MainsFrequency2.0
| Project MainsFrequency2.0 | |
|---|---|
| |
| A simple mains frequency counter | |
| Status | In progress |
| Contact | bertrik |
| Last Update | 2023-03-16 |
Introduction
This project is a reboot of this earlier main frequency counter, aiming for more accuracy and lower latency.
It's based on the Arduino platform, using an ESP8266 to do the wifi/network/MQTT stuff. The frequency measurement principle is to measure the time between zero crossings (in a statistically robust way).
Concept
Instead of just counting pulses from zero-crossings, we sample the actual 50 Hz waveform and try to estimate the zero-crossing as accurately as possible.
Desired end result
- get more accurate frequency measurement
- get more responsive frequency measurement, i.e. instantaneous value, not a running average over 50 seconds.
A suitable module for relatively safely sampling the mains voltage is this ZMPT101B module. It contains a transformer and an op-amp circuit.
More information about this module:
Algorithm for getting accurate instantaneous frequency out of it:
- Sample the mains frequency waveform during approximately one second (say 5 - 10 kHz)
- Determine the median, lower and upper quartiles of the waveform, and shift data so its values are symmetrically around zero
- Calculate zero crossings by doing linear regression to find the zero-crossing of the wave in the region around the zero crossing (in between the quartile values), this gives sub-sample time resolution
- The linear regression runs a kind of state machine, just keeping track of sum(x), sum(y), sum(x*x), sum(x*y) is enough to perform a linear regression at any time
- Do this over the approximately 50 cycles contained in the one second data and determine average frequency
-> this should give about 1 millihertz frequency resolution in one second
Hardware
For measurement with an ESP8266, like a Wemos D1 mini or nodemcu, you need to put a 180k ohm resistor in line with the output from the ZMPT101B to the A0 input. The A0 input already has a 220k/100k resistive divider, effectively becoming a 400k/100k resistive divider with the series resistor, scaling down the 0-5V range to the 0-1V range required for the ADC on the ESP8266.
The "blue pill" seems to have too low accuracy of the built-in crystal, about 100 ppm, while we need about 20 ppm to get 1 mHz resolution. Notes about blue pill crystal accuracy: https://sparklogic.ru/arduino-for-stm32/accurate-blue-pill-clock-frequency-adjustment.html
Software
Github project: https://github.com/bertrik/MainsFrequency