WITMOTION WT931 Inclinometer Sensor User Manual

1 Introduction

The WT931 is a multi-sensor device capable of detecting acceleration, angular velocity, angle, and magnetic field. Its compact size makes it ideal for industrial retrofit applications such as condition monitoring and predictive maintenance. By interpreting sensor data through smart algorithms, the device can be configured to address a wide range of use cases.

The WT931 is scientifically known as an AHRS IMU sensor. It measures 3-axis angle, angular velocity, acceleration, and magnetic field. Its primary strength lies in its algorithm, which accurately calculates three-axis angles. The WT931 is employed in applications requiring the highest measurement accuracy and offers several advantages over competing sensors:

• Heated for best data availability: Features a new WITMOTION patented zero-bias automatic detection calibration algorithm that outperforms traditional accelerometer sensors.
• High precision output: Provides Roll, Pitch, Yaw (X, Y, Z axis) data for Acceleration, Angular Velocity, Angle, and Magnetic Field.
• Low cost of ownership: Offers remote diagnostics and lifetime technical support from the WITMOTION service team.
• Developed tutorial resources: Includes a manual, datasheet, demo video, free software for Windows computers, and sample code for MCU integration (e.g., 51 serial, STM32, Arduino, Matlab, Raspberry Pi), along with a communication protocol for project development.
• Industry recognition: WITMOTION sensors have been praised by thousands of engineers as a recommended solution for attitude measurement.

1.1 Warning Statement ⚠️

• Applying more than 5 Volts across the sensor wiring of the main power supply can lead to permanent damage to the sensor.
• VCC cannot be directly connected to GND, as this will burn the circuit board.
• For proper instrument grounding, use WITMOTION with its original factory-made cable or accessories.
• For secondary development projects or integration, use WITMOTION with its compiled sample code.

2 Use Instructions with PC

2.1 Connection Method

The PC software is compatible only with Windows systems. The software is continuously improved and upgraded based on user feedback, so minor differences may exist between versions. Your understanding is appreciated.

? Link to WT931's demo video

2.1.1 Serial Connection

Step 1: Embed the WT931 in an evaluation board and connect the sensor using a serial converter or Micro-USB cable. The required PIN connections are:

• VCC - 5V
• TX - RX
• RX - TX
• GND - GND

(When connecting to a computer, VCC-5V is recommended.)

Diagram: TTL Serial PIN Definition shows color-coded connections: RED for VCC 5V, YELLOW for TX, GREEN for RX, and BLACK for GND. A separate diagram illustrates a sensor connected via TX, RX, VCC, and GND to a USB-to-serial converter, which is then connected to a computer.

Recommended tools:

Image: An image of a 3-in-1 serial converter and an image of a 6-in-1 serial converter.

Step 2: Unzip the software and install the driver CH340 or CP2102 (depending on the accessory used).

? Link to tutorial of 3-in-1 serial converter/ TTL serial cable (CH340 driver)

? Link to tutorial of 6-in-1 serial converter (CP2102 driver)

Step 3: Connect the converter to your computer and confirm the "COM port" in the Device Manager.

Screenshot: Windows Device Manager showing "Silicon Labs CP210x USB to UART Bridge (COM15)" listed under Ports (COM & LPT).

Step 4: Open the software (Minimu.exe). Data will appear after auto-search finishes.

Notice: If the connection is not successful, please operate manually. Choose the correct COM port and set the baud rate to 9600; data should then be displayed on the software.

2.2 Software Introduction

? Link to download software

2.2.1 Main Menu

Screenshot: The WITMOTION software's main interface, titled "WitMotion Shenzhen Co., Ltd Attitude Measurement System". It displays current angle readings (Angle X: 74.77°, Angle Y: 69.63°, Angle Z: -71.00°) and a compass graphic. A table lists the software's main menu options (File, Tools, Record, 3D, Config, Help, Auto-search) and their corresponding functions, such as launching recorded files, hiding toolboxes, recording data, 3D DEMO, configuration settings, language selection, firmware updates, and factory tests.

