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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I have put together a breadboard version using a PLL, low-pass loop filter and a POT for fine-tuning VCOin. My tank's resonant frequency is about 76kHz; the PLL low and high range are 65 and 85kHz. When I turn it on it gets close and the POT gets me dead on.
Now, I am wondering if there is anyone holding out on this question: is there a way to have a circuit such that I can put the workpiece in the coil, turn on the unit and not have to make any tuning adjustment? I would like the unit to hit the exact resonant frequency completely by itself. Maybe it is not possible, and I will have to settle for making one initial adjustment.
I tried feeding PCA1 into a simple integrator with a -1 gain after to get the sign correct. I sent the signal to VCOin. The signal was all over the place, as if it could not lock onto a correct frequency. I tried adjusting the RC constant from low to high to no avail.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Many PLL circuits need "wobbulated" to get them into lock initially. You hook up a sawtooth oscillator that wobbles the VCO voltage around until it locks, at which point a lock detector turns the oscillator off. I've seen this arrangement in radio equipment and the like. All it is is an automated version of a guy twiddling the pot until it locks.
When I used my Mk2 driver to run an induction heater, it wouldn't lock by itself either: I had to turn the pot to make it jump into lock, but once it had locked, it stayed locked and would track changes as workpieces were inserted and removed. I would get it locked running off a variac at a low voltage, then crank it up full. I never got round to adding a wobbulator.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
This is where I am at on the breadboard. I actually get it to lock when I turn up the variac and it is really close. I then use a pot to get it precise and it stays just about there the whole time.
I noticed that when at resonance I have 7.5v (Vcc/2) on the PCA1 going to the low-pass filter when I have adjusted a POT going to VCOin set near 8.8v. So the resonant frequency is a little above the natural PLL middle frequency and I need to up the frequency using the additional 1.3v.
I need a means, without using a programmable chip, that will keep raising or lowering the VCOin voltage until PCA1dc is 7.5v. Would two unit op amps (two used so the gain is x1 and not -1) with a capacitor from the output to ground work? V- will be 7.5v. If PCA1 is low the opamp pair will put out 15v which will slowly increase the capacitor voltage which will go to VCOin. When the voltage is high enough the frequency will increase to the point that PCA1 is greater than 7.5v and then the opamp will switch. The capacitor will average out the alternating states to maintain the correct voltage to keep PCA1 at 7.5v.
As power is applied, the LM358 at the extreme right modulates the VCO-in of the 4046 (via a diode). (if I understand the circuit correctly, that is).
Found it somewhere on the net - sorry, don't recall where. There are some other induction heater schematics in my gallery too, some of which PLL controlled, as I'm gathering information on the topic in preparation of my own attempt in building one (but will have to wait till after the sinker EDM is finished).
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Thanks Peter. That is a non-inverting unity gain active low-pass filter. I guess it is a little better than the passive low-pass. I have been playing with various configurations and have found that an integrator with a -1 gain opamp actually tracks the resonance with hardly any drift at all. It worked better than the low-pass filter design. I added a POT in my breadboard to make sure I was near resonance, and when I turned the unit on it locked at resonance and followed it through the heating cycle.
Your reference did give me an idea to try and add some LED configuration to PCA2 to alert to a lock condition.
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Induction heater: PLL and automatic lock
