Temperature two-point controller

For these devices, you can configure them to work autonomously meaning you can set them up in InstaCal with temperature thresholds (alarms) for digital IO on or off, and then position them to run remotely.  All you need is their power supply.   These device's digital outputs operate 5V or 3.3V with various amperage specs.

Here is a table summary:

InstaCal refers to these settings as alarms, and that is one use.  Another use is to use them to operate a closed loop system using any of these devices as a two point controller.  As stated many HVAC systems require a 24 VAC signal to be operated.  Here you see in most cases, the alarm (digital out) signal is a 5 V TTL logic signal, with low current.  In all cases, none of the above can output an AC voltage.  To remedy this, you can add a transistor driver and/or relay or TRIAC to control an HVAC or any other required control signal.

If you wanted to use channel 0 of an E-TC to monitor a room’s temperature and control a heater to keep that room at 25 °C, you can configuration the Alarm dialog box like this:

Here we see Channel 0’s alarm set to enabled.  It is set to turn on when a threshold is exceeded, and the output will use active high logic.

The threshold is set using the second option; to turn on the alarm when the measured temperature goes below 25 °C, and to turn off when the measured value goes above 26 °C.

Once the unit is configured, click OK, and close InstaCal.  

Below is a schematic for how you might wire an E-TC to a type T thermocouple, solid state relay and heater.

The E-TC and external wiring to the heater:

DASYLab has a built in module for 2 point control as part of all versions except DASYLab Lite. Here is what a basic two point control worksheet would look like:

LabVIEW does not have a specific 2 point control vi, but it can easily be resolved using a Less than/Equal vi as the decision maker.  Here is what a basic 2 point controller would look like in LabVIEW:

This is the basic 2 point control implementation.  From the front panel, you can adjust the set point, and monitor the performance of the system.  

Writing a program in Visual Basic .NET using the Universal Library provides greater flexibility.  In DASYLab and LabVIEW, there are a few more things going on that you don’t see.  Things handled by the DASYLab and LabVIEW environments such as finding the device and the handling of errors.  For device discovery, all the user needs do is pick the device from the list provided.  In syntax programming for a VB app, the programmer and user can be the same person, and so the programmer/user has to generate all the code, or get it from another example app.  The programmer will usually write the code segment once, and use it in as many places as possible making slight changes along the way.  This is exactly what I have done here.  So though I wrote the device discovery and error handling routines, I’m not going to show them here, but that code can be examined and freely used, please see the attachment.  Another thing not being shown is the code behind the LED, thermometer, and strip chart.  I created these objects as a way to make the data read and generated in my sample programs easier understand.  So, again, I am not going to show the code behind them, but if you want to use them, they are free to use for your own use.

Here is the routine in the Timer_tick event used to loop through reading the thermocouple, determine if the 2 point controller should be on or off, and operate the digital bit controlling the relay and heater.

As you can see, using Measurement Computing temperature measurement devices and software products provide many ways to resolve the need to implement a two point temperature controller.  Pick the combination of hardware and software meeting your needs.

If you need better temperature control than a two point limit, then perhaps you need a proportional controller or PID controller. But that’s a topic for another knowledgebase article.