Archive → February, 2013
This project started because of some reported problems with a product I worked on in the past. Customers were reporting strange errors which seemed to be heat related. To reproduce the problem, I really needed a thermal chamber. Unfortunately, that’s not something I have in my lab, and getting involved in selecting heating elements and rigging up an enclosure sounded time consuming.
I did, however, have an inexpensive toaster oven that I bought for small SMD reflow assembly sitting nearby. While not ideal, I figured I should be able to add a relay to control the on/off power to the oven, and with a temperature sensor and some Arduino code, create a poor man’s environmental chamber.
Again, to keep the project quick and simple, I purchased the PowerSwitch Tail II (which is a pre-package AC relay control box), and a “high temperature” Dallas 1-wire temperature sensor from Adafruit. In this case, high temperature only means +125°C, but that was plenty for my application where I only wanted to get things up around +38°C to +50°C (+100°F to +120°F).
Wiring everything up to the Arduino was straight forward. Everything is driven directly by the Arduino with pins configured for digital mode. The only thing to remember is the 4.7K pull-up resistor for the 1-wire data line (which Adafuit conveniently included with the sensor). Here is a quick schematic:
There are two “off the shelf” libraries I used in my Arduino sketch: the Arduino PID Library and the OneWire Arduino Library. I followed the PID library example for using “relay output”, and coupled it with a function to read the temperature sensor at one second increments. I dumped the current readings to the serial port so that I could capture them, and plot the data in OpenOffice Calc. Here is the Arduino code I used in my experiments: toasteroven.zip
Everything so far had seemed to go quick and smooth, but when I started trying to tune the PID parameters, I started to realize this was going to take a little more time. Partly because after each “test run”, I had to turn things off and wait for everything to cool down. Working in the evenings, I generally only got one or two runs in.
It’s been years since I’ve done anything with PID control, but I knew I should start simple with just proportional control. Setting the tuning parameters (Kp, Ki, and Kd) to (5,0,0) and my setpoint to 38°C, I recorded the data shown by the yellow line in the graph below. Lots of overshoot! So I tried again, with a proportional gain of 2, and no integral or derivative (2,0,0). See the red line, which is not much better. One more try, this time with a proportional gain of 1. This is shown by the blue line, which actually doesn’t look too bad, but it still has an overshoot of nearly seven degrees.
To try to improve things, I added some derivative gain. In the next graph, I compared the “proportional only” data with that of “proportional and derivative”. I used the same Kp of 1, and set Kd to 100, just as a starting point, based on other examples I had seen. My overshoot is gone, but it takes even longer to get up to temperature.
Even when it did get up to temperature, the (1,0,100) settings didn’t seem to hold temperature very well, as shown in this graph…
To try to make things a little more aggressive, I bumped up the Kp back to 2, and this helped the steady state, but gave me overshoot again (see the blue 2,0,100 line below). Having had luck with increasing Kd to remove overshoot, I tried that again, and actually got something that looked pretty good. Check out the red (2,0,200) line!
Looking at the data, I get up to temperature in about 210 seconds, and then I’m never more than +/- 0.87°C from my setpoint. One final test is shown below– This time, over several setpoints, and for a longer period of time. I could probably spend even more time tuning things, but it seems fine for my purposes right now, and I need to get back to investigating that customer problem that initiated this whole project!