Design Considerations

Although the immediate objective is to get one node up, running and logging it’s important to try and assess where the project might go next. It’s particularly true of programming in order to save much time consuming software rewriting and to ensure the programs have a suitable structure.

So what follows is a short analysis of the project

What: To measure and record the temperature at a location.

From this simple statement a surprising number of topics arise.

measure	Units	Celcius
	Accuracy	0.5 deg
	Device	XBee node identifier
	Frequency	variable
record	Data storage system	Could be a simple flat file or a relational database system
	record specification	date, time, temperature
location	where	node location

So, looking at the table above, the measurement process encompasses the TMP36 sensor, how it is to be used in practice and the XBee’s analogue to digital conversion(DAC). Then there are the calculations to be performed on the returned data to convert a voltage value into a temperature. Also, if 0.5 degrees is chosen as the required accuracy then this needs to be checked against the TMP36’s sensitivity and XBee DAC conversion process.

In terms of recording the data then it’s important to record more than just temperature. It’s important to know when and where the temperature was measured and probably by which sensor.

Looking ahead, if the project continues, there will be multiple sensors of various types and this suggests a structured database rather than a separate file for each XBee node (or measurement point since an XBee has multiple inputs). (The open software MySQL looks a good candidate for a database solution especially since there is good documentation for using it with Java).

Why: To correlate the effect of temperature on energy consumption

The main objective of the project is to measure, record, compare and analyse. With two sources of data this will not be so hard but it can quickly be envisaged that if the sources of data grow in number, the data will have to be saved and structured in a very flexible and organised way.

When: For specified periods of the day

It is anticipated that the measurement process will be continuous and the specified periods of interest will be selected at the time of analysis from the recorded data.

Where: Inside and outside

The most obvious temperatures to measure are inside and outside but this could, for example, be as varied as a pipe temperature or air temperature. This highlights that multiple nodes will be required. (No problem because an XBee network can in theory support over 4 billion nodes). But it also raises questions about construction. If a node is outside it will need protecting from the elements. Not a programming issue but very important all the same.

How: Using a wireless network.

At the moment the wireless network will be using XBee’s. But as mentioned in previous posts there are alternatives that could be much less expensive. With that in mind it might be wise to keep the software interface that collects data from the nodes as modular as possible. Being wireless also implies battery powered and battery life will be important.

XBee’s do have a facility whereby they can sleep and use minimal power but to use this facility requires configuration.