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Yeeeeha! Microprocessor controlled induction heater

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IamSmooth

Mon Aug 16 2010, 12:05PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

Mike wrote ...

As for the circuit, I was looking at

You are looking at the old schematic. If you follow the web pages the uP based controller is at the end in the induction heater levitation section.

The web tutorial is a progression, starting with my initial endeavor, and culminating with my uP based solution. The uP also allows me to have current limiting algorithms for situations when I am at full power and the workpiece is suddenly removed.

Paragon

Mon Aug 16 2010, 12:26PM

Registered Member #3007 Joined: Sat Jul 17 2010, 02:35PM
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Posts: 12

Hello All,

I have been working on a similiar aproach in using a PIC micro to auto lock frequency into a 4046 PLL.
But am having some issues that I would very much appreciate some help with.

The current setup I am employing is as follows.

Commparator 1 is used.
A square wave from a function generator is connected to PCAin1 of the XOR (pin 14) of the 4046.
The output of the VCO (pin4) connects to PCAin2 of the XOR (pin3).
The output of the XOR PCout1 connects to the PIC ADC (an0) via a filter network to give a voltage between 0-5v depending on phase.

The pic monitors this voltage and uses a pwm output via a filter network to produce a voltage between 0-5v this connects to the VCOin (pin9) of the 4046 and is used to ajust the frequency of the VCO.

The problem I have is with the XOR output going crazy and scrolling along the oscilloscope even when both input frequencys are the same!?

The PLL locks if I feed the output of the XOR to VCOin but this defeats the the object but proves the PLL will lock with its own feed back.

The PIC PLL is 10bit resolution giving 1024 steps for 0-5v output (or 0.005v per step) so I think the resolution is OK.

I should mention that I am using a MC14046BCP and that the supply voltage is 5v does this make a difference over say the HEF4046 that runs at upto 15V with regards to stability?

Does anyone have any ideas why the XOR out is not giving a steady output when frequency match on the inputs?

TIA
SRG

IamSmooth

Mon Aug 16 2010, 02:37PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

Without seeing your schematic, what is your filter network that takes the XOR output and goes to the uP? I put a lot of working getting it right. If your filter is off it will take too long to settle on a value, and you will oscillate. If it does not filter enough you will get ripple from the pulsed output and this can cause oscillations.

Paragon

Mon Aug 16 2010, 03:30PM

Registered Member #3007 Joined: Sat Jul 17 2010, 02:35PM
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Posts: 12

Thanks for the reply IamSmooth,
BTW are you John?
It's just that I found this schematic on John's site which looks to be almost exactly like the setup that I am employing except my devices are at 5v Vdc.

1281971432 3007 FT85701 Upcontrolleddriver

Coincidently the filter network is made of the same values as the above.

Here is the thing when I put a 10k pot between 0-5vdc and the center via 100k resistor to the VCOin I can get a lock by manualy ajusting the pot and the square wave from the generator, but it's not completly stable. The closer it gets to 90degree phase the more unstable it becomes.
The lock is also in a very small range and when the freq is ajusted towards 90 degree phase or Vdd/2 (2.5v) it jumps above or below it when measuring the voltage at the filter output. BTW the output of the filter was not fed back into the VCO in the above test but was only used for monitoring voltage

What determines the lock width is it the filter?

Kind Regards,
SRG

IamSmooth

Mon Aug 16 2010, 04:37PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

What are you measuring to compare the phase? What are you driving with the signal and what points are you measuring to compare the phase?

Also, what coil are you driving (physical dimensions) and is there a workpiece inside of it? I have seen for very large coils with no workpiece a very narrow range for resonance.

Paragon

Mon Aug 16 2010, 09:30PM

Registered Member #3007 Joined: Sat Jul 17 2010, 02:35PM
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Posts: 12

Tried posting sooner but the site appeared o be down.
Sorry for the confusion...
At the moment the driver is not connected to the inverter.
I am driving the PLL with a function generator into comparator1 (PCAin pin14) and VCOout > PCBin (pin 3) then adjusting the voltage on VCOin manually with a pot or via PWM (PIC) for this test.
I am using the scope to monitor :-

(A) PC1out (comaprator1 out) for freq lock and a 50/50 duty cycle ie half supply voltage 2.5v = 90 degree phase
(b) PC1out also goes through a filter network which produces a linear voltage and I am looking for for 2.5v for 90 degrees phase.
(C) Function Generator Waveform.
(D) VCO output waveform.

It's the comparator that I am having difficulty with, in that, by adjusting either the function generator square wave freq or the VCOin voltage, either manually or via PWM (PIC) the lock is very very small and jumps high or low when the COMPout nears the 2.5v centre mark.

When I mention lock I mean the area where the comparator gives a clean pulse width signal on it's output and not the fussy output when inputs are out of phase / freq.

Cheers,
SRG

IamSmooth

Mon Aug 16 2010, 11:31PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

You are looking at it the wrong way. First, Vs/2 will not always be the correct point, so don't get fixed on this. Vs/2 is simply the mid-point of your low and high frequency range determined by your R11 and R12 values. It is not necessarily going to be the point when your two independent waveforms are 90 degrees out of phase.

I would put a scope on the inputs to the PLL (pints 14 and 3) and make sure they are clean square waves. Make sure your inputs go from 0 to 5v; your function generator might be going from -5 to 5, although I don't know what effect this will have. Disconnect your filter from the PC1out and just monitor the raw output. The pulses should smoothly grow as the two waveforms get further out of phase.

Is this on a breadboard?

Paragon

Tue Aug 17 2010, 07:59PM

Registered Member #3007 Joined: Sat Jul 17 2010, 02:35PM
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Posts: 12

Iamsmooth,

How are you monitoring the output and locking the system at 90 degree phase?
Looking at the diagram I posted previously voltage is monitored from the comparator out filter into an analogue pin of the micro.
A PWM output is filtered supplying a voltage to VCOin.

As I understand it when two inputs into an xor are 90 degree out of phase the xor out put will be a 50% square wave. When this output is filtered it give a voltage of VDD/2

Could you please explain how you accomplished this on your setup?

Kind Regards,
SRG

IamSmooth

Tue Aug 17 2010, 09:22PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Again, the proper voltage to the VCO is not necessarily Vs/2 for the 90 phase difference. You will need to determine this value emperically.

A filtered DC input goes to the analog pin of the uP. The uP increases or decreases the duty of a pwm output, which is filtered into a DC voltage going to the VCO input of the PLL, based on the value of the analog input. You need to program an algorithm to monitor the analog input and determine which course of action to pursue. You can also monitor other inputs for over-current or over-voltage condition.

I would say I could just program a chip for you, but depending on the stray inductance values, length of your tracings and where you are monitoring the signals the proper voltage value for 90 degrees phase difference will not be the same. You will need to determine this for your unit.

IamSmooth

Sun Sept 26 2010, 04:46AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I just added some videos showing 10kw for heating a blade blank and heating a 4 lb brick of iron for forging. Without the resonant lock the mosfets may have blown as the handler is moving the blade in and out of the coil. Instead, the uP holds onto the right frequency. The water-cooling for the mosfets has also proved to be a big success allowing me to run the unit in a 95F garage without any worries.

The videos are still processing.

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