Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
2002-02-12
2003-07-08
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
Reexamination Certificate
active
06590390
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of sensors and, move particularly to the fields of compact sensors and methods of mounting compact sensors.
BACKGROUND OF THE INVENTION
Over the years sensors we have been developed which include transducers that possess a specific preferred orientation in relation to an electrical field, or a magnetic field or mechanical force to be sensed. To maximize the response of the sensor, the transducer must be oriented in the direction of this field or force. Some examples of electrical or magnetic field sensors are position and proximity sensors such as Hall effect, magneto resistor, capacitive, and inductive sensors and electrical current field sensors. Mechanical force sensors generally measure the flow or pressure of a liquid or gas, the mechanical stress or weight of an object, or the acceleration of an object. These sensors generally have an orientation of the transducer to the electrical or magnetic field or to the physical force being sensed in order to maximize the sensitivity of the transducer.
Also, there may be other extraneous electrical or magnetic fields or mechanical forces in the system. The transducer may have to be oriented relative to these extraneous fields or forces in a specific direction to reduce the sensitivity of the transducer to them. This helps to eliminate sensing errors or noise caused by the movement of other objects or caused by the presence of other fields or forces.
These sensors also conventionally employ signal conditioning circuitry or a signal conditioner to amplify or otherwise condition the transducer signal. The signal conditioner is needed, for example, because the transducer signal is usually too low in magnitude to overcome noise or contains a large offset or other error signal that overdrives sensitive monitoring equipment. Otherwise, the transducer signal is not conducive to transmission over a distance to a remotely located sensor monitoring circuit.
Additionally, the sensors are often used in mechanical systems that have restrictions on overall size, weight, structural integrity, reliability, and cost. For these reasons, the sensor is usually made as small as possible by using transducers and signal conditioners that are electronic or electrical devices manufactured on semiconductor wafers.
A first significant problem with transducers and signal conditioners which are manufactured as semiconductor devices, however, is that the electrical conductivity or other operating characteristics change significantly in response to changes in temperature. This can result in a significant change in transducer output as a function of temperature. Because most of the mechanical systems in which these sensors operate can experience rather significant changes in operating temperature, the effects of these temperature changes on the sensor output constitutes an error signal and should therefore be eliminated or reduced if possible.
The elimination of this error signal is usually a function of the signal conditioner and is usually accomplished in several related ways. The transducer and the signal conditioner that are to be used together in any single sensor are usually manufactured at the same time using the same manufacturing process and are located as closely as possible in relation to each other on the same semiconductor wafer. This is done primarily to make the physical proportions of the components that comprise the transducer and the signal conditioner equal or proportional in width, length, and depth of features and to have equal relative concentrations of the various semiconductor materials used to form the components. For instance, all transistor bases, emitters, and collectors will be essentially the same relative size even if they are manufactured slightly larger or smaller than intended, and will have generally the same concentrations of materials regardless of whether they are at the intended levels of these various concentrations. The electrical conductivity of any particular electrical component in the transducer or the signal conditioner is proportional to the size of its features as well as the concentration of materials from which the component is manufactured. Any two components on the wafer located in close proximity to each other with the same dimensions and formed from the same relative concentrations of materials generally will have equal electrical conductivities if they are at the same temperature. Also, any component located near another component which has the same concentrations of materials but whose dimensions are not equal but are proportional to the other component will have an electrical conductivity that is proportional in the same degree as the dimensions if they are both at the same temperature. Because the size and composition of these components are set during manufacture, any short term changes in their electrical conductivity under identical electrical conditions are generally caused only by changes in the temperature of the component.
In this manner any specific component or collection of components on the transducer required for proper operation can be duplicated in the signal conditioner at the same size or at a specific proportional scale and with equal concentrations of materials. For example, some transducers employ four resistors in a Wheatstone bridge configuration. Any one or more of these resistors can be made with equal dimensions and with equal composition of materials on the signal conditioner. Under these conditions, the electrical conductivity of both pairs of resistors generally will be equal if their temperatures are equal. In any case, if the temperature of the transducer components is the same as the temperature of the signal conditioner components, both the transducer and signal conditioner will contain components that generally experience equal or proportional electrical conductivity due to the effects of temperature alone.
One of two methods are generally used in association with a signal conditioner to determine this change in electrical conductivity and then to produce a corresponding signal that cancels the effects of this change on the transducer output. First, if size allows, a complete duplicate of the transducer can be made on the signal conditioner. This duplicate transducer is then electrically, magnetically, or physically shielded from the field or force being sensed or is in some manner made unresponsive to the sensed parameter. An equal excitation or drive signal is then applied to both the components comprising the active transducer and the components comprising the duplicate passive transducer on the signal conditioner. The output of the signal conditioner passive transducer is then relative only to temperature and is then subtracted from the output of the active transducer that responds to the field or force. This is usually accomplished in a differential amplifier or a similar electronic circuit.
A second method, for example, can be used where space for the signal conditioner is more limited. A representative part of the transducer at any proportion can be duplicated on the signal conditioner. This representative part can be chosen to be a part that is unresponsive to the parameter being sensed or can be physically oriented to a position where it is not affected or otherwise shielded from the parameter being sensed. In any case, it is designed so its electrical conductivity is proportional only to changes in temperature. The change of the transducer output due to the cumulative changes of electrical conductivity of all transducer components due only to changes in temperature is determined by direct measure or by mathematical calculation during the sensor design phase. This yields a specific level of transducer output change per degree of change in temperature. This information advantageously can be used to design a circuit with a specific amount of gain determined by the relationship of the change in electrical conductivity of the signal conditioner duplicate component to the change in transducer output caused
Smith, Jr. Marshall E.
Stettler Richard W.
Wolff Peter U.
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Patidar Jay
Wolff Controls Corporation
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