Issue 154 May 2003
Automatic Temp Controller
Data Logger for Slow Cooker

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ON/OFF CONTROL

If the cooker temperature is below the desired level, the simplest control algorithm is to turn on the fan fully. Turn it off if the cooker temperature is above the desired value. The resulting cooker temperature will tend to oscillate around the desired value, because the thermal inertia in the cooker will cause the temperature to continue to rising after the fan is turned off and falling after the its turned on. As long as the average temperature is what you want and the swings aren’t too big, this is perfectly acceptable. After all, this is how most kitchen ovens work, and the goal is to be as good as one of those.

PID CONTROL

A more sophisticated approach is to set the fan to the exact speed needed to reach a given temperature, and then incrementally adjust the speed to maintain that temperature as conditions change. The PID algorithm uses proportional, integral, and derivative calculations to do just that.

A measure of current conditions, the proportional part of the calculation is based on the size of the error (i.e., the difference between the desired temperature and the actual temperature). The integral portion of the calculation is based on the sum of all the previous error values; it’s a measure of any longer-term error trend. The derivative part of the calculation is based on the change in the error, not the temperature, and it measures how quickly the error is changing.

Each of the three components can be given a different weighting factor. If the weighting factors are chosen correctly—through a process called "tuning"—the PID algorithm will adjust the fan speed to bring the actual temperature to the desired temperature with no overshoot, and it will maintain that temperature with little or no oscillation even if conditions change. Parallax’s Industrial Control Student Guide Version 1.1 provides a straightforward introduction to the PID algorithm.

Although the hardware was designed with a variable fan speed so that PID could be used, I ran the initial tests with an On/Off algorithm. The results are shown in Figure 3. The goal was to build a device that is as good as an indoor oven. But, even with a simple On/Off control, the ceramic cooker is considerably better. So, the final controller uses On/Off control, and the more complex PID algorithm isn’t needed.

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Figure 3—Compare the temperature values over a 45-min. period for an indoor kitchen oven (dark blue) and the automatically controlled ceramic cooker (light blue). In this instance, both are set at 225°. As you can see, the ceramic cooker shows much smaller temperature swings and a more accurate average temperature.

USING THE SYSTEM

Photo 7 shows the ceramic cooker with the automatic temperature controller in a typical setup. The meat, rubbed with secret barbecue spices, is on the top grill with a drip pan underneath it. Beneath the drip pan is a foil-covered pizza stone, which prevents the bottom of the meat from charring. A full load of hardwood lump charcoal is under the pizza stone. I added chunks of hickory and cherry wood to add a smoked flavor.

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Photo 7—A typical setup includes food, oven and food temperature probes, and the controller.

After the temperature probes are inserted and the lump lit, close the lid. The upper draft control is opened a half turn, and the cooker temperature and food alarms are set. As soon as the alarm sounds, you can check the meat to see if it’s done. You aren’t required to monitor or adjust the cooker temperature, although some barbecue styles require you to periodically baste and turn the meat.

Figure 4 shows the log of an actual cooking session. The meat required approximately 17 h to reach a final temperature of 192°F. The cooker temperature was set at 225°F. It stayed within 3° of that setting more than 98% time, and remained within 1° more than 85%.

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Figure 4—The data log shows a stable ceramic cooker temperature (in blue) and a slowly rising meat temperature (in red). The initial temperature dip occurred when I opened the lid to the preheated cooker to add the food.

WHO’S TENDING THE FIRE?

Traditionalists may question the idea that real barbecue can be achieved without an all-day or all-night fire-tending vigil. But modern technology can make this cooking style accessible to those who otherwise wouldn’t have the time for it.

The system is useful for more than barbequing. After the fire is lit, the cooker is as easy to use as a kitchen oven but has the advantage of better temperature control. In addition, it makes it easy to monitor food temperature, and it gives you the ability to add a smoked flavor to anything you cook.