Packaged integrated accelerometer

Disclosed are method and apparatus for creating a cantilever accelerometer beam by mechanically forming an accelerometer from a piezoelectric substrate. The inherent structure of the accelerometer provides a means for employing surface mount technology (SMT), or a protective package may be utilized to couple the accelerometer to the hybrid circuit within an implantable pacemaker. The sensor's structure is defined by three members. The first and second members are used to couple the sensor to the hybrid circuit and the third member defines the cantilever accelerometer beam, which generates an electrical output signal indicative of a patient's activity.

BACKGROUND OF THE INVENTION 
The present invention relates generally to implantable cardiac pacemakers, 
and in particular to a method for surface mounting a piezoceramic 
accelerometer directly to a hybrid circuit thereon within a hermetically 
sealed pacemaker housing. 
Present day piezoceramic cantilevered beams are well understood in the area 
of cardiac pacing, as well as the equations which govern their 
characteristics. Conventional electrical and mechanical connection of the 
beam is typically clamped on the short edge of the beam producing a 
cantilever configuration, which defines the overall beam length. 
U.S. Pat. No. 4,140,132, issued to Dahl describes one of the first uses of 
piezoceramic material as a physical activity sensor. The issued patent 
describes an elongated piezoelectric cantilevered element with a weighted 
mass on one end of the element enclosed within an implanted cardiac 
pacemaker. 
U.S. Pat. No. 5,235,237, issued to Leonhardt discloses a piezoceramic 
bending beam accelerometer enclosed within a housing and employs surface 
mount technology for mounting the packaged accelerometer by clamping down 
one end of the accelerometer within the enclosed package. 
U.S. Pat. No. 4,653,326, issued to Danel et al. cites an accelerometer 
capable of measuring a component of acceleration by means of a variable 
capacitance capacitor. 
U.S. Pat. No. 5,031,615, issued to Alt cites a pacemaker which employs an 
accelerometer comprising a miniaturized mechanoelectrical converter or 
transducer formed in a semiconductor device. 
However, the aforementioned disclosures have disadvantages. For example, 
the beam connection to the package or pacemaker shield becomes a dominant 
factor in determining the sensitivity output of the accelerometer when 
employing a bonding medium of either solder or conductive epoxy. When 
bonding, the medium may bleed onto the beam resulting in a reduced 
effective net length of the beam and an attenuation of piezoceramic 
sensitivity. Hence, the bonding step can adversely affect the overall beam 
performance and contribute to manufacturing yield loss. Also, the bonding 
method, supra, requires complex and expensive packing techniques to ensure 
a robust design. 
SUMMARY OF THE INVENTION 
The present invention overcomes the disadvantages of the prior art by 
providing a method of and apparatus for coupling an accelerometer within a 
cardiac pacemaker. The way in which this solution is achieved by the 
invention will be understood by considering the following description. 
In a preferred aspect, the present invention provides a cardiac pacemaker 
with a piezoceramic accelerometer directly coupled to the pacemaker hybrid 
circuit thereto via surface mount technology, (SMT). 
More specifically, the accelerometer crystal is wider than the prior art 
with two incisions made from the same side thereby forming three members. 
The outer members are used for coupling the accelerometer crystal to the 
hybrid circuit and define the inactive areas of the crystal. The third and 
center member delineates the active area of the crystal, that is, the 
piezoceramic cantilevered beam which generates an electrical signal based 
upon patient activity. Unlike the prior art, the invention intregrates the 
coupling members as part of the accelerometer and renders an accelerometer 
design independent of the coupling method employed. 
In another aspect of the invention, the accelerometer is custom packaged in 
a metal housing prior to surface mounting to the hybrid circuit. Unlike 
the inherent disadvantages associated with the prior art, the invention 
employs a custom package to contact only the outer legs of the 
accelerometer thereby allowing the entire cantilever beam to move freely 
within the z-plane of the sensor. 
An advantage of the method of the present invention is the outer dimensions 
can be held constant and the electrical output signal can be mechanically 
adjusted by the depth and width of the aforementioned incisions, thereby a 
family of accelerometer crystals of different outputs could employ the 
same package. 
