Concept renderMotion sensors come in two groups: room sensors such as PIR, microwave and ultrasonic, and sensors inside the moving device. Wearables use the second: an accelerometer for steps, tilt and falls, a gyroscope for rotation, a magnetometer for heading and a barometer for height. An IMU combines an accelerometer and a gyroscope.
Search for motion sensors and most results describe the PIR sensor over a garage door. A wearable needs a different family: small MEMS chips that measure the motion of the device itself. This guide explains each type, what it does in a watch, band, ring or earbud, and which datasheet lines to compare. Every figure comes from a vendor datasheet or product page linked below, read on November 5, 2026. It contains no Duobay price and no test result.
What are the types of motion sensors, and which go in a wearable?
Motion sensors fall into two groups. One group watches a space and detects a person moving through it. The other sits inside the moving object and measures its own motion. Wearables use the second group, called inertial sensors, plus a barometer for height. For the other sensor types in a wearable, such as PPG, ECG and skin temperature, see wearable sensors: types, power and placement.
| Type | What it senses | In a wearable? |
|---|---|---|
| Passive infrared (PIR) | Body heat moving across its view | No. It watches a room from a wall or ceiling. |
| Microwave or radar | Radio waves reflected by a moving object | No, for the same reason. |
| Ultrasonic | Sound above human hearing, reflected back | No, for the same reason. |
| Accelerometer | Acceleration of the device itself, including gravity | Yes. Most features in this guide start here. |
| Gyroscope | Rotation rate of the device itself | Often, for fast or precise orientation. |
| Magnetometer | Magnetic field on three axes | Sometimes, for heading. |
| Barometer | Air pressure, which changes with height | Sometimes, for floors and climbs. |
| IMU | An accelerometer and a gyroscope in one package | Yes, when you need both signals in sync. |
How do gyroscope sensors work, and how do accelerometers differ?
Both are MEMS parts: tiny mechanical structures etched in silicon, read out by a chip in the same package. What follows is the general working principle. The datasheets we read do not describe each vendor's internal design.
Accelerometer
A small proof mass hangs on silicon springs. When the device accelerates, the mass shifts a little relative to its frame. Fixed electrodes next to the mass form capacitors, and the readout chip turns the change in capacitance into acceleration on three axes. Gravity also acts on the mass, so a still accelerometer shows a steady reading along the vertical. That is how a band knows its tilt.
Gyroscope
A MEMS gyroscope keeps a small mass vibrating. When the device rotates, the Coriolis effect pushes the vibrating mass sideways, at right angles to its vibration. Sense electrodes pick up that sideways motion, and the chip converts it to a rotation rate in degrees per second. In the datasheets below, the gyroscope draws far more current than the accelerometer.
Magnetometer and barometer
A magnetometer measures the magnetic field on three axes. Earth's field gives a compass heading, and nearby magnets or steel distort it. A barometer measures air pressure, which drops as you climb.
What is an IMU sensor, and when do you add a magnetometer or barometer?
An IMU, or inertial measurement unit, puts an accelerometer and a gyroscope in one package with shared timing. The Bosch BMI270 datasheet describes a 16-bit 3-axis accelerometer and a 16-bit 3-axis gyroscope in a 2.5 x 3.0 mm package, 0.83 mm high, as of November 2026. It also lists an auxiliary interface for one external sensor, such as a magnetometer or pressure sensor, with data synchronized to the IMU. A 9-axis design adds a 3-axis magnetometer to a 6-axis IMU.
Add a magnetometer when the product needs a heading that does not drift. The BMM150 datasheet quotes a zero-B offset of ±2 µT only after calibration with Bosch eCompass software and movements through various orientations, as of November 2026. Plan that calibration step into the user flow.
Add a barometer for stairs, floors and hikes. The BMP390 datasheet gives a relative accuracy of ±3 Pa, which it equates to ±0.25 m, between 700 and 1,100 hPa and 25 to 40 °C, as of November 2026. Relative accuracy is the figure that matters for a change in height.
What does each motion sensor do in a wearable?
