November 2005, Issue 184

Water Pressure Sensor
ZigBee-Based NozzleMon Aids Firefighters

SOFTWARE DEVELOPMENT

The software for this project is based on the lighting demonstration software that comes with Freescale’s SMAC package. The pump panel unit’s code simply sends out a polling message for the nozzle unit to respond and then waits for a response or a timeout. The only possible responses from the nozzle unit are either a nozzle pressure to report or a “nozzle out of control” message, which is encoded as a nozzle pressure of 255 psi (an otherwise impossible reading).

Managing the front panel switches and displays is a straightforward process, especially because the LCD assembly contains a Hitachi HD44780U controller that’s compatible with the routines included in the LCD interface in the SMAC package. More sophisticated communication and display management will be required with multiple nozzles to monitor.

Multiple nozzles can be accommodated in the same software structure by polling to see how many nozzles are enabled and adjusting the front panel to display the pressure and state of the nozzles. Polling each of the nozzles enables the pump panel unit to control the flow of the data and manage the available bandwidth as opposed to having the nozzle units broadcast continuously. This should keep enough bandwidth available for other 802.15.4 applications to be used on the fire scene.

The only computations performed by the pump panel unit are the current average pressure and the status of the set pressure warning. The average pressure is the sum of the last 16 valid readings right-shifted four places. The set pressure is determined by storing the current average when you depress the red button. After it’s set, the current average is compared with the stored pressure upon each valid pressure reading. If the current average is more than 10 psi above or below the set pressure, the LED lights up and a message appears.

The nozzle unit code is also simple. It waits for a polling request from the pump panel unit and takes readings from its sensors when a request is received. The response is a single byte that’s either the current pressure or a flag that the nozzle is out of control. The pressure value returned is derived from using the actual reading from the sensor to index into a look-up table of values that was collected during the device’s calibration (using the pressure gauge mounted on the brass nipple).

Listing 1 shows the code to read the pressure sensor and index into the look-up table. Keeping the response payload to a single byte reduces the required bandwidth and pump panel unit’s computational requirements.