Another advantage of the present invention is the electrical output signal 
can be further adjusted by the attachment method. That is, by decreasing 
or increasing the bonding area of the outer members of the hybrid adds or 
subtracts to the output signal of the sensor respectively. 
Yet another advantage of the present invention is the outer members provide 
an option for a simple attachment of the sensor to a surface mounted 
package thereby eliminating output signal variability due to the 
attachment of sensors without outer members. 
Moreover, yet another advantage of the present invention is that the 
electrical output signal of the accelerometer can be functionally adjusted 
by reducing a small amount of metalization from the top portion of the 
third member by laser trimming, thereby reducing the effective net length. 
The present invention is specifically concerned with a method of 
manufacturing a surface mountable piezoceramic accelerometer, and in 
particular an accelerometer with adjustable sensitivity which corresponds 
to the inventive shape of the sensor. The preferred method involves 
surface mounting via a protective package to the hybrid circuit of the 
pacemaker. This method further provides pre-circuit attachment testing and 
adjustment for performance centering and yield enhancement. An alternative 
coupling method is also disclosed. The alternative method of mounting is 
by direct surface mounting of the sensor to the hybrid circuit of the 
pacemaker. 
Other features, advantages and objects of the piezoceramic accelerometer 
sensor and method of manufacture of the present invention will hereinafter 
become more fully apparent from the following description of the drawings, 
which illustrate the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, there is illustrated the placement of a pacemaker 10 
in accordance with one embodiment of the present invention. Pacemaker 10 
is shown in FIG. 1 as it would be implanted in a patient 11. The preferred 
embodiment of the invention includes an activity sensor 12, which is a 
piezoceramic accelerometer disposed on the hybrid circuit and isolated 
from the housing 14 of pacemaker 10. Pacemaker 10 may additionally include 
other sensors, such as a pressure sensor or the like implanted within 
heart 16 or disposed on the distal end of pacemaker lead 18. 
A pacemaker which measures the physical activity of a patient by means of a 
piezoelectric transducer which is disposed on the housing of the pacemaker 
is disclosed in U.S. Pat. No. 4,485,813 to Anderson et al. and assigned to 
the assignee of the present invention, which patent is incorporated herein 
by reference in its entirety. Also in U.S. Pat. No. 5,031,615 issued to 
Alt as disclosed, supra, is another example of an activity-sensing cardiac 
pacemaker which uses an integrated miniaturized accelerometer. 
It is to be understood that the present invention is not limited in scope 
to either single-sensor or dual-sensor pacemakers, and that other sensors 
besides activity and pressure sensors could be used in practicing the 
present invention. Nor is the present invention limited in scope to 
single-chamber pacemakers. A multiple-chamber (e.g., dual-chamber) 
pacemaker can also be used in practicing the present invention. 
Turning now to FIG. 2, a block diagram illustrating the constituent 
components of a pacemaker 10 in accordance with the presently disclosed 
embodiment of the invention is provided. Although the present invention 
will be described herein in conjunction with a pacemaker 10 having a 
microprocessor-based architecture. It will be understood that the present 
invention may be utilized in conjunction with other implantable medical 
devices, such as cardioverters, defibrillators, cardiac assist systems, 
and the like. 
In the illustrative embodiment shown in FIG. 2, pacemaker 10 includes an 
activity sensor 12, which as previously noted is a piezoceramic 
accelerometer bonded to the hybrid circuit inside of the pacemaker 
housing. Piezoceramic accelerometer sensor 12 provides a sensor output 
which varies as a function of a measured parameter that relates to the 
metabolic requirements of patient 11. 
Pacemaker 10 of FIG. 2 is programmable by means of an external programming 
unit (not shown in the figures). One such programmer suitable for the 
purposes of the present invention is the Medtronic Model 9790 programmer 
which is commercially available and is intended to be used with all 
Medtronic pacemakers. The programmer is a microprocessor device which 
provides a series of encoded signals to pacemaker 10 by means of a 
programming head which transmits radio-frequency (RF) encoded signals to 
pacemaker 10 according to the telemetry system laid out, for example, in 
U.S. Pat. No. 5,312,453 issued on Jul. 7, 1992 to Wyborny et al., which is 
hereby incorporated by reference in its entirety. It is to be understood, 
however, that the programming methodology disclosed in Wyborny et al. 
patent is identified herein for the purposes of illustration only, and 
that any programming methodology may be employed so long as the desired 
information is transmitted to the pacemaker. It is believed that one of 
skill in the art would be able to choose from any of a number of available 
programming techniques to accomplish this task. 
