Power system overview

1
Power system overview

• Life in the Atacama Design ReviewDecember 19,
  2003
• J. TezaCarnegie Mellon University

2
Power system - function

• Sources
• solar panel
• shore power
• Storage
• daylight operation with reduced insolation
• night operations (science)
• hibernation
• Control
• operation of subsystems
• power distribution
• Measurement
• engineering logging
• health monitoring

3
Power Simplified Architecture
Solar Array
MPPT
Main DC Bus
DC/DC Converters
Li Polymer Battery
Amplifier/ Motors
What is an appropriate battery?
4
Simulation effect of battery capacity
Input power
Load profile
Battery capacity 1500 Wh
1000 Wh
500 Wh
5
Battery - requirements

• Energy capacity
• at least 1000 Wh
• Voltage
• within requirement of locomotion system
• (75V lt Vnominal lt 90V)
• Current capacity
• sufficient for obstacle climbing
• Weight
• less than 15 kg
• Thermal - operating range
• 0 to 40o C
• Reliability
• Safety during operation and shipping
• Cost
• Schedule

6
Battery trade study
7
Battery trade

• Sealed lead acid
• Low specific energy
• simple, reliable, cheap
• NiMH
• Fair specific energy
• Problems - charge control, cost, reliability,
  thermal, configuration
• Li Ion
• Good specific energy
• Component and NRE costs, lead time, control,
  safety
• Voltage required makes design complex
• Li Ion Polymer
• Good specific energy
• Reliability / Risk (?)
• Cost limits spares, redundancy

8
Battery implementation
9
Battery implementation Li polymer

• Worley
• Li Polymer
• Capacity 1.4 kWh, 78 V (nominal)
• Cost 18K (two batteries, one controller)
• Delivery 6 (to 8) weeks
• Vendor claims no shipping restrictions on
  assembled battery
• Fabrication - Singapore

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Battery implementation Li polymer

• 3.30 Ah (rated) 3.7 V Li polymer cell
• Six cell parallel module
• Module size 64 x 100 x 36 mm (approximate)
• 21 modules in series
• Voltage 63 to 88.2 V, 78V nominal
• Capacity 19.8 Ah (rated)
• Maximum current 35 A
• Battery dimensions
• For example
• 128 x 110 x 378 mm (2 x 1 x 11)
• Volume 0.0053 m3
• Mass 8.2 Kg, plus wiring, fuses, enclosure

One module
11
Battery Li Polymer - Cell
Capacity dependent on Charge / discharge rate
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Battery Li Polymer - controller

• Lithium battery safety unit Worley LBSU-4-100
• Monitor individual cell voltages
• Monitor battery current
• Monitor battery temperature
• Shut off battery if out of limit condition occurs
• Allows external reset of battery (circuit
  closure)
• Allows control of external battery relay
• Serial (RS-232) communication
• voltage, current, temperature, fault condition
  reported every minute
• Is this control sufficient?

One module
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Battery Issues

• Reliability
• Components, vendor
• Single string no redundancy for computing load
• Life cycle limited (100 200 cycles)
• Cost limits redundancy, spares testing
• Testing limits life cycles
• Spares cold or hot?
• Fall back / risk mitigation
• Substitute other technology (SLA or ?)
• Impact of change of technology
• Reduction in capacity / increase in mass
• Effect on Solar power tracker / solar array
  requirements (?)

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DC bus

• nominal 78V
• Typical range - 75.6 to 79.8 V
• Maximum range - 63 to 88 V
• Issues
• Maximum too close to amplifier limit
• Switching light weight components limited
• Fusing circuit breakers (?) or fuses -
  reliability
• Control solid state relays (typical failure
  mode for MOSFET is to fail open)

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DC sub buses

• Typical bus voltages
• 5,12, 24 V
• others 3.3, /- 12, /- 15 V
• DC / DC converters
• Implementation
• Vicor input 55 to 100V (72V nominal)
• High efficiency
• 25 to 200 W units, Mega-modules, VI-200 or VI-J00
  series
• -10 to 40 C temperature, can be paralleled
• VI-200 have over-temp and over-current protection
• Can be shut down with gate control

