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Reflow Skillet

I created a computer-controlled frypan to assist me with surface mount soldering. I was inspired by the project from Ladyada  (part 1 and part 2).

The new Generation 3 electronics for the Reprap and Cupcake CNC controller is out, but they've moved almost exclusively to surface mount technology. Whilst I've soldered surface-mount components by hand before, I'd much prefer a more automated way of doing so.

Professional PCB manufacturers use special million dollar conveyor fed multiple-thermal-zone ovens in order to ensure circuit boards are soldered optimally. If I can emulate this in my kitchen for a hundred bucks, I'll be pretty happy.


Hardware:

Skillet

The skillet is from my local Go-Lo (Aussie bargain store) for $50. It has a two kilowatt heater and a rudimentary thermostat inbuilt.

The performance of the thermostat is, to put it bluntly, crap. Here are the results of heating using each of the 5 dial positions:
For example if the dial is set to the first notch the temperature can be anywhere from 100*C to about 200*C. Such a swing completely rules out using the internal dial for soldering purposes.

Temperature Sensor

I used the thermistor sensor from the RepRap foundation. It has a range up to mid 200 degrees, but isn't that accurate at lower temperatures. However it's a lot simpler to use than thermocouple based solutions (no extra chips or zero-point compensation circuitry) and it seems less susceptible to noise.

I used JB-weld to glue the thermistor to the bottom centre of the pan. It holds strongly and should be good up to the temperatures I need.


A little more JB weld secured the terminal blocks. I can't solder it since the temperature may go above the melting point.


Controller

I used the Arduino microcontroller . I love using it for projects. Since it doesn't need an extra programmer, and can send data directly to the PC, it's way simpler than a lot of others. It's also cheap enough for me to keep a couple spare, just in case.

To control the skillet I used a solid-state relay. It was very simple to use, the microcontroller can drive it directly, without even a limiting resistor! It's difficult to get more simple than that. The downside is cost, it was about $50.


For display I used a Hitachi LCD and the serial backpack from SparkFun electronics.

Software Design:

Bang-Bang control, Overshoot and Hysteresis

First off I tried running using an extremely simple method. If the temperature is below the set point then turn on the heater. If the temperature is above the set point then turn off the heater. Since the system moves abruptly from OFF-->ON and vice versa, this is known as 'Bang-Bang Control'. This worked, but it didn't behave very nicely. For example if I started from a cold start and made the target temp 50*C, it would overshoot to 95*C before finally settling down:

A way to 'neaten up' the response from the controller is to use something called Hysteresis. For example if the controller is supposed to turn on at 100*C, then with a 10*C hysteresis it won't turn off until it drops below 90*C. This sort of behavior is good at stopping the system chattering around the setpoint, but it doesn't address my problem with overshooting the mark. For that I needed a way to throttle the output to the heater....

Output PWM

However the relay can only be either off or on, I can't control to be half-way on. (This is actually a Good Thing, since a relay which continually let only half the power into the heater must be dissipating half the power itself, and would immediately melt itself into the ground)

To get around this we use a technique known as Pulse-Width-Modulation (PWM). This means that the relay is switched ON and OFF rapidly, but the length of the ON and OFF times are adjusted. The ratio between the ON time and the total ON & OFF times is known as the Duty Cycle. If the duty cycle is 30% then it means that 30% of the time the relay is ON, and 70% of the time it is OFF.

A word of warning: Don't try this technique unless you A) use a solid-state relay, and B) have a load that doesn't 'mind' being switched on and off rapidly. Some things I would never try dimming using PWM include fluorescent lights, some motors, and anything with a filament or that may suffer thermal shock. Using a old-style non solid-state relay will quickly result in the relay dying due to overuse (old relays had moving contacts which would wear out. Newer ones have no moving parts and hence are known as solid-state).

In this case I chose to use a period of 1 second for the output cycling. This is quite large by PWM standards. The arduino actually has dedicated hardware outputs to do PWM at 500Hz, however it is tricky to use this with mains frequencies. It is possible to do this, but it typically requires extra circuitry to allow the microcontroller to sync its outputs to the starts of the mains cycles, and frankly I couldn't be bothered.

So now, using the PWM technique on the output, we have a sort of 'virtual dial' that we can use to control the output to the heater.

I ran the test again, using 'Bang-Bang' control, a target temperature of 50*C,  and having the heater output constantly throttled to 50% duty cycle. This helped reduce the overshoot, instead of peaking at 95*C, it stopped just above 70*C. However the 'rise-time' was really increased.
This was not too surprising, as I had basically simulated having a heater of half the power.

So, we need a control strategy which allows us to get to the target temperature faster, but has less overshoot. That brings us to...

PID Loop

To be done.

Temperature Profile

To be done.

Commissioning:

Still in progress. Either I've made a mistake, or both Ziegler and Nichols are full of shit. Since their method has been used by 3 generations of engineers without issue, I think the problem lies with me.

Results:


Thermographic Images:

Since the skillet has a single heating element, which is arranged in a ring around the base of the pan, I was concerned whether it would heat evenly.

So, I stole a $50k thermographic camera and started taking pictures.

                                                                                                        Before I started. Warm laptop and cool beverage:                                                    After warming up a little, pretty uneven:
      
                                                                                                        After running for a minute or two:                                                                                 Overview shot:


Although the warm-up is a little uneven, it becomes homogeneous very quickly. I don't think I'll have any problems with it, but I'll make sure to put the PCBs near the centre, just to be on the safe side.





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