Thursday, December 14, 2017

Sense Energy monitor teardown - sampling in MHz

Recently I obtained a Sense Energy monitor via US from Margaret of BitKnitting. She is doing a very interesting neighbourhood energy efficiency project. As usual I could not contain my curiosity and opened it up to have a look. I will start off with an analogy - the closest bit of open-source kit that I have to do half the amount of analog functions as the Sense is the PRUDAQ on the BeagleBone wifi as pictured below.
Beaglebone Wireless + PRUDAQ
The case is very well built from high density plastic, however it is clipped together neatly and easy to take off without permanent damage. The wifi antenna is screwed on with a neat water sealed connector. The power/voltage and current sampler connectors are Molex Microfit and Nanofit respectively. I rather liked the Molex Nanofit and adopted it for my own DIN Rail sampler board.
Enclosure and main boards
Inside this very nice exterior are a couple of very dense sandwiched PCB's. One is Green, almost a generic power supply board, but with additional function to sample the split phase AC in the North American grid. The other is a black board where the magic happens. It is essentially a beaglebone wireless with a custom energy monitor system put together using high-speed ADC's and a CPLD
Green Power board, Black Digital+Analog board

Main PMIC, same one is used on the BeagleBone Black
The image below shows the subsection that performs energy measurements, it is composed of x2 high sample rate ADC's (14bit 2MSPS) and an Altera Max V 64pin CPLD. I assume the cells inside the CPLD perform the reading from the ADC's and multiplication/accumulation to compute energy usage, then transmit computation results to the TI CPU.

Custom energy monitor with high-speed (2MSPS) ADC's and Altera CPLD

Wifi Module
Switcher IC's on power supply board

The main CPU is a Linux compatible AM3352 CPU from TI. It does not have a PRU co-processor like the one used on the BeagleBone, hence the need for a CPLD to perform some of the real-time computation required.
TI Arm CPU and RAM/Flash

Overall the Sense energy monitor hardware is a great design, though a bit specific to the USA market. I am looking forward to a release in the Australian market. The purchase to study one was certainly worth it. Now off to install my own linux build on it and send the data elsewhere.
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