Sensor acquisition

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Introduction

This article helps you learn how to create the Sensor Acquisition Program, which can be used to show the Temperature, Humidity and Light Intensity in the vicinity of Tmote Sky.

How to start an application on Contiki

Every process should start with the PROCESS macro. It takes two arguments

• name: The variable name of the process structure.
• strname: The string representation of the process name.

PROCESS(name,strname)

Then comes another macro AUTOSTART_PROCESS(struct process &). AUTOSTART_PROCESSES automatically starts the process(es) given in the argument(s) when the module boots.

• &name: Reference to the process name.

AUTOSTART_PROCESS(struct process &)

Then we call the PROCESS_THREAD function. This function is used to define the protothread of a process. The process is called whenever an event occurs in the system.Each process in the module requires an event handler under the PROCESS_THREAD macro.

• name: The variable name of the process structure.
• process_event_t: The variable of type character.If this variable is same as PROCESS_EVENT_EXIT then PROCESS_EXITHANDLER is invoked.

PROCESS_THREAD(name, process_event_t, process_data_t)

Then comes the PROCESS_BEGIN macro. This macro defines the beginning of a process, and must always appear in a PROCESS_THREAD() definition.

PROCESS_BEGIN()

Then comes the set of C statements that you require to write your application.

At the end we use another macro PROCESS_END. This macro defines the end of a process. It must appear in a PROCESS_THREAD() definition and must always be included. The process exits when the PROCESS_END() macro is reached.

PROCESS_END()

Sensor Acquisition Program on the Tmote Sky!

Step 1

Using the opened terminal window compile and upload the Sensor Acquisition program on the Tmote Sky.

make TARGET=sky savetarget (This save the target for any future compilations)

make sensor-acq.upload (This will upload the code on the Tmote Sky)

make login (This will enable us to view the output. If permission error occurs, use sudo command at the beginning)

See the following link for troubleshooting - http://anrg.usc.edu/contiki/index.php/Troubleshooting

Step 2

Press the reset button on the Tmote Sky. The following message will appear on the terminal window

Temperature=28.04 C (6764)

Humidity=73.20% (2259)

Light=38.26 lux (94)

Values inside the round brackets (ex 6764, 2259 and 94) are the actual sensor values and the calibrated values can be obtained by performing calculations on these sensor values. You can find the calculations at http://tinyos.stanford.edu/tinyos-wiki/index.php/Boomerang_ADC_Example

Step 3

Press Ctrl-C to quit.

Code

PROCESS(sensor_acq_process,"Sensor Acquisition");
AUTOSTART_PROCESSES(&sensor_acq_process);

PROCESS_THREAD(sensor_acq_process,ev,data)
{ 
      static struct etimer et;
      static int val;
      static struct sensors_sensor *sensor;
      static float s = 0;
      static int dec;
      static float frac;

      PROCESS_BEGIN();

      printf("Starting Sensor Example.\n");
      
      while(1)
      {
	   etimer_set(&et, CLOCK_SECOND * 2);
	   SENSORS_ACTIVATE(light_sensor);
      	   SENSORS_ACTIVATE(sht11_sensor);
        
	   PROCESS_WAIT_EVENT_UNTIL(etimer_expired(&et));


           val = sht11_sensor.value(SHT11_SENSOR_TEMP);
      	   if(val != -1) 
      	   {
		s= ((0.01*val) - 39.60);
      	  	dec = s;
      	  	frac = s - dec;
      	  	printf("\nTemperature=%d.%02u C (%d)\n", dec, (unsigned int)(frac * 100),val);               
           }

	   val=sht11_sensor.value(SHT11_SENSOR_HUMIDITY);
	   if(val != -1) 
      	   {
		s= (((0.0405*val) - 4) + ((-2.8 * 0.000001)*(pow(val,2))));  
      	  	dec = s;
      	  	frac = s - dec;
      	  	printf("Humidity=%d.%02u % (%d)\n", dec, (unsigned int)(frac * 100),val);               
           }

           val = light_sensor.value(LIGHT_SENSOR_TOTAL_SOLAR);
      	   if(val != -1) 
      	   {
      		s = (float)(val * 0.4071);
      	  	dec = s;
      	  	frac = s - dec;
      	  	printf("Light=%d.%02u lux (%d)\n", dec, (unsigned int)(frac * 100),val);               
           } 
	
	   etimer_reset(&et);
    	   SENSORS_DEACTIVATE(light_sensor);
    	   SENSORS_DEACTIVATE(sht11_sensor);

      } //end of while
    
      PROCESS_END();

}

 

Understanding the Code

etimer_set(&et, CLOCK_SECOND * 2);

This will set the timer to repeat the iterations every 2 seconds.

SENSORS_ACTIVATE(light_sensor);

SENSORS_ACTIVATE(sht11_sensor);

We need to activate light_sensor for measuring the light intensity and sht11_sensor for the measurement of temperature and humidity.

val = sht11_sensor.value(SHT11_SENSOR_TEMP);

Here we are capturing the actual sensor value.

s = ((0.01*val) - 39.60);

We need to calibrate the sensor values by doing some calculations. You can find the calculations here: http://tinyos.stanford.edu/tinyos-wiki/index.php/Boomerang_ADC_Example

SENSORS_DEACTIVATE(light_sensor);

SENSORS_DEACTIVATE(sht11_sensor);

After we are done with the calculations, we need to deactivate the sensors.

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Edited by : Nitin