Calibration and testing

Why this matters

A flight controller knows the world only through its sensors. If the accelerometer believes level is tilted by 3°, the drone drifts one way all the time. If the compass is disturbed, the drone may circle in a growing spiral (“toilet bowling”). And if the battery monitor reads the voltage wrongly, the failsafe triggers too late and the battery runs out in the air. Calibration teaches every sensor to read the truth; a test flight and its log provide the evidence that it worked.

The calibration sequence

The order matters, because later steps depend on earlier ones. If the board orientation is wrong, for example, the whole accelerometer calibration will be wrong too.

1. Firmware version and frame type: always record the version, because parameter names and procedures change between versions
2. Board orientation: tells the system which way the board faces relative to the airframe
3. Accelerometer, six positions: hold the aircraft still in six attitudes (level, left side, right side, nose down, nose up and upside down) to find each axis’s offset and scale
4. Level: defines the aircraft’s level attitude. The ArduPilot documentation notes it can compensate for a board mounted up to about 10° off level
5. Compass: rotate the aircraft through every direction, outdoors and away from steel structures and cars. Recalibrate after installing new equipment near the compass
6. Radio: tells the system the end points and centre of each channel
7. ESCs and motor test: PWM ESCs need throttle-range calibration; DShot and DroneCAN ESCs do not. Then run motor test one motor at a time with the propellers removed to confirm order and direction against the frame’s motor map
8. Battery monitor and failsafe (next sections)
9. Pre-arm: the system checks itself before allowing arming. All checks must pass; do not disable checks just to be able to fly

PX4 uses a similar standard sequence through QGroundControl: sensor orientation, compass, gyroscope, accelerometer and level horizon.

Calibrating the battery monitor

The power module reports voltage and current through multipliers in the parameters. If a multiplier is wrong, every decision based on voltage or remaining energy is wrong too. To check, measure the real voltage with a multimeter, and compare the charge used according to the log with the charge the charger puts back.

old_volt_mult = 10.10
reported_v, meter_v = 22.05, 22.41      # system reading and multimeter reading, taken together
new_volt_mult = old_volt_mult * meter_v / reported_v
print(f"new voltage multiplier {new_volt_mult:.3f}")

logged_mah, charger_mah = 3620, 3980    # charge used according to the log, and charge put back by the charger
print(f"current reading low by {1 - logged_mah / charger_mah:.1%}; scale current by {charger_mah / logged_mah:.3f}")

new voltage multiplier 10.265
current reading low by 9.0%; scale current by 1.099

Battery failsafe

A failsafe is an automatic action when something goes wrong, such as returning home or landing when the battery is low.

• ArduPilot has two levels, low and critical, each settable by voltage and by remaining charge. The default low voltage is 10.5 V, which suits a 3-cell battery, so a 6-cell battery must be configured. By default the failsafe triggers only when voltage stays below the threshold for 10 seconds, so a momentary sag during a climb does not trigger it falsely, and the action can be chosen, for example land or return
• PX4 estimates remaining charge from voltage per cell, treating 3.6 V per cell under load as “empty” and 4.05 V as “full” by default, with thresholds of 15% for low, 7% for critical and 5% for emergency

Suitable values depend on the battery, the mission and the distance home. Set them from testing and record the reasons; do not accept defaults unchecked.

Testing vibration from the log

Excessive vibration corrupts accelerometer readings, and the position estimator may compute the wrong altitude, even making the drone climb on its own. ArduPilot logs a VIBE message with the vibration level of each axis (m/s²) and a cumulative count of how many times an accelerometer hit the limit of its measuring range (clipping).

The ArduPilot documentation says levels below 30 m/s² are normally acceptable, levels above 60 m/s² nearly always cause problems, and clipping should be zero (increases only during hard landings may not be a problem). PX4 does not give numeric thresholds like these but recommends inspecting the vibration plots in Flight Review.

import pandas as pd

vibe = pd.read_csv("vibe_log.csv")
axes = ["VibeX", "VibeY", "VibeZ"]
for flight, g in vibe.groupby("flight", sort=False):
    worst = g[axes].max(axis=1)
    new_clips = g["Clip"].iloc[-1] - g["Clip"].iloc[0]
    means = ", ".join(f"{a[-1]} {g[a].mean():.1f}" for a in axes)
    print(f"{flight:<7} mean {means} | over 30: {(worst > 30).mean():.0%}, "
          f"over 60: {(worst > 60).mean():.1%}, new clips {new_clips}")

before  mean X 24.7, Y 26.8, Z 42.0 | over 30: 99%, over 60: 2.8%, new clips 17
after   mean X 11.0, Y 12.0, Z 17.1 | over 30: 0%, over 60: 0.0%, new clips 0

Module lab

Common mistakes

Summary

• Calibrate in order, because later steps depend on earlier ones, and always record the firmware version
• PWM ESCs need range calibration; DShot and DroneCAN do not. Always remove propellers for motor tests
• The voltage multiplier is adjusted by the ratio of meter reading to system reading
• ArduPilot’s vibration guidance: below 30 m/s² acceptable, above 60 m/s² problems likely, and clipping should be zero
• The battery failsafe must match the cell count and the mission

Check your understanding

1. The voltage multiplier is 10.0, the system reads 20.0 V and a meter reads 20.5 V. What is the new multiplier?
2. Mean Z-axis vibration is 45 m/s². Which range is that in, by ArduPilot’s guidance?
3. Which kinds of ESC do not need throttle-range calibration?
4. Why does the low-voltage failsafe wait for voltage to stay low for a while before acting?
5. The log shows 3000 mAh used but the charger puts back 3300 mAh. By about what percentage does the system under-read current?