Electricity: measuring and testing – Magnetic – With compensation for test variable
Reexamination Certificate
1994-07-22
2001-03-27
O'Shea, Sandra L. (Department: 2215)
Electricity: measuring and testing
Magnetic
With compensation for test variable
C324S202000, C324S207170
Reexamination Certificate
active
06208135
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to electromagnetic shielding for shielding electromagnetic pickups, other types of electronic equipment, and specific regions of space from electromagnetic radiation, and more particularly to active electromagnetic shielding for providing an electrical cancellation signal for canceling electromagnetic radiation or canceling the response of an electronic device to electromagnetic radiation.
It has long been known that voltages are induced in all conductors exposed to changing magnetic fields regardless of the configuration of such conductors. Electromagnetic radiation will induce electrical signals in electronic devices according to the laws of magnetic induction. Thus it has been desirable in some applications of electronic instrumentation to reduce the inductive noise caused by electromagnetic radiation.
A common method for providing electromagnetic shielding involves the use of passive electromagnetic shielding. A shield consisting of layers of high and low permeability material may be used to attenuate electromagnetic radiation passing through it. However, this passive electromagnetic shielding adds substantial bulk and weight to the system which it shields.
Another method for providing electromagnetic shielding is to utilize cancellation coils for generating a cancellation electromagnetic radiation in opposition to incident radiation produced by external sources in order to cancel the effects of the incident radiation. In U.S. Pat. No. 5,066,891, Harrold presents a magnetic field sensing and canceling circuit for use with a cathode ray tube (CRT). Magnetic flux gate sensors provide output signals that are functions of detected fields. These signals are then used to control the current in the cancellation coils which produce a cancellation magnetic field. Harold explains that it is of great importance that the CRT in a color monitor be protected from the effects of external magnetic fields, and, in particular, time-varying magnetic fields. However, this method provides no compensation to the frequency-dependent amplitude and phase responses of the sensor that picks up incident electromagnetic radiation and the device which generates the cancellation radiation.
Likewise, in U.S. Pat. No. 5,132,618, Sugimoto shows a magnetic resonance imaging system that includes active shield gradient coils for magnetically canceling leakage fields that would otherwise produce eddy currents in the heat shield tube.
A common method for providing shielding to an electromagnetic pickup is to utilize identical pickup coils connected in series or in parallel so as to cancel the effects of uniform electromagnetic radiation. Pizzarello shows such a system in U.S. Pat. No. 5,045,784 for reducing inductive noise in a tachometer coil. An electric tachometer is a coil of wire that may be attached to a moving part of a motor that passes through a stationary magnetic field. The motion of the wire through the magnetic field induces a voltage that is indicative of the motor's speed. However, if the motor is powered by electricity, changes in the current powering the motor will cause a magnetic flux, which will also produce a voltage in the coil. Pizzarello shows a stationary pickup coil that is responsive to magnetic flux, and a means for subtracting the pickup voltage from the tachometer voltage.
Likewise, in U.S. Pat. No. 4,901,015, Pospischil shows a cancellation circuit for canceling the response of a magnetic pickup generators to ambient electromagnetic fields. Pickups used in integrated drive generators are responsive to ambient electromagnetic fields produced by the generator. A first magnetic pickup responsive to the ambient magnetic field and a modulated flux field produced by a rotating shaft is combined with a second magnetic pickup responsive to the ambient electromagnetic fields in order to cancel the inductive effects of the ambient electromagnetic field. Such pickup assemblies are also used with electric guitars and are known as “hum-bucking” pickups. This technique is not effective for providing a high degree of cancellation because slight differences between the pickups, even pickups that are substantially identical, cause the frequency-dependent amplitude and phase response of the pickups to differ significantly from each other. Thus the pickup signals will not be exactly out of phase and equal in amplitude when they are combined.
A prior-art method for providing shielding to an electromagnetic pickup from an electromagnetic source that produces a non-uniform field is to “unbalance” either the pickup device or the electromagnetic source. Such a method is described by Hoover in U.S. Pat. No. 4,941,388. Hoover uses amplitude-adjustment techniques to compensate for amplitude variations between the responses of separate pickups to electromagnetic radiation generated by an electromagnetic sustaining device for driving the vibrations of a string on an electric guitar. However, Hoover does not compensate for differences in the pickup coils which cause the amplitude-variation of the responses of the pickups to be frequency-dependent, thus resulting in poor cancellation over a broad range of frequency. Furthermore, Hoover does not compensate for phase-variations that occur between different pickup coils. The resulting cancellation from the unbalancing method is poor.
Hoover describes the operation of negative feedback in a system where a magnetic pickup provides an electrical signal to a magnetic driver which generates a magnetic field to which the pickup responds. Hoover mentions that the system tends to drift from the negative feedback condition at higher frequencies, and identifies the cause of this drift as distortions in the phase-response of the system resulting from the pickup, driver and amplifier in the system. Hoover neither presents an effective method for controlling the phase-response of the system, nor shows the mathematical relationships between phase and frequency resulting from the driver and pickup coils. Rather, Hoover proposes the use of a low-pass filter to reduce the gain of the system at which the negative feedback condition breaks down.
Methods of active phase-compensation are described by Rose in U.S. Pat. Nos. 4,907,483, 5,123,324, and 5,233,123. Rose uses active circuits for determining the frequency or frequency range of an electrical signal from an electromagnetic pickup. Active phase-adjustment is applied to the pickup signal, which is used to power an electromagnetic driver that generates an electromagnetic driving force on a vibratory ferromagnetic element of a musical instrument. The purpose of the phase-adjustment of the pickup signal is to provide a driving force to the vibratory element that is substantially in-phase with its natural motion. Because the purpose of Rose's invention is to improve the efficiency of the electromagnetic drive force on the element, it is apparent that a passive phase-compensation circuit would be preferable to Rose's active phase-compensation circuit. However, Rose does not realize the mathematical relationships between phase and frequency which provide the basis for constructing a passive phase-compensation network. Furthermore, Rose's invention does not provide simultaneous phase-compensation to more than one harmonic.
Another method for providing electromagnetic shielding is to orient the angle of a pickup coil to incident electromagnetic radiation such that the electrical current induced in the coil by the electromagnetic radiation will substantially cancel. One application of this method is shown by Burke in the
Handbook of Magnetic Phenomena
, published in 1986. Burke uses a transmitting coil that produces electromagnetic radiation and a receive coil which senses radiation. The two coils can be configured in such a way that no energy is transferred between the transmitting and receiving coils. Burke shows the receiving coil oriented with the axis of its turns at right angles to the direction of the magnetic field produced by the transmitting coil. Bu
O'Shea Sandra L.
Patidar J. M.
LandOfFree
Inductive noise cancellation circuit for electromagnetic... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Inductive noise cancellation circuit for electromagnetic..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Inductive noise cancellation circuit for electromagnetic... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2538145