Tercio IMU

The Tercio IMU streams the attitude of your machine on the same bus as your motors: a quaternion, roll, pitch and yaw, acceleration and angular rate. It speaks protocol v2 like the Tercio S1, and Tercio Control finds it by itself.

Sensor

Bosch BNO055, 9-axis with on-chip fusion

Output

Quaternion, roll / pitch / yaw, acceleration, rate

Rate

100 Hz fusion, telemetry up to 100 Hz

Bus

CAN-FD, protocol v2, node 100 by default

Processor

STM32G431 at 170 MHz

Board

25 × 51 mm

100Hz

Sensor fusion

9axes

Accel, gyro, magnetometer

62bytes

One telemetry frame

100

Default node ID

What it does

Attitude, two waysA quaternion for your maths, and roll, pitch and yaw for your eyes, both relative to the zero you choose.

Zero anywhereMake this attitude zero, or only this heading. Zeroing works on the quaternion, so angles never jump at ±180°.

Calibrate onceSave the sensor’s calibration to flash. It is restored at every power-up, so the IMU starts calibrated.

Happy next to motorsThe default IMU fusion mode ignores the magnetometer, so stepper magnets and steel cannot pull the heading.

Fixes itselfIf the sensor stops answering, the firmware frees the I²C bus, resets it and carries on, and reports what happened.

One protocolDiscovery, node IDs, parameters, Save, telemetry and fault events work exactly as on the Tercio S1.

How it works

1. 01
   SenseThe BNO055 fuses accelerometer, gyroscope and magnetometer on-chip, 100 times a second.
2. 02
   ReadThe STM32 reads it over I²C every 10 ms, without ever blocking the CAN side.
3. 03
   ZeroThe attitude is expressed relative to your zero: q_ref⁻¹ · q, then roll, pitch and yaw.
4. 04
   StreamTelemetry goes out on 0x200 + node at your chosen rate; faults go out at once.

Fusion modes

Default

IMUAccelerometer and gyroscope. Roll and pitch are absolute (gravity); yaw is relative to where the IMU started or was zeroed, and drifts slowly. Immune to magnets and steel, which is what you want on a machine full of motors.

Absolute heading

NDOFAdds the magnetometer for an absolute heading that does not drift. Needs a clean magnetic environment and a magnetometer calibration (a slow figure of eight).

Choose under Settings → Fusion mode in Tercio Control, or set parameter 0x70. The change applies at once; Save keeps it.

Calibration

The BNO055 calibrates itself while it moves, and reports how far it got: 0 to 3 for each sensor. The IMU is calibrated when the gyroscope and accelerometer reach 3 (in NDOF mode, the magnetometer and the system too).

1. Gyroscope: keep the IMU still for a few seconds.
2. Accelerometer: hold it still in six orientations, each face pointing up in turn, a few seconds each.
3. Magnetometer (NDOF only): move it slowly in a figure of eight.
4. Save: when the meters are full, select Save calibration in Tercio Control (imu.save_calibration() in Python). The calibration goes to flash and is written back to the sensor at every power-up.

The Not calibrated warning stays on until calibration is complete. The sensor keeps refining a restored calibration as it moves.

Zeroing

	Roll	Pitch	Yaw	Use it for
Zero	0	0	0	A machine whose rest position is not level: its own rest attitude becomes zero.
Zero heading	stays	stays	0	Keep roll and pitch referenced to gravity, but point yaw where your machine faces.
Clear zero	sensor	sensor	sensor	Back to the sensor’s own reference.

A zero applies at once. Save keeps it after a power cycle, like every setting.

Mounting

The sensor chip sits on the underside of the board. If the axes do not match your machine, choose the matching BNO055 placement P0–P7 under Settings → Mounting (parameter 0x71). P1 is the sensor’s default. Then zero in the machine’s rest position.

Acceleration and angular rate are reported in the sensor’s axes, after that mapping. Acceleration includes gravity: lying still and level, about 9.81 m/s² on one axis.

In Tercio Control

The IMU appears under Sensors in the side bar and on the bus overview. Its page has:

AttitudeAn artificial horizon for roll and pitch, a compass for yaw, and the three angles.

Live chartRoll, pitch and yaw over the last 5, 10 or 30 seconds, with freeze.

CalibrationA meter per sensor, the steps to take, and Save calibration.

Settings and diagnosticsFusion mode, mounting, rate and node ID; identity, link quality and faults.

In code

from TercioBridge import Bus, Imu, FusionMode

with Bus("COM5") as bus:
    imu = Imu(bus)                     # node 100
    imu.wait_calibrated()              # blocks until the sensor is calibrated
    imu.save_calibration()             # restored at every power-up
    imu.zero_heading()                 # yaw 0 here, roll and pitch stay level-referenced
    imu.save()                         # keep the zero

    t = imu.telemetry
    print(f"roll {t.roll:.1f}°  pitch {t.pitch:.1f}°  yaw {t.yaw:.1f}°")
    print(t.quaternion, t.acceleration, t.rate, t.calibration)

The Arduino library has the same in C++: Tercio::Imu imu(bus); imu.zero(); imu.telemetry(t);. See Python and Arduino.

Specifications

Sensing

Sensor

Bosch BNO055: accelerometer, gyroscope, magnetometer, on-chip fusion

Fusion rate

100 Hz

Modes

IMU (no magnetometer) or NDOF (absolute heading)

Mounting

Axis mapping P0–P7

Board temperature

NTC on the board, in telemetry

Bus

Bus

CAN-FD, 500 kbit/s + 2.5 Mbit/s, protocol v2

Node ID

1–127, 100 by default

Telemetry

0–100 Hz, 50 Hz by default

Data

Quaternion, roll / pitch / yaw, acceleration, rate, calibration, temperature

Hardware

Processor

STM32G431, Cortex-M4F at 170 MHz

Board

25 × 51 mm

Connectors

JST XH 3-pin and a 2-pin screw terminal at each end

Firmware

Source

Kerol-Dev/Tercio-imu, PlatformIO

Updates

Over SWD with an ST-Link

Version

2.0.0 (protocol v2)

Coming soon

Supply range, connector pinouts and termination details for the IMU board. Until then, wire it into the bus like a driver: CAN-H, CAN-L and GND from the previous node and on to the next.

The frames, commands and parameters are in IMU protocol.