EXPERIMENTING WITH TILT AND GRAVITY SENSORS in Software

Generation ECC200 in Software EXPERIMENTING WITH TILT AND GRAVITY SENSORS

lights. As the 150 is tilted horizontally, the output decreases, and the red LED lights instead. You ll need to experiment with the setpoint (I used 825), depending on the actual values provided by your ADXL150 circuit.

' ADXL150 test program ' For use with BasicX-24 microcontroller ' Output of ADXL150 is connected to pin 13 (IO line 7) ' of the BasicX-24 Dim Voltage As Integer, BlinkTotal As Integer Dim Total As Long Const PinNumber As Byte = 13 Const GreenLED As Byte = 26 Const RedLED As Byte = 25 Const LEDon As Byte = 0 Const LEDoff As Byte = 1 Sub Main() Dim x as Byte Total = 0 Do For x = 1 to 254 Voltage = GetADC (PinNumber) Total = Total + CLng(Voltage) Next Total = Total = Total \ 254 BlinkTotal = CInt(Total) Call LEDs Call Delay(0.1) Loop End Sub Sub LEDs() If BlinkTotal > 825 Then Call PutPin(RedLED, LEDoff) Call PutPin(GreenLED, LEDon) Else Call PutPin(GreenLED, LEDoff) Call PutPin(RedLED, LEDon) End If End Sub

The ADXL150 single-axis accelerometer, described in detail in the last section, has a close sibling: the ADXL250, which is like two accelerometers in one. The ADXL250 is a dualaxis device in which the axes are oriented at right angles to one another. When the accelerometer is positioned horizontally, it can therefore detect motion in 360 (it cannot detect up and down motion when in this position, however). Using the ADXL250 is very similar to using the ADXL150 you just duplicate the interface electronics for the second axis. Refer to the data sheet for the ADXL250 on the pinout diagram for the device.

Analog Devices makes a less expensive line of accelerometers that is specifically designed for consumer products. Their ADXL202 is a dual-axis device with a 2-g sensitivity (if you need more g s, check out the ADXL210, which is rated at 10 g s). Besides being cheaper than the ADXL150/250, the 202 has a simplified output: instead of a linear voltage, the output is purely digital. As acceleration changes, the timing of the pulses at the output of the ADXL202 changes. This change can be readily determined by a PC or microcontroller, using simple software (see the example for the Basic Stamp later in the chapter). No op amp or scaling adjustment components are necessary, which makes the ADXL202 a breeze to use. Like the ADXL150, the 202 is a surface-mount component. See the discussion in the section on the ADXL150 about alternative ways to interface the 202 electronics in your robot. By a long measure, the ready-made ADXL202 Evaluation Board is the easiest way to use this device. It comes on a small postage stamp carrier, which can be directly soldered to the Basic Stamp or other microcontroller.

The basic hookup diagram for the ADXL202 is shown in Fig. 41.5. Note that except for two filter capacitors and a single resistor, there are no external components. I have specified a rather low bandwidth of 10 Hz for the device. According to the ADXL202 data sheet, the value of C1 and C2 for this bandwidth should be 0.47 F. Resistor R1 sets the value of the timing pulse used for the output of the X and Y axes of the 202 chip. I have specified a modest timing pulse of 5 milliseconds; according to the data sheet this requires a nominal value of about 625K for R1 (I specified a reasonably close standard resistor value). Note that the exact timing of the pulse is not critical, as any variation will be accounted for in the software. You will want to select a higher or lower timing pulse based on the capabilities of the PC or microcontroller you are using and the resolution you desire.