction. One or more humans are involved in console and work area monitoring, and provide the safety backup to the main computers, or teleoperator error.

Brainstem Data Processing

To leverage rapid advances in computer technology, the brainstem processing and I/O has up to now been commercial off the shelf (COTS) hardware. The current Robonaut computer chassis is 6U VME based and contains three 604 PowerPC computer boards, and several COTS I/O boards to perform external data communication. A second-generation CPU rack is under development which will be embedded in Robonaut and utilize a COTS 3U CPCI PowerPC 750 CPU with custom I/O boards. The I/O boards will improve system performance by reducing the CPU overhead for bus communications, and by being able to perform local I/O stream data processing. While the new CPU is a COTS product, a space rated computer with very similar hardware specifications has been identified for use in a flight configuration of Robonaut.

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