[General] Feedback controlling method understand

[thannara123]

In sinewave inverter like products ,How the feedback control Works ? (transformer Based )

The feedback sinewave signal properly shaped and connected to the ADC of the controller .

Is the ADC reads, the first Half cyle and calculate the RMS of the signal . Then correct the PWM switching to maintain the voltage stable?

Or
is the instantenious value of the ADC reads is correct the PWM to maintain the voltage stable ?

The first one takes more time in 50 Hz system it takes 10 milli second .

what is the normal type is there any other logic ?

how the settiling time / Response time ?

 

[KlausST]

Hi,

you are talking abut "voltage regulation"?

There are several ways:
* Some use analog circuit the get an information about RMS voltage as DC. Then this is fed to the ADC and processed.

*****

I´m for the more perfect solution:
Voltage signal (PWM) --> analog LPF fiter --> (sine) --> DC offset --> ADC --> digital HPF --> square, average over an integer of period time

And because taking the square root (especially on smaller microcontrollers) takes a lot of processing time I avoid it:
Instead I square the setpoint value and compare it with the (not_R)MS value.

You may use a PID regulation.
in the end one multiplies the duty_cycle of each PWM cycle with the correction factor to adjuast sinewave amplitude.

*******

But there are many other ways. It depends on your requirements ... and skills (math, hardware, software)


Klaus

 

[thannara123]

another way of thinking is
to comparing the instantaneous value obtained from an ADC read with the corresponding sine value from your sine table
is it fast and good way >?
we get a real time correction ?

 

[KlausST]

to comparing the instantaneous value

The "instantaneous value" of the output is PWM´d DC voltage. so it somehow is ON and OFF.
And the DC (bus bar) voltage will also fluctuate.

To me your whole signal flow is unclear.
Draw a sketch ... with signal waveform and timings and so on....

Klaus

 

[thannara123]

sir the signal is taken from the output of the transformer -> stepdown transformer-> opamp + shifting DC bias-> filter -> ADC so we get the sinewave

I meant that "the instatanious value" the feedback signal which is properly shifted and connected to the ADC of the controller
That means not takes the value of array ( adc value) ,No avraging ,no calculation about RMS value
only one adc read ,these value is comparing with the corresponding sine value (from the truth table)

so each adc read add or subbstract the PWM duty cycle to control the output voltage

is it necessory that taking RMS value ?
if use RMS value ,here need to read atleast half cycle it require 10 millisecond ,so the resoponse time is above 10 milli second

 

[KlausST]

Hi,

for me ... a single ADC value is too unreliable.
There may be noise, there may be sinewave distortion, there may be filter delay ... and others.
Thus the measured value is lekely not to match your expected value.

is it necessory that taking RMS value ?

It´s not necessary. You are free to do want you like..
But if you want to comply with any standard on how to measure AC (mains) voltage, then true RMS is the way to go.

Klaus

 

[FvM]

I read "instantaneous value" as estimated average over a PWM cycle, e.g. inverter output with typical filter removing PWM carrier or an additional dedicated low-pass. I believe however that the idea behind post #3 involves some problems:
1. The high bandwidth can easily result in an unstable control loop due to digital and analog delays.
2. It attempts to equalize waveform distortions caused by non-linear loads which is most likely beyond inverter power capabilities

In practice, a kind of mean value, either RMS or rectified value averaged over a half or quarter cycle will allow a smooth control with reasonable loop bandwidth.