TFRPMMAG01 - Magnetic RPM Sensor with I2C Interface
The TFRPMMAG01 is a compact sensor designed for measuring the RPM of diametrically magnetized rotating parts, primarily in UAV and drone applications. It supports direct connection to Pixhawk-based autopilot systems with PX4 firmware. The high-speed measurement capability and compact design make it a robust solution for UAV propulsion monitoring and general rotary speed sensing.
Features
- Designed for UAVs: Compact round form factor optimized for drone integration
- High-Speed Measurement: Accurate up to 28,000 RPM
- I2C Interface: Simple connection to autopilots and controllers
- Pixhawk Compatibility: Plug-and-play support with PX4 firmware
- High Resolution: 14-bit precision for accurate RPM detection
Where to Buy?
The TFRPMMAG01 is available from ThunderFly s.r.o..
For a quotation or support, contact us at sale@thunderfly.cz.
Technical Specifications
| Parameter | Value | Description |
|---|---|---|
| RPM Measurement Principle | Diametrically magnetized rotating part | Ensures dust-reliable speed detection |
| Max. Operating Speed | 28,000 RPM | High-speed UAV and motor applications |
| Resolution | 14-bit | Provides precise RPM and position data |
| Interface | I2C | Standard 4-pin JST-GH connector |
| I2C Address | 0x40 | Default address, configurable in PX4 |
| Input Voltage | 3.3 – 5 V | Supports common Pixhawk UAV bus voltages |
| Connector | JST-GH 4-pin | Pixhawk standard Pixhawk pinout |
| Dimensions | 18 × 18 × 5 mm | Compact circular PCB |
| Mass | 2 g | Without cable |
Pin Configuration
Interface conforms to the Pixhawk Connector Standard.
| Pin | Name | Type | Description |
|---|---|---|---|
| 1 | VDD | Power | Power supply (3.3–5.5 V) |
| 2 | SCL | Input | Serial Clock Line (I2C) |
| 3 | SDA | I/O | Serial Data Line (I2C) |
| 4 | GND | Ground | Ground connection |
Design and Functionality
The sensor detects RPM by measuring the rotation of a diametrically magnetized part. Its compact design allows for direct mounting in UAV propulsion systems. An indicator LED provides a visual pulse once per revolution, aiding in testing and calibration.
Place a diametrically magnetized disc magnet above the sensing area on the TOP side and keep it centered. Maintain a small axial air gap (< 1 mm) and avoid axial magnetization. Any rotation angle is acceptable; RPM is derived from the change of angle over time.
Dimensions of the sensor follow. The overall thickness is 0.6 mm PCB + 1.2 mm Sensor IC height. Therefore, 1.8mm in total. A 2 mm wide slit is usually a good design value, which allows manipulation with the sensor during maintenance.
Nominal hole diameter in PCB is 2.2 mm. Acceptable screw head diameter is up to 4.4 mm.
PX4 Autopilot Integration
The PX4 firmware includes a dedicated driver for the TFRPMMAG01 based on the S35770 I2C counter IC. The driver is part of the rpm_sensor module group.
Enabling the Driver
The driver can be enabled permanently by setting the SENS_EN_S35770 parameter to 1 in QGroundControl or via the MAVLink console:
param set SENS_EN_S35770 1
A reboot is required after changing this parameter. After reboot the driver starts automatically.
Manual Start via Console
To start the driver manually from the PX4 NSH console (e.g., via MAVLink Shell), run:
s35770 start -X -a 50
Where -X selects the external I2C bus and -a 50 sets the I2C address (decimal 50 = 0x32). To verify the driver is running:
s35770 status
To stop the driver:
s35770 stop
Driver Parameters
| Parameter | Default | Unit | Description |
|---|---|---|---|
SENS_EN_S35770 | 0 | — | Enable driver: 0 = disabled, 1 = enabled. Requires reboot. |
S35770_POOL | 1 000 000 | µs | Sensor poll interval. Requires reboot. |
S35770_MAGNET | 1000 | counts/rev | Number of counter pulses per one full rotation of the actuator. Adjust to match the number of pole pairs or magnet pulses. Requires reboot. |
The S35770_MAGNET parameter is the most important one to configure correctly — it determines the RPM calculation accuracy. Set it to the number of magnetic pulses generated per revolution by your specific rotating assembly.
Applications
- UAV propulsion monitoring
- RPM measurement in BLDC motor systems
- General-purpose rotary speed sensing