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Clean up, simplify, and correct some energy meter code #810

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Clean up, simplify, and correct some energy meter code #810

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45 changes: 8 additions & 37 deletions src/energy_meter.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -119,7 +119,7 @@ void EnergyMeterData::deserialize(StaticJsonDocument<capacity> &doc)
// old OpenEvse total_energy imported flag
imported = doc["im"];
}
if (doc.containsKey("el") && doc["el"].is<uint32_t>())
if (doc.containsKey("el") && doc["el"].is<double>())
{
// elapsed
elapsed = doc["el"];
Expand All @@ -134,7 +134,6 @@ void EnergyMeterData::deserialize(StaticJsonDocument<capacity> &doc)
EnergyMeter::EnergyMeter() : _last_upd(0),
_write_upd(0),
_rotate_upd(0),
_elapsed(0),
_switch_state(0){};

EnergyMeter::~EnergyMeter()
Expand Down Expand Up @@ -230,51 +229,23 @@ bool EnergyMeter::update()
if (_monitor->isCharging())
{
// increment elapsed time
_data.elapsed += dms / 1000U;
_data.elapsed += dms / 1000.0;

DBUGLN("Energy Meter: Incrementing");
// accumulate data
uint32_t mv = _monitor->getVoltage() * 1000;
uint32_t ma = _monitor->getAmps() * 1000;

/*
* The straightforward formula to compute 'milliwatt-seconds' would be:
* mws = (mv/1000) * (ma/1000) * dms;
*
* However, this has some serious drawbacks, namely, truncating values
* when using integer math. This can introduce a lot of error!
* 5900 milliamps -> 5.9 amps (float) -> 5 amps (integer)
* 0.9 amps difference / 5.9 amps -> 15.2% error
*
* The crazy equation below helps ensure our intermediate results always
* fit in a 32-bit unsigned integer, but retain as much precision as
* possible throughout the calculation. Here is how it was derived:
* mws = (mv/1000) * (ma/1000) * dms;
* mws = (mv/(2**3 * 5**3)) * (ma/(2**3 * 5**3)) * dms;
* mws = (mv/2**3) * (ma/(2**3) / 5**6 * dms;
* mws = (mv/2**4) * (ma/(2**2) / 5**6 * dms;
*
* By breaking 1000 into prime factors of 2 and 5, and shifting things
* around, we almost match the precision of floating-point math.
*
* Using 16 and 4 divisors, rather than 8 and 8, helps precision because
* mv is always greater than ~100000, but ma can be as low as ~6000.
*
* A final note- the divisions by factors of 2 are done with right shifts
* by the compiler, so the revised equation, although it looks quite
* complex, only requires one divide operation.
*/
double mws = (mv / 16) * (ma / 4) / 15625 * dms;
double v = _monitor->getVoltage();
double a = _monitor->getAmps();
double mws = v * a * dms;
if (config_threephase_enabled())
{
// Multiply calculation by 3 to get 3-phase energy.
mws *= 3;
}

// convert to w/h
double wh = mws / 3600000UL;
double wh = mws / 3600000;
_data.session += wh;
double kwh = wh / 1000UL;
double kwh = wh / 1000;
_data.total += kwh;
DBUGVAR(_data.session);
_data.daily += kwh;
Expand Down Expand Up @@ -318,7 +289,7 @@ bool EnergyMeter::publish()

void EnergyMeter::createEnergyMeterJsonDoc(JsonDocument &doc)
{
doc["session_elapsed"] = _data.elapsed; // sec
doc["session_elapsed"] = (uint32_t)_data.elapsed; // sec
doc["session_energy"] = _data.session; // wh
doc["total_energy"] = _data.total; // kwh
doc["total_day"] = _data.daily; // kwh
Expand Down
3 changes: 1 addition & 2 deletions src/energy_meter.h
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Expand Up @@ -39,7 +39,7 @@ class EnergyMeterData
double weekly; // kwh
double monthly; // kwh
double yearly; // kwh
uint32_t elapsed; // sec
double elapsed; // sec
uint32_t switches; // homw many switches the relay/contactor got
bool imported; // has imported old counter already
EnergyMeterDate date;
Expand All @@ -57,7 +57,6 @@ class EnergyMeter
uint32_t _write_upd;
uint32_t _event_upd;
uint32_t _rotate_upd;
uint32_t _elapsed;
uint8_t _switch_state; // 0: Undefined, 1: Enabled, 2: Disabled

EvseMonitor *_monitor;
Expand Down
2 changes: 1 addition & 1 deletion src/evse_man.h
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Expand Up @@ -348,7 +348,7 @@ class EvseManager : public MicroTasks::Task
bool publishEnergyMeter() {
return _monitor.publishEnergyMeter();
}
void createEnergyMeterJsonDoc(JsonDocument &doc) {
void createEnergyMeterJsonDoc(JsonDocument &doc) {
_monitor.createEnergyMeterJsonDoc(doc);
}
long getFaultCountGFCI() {
Expand Down