2.2.2 Menu of Configuration

Screenshot: The WITMOTION software's "Menu of Configuration" window. This window allows users to configure system settings, calibration, range, and communication parameters. Key settings include:

• System: Reset, Sleep, Alarm, Algorithm (9-axis), Install Direction (Horizontal/Vertical), Instruction Startup.
• Calibrate: Acceleration, Magnetic Filed, Reset Z-axis Angle, Gyro Auto Calibrate, Reset Height, Angle Reference.
• Range: Acceleration (e.g., 16 g), Gyro (e.g., 2000 deg/s), Band Width (e.g., 20 Hz), GPS Time Zone (e.g., UTC-12).
• Communication: Baud Rate (e.g., 9600), Output Rate (e.g., 10Hz), Device Address.
• Content: Checkboxes to select data outputs like Time, Acceleration, Velocity, Angle, Magnetism, Port, Pressure, Location, PDOP, Quaternion, Positioning Accuracy, and GPS Original.
• Port: Configuration for D0-D3 models, including model selection (e.g., AIN), pulse width, and cycle.

Table: "Menu of Configuration" buttons include Read Config, Lock, Unlock, Calibrate Time, and Save Config, each with its specific function.

Screenshot: Displays the "Menu of System" and "Menu of Calibrate" sections. The System menu covers reset, sleep, alarm, algorithm selection (6-axis or 9-axis), installation direction, and startup instructions. The Calibrate menu details functions for Accelerometer, Magnetic Field, Reset Height, Reset Z-axis Angle, Angle Reference, and Gyro Auto Calibrate.

Screenshot: Shows the "Menu of Range", "Menu of Communication", and "Menu of Content" sections, detailing available settings and data output options.

Screenshot: Illustrates the "Menu of Port" section, detailing configuration options for extended ports (D0-D3) including model selection, pulse width, and cycle, accompanied by a visual representation of signal lines.

2.3 Calibration

Preparation: Ensure the sensor is "Online". Calibration using the PC software is required for the first-time usage.

2.3.1 Accelerometer Calibration

Purpose: To remove the zero bias of the accelerometer. Calibration ensures accurate measurements by correcting initial bias errors.

Methods:

1. Keep the module horizontally stationary.
2. Click the "Accelerometer calibration" button.
3. Click "Start calibration" and wait for 3 seconds.
4. Click “Complete Calibration”.

Screenshot: The "AccCal" (Accelerometer Calibration) window shows input fields for X, Y, Z values and provides operating instructions for placing the module face up on a horizontal plane, starting calibration, and saving parameters.

Step 5: Judge the result – confirm if there is 1g on the Z-axis acceleration. After calibration, the three axial acceleration values should be approximately 0, 0, 1, and the X/Y axis angles should be around 0°.

Screenshot: The main WITMOTION software window displaying real-time sensor data, including Acceleration, Angular Velocity, Angle, Magnetic Field, Pressure, Angle, and Quaternion values. A compass graphic is also visible.

Note: When the module is placed horizontally, the Z-axis acceleration should read approximately 1g (gravitational acceleration).

2.3.2 Magnetic Field Calibration

Purpose: To remove the zero bias of the magnetic field sensor. Uncalibrated magnetic sensors can introduce significant measurement errors, affecting the accuracy of the Z-axis angle (heading).

Preparation: Ensure the sensor is at least 20CM away from magnetic and iron materials.

Methods:

1. Open the "Config" menu.
2. Click the "Magnetic Filed" calibration button and then click "Start calibration".
3. Slowly rotate the module 360° around the X, Y, and Z axes accordingly.
4. After rotation, click “End calibration”.

Screenshot: The "MagCal" (Magnetic Field Calibration) window displays charts (chartXZ, chartYZ, chartXY) representing magnetic field data during calibration, along with calibration method options (Ellipse fitting), current values, offset, and range settings.

Successful result: Most data dots should fall within the ellipse on the charts. If unsuccessful, move away from potential sources of magnetic field interference.

