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Disclaimer: This page was created by G. Nash and S. Kibler.
All opinions expressed here are those of their author(s) and not of D. Raskovic
WIRELESS TILT SENSOR
George Nash and Steven Kibler
The purpose of the wireless tilt sensor is to obtain the orientation (tilt) information of a handheld device, and send the data to a computer, via a receiver base station, for visual display and data logging. Two separate programs show the data received from the tilt sensor, one that virtually represents the box, mimicking the movements of the box, and the other is a plotting program that plots in real time the acceleration data from the device.
The Handheld Device
The handheld device is 3 inches square, and contains an Analog Devices’ 3-axis, ±3g accelerometer, a Texas Instruments MSP430F149 microprocessor, and a Nordic Semiconductor’s nRF24L01 transceivers, transmitting 2 Mbps at 2.4 GHz. The accelerometer outputs its data as an analog voltage, which is read by the MSP430’s onboard 12-bit analog-to-digital converter. The 12-bit value for each axis is read 50 times a second.
Figure 1 - Wireless Device w/o Processor
Figure 2 - Wireless Device with Processor
Notice in figure 1 that the tilt sensor (accelerometer) is located in the exact center of the device, mounted underneath the processor breakout board. This was done to minimize the size of the board as well as to eliminate errors in tilt due to centrifugal force.
Base Station
The base station has the simple duty of gathering the data from the receiving radio, and sending it along the USB wires to the computer for display. No data processing occurs in the base station’s microprocessor, it is only acting as a wireless-to-wire converter. In Figure 3 below, starting left to right, the components are the USB to UART Bridge, then the MSP430F149 microprocessor, and then, seen edge on, the nRF24L01 radio transceiver.
Figure 3 - Base Station
Computer Interface
The idea behind the computer interface is to show visually what the tilt sensor is “seeing”. This was accomplished in two separate programs. The first displays a virtual box that moves in relation to the wireless tilt sensor. Different faces of the virtual box will face forward when the corresponding axis on the tilt sensor is either pointing up or pointing down. In the next version of our device, we will include a rate sensor (gyroscope) to include “twisting” information. With this information, we will be able to make the virtual box mimic the real box, their movements in exact synchronization. This program uses an averaging function to reduce the “wiggles and jiggles” associated with handheld devices. A screenshot of this program is shown here in Figure 4.
Figure 4 - Virtual Box Tilt Interface
The other program displays real-time information from each of the axes. Rather than just displaying the tilt information, this program also displays the movement horizontally and vertically of the handheld device, up to + 3.3 g and down to -3.3 g. This allows the user to see all of the movement of the device, including all the “wiggles and jiggles”. Figure 5 shows a screen shot taken during the operation of this program. Note in figure 5, at the beginning of the program, only tilt information was being obtained, resulting in data between 1g and -1g. At about 7.5 seconds, movement along the Z-axis was obtained, followed at 8.5 seconds by movement of the Y-axis, followed at 9 seconds by movement of the X-axis.
Figure 5 - Real Time Interface
The Team
Figure 6 - George Nash (Holding the Base Station) and Steven Kibler (Holding The Wireless Device)
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