The programmer facilitates the selection by a physician of the desired 
parameter to be programmed and the entry of a particular setting for the 
desired parameter. For purposes of the present invention, the specifics of 
operation of the programmer are not believed to be important. 
Pacemaker 10 is schematically shown in FIG. 2 to be electrically coupled 
via a pacing lead 18 to a patient's heart 16. Lead 18 includes an 
intracardiac electrode located near its distal end and positioned within 
the right ventricular (RV) or right atrial (RA) chamber of heart 16. Lead 
18 can carry either unipolar or bipolar electrodes as is well known in the 
art. Although an application of the present invention in the context of a 
single-chamber pacemaker will be disclosed herein for illustrative 
purposes, it is to be understood that the present invention is equally 
applicable in dual-chamber pacemakers. 
Lead 18 is coupled to a node 150 in the circuitry of pacemaker 10 through 
input capacitor 152. In the presently disclosed embodiment, piezoceramic 
accelerometer 12 is attached to the hybrid circuit inside of the pacemaker 
14 (not shown in FIG. 2), as noted with reference to FIG. 1. As shown in 
FIG. 2, the output from piezoceramic accelerometer 12 is coupled to an 
input/output circuit 154. 
Input/output circuit 154 contains the analog circuits for interface to 
heart 16, piezoceramic accelerometer 12, an antenna 156, as well as 
circuits for the application of stimulating pulses to heart 16 to control 
its rate as a function thereof under control of the software-implemented 
algorithms in a microcomputer circuit 158. 
Microcomputer circuit 158 comprises an on-board circuit 160 and an 
off-board circuit 162. Unit 158 may correspond to the microcomputer 
circuit employed in U.S. Pat. No. 5,312,453 issued to Shelton et al. on 
May 7, 1994., which is hereby incorporated by reference in its entirety. 
On-board circuit 160 includes a microprocessor 164, a system clock circuit 
166, and on-board RAM 168 and ROM 170. In the presently disclosed 
embodiment of the invention, off-board circuit 162 comprises a RAM/ROM 
unit. On-board circuit 160 and off-board circuit 162 are each coupled by a 
data communication bus 172 to a digital controller/timer circuit 174. 
Microcomputer circuit 158 may be fabricated of a custom integrated circuit 
device augmented by standard RAM/ROM components. 
It will be understood that the electrical components represented in FIG. 2 
are powered by an appropriate implantable battery power source 176, in 
accordance with common practice in the art. For the sake of clarity, the 
coupling of battery power to the various components of pacemaker 10 has 
not been shown in the figures. 
Antenna 156 is connected to input/output circuit 154 for purposes of 
uplink/downlink telemetry through RF transmitter and receiver unit 178. 
Unit 178 may correspond to the telemetry and program logic employed in 
U.S. Pat. No. 4,566,063 issued to Thompson et al. on Dec. 3, 1985 or in 
the above-referenced Wyborny et al. patent, both of which are incorporated 
herein by reference in their entirety. The particular programming and 
telemetry scheme chosen is not believed to be important for the purposes 
of the present invention so long as it provides for entry and storage of 
values of rate-response parameters discussed herein. 
A V.sub.REF and Bias circuit 182 generates a stable voltage reference and 
bias currents for the analog circuits of input/output circuit 154. An 
analog-to-digital converter (ADC) and multiplexer unit 184 digitizes 
analog signals and voltages to provide "real-time" telemetry intracardiac 
signals and battery end-of-life (EOL) replacement function. 
The operating commands for controlling the timing of pacemaker 10 are 
coupled by data bus 172 to digital controller/timer circuit 174 wherein 
digital timers and counters are employed to establish the overall escape 
interval of the pacemaker, as well as various refractory, blanking, and 
other timing windows for controlling the operation of the peripheral 
components within input/output circuit 154. 
