Cati Vaucelle
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HARDWARE - by Cati Vaucelle
The hardware deals with the inputs into the software: gesture detection with accelerometers; circuit design for the acceleremoters; and wireless communication to the computer.
PIEZO VIBRATION SENSOR
CHARGED AMPLIFIER
I use a Piezo vibration sensor, a PVDF (polyvinylidene fluoride). Because piezoelectric materials create electric charge when mechanically stressed, in Picture This I use the Piezo sensor as an accelerometer.
In the piezo vibration sensor, crystal structures are stressed which generate voltage that can be converted to acceleration. I am using the MiniSense 100, a vibration sensor loaded by a mass to offer high sensitivity at low frequencies. Impacts containing high frequency components will excite the resonance frequency (100hz).
I distinguish between vertical and horizontal motions while my sensor is just a single axis accelerometer (detects mechanical stress).
To do this trick, I detect small variations of the off-axis motion with the on-axis accelerometer and I am able to categorize strong motion in one axis and weak motion in the orthogonal axis (see the software section of the Picture This! website for filtering algorithm).
To amplify the signal from the Piezo sensor, I used a charged amplifier (see circuit diagram below) instead of a voltage amplifier. For a cut-off frequency 1/(2πRC) at approximately 30hz I have Cf=6.8uF and R=10k.
The advantage of using a charge amplifier is to minimize charge leakage through the stray capacitance around the sensor. For the future iteration I will use surface mounted components in my circuit design to only need a voltage amplifier, that exhibits less temperature dependence.
The output voltage will depend on feedback capacitance not the input capacitance. The advantage of using a charged amplifier in my design is that it allows me to use a long cable between the Piezo film sensor (in the bag of the doll) and the electronics; this technique is used for the first prototype of Picture This!. For the second prototype, the data are sent via wireless communication (see next section).
Reference about Piezo sensors see:
MSI Piezo Guide, Piezo Film Sensors, technical manual, Measurement Specialties, Inc. www.msiusa.com
Link: http://www.meas-spec.com/myMeas/sensors/piezo.asp
Let's look at other hardware methods to detect gestures in toys and wearables:
- Tilt mechanical switch: ON/OFF.
- Capacitive sensing: measuring the distance between an object and its reference point. Change in capacitance related to acceleration.
- Piezo resistive effect: resistance changes with acceleration.
- Hall effect: motion converted to an electrical signal detecting change in magnetic fields.
- Heat transfer: location of heated mass tracked during acceleration by sensing temperature.
- Magnetoresistive: material resistivity changes in presence of magnetic field.
Funnel board + XBee wireless modules.
Each toy carries on a XBee wireless module attached to its funnel board and battery. The piezo sensor is plugged into one of the pin of the funnel board. By default the XBee baud rate is at 9600 that I synchronize on the software end.
Some advice for novice users of XBee:
Step1. Download the X-CTU app on windows and update the firmware for all XBees (important because the firmware is not up to date)
Step2. Then plug the XBee in the XBee config tool, select port, coordinator (computer side), bring in a PANID, ex. 335.
Step3. Do the same with the other XBee, that will be the end device (funnel side).
Step4. The pairing process is done via XBee config tool (same ID).
The rest is pretty straightforward.