kopia lustrzana https://github.com/peterhinch/micropython-samples
110 wiersze
4.7 KiB
Markdown
110 wiersze
4.7 KiB
Markdown
# A phasor power meter
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This measures the AC mains power consumed by a device. Unlike many cheap power
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meters it performs a vector measurement and can display true power, VA and
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phase. It can also plot snapshots of voltage and current waveforms. It can
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calculate average power consumption of devices whose consumption varies with
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time such as freezers and washing machines, and will work with devices capable
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of sourcing power into the grid. It supports full scale ranges of 30W to 3KW.
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[Images of device](./images/IMAGES.md)
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###### [Main README](../README.md)
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## Warning
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This project includes mains voltage wiring. Please don't attempt it unless you
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have the necessary skills and experience to do this safely.
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# Hardware Overview
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The file `SignalConditioner.fzz` includes the schematic and PCB layout for the
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device's input circuit. The Fritzing utility required to view and edit this is
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available (free) from [here](http://fritzing.org/download/).
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The unit includes a transformer with two 6VAC secondaries. One is used to power
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the device, the other to measure the line voltage. Current is measured by means
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of a current transformer SEN-11005 from SparkFun. The current output from this
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is converted to a voltage by means of an op-amp configured as a transconductance
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amplifier. This passes through a variable gain amplifier comprising two cascaded
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MCP6S91 programmable gain amplifiers, then to a two pole Butterworth low pass
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anti-aliasing filter. The resultant signal is presented to one of the Pyboard's
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shielded ADC's. The transconductance amplifier also acts as a single pole low
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pass filter.
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The voltage signal is similarly filtered with three matched poles to ensure
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that correct relative phase is maintained. The voltage channel has fixed gain.
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## PCB
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The PCB and schematic have an error in that the inputs of the half of opamp U4
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which handles the current signal are transposed.
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# Firmware Overview
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## Dependencies
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1. The `uasyncio` library.
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2. The official lcd160 driver `lcd160cr.py`.
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Also from the [lcd160cr GUI library](https://github.com/peterhinch/micropython-lcd160cr-gui.git)
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the following files:
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1. `lcd160_gui.py`.
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2. `font10.py`.
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3. `lcd_local.py`
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4. `constants.py`
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5. `lplot.py`
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## Configuration
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In my build the above plus `mains.py` are implemented as frozen bytecode. There
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is no SD card, the flash filesystem containing `main.py` and `mt.py`.
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If `mt.py` is deleted from flash and located on an SD card the code will create
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simulated sinewave samples for testing.
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## Design
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The code has not been optimised for performance, which in my view is adequate
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for the application.
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The module `mains.py` contains two classes, `Preprocessor` and `Scaling` which
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perform the data capture and analysis. The former acquires the data, normalises
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it and calculates normalised values of RMS voltage and current along with power
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and phase. `Scaling` controls the PGA according to the selected range and
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modifies the Vrms, Irms and P values to be in conventional units.
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The `Scaling` instance created in `mt.py` has a continuously running coroutine
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(`._run()`) which reads a set of samples, processes them, and executes a
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callback. Note that the callback function is changed at runtime by the GUI code
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(by `mains_device.set_callback()`). The iteration rate of `._run()` is about
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1Hz.
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The code is intended to offer a degree of noise immunity, in particular in the
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detection of voltage zero crossings. It operates by acquiring a set of 400
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sample pairs (voltage and current) as fast as standard MicroPython can achieve.
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On the Pyboard with 50Hz mains this captures two full cycles, so guaranteeing
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two positive going voltage zero crossings. The code uses an averaging algorithm
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to detect these (`Preprocessor.analyse()`) and populates four arrays of floats
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with precisely one complete cycle of data. The arrays comprise two pairs of
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current and voltage values, one scaled for plotting and the other scaled for
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measurement.
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Both pairs are scaled to a range of +-1.0 with any DC bias removed (owing to
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the presence of transformers this can only arise due to offsets in the
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circuitry and/or ADC's). DC removal facilitates long term integration.
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Plot data is further normalised so that current values exactly fill the +-1.0
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range. In other words plots are scaled so that the current waveform fills the
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Y axis with the X axis containing one full cycle. The voltage plot is made 10%
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smaller to avoid the visually confusing situation with a resistive load where
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the two plots coincide exactly.
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## Calibration
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This is defined by `Scaling.vscale` and `Scaling.iscale`. These values were
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initially calculated, then adjusted by comparing voltage and current readings
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with measurements from a calibrated meter. Voltage calibration in particular
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will probably need adjusting depending on the transformer characteristics.
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