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Key to ControLeo2's success was "learning mode", where the oven would learn how much power each heating element needed to
follow the reflow profile. The goal was to keep the temperature as stable as possible, avoiding surges of power that would create a hot spot - or a void of power that would produce a cold spot. The downside of this method was that it was next-to-impossible for users to create custom reflow profiles better suited to their needs.
We had a lot of requests to support custom reflow profiles, and it was clear that we needed to support this. The obvious solution was to implement a PID (Proportional-Integral-Derivative) algorithm, but PID has some shortcomings:
For all of PID's limitations, this is still the best algorithm to use. It just needed to be improved and adapted ...
- PID tuning is notoriously difficult to do, and is often compared to black magic!
- A successful reflow needs consistent temperatures throughout the oven. This is difficult to control when using PID; the elements may be on or off for extended periods of time which can create hot or cold spots.
- During a reflow, it is useful to be able to direct where the heat comes from.
Controleo3 spends around an hour figuring out how well your oven was built and how well it performs:
- It learns how much power is needed to maintain a steady temperature in your oven.
It measures the combination of all elements, as well as individual elements. In the example below, the elements are on only 13% of the time to maintain a steady 120°C. The bottom element needs to be on 32% of the time to maintain the same temperature. This gives Controleo3 a general impression of the overall power and insulation of your oven.
- It learns how quickly your oven responds to requests for an increase in temperature.
It also measures the over-shoot. This gives Controleo3 an idea of the thermal inertia of your elements. In the reflow phase, the ability to quickly increase the temperature is very desirable. In the example below, it took 41 seconds to raise the temperature of the oven from 120°C to 150°C.
- Given the high temperatures required for lead-free reflow, your oven should be well insulated.
Controleo3 times the drop in temperature from 150°C to 120°C to get an idea of how well your oven is insulated.
The results for each measurement are analyzed and the appropriate smiley-face is displayed. The oven is given an overall score to reflect how well it is able to handle high-temperature lead-free reflows!
Controleo3 uses a smart PID algorithm that uses all these data points to carefully and accurately follow the required profile. The profile is able to specify the top-bottom bias, and Controleo3 is able to adjust output power to suit.
Your oven's score is based on 3 factors. You can find the formula in function "ovenScore( )" at the bottom of this file on GitHub.
The smiley faces give you some idea of how each area performed. The grades are all given assuming the desire is for an oven capable of high-temperature lead-free reflow. If your oven obtains a low score, you can attempt further improvement of your oven to increase the score. You can consider using a low-temperature paste, like Chip Quik SMDLTLFP10.
- 40% - power required to maintain 120°C
12% = excellent, 30% = poor
- 40% - time needed to increase the temperature by 30°C
36 seconds = excellent, 60 seconds = poor
- 20% - insulation
130 seconds = excellent, 80 seconds = poor
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