Bob's Sensors and Sensor Network Blog

Cats of Engineering - Mungo Jere

Here's Mungo Jere's photo for the Cats of Engineering layout at Adafruit.

 

Energy Scavenging WSN

Putting together all the pieces now. Here is a complete Open Source Hardware (OSHW) Wireless Sensor Node (WSN) that energy scavenges. Have a Peltier thermo-electric generator resting on top of a 20W wirewound resistor and scavenging the energy from the waste heat of that resistor.  The Peltier device then powers a Linear Technology LTC3108 chip (described here) which powers a wireless sensor node (WSN) that is built from a NXP LPC1114 MCU and a Atmel AT86RF212 800 MHz radio (described here).  The wireless stack is the Chibi stack (described here).  The WSN transmits data every 2 minutes, which gives the storage capacitor in the energy scavenger plenty of time to recover.  The sleep state of the WSN draws about 25 micro-amps and the wake state when polling the sensors and the radio on draws about 11 ma.  The radio is on for about a second every 2 minutes. 

Click on image to enlarge

 The peltier device generates electrical power proportionate to the heat flow through the device.  The power output can be increased then by blowing gently across the top surface of the peltier device which increases the convective heat loss on the top surface and increases the flow of heat through the device from the hot side where the resistor is generating heat to the top cool side where heat is lost convectively.

Energy Scavenging LTC3108 OSHW SSOP package version

Just finished testing the LTC3108 board with the SSOP package.  Works great!   More to come.

Having fun with Sparkfun Ultimate IMU

Just got my Sparkfun Ultimate IMU and it is performing flawlessly.  Next I plan some changes to both hardware and software but as it is, right out of the box, it's amazing.  The IMU (with a quarter for size comparison) is powered by a LiPo battery.   It uses an Xbee to link to the PC (IMU Xbee is on the underside of the board).  This is a 9 axis part, that is there's 3 axis accelerometer, 3 axis gyro, and 3 axis magnetometer (to help manage gyro drift).   Thinking of agriculture related applications amongst other things.  I'll comment more on my progress and the changes in other posts.  One idea I have is to redo this using roughly the same software but with a updated MCU, a LPC1343 Cortex M3 vs the  LPC2148 M7 that comes with it.  Also will add a SWD JTAG connection so it can be used with a debugger (the Sparkfun unit does not come with any connection for JTAG).  Want to add a version of Kalman filtering to improve the performance as well.  More on that later.

LTC3108 Eagle files and component library posted on github

I've posted the Eagle files and the component library for the LTC3108 breakout board (both DFN and SSOP) on my public github site, https://github.com/wa7iut

Energy Scavenging LTC3108 OSHW SSOP package version coming

Sent off a version of the LTC3108 Energy Scavenging board that uses the SSOP version of the package.   The advantage will be that manual assembly of the board will be simplied.  Otherwise, the features are the same as the earlier DFN package version of the board.

ADXRS649 20,000 deg/sec Gyro (Twenty Thousand deg/sec!)

Just received an Analog Devices ADXRS649 20,000 deg/sec gyro and they're tiny. Got it on the eval board so I didn't have to deal with the BGA, even though that doesn't look too bad, just 64 pins. Also, this unit is supposed to have much higher immunity to shock and vibration than most mems gyros. Looking forward to trying it out.

 

 

LTC3108 Start-up Sequence

 The input section of the LTC3108 consists of a step-up oscillator circuit formed from an internal depletion mode N-channel MOSFET together with an external 1:100 transformer and coupling capacitors.  The oscillation resonant frequency can vary between 20kHz and 200kHz for typical values of the components (depends mostly on the inductance of the secondary winding of the transformer).

 

 

   

A LT Spice simulation of this circuit shows the start up current waveform in the secondary winding of the transformer.    This is in response to a step function input of 30 mV and shows the first second of operation.   In this case, C1 = 330 pF, C2 = 1000 pF, Primary winding = 7.5 uH, Secondary winding = 75 mH.

 

 This is a "zoomed" version of the same data showing what happens in the first few milliseconds. (click on picture to see full size).

LTC3108 Breakout Board Pin Names and Function

Here's a list of the functions of the pins on the LTC3108 Breakout Board.

Pin Name                                            Function

V-IN                                          Input Voltage, range is 20 mV to 400 mV.

GND                                         Board Ground.

C-STORE EXTERNAL             Auxiliary External Storage Capacitor, positive side.

Discharge                                 Provides discharge path for storage capacitors if needed, e.g. taking                                                  board out of service.

VOUT                                       Output Voltage, set to 3.3V by design of the board.

VOUT2                                     Switched output 3.3V for sensors, etc  that don’t have a sleep mode,                                                  controlled by VOUT2_EN  (provides for MCU control of VOUT2).

VLDO                                       2.2 V power for MCU in sleep mode.

PGD                                         Power GooD  is high (2.2V) when VOUT is within 7.5% of 3.3 V,                                                             goes low (0V) when VOUT drops 9% below 3.3 V.

VOUT2_EN                              VOUT2 enabled when high, 5M internal pulldown resistor. 

 

 Here is how the pins on the board relate to the block diagram of the LTC3108 (click to enlarge).

Efficiency and Storage in LTC3108 Circuit

The diagram below (steampunk version) suggests some of the issues that a designer has to deal with using the LTC3108.  In this case, a thermoelectric generator (TEG) converts heat flow into low level voltages (20 mV to 400 mV) and moderate currents (10's of ma).

 

 

The main idea is that there's a low level source of energy at voltages that the LTC3108 can transform into higher level voltages useful for powering microcontroller units and radios and so forth.  Unfortunately, this doesn't come for free.  The conversion efficiency can vary anywhere between 5% to 40%, which means input losses in power from 95% to 60%!  That may or may not be a problem depending on the available ambient energy.

When the unit starts up, the storage capacitor is charged to 5.5V at a rate determined by the available current (power) from the input source.  The external circuitry (microcontroller, etc.) draws it's power from the LTC3108, but only sustainably if it the power required is proportionate to the input power given the efficiency.   If the input power goes away, the exteral circuitry draws power from the storage capapcitor and the output voltage is regulated until the storage capacitor drops in voltage to the same level as VOUT (normally 3.3V).

 

Perhaps a more common mode of operation for the external circuitry is to remain in a very low current (power) sleep state most of the time, coming alive periodically to take measurements or do other work and the to return to the sleep state.  During the time the circuti is "awake" it draws current (power) from the storage capacitor at a rate that may greatly exceed the capacity of the LTC3108 to deliver from the input source, however, as long as the external circuit awake state is shorter than the time it takes to drain the storage capacitor, all is well.  Sizing the storage capacitor to the intended external circuitry is a critical part of using the LTC3108.