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Power - Architecture
Solar Array
MPPT
Main DC Bus 78V (63 to 88 V)
DC/DC Converters

Li Polymer Battery
Battery Controller
Sub DC Buses
(5, 12, , 24V)
Amplifier/ Motors
PMAD Controller
DC/DC Converter
Li ion Battery
17
Power Architecture Shore power
CC/CV DC supply
Shore Power
Main DC Bus 78V (63 to 88 V)
DC/DC Converters
Li Polymer Battery
Battery Controller
Amplifier/ Motors
PMAD Controller
DC/DC Converter
Li ion Battery
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Power - Architecture split solar array
Main DC bus
DC/DC Converters
Amplifier/ Motors
PMAD Controller
DC/DC Converter
Li ion Battery
Reduce effect of shadowing and single point
failure
19
Power Architecture battery redundancy
OR diodes drive main DC bus
DC/DC Converters
Amplifier/ Motors
PMAD Controller
DC/DC Converter
Li ion Battery
Reduces chance of system fault due to a battery
fault
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PMAD controller - requirements

• Controls
• Hibernation of main computer
• Power for subsystems computing, sensors,
  instruments
• Battery controller reads status and internal
  values (cell voltage and temps), reset via serial
  interface
• Solar MPPT via CAN bus interface
• Acquires system measurements
• Solar panel, bus voltages and currents
• temperatures
• Logging on main computer or internally when main
  computer is off line
• Communicates via main computer or external serial
  port
• Has own battery backup
• Provides status display on exterior panel of robot

21
Power Architecture PMAD
Solar Panels
MPPT
Main DC Bus 78V (63 to 88 V)
V, I
V, I
DC/DC Converters
analog
CAN bus
analog
Li Polymer Battery
Battery Controller
digital
Amplifier/ Motors
RS-232
PMAD Controller
DC/DC Converter
Li ion Battery
PMAD control and data acquisition
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PMAD controller requirements

• I/O required
• CAN bus
• Serial three ports
• Digital - input / output
• opto-isolated
• number - TBD
• Analog input range, number TBD
• LCD display driver

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PMAD controller - implementation

• PC104
• Low power CPU
• Compact flash
• Real time clock
• Watchdog timer
• Battery backup
• Can bus, Digital and analog I/O, serial ports
• Operating system - Linux (w/ minimal kernel)
• Example system
• Arcom Viper, AIM104-CAN, AIM104-ADC16/IN8,
  ViperUSP
• Total power 4.5W _at_ 5V with battery backup for 1
  hr in full power mode or 18 hr in low power mode
• Provision for LCD display

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Exterior display / control panel

• Displays
• Battery status charge, discharge, on/off line,
  fault condition, voltage, current, maximum
  temperature
• Main system state hibernation, normal, fault
• Planner system state on/off
• Controls
• Main power control (manual switch)
• Manual reset of battery controller
• Manual rest of PMAD controller
• Reset / halt of motion controller
• Joystick input
• E-Stop control

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Mechanical - thermal

• Ebox compartmentalization
• Battery
• Ventilation, isolation, battery change out
• Power distribution and locomotion
• PMAD (core CPU), MPPT, distribution buses, fuses
• Locomotion - Amplifier, motion controller I/O
• Computing
• Autonomy, planner, motion controller CPU, science
  computer (?)
• Science provide mechanical support, power,
  communication for
• Chlorophyll detector
• Fluorescence camera
• VisNIR spectrometer
• Additional instruments

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Mechanical thermal - issues

• Thermal ventilation not feasible
• Maximize conduction dissipation
• Layout - packaging
• Cabling
• Fabrication and field access

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Power requirements load

• Locomotion
• Motion controller 9W
• Motors -
• Computing
• Main 20 W
• Planning 30 W
• Core (PMAD and hibernation) 5 W
• Communications
• Ethernet - 6.3 W
• Low BW - ?
• Sensing
• Nav pair 3W
• SPI pair 3W
• Localization FOG 3W, SBC 2.2W
• Crossbow Tilt sensor 0.24W
• Pan/tilt 18W (operating)
• Workspace cams ?
• Sick laser 17W
• Novatel GPS 12W