Download Electromagnetic Actuation Technologies

Electromagnetic actuation
technologies
Prof Phil Mellor
Department of Electrical
and Electronic Engineering
2
Overview
• Review developments in electromagnetic actuation
– More electric aircraft
– Our research experience
• Back of envelope system discussion
3
Static performance capabilities
Huber, J.E., Fleck, N.A., and
Ashby, M.F., The selection of
mechanical actuators based
on performance indices.
Proceedings of the Royal
Society of London Series aMathematical Physical and
Engineering Sciences, 1997.
453(1965): p. 2185-2205.
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Why consider electrical actuation?
• Benefits include
– High efficiency
– High reliability
– Low maintenance and easy to replace
– Easy to control with good dynamic response
– Low infrastructure and running costs
• Challenges
– Realising high specific force
– Fault tolerance and benign failure modes
– Technology maturity: bespoke designs needed for
each application
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Technology advances
•
Permanent Magnets
•
Digital Control
•
Power Electronics
•
Sensors
Source: Group Arnold
Source: International Rectifier
Source: SIKO GmbH
Source: Texas Instruments
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Developments in the more-electric aircraft
• Rudder
• Aileron
• Elevator
• Flaps
• Slat
• Landing gear
• Brakes
Aileron
Rudder
Flap
Slat
Elevator
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Aircraft primary surface actuator
Force (kN)
140
30
0
50
80
Speed (mm/s)
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EHA (Electro-Hydrostatic Actuator)
•
•
•
•
•
The EHA consists of an hydraulic pump driven by an electric
motor as part of an actuator based around an hydraulic ram.
Control is achieved by running the motor at varying speeds
and directions and driving a fixed volume pump
Poor static load holding leads to reduced thermal
performance and low speed rotation of pump can give rise to
high pump wear rates
Typically powered by permanent magnet synchronous
machines (PMSM) with power electronic control
High inertial losses due to frequent motor reversals
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EHA actuator schematic
Demanded
Surface
Position
Accumulator
Electronic control
Surface
Position
Control
Motor
Control
Shaft
Position
Feedback
pump
M
motor
P
Crossport
Relief
Surface
Position
Feedback
By-Pass
Valve
Controlled
Surface
Actuator
Existing hydraulic
technology
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EHA on an inertia simulator
pivot
Resistive
force
EHA
Inertial
mass
- 350kN peak force
- <2Hz response
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EMA (Electro-Mechanical Actuator)
•
•
•
•
•
The EMA consists of a gearbox driven by an electric motor.
The resultant output may then drive a rotary to linear
conversion e.g. a ballscrew or roller screw
Control is achieved by running the motor at varying speeds
and directions
Significant static load holding will lead to reduced
performance
Typically powered by permanent magnet synchronous
machines (PMSM) requiring power electronic control
High inertial losses due to frequent motor reversals
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Typical EMA/EHA actuator motors
25kW Brushless PM
20kW Brushless PM
40kW Brushless PM
5kW Switched Reluctance
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Electromagnetic direct-drive actuators
Advantages
Disadvantages
Simple construction
Higher cost
Good positioning accuracy
No mechanical advantage
Good dynamic performance
More complex specification
Reconfigurability
Non-standardised
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PM linear actuator topologies
LINEAR MACHINES
THRUST MACHINES
LEVITATION MACHINES
LONG ARMATURE
SHORT ARMATURE
LONG STROKE
ATTRACTION TYPE
REPULSION TYPE
SHORT STROKE
STATIONARY ARMATURE
MOVING ARMATURE
PLANAR MOTOR
TUBULAR MOTOR
SINGLE SIDED
TRANSVERSE FLUX
BRUSHED DC
DOUBLE SIDED
LONGITUDINAL FLUX
LINEAR SYNCHRONOUS MOTOR
SWITCHED RELUCTANCE
LINEAR INDUCTION MOTOR
PM BRUSHLESS
STEPPER
COMPOSITE SECONDARY
SHEET SECONDARY
AC
LADDER STRUCTURE
RELUCTANCE
HYBRID
DC
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PM linear actuator topologies
TUBULAR
PLANAR
y
r
z (direction of travel)
x
z (direction of travel)
θ
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Tubular construction
+ Balanced electromagnetically
(single-sided planar has up to ~1000% normal force to continuous
force capability)
+ No end windings leads to a better utilisation of copper
and hence improved motor constant
- Limited length and sag of tubular rod
- Radial field orientation makes it difficult to laminate back
iron
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Tubular topologies - armature options
(a) Slotless motor with magnetic sleeve
(b) Slotless motor without magnetic sleeve
(c) Conventional slotted motor
(d) Longitudinal flux motor topology
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Tubular topologies - magnetisation options
(a) Axially magnetised primary
(c) Ideal Halbach array
(b) Radially magnetised primary
(d) Discretised Halbach array
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Air-cored or Iron-cored
Air-cored
No cogging force
Small or zero saliency force
Lower force per amp and per volume
Lower mass per volume
Higher acceleration (up to 100g)
Lower thermal resistance
Iron-cored
Cogging force
Saliency force
Higher force per amp
Higher mass per volume
Lower acceleration (up to 22g)
Higher thermal resistance
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Pros and cons of tubular PM linear actuators
• Good force per amp capability (>50N/A)
• High peak force capability (~400%)
• Zero normal (attraction) force
• High force bandwidth
• High speed operation (>5m/sec)
• No backlash - bearing friction only
• Accuracy (<5µm) & repeatability
• Quiet
