Project Regarding The Use of Temperature Controller

[mrwheezy]

My teacher and I want to make an incubator that will provide an environment with a optimal temperature of 37C. This is our final year project since now that AP exams are over...

Our problem is trying to get a controller that will regulate the temperature within the incubation tank.

I proposed two methods,
1) To use a computer to monitor the temperature by using the temperature sensor built into motherboards/CPUs/etc and to regulate fan speed within the incubation tank through the computer as well.
2) Buy a temperature controller that's used in a thermostat, a cheap on/off system.

However, the problem that I'm facing for both methods is
1) How do I connect our heat source (a heating pad) to the temperature monitor, be it a on/off system or the computer.
2) I don't have any clue about circuit boards or electrical engineering or programming. But I'm more than willing to learn.

Does anyone know the best way to go about this? Which method would work out best (hopefully with the cheapest price)?

 

[berkeman]

My teacher and I want to make an incubator that will provide an environment with a optimal temperature of 37C. This is our final year project since now that AP exams are over...

Our problem is trying to get a controller that will regulate the temperature within the incubation tank.

I proposed two methods,
1) To use a computer to monitor the temperature by using the temperature sensor built into motherboards/CPUs/etc and to regulate fan speed within the incubation tank through the computer as well.
2) Buy a temperature controller that's used in a thermostat, a cheap on/off system.

However, the problem that I'm facing for both methods is
1) How do I connect our heat source (a heating pad) to the temperature monitor, be it a on/off system or the computer.
2) I don't have any clue about circuit boards or electrical engineering or programming. But I'm more than willing to learn.

Does anyone know the best way to go about this? Which method would work out best (hopefully with the cheapest price)?

Can you just use a normal home heater thermostat, and use that to control a relay that turns power on and off to the heating element? That's how many home thermostats & heaters work.

 

[spacester]

What you need is available off the shelf and is quite affordable at ~$100.

A thermostat will always produce a temperature cycle due to the "deadband" between on and off settings. You need much tighter control for your experiment.

What you need is a controller that will decrease the amount of heating as you get closer to the set point, and then provide just the right amount of current to maintain the temperature within a very narrow band. This is a very common task in industry, controllers abound for this, and is called Proportional Control. It is actually the simplest form of a very sophisticated control scheme called Proportional Integral Derivative.

http://en.wikipedia.org/wiki/PID_controller

The devil is in the details, and there are a lot of them. You will need to do some studying, but it will be worth it. The short version is that you can get a panel-mounted controller that will use a thermistor or thermocouple sensor and directly control the current to your heater and give you very nice control indeed.

You can buy that controller, open the box, then take a year or two to learn Japanese so you can read the manual and program the thing. :-) It will have an English version but they totally assume you know about PID control systems. They can't have just anybody doing this stuff you know.

How many watts do you figure you will need that heater to deliver?

 

[Feldoh]

Hmm to be honest making your own computer controlled heating element is rather difficult if you've never done it before.

Using a crude on/off heating element will allow the temperatures to fluctuate by a decent amount (Somewhere in the range +/- 5 to 10 degrees). If this doesn't matter than you job is a lot easier.

If however you want to control the temperature in the range of +/- 1 degree or less as spacester said you're going to want a Proportional, integral, or differential control of the heat, or some combination of all three (i.e. the well recognized PID control)

In terms of simple electronics you'll want a thermistor attached to a heating element if the space you're trying to heat is small enough. Thermistor's allow you to essentially relate a voltage reading to a temperature. Using a digital multimeter you could easily digitally acquire a voltage reading which could be read into a computer and converted into a temperature.

Once the computer knows the temperature of the environment you have two choices:

1.) Manually control the power the heating element receives which has the obvious downsides

2.) Regulate the power with the computer using something like PID control. For this you'd need some type of voltage source that allows for computer control. Most digital power supplies allow this.

Some other things you'll need to know are more on the engineering side: how many watts is your heating element rated for? how much power can you supply from your power supply, etc...