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Hybrid Hydraulic-Electric Architecture for Mobile Machines
Jacob Siefert and Arpan Chatterjee
Advisor: Perry Li
Outline
•Motivation
•HHEA Architecture
•Energy saving study
•Controls
•Summary
Motivation – Conventional Off-Road Machines
• Consume large amount of energy• Excavators (20-30 ton): 300 Trillion BTUs• Turfgrass Mowers: 57 Trillion BTUs
• Hydraulically actuated
• Control via throttling valves• State-of-the art: Load-Sensing
• Average Efficiency = 21%
Fluid Power Trends• Better Efficiency• Electrification
Hydraulics vs Electronics
Hydraulics:
• Power Density (+)
• Reliability/Robustness (+)
• Familiar (+)
• Can be inefficient (-)
• Efficiency vs Control vs Size (-)
• Poor energy storage density (-)
• Not too flexible, leakage, NVH (-)
Electronics:
• Good Efficiency
• Control Performance
• Good energy storage density
• Clean
• Flexible
• Low power density (-)
• High cost for high power (-)
Hydraulics & electrics are complementary!
Electro-Hydraulic Actuator (EHA)
Benefits
• No Throttling & Regenerative
• Good Efficiency & Control Performance
• Fixed Displacement Pump/Motor
• Electrical Wiring
Limitation:
•Full power from electric drive
• Expensive and bulky for high
power (> 20kW) systems
Project Objective
Demonstrate a target efficiency (from engine output) of ≥65% in hydraulically powered off-highway vehicles through development of an integrated hydraulic and electric system architecture applicable to a wide range of multi degree-of-freedom mobile machines.
•36 month
•Start date: 9.1.2018
•Benefits of efficiency and control performance
•Integrated hydraulic-electric components
increase power density, efficiency and reduce
cost
Hybrid Hydraulic-Electric Architecture (HHEA)
(Optional)
Multiple
pressure rails
Provisional Patent Filed
Hydraulic-Electric Control Module (HECM)
• Pick pressure rails to provide force/torque closest to load
• Use electric drive to make up difference
Rotary HECMLinear HECM
HHEA: Electrical Power
(PA Ac - PB Ar ) + FHECM = Fload
Pressure Rail Force Electric
Example: 3 rails: (0, ½, 1) 𝑃𝑚𝑎𝑥
⇒ 9 possible pressure rail forces
1.Electric drive needs only provide difference
2.Throttle-less and regenerative
HECM
EHA
An Example of 2 vs 3 CPRs
H+T
CPRsTank: TMiddle: MHigh: H
CPRsTank: THigh: H
H+HT+T
0+T
An Example of 2 vs 3 CPRs
2 CPRs
3 CPRs
• Rail pressures uniformly distributed pressures• not always optimal
• Area ratio = 1.5• Minimum size e-components• Not all rails are usable (e.g. cavitation)
• Optimal depends on area ratio and span of duty cycle force
E-component downsizing vs # Pressure Rails
1 rail = EHA
Pressure Rail Management
(Current idea - work in progress)
Centralized
Main pump
•Main pump can operate at full
displacement ⇒ more efficient
•Apportion flow to pressurized rails
or tank to maintain pressure near
target levels
HHEA Benefits•Allows downsizing of e-motor/drives cf with EHA
⇒ makes electrifying high power machines
possible
•Efficiency benefits:
• Throttle-less;
• Regenerative - hydraulically and electrically
• Fixed displacements P/Ms
•Control performance benefits:
• High bandwidth flow metering via e- drive
•Readily available components
• Tight integration offers even greater benefits
Machine Results
• HHEA Efficiency and Energy Breakdown• # Rails and placement of rails
• Static energy loss model
• Use Lagrange Multiplier Method for Optimal Control
• Compared to Load-Sensing• Pressure Margin
• Some regenerative pathways
• “Backside” pressure for control
A Note About Efficiency
𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =𝑃𝑜𝑠𝑖𝑡𝑖𝑣𝑒 𝑊𝑜𝑟𝑘
𝐼𝑛𝑝𝑢𝑡 𝐸𝑛𝑒𝑟𝑔𝑦
𝑃𝑜𝑠𝑖𝑡𝑖𝑣𝑒 𝑊𝑜𝑟𝑘 = න𝑡𝑚𝑖𝑛
𝑡𝑚𝑎𝑥
𝑖=1
𝑛𝑎
𝑓𝑖 𝑑𝑡
𝑓𝑖 𝑡 =0 𝑖𝑓 𝐹𝑖 𝑡 𝑣𝑖 𝑡 < 0
𝐹𝑖 𝑡 𝑣𝑖 𝑡 𝑖𝑓 𝐹𝑖 𝑡 𝑣𝑖 𝑡 ≥ 0
Weight
Motor
Battery
Machine Results: 22 Tonne Excavator
HHEA: 127%LS: 40%Energy Saving: 69%
HHEA: 112%LS: 25%Energy Saving: 78%
HHEA: 108%LS: 37%Energy Saving: 66%
Machine Assumptions• 4 CPRs• No DCV: Rod-Side Only• Capped HECM Torques
(+)
MP
(-)
Co
ntr
ol
Inp
ut
(+)M
P
(-)C
on
tro
l
(+)
MP
(-)
Co
ntr
ol
Inp
ut In
pu
t
Machine Results: Wheel Loader
HHEA: 127%LS: 37%Energy Saving: 71%
HHEA: 114%LS: 41%Energy Saving: 64%
Machine Assumptions• 4 CPRs• No DCV: Rod-Side Only
*Can be significantly reduced if optimized for torque vs energy.
