Simpliciti Acoustic Event Sensor - MSP430

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by N.Z.Kolev, Varna, Bulgaria

The main goal is the development of a network acoustic sensor suitable for detection of sound pulses in the time domain with the SimpliciTI network as a backbone. A microphone preamplifier was designed and fabricated with TINA electronic workbench for use with EZ430-RF2500. On the basis of SimpliciTI example project “AP as a Data Hub” an application software was developed for distributed detection of acoustic pulses in star configuration of the network with relative to the AP time. An experimental setup was developed with Labview control and VI’s for UART and sound card stimulus. The data is recorded on a text file for later use. The deveoped experimental system was tested functioning with one AP and up to four ED.

Design of a Microphone PreAmplifier

The idea of the design of the microphone preamplifier is to increase the overall gain and sensitivity of the device to serve for more specific sound processing tasks than the used in the simple glass breakage detector in [3]. The configuration of the input stage is given on Fig.1:

EZ430-RF2500 Microphone Preamplifier TINA Layout.png

An aditional transistor input stage was set in order to increase the total gain of the amplifier and also as a buffer between EZ430 linear stages (even though that the electret microphone has a FET transisor - mainly for coupling with the highly capacitive microphone). The choice of opportunity was BC847 – a SMD transistor with good parameters. The final schematic was analized with TINA - Version 7.0.30.267 SF-TI. The AC transfer characteristic is shown on Fig. 2.

AC Transfer.png

The average AC gain of the adaptive passband preamplifier in the used microphone audio range according to fig. is 350. The OA0 is used as an inverting PGA with set gain of 7. The software configuration is as follows:

  • OA0CTL0 = OAN_1 + OAPM_3 + OAADC0 - Inverting input is OA0I1, fast mode,

OA1OUT connected to external pins.

  • OA0CTL1=OAFBR_6 + OAFC_6 + OANEXT - Inverting PGA with gain = 7, OA0

inverting input externaly available

The OA1 is used as a two pole Sallen – Key low-pass filter. The pass band may be controlled with C5,C6, R8, R9. For our case 10K resistors and 1n capacitors were used assuring pass band to 10K. In the simulations later the frequency of the sampled and analized pulses is 2 KHz. The software configuration of OA1 is as follows:

  • OA1CTL0 = OAP_0 + OAPM_3 + OAADC0 - noninverting input is OA1I0, fast

mode, OA1OUT connected to external pins and ADC input A1, A3, or A5.

  • OA1CTL1 = OAFC_2 – unity gain buffer

The board was designed with EAGLE 5.6.0 Light.

Fig.4. The EZ430-RF2500 microphone preamplifier EAGLE schematic. Fig.5. The EAGLE layout of EZ430-RF2500 microphone preamplifier – board size is under 1 inch - 0.9x0.9.

Acoustic Event Sensor Software API

The AP receives a packet from LABVIEW . After that it sends broadcast message to all ED. This message contans a time stamp of the AP clock. The ED’s start ADC sampling of the acoustic microphone channel after receiving the AP broadcast message. The acoustic event is detected by meeting two criteria. The first one is when the maximum of the signal in a given frame with respect to the average value exceeds the set threshold. The second one is when the number of signal zero crossings is in a given range. The first theshold depends on the sound intensity level. The second one is dependant of the sound frequncy of interrest ( the signal is low pass filtered in the analog circuitry). A block diagram of the software API is given on fig. 6. SIMPLICITI ACOUSTIC EVENT SENSOR API VISA UART WRITE ACOUSTIC EVENT TIME VISA UART READ HIST BUFFER/ MOVING WINDOW DATA AVERAGING TEXT FILE DATA LOGGER UART/PKT TX ACOUSTIC EVENT TIME/ED NR RX SIMPLICITI SOUND SIGNAL ADC SAMPLING, PROCESSING AND AE TIME FRAME ESTIMATION AP/BCAST RX START LABVIEW/PC TIME STAMP BROADCAST FOR ED AP SOUND PULSE GENERATION UART/PKT RX ACOUSTIC EVENT TIME/ED NR TX EZ430-RF2500 ED Fig.6.

Conclusions

With the developed SimpliciTI Acoustic Event Sensor system several experiments were carried out. There are time offsets in the ED clocks caused by voltage and temperature differences. The main reason however is in the principle of interrupt driven packet transmission. It is possible to achive enough accuracy for some applications with moving window time averaging of the ED time data. The design is new and is subject still to software optimization. Further more elaborate time synchronization may be used. The schematics, board application designs and full CCS projects are included in the accompanied zip folder.

References

1. Larry Friedman, SimpliciTI Developers Notes, Texas Instruments 2009. 2. SimpliciTI Sample Application User’s Guide Document Number: SWRA243 Texas Instruments,2009. 3. Kammel Robin, Venkat Kripasagar A Simple Glass Breakage Detector Using the MSP430, SLAA351. Fig.7. A Labview VI of the main control loop for the experimental setup