01 | WHAT THE INERTIAL SENSOR MEASURES

The V5 Inertial Sensor combines gyroscope and accelerometer measurements. For autonomous turning, the most useful starting values are heading, accumulated rotation and angular rate.

It measures turning differently from motor encoders. Wheels may rotate without the robot turning as expected; the inertial sensor observes the chassis’s angular motion.

It is not a field-position sensor or a compass that automatically knows geographic north. A heading reference must be established for your route.

02 | MOUNT AND CONFIGURE IT FIRST

Mount the sensor firmly in the correct orientation for your build and installation guidance. A loose mount or a changing sensor orientation invalidates your interpretation of its readings.

inertial_sensor = Inertial(Ports.PORT20)

Port 20 is an example. Record the actual Smart Port and confirm the cable connection. Use either this manual declaration or a Devices-panel definition, not both.

The diagnostic on this page contains no motor commands. Keep the robot disabled while moving it by hand for sensor checks.

03 | CALIBRATION AND ZEROING ARE DIFFERENT

Keep the whole robot still during calibration. Moving, vibrating or tilting the sensor can affect the result.

Only after calibration should you establish the route’s reference. Place the robot facing the chosen starting direction and set zero deliberately. Resetting numbers cannot repair a bad calibration.

04 | WAIT CORRECTLY AND HANDLE FAILURE

Check installed() before trusting readings. A false is_calibrating result alone does not establish that a sensor is connected or functioning.

The current VEX documentation describes calibrate() as waiting for completion. Behaviour and available options can vary with the installed runtime, so check its Help. A loop after a blocking call cannot enforce a deadline on that call itself.

The complete diagnostic starts calibration in a worker thread and bounds the supervisor’s wait. If that wait expires, it refuses to use the sensor. The worker may still be calibrating: the deadline does not cancel the sensor’s physical calibration.

Do not repeatedly launch new calibration workers or automatically enable movement after a late completion. Investigate the connection and environment, then restart deliberately.

05 | HEADING WRAPS AT ONE REVOLUTION

Heading is a direction from 0° up to just below 360°. After passing 359.99° in the right-positive direction, it wraps to zero.

A heading tells you which direction the robot faces now. It does not tell you how many complete turns it made to get there.

06 | ROTATION KEEPS THE ACCUMULATED TURN

Rotation can increase beyond 360° or decrease below zero. With the standard right-positive convention, right turns add and left turns subtract.

Use heading to express an orientation target. Use rotation differences to measure a relative turn, particularly when it crosses zero or includes multiple revolutions.

If you change either reference, record that reset. Check both reported values after reference-setting calls rather than assuming old readings remain comparable.

07 | RAW SUBTRACTION FAILS AT WRAPAROUND

Suppose the robot faces 350° and should face 10°. Raw target minus current gives −340°, but the short path is 20° right.

def heading_error(target_degrees, current_degrees):
    return (target_degrees - current_degrees + 180) % 360 - 180

This wraps the difference into the interval from −180° up to below +180°. Positive values mean right and negative values mean left under the convention used here.

The −180° tie is a deliberate mathematical choice in this helper, not a promise about which direction a VEX heading command chooses. Use an explicit relative LEFT or RIGHT turn if that direction matters.

08 | TRY A HEADING ERROR

This calculator shows the angle arithmetic only. It does not communicate with a robot.

09 | MEASURE A RELATIVE TURN

start_rotation = inertial_sensor.rotation(DEGREES)

# Perform the intended turn using your tested movement controller.

change = inertial_sensor.rotation(DEGREES) - start_rotation

A change from 350° accumulated rotation to 440° is +90°, even though the corresponding heading crosses zero. There is no need to reset rotation before each movement.

Capture the starting value once. Do not subtract each loop’s reading from a continuously updated baseline if your goal is the total turn from the action’s start.

10 | COMPLETE SENSOR-ONLY DIAGNOSTIC

Keep generated imports and Brain setup above this block. Paste below # Begin project code, outside the generated region. Replace any existing movement program for this diagnostic test.

There are no motor declarations or motor commands. The sole configured device is the inertial sensor on the example port 20. Use a stationary disabled robot for startup, then rotate it by hand only after readings appear.

# =============================================================================
# TEAM [MY TEAM NAME / NUMBER] | INERTIAL DIAGNOSTIC
# =============================================================================
# SENSOR CONFIGURATION
# - V5 Inertial Sensor: Smart Port 20 (change to your actual port)
# - NO motor declarations or movement commands in this diagnostic.
# Keep generated imports and Brain setup above this user code.

