System and method for servo control of nonlinear electromagnetic actuators

Abstract

Servo control using ferromagnetic core material and electrical windings is based on monitoring of winding currents and voltages and inference of magnetic flux, a force indication; and magnetic gap, a position indication. Third order nonlinear servo control is split into nested control loops: a fast nonlinear first-order inner loop causing flux to track a target by varying a voltage output; and a slower almost linear second-order outer loop causing magnetic gap to track a target by controlling the flux target of the inner loop. The inner loop uses efficient switching regulation, preferably based on controlled feedback instabilities, to control voltage output. The outer loop achieves damping and accurate convergence using proportional, time-integral, and time-derivative gain terms. The time-integral feedback may be based on measured and target solenoid drive currents, adjusting the magnetic gap for force balance at the target current. Incorporation of permanent magnet material permits the target current to be zero, achieving levitation with low power, including for a monorail deriving propulsion from the levitation magnets. Linear magnetic approximations lead to the simplest controller, but nonlinear analog computation in the log domain yields a better controller with relatively few parts. When servo-controlled solenoids provide actuation of a pump piston and valves, electronic LC resonance measurements determine liquid volume and gas bubble volume.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] The present application is a continuation of commonly assigned copending U.S. patent application Ser. No. 09 / 771,892, which was filed on Jan. 30, 2001, by Joseph B. Seale et al., for a SYSTEM AND METHOD FOR SERVO CONTROL OF NONLINEAR ELECTROMAGNETIC ACTUATORS, which is a divisional application of U.S. patent application Ser. No. 08 / 882,945, which was filed on Jun. 26, 1997, by Joseph B. Seale et al., for a SYSTEM AND METHOD FOR SERVO CONTROL OF NONLINEAR ELECTROMAGNETIC ACTUATORS now U.S. Pat. No. 6,208,497, which was issued on Mar. 27, 2001, and each of which is hereby expressly incorporated by reference.BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates to systems and methods for controlling the movement of mechanical devices. More particularly, the present invention relates to the servo control of electromagnetic devices. Still more particularly, the present invention relates to the serv...

AI Technical Summary

Benefits of technology

The invention is about a new way to measure the change in current in a solenoid. Unlike previous methods, this method uses a very strong signal from the powering of the solenoid, without needing an extra oscillator. The switching noise that occurs in the regulator is also used as a position-indicating signal. This allows for the measurement of the current in the solenoid without needing to extract information from a current sense resistor. Overall, this method is more robust and accurate for measuring the current in a solenoid.

Problems solved by technology

Magnetic steel solenoid parts are typically solid rather than laminated, because eddy current losses in dynamic operation are not a design consideration.

The result is to cause the solenoid to stick in its closed position after external current is removed.

This is a failure mode for print wire solenoids.

While these relationships are needed building blocks in the conception of the instant invention, they are not an adequate basis for a servo system generating large mechanical motions and correspondingly large changes in solenoid inductance.

First, there are limitations to the linearized small-perturbation models taught by Jayawant et al for controlling large solenoid motions.

Second, dynamic stability problems would remain even with a more complicated and costly servo implementation using non-linear circuit models, e.g., computing position as the ratio of current / flux and force as the square of flux, instead of Jayawant's tangential linear approximations of the ratio and square law relations.

It is understood that servo control over a third order system is prone to instability since phase shifts around the control loop, tending toward 270 degrees at high frequencies, readily exceed 180 degrees over the bandwidth for which control is desired.

A combination of high efficiency and tight control spell problems for loop stability, for even with single-pole phase lead compensation, minor resonances, e.g., from mechanical flexure, can throw the servo system into oscillation.

The mixed actuation voltage was usually well in excess of the minimum requirement, and the result was actuation with excessive force and resulting severe contact bounce.

When a solenoid begins to close, the resulting “back EMF” due to armature motion tends to reduce electric current, in relation to gap, to maintain a constant magnetic flux, with the result that increases in force with gap closure are only moderate.

Even under conditions where sufficiently soft landing is achieved, it is at the cost of a substantial excess energy consumption to generate a long ramp of pulse duty cycle and current, only the middle portion of which causes actuation.

Though such a system responds to some of the limitations of Wieloch, it is not readily adaptable to an actuation system that must respond to changing conditions of starting position and the load force curve while achieving quiet, impact-free, efficient operation.

Magnetic characteristics in this region have presumably been considered too nonlinear for practical control.

In particular, the region of operation approaching full closure and contact of mating magnetic surfaces presents a very steeply changing inductance and correspondingly steep change in the sensitivity of force to change in coil current.

Yet problems with stability and non-linearity inherent to magnetically soft ferromagnetic-core solenoids have impeded the development of servo solenoids, and therefore have prevented the potential efficiency advantages just described.

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