How to calculate total energy consumption using Cooja

Question

I'm working with wireless sensor network lead to evaluate its performance in my work. I want to measure the latency and total energy consumption to find the remaining energy in each node. So my problem is that I have some values of tx rx cpu cpu_idle and I don't how to use them to calculate what I need. I found some rules that take the calculation but i don't understand exactly how to apply it in my case.

Energy consumed in communication:

Energy consumed by CPU:

What is the meaning of 32768, and why do we use this number? Is it a standard value?

Where did you find that case study, please declare the source of these pictures or paper. In general, there aren't any rules for a good energy calculation. It depends on switched on peripherals, used processor, speed, etc. — Tom Kuschel

kfx kfx · Accepted Answer · 2017-08-12T11:10:50

The powertrace output is printed in timer ticks.

tx - the number of ticks the radio has been in transmit mode (ENERGEST_TYPE_TRANSMIT)
rx - the number of ticks the radio has been in receive mode (ENERGEST_TYPE_LISTEN)
cpu - the number of ticks the CPU has been in active mode (ENERGEST_TYPE_CPU)
cpu_idle - the number of ticks the CPU has been in idle mode (ENERGEST_TYPE_LPM)

The elements of the pair tx and rx are exclusive, as are cpu and idle - the system can never be in both modes at the same time. However, other combinations are possible: it can be in cpu and in tx at the same time, for example. The sum of cpu and idle is the total uptime of the system.

The duration of timer a tick is platform-dependent and defined as the RTIMER_ARCH_SECOND constant. 32768 ticks per second is a typical value of this constant - that's where the number in your equation comes from. For example:

ticks_in_tx_mode = energest_type_time(ENERGEST_TYPE_TRANSMIT);
seconds_in_tx_mode = ticks_in_tx_mode / RTIMER_ARCH_SECOND;

To compute the average current consumption (in milliamperes, mA), multiply each of tx, rx, cpu, cpu_idle with the respective current consumption in that mode in mA (obtain the values from the datasheet of the node), sum them up, and divide by RTIMER_ARCH_SECOND:

current = (tx * current_tx_mode + rx * current_rx_mode + \
          cpu * current_cpu + cpu_idle * current_idle) / RTIMER_ARCH_SECOND

To compute the charge (in millicoulumbs, mC), multiply the average current consumption with the duration of the measurement (node's uptime) in seconds:

charge = current * (cpu + cpu_idle) / RTIMER_ARCH_SECOND

To compute the power (in milliwats, mW) multiply the average current consumption with the voltage of the system, for exampe, 3 volts if powered from a pair of AA batteries:

power = current * voltage

Finally, to compute the energy consumption (in millijoules, mJ), multiply the power with the duration in seconds or multiply the charge with the voltage of the system:

energy = charge * voltage

The first formula above computes the energy consumption for communications; the second one: for computation.

This site might be helpful to break down the numbers.

How to calculate total energy consumption using Cooja

2 Answers