inertial_sensor = Inertial(Ports.PORT20)
CALIBRATION_WAIT_MS = 5000
REPORT_MS = 200
calibration_done = False
calibration_error = False
sensor_ready = False

def heading_error(target_degrees, current_degrees):
    # Positive means right with the standard right-positive convention.
    # Exact 180-degree ties deliberately return -180 (left).
    return (target_degrees - current_degrees + 180) % 360 - 180

def show_status(message):
    brain.screen.clear_screen()
    brain.screen.set_cursor(1, 1)
    brain.screen.print(message)

def calibration_worker():
    global calibration_done, calibration_error
    try:
        inertial_sensor.calibrate()
    except Exception:
        calibration_error = True
    finally:
        calibration_done = True

def prepare_sensor():
    global sensor_ready
    sensor_ready = False
    if not inertial_sensor.installed():
        show_status("Sensor not installed")
        return False
    show_status("Keep robot still: calibrating")
    # Some API versions wait inside calibrate(). The supervisor can still time out.
    worker = Thread(calibration_worker)
    start_ms = brain.timer.time(MSEC)
    while not calibration_done:
        if brain.timer.time(MSEC) - start_ms >= CALIBRATION_WAIT_MS:
            show_status("Calibration wait timed out")
            # Do not read or start motors. The worker may still be calibrating.
            # Restart deliberately after investigating; do not launch another worker.
            return False
        wait(20, MSEC)
    if (calibration_error or not inertial_sensor.installed()
            or inertial_sensor.is_calibrating()):
        show_status("Calibration not ready")
        return False
    # Face the intended reference direction before starting this project.
    inertial_sensor.set_heading(0, DEGREES)
    inertial_sensor.set_rotation(0, DEGREES)
    sensor_ready = True
    return True

# Run this diagnostic while disabled, then rotate the robot by hand after ready.
if prepare_sensor():
    while sensor_ready:
        if not inertial_sensor.installed() or inertial_sensor.is_calibrating():
            sensor_ready = False
            show_status("Sensor unavailable")
            break
        heading = inertial_sensor.heading(DEGREES)
        rotation = inertial_sensor.rotation(DEGREES)
        error_to_zero = heading_error(0, heading)
        brain.screen.clear_screen()
        brain.screen.set_cursor(1, 1)
        brain.screen.print("Heading: " + str(round(heading, 1)))
        brain.screen.set_cursor(2, 1)
        brain.screen.print("Rotation: " + str(round(rotation, 1)))
        brain.screen.set_cursor(3, 1)
        brain.screen.print("Error to 0: " + str(round(error_to_zero, 1)))
        print("INERTIAL", heading, rotation, error_to_zero)
        wait(REPORT_MS, MSEC)

# Keep the main task alive after a failure; no automatic calibration retry.
while True:
    wait(100, MSEC)

Read and compare heading, rotation and the wrapped error to zero. Make a slow quarter-turn, cross the zero boundary, then try a full revolution.

The worker thread only calibrates the sensor. It does not own motors. The program never retries calibration automatically or begins a driving routine.

11 | CHECK DRIFT AND PHYSICAL AGREEMENT

1. Calibrate with the robot still, then leave it untouched for a short observation interval.
2. Record initial and final readings to inspect stationary drift.
3. Rotate through a physically marked quarter-turn and compare the reported change.
4. Cross the 0° boundary slowly and confirm heading wraps while rotation remains continuous.
5. Return to the starting orientation and compare the physical reference with the readings.

Do not expect perfectly constant readings forever. Log the observed variation, calibration conditions and mounting. Recalibrate deliberately when needed rather than repeatedly zeroing drift away during a route.

If signs are reversed, inspect mounting and configured turn-direction conventions. Confirm the sign physically before applying it to a controller.

12 | USE THE SENSOR IN A MOVEMENT PROJECT

After the diagnostic passes, integrate one sensor instance into the SmartDrive configuration and use one agreed reference for the autonomous route.

A standalone diagnostic does not need a drivetrain. In a SmartDrive project, check the relationship between sensor and drivetrain references, and set the reference through the intended API consistently.

Calibration should finish before enabled movement. Keep the setup-ready gate, movement deadlines, cancellation and outcome checks from the earlier guides. Validate installed status and sensible readings as part of startup.

Heading can support turn correction, but it does not verify translation. Avoid estimating reliable field position simply by integrating accelerometer readings; small biases accumulate into large errors.

13 | COMMON MISTAKES

• Moving during calibration: the measurement baseline may be unreliable.
• Confusing calibration with zeroing: changing a reference is not sensor calibration.
• Using raw heading subtraction: wraparound creates a misleading large error.
• Using heading for a multi-turn target: heading loses the revolution count.
• Resetting every loop: the route’s reference keeps changing.
• Assuming a wait timeout cancels calibration: the sensor operation may still be running.
• Trusting a disconnected sensor: check installation and physical response.

CHALLENGES | PROVE YOUR ANGLE REFERENCES

1. Explain: distinguish calibration, heading zero and rotation zero.
2. Configure: record sensor port and mounting orientation.
3. Predict: compare heading and rotation after right 450° from a shared zero.
4. Calculate: find shortest heading error from 355° to 5° and from 5° to 355°.
5. Observe: cross zero and record heading and rotation together.
6. Measure: record drift over a fixed stationary interval.
7. Validate: compare a marked physical quarter-turn with the sensor’s change.
8. Integrate: describe the checks required before permitting an autonomous turn.

OFFICIAL REFERENCES

• VEX inertial calibration, heading and rotation API
• VEX worker threads

This is an independent learning resource; VEX and VEXcode are trademarks of their respective owners.