Taming the Electromagnetic Solenoid: Building a System That Achieves a Soft Landing Gary Bergstrom Magnesense
Gary Bergstrom, Magnesense Simplified valve
Gary Bergstrom, Magnesense Flux in an E-core
Gary Bergstrom, Magnesense Electrical Rtotal=Rdrive+Rsolenoid L is inductance of solenoid Rsolenoid is a function of temperature Inductance is a strong function of position + - Drive Solenoid L Rsolenoid Rdrive
Gary Bergstrom, Magnesense Inductance vs. Position
Gary Bergstrom, Magnesense Mass, spring damper – mechanical model x m mass, Kg c damping coeff k spring coeff, N M F force, N x displacement, M x velocity, M/S x acceleration, M/S^2 m is all moving mass, including part of springs k is the net restoring force from all springs F is the net electromagnetic force from both stators c is damping from mechanical friction and gas flow x is displacement, symbolized by a pointer moving along scale m x + c x + k x = F k c m F
Gary Bergstrom, Magnesense Force vs. Position, various flux densities gap in meters force in N
Gary Bergstrom, Magnesense Force vs. Flux density, various gaps Flux density in T Force in N gap
Gary Bergstrom, Magnesense 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
Gary Bergstrom, Magnesense Excel spreadsheet of simulation
Gary Bergstrom, Magnesense 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
Gary Bergstrom, Magnesense Position, voltage and current
Gary Bergstrom, Magnesense Flux density and force
Gary Bergstrom, Magnesense Voltage drive details 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
Gary Bergstrom, Magnesense Position, voltage and current
Gary Bergstrom, Magnesense Flux density and force
Gary Bergstrom, Magnesense Position, voltage and current
Gary Bergstrom, Magnesense Position, voltage and current
Gary Bergstrom, Magnesense Position, voltage and current 30V supply
Gary Bergstrom, Magnesense 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
Gary Bergstrom, Magnesense 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
Gary Bergstrom, Magnesense Position, voltage and current
Gary Bergstrom, Magnesense Flux density and force
Gary Bergstrom, Magnesense Position, voltage and current
Gary Bergstrom, Magnesense Position, voltage and current
Gary Bergstrom, Magnesense Position, voltage and current 30V supply
Gary Bergstrom, Magnesense 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
Gary Bergstrom, Magnesense Flux drive Flux sensor needed This design uses full bridge drive More parts, more performance
Gary Bergstrom, Magnesense Position, voltage and current
Gary Bergstrom, Magnesense Flux density and force
Gary Bergstrom, Magnesense Position, voltage and current
Gary Bergstrom, Magnesense Position, voltage and current
Gary Bergstrom, Magnesense Position, voltage and current 30V supply
Gary Bergstrom, Magnesense 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
Gary Bergstrom, Magnesense So how do we sense flux? Hall effect sensor Sense coil “Sensorless”
Gary Bergstrom, Magnesense Hall effect sensor Good points: Simple DC response Low cost Small Bad points: Temperature (reliability) Some cost Extra wires Measurement position
Gary Bergstrom, Magnesense Sense coil Good points: Simple circuit Rugged Low cost No temperature problems Bad points: More parts Higher cost Takes up core area Extra wires
Gary Bergstrom, Magnesense “Sensorless” 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 Flux existing drive Rtotal MULT Rsense INT