Sensors and sensing

Learning outcomes

Understand

what a sensor is
what sensor deviation (or error) is
how to acquire sensor readings using the Telos Mote

What is a sensor?

“A sensor is a transducer whose purpose is to sense (that is, to detect) some characteristic of its environs.” — wikipedia

A perfect sensor is only sensitive to the measured property and insensitive to other properties while not influencing the environment.

Sensor deviation

A sensor measurement may be different from the true value due to systematic errors, e.g.:

bias
non-linearity
drift
hysteresis (or memory effects)

or random errors, e.g.:

digitization errors
noise

How to reduce sensor deviation?

Bias and non-linearity are generally lessened through calibration

Noise is usually reduced by filtering

e.g. moving average, Kalman filter, etc

However, do not forget that some noise may be resolved by

better sensor design (improved analogue circuitry)
shielding cables from interference

Sensor types

temperature

humidity

gas concentration (e.g. CO2)

electricity (W or kWh)

switch or push button

pressure

acceleration

acoustic

hydrophone

fluid or gas flow rates

radiant heat

proximity (infrared)

magnetic (e.g. compass)

location (GPS)

gyroscopic

also see http://en.wikipedia.org/wiki/List_of_sensors

Telos Mote Hardware (front)

Telos expansion connector

Obtaining sensor data in Contiki

Driver files for the Telos mote are available under arch/platform/sky/dev

To find a file starting with sht11 use:

find . -name "sht11*"

Each sensor needs to be declared as:

SENSORS_SENSOR(sensor, NAME, value, configure, status);

For example, the battery sensor for the Telos is in arch/platform/sky/dev/battery_sensor.c and in there you will see:

SENSORS_SENSOR(battery_sensor, BATTERY_SENSOR, value, configure, status);

Things you can do to a sensor

The following C will activate, deactivate, and obtain the value from a sensor.

SENSORS_ACTIVATE(sensor)    
SENSORS_DEACTIVATE(sensor)  
sensor.value(type);

The type parameter depends on the sensor and refers to the type of reading you are requesting (e.g., temperature or humidity)

Battery example

#include "dev/battery-sensor.h"
#include "stdio.h"
/*---------------------------------------------------------------------------*/
PROCESS(test_battery_process, "Battery Sensor Test");
AUTOSTART_PROCESSES(&test_battery_process);
/*---------------------------------------------------------------------------*/
PROCESS_THREAD(test_battery_process, ev, data)
{
  static uint32_t battery;
  PROCESS_BEGIN();
  SENSORS_ACTIVATE(battery_sensor);

  while(1) {
    battery = battery_sensor.value(0);
    battery *= 5000;
    battery /= 4095;
    printf("Battery: %u.%03uV\n", (uint16_t)battery / 1000, (uint16_t)battery % 1000);
  }
  PROCESS_END();
}

Excerpt From: Antonio Liñán Colina, Alvaro Vives, Antoine Bagula, Marco Zennaro and Ermanno Pietrosemoli. “Internet of Things (IoT) in 5 days”. (added include for stdio.h)

Telos sensor example

#include "stdio.h"
#include "contiki.h"
#include "dev/sensor/sht11/sht11.h"
#include "dev/sensor/sht11/sht11-sensor.h"

PROCESS(temp, "Temperature");
AUTOSTART_PROCESSES(&temp);
PROCESS_THREAD(temp, ev, data)
{
  static struct etimer timer;
  PROCESS_BEGIN();

  /* Setup a periodic timer that expires after 10 seconds. */
  etimer_set(&timer, CLOCK_SECOND * 10);
  SENSORS_ACTIVATE(sht11_sensor);
  while (1) {
    PROCESS_WAIT_EVENT_UNTIL(etimer_expired(&timer));
    int temp = sht11_sensor.value(SHT11_SENSOR_TEMP);
    printf("temp: %d\n", temp);
    etimer_reset(&timer);
  }

  PROCESS_END();
}

Notes

The above code does not interpret the integer value returned. According to the data sheet, this value must be divided by 100 and 39.6 subtracted \[ x/100-39.6 \] In C, you need something like:
```
float s = ((0.01*val) - 39.60);
int dec = s;
float frac = s - dec;
    
```
Other sensors can also be accessed by changing the parameter to value to SHT11_SENSOR_?. See the header files (in the arch/dev/sensor/sht11 directory) for a complete list of options.

Proxy or virtual sensors

Some sensors can be used to measure something seemingly unrelated - think about what the following might measure

light sensor inside cupboard
humidity sensor in a bathroom
CO2 sensor in bedroom (!)
CO2 sensor in a car cabin
temperature sensor in a pendant or worn device
pressure sensor at the foot of a quay (underwater)

Key things to remember

read the data sheet
use sample code from Internet (warily!)
be aware of conflicting use of pins
check conversion
- what is the true maximum adc value?
- max / min voltage?
deal with noise in hardware first (but software later)
- twist lead wires together
- reduce lead wire distance if possible
- filtering capacitor needed? placed where?

What we covered

what a sensor is
what sort of sensors are on the Telos mote
how to interface to them using Contiki code
how to obtain detailed interfacing information for specific devices

Additional reading

datasheet for SHT11 (temperature and humidity sensor on Telosb)
TelosB datasheet (search on web)

rwanda-summer-school-2022/10-sensing.org