Power control switch for 115VAC wall outlet to operate resistance heating element.

Question

I am very new to LabView software as well as hardware
interfacing, so don't presume that I know anything.  My problem seems
simple, I want to be able to control and automate via LabView software a resistance
heating element that can plug into a standard 115VAC wall outlet.  I
searched through the Knowledge Base and found this comment:
http://digital.ni.com/public.nsf/allkb/855AF9CBDAACE3018625725A00638653
The PXI 2564 seemed ideal but I have no clue how to connect the serial port to
my heating element, or even if this is enough current (5A, do heating elements
need a certain wattage?)  I had originally envisioned a simple surge
protector like device that had computer control options.  Many devices
like this are available, however all the ones I have found are web-accessed via IP address
and I am unsure how to interface this type of device with LabView if it has its
own GUI.  Obviously I have no idea what I am doing here so any help would
be much appreciated.  The heating element requires 115VAC or DC it does
not matter, and all I need to do is turn the power on and off, no variance is
required, although that could be very useful.  I have implemented a NI
CompactDAQ for other control and measurement (24VDC Digital I/O and Thermocouple) if this
could somehow play into my solution, but I believe the limits on voltage for
this device are much lower.  Thanks!

Answer 1

Hi Anfu,
LabVIEW gives you the power to control and automate all National Instruments hardware using NI drivers (i.e. NI-DAQmx).  I have a couple of NI hardware suggestions, but it's up to you to decide on the hardware that will best fit your application.  Even if you end up using third-party hardware, I think you will find LabVIEW is a great interface to control and/or automate your hardware.
The resistance heating element you want to control...you state it is powered via wall voltage, but you did not state the required current.  This is important information you should be able to find in the heating element's data sheet.  The amount of current you need at 115 VAC (or DC) will determine your desired relay characteristics.  Once you know the voltage and current (or power) the heating element requires, you can calculate the hardware specifications that are necessary for your application - and can confidently select the correct hardware.
Starting from the fact you need to switch wall voltage, you will want a switch rated for Measurement Category II (CAT II).  This classification ensures you will be kept safe from the high voltages being switched.  National Instruments has PXI hardware that can switch wall voltages, as you mentioned previously, and one of these switches is the PXI-2564.  Your computer would communicate with the PXI-2564 through its PCI bus, not its serial port.  Several methods to connect your PCI bus to PXI is to purchase a MXI-4 card (this would fit in one of your computer's PCI slots) that cables to a PXI chassis (in which the PXI-2564 is located).  Also, you can purchase an embedded PXI controller (then you wouldn't need a separate computer, as the embedded PXI controller houses a laptop-like computer).  In total, you would need a PXI-2564, a PXI chassis, and either a MXI-4 or embedded PXI controller.
If PXI doesn't seem ideal, instead you could create a Plug-and-Play (PnP) SCXI system!  First, choose a SCXI switch module (such as a SCXI-1127).  Then, you can purchase a USB-1357 to control the SCXI-1127 module from one of your computer's USB slots.  In total, you would need a SCXI-1127 module, a 4-slot SCXI chassis, and a USB-1357.
Another option is the SCXI-1161.  Unfortunately, the USB-1357 doesn't connect to the rear of the SCXI-1161, but there is there are two additional options available for controlling the SCXI-1161.  One is to purchase a PCI DAQ module (or use one if you already have one) and connect it to the SCXI-1161 using the SCXI-1349.  The other is to purchase a SCXI-1600 (which is a DAQ board, SCXI form factor) that you control via USB.  In total, you would need a SCXI-1161, a 4-slot SCXI chassis, and a SCXI-1600 or a DAQ board and a SCXI-1349.
Either way, the PXI and SCXI options both give you the full functionality of a modular, LabVIEW-controlled switch.  Upon purchasing the switch module, you also get the required drivers, getting-started and troubleshooting tools (such as the NI-SWITCH Soft Front Panel), and examples to get you up and running quickly.  
In reference to the simple surge protector like device that has computer control options - I'm sure someone from the LabVIEW forum could assist you in communicating to one of these devices via the web.  Don't worry, if the device can be controlled via the web, you're good to go. 
Lastly, in response to your final statements, it is important to find out how much current or power the heating element requires because that will directly determine your required switch hardware specifications.  You mention no variance is required, which is good, because the switch will simply turn on or off the power to the heating element.  I agree a NI CompactDAQ option could be very useful for an application such as this, however there are no CAT II relay or switch modules for this form factor at this time.
Hope this helps!
Chad Erickson
Switch Product Support Engineer
NI - USA