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Relate Cube - indoor positioning using ultrasonics

Written: December 2009

Relate Cube.

After the Dots came the Cube. The Relate project required a prototype 3D ultrasonic ranging device. We also wanted enhanced embedded processing capability to enable the application of fancy algorithms to test out signal processing algorithms. So I designed and built the Cube. This has a dsPIC128GP706 16 bit digital signal processor (DSP) as an embedded processor. The basic cube consists of four stacked boards with a Teco Particle computer providing the wireless communications - this clips onto the Communications board on top of the device. The figure to the left shows the cased cube with the communication board’s antennae coming out of the top. The programming cable connects to a Microchip ICD2 for debugging. This photo made it into the Faculty Christmas booklet in 2008. The guy in the background isn’t me. It’s some poor sap who signed up for a signal processing PhD and got lumbered with my hardware.

Cubes being used in anger for algorithm optimisation research.

DSP Board

The Cube DSP boards are around 45mm on an edge with an embedded Microchip dsPIC128GP706. The dsPICs are big brothers of the 8-bit PICs manufactured by Microchip. I put on a 2x5 row 0.1" programming header to enable the use of both Microchip’s ICD2 programmer and the LV24-33 programmer from Mikroelektronika. The LV24-33 is a more robust programmer which will power the DSP directly, but needs all 10 pins of the programming header, whereas the ICD2 only needs one of the rows, but requires the board to be externally powered.

 Later versions of the DSP board went down to a single 5 pin connector to avoid confusing the other folk in the project who only have the pre-diluvian ICD2 programmer. 

The dsPIC digitises the amplified analog signals received by the transducers on the Amplifier board.  

Relate Cube DSP board.

Transmitter & Battery Board

Each of the six transducers can be independantly set to transmit. A 15V peak to peak voltage for driving the transducers is generated by utilising LTC1348CG RS232 transceiver ICs, which have built in charge pumps to ramp up the 3V supply. An LM3658SD-B lithium battery recharging IC allows the lithium battery connected to the base of the board to be recharged via the mini-USB connector on the communication board. The ‘B’ variant of the LM3658SD family allows the cube to operate without a battery being attached, instead drawing the supply current from the USB connector. The figure shows the board being tested with an external PSU. The red inidicator light shows that the battery is charging. However the cube can still be used while the battery recharges.

Relate Cube transmitter board.

Communications Board

 A Particle computer communications board is connected to this board via a header, to allow for wireless communications from the Cube. Serial communication via a mini-USB connector is provided using one of the FTDI USB to serial chips. This same mini-USB connector passes the 5V voltage from the USB rail to the Transmitter and Battery Board to recharge the lithium battery mounted under that board.

Amplifier Board 

 Six ultrasonic transducers are mounted on this board to allow for 3D ultrasonic ranging. The driving signals for the transducers originate on the Transmitter and Battery board. The received signals are amplified by a two stage amplifier circuit and passed to the DSP board as an analog signal. Plans have been made to construct an amplifier board where the analog signals are digitised then passed the the DSP board via an SPI interface. The image only shows four of the six transducers attached to the board - the last two are connected via flying leads to sockets on the board.

Relate Cube amplifier board. Six channel.

An amplifier board sporting variable gain amplification (VGA) is in progress. This allows real time compensation for the spherical attenuation of the ultrasonic signal. The AD605 VGA will be incorporated which allows logarithmic amplification over a 40dB range. Exponential gain is supplied from a linear control voltage. The control voltage comes from a DAC controlled via SPI. Six channels of this malarkey has to be shoehorned onto the board area of a large postage stamp. Joy.

Case

The cases are 3mm acrylic plastic, lasercut by Ponoko. The flat sides clip together. Judicious placement of small raises inside the teeth enables a friction fit. So far we have black, red and clear cases. Soon we will have all the colours of the rainbow. Nice.

Relate Cube in a laser cut case.