Digital controller/timer circuit 174 is coupled to sensing circuitry 
including a sense amplifier 188, a peak sense and threshold measurement 
unit 190, and a comparator/threshold detector 192. Circuit 174 is further 
coupled to an electrogram (EGM) amplifier 194 for receiving amplified and 
processed signals picked up by the electrode disposed on lead 18 which 
signals are representative of the electrical activity of the patient's 
heart 16. Sense amplifier 188 amplifies sensed electrical cardiac signals 
and provides this amplified signal to peak sense and threshold measurement 
circuitry 190, which provides an indication of peak sensed voltages and 
the measured sense amplifier threshold voltage on multiple conductor 
signal path 67 to digital controller/timer circuit 174. The amplified 
sense amplifier signal is then provided to comparator/threshold detector 
192. Sense amplifier 188 may correspond, for example, to that disclosed in 
U.S. Pat. No. 4,379,459 issued to Stein on Apr. 12, 1983, incorporated by 
reference herein in its entirety. The electrogram signal developed by EGM 
amplifier 194 is used on those occasions when the implanted device is 
being interrogated by an external programmer, not shown, to transmit by 
uplink telemetry a representation of the analog electrogram of the 
patient's electrical heart activity, such as described in U.S. Pat. No. 
4,556,063, issued to Thompson et al., assigned to the assignee of the 
present invention and incorporated herein by reference. An output pulse 
generator 196 provides pacing stimuli to the patient's heart 16 through 
coupling capacitor 198 in response to a pacing trigger signal developed by 
digital controller/timer circuit 174 each time the escape interval times 
out, or an externally transmitted pacing command has been received, or in 
response to other stored commands as is well known in the pacing art. 
Output amplifier 196 may correspond generally to the output amplifier 
disclosed in U.S. Pat. No. 4,476,868 issued to Thompson on Oct. 16, 1984 
also incorporated herein by reference in its entirety. 
While specific embodiments of input amplifier 188, output amplifier 196, 
and EGM amplifier 194 have been identified herein, this is done for the 
purposes of illustration only. It is believed by the inventors that the 
specific embodiments of such circuits are not critical to the present 
invention so long as they provide means for generating a stimulating pulse 
and provide digital controller/timer circuit 174 with signals indicative 
of natural and/or stimulated contractions of the heart. 
FIG. 3 illustrates the manufacture of a piezoceramic accelerometer. The two 
sheets of piezoelectric material 30 and 32 are bonded together to form a 
bimorph with a platinum metal 34 between. This structure is then co-fired 
and subsequently nickel electrode layers are 100 and 101 plated on the 
upper and lower surface of the bimorph sheet. Next, the piezoelectric 
sheet is poled by conventional means to yield piezoelectric properties, 
that is the electrical axes are set during polling to established the 
orientation of the electrical properties. The sheet 107 is then cut up 
into smaller rectangular elements with a ceramic cutting saw (not shown in 
the figures). The process of manufacturing piezoelectric material is well 
known and believed that one skilled in the art would be able to accomplish 
such a task. 
Next, the step of defining active area 36, the cantilever beam, and 
inactive areas 38 which are employed to couple the accelerometer to the 
pacemaker hybrid circuit is performed. FIG. 3 shows a piezoceramic sensor 
102. Incisions 103 and 104 are made which separate the inactive and active 
areas of the sensor. The incisions into the piezoelectric material are 
also employed to define the sensitivity of the sensor by controlling the 
resulting beam width 105 and length 106. The accelerometer in FIG. 3 is 
then surface mounted onto the hybrid circuit as shown in FIG. 5. 
Turning to FIG. 5, a finished medical device (e.g.a cardiac pacemaker) is 
formed by mounting one or more feed throughs 509 to one or more of the 
shield halves 505 and 507, enclosing the internal electronics 501, (e.g. 
pulse generator circuitry) including a piezoelectric accelerometer 512, 
and the battery cell 503 within the shield halves 505 and 507, coupling 
the battery 503 to the circuitry, coupling the circitry to the 
feedthroughs 509 and subsequently laser welding the shield halves together 
along their edges to form a substantially hermetic enclosure. A molded 
plastic connector block assembly (not illustrated) containing electrical 
connectors for attachment to the feedthroughs 509 is typically installed 
thereafter. 