2.3.3 Gyroscope Automatic Calibration

The gyroscope calibration calibrates the angular velocity. The sensor performs automatic calibration. It is recommended to inactivate automatic gyroscope calibration only if the module rotates at a constant speed.

2.3.4 Reset Z-axis Angle

Note: To mitigate magnetic interference, you can switch the algorithm to 6-axis mode and use the "Reset Z-axis angle" function. The Z-axis angle is an absolute angle referenced to the northeast sky. Resetting the Z-axis to 0° makes the initial angle relative. If the Z-axis drifts significantly during use, this function can recalibrate it. Upon power-on, the Z-axis will automatically return to 0°.

Calibration Method: Keep the module static, open the "Config" menu, and click "Reset Z-axis Angle". The Z-axis angle will reset to 0° in the module's data bar.

2.3.5 Reset Height to 0

This function is available only for modules with a built-in barometer (e.g., WT901B, HWT901B, WTGAHRS1, WTGAHRS2).

2.4 Configuration

2.4.1 Return Content

Setting Method: The content of the data return can be customized. Click the configuration option bar and select the desired data content to be output. For the WT931, the default output includes acceleration, angular velocity, angle, and magnetic field.

Notice: Selecting "GPS Original" will disable all other data outputs. It is generally not recommended to check "GPS Original" under normal circumstances.

Screenshot: The WITMOTION software's configuration window showing options for data output content such as Time, Acceleration, Velocity, Angle, Magnetism, Port, Pressure, Location, PDOP, Quaternion, Positioning Accuracy, and GPS Original.

2.4.2 Output Rate

The default return rate of the module is 500Hz. When the PC software's output rate is set to "single" (单次), and the baud rate is 921600, the rate is 500Hz. The return rate supports up to 500Hz.

500Hz means 500 data packets are returned per second. By default, each data packet is 44 bytes.

Note: If there is a large amount of backhaul content and the communication baud rate is low, data transmission may be limited. In such cases, the module will automatically reduce its frequency to output at the maximum allowable rate. For higher return rates, a higher baud rate (generally 921600) is recommended.

Screenshot: The WITMOTION software's configuration window, highlighting the "Output Rate" setting.

2.4.3 Baud Rate

The module supports multiple baud rates, with the default being 921600. To change the baud rate, ensure a correct connection between the software and the module. Select the desired baud rate from the communication rate dropdown menu in the "Normal-Config" bar. After a few seconds, the module will update its baud rate. If data is not displayed, power cycle the module.

Note: After changing the baud rate, the module may initially output at the original, incorrect rate.

Screenshot: The WITMOTION software's configuration window, showing a dropdown menu for selecting Baud Rate, with options including 4800, 9600, 19200, 38400, 57600, 115200, 230400, 460800, and 921600.

2.4.4 Data Recording

Method:

1. Click "Record" and then "Begin".
2. Click "Stop" to end recording.

Screenshot: The WITMOTION software's interface with "Record" and "Begin" buttons active, indicating data recording is in progress. A compass graphic is visible.

Step 3: Extract the recorded data as a "txt" file.

Screenshot: Shows the process of extracting recorded data as a TXT file, with a sample data table displayed in a Notepad window.

Notice: Repeated "TIME" entries may occur due to the low resolution of the Windows system's time display. Other data changes are accurate. It is highly recommended to paste the data into an Excel file for proper ordering and viewing.

Data Field Explanations:

• Time: Represents the timestamp.
• ax, ay, az: Acceleration values along the X, Y, and Z axes.
• wx, wy, wz: Angular velocity values along the X, Y, and Z axes.
• AngleX, AngleY, AngleZ: Angle values for the X, Y, and Z axes.
• T: Temperature.
• hx, hy, hz: Magnetic field values along the X, Y, and Z axes.

2.4.5 Data Playback

New Function: When a recorded file is created, a BIN file is generated in the "recordFile" folder within the installed software path. This allows for recorded data playback.

Recorded Data Playback Method:

1. Disconnect the sensor.
2. Click the "File" button and then click "Load".
3. Choose the original path of the software installation and load the BIN file.
4. Click "Run". The binary file will then be played back. The playback rate can be edited.

