Reduction of physiological metric error due to inertial cadence

1. A heart rate monitor for sensing a heart rate of a user, the heart rate monitor comprising:

a photoplethysmograph (PPG) sensor configured to:

sense light scattered from a body of the user; and

provide PPG information responsive to the sensed light;

a step rate sensor configured to:

sense motion from the user; and

provide motion information responsive to the sensed motion;

at least one processor circuit configured to:

determine a step rate of a user from the motion information provided by the step rate sensor;

determine an initial instantaneous heart rate from the PPG information provided by the PPG sensor and the motion information provided by the step rate sensor;

compute a difference between the initial instantaneous heart rate and the step rate;

determine a second instantaneous heart rate as a function of the difference; and

output a revised heart rate as a function of the second instantaneous heart rate and a rate limit.

2. The heart rate monitor of claim 1 wherein the at least one processor circuit determines the second instantaneous heart rate by:

comparing the initial instantaneous heart rate to a current filtered heart rate;

revising the initial instantaneous heart rate based on the comparison between the initial instantaneous heart rate and the current filtered heart rate by computing a revised filtered heart rate as a function of the current filtered heart rate and the rate limit, and based on the comparison between the initial instantaneous heart rate and the current filtered heart rate;

if the revised filtered heart rate is within a crossover window relative to the step rate, determining the second instantaneous heart rate as a function of a difference between the revised filtered heart rate and the step rate; and

else; setting the second instantaneous heart rate equal to revised filtered heart rate.

3. The heart rate monitor of claim 2 wherein the at least one processor circuit:

is further configured to update a lock count based on whether the second heart rate is within a crossover window relative to the step rate; and

determines the second instantaneous heart rate as a function of the difference between the step rate and the initial instantaneous heart rate by:

selecting a filter responsive to a comparison between the lock count and a threshold; and

determining the second instantaneous heart rate as a function of the selected filter.

4. The heart rate monitor of claim 1 wherein the at least one processor circuit determines the initial instantaneous heart rate by determining the initial instantaneous heart rate using a combination of two or more dominant signals in said PPG information provided by the PPG sensor when the initial instantaneous heart rate is within a crossover window relative to the step rate.

5. The heart rate monitor of claim 1 wherein the at least one processor circuit determines the initial instantaneous heart rate by determining the initial instantaneous heart rate using a combination of two or more dominant signals in said PPG information output by the PPG sensor when at least one of the two or more dominant signals is within a crossover window relative to the step rate.

6. The heart rate monitor of claim 1 wherein the at least one processor circuit determines the second instantaneous heart rate as a function of the difference between the step rate and the initial instantaneous heart rate by;

if the initial instantaneous heart rate is within a crossover window relative to the step rate, determining the second instantaneous heart rate as a function of the difference between the initial instantaneous heart rate and the step rate; and

else, setting the second instantaneous heart rate equal to the initial instantaneous heart rate.

7. The heart rate monitor of claim 1 wherein the at least one processor circuit determines the second instantaneous heart rate as a function of the difference between the step rate and the initial instantaneous heart rate by:

selecting a filter responsive to a comparison between the difference and a threshold; and

determining the second instantaneous heart rate as a function of the selected filter.

8. A method of sensing a heart rate of a user comprising:

sensing light scattered from a body of the user using a photoplethysmograph (PPG) sensor to determine PPG information;

sensing motion information from the user using a step rate sensor;

determining a step rate of a user from the motion information provided by the step rate sensor;

determining an initial instantaneous heart rate from the PPG information provided by the PPG sensor and the motion information provided by the step rate sensor;

computing a difference between the initial instantaneous heart rate and the step rate;

determining a second instantaneous heart rate as a function of the difference; and

outputting a revised heart rate as a function of the second instantaneous heart rate and a rate limit.

9. The method of claim 8 wherein determining the second instantaneous heart rate comprises:

comparing the initial instantaneous heart rate to a current filtered heart rate;

revising the initial instantaneous heart rate based on the comparison between the initial instantaneous heart rate and the current filtered heart rate by computing a revised filtered heart rate as a function of the current filtered heart rate and the rate limit, and based on the comparison between the initial instantaneous heart rate and the current filtered heart rate;

if the revised filtered heart rate is within a crossover window relative to the step rate, determining the second instantaneous heart rate as a function of a difference between the revised filtered heart rate and the step rate; and

else, setting the second instantaneous heart rate equal to revised filtered heart rate.

10. The method of claim 9 :

further comprising updating a lock count based on whether the second heart rate is within a crossover window relative to the step rate;

wherein determining the second instantaneous heart rate as a function of the difference between the step rate and the initial instantaneous heart rate comprises;

selecting a filter responsive to a comparison between the lock count and a threshold; and

determining the second instantaneous heart rate as a function of the selected filter.

11. The method of claim 8 wherein determining the initial instantaneous heart rate comprises determining the initial instantaneous heart rate using a combination of two or more dominant signals in said PPG information provided by the PPG sensor when the initial instantaneous heart rate is within a crossover window relative to the step rate.

12. The method of claim 8 wherein determining the initial instantaneous heart rate comprises determining the initial instantaneous heart rate using a combination of two or more dominant signals in said PPG information output by the PPG sensor when at least one of the two or more dominant signals is within a crossover window relative to the step rate.

13. The method of claim 8 wherein determining the second instantaneous heart rate as a function of the difference between the step rate and the initial instantaneous heart rate comprises;

if the initial instantaneous heart rate is within a crossover window relative to the step rate, determining the second instantaneous heart rate as a function of the difference between the initial instantaneous heart rate and the step rate; and

else, setting the second instantaneous heart rate equal to the initial instantaneous heart rate.

14. The method of claim 8 wherein determining the second instantaneous heart rate as a function of the difference between the step rate and the initial instantaneous heart rate comprises:

selecting a filter responsive to a comparison between the difference and a threshold; and

determining the second instantaneous heart rate as a function of the selected filter.