We present the switched nozzle valve (SNV) approach for feed-forward gas flow rate control, the considerations included in the design of a prototype, and the results of its evaluation using a bell prover.&nbsp; The SNV provides inexpensive feed-forward gas flow rate control in contrast to closed-loop systems with thermal mass flow sensors.&nbsp; Attempts to meter gases in a feed-forward manner are usually impractical due to the difficulty of reliably locating a throttle body without some kind of feedback measurement.&nbsp; The SNV uses a statically machined converging-diverging nozzle to provide stable transonic flow insensitive to the application back-pressure.&nbsp; In this way, the flow is stable and repeatable when the valve is open and the flow is practically zero when the nozzle is obstructed.&nbsp; To simultaneously address the throttle body problem and provide an interface convenient for digital control, a pulse-width modulation scheme is applied through a high-speed magnetic actuator to vary the percentage of time the nozzle is unobstructed rather than continuously vary the size of the nozzle.The study of the actuator includes an investigation of the prototype’s repeatability, linearity, actuation parameter sensitivity, and mechanical tuning sensitivity.&nbsp; Data collected under a bell prover show the properly tuned valve calibration of flow versus duty cycle offers better than 1%FSE nonlinearity and repeatability on the order of 0.5%FSE.&nbsp; However, linearity worsens severely if the current control or mechanical tuning are compromised.&nbsp; The conclusion is that inexpensive digital gas flow rate control is practical using the SNV, but mechanical and electrical tuning parameters are essential to achieve the desired performance.We describe an embodiment of the concept that uses a ring topography solenoid actuator to increase operator force-to-mass ratio in comparison with conventional solenoid operators.&nbsp; This is used to achieve fast opening and closing.&nbsp;Since long-term stability of the opening and closing times is critical to the valve’s calibration, the paper describes the care taken to minimize the impact of fluid forces and wear on the valve’s performance.&nbsp; Seat geometry is optimized to increase stroke length in favor of minimizing delivery pressure on the valve opening characteristics.&nbsp; The SNV prototype further deviates from conventional valve design by using tool steel for the nozzle tip and a sapphire valve seat are used to mitigate wear over the billions of cycles of operation required for the PWM control scheme.&nbsp;Electrical control is achieved with a high-speed current control circuit.&nbsp; Switching times on the order of 2ms are achieved by over-driving the valve with a brief pulse before backing to a milder holding current.&nbsp; Current pulse shaping is performed using a PI current control, but the work also includes recommendations for simpler switching approaches.&nbsp; Like the problem of switching field-effect transistors in power electronics applications, the system’s performance suffers the longer it takes to transition through “partially on” states.

Digital Feed-Forward Gas Flow Rate Control With a Switched Nozzle Valve

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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