Department of Electrical and Electronic Engineering

Electromagnetic actuation

technologies

Prof Phil Mellor

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 a-Mathematical Physical and Engineering Sciences, 1997. 453(1965): p. 2185-2205.

4 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

5 Technology advances

Source: Group Arnold

• Permanent Magnets

• Digital Control

• Power Electronics

• Sensors

Source: Texas Instruments

Source: International RectifierSource: SIKO GmbH

6 Developments in the more-electric aircraft

Elevator

Rudder

Slat

Aileron• Rudder• Aileron• Elevator• Flaps• Slat• Landing gear• Brakes

Flap

7 Aircraft primary surface actuator

30

140

0

Force (kN)

Speed (mm/s)50 80

8 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

9 EHA actuator schematic

Electronic control

Existing hydraulictechnology

DemandedSurfacePosition

SurfacePositionControl

MotorControl

M

P

Accumulator

ControlledSurface

Actuator

CrossportRelief

By-PassValve

SurfacePosition

Feedback

ShaftPositionFeedback

motor

pump

10

pivot

Inertialmass

Resistiveforce

EHA

EHA on an inertia simulator

- 350kN peak force - <2Hz response

11 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

12Typical EMA/EHA actuator motors

40kW Brushless PM25kW Brushless PM

5kW Switched Reluctance20kW Brushless PM

13 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

14 PM linear actuator topologies LINEAR MACHINES

LEVITATION MACHINES

ATTRACTION TYPE REPULSION TYPE SHORT ARMATURE LONG ARMATURE

LONG STROKE SHORT STROKE

MOVING ARMATURE STATIONARY ARMATURE

TUBULAR MOTOR

PLANAR MOTOR

SINGLE SIDED DOUBLE SIDED

TRANSVERSE FLUX LONGITUDINAL FLUX

LINEAR INDUCTION MOTOR

LINEAR SYNCHRONOUS MOTOR

COMPOSITE SECONDARY SHEET SECONDARY

BRUSHED DC

STEPPER

SWITCHED RELUCTANCE

RELUCTANCE HYBRID

AC DC

THRUST MACHINES

PM BRUSHLESS

LADDER STRUCTURE

15 PM linear actuator topologies

z (direction of travel)r

θ

TUBULAR

y

x

z (direction of travel)

PLANAR

16 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

17Tubular topologies - armature options

(a) Slotless motor with magnetic sleeve (c) Conventional slotted motor

(d) Longitudinal flux motor topology(b) Slotless motor without magnetic sleeve

18 Tubular topologies - magnetisation options

(b) Radially magnetised primary(a) Axially magnetised primary

(c) Ideal Halbach array (d) Discretised Halbach array

19Air-cored or Iron-cored

Air-cored Iron-cored No cogging force Cogging force Small or zero saliency force Saliency force Lower force per amp and per volume Higher force per amp Lower mass per volume Higher mass per volume Higher acceleration (up to 100g) Lower acceleration (up to 22g) Lower thermal resistance Higher thermal resistance

20 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

21 Pros and cons of tubular PM linear actuators

• Finite length (not for planar)

• Vertical operation problematic (failure)

• Cost

• Cogging force (high accuracy displacement)

• Environmental sealing

22 Moving secondary

Stator

Position sensors

Traversing guide stripsEnd

stop

Pipe adapter

Magnet array

WindingKevlar fibre composite

Coolantrz

10Hz Yarn traverse:Max acceleration >50gMax speed 2ms-1

Traversal 0.2m

23 External armature

• Longitudinal flux motor• 2-phase BLAC machine• Max speed 5 ms-1

• 1.0kN pk force• 10g self acceleration• Traversal 600mm

iron sleevecoil

magnet

24 Electrodynamic shakers

50mm displacement90kN peak force (sine)3m/s max velocity

• Large voice coil actuators• High bandwidth• Limited displacement• Big and expensive

25 Electromagnetic control surface actuator

• 500N force

• ~1.2kg, >10J/kg

• 21Hz operation

• +/-3mm displacement

26 Force capability

Magnetic flux density B (Tesla)

Ampere stream Q (A/m)

Magnetic flux density B (Tesla)

Ampere stream Q (A/m)

Magnetic stress σ = Ku B Q

L

D

σ

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 60Longitudinal flux PM

80-100Transverse flux PM

40Radial field/linear PM

15Induction

σ (kPa)

60Longitudinal flux PM

80-100Transverse flux PM

40Radial field/linear PM

15Induction

σ (kPa)

27 Composite realisation of transverse flux

28 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

29 Typical table actuator requirements

• 6 axes with 8 actuators

• 50Hz maximum bandwidth

• 40kN force

• +/-150mm displacement

• 1ms-1 peak speed

• 6g acceleration

• Around 20kW rms power per actuator

10

40

0

Force (kN)

Speed (m/s)0.5 1.0

30 Moving magnet tubular actuator example

• 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

31Possible 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

x100 installed filter capacitor

< inertia of a pump motor

Actuator(s)

Active rectifier(supplies losses only)

Commercial induction motor drive acting as a flywheel

415V ac

32 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

33Observations

• 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

34 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

35 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

36 Comparison (source: CLD Inc)

Tubular motors Mechanical Hydraulics Pneumatic Speed 100 in./sec 10 in./sec 10 in./sec 20 in./sec Accuracy 0.001 in. 0.001 in. 0.01 in. 0.1 in. Stiffness High Medium Medium Low Friction Medium Medium High High Temperature 125°C 125°C 50°C 50°C Shock loading High Medium High High Efficiency 50% 40% 25% 25% Noise 40dB 80dB 120dB 120dB Environmental None Minimal Oil

leaks/disposal Oily air mist

Controllability Fully (no backlash)

Fixed move profiles (cams)Backlash

Limited move profiles

Mostly bang/bang