Temperature control for a sous vide water bath is a popular application for hobbyist microcontroller kits, with lots of well documented projects out there. My favourite is the Sous Vader – if only for the attention to detail in the decal on the box.

I’ve been planning my own for a little while, and got some DS18B20 temperature probes a little while ago. The long Easter weekend break finally brought me some time to progress the project.

Danger – High Voltage

I have an old slow cooker that’s perfectly good for doing pot roast. It’s been falling to bits for some time, and I’ve patched it up with a Sugru sample I got at Monkigras. I had thoughts of hacking it directly (and perhaps even building in a Raspberry Pi), but common sense took hold and I decided on a route that didn’t involve directly manipulating mains electricity.

To turn the slow cooker on and off I got hold of a radio controlled mains socket from Maplin – they’re available without the controller (which I didn’t need) for £7.99.

I found a library to control the remote socket on github – Raspberry Strogonanoff (thanks Duncan McGregor, and a clever choice of project name). This uses a Sparkfun 434MHz RF Link Transmitter (which as suggested by Duncan I got from Protopic).

Getting Raspberry Strogonanoff working was the easier part of the whole set up. Having looked at the socket to establish that it was set to channel 4 button 1 I simply ran this line to turn it on:

sudo ./strogonanoff_sender.py --channel 4 --button 1 --gpio 0 on

and this to turn it off again:

sudo ./strogonanoff_sender.py --channel 4 --button 1 --gpio 0 off

Getting control

A water bath is a good example of a control system. The water and pot collectively have quite a high specific heat capacity, and there’s quite a bit of latency between turning off power and the rise in temperature stopping. The standard way of dealing with this is to use a PID Controller (see this explanation for a BBQ application for something more straightforward than the Wikipedia article). The Sous Vader seems to have worked with just a PI controller (effectively D=0), so I tried that first. There is a python implementation of a PID controller, which I didn’t spend the time to integrate.

Tuning the control loop

This is the tricky and time consuming part. In theory there should be a nice library out there that figures out correct settings for P, I and D based on observed data, but in practice it seems that most people resort to manual tuning.

The hardware

I started out with things on a breadboard using my usual home made Pi Cobbler, but as I want to use this in the kitchen I’ve created a more permanent setup using a Ciseco Slice of Pi, which at £3.90  is a very inexpensive small project board. I used one of the header strips in the Slice of Pi kit as a place to plug in the 434MHz transceiver and the temperature probes (so it would be easy to use parts in other projects). It was then simply a case of soldering some connecting wires to the appropriate power (5v for the transceiver and 3.3v for the DS18B20), GPIO (0 for the transceiver and 7 for the DS18B20) and GND:

The system pictured above has an Edimax WiFi dongle so that I can control and monitor the whole thing when it’s in the kitchen.

The software

Update 14 Apr 2013 – the code (slightly improved over what’s below) is now available on GitHub along with (hopefully comprehensive) installation instructions.

It’s not especially pretty, but seems to be doing the job:

import os
from subprocess import Popen, PIPE, call
from optparse import OptionParser
from time import sleep

def tempdata():
    # Replace 28-000003ae0350 with the address of your DS18B20
    pipe = Popen(["cat","/sys/bus/w1/devices/w1_bus_master1/28-000003ea0350/w1_slave"], stdout=PIPE)
    result = pipe.communicate()[0]
    result_list = result.split("=")
    temp_mC = int(result_list[-1]) # temp in milliCelcius
    return temp_mC

def setup_1wire():
  os.system("sudo modprobe w1-gpio && sudo modprobe w1-therm")

def turn_on():
  os.system("sudo ./strogonanoff_sender.py --channel 4 --button 1 --gpio 0 on")

def turn_off():
  os.system("sudo ./strogonanoff_sender.py --channel 4 --button 1 --gpio 0 off")
#Get command line options
parser = OptionParser()
parser.add_option("-t", "--target", type = int, default = 55)
parser.add_option("-p", "--prop", type = int, default = 8)
parser.add_option("-i", "--integral", type = int, default = 2)
(options, args) = parser.parse_args()
target = options.target * 1000
print ('Target temp is %d' % (options.target))
P = options.prop
I = options.integral
# Initialise some variables for the control loop
interror = 0
pwr_cnt=1
pwr_tot=0
# Setup 1Wire for DS18B20
setup_1wire()
# Turn on for initial ramp up
state="on"
turn_on()

temperature=tempdata()
print("Initial temperature ramp up")
while (target - temperature > 3000):
    sleep(15)
    temperature=tempdata()
    print(temperature)

print("Entering control loop")
while True:
    temperature=tempdata()
    print(temperature)
    error = target - temperature
    interror = interror + error
    power = ((P * error) + ((I * interror)/100))/100
    print power
    if (power > 0):
        pwr_tot = pwr_tot + power
    pwr_ave = pwr_tot / pwr_cnt
    pwr_cnt = pwr_cnt + 1
    print pwr_ave
    # Long duration pulse width modulation
    for x in range (1, 100):
        if (power > x):
            if (state=="off"):
                state="on"
                print("On")
                turn_on()
        else:
            if (state=="on"):
                state="off"
                print("Off")
                turn_off()
        sleep(1)

Conclusion

I’ve not had the chance to use the water bath for cooking yet, but I’m very much looking forward to trying out a few steaks and maybe some Sunday roasts. I’ll report back on the results.