Most wearable features start in the accelerometer. The table maps each feature to its sensor, what the datasheets offer on the chip as of November 2026, and what you still check on your own device.
| Feature | Sensor | What the datasheet offers | What to check on your device |
|---|---|---|---|
| Step counting | Accelerometer | BMI270: a wrist-worn step counter tuned for watches and bands, plus a low-latency step detector. BMA400: a step counter with overall device current of 4 µA. | Count steps at the wear position you will ship, against hand-counted steps, at slow and fast pace. |
| Activity recognition | Accelerometer | BMI270: still, walking, running and unknown, with an interrupt only when the state changes. BMA400: walking, running and standing still. | Make sure the labels you market match the states the chip reports. |
| Wrist tilt to wake the screen | Accelerometer | BMI270 wrist wear wakeup detects the hand moving to look at a watch. It assumes the sensor axes match the device axes, and the datasheet points to axis remapping. | Confirm the sensor orientation on your PCB, then test on both wrists. |
| Wake on motion | Accelerometer | BMI270 any-motion compares the slope between samples with a threshold. BMA400 low-power mode runs at a fixed 25 Hz for self wake-up. | Tune the threshold so the device sleeps on a nightstand and wakes in use. |
| Fall detection | Accelerometer, often with a gyroscope | BMI270: the accelerometer reports zero in free fall. None of the documents we read lists an on-chip fall detection feature for people, so the algorithm is yours. | Pick a range that does not clip impacts in your own recordings. Log false alarms per day of wear. |
| Heading | Magnetometer plus accelerometer | BMM150: heading accuracy depends on hardware and software, and assumes a calibrated sensor and ideal tilt compensation. | Test near the battery, speaker magnet and charging contacts. |
| Floors and climbs | Barometer | BMP390: lists floor detection and elevator detection among its uses. | Check the air path through the enclosure, so a pressed button does not read as a climb. |
A health band also uses the accelerometer as a motion reference for the heart rate sensor. Our guide to PPG motion artifacts covers that bench test.
Which datasheet specs should you compare?
Four lines decide most choices: range, noise density, output data rate and current. The table lists typical values from five vendor documents, as of November 2026. Conditions differ from row to row, so read the notes below before you rank parts.
| Part | Range | Noise | Output data rate | Typical current | Source |
|---|---|---|---|---|---|
| Bosch BMI270 (6-axis IMU) | Accel ±2 to ±16 g. Gyro ±125 to ±2,000 dps. | Accel 0.16 mg/√Hz (normal mode, 8 g). Gyro 0.007 dps/√Hz performance mode, 0.010 normal. | Accel 0.78 Hz to 1.6 kHz. Gyro 25 Hz to 6.4 kHz. | Both sensors 685 µA normal, 970 µA performance. Accel only 210 µA normal, 10 µA low power at 25 Hz. Suspend 3.5 µA. | BMI270 datasheet, rev 1.6, March 2026 |
| TDK ICM-42688-P (6-axis IMU) | Accel ±2 to ±16 g. Gyro ±15.6 to ±2,000 dps. | Accel 70 µg/√Hz. Gyro 2.8 mdps/√Hz. | Not on the product page. | 0.88 mA, 6-axis low-noise mode. | ICM-42688-P product page |
| Bosch BMA400 (accelerometer) | ±2 to ±16 g. | 180 µg/√Hz on X and Y, 240 on Z (normal mode, highest oversampling, 4 g). | 12.5 to 800 Hz normal. 25 Hz in low-power mode. | 14.5 µA normal, highest oversampling. 850 nA low-power. 160 nA sleep. | BMA400 datasheet, rev 2.3, July 2025 |
| Bosch BMM150 (magnetometer) | ±1,300 µT on X and Y, ±2,500 µT on Z. | 1.0 µT rms on X and Y, low power preset. 0.3 µT rms, high accuracy preset. | 10 Hz low power. 20 Hz high accuracy. | 0.17 mA low power. 4.9 mA high accuracy. | BMM150 datasheet, rev 1.4, April 2020 |
| Bosch BMP390 (barometer) | 300 to 1,250 hPa. | Relative accuracy ±3 Pa, equal to ±0.25 m per the datasheet. Absolute ±50 Pa. | Not compared here. | 3.2 µA at 1 Hz. 1.4 µA sleep. | BMP390 datasheet, rev 1.7, March 2021 |
The BMA400 datasheet gives 160 nA for sleep in its spec table and 200 nA in its power-mode text. When two lines disagree, ask the vendor which applies. The TDK row comes from the product page. We did not read the full ICM-42688-P datasheet.
Range
Range is the largest signal the sensor reports before it clips. A wider range catches hard impacts at lower resolution per bit. BMI270 accelerometer sensitivity drops from 16,384 LSB/g at ±2 g to 2,048 LSB/g at ±16 g. Keep range apart from survival ratings. The same datasheet lists absolute maximum ratings of 20,000 g for shocks up to 200 µs and a 1.8 m free fall onto hard surfaces. Those describe what the part survives, and say nothing about what it can measure.
