Sitl model

Matteo_Scanavino · December 12, 2018, 3:30pm

Hello,

I’m using px4 SITL and HITL with iris model. I’d like to integrate my copter and other sensors to perform simulation in gazebo.

I wonder if someone can give me some details on the mathematical model implemented in SITL. I think the dynamic behaviour of the copter is based on the equations for forces and moments of the rigid body. I wonder how the propulsion system is modeld, as well as all the other sensors provided in the simulation (i.e. IMU, GPS etc.). How about the aerodynamic forces for the fixed wing simulation? Are they based on real aerdodynamic coefficients?

Moreover, I’ve noted that when I’m testing SITL or HITL the battery level goes down to 51% but then it doesn’t change even if I run the simulation for a long time. How is implemented the battery behaviour?

Thankyou in advance

JulianOes · December 12, 2018, 4:06pm

The battery simulation happens here:

github.com

PX4/PX4-Autopilot/blob/f8519781011a6d0ca3114a8854b2b8f06832a283/src/modules/simulator/simulator_mavlink.cpp#L359-L384


      
          			// battery simulation (limit update to 100Hz)
          			if (hrt_elapsed_time(&_battery_status.timestamp) >= 10000) {
          
          				const float discharge_interval_us = _battery_drain_interval_s.get() * 1000 * 1000;
          
          				bool armed = (_vehicle_status.arming_state == vehicle_status_s::ARMING_STATE_ARMED);
          
          				if (!armed || batt_sim_start == 0 || batt_sim_start > now) {
          					batt_sim_start = now;
          				}
          
          				float ibatt = -1.0f; // no current sensor in simulation
          				const float minimum_percentage = 0.5f; // change this value if you want to simulate low battery reaction
          
          				/* Simulate the voltage of a linearly draining battery but stop at the minimum percentage */
          				float battery_percentage = (now - batt_sim_start) / discharge_interval_us;
          				battery_percentage = math::min(battery_percentage, minimum_percentage);
          				float vbatt = math::gradual(battery_percentage, 0.f, 1.f, _battery.full_cell_voltage(), _battery.empty_cell_voltage());
          				vbatt *= _battery.cell_count();

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Matteo_Scanavino · December 12, 2018, 9:56pm

Thank-you! That’s interesting!
Do you know if ground effect is modeled in SITL?

JulianOes · December 13, 2018, 6:33am

From a search I didn’t see anything:
https://github.com/PX4/sitl_gazebo/search?p=3&q=ground&unscoped_q=ground

shrit · December 16, 2018, 10:09pm

Gazebo can simulate an entire world.
You need to have look at the following:
http://gazebosim.org/tutorials?tut=architecture&cat=get_started

Gazebo have good tutorial, it worth to speed a time reading it.

Finally, you can choose for the simulated model wether if should be affected by
the real world physics or not.