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Silicon Altimeters for Consumer Electronics Apps

Pressure sensors have long found use in various niche industrial designs, as well as in at least one notable high volume application: as a tire deflation early-warning indicator. More recently, traditional mechanical sensors have been supplemented (or perhaps more accurately, supplanted) by MEMS-implemented semiconductor approaches that can deliver improved accuracy and reliability, along with reduced cost. Now another volume opportunity has arisen, led by a handful of smartphones, tablets, and watches. What uses might such systems find for a silicon pressure sensor, and how can other technology advances both augment its capabilities and compensate for its shortcomings? This article will address these questions.

Elevation determination

An altimeter does not actually measure altitude directly, but rather just atmospheric pressure. So an altimeter is actually a barometer created for a specific purpose. This can be done because the pressure at any given point in the atmosphere around the earth is a result of the weight of the atmosphere above it (pulled down by gravity). So the higher in the atmosphere you are, the less atmosphere is above you, and the less pressure is exerted on you.

In this particular application there needs to be some means of discerning whether (for example) a decrease in air pressure is due to the fact that the user has gained altitude on a hike or climb or is about to experience the effects of an incoming storm front. On a day full of weather changes, such as an approaching cold front, air pressure could change by as much as 5 mbar, or more. This could result in a skewed altitude reading of up to 130 feet (40 meters), or more.

Elevation measurement is the predominant motivation for putting pressure sensors in Android-based devices such as the Motorola Xoom and Galaxy Nexus. Motorola's Xoom tablet computer was the first hardware platform to support Android 3.0, aka "Honeycomb,” and Samsung's Galaxy Nexus cellular handset is the premier implementation of the latest-generation Android 4.0 "Ice Cream Sandwich."

In addition to a dual-core Tegra 2 applications processor from Nvidia Corp., Motorola’s Xoom media tablet includes a pressure monitor (BMP085) from Bosch Sensortec. (Figure 1).

Figure 1: Motorola's Xoom tablet (top) includes (bottom) a Bosch MEMS-based pressure sensor to assist GPS functionality.

The BMP085 is based on piezo-resistive MEMS technology and is claimed to offer an absolute accuracy of 2.5hPa and a noise level down to 0.03 hPa (equivalent to an altitude change of only 0.25 m). Power consumption is only 3 µA, which combined with its thin (1.2 mm height) 8-pin package makes it well-suited for mobile device designs.

According to a Google engineer's recent discussion forum post1, the pressure sensor is included specifically as a means of assisting the systems' GPS subsystems in quickly and accurately determining a user's location.3 Ironically, GPS is a particularly poor means of ascertaining altitude, even if triangulated data from multiple satellites is employed. Conversely, AGPS (assisted GPS) altitude data from a pressure sensor, along with other location-approximation information from nearby Wi-Fi and cellular tower broadcasts, can compensate for GPS satellites that are blocked or whose signals are multipath-distorted by reflections caused by intermediary objects such as buildings.

And since devices such as the Xoom and Galaxy Nexus (which houses the Bosch BMP180, a first pressure sensor implementation for smartphones and at 0.93 mm (H) thinner even than the aforementioned BMP085 ) contain various network connectivity capabilities (Bluetooth, Wi-Fi, EV-DO, HSPA, LTE, etc.), the consequent access to supplemental data can also provide the pressure sensor with useful assistance . For more information on pressure sensors and GPS see the previous TechZoneSM article “How Pressure Sensors Can Supplement GPS in Navigation Systems”

Another frequently cited application is in 'crowd sourcing' barometric pressure measurements obtaining data from mobile electronics devices as a means of augmenting the information coming from traditional static weather stations. This idea is analogous to the already implemented, dynamically updated database of radiation measurements taken by Geiger counter-toting users nearby the Fukushima Daiichi nuclear plant disaster site.

Before the advent of tablet computers a number of ruggedized watches, such as Suunto's trendsetting Vector and Timex's follow-on Helix WRKS, offered built-in altimeters. A manual elevation override function is offered on these watches so that the wearer can periodically recalibrate the watch upon encountering a known altitude metric; a survey marker, for example, or a location on a topographical map, or a trail sign. The override is provided because the accuracy of an altimeter can be adversely affected by evolving weather conditions. In systems such as these, by the way, data logging capabilities (to flash memory or DRAM, for example) are also recommended so that the user can later download the time-versus-elevation database for archive and analysis.

Weather analysis

As executed in the Suunto Core altimeter watch, for example, the watch displays your current elevation, shows how much you’ve climbed or descended, and records your entire session for later analysis. A veritable weather station on your wrist, the Suunto Core barometer also measures and records air pressure to help you predict changing conditions, from sunny skies to hair-bending electrical storms. Resolution is said to be 0.05 inHg/1 mbar.

Weather applications don't typically require precise pressure measurements. And depending on the specific implementation, even less precision may be necessary. For a weather station implementation, note that barometric pressure typically provided in weather reports (on TV, the radio, and via weather-themed websites, for example) is mean sea level pressure, which is not the pressure at a particular location's actual elevation but instead is the estimate at an altitude-independent sea level equivalent5

Part and kit options

Freescale's DEMOAPEXSENSOR Altitude Pressure EXperimental (APEX) Sensor Board provides an interesting implementation case study (Figure 2). It contains two absolute pressure sensors, the MPL115A with a 50 - 115 kPa measurement range and an I²C/SPI digital system interface, and the MPXM2102A with a 0 - 100 kPa measurement range and analog interface. The latter sensor is mated to a 24-bit ADC. This kit also has MPXV5004G pressure sensors with an MMA7361L accelerometer and makes use of Freescale’s Flexis MC9S08JM60 8-bit microcontroller.

Figure 2: Freescale's Altitude Pressure EXperimental Sensor Board contains two different air pressure sensors with varying precisions, measurement ranges, and system interfaces, thereby targeting both overlapping and dissimilar applications.

The DEMOAPEXSENSOR has variety of applications built into the demo. These include: Altimeter, Barometer, Barometer Alarm, Weather station, Waterlevel, Waterlevel Alarm, Tilt, Freefall, and Temperature Sensing. The board has been designed as a standalone board to be interfaced via an LCD display and menu buttons.

Freescale’s MPL115A1 is primarily intended for barometer applications, while the higher precision (albeit with higher bill of materials cost) of the MPXM2102A-plus-ADC combo also make it relevant for altimeter implementations. The MPL115A1 employs a MEMS pressure sensor with a conditioning IC to provide accurate pressure measurements from 50 to 115 kPa. An integrated ADC converts pressure and temperature sensor readings to digitized outputs via an SPI port. Factory calibration data is stored internally in an on-board ROM. Its 5 by 3 by 1.2 mm LGA package makes it well-suited for the space constrained requirements of portable electronic devices. Its low current consumptions of 5 μA during Active mode and 1 μA during Shutdown (Sleep) mode are essential when focusing on low-power applications.

Freescale’s MPXM2102 series devices are silicon piezoresistive pressure sensors providing an accurate and linear voltage output directly proportional to the applied pressure. The sensor is a single, monolithic silicon diaphragm with the strain gauge and a thin-film resistor network integrated on-chip.

Pressure sensors have only just begun to inspire product developers in mass market, mobile consumer electronics. These sensors can noticeably boost the usage of both established and new products by providing additional functionality (such as determining elevation or upcoming changes in weather ) or assist with existing features (such as GPS in the Motorola Xoom). For more information use the provided links to product pages on the Hotenda website.