Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2001-10-15
2004-12-07
Jastrzab, Jeffrey R. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C600S486000, C128S901000
Reexamination Certificate
active
06829507
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of medical implants. More specifically, the present invention relates to an apparatus for providing a signal representing the status of a sensor in a medical implant.
2. Description of the Prior Art
Ever since the introduction of rate responsive implanted cardiac stimulators, a number of different parameters have been used for determining the activity level of the patient, which in turn is used for controlling the rate at which the heart of a patient is to be stimulated by the pacemaker. One of the most common sensors used is the piezoelectric accelerometer.
Another form of sensor is the intracardiac piezoelectric pressure sensor.
Unlike the piezoresistive and piezocapacitive sensors piezoelectric sensors are not energy consuming, on the contrary they generate their energy themselves. Piezoelectric sensors are also arranged to alter the mechanical stress of the piezoelectric material in response to a change of loads emanating from for instance an acceleration of a seismic mass or from a change in pressure acting on the sensor. This results in a transport of electrons or electrical charges within the material, which provides a change in voltage across the piezoelectric sensor. This voltage corresponds to the load to which the sensor is subjected.
A problem related to measuring the voltage across a piezoelectric sensor is the leakage of charges that occurs, negatively affecting the accuracy of the measurements. In an attempt to solve this problem, use has been made of a voltage amplifier having a very high input impedance. This requires, however, a very large resistance component, which is undesired within a medical implant. Furthermore, the problem related to leaking charges is still not completely eliminated, and the use of a memory function of some sort would be required. The problem of leaking charges is of particular interest when the piezoelectric sensor is subjected to relatively small changes in load over long time periods, such as small changes of pressure over a long time or changes in posture.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method and apparatus for determining the status of a piezoelectric sensor that takes the leakage of charges mentioned above into account.
A further object of the present invention is to improve the possibilities of evaluating the status of a piezoelectric sensor.
These objects are achieved in accordance with the present invention wherein status related sensor output changes are substantially continuously detected, and based thereupon a signal representing the actual status of a sensor is generated. Advantageously, said signal is generated by integration of the sensor output changes.
Preferably, use is made of a sensor of the type in which status changes generate changes regarding electrical charges in the sensor. Thus, the sensor is suitably of the piezoelectric type.
In accordance with a preferred aspect of the invention, positive and negative charges generated by the sensor, as more closely discussed below, are substantially continuously detected and removed from the sensor, thereby keeping the output voltage of the sensor at a substantially constant zero level, while at the same time providing an output current which can be the basis for an integration in order to produce the signal.
According to an embodiment of the invention, this can be accomplished by connecting the charge-producing sensor to a circuit having the characteristics of an input impedance that is extremely low or redundant. As a result, charges generated by the piezoelectric sensor will immediately leak to, or be collected or removed by, the connected circuit. This also means that there will be no problem with uncontrolled leakage of charges from the sensor, as is the case in the prior art.
As indicated above, a change of load generates a change of charges in a sensor of the piezoelectric type, all charges generated being collected, i.e. detected and removed from the piezoelectric sensor, by the connected circuit. A change of load can be either positive or negative. A positive change of load will generate an internal transport of charges in a direction opposite that caused by a negative change of load. Furthermore, if a transport of charges in one direction generates a positive voltage across the piezoelectric sensor, a transport of charges in the opposite direction generates a negative voltage. Hence, for restoring a zero voltage level across the sensor from a positive voltage level, there must be a transport of “actual” charges from the sensor, while for restoring a zero level from a negative voltage, there must be a transport of “actual” charges from the connected circuit to the sensor.
In accordance with the above, the actual charges supplied to the sensor for restoring a zero level will hereinafter be referred to as collected or removed negative charges, and the resulting current will be referred to as a negative current. Correspondingly, the actual charges removed from the sensor will be referred to as positive charges, and the resulting current as a positive current. Therefore, both the supply and the removal of charges to and from the sensor hereinafter will be referred to as a collection of charges, wherein a supply of charges to the sensor will be referred to as a collection of negative charges, and a removal of charges from the sensor will be referred to as a collection of positive charges.
The charges generated in a sensor of the type discussed correspond to the load (e.g. acceleration and/or gravitational force or pressure) to which the sensor is subjected. Accordingly, each generated charge represents a certain change of load. A greater change of load generates more charges; a more rapid change of load provides a more rapid generation of charges; and an change of load in one direction generates positive charges and an change of load in the opposite direction generates negative charges (in accordance with the above stated definition of positive and negative charges). Hence, the electric charges generated by the sensor per time unit, i.e. electric current, correspond to the amount and the direction of the change of load and, hence, to the time derivative of the load to which the sensor is subjected.
The charges generated by a piezoelectric sensor, as described above, are provided to a circuit for detecting and removing these charges. Since the number of generated charges per time unit, hereinafter referred to as the sensor current or sensor output current, is proportional to the time derivative of the change of load, an integration of said current will result in an integrated value or signal that is proportional to the load.
The circuit for receiving the current (detecting and removing the charges) is, according to the invention, arranged to integrate said current, i.e. to quantify and to cumulate the charges generated by the sensor. Accordingly, the resulting value from this integration will represent the net amount, i.e. considering the sign of the generated charges, of charges generated by the sensor. Thus, the integrated value, or signal, will be directly representative of the load to which the sensor currently is subjected. The integrated value can therefore be seen as a recreation of the voltage that would have existed in the sensor, provided that there would have been no leakage or deliberate removal of charges at all. Thus, the present invention solves the problem regarding obtaining an absolute value representative of the level of for instance a constant acceleration or gravitational force or pressure by the use of a piezoelectric type sensor.
As stated above, the restoring in the sensor of a zero level from a negative voltage level would require a supply of charges from the connected circuit to the sensor. According to an embodiment of the invention, the supply of charges can be provided by connecting a constant direct current, hereinafter referred to as a DC signal, to the sensor and the circuit. If the mag
Kjellman Charlotte
Landelius Lars
Lidman Johan
Nilsson Kenth
Jastrzab Jeffrey R.
Oropeza Frances P.
Schiff & Hardin LLP
St. Jude Medical AB
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