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Heart rate monitor with programmable SoC

14 Oct 2013  | Asha Ganesan, AnandaGanesh M S

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This article provides a glimpse of what a heart rate monitor is and why it is needed. Next, the design considerations of a simple hand held heart rate monitor are discussed. Finally, it is shown how to implement it using a single programmable system-on-chip (PSoC).

What is a heart rate monitor?
A heart rate monitor is a device used to monitor the rate of heart-beat for human beings. The heart rate is expressed in units of number of beats per minute (BPM). The heart rate of a human body varies depending on daily physical activities, sleep, and general health.

Heart rate monitors are typically used in fitness equipment and in certain hospital equipment to diagnose cardiovascular diseases like tachycardia and arrhythmia. Many OEMs are now moving towards smartphone-connected and handheld/wearable health-related monitoring products.

Need for a heart rate monitor in fitness equipment
Keeping track of the heart rate during physical exercises enables people to ensure the safety of their fitness program. The normal heart rate for a person during rest is 60-100 BPM. However, the rate increases when the person does physical activity. The maximum allowable heart rate depends on the person's age and decreases as the age of the person increases. Another good reason to record one's heart rate during exercising is to gauge the progress of their health.

Many users also want to know how many calories they have burned throughout the day. Most fitness machines used in a gym have a built-in heart rate monitor along with a calorie counter. However, people who prefer jogging or walking instead of a machine assisted workout need portable equipment in hand to monitor their heart rate.

Behaviour of blood flow during a heart beat
The contraction and relaxation of cardiac muscles causes blood to flow in and out of the heart. During each cardiac cycle, a group of tissues in the heart called the sinoatrial node generates electrical impulses that spread through the heart and cause rhythmic contraction and relaxation of heart muscles. During each cardiac cycle, the blood vessels pulsate in order to carry blood to/from different parts of the human body.

The general places used to measure heart rate are inside the elbow, ventral part of wrist, neck, behind the knee (popliteal artery) and chest (where a stethoscope is usually used). Heart rate monitoring machines are designed so as to measure the heart beat at these spots. However, these machines turn out to be bulky and difficult to carry as the methodology used to monitor the heart rate at these spots involves a complex process. On the other hand, the heartbeat may also be palpated in any place such as tip of the finger or the bottom of the ear lobe. This provides flexibility in bringing out portable heart rate monitors.

Counting calories based on heart rate
The calorie count burned during a workout depends upon the average heart rate, and workout duration. The other terms used in calorie computations are constant. Hence, the portable heart rate monitor can also have the feature of displaying the calories burned.

The following equations provide details on how to calculate the calories burned depending on the heart rate.

Calories Burned for Men (kCal) =

[(Age in years • 0.202) + (Weight in kg • 0.199) + (BPM • 0.631) – 55.1] • Duration in minutes / 4.18

Calories Burned for Women (kCal) =

[(Age in years • 0.074) + (Weight in kg • 0.126) + (BPM • 0.447) – 20.4] • Duration in minutes / 4.18

The person's age and weight are manually entered into the machine before it can begin this computation.

Photoplethysmography technique in heart rate monitoring
Depending on the heartbeat rate, the volume of blood flowing through the fingertip or earlobe varies. Hence we need some mechanism to detect the change in the volume of blood so as to determine the rate at which the heart is beating.

One of the most common methods to monitor heart rate is to use an optical transceiver that contains a built-in IR transmitter and receiver. When light radiation such as infrared is passed through a blood vessel in a finger or earlobe, the received light signal is periodic and varies due to the rhythmic flow and absorption properties of blood. (In general, human blood tends to reflect IR waves incident on it.) This methodology is known as Photoplethysmography.

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