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