Accelerometers

• single-axis or dual-axis
• analog or digital output
• dynamic and/or static (tilt) measurements

Theory of Operation (ADXL family and others):

• micro-machines mass-spring system on top of silicon wafer

  

• acceleration is proportional to displacement of the mass: a = k x / m
• uses differential capacitor setup, which is proportional to x for small displacements

  

ADXL202: dual-axis, digital output

• duty-cycle output

  

• complete cycle-period set with a resistor (R_set)
• tilt sensing:

  

• bandwidth adjustment using capacitors (C_x, C_y)
• voltage outputs also available

Interfacing ADXL202 to the BASIC Stamp:

• Connect X_out and Y_out directly to I/O pins of the BASIC Stamp.
• Use the PULSIN routine to measure (count) the duty-cycle pulse width.

PULSIN Pin, State, Variable

where:

• Pin is I/O pin the circuit is connected to.
• State specifies whether the pulse state to measure is low (0) or high (1). A low pulse begins with a 1-to-0 transition and vice-versa.
• Variable is a variable (usually a word) in which the pulse duration measurement is stored.

PULSIN is like a stopwatch which is triggered by a change in state on the input pin. The unit of measured time depends on the Stamp type being used (BS2/BS2e: 2 microseconds; BS2sx: 0.8 microseconds).

'{ $STAMP BS2 } accx VAR WORD accy VAR WORD loop: PULSIN 12, 1, accx 'read high state on pin 12 DEBUG home, ? accx PULSIN 13, 1, accy 'read high state on pin 13 DEBUG ? accy PAUSE 100 GOTO loop