Electromagnetic actuation. technologies. Prof Phil Mellor

Transcription

1 Electromagnetic actuation technologies Prof Phil Mellor Department of Electrical and Electronic Engineering

2 2 Overview Review developments in electromagnetic actuation More electric aircraft Our research experience Back of envelope system discussion

3 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, (1965): p

4 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 5 Technology advances Permanent Magnets Digital Control Power Electronics Sensors Source: Group Arnold Source: SIKO GmbH Source: International Rectifier Source: Texas Instruments

6 6 Developments in the more-electric aircraft Rudder Aileron Aileron Elevator Flaps Slat Rudder Landing gear Flap Slat Elevator Brakes

7 7 Aircraft primary surface actuator Force (kn) Speed (mm/s)

8 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 9 EHA actuator schematic Demanded Surface Position Accumulator Surface Position Control Motor Control M Shaft Position Feedback Electronic control pump P Crossport Relief motor Surface Position Feedback Actuator By-Pass Valve Controlled Surface Existing hydraulic technology

10 10 EHA on an inertia simulator pivot EHA Resistive force Inertial mass - 350kN peak force - <2Hz response

11 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

12 12 Typical EMA/EHA actuator motors 25kW Brushless PM 40kW Brushless PM 20kW Brushless PM 5kW Switched Reluctance

13 13 Electromagnetic direct-drive actuators Advantages Simple construction Good positioning accuracy Good dynamic performance Reconfigurability Higher cost Disadvantages No mechanical advantage More complex specification Non-standardised

14 14 PM linear actuator topologies LINEAR MACHINES THRUST MACHINES LEVITATION MACHINES SHORT ARMATURE LONG ARMATURE ATTRACTION TYPE REPULSION TYPE LONG STROKE SHORT STROKE MOVING ARMATURE STATIONARY ARMATURE PLANAR MOTOR TUBULAR MOTOR SINGLE SIDED DOUBLE SIDED TRANSVERSE FLUX LONGITUDINAL FLUX BRUSHED DC SWITCHED RELUCTANCE LINEAR SYNCHRONOUS MOTOR LINEAR INDUCTION MOTOR STEPPER PM BRUSHLESS COMPOSITE SECONDARY SHEET SECONDARY LADDER STRUCTURE RELUCTANCE HYBRID AC DC

15 15 PM linear actuator topologies TUBULAR PLANAR y r z (direction of travel) x z (direction of travel) θ

16 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

17 17 Tubular topologies - armature options (a) Slotless motor with magnetic sleeve (c) Conventional slotted motor (b) Slotless motor without magnetic sleeve (d) Longitudinal flux motor topology

18 18 Tubular topologies - magnetisation options (a) Axially magnetised primary (b) Radially magnetised primary (c) Ideal Halbach array (d) Discretised Halbach array

19 19 Air-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 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 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 22 Moving secondary End stop Traversing guide strips Stator Winding Kevlar fibre composite Pipe adapter Position sensors Magnet array r z Coolant 10Hz Yarn traverse: Max acceleration >50g Max speed 2ms -1 Traversal 0.2m

23 23 External armature Longitudinal flux motor 2-phase BLAC machine Max speed 5 ms kN pk force 10g self acceleration Traversal 600mm magnet coil iron sleeve

24 24 Electrodynamic shakers Large voice coil actuators High bandwidth Limited displacement Big and expensive 50mm displacement 90kN peak force (sine) 3m/s max velocity

25 25 Electromagnetic control surface actuator 500N force ~1.2kg, >10J/kg 21Hz operation +/-3mm displacement

26 26 Force capability Magnetic flux density B (Tesla) D σ Ampere stream Q (A/m) L Achievable values: B = 1T for a PM armature Magnetic stress σ = K u B Q Induction Q = 50,000 A/m rms cont. for a liquid cooled actuator, peak values Radial field/linear PM 40 x5 cont. not uncommon Longitudinal flux PM 60 Transverse flux PM σ (kpa)

27 27 Composite realisation of transverse flux

28 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 29 Typical table actuator requirements Force (kn) 6 axes with 8 actuators 50Hz maximum bandwidth 40kN force +/-150mm displacement 1ms -1 peak speed 6g acceleration Around 20kW rms power per actuator Speed (m/s)

30 30 Moving magnet tubular actuator example 40kN peak, 28kN rms 0.56m 2 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=ω 2 x x=0.15m ω max =61.7rads -1 ; 9.8Hz v max =ωx=9.2ms -1

31 31 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 = 0.53kgm 2 Active rectifier (supplies losses only) x100 installed filter capacitor < inertia of a pump motor 415V ac Actuator(s) Commercial induction motor drive acting as a flywheel

32 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

33 33 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

34 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 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 36 Comparison (source: CLD Inc) Tubular motors Mechanical Hydraulics Pneumatic Speed 100 in./sec 10 in./sec 10 in./sec 20 in./sec Accuracy in in 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 Oily air mist Controllability Fully (no backlash) Fixed move profiles (cams) Backlash leaks/disposal Limited move profiles Mostly bang/bang