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Taming the Electromagnetic Soleno i d: Building a System That Achieves a Soft Landing

Taming the Electromagnetic Soleno i d: Building a System That Achieves a Soft Landing. Gary Bergstrom Magnesense. Simplified valve. Flux in an E-core. Electrical. Rtotal=Rdrive+Rsolenoid L is inductance of solenoid Rsolenoid is a function of temperature

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Taming the Electromagnetic Soleno i d: Building a System That Achieves a Soft Landing

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  1. Taming the Electromagnetic Solenoid: Building a System That Achieves a Soft Landing Gary Bergstrom Magnesense

  2. Simplified valve

  3. Flux in an E-core

  4. Electrical • Rtotal=Rdrive+Rsolenoid • L is inductance of solenoid • Rsolenoid is a function of temperature • Inductance is a strong function of position Rdrive Rsolenoid + Drive - L Solenoid

  5. Inductance vs. Position

  6. Mass, spring damper – mechanical model F m mass, Kg m c damping coeff k spring coeff, N M F force, N x displacement, M x velocity, M/S x x acceleration, M/S^2 m x + c x + k x = F k c m is all moving mass, including part of springs c is damping from mechanical friction and gas flow k is the net restoring force from all springs F is the net electromagnetic force from both stators x is displacement, symbolized by a pointer moving along scale

  7. Force vs. Position, various flux densities 1200 1000 0.1 0.3 0.5 800 0.7 0.9 1.1 1.3 force in N 600 1.5 1.9 1.7 1.7 1.9 400 1.5 200 0 0.00000 0.00100 0.00200 0.00300 0.00400 0.00500 0.00600 0.00700 0.00800 gap in meters

  8. Force vs. Flux density, various gaps 1200 1000 0.00000 800 gap 0.00000 0.00117 Force in N 600 0.00218 0.00400 400 0.00117 200 0.00218 0.00400 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Flux density in T

  9. Flux summary Flux resists changes V=L*dI/dt only when: x doesn’t change no eddy current no saturation Flux is the integral of inductive voltage Force goes as the square of flux and is a non-linear function of position

  10. Excel spreadsheet of simulation

  11. Voltage drive • I=V/Rtotal • if V=40V and Rtotal=.25 then I=160 Amps • This can occur at saturation • Power lost is I^2 * Rtotal so we want to minimize R

  12. Position, voltage and current

  13. Flux density and force

  14. Time is in seconds Position 4.5 mm to 0 mm (plot starts near “middle”) Voltage 0 to 40 volts Flux density in Teslas Force is in Newtons Flux must = ~1.65 T to hold in this example “bounce” was set to 70% of the incoming velocity (or ½ the energy) Flux goes as integral of applied inductive voltage Force is function of position and square of flux Voltage drive details

  15. Position, voltage and current

  16. Flux density and force

  17. Position, voltage and current

  18. Position, voltage and current

  19. Position, voltage and current 30V supply

  20. Voltage drive summary • Sensitive to changes in power supply • Very prone to saturating core, but need to run close to saturation due to size considerations • No good correlation between applied voltage and resulting force • Cannot always achieve soft landing and holding flux level at same time with simple drive • Landing time very sensitive to changes in initial energy

  21. Current drive • Rs (current sense) should be small (more I^2 * R loss) • R1/R2 gain circuit is to reduce noise • Diode must include both Solenoid and Rs in loop

  22. Position, voltage and current

  23. Flux density and force

  24. Position, voltage and current

  25. Position, voltage and current

  26. Position, voltage and current 30V supply

  27. Current drive summary • Not very sensitive to power supply changes • Saturation is not as big a problem (current is limited, saturation still occurs) • Unstable – the current changes in the opposite direction from what is needed for a soft landing • Back EMF forces the current around in counter-intuitive ways

  28. Flux drive • Flux sensor needed • This design uses full bridge drive • More parts, more performance

  29. Position, voltage and current

  30. Flux density and force

  31. Position, voltage and current

  32. Position, voltage and current

  33. Position, voltage and current 30V supply

  34. Flux drive summary • Less sensitive than voltage drive to changes in power supply • Stable like voltage drive but without the saturation problem • Flux, therefore force is known (if position is known) • Allows position to be calculated since: x ~ current / flux • Position PID loop can now be closed giving us closed loop position drive, with a well behaved open loop system

  35. So how do we sense flux? • Hall effect sensor • Sense coil • “Sensorless”

  36. Good points: Simple DC response Low cost Small Bad points: Temperature (reliability) Some cost Extra wires Measurement position Hall effect sensor

  37. Good points: Simple circuit Rugged Low cost No temperature problems Bad points: More parts Higher cost Takes up core area Extra wires Sense coil

  38. Good points: No wires Reliable No size (at valve) Can be done in software Bad points: Small temperature sensitivity Even more parts Difficult to develop Difficult to understand INT “Sensorless” Flux existing MULT drive Rsense Rtotal

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