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An Overview of Accelerometer Data on the GO Device

September 2020

Table of contents

About Accelerometers

What is an Accelerometer?

An accelerometer is a micro-electromechanical system (MEMS) that measures proper acceleration. Acceleration may be static (such as gravity pulling at your feet) or dynamic, caused by forces on a vehicle (stepping on the brakes, swerving, or slamming the door). Many consumer electronics include accelerometers, such as cellphones, laptops, fitness trackers, video game controllers, and drones.

TECHNICAL NOTE: Proper acceleration is acceleration that is measured relative to a free-fall observer who is momentarily at rest relative to the object being measured. So, the sensor assumes that it is constantly falling at the same rate as gravity and reports data by how it differs from that assumption. If you dropped an accelerometer from a sufficient height, it would report zero acceleration because it is in free-fall. An accelerometer on a table would report that it is accelerating upward at the rate of gravity because the table is preventing the sensor from falling downwards freely at that same rate. This is why Z-axis acceleration data in MyGeotab is positive.

Acceleration and G-forces

MyGeotab records the rate of acceleration in meters per second squared (m/s2), the international standard unit for acceleration. Gravitational force equivalent (known as g-force or G’s), is another common unit used to measure acceleration. To convert m/s2 to g-force, divide the value by 9.81 m/s2.

For example:

  1. 5.292 m/s2 = 0.54 g or 540 milli-g (mg).
  2. 1 g = 1000 mg = 9.81 m/s2 (the standard value of gravitational acceleration at sea level on Earth).

Accelerometers and the GO Device

What Hardware Supports Accelerometer Data?

All Geotab devices have a built-in accelerometer. The GO7 and GO8 have a higher quality sensor than their predecessors, which allows for better precision. The GO9 has an even higher quality sensor and an added gyroscope to further improve the stability of the accelerometer data.

TECHNICAL NOTE: When a GO device is installed in a new vehicle, the orientation of the device is unknown. The device uses data from the accelerometer, such as gravity and driver behavior, to estimate the initial orientation. However, sustained acceleration (i.e. highway on-ramp, slow banking turn) can be difficult to distinguish from a change in orientation while the vehicle is moving, which can potentially lead to minor errors with the orientation estimate, as well as accelerometer data. Since a gyroscope measures change in orientation over time, it helps to maintain a stable orientation estimate while the vehicle is moving.

Sample Rate

Different GO devices have different sensors. Therefore, they sample data in different ways. The GO device samples the accelerometer faster so it can filter out noise (i.e. vibrations from the engine, suspension, or horn). The filtered data is then reduced to 100 Hz for analysis.

Curve Logic

If you are familiar with Geotab products, you have likely heard the term curve logic before. For a detailed explanation of how the curve logic works, please see this blog post written by CEO Neil Cawse or watch a short video. The acceleration data is logged using the curve logic and uploaded to MyGeotab.

Accelerometer Calibration

The accelerometer must be calibrated before the GO device can use the acceleration data properly because it needs to determine the relative XYZ axes. The calibration requires the GO device to meet the conditions listed below. Usually, this happens after one or two trips on a paved road.

The GO device requires three conditions for the accelerometer to calibrate properly:

  1. Proper Accelerometer data: Events in the XY plane. If the device is loose or continually going over irregular terrain, the Z component (gravity) will not be constant and the accelerometer will not be calibrated.
  2. Acceleration event: A significant movement that triggers an acceleration event.
    1. Event threshold: > 1.5 m/s2
  3. Good GPS Data: Must have valid GPS latch when the acceleration event is detected and must see a GPS event with a similar magnitude as the accelerometer event.
    1. Event threshold: > 1.4 m/s2

Until the device accelerometer is calibrated, the device generates an ‘Accelerometer calibration in progress (more driving required)’ fault. If the device is unable to finish the calibration after multiple trips, it means the device is not securely properly or the road is too irregular.

Collision Detection

The GO device uses the acceleration data to detect a potential collision. The device triggers a collision event when the following events happen:

  1. An impact of 2.5G or greater in the XY plane. The firmware will not use up/down (Z-axis) in this calculation. The formula used to obtain the magnitude is √(X2+Y2), where X is forward/backward and Y is side to side.
  2. A negative acceleration greater in magnitude than 0.9G in the Z direction (roll over).

When the device detects a collision event, it records a high resolution GPS data (second by second), and brake pedal data (GENERIC_BRAKE_PEDAL 1) – if the vehicle supports it. This data is uploaded to MyGeotab.

However, accelerometer data during a collision is still logged using the curve logic with no collision specific logic. The curve logic will detect that there is a change in acceleration and send the appropriate accelerometer points.

The collision accelerometer data will not be available on the base mode, but the collision fault will still be logged.

Reverse Gear Detection

Usually, the GO device reads gear position data through J1708 or J1939 protocols. Some passenger vehicles provide these data differently through OBDII and sometimes the device is unable to decode the information.

In that case, we use accelerometer data to determine if the vehicle is moving forward or backward for the gear position. We integrate the acceleration signal to get a velocity and compare the data to engine road speed and GPS speed for the reverse gear detection.

Accelerometer Disabled Fault

The accelerometer can disable itself for two reasons: excessive data logging or excessive communication failures with the sensor itself. Data logging is considered excessive when it sends more than 500 logs in 10 minutes.

In both cases, the accelerometer can be re-enabled by resetting the device. This resets both counters to zero and allows for normal functionality. Accelerometer data for collision level events is still logged even if the accelerometer is disabled due to excessive logging.

Recording Acceleration Data

Base Mode

The GO device on Base mode does not log acceleration data on MyGeotab. However, the accelerometer still detects harsh acceleration, harsh braking and collision events then creates corresponding faults on MyGeotab.

Pro Mode/ProPlus Mode

The GO device on Pro or ProPlus mode uses the acceleration data for collision detection, reverse gear detection, and harsh driving conditions. The acceleration data is logged on MyGeotab using the curve logic. The curved acceleration data can be seen on the Engine Measurement page under Acceleration forward or braking (X-axis), Acceleration side to side (Y-axis) and Acceleration up down (Z-axis) diagnostics. However, Acceleration up down (Z-axis) data is normally only logged when there are corresponding XY axes data points.