Noise density
Noise density is noise per square root of bandwidth. Multiply by the square root of your bandwidth to estimate total noise. The BMI270 figures check out: 0.16 mg/√Hz times √80 Hz is about 1.43 mg, and the datasheet lists 1.51 mg rms at 80 Hz bandwidth and 200 Hz output rate. Convert to one unit before comparing. 0.16 mg/√Hz is 160 µg/√Hz, next to 70 µg/√Hz for the TDK part and 180 µg/√Hz for the BMA400. Each was measured under its own conditions, so treat the ranking as a first filter.
Output data rate and bandwidth
The output data rate (ODR) is how often the sensor delivers a sample. The filter bandwidth sets how fast a change shows up in it. In the BMI270 bandwidth table, a 25 Hz ODR gives 11 Hz of bandwidth and a 200 Hz ODR gives 89 Hz. Pick the lowest ODR each feature can live with.
Current in the mode you will run
The headline current is rarely the one your product sees. On the BMI270, accelerometer-only normal mode is 210 µA, gyroscope-only is 600 µA, and both together are 685 µA. The BMA400 low-power mode is 850 nA, about 800 times less than the BMI270 with both sensors on. The BMI270 gyroscope takes 45 ms from suspend to normal mode, or 2 ms in fast start mode. A common pattern: keep the accelerometer in low power, wake the gyroscope only when a feature needs it, and let the sensor FIFO fill while the main processor sleeps. The FIFO is 2 KB on the BMI270, 1 KB on the BMA400 and 512 bytes on the BMP390.
What should you ask your hardware team?
Take this list to your team, or to any supplier that proposes a sensor.
- Which features need the gyroscope, and does the accelerometer wake it only for those?
- Did you check the range against recorded impacts and fast arm swings?
- Were both parts in the same mode and range when you compared noise?
- What output data rate does each feature need, and where is that written down?
- What is the sensor current in the mode and rate we will actually run?
- Does the main processor sleep while the sensor FIFO fills?
- Do the firmware axis settings match the sensor orientation on the PCB?
- Is the magnetometer clear of the battery, speaker and charging magnets?
- How do we test step, activity and fall features on real wear positions before we write a claim?
For the category background, see our health wearable development page. Health features there are general wellness only, with no medical claims.
Questions about motion sensors in wearables
What is the difference between an accelerometer and a gyroscope?
An accelerometer measures acceleration, including gravity, so it also tells you which way is down. A gyroscope measures how fast the device rotates. The Bosch BMI270 datasheet says the accelerometer reports zero in free fall and the gyroscope reports zero at rest (BMI270 datasheet, as of November 2026).
Do I need a gyroscope to count steps?
Not always. The BMA400 is an accelerometer-only part, and its datasheet lists a step counter with an overall device current of 4 µA, as of November 2026 (BMA400 datasheet). Add a gyroscope when a feature needs rotation rate.
Can a wellness wearable advertise fall detection?
How you describe the feature matters, especially for older adults. Read our summary of FDA general wellness rules for wearable claims and have counsel review the wording. This article is an engineering summary and is not legal advice.
How could Duobay help with motion sensor choices?
Initial calls are free, and we do not publish prices. A Gate 0 Feasibility study can review your feature list, the sensors it needs, a current budget and a test plan for your device. Tell us about your device and we will explain how it works.
If you want help choosing sensors and writing the test plan for your wearable, tell us about your device.
Sources
All pages accessed November 5, 2026. Vendors revise datasheets, so check the current revision before you design around a figure.
- Bosch Sensortec, BMI270 datasheet, BST-BMI270-DS000-08, rev 1.6: https://www.bosch-sensortec.com/media/boschsensortec/downloads/datasheets/bst-bmi270-ds000.pdf
- Bosch Sensortec, BMA400 datasheet, BST-BMA400-DS000-14, rev 2.3: https://www.bosch-sensortec.com/media/boschsensortec/downloads/datasheets/bst-bma400-ds000.pdf
- Bosch Sensortec, BMM150 datasheet, BST-BMM150-DS001-05, rev 1.4: https://www.bosch-sensortec.com/media/boschsensortec/downloads/datasheets/bst-bmm150-ds001.pdf
- Bosch Sensortec, BMP390 datasheet, BST-BMP390-DS002-07, rev 1.7: https://www.bosch-sensortec.com/media/boschsensortec/downloads/datasheets/bst-bmp390-ds002.pdf
- TDK InvenSense, ICM-42688-P product page: https://invensense.tdk.com/products/motion-tracking/6-axis/icm-42688-p/
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