Hardware Design




Our hardware design was quite simple.  It consisted of two KXM52 tri-axis accelerometers from Kionix, mounted on a custom proto-board, with long wires connected to the prototype board designed for this course.  Appendix B contains the full project schematic.



KXM52 Accelerometer


The KXM52 is a three-dimensional (tri-axis) accelerometer that measures accelerations in the range of +/- 2g with a linear relation between acceleration and output voltage of the chip.  The chip is composed of three differential-capacitive inertial sensors that have a linear relationship with gravity.  The sensor output is then amplified with an output resistance of 32K [k1].  Thus, the output can be bandwidth-limited by placing a capacitor to ground on the output.  The functional diagram is shown below in figure (7).


Figure (7): KXM52 Functional Diagram



Power supply voltages can range from 2.5 to 5.5 volts, so the Vcc rail (5V) on the prototype board was chosen as the power supply voltage.  The bandwidth of the output signal is determined by equation (9) below,


Equation (9): Output Capacitance as a Function of Bandwidth [k2]


Where Ci is the ith output capacitance value.  The bandwidth has an upper limit of 3KHz for X and Y sensors and 1.5KHz for the Z sensor.


Shown below in figure (8) is a typical application circuit.


Figure (8): KXM52 Application Circuit [k2]


A .1uF decoupling capacitor is used between Vcc and ground [k2].


The KXM52 comes in a Dual Flat No-lead (DFN) package, which is impossible to solder without highly sophisticated and expensive soldering equipment.  Luckily, a student sample kit is available for free, which contains 2 stand-alone chips and 1 chip mounted onto a custom PCB board.  We ordered several of these kits for free and included the demo board for our application.  The evaluation board is shown below in figure (9).


Figure (9): KXM52 Student Evaluation Board [k3]



C1 through C4 are all chosen to be .1uF on the evaluation board, giving an effective bandwidth of approximately 50 Hz on each channel.  Thus, the system should accurately reconstruct “slow” (human-speed) motions effectively.


We connected stranded wire (for greater flexibility) to pins 1,3,6,7 and 8, which are in turn connected to a pin header on the main prototype board.  Pin2 (power shutdown) was connected directly to Vcc to enable operation at all times.


The KXM52 has a linear output curve based on the measured acceleration.  Equation  (10) demonstrates this relationship.


Equation (10): Voltage Output As a Function of Acceleration [k2]


Notice that the zero-g point is centered at half of Vcc.  Accelerations in either direction either low the voltage (to ground, indicating –2g) or raise the voltage (to Vcc, indicating +2g).  The zero-g point has high stability, fluctuating within 5% for a reasonable temperature (-40 to 85 Celsius).



Human Interface


The KXM52 accelerometer is very sensitive and can pick up small movements (shaking) of the chip if it is not attached to a rigid body.  Thus, to ensure maximum accuracy and stability, the evaluation board must be attached firmly to the human body.  We used a Velcro strap attached to the board to secure the board to the body.  Also, the wires (stranded) that were attached to the board were bundled together.  The wire-board junction was taped down using electrical tape.  An accelerometer board is shown below in figure (10).


Figure (10): Finished Tracking Sensor


Another concern of attaching the board directly to the body is safety.  There are voltages on the chip, which may come into contact with the body.  We have went through the precaution of isolating the wire connections from the body by wrapping the connections in electrical tape.  There is no concern of getting a shock from the evaluation board itself.


For best results, the tracking sensors should be placed on the top of the limb (when the limb is only in the y-axis).



Prototype Board


We used the PCB board designed by Professor Land to prototype this design.  The RS-232 interface was installed along with a power on-off switch.  We used the Mega32 with a 16MHz crystal and a diode attached to pin D.7 for visual feedback.  Figure (11) shows our PCB board.


Figure (11): PCB Prototype Board


The Power switch is a simple 3-pin switch, where in the left position it connects the left and center pins together and in the right position, it connects the right and center pins together.  The left pin was removed and the right and center pins were connected across the Vcc (Power)  plug power pins.  The result is no power to the system in the left position and power on in the right position.


We created a custom header for the KXM52 accelerometer boards.  This made prototyping much easier as a sensor had to only be plugged in, with no worry of pulling out wires.  The pinout and header diagram are shown below in Figure (12).


Figure (12): Custom KXM52 Accelerometer Header


Where pin1 faces the right side of the board in Figure (11).  Pin 1 is marked on both the board and the plug.


Two accelerometers are used in this project.  The connections are shown below in Table (1).


Table (1): Accelerometer Connections

Pin #


to (point1)

to (point2)