What is an accelerometer and a gyroscope, and how do they differ?

An accelerometer is a sensor that measures acceleration or translational motion. A gyroscope is an angular rate sensor - it measures the rate of angular rotation. While angular accelerometers (devices that measure the rate of change of angular rotation) do exist, they are rarely used. Generally, accelerometers are thought of as devices that measure linear acceleration.

What is an inertial sensor?

Strictly speaking, an inertial sensor is a device that uses inertia to perform a measurement. As a practical matter, when people say "inertial sensor" they are referring to an accelerometer or a gyroscope.

What are the shock ratings or limits for the MEMS devices?

The MEMS elements are typically rated between 2000 g's to 3500 g's of shock. The specific device data sheet should be refered to for the actual absolute maximum limits.

Be aware of the device sensitivity to shock, as the limits can be exceeded during PCB separation, cleaning, dropping, or bumping and a variety of other board processing steps.

What are the moisture absorption specifications or considerations for the iSensor products?

Each of the iSensor LGA products has an MSL rating per JEDEC J-STD-033. Per J-STD-033, all iSensor LGAs are stored and shipped from ADI in trays, which are vacuum packed inside antistatic bags with desiccant bags and moisture indicator strips. The MSL rating is clearly labeled on the outside of the antistatic bag, normally with a white stick with blue text.

It's important to check with your manufacturing resources regarding their MSL handing procedures and verify that it gets appropriate consideration. Always inspect incoming materials to verify packaging per
J-STD-033, including the moisture indicator strip, which can alert inspectors to broken seals and potential exposure to moisture. If the integrity of the packaging is in doubt, use a bake-out per J-STD-033 and manage the devices exposure to moisture prior to solder reflow.

What are some of the mechanical stress considerations when mounting iSensor devices?

Packages with small lead structures can be fractured by any process that causes printed circuit board (PCB) tourque. This flexing, or bending of the PCB, can result in fractures and device separation.

Do not assume that the PCB score and snap processes are benign. It is very difficult to quantify the amount of stress it can place on a single lead. The iSensor LGA evaluation boards use a routing bit and clamping system to greatly reduce stress on the lead structure during PCB separation. The use of underfill systems, such as Hysol 4450, can increase the attachment area underneath a device and help prevent lead fracture.

Where would you use a gyroscope?

Gyroscopes are used when you want to know how fast or how much something is turning.There are many ways to determine rotational rate (optically, magnetically, etc), but a gyroscope is unique in requiring nothing external to make this measurement. Here are a few examples:

In automobile electronic stability control systems the rotation rate of the car is measured by the gyro and compared to what is expected from the wheel speed and steering wheel sensors. If there is a discrepancy between them, differential braking is applied to bring the car back into control.

Optical image stabilization in digital cameras is accomplished by having a gyro measure the unintentional rotational movement of the camera. The image is then steered by moving lens elements such that it remains relatively stationary compared to the image sensor. In this way images remain blur-free even though the camera moved during exposure.

In navigation systems, a gyro's angular rate output is integrated to determine angular heading which in turn is used to determine if you have been turning and by how much. When combined with displacement information you can figure out your position.