Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
1999-05-19
2001-04-17
Strecker, Gerard R. (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C324S228000, C324S258000, C033S361000, C340S870330, C361S156000
Reexamination Certificate
active
06218831
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not Applicable.
REFERENCE TO MICROFICHE APPENDIX
Not Applicable
FIELD OF THE INVENTION
The present invention relates to oscillating driver circuits, and more particularly to oscillating driver circuits for driving inductive components, such as magnetic fluxgate sensors within position encoders.
BACKGROUND OF THE INVENTION
In general, position encoders are devices which determine the instantaneous physical position of a movable object with respect to a fixed reference point, and translate such position information into a form that can be utilized by a processing, analytical or other type, tool. A position encoder typically transforms position information into an electrical signal, and provides the electrical signal to an analog or digital signal processor. Position encoders may determine angular position, as in the case of a rotatable shaft or toroidal structure (e.g., an automobile tire), or they may determine linear position, as in the case of a slidable control actuator. An ideal position encoder produces an output signal that is a linear function of the position of the movable object. An improved position encoder is described and claimed in my copending application, U.S. patent application Ser. No. 09/315,205, filed contemporaneously herewith, and assigned to the present assignee (Attorney Docket No. ADL-091). Instantaneous position information, sampled over time, may be used to determine higher derivatives of position such as velocity and acceleration.
Typical position encoders operate either mechanically, optically or magnetically. A mechanical encoder relies upon physical contact with the movable object; actuators on the movable object physically interact with an electro-mechanical transducer to produce an electrical signal. An optical encoder receives light reflected from illuminated markings associated with the movable object and translates variations in the received light into an electrical signal. Magnetic encoders typically utilize either fluxgate sensors or Hall effect sensors. A fluxgate sensor magnetic encoder uses fluxgate sensors to detect the magnetic field generated by magnetic elements attached to the movable object, and translates aspects of the magnetic field such as magnitude and direction into an electrical signal corresponding to the position of the object. A Hall effect sensor magnetic encoder translates the Hall effect of a magnetic field on a current carrying conductor to produce a signal corresponding to the position of the object. Fluxgate position encoders are several orders of magnitude more sensitive than Hall effect position encoders and are thus preferred in applications where it may be difficult to have the sensors in close proximity of the magnetic element producing the magnetic field. For example, in an application to determine the angular position of an automobile tire, the close proximity a Hall effect sensor requires is difficult to maintain because of the harsh environment created by road dirt, oil, grease, ice and snow.
A fluxgate sensor includes one or more turns of an electrical conductor wound about a core, which is disposed along a sensing axis. The core may be any material, including air, although high permeability materials such as iron or nickel are usually preferred. An external driving circuit alternately drives the sensor into saturation in one polarity and then into the opposite polarity. The external driving circuit drives current through the windings in one direction until the core saturates. Once the core saturates, the driving circuit reverses current in the windings until the core saturates in the opposite polarity. In the absence of an external magnetic field, the amount of time the driving circuit drives current in each direction is the same; i.e., the current waveform through the windings as a function of time is symmetrical. The presence of an external magnetic field “helps” (i.e., enhances) the saturation of the core in one polarity, while the external magnetic field impedes the saturation of the core in the opposite polarity. Thus, in the presence of an external magnetic field, the waveform of the current through the windings as a function of time is asymmetrical, since saturation occurs more quickly for the polarity of the saturation enhanced by the external field. The amount of asymmetry may be used to determine characteristics of the external magnetic field, such as magnitude and direction.
The amount of current necessary to drive an inductor coil into saturation varies with the number of windings, the core material, etc. However, for a typical flux gate sensor, the amount of current necessary to drive the sensor into saturation will be on the order of tens of mA. Since this current is entirely supplied by the driver circuit, the input power requirements of such a driver circuit are defined by the saturation current of the fluxgate sensor. For example, U.S. Pat. No. 4,859,944, “Single Winding Magnetometer With Oscillator Duty Cycle Measurement,” invented by Spencer L. Webb, discloses a driver circuit which essentially alternately connects a positive voltage source and a negative voltage source across an inductor coil. A few tens of mA is not generally considered a large amount of current. However, in low power applications, such as portable electronic systems which operate from a battery power source, current requirement goals are typically in the micro-ampere range.
It is an object of this invention to provide a position encoder that substantially overcomes or reduces the aforementioned disadvantages while providing other advantages which will be evident hereinafter.
SUMMARY OF THE INVENTION
The present invention is a system for cyclically driving an electrical current through an inductor in alternating directions so as to produce a substantially periodic current waveform, and for producing an output signal representative of the current waveform. In one aspect of the invention, the output signal corresponds to the strength of an external magnetic field in the presence of the inductive component. The system includes a first capacitor and a second capacitor electrically coupled in series. The first capacitor is coupled between a voltage source and a junction node, and the second capacitor is coupled between the junction node and a system ground. A first terminal of the inductor is electrically coupled to the junction node, and the junction node has an associated junction voltage. The system further includes a switching network for alternately interconnecting the first capacitor, the second capacitor and the inductor in a first state and a second state. The first state is characterized by the inductor and the first capacitor being electrically coupled in parallel, and the second state is characterized by the inductor and the second capacitor being electrically coupled in parallel. The system also includes a controller for configuring the switching network to the first state for a first time interval until the inductor saturates, thereupon configuring the switching network to the second state for a second time interval until the inductor saturates, such that the switching network cyclically alternates between the first state and the second state. The system further includes a signal processor with an input terminal that is electrically coupled to the junction so as to receive the junction voltage. The signal processor also has an output terminal that produces the output signal. The output signal is a function of the first time interval and the second time interval.
In another embodiment, the inductor includes at least one flux gate sensor.
In another embodiment, the inductor includes at least two fluxgate sensors electrically coupled in series opposition.
In yet another embodiment, the switching network includes a digital driver having a totem-pole output architecture electrically coupled between the voltage source and the system ground.
In another embodiment, the system further includes a current source having an output terminal electrically coupled to the junctio
Arthur D. Little Inc.
McDermott & Will & Emery
Strecker Gerard R.
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