In FIG. 5, the outer members 514 and 516 of the sensor permit direct 
attachment to the hybrid circuit during final assembly by employing either 
a solder reflow or a conductive epoxy film, which provides an electrical 
connection to the bottom side electrode, thereby elevating the sensor from 
the hybrid substrate surface and providing satisfactory vibration space 
for the center member 518 to move in a perpendicular arc relative to the 
planar surface of the sensor. Furthermore, the attachment process, supra, 
eliminates the variability of the electrical output signal by providing a 
consistent attachment means of the accelerometer to the pacemaker hybrid. 
Moreover, the electrical output signal of the accelerometer can be 
functionally adjusted by reducing a small amount of metalization from the 
top portion of the center member by conventional laser trimming, (not 
shown in the figures), thereby reducing the effective net length. 
Referring now to FIG. 4, the outer members 49 of the accelerometer 12 are 
employed to mount the accelerometer within a low cost protective package 
in accordance with the preferred embodiment of the present invention is 
diagrammatically illustrated. In FIG. 4A, a Ni/Au plated lid 41 is formed 
into a "U" shape 43 to allow clearance for perpendicular movement of the 
center member (cantilever beam) 36. The lids 41 and 45 have been formed to 
contact only the outer members of the "M" in 12. After the metal housings 
are made and plated, a conductive epoxy 47 is added to lid 45, which 
contacts the outer members 49 of the accelerometer 12 in FIG. 4C. The 
epoxy 47 forms an electrical contact between the nickel plated surfaces of 
the piezoelectric accelerometer and the metal housing. The conductive 
epoxy also provides a mechanical bond, which holds all three pieces 
together and is cured under pressure in FIG. 4D, wherein the latter curing 
step is conventional to the present art. The center member 36 (active area 
of the accelerometer) of the accelerometer 12 is isolated from the 
packaging assembly. Once the assembly process 40 is complete as 
illustrated in FIG. 4E the device defines an accelerometer component which 
can be tested and characterized prior to coupling one terminal 59 of the 
device with solder 53 to the hybrid, (not shown in the figures), and 
electrically coupling by an ultrasonic wire bond process the other 
terminal 61 to the hybrid via a wire bond 55. Hence, this protective 
packaging process enhances manufacturing yields by obviating the 
variability of the output signal by providing a consistent attachment 
means of the accelerometer to the protective package. 
Moreover, the output signal may be augmented to by a twisting movement at 
the base of the sensor 35 which adds to the output signal generated by the 
center member 36. The fashion by which the outer members of the sensor are 
bonded may add to the output signal or subtract from the output signal. 
For example, if conductive epoxy is applied from the base of the sensor to 
the opposite end of the base of the sensor along the outer members, this 
bonding method will reduce the twisting of the sensor at the base, hence 
subtract from the overall output signal. As the epoxy is removed from the 
base towards the opposite end of the base of the sensor along the outer 
members, this bonding method will increase the twisting of the sensor at 
the base, hence add to the overall output signal. 
While only a single embodiment of the invention has been illustrated and 
described, it is not intended to be limited by the aforementioned 
embodiment of the invention and the following alternative embodiment 
should be considered. 
A method of manufacturing a sensor for a medical device by providing a 
bimorphic piezoceramic substrate and sculpturing a first member and second 
member from the piezoceramic substrate, wherein the first and second 
members are integral and define a sensor 520 for a medical device in FIG. 
5B. The first member 522 is employed to couple the sensor within the 
medical device, and the second member 524 defines the active area of said 
sensor, which is also known as the cantilever beam of the sensor. Both 
members can be positioned to form a substantially "T" shape as shown in 
FIG. 5B. Note that the accelerometer 512 in FIG. 5A can be replaced with 
piezoelectric sensor 520 in FIG. 5B. 
Although specific embodiments of the invention have been set forth herein 
in some detail, it is to be understood that this has been done for the 
purposes of illustration only, and is not to be taken as a limitation on 
the scope of the invention as defined in the appended claims. It is to be 
understood that various alterations, substitutions, and modifications may 
be made to the embodiment described herein without departing from the 
spirit and scope of the appended claims.