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9
9
9
9
9
9
9
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Pros and cons of tubular PM linear actuators
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Vertical operation problematic (failure)
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Cost
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Cogging force (high accuracy displacement) 8
Environmental sealing
8
• Finite length (not for planar)
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•
•
•
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Moving secondary
End
stop
Pipe
adapter
Traversing
guide strips
Position
sensors
Kevlar fibre
composite
Stator
Winding
Magnet
array
r
z
Coolant
10Hz Yarn traverse:
Max acceleration >50g
Max speed
2ms-1
Traversal
0.2m
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External armature
• Longitudinal flux motor
• 2-phase BLAC machine
• Max speed 5 ms-1
• 1.0kN pk force
• 10g self acceleration
• Traversal 600mm
magnet
coil
iron sleeve
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Electrodynamic shakers
•
•
•
•
Large voice coil actuators
High bandwidth
Limited displacement
Big and expensive
50mm displacement
90kN peak force (sine)
3m/s max velocity
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Electromagnetic control surface actuator
• 500N force
• ~1.2kg, >10J/kg
• 21Hz operation
• +/-3mm displacement
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Force capability
Magnetic flux
density B (Tesla)
D
σ
Ampere stream
Q (A/m)
L
Magnetic stress σ = K B Q
u
Achievable values:
– B = 1T for a PM armature
– Q = 50,000 A/m rms cont. for a
liquid cooled actuator, peak values
x5 cont. not uncommon
σ (kPa)
Induction
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Radial field/linear PM
40
Longitudinal flux PM
60
Transverse flux PM
80-100
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Composite realisation of transverse flux
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The route to increased specific outputs
• Novel topologies
– >B: improved magnetic properties, multipole magnetisations
– >Q: better winding utilisation, improved cooling
• Higher operating stresses
– mover mechanical integrity, use of composites
• Higher operating temperatures
– high temperature magnets and insulation
– better understanding of thermal behaviour and loss
mechanisms
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Typical table actuator requirements
Force (kN)
• 6 axes with 8 actuators
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• 50Hz maximum bandwidth
• 40kN force
• +/-150mm displacement
•
1ms-1
peak speed
• 6g acceleration
10
0
• Around 20kW rms power per actuator
0.5
Speed (m/s)
1.0
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Moving magnet tubular actuator example
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•
•
•
•
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40kN peak, 28kN rms
0.56m2 active surface: D=0.3m, L=0.6m
40mm pole pitch with 10mm thick magnet array
Magnet mass 45kg
Composite carrier 25kg mass including bearings
58g self acceleration
– Accel=ω2x x=0.15m ωmax=61.7rads-1; 9.8Hz
– vmax=ωx=9.2ms-1
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Possible system configuration
• Key issue is dynamic energy storage
• Capacitors or flywheels are possible solutions
• 10 ton at 1ms-1 = 5kJ
– 100V excursion on a 600V dc link = 77mF
– 300rpm variation in 3000rpm flywheel = 0.53kgm2
Active rectifier
(supplies losses only)
x100
installed filter
capacitor
< inertia of a
pump motor
Actuator(s)
415V ac
Commercial induction
motor drive acting as a
flywheel
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Typical commercial power electronic drive
• 8-16kHz inverter switching
• 1kHz current/force control loop
• 100Hz speed loop
• 10Hz position loop
• Same controller regardless of scale
• ~100Euro/kVA (excludes actuator)
600kVA installation
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Observations
• Bespoke actuator design is required
– A typical test cycle is less that 1 minute hence thermal
issues may not be a problem
– PM machines have a high peak to mean capability 10:1
possible, performance ultimately thermally limited
• Commercial industrial power electronic equipment
would be suitable.
– A standard induction motor could be used for load levelling
– Power draw from mains supply limited to losses
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Piezoelectric solutions
• High stress per volume/weight
• Unidirectional
– Back to back arrangement
– Piezo element must always be in compression
• Low strains – mechanical gearing required
• High voltage operation
• Low energy density
• Stored energy in field comparable to work done
(Similarly issue with electromagnetics where inertia
of armature/rotor is significant)
• New high strain materials on there way
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Conclusions
• A range of direct drive and geared electric actuation
technologies are available
• Examples exist with demonstrated performance
elements that exceed typical earthquake table
requirements:
– x10 force capability
– x5 maximum speed
– x10 acceleration
• Whilst an a specific actuator solution does not exist
which can meet the full performance, although
challenging, indications are that such a device would
be feasible
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Comparison (source: CLD Inc)
Speed
Accuracy
Stiffness
Friction
Temperature
Shock loading
Efficiency
Noise
Environmental
Controllability
Tubular motors
100 in./sec
0.001 in.
High
Medium
125°C
High
50%
40dB
None
Fully
(no backlash)
Mechanical
10 in./sec
0.001 in.
Medium
Medium
125°C
Medium
40%
80dB
Minimal
Hydraulics
10 in./sec
0.01 in.
Medium
High
50°C
High
25%
120dB
Oil
leaks/disposal
Fixed
move Limited move
profiles (cams) profiles
Backlash
Pneumatic
20 in./sec
0.1 in.
Low
High
50°C
High
25%
120dB
Oily air mist
Mostly
bang/bang