(+) MP
(-) Co
ntr
ol
(+)
MP
(-)
Co
ntr
ol
Inp
ut In
pu
t
Machine Results: 5T Mini Excavator
HHEA: 119%LS: 30%Energy Saving: 75%
HHEA: 64%LS: 20%Energy Saving: 69%
HHEA: 95%LS: 31%Energy Saving: 67%
Machine Assumptions• 2 CPRs• MP Driven by Electric
Motor/Gen• DCV: Cap or Rod-Side• Capped HECM Torques
(+)
MP(-)
Co
ntr
ol
(+)
MP
(-)
Co
ntr
ol
(+)
MP
(-)
Co
ntr
ol
Inp
ut Inp
ut
Inp
ut
Objective for control performance
Fd(t)
Control Input : • E-Motor torque• Limited torque
Objective:• Tracks desired
position xd(t)Given :• Desired position, velocity and load force• Pressure Rail selection
Passivity based backstepping approach
• Robust non-linear control method • Uses systems intrinsic energy function as
Lyapunov function • Successive design methodology : assuming
• Velocity as input• Force / pressure as input • Flow as an input• Torque as an input
P1
P2
Fl
X, v
T
PmT Ph
PmT Ph
Q
Passivity based backstepping approach
Backstepping level 1 (Velocity as input)
Velocity input :
ሶ𝑒 = −𝜆𝑝𝑒 + 𝑒𝑣
𝑒𝑣 = ሶ𝑥 − 𝑟
P1
P2
Fl
X, v
T
PmT Ph
Pm
T Ph
Q
Backstepping level 2 (Pressure/force as input)
𝑃𝑑𝐴2 = 𝑃1𝐴1 − 𝐹𝑙 −𝑀 ሶ𝑟 + 𝐾𝑣𝑒𝑣 + 𝐾𝑝𝑒
𝑀 ሶ𝑒𝑣 = 𝐾𝑣𝑒𝑣 +𝐾𝑝𝑒 - 𝐴2 ෨𝑃
Backstepping level 4 (Torque as input) :
Backstepping level 3 (Flow as input) :
Passivity based backstepping approach
d
Successful Position Tracking without torque saturation
Max error < 0.2mm
Rail Switching events
Peak Torque : 2000 Nm
Effect of torque limitation during switching
Torque limit : +/-250 Nm
Max error = 7.1mm
P1P2
Fl
Pump + Motor unit
Pr1 (increases)Pr2 (increases)
Switching Event
(slowly decrease)(decrease)
x
Q
xd
• Cap side pressure switches near instantaneously
• Rod side pressure dynamics limited by pump flow/torque
• This induces pressure and tracking error
Solution :• Reduce switching frequency• Delay cap side switching • Preemptive control of rod side pressure
Switching Penalty
• Avoid frequent rail switching • Switch only if there is significant benefit or
sufficient time has passed• An addition loss is virtually added to the system• Modified Loss :
𝐽𝑚𝑜𝑑 𝑃𝑟 , 𝑡 = 𝐽 𝑃𝑟 , 𝑡 + 𝐴 𝑃𝑟 , 𝑃𝑟∗ 𝑡𝑠 𝑒 −𝜆∗ 𝑡−𝑡𝑠
- 𝑃𝑟∗ 𝑡 = current selected pressure rail
- 𝐴 𝑃𝑟 , 𝑃𝑟∗ 𝑡𝑠 = 0 if 𝑃𝑟 = 𝑃𝑟
∗
- ts = last time of rail switch
Next selected 𝑃𝑟∗ 𝑡 = argmin(𝐿𝑜𝑠𝑠𝑚𝑜𝑑(Pr, t) )
Comparison of cumulative error between rail switch with and without penalty
Cumulative error = 𝟎𝒕𝒆 .𝒅(𝒕)
Delayed cap side switching
No anticipation
With anticipation
Increase P
Decelerate pump
Error Comparison
7.1mm
0.5mm
Summary
• Blended hydraulic and electric actuations• Multiple pressure rails provide majority of power
• (Small) electrical components modulates power and meters flow
• Preliminary energy savings analysis is promising
• Control strategy compensates for effect of pressure rail switching
• Future work• Refined energy analysis
• Hardware in the loop control test
Acknowledgement :
US DOE EERE Grant DE-EE0008384