VEX Analog Accelerometer V0.5

Connecting the Accelerometer

Each channel used must be connected to an analog input on the VEX Microcontroller using a standard servo extension cable. You don’t have to hook up all the channels; you only need to connect the ones required for your application. The white (signal) wire of each extension cable goes near the ‘X’, ‘Y’, or ‘Z’ silkscreened on the board. The black (ground) wires go at the other end, adjacent to the ‘B’ silkscreened on the board. The center wire is for +5 volts. Also, the mounting holes are electrically isolated from the circuit.

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Acceleration and Gravity

The sensor will measure acceleration in both directions along each of the 3 axis. Acceleration along the X or Y axis in the direction of the silkscreened arrows will produce a larger reading, while acceleration in the opposite direction will produce a smaller reading. For the Z axis, upward acceleration (in the direction of the board’s face) produces larger values, and downward acceleration (toward the board’s back) produces lower values.

Gravity is indistinguishable from upward acceleration, so the sensor will detect a constant 1.0G while at rest. If the board is mounted horizontally, gravity will effect only the Z axis. If the sensor is tilted away from the horizontal, the gravity reading on the Z axis will diminish, and the readings on the other axis will change depending on which way you are tilting it.

Sensitivity Ranges

The accelerometer has four sensitivity ranges, selected by Jumpers 1 and 2. Pin 1 of the Jumper (the pin closest to the “Y” marking) connects directly to the MAA7260 with a 1K pulldown resistor. Pin 2 of the Jumper connects to +3.3 volts. The easy way to remember the jumper settings is to add the values of the installed jumpers; The larger the sum, the larger the range.

Jumpers
Sum
Range
Output (-1g to +1g)
none
0
± 1.5g
1.3 volts to 3.7 volts
1 only
1
± 2.0g
1.6 volts to 3.4 volts
2 only
2
± 4.0g
2.0 volts to 3.0 volts
1 & 2
3
± 6.0g
2.2 volts to 2.8 volts

You can control the Sensitivity Ranges remotely by connecting your control signal to pin 1 of the Jumpers. The Max Input voltage for the Jumpers is +3.3 Volts. For more details, please refer to the data sheet on the Freescale MMA7260QT chip.

History

2008-09-16 – The VEX Analog Accelerometer V1.0 was first described in this forum thread [3].
2009-01-07 – The VEX Analog Accelerometer V0.5 was released, as announced in this forum thread [4]. Notably, this product is the first VEX sensor that is not fully encased in a protective plastic shell.
2009-02-05 – IFI is adding a conformal coating to the top and bottom of the accelerometer for all future shipments. This coating will minimize accidental shorting, improve FOD (foreign object damage) tolerance from metal filings, and reduce ESD susceptibility. IFI has no plans to add covers due to mold costs and the need to continually evolve the product.

External links

Motion-Sensing MEMS Gyros And Accelerometers Are Everywhere, from Electronic Design

ROBOTC Sample Code

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CMU Robotics Academy Resources

easyC Sample Code

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