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Publication Date: 2010-Aug-11
Document File: 3 page(s) / 145K

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A cardiac rhythm management (CRM) device includes a multi-axis accelerometer and a processor configured to detect a fault in the accelerometer.

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Cardiac Rhythm Management (CRM) devices include implantable pacemakers, implantable cardioverter defibrillators (ICDs), and wearable cardioverter defibrillators (WCDs).  A CRM device may contain a multi-axis posture sensor such as a 3-axis accelerometer. Undetected or mitigated faults in the posture sensor or posture sensor processing algorithms could lead to undesirable, and potentially hazardous, system behavior.

Fault Detection

In some examples, the three active axes of a 3-axis accelerometer include three mutually orthogonal accelerometers; one accelerometer for each axis.  If the only acceleration experienced by the patient is acceleration due to gravity, the squared outputs of the accelerometers should sum to 1 g ± a reasonable tolerance (e.g., +0.1 g).

The output of each individual accelerometer is:

Accel = Cos(θ),           (Eq. 1)

where θ is the angle between the accelerometer's active axis and the direction of the acceleration.  The outputs of the individual sensors are summed as shown below in Equation 2.  The summed output, Check Signal, should be equal to 1 g ± some small tolerance.

Check Signal = Accelx2 + Accel y2 + Accel z2, (Eq. 2)

where Accelx, Accely and Accelz are the outputs of three accelerometers.  An example is shown in Figure 1 below.  In the example, two axes are oriented 45o to gravity and 1 axis (orthogonal to the page) is oriented 90o to gravity.  The output, Check Signal, is equal to 1 g.

Figure 1.

            Another example is shown below in Figure 2.  In the example, all three axes are oriented 54.7o to gravity.  The output, Check Signal, is again equal to 1 g.  If the summed output is not equal to 1 g, there is a fault in the posture sensor.


Figure 2.

Two conditions are useful in acquiring a measurement for fault detection.  The first condition is that the patient should be experiencing acceleration only due to gravity. Additional acceleration or accelerations can confound the test.  One or more methods may be used to ensure that only acceleration due to gravity is used to determine the presence of a fault in the posture sensor. These methods can include:

1) Only conducting the fault test when there is no (or extremely little) dynamic component to an accelerometer output signal.  This is useful to avoid errors caused by dynamic acceleration.

2) Processing an output signal generated by an accelerometer through a low pass filter.  This is also useful to avoid errors caused by dynamic acceleration.

3) Calculating the summed output, Check Signal, multiple times...