What Is an Accelerometer?

Jul 29, 2020

Accelerometers are small axis-based devices that can be affixed to items and indicate orientation. In a smart phone, it’s the accelerometer that auto-rotates the screen and tells your stargazing app where it’s pointing in the sky. In a laptop, the accelerometer senses freefall and parks the hard drive—usually—before the device hits the ground.

In a shipping impact monitor, an accelerometer is integrated into the monitor to detect impacts and vibrations. Impact monitors provide detailed information that helps logistics managers determine whether their cargo was dropped, handled incorrectly or experienced 10 miles of rough roads. By measuring even minuscule changes in velocity—distance divided by time—they can determine the direction and level of impacts and vibrations that occur during transit and storage.

How do Accelerometer Sensors Work?

Accelerometer sensors are small circuits that typically are integrated into other devices. They work in either of two ways. The most common uses the piezoelectric effect. When the microscopic crystals within the sensor are stressed by acceleration, they create voltage. The accelerometer translates the changes in voltage to velocity or impact and orientation.

Piezoelectric accelerometers use either an external amplifier or an internal charge converter to amplify the electric charge and minimize susceptibility to external noise, such as vibrations below pre-determined thresholds. The amplifier accepts a constant current and varies its impedance based on the varying charge in the piezoelectric crystal. Changes in impedance, therefore, indicate vibrations or impacts.

The alternative method detects fluctuations in capacitance—the ability to store an electric charge—between the microstructures adjacent to the device. Changes in orientation or velocity move those structures, changing their ability to hold a charge.

Sensitivity and Measuring Amplitude

When choosing an accelerometer, engineers pay attention to sensitivity. Sensitivity for accelerometers refers to the conversion between vibration and voltage at a certain frequency. Low sensitivity accelerometers are used to measure high amplitude signals. Conversely, high sensitivity accelerometers are used to measure low amplitude signals.

Sensitivity is one of the most important factors in selecting impact indicators or monitors, too. Here, impact indicators—simple go/no-go devices—are factory set for a specific impact threshold. SpotSee’s indicator thresholds range from 5 to 100G.

Recorders, in contrast, are generally programmable. You can set the impact threshold and can filter out high frequency noise which may be appropriate for the application. For instance, the ShockLog 298 records acceleration between 0.1 and 200Gs. Its frequency filters may be set at 10Hz, 40Hz, 50Hz, 90Hz, 120Hz, or 250Hz.

Adding an impact indicator to cargo means that when a package is handled roughly or dropped, the impact indicator catches the mishandling and changes color. With an impact monitor / recorder, the accelerometers can be programmed to register the vibrations or impacts that would be damaging to the cargo. Because some impacts and vibrations are normal, impact records focus on threshold events—impacts and vibrations that can cause damage. The threshold is selected by the shipper based upon the specifications of the items being shipped.

Accidents caused at least one spacecraft to break from its tie-downs and roll during shipping, causing damage that delayed its launch. Countless other events—from packages falling, to impacts caused by rough freight car couplings—have caused noticeable, and not so noticeable, damage. Impact indicators or monitors document the event, the impact level, and, depending on the device, the direction and duration of impact.

When cargo—whether small packages or huge industrial equipment—experiences impacts above the selected thresholds, the device triggers an alert. SpotSee indicators, for example, turn red, providing a visual signal that tells receivers to check the cargo thoroughly.

SpotSee monitors, however, provide more detailed information. SpotBot BLE, for example, records 70,000 separate events and has a two-year battery life. Other products, including ShockLog Cellular and ShockLog Satellite, send an unlimited number of impact events to the cloud.

Data can be downloaded directly from SpotSee impact monitors, from the cloud as well, so you can see the conditions your cargo experiences.
Accelerometers are the basis for monitoring modern supply chains, providing detailed impact and vibration data needed to protect current and future shipments.

To learn more, contact SpotSee.