Altitude measurements help to keep you fit

The market for wearable activity monitors has risen steadily over the last few years. Wrist and belt worn devices have become more common as people try to be mindful of the levels of exercise they are achieving. One of the most popular brands in this sector is Fitbit, which makes the Fitbit Flex, a plastic wristband that monitors the user’s steps, distance and calories burned. At night, it tracks sleep quality and wakes the user silently at the right time. Another product, the Fitbit One, is clipped to the user’s belt or clothing and can measure stairs climbed as well.

Wearable activity monitors: Fitbit Flex (left) is a wristband, while Fitbit One (right) clips to clothing

These devices are intended to be small and unobtrusive enough for daily wear, motivating users to be more active by collating data about their lifestyles. They work well as part of a personal area network, including a mobile phone and PC, to provide Internet connectivity, as well as other fitness related sensors, such as a heart rate monitor.

For wearable devices to continue to gain market acceptance, the highest levels of accuracy are required. Where and how a person moves is key to determining the amount of calories they burn. The typical way to monitor movement would be to use a MEMS accelerometer inside the device, but this type of movement sensing is more accurate when used in combination with other sensors. For example, a wrist worn accelerometer alone wouldn’t sense much difference between walking on a level surface and walking upstairs, even though walking upstairs burns significantly more calories. A MEMS air pressure sensor can beintegrated into the design to sense the difference in relative air pressure between the bottom of the stairs and the top. The change in altitude can help the activity monitor determine whether the person went up or down the stairs. The effect is even more pronounced for walking or running uphill, or for activities that involve rapid changes of altitude, like skiing.

Luckily, MEMS pressure sensors used for altitude measurements are widely available with sufficient accuracy for the type of application presented by activity monitors. These sensors have tiny footprints and are cost-effective and energy-friendly enough for battery powered devices.

As an example, the Omron 2SMPB-01-01 MEMS air pressure sensor (right) is well suited for wearable activity monitors as it can determine changes in altitude as small as 50cm (corresponding to 6Pa difference in air pressure). It achieves this very high level of accuracy by incorporating a temperature sensor, so that fluctuations in air pressure caused by ambient temperature changes can be compensated for. Also for accuracy, each part is calibrated individually on the production line, with the calibration data stored in a one-time programmable memory. In high accuracy mode, the sensor can deliver an air pressure reading in 17ms.

Omron’s 2SMPB-01-01 suits battery powered devices, as its supply voltage is 2.5V. Current consumption is 9uA for high accuracy mode (1 sample per second), which is reduced to 0.5uA in sleep mode. Sleep mode entails powering down non-working circuits to preserve battery power.

Aside from using altitude to help measure calories burned, knowing the air pressure may be useful for some secondary functions in portable fitness devices. For example, altitude measurement can be used to back up GPS data from a smartphone to determine how far the user walked or ran, when the GPS signal is absent or distorted. Or perhaps indoor navigation is required, for which knowing which floor the user is on is a pre-requisite. Absolute air pressure may also be used to predict the weather, especially when the sensor is a combination air pressure, temperature and humidity sensor; in a wearable activity monitor, this might lead to an alert advising the user to get their morning run in before it starts raining.

Sensor fusion, or combining data from different sensors to more accurately measure the environment around us, will become more and more commonplace as small sensor nodes proliferate as part of the Internet of Things. These nodes will upload their data to the Internet for processing and interpretation. Wearable activity monitors are just one way that MEMS air pressure sensors can be used, but they are a very important one, since more and more consumers look to quantify factors that affect their health, in order to improve their lifestyles.

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