Transcript Architectural Issues and Actuation Modes in Robots with

The Current Status of Hydraulic
Hybrid Powertrain Technology
Kenneth J. Waldron
Professor (Research), Stanford University
Professor, University of Technology, Sydney
We’ve Been There Before
There was an upsurge in research on high
efficiency powertrains in the wake of the OPEC oil
crisis in ‘70’s and ‘80’s
– Included lots of projects on hydraulic CVT’s with energy
storage and regeneration: hydraulic hybrid powertrains
– Hydraulics were the primary interest, relatively little on
electric powertrains
– In contrast to the present, the preferred energy storage
device was a flywheel
– Current hydraulic systems seem all to be focused on
accumulators
WHY?
Adaptive Suspension Vehicle
Displacement controlled 18 degree of freedom
system
– Included regeneration using an energy storage flywheel
ASV Power Train
Displacement controlled hydraulic actuation
– Power actuators: equal area linear actuators
– Pressure regulated, valve controlled primary system with
fixed displacement rotary actuators to activate swash
plates
– Reservoirs and small accumulators local to actuators
≅ series hybrid with 18 “motors”
Incorporated energy storage flywheel
Powered by motorcycle engine
Flywheel
Primary inertia: martensitic steel rim
Containment: unidirectional kevlar-epoxy
composite is part of rotating mass
Rim is press fit on hub plate: designed to drop off
at 25% overspeed
Sealed aluminium casing is evacuated to low
vacuum by bearing oil pump
Tested to Destruction
Test intended to demonstrate effectiveness of
containment concept
Wheel was crippled by drilling hole through rim to
ensure failure within the limits of the available
drive
– Failure speed above rated upper speed limit
Containment was successful in preventing ejection
of rim fragments
Hydraulic Hybrids
Conventional wisdom is best suited for heavy
vehicles used in frequent start-stop conditions
– Favored by superior regeneration performance
– High power density also relevant
– Hydraulics best at low speed

Will run fast, but losses increase nonlinearly with speed
– Relatively high energy losses over time are not important
in this application
New control capabilities and new materials may
extend these benefits to lighter vehicles
Parallel Hybrid Configuration
Series Hybrid Configuration
Power Split Hybrid
Center for Compact and Efficient Fluid Power
Hydraulic Configuration
Technology Differences from
Eighties
Lithium-ion battery technology was not
commercially available
– Lead-acid was standard technology for automotive
electrics
Graphite composite material technology was not
well developed
– Steel accumulator weighs an order of magnitude more
than graphite composite
Digital control hardware was relatively primitive
– PWM was brand new technology
Modern integrated digital controllers are much
more powerful
Hydraulics Compared to
Electrics
Hydraulic pump/motor is lighter, more compact and less
expensive than electric motor/generator of same power
Hydraulic motors are not subject to overheat at stall
Hydraulic pump/motors can absorb very high power densities
Regeneration is significantly more efficient in a hydraulic
system
Hydraulic pump/motors require a primary actuator to drive
the swash plate shaft
Electric motor/generators need solid state switches
Hydraulic systems require a parasitic generator to drive
electric accessories
New technology batteries have the best energy storage
characteristics
Batteries are more expensive than competing technologies
Flywheel or Accumulator versus
Battery Storage
Flywheel Performance Limits
Theoretical maximum energy density
K


M 2
K is kinetic energy in system
M is system mass
 of rim (hoop stress)
σ is strength
ρ is density of rim
No limit on power density
Flywheel Issues
Simple analysis assumes thin rotor
– Uniform disk reduces energy density by half
Gyroscopic moments affect handling
– Use counter rotating rotors
Need speed reducer to pump/motor
Minimize windage and bearing losses
– Run in low vacuum
– Magnetic bearings? Hydrostatic? Hydrodynamic?
Need containment
– Composite ring? Include in rotating mass?
Accumulator Performance Limit
Theoretical maximum energy density
U


M 2.4 
Difficult to approach in practice due to upper limit
imposed by system operating pressure

Relatively
rapid leakage due to heat transfer
Accumulator Issues
Theoretical energy density not practical
– Composite thin walls are fragile, back with aluminium or
titanium
Accumulators are bulky, difficult to package in
vehicle
Hydraulic System Issues
Noise
–
–
–
–
Principal source is valve porting
Reduced by running slower: means larger pump/motors
Use gear trains at engine and flywheel (if used)
Acoustically isolate pump/motors
Throttling losses
– Avoid control valves
Leakage
– Eliminated with proper design and maintenance
Peaky efficiency/speed characteristics
– Actually no worse than electric machines
 Swash plates need significant muscle
Pump/Motor Circuit
EPAM Actuators
Configured as capacitor with very extensible
dielectric, compliant electrodes
Pretension to maximum actuation force
– Excitation causes relaxation in stretch direction
– Largest force produced when passive
Good force to weight ratio
Fast response
Moderate efficiency
Cross-Pull EPAM Actuator
Electrostrictive polymer actuator
– Thin polymer layer with compliant electrodes deposited
on both sides
– Sheet is pre-tensioned
– Relaxes when excited
– Needs high voltage, small current
– Nonlinear characteristics
Two sheets tensioned across diagonals of
parallelogram frame
Durability issues
Binary Configuration
Cross-pull EPAM’s work well as bistable actuators
– Extensive practical experience with this mode
– Proven durability
Suggests use to actuate switching valves
–
–
–
–
Needed to switch from motor to pump operation
Effort needed is moderate
Normally use solenoid valves
Efficiency is moderate, but also true for solenoids
EPAM on Swashplate?
Swashplate shaft actuator
should be fast and accurate.
Does not need large motion
range. Should be low loss.
Alternatives are fixed
displacement hyd. motor,
electric motor, EPAM
– Hydraulic motor entails severe
valve losses
– Electric motor is heavy, bulky
– EPAM is light with good
bandwidth, adequate motion
range
Summary
New technology options justify a new look at
hydraulic hybrids
Propose optimal configuration study
Mechanical complexity versus electrical
complexity
Serial or split configurations are most attractive
EPAM’s may provide viable option for swash plate
and switching valve actuation