Inertial Sensor (IMU)

The Inertial Sensor is a combination of an Accelerometer and Gyroscope.

The housing of this sensor has a single mounting hole which allows it to be easily mounted to the robot’s structure. In addition, there is a small indentation in front of the mounting hole which marks the sensor’s reference point. On the bottom of the housing, there is a round boss which is sized to be inserted into a square hole of a piece of structural metal. This will keep the sensor fixed to its attachment point. On the back of the sensor housing is a V5 Smart Port.

Both the accelerometer portion and the gyroscope portion of this sensor produce a smart signal feedback to the V5 Brain.

The accelerometer measures how fast the sensor changes its motion (accelerates) along the X-axis, the Y-axis, and/or the Z-axis. These axis are determined by the orientation of the Inertial sensor. For example, one orientation could have a robot’s X-axis as its forward and backward motion, its Y-axis as its side to side motion, and its Z-axis as its up and down motion (such as the robot lifting itself up off the field on a suspension pole).

The accelerometer measures a change in motion when its internal electronics detect a change in inertia and this creates a change in its reading. The faster the change in motion the more the reading changes. Note: This may be a larger positive value or a larger negative value depending on the direction of the motion along the axis.

Acceleration is measured in g’s (unit of gravitational acceleration). The maximum measurement limit for the accelerometer portion of the Inertial Sensor is up to 4g. This is more than enough to measure and control most robot behaviors.

The gyroscope, rather than measuring linear motion along the 3 axis, measures the rotational motion around the 3 axis. The sensor measures this rotation when the internal electronics create a fixed reference point. As the sensor rotates away from this reference point it changes the output signal.

It takes a short period of time for a gyroscope to establish its reference point (calibration). This is commonly called the initialization or startup time. (Note: It is recommended using 2 seconds for a calibration time or start the sensor’s calibration within the pre-auton portion of the competition template. When using the sensor within the VEXcode V5 Blocks/Text drivetrain functions, the calibration is included within the function.)

An electronic gyroscope also has a maximum rate of rotation. That is, if the object the sensor is measuring is spinning faster than the gyroscope can measure its rotation, the sensor will return incorrect readings. The maximum rotation rate for the Inertial Sensor is up to 1000 degrees/second. Once again, this is more than enough to measure and control all but extreme robot behaviors.

Teams Contributed to this Article:

  • BLRS (Purdue SIGBots)

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