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I’ve been very happy with the results from my Raspberry Pi controlled water bath for sous vide cooking, but I knew that the control loop could be improved. Past runs show fairly continued oscillation:
Roast beef temps2
Roast beef run at 60C
I’ve been keeping track of the average power for my control loop, which has been coming out at 22%. So i modified the code to have a bias of 22%, and here’s the result:
Test run at 55C
Test run at 55C
Overall much more stable. The occasional hiccups are probably caused by the remote socket failing to receive off commands. There’s a 3C overshoot at the start, which I hope to have fixed by entering the control loop from initial warm up 3C earlier.
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 = 6) parser.add_option("-i", "--integral", type = int, default = 2) parser.add_option("-b", "--bias", type = int, default = 22) (options, args) = parser.parse_args() target = options.target * 1000 print ('Target temp is %d' % (options.target)) P = options.prop I = options.integral B = options.bias # 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 > 6000):     sleep(15)     temperature=tempdata()     print(temperature) print("Entering control loop") while True:     temperature=tempdata()     print(temperature)     error = target - temperature     interror = interror + error     power = B + ((P * error) + ((I * interror)/100))/100     print power     # Make sure that if we should be off then we are     if (state=="off"):         turn_off()     # 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)
http://blog.thestateofme.com/2013/05/12/improved-temperature-control-for-raspberry-pi-sous-vide/
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.
来自：http://blog.thestateofme.com/2013/03/31/raspberry-pi-sous-vide-water-bath/
参考：
http://blog.thestateofme.com/2013/01/28/ds18b20-rpi/
http://www.flashingleds.net/sousvader/sousvader.html
自己参考：
（1）Raspberry Pi PWM编写的方法：
http://jackxiang.com/post/6591/
（2）树莓派Raspberry和ds18B20接合,可以接多个Ds18B20：http://jackxiang.com/post/6588/

作者：jackxiang@向东博客 专注WEB应用 构架之美 --- 构架之美，在于尽态极妍 | 应用之美，在于药到病除
地址：http://jackxiang.com/post/6599/
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