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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I found the AT90PWM chip which has a built-in PLL that can run off an internal 64Mhz clock. Are there any methods for digitally monitoring the voltage and current internally with the uP so I eliminate the RC filter network that is used to convert pwm signals to DC voltages?
Since I am using a uP, would it be better if I monitored the inverter voltage and current? I could convert the current waveform to a square-wave and time the difference between the two positive-going (leading) edges. I could then adjust the output to increase or decrease the difference.
Again, is there any reason to try this approach? Does anyone know what approach for professionally made industry units?
Registered Member #2478
Joined: Mon Nov 23 2009, 03:24AM
Location: Texas A&M University
Posts: 47
A slightly old (late 90s) book I found at the library suggested that the commercial controllers monitor the input power and sweep frequency starting from the 'safe end' until the desired power is reached, the input power changes slope, or safe power levels are exceeded.
Where the 'safe end' is high frequency for series resonant voltage source inverters and low frequency for parallel resonant current source inverters.
You don't need to monitor the inverter voltage. Unless you have some crazy propagation delay, you should be able to assume that the inverter voltage follows the gate drive signal in frequency and phase.
If you want the uC to do all the work, you could set a falling edge-triggered interrupt on the inverter current. The interrupt handler would store the current time in $inv_cur. The (inline) function that switches the gate drive signal low would then store the current time in $inv_volt.
($inv_volt - $inv_cur) then yields an error term with which to implement a PID loop.
However, there might be a brief glitch when the precision timer rolls over.
If reducing component count isn't critical, it might be easier to use a 4046.
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Other approach for uP controlled resonant locking for an induction heater
