Robonaut Avionics Subsystem
Embedded Motor Control
The approach used to efficiently package the motor control for 15 degrees of freedom in the dexterous hand and wrist has been to design a 3 axis FPGA controller coupled with a custom 3 axis hybrid power motor driver. Using this approach the number of wires for the control of the Robonaut were reduced from greater than 400 to less than 250. The hand motors are clustered in four 3 motor packs and each motor pack is interfaced to a hybrid power driver and FPGA using flexible printed circuit boards (PCB) and nano-connectors.
The functions that the FPGA motor control provides are six step commutation of the three phase brushless DC motors using the hall sensor feedback, pulse width modulation (PWM) to control the six switches in the driver, open loop PWM or velocity loop control of the motor using the motor shaft incremental encoder feedback, over temperature monitoring and shutdown using thermostat feedback, velocity and status feedback, and bi-directional host CPU communication with address decoding and data integrity checksum using a synchronous serial data line with RS-485 drivers. The FPGA motor control PCB is a six layer board which measures 1.5" x 2.0" x 0.25" and is populated double sided with surface mount device (SMD) components.
The functions that the hybrid motor driver performs are the translation of logic level control signals, gate drive of the high and low side MOSFETS of the three phase power bridge, and motor phase sourcing and sinking using the MOSFETS and ultra-fast recovery flyback diodes. The hybrid driver measures 2.88" x 0.8" x 0.175" and is rated to deliver 2A continuously at 28 VDC from -55C to +125C.
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 three 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 Robonaut I hand/wrist module contains 42 sensors for feedback and control, 28 of which are analog and require signal conditioning and digitization. The arm module contains a similar number of sensors with similar requirements. To perform this function a modular, compact, ruggedized data acquisition system (DAS) was specified to meet environmental test conditions. A thorough investigation of COTS DAS's was performed to determine the feasibility of using existing systems versus designing and building a custom DAS for the Robonaut. Several vendors produce DASs certified to MIL-STD-810B for military, and aerospace applications which meet many of the Robonaut requirements. A DAS produced by Acra Controls and distributed in the U.S. by Nicolet was selected as best meeting the size, modularity and environmental requirements of Robonaut. The DAS has been delivered and integrated with the sensors and VME computer system which provides Robonaut's primary control. The DAS is currently configured to accept 24 channels of strain gage input, 24 channels of programmable 0-5V analog input, 64 channels of fixed 0-5V analog input, and 15 channels of thermocouple data. The data is encoded, packetized, and transmitted in an IRIG-106-93 PCM format.
Related Development Work
Smart Motor Controller Chip for Brushless DC Motor
The Smart Motor Controller Chip for Brushless DC Motor (SmartChip) project is a collaborative effort between JSC and JPL and funded by HQ code S. It is a new start in FY99 building on previous work. The goals of the SmartChip project are to reduce by an order of magnitude the control and power electronics associated with small brushless DC motors of 1" and smaller in diameter, and enabling integrated packaging of the electronics with the motor. The focus of the first year's effort was on building component technology for bench level testing. JPL has expertise in Micro Electro-Mechanical Systems (MEMS) switches for motor commutation and is adapting this technology for a miniature custom encoder. JSC is combining Field Programmable Gate Array (FPGA) based motor control expertise with radiation tolerant Application Specific Integrated Circuit (ASIC) design to produce a miniature motor control ASIC which can be combined with JPL's MEMS encoder and commutation components.
The Robonaut project is currently developing FPGA based motor controller technology which is being utilized in the development of the SmartChip ASIC design. In addition, the SmartChip is using JSC's custom developed standard cell CMOS logic libraries (R. Shuler/ EV131) designed to provide latch-up immunity. The preliminary SmartChip has been fabricated using the HP 0.5u batch CMOS fabrication process through the MOSIS service which provides low cost, small quantity fabrication runs. The HP 0.5u batch CMOS fabrication process has been statistically tested and demonstrates radiation tolerance to total ionizing dose radiation in the range of 100K rad. The SmartChip design was developed using the VHDL hardware description language, the Exemplar level 1 Synthesis tool, the Tanner Tools Pro standard cell ASIC place and route tools, OrCad Express R9 for front-end and extracted simulation, and SimGen, a custom designed tool (Shuler) for streamlining integration between Tanner, Exemplar and OrCad. The SmartChip is an all digital motor controller. The SmartChip design provides a serial communication front-end with multi-axis motor capability, a six-step motor commutation section, open loop PWM control mode, and a velocity servo loop mode. Future implementations will include the capability for encoder position accumulation and feedback, pseudo-sinusoidal motor commutation using hall and encoder feedback signals, and current feedback and control using an externally digitizing current sensor.
The current status of the SmartChip ASIC is the first generation (G1) design has been fabricated and delivered. A bench test circuit board has been designed and fabricated. The SmartChip G1 testing has begun and the following results have been obtained: communication data is being received and commands are passing to the motor controller section; the PWM and commutation sections are operating and sending signals to the appropriate motor phases. More detailed testing is ongoing to determine the actual SmartChip performance relative to the design simulations. While the project funding has not been renewed for a second year the second generation (G2) Robonaut FPGA motor controller design is being applied to the SmartChip and the G2 ASIC should be fabricated in the coming year and applied to the Robonaut project. The third generation (G3) Robonaut FPGA motor controller design is currently in progress and should be tested this year as well.
For more information on Robonaut's avionics, contact Scott Askew
NASA Johnson Space Center
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Last Update: 28 June 2000
Curator: Rob Ambrose