Screenshot: The WITMOTION software's data playback interface, showing the "Load" button and a file selection dialog for BIN files. A compass graphic is visible.

Screenshot: The WITMOTION software's data playback interface, showing the "Run" button and the playback of a BIN file with an adjustable rate. A compass graphic is visible.

2.4.6 Standby and Wake Up

Sleep: The module pauses its operation and enters standby mode, reducing power consumption.

Wake Up: The module returns to its working state from standby mode. By default, the module is in a working state. In the "Config" menu, click "Sleep" to enter sleep state, and click "Sleep" again to release it.

Screenshot: The WITMOTION software's configuration window, highlighting the "Sleep" option under the "System" menu.

2.4.7 Placement Direction

The default installation direction is horizontal. For vertical installation:

1. Rotate the module 90 degrees around the X-axis.
2. Place the sensor 90 degrees vertically.
3. Click "Vertical" in the "Install Direction" option on the "Config" menu.

Screenshot: The WITMOTION software's configuration window, showing the "Vertical" option selected for "Install Direction". A compass graphic is visible.

2.4.8 Bandwidth

The default bandwidth is 20Hz.

Function:

• A higher bandwidth setting leads to greater fluctuation in the data waveform. Conversely, a lower bandwidth results in smoother, more fluent data.
• Example: With a bandwidth of 20Hz and an output rate of 10Hz, the waveform is very steady.

Screenshot: The WITMOTION software's configuration window, highlighting the "Band Width" setting.

Screenshot: Illustrates the effect of different bandwidth settings (20Hz vs 256Hz) on data waveforms displayed in the WITMOTION software.

A higher bandwidth setting can help resolve data-repeating issues. For instance, if the bandwidth is 20Hz and the retrieval rate is 100Hz, there might be 5 repeating data points. To ensure no repeating data, increase the bandwidth to over 100Hz.

2.4.9 Restore Factory Setting

There are two methods to restore factory settings: the short-circuit method and the command method.

• Short-circuit method: Short-circuit the D2 pin and VCC pin of the module with wires, then power on the module. The module's LED will stay on for approximately 2 seconds before turning off, indicating the factory reset is complete.
• Command method: Connect the WT931 module to the computer via a USB-TTL module. Click the setting tab and select "Restore Default". After restoring, power cycle the module.

Note: The command method requires knowing the module's baud rate in advance. If the baud rate does not match, the instruction will not take effect. Try the short-circuit method for recovery if needed.

2.4.10 6-axis/ 9-axis Algorithm

• 6-axis algorithm: The Z-axis angle is primarily calculated based on the integral of angular velocity. This method may introduce calculated errors on the Z-axis angle.
• 9-axis algorithm: The Z-axis angle is calculated and analyzed based on the magnetic field. This method may result in some drift in the Z-axis angle.

The WT931 defaults to the 9-axis algorithm. If magnetic field interference is present in the operating environment, it is recommended to switch to the 6-axis algorithm for angle detection.

Method:

1. Switch to the "6-axis" algorithm in the "Config" menu.
2. Perform "Accelerometer calibration" and "Reset Z-axis angle" calibration.

After calibration, the device can be used normally.

Screenshot: The WITMOTION software's configuration window, showing the "Algorithm" dropdown menu with options for "6-axis" and "9-axis". A compass graphic is visible.

3 MCU Connection

Diagram: Illustrates the WT931 sensor connected to a microcontroller (MCU) unit. Connections include VCC, GND, SWCLK, SWDIO, SCL, SDA, TX, and RX pins.

? Link to download all sample code

? Link to sample code instructions demo

Notice: Sample code is not currently provided for Linux or Python systems.

3.1 Arduino

? Download link

? Arduino UNO3 Demo Link

3.2 STM32

? Download link

3.3 Raspberry pi

? Tutorial link

3.4 C#

? DEMO link

3.5 C++

? DEMO link

3.6 Matlab

? Receive Sample Code

? Dataplot DEMO