6. Examples

6.1. Experimental Procedure for LED Control Experiment

The flashing frequency and color of the LED on the Pixhawk hardware can be controlled by designing controller in Simulink. This section takes a simple LED light control experiment [1] as an example to introduce the operation process of the hardware and software components of the experimental platform released with this book.

6.1.1. Experimental Objective

As shown in Fig. 3.54, use two channels from CH1 to CH5 of the RC transmitter to control the LED light on Pixhawk in two different colors and two different modes.

Fig. 3.54 Hardware connection of LED control experiment

6.1.2. Experimental Procedure

The experiment uses the “RGB_LED” module in the PSP toolbox introduced in the previous chapter (see Fig. 3.55) to control the LED light on Pixhawk.

(1). Create and open a new Simulink model file. As shown in Fig. 3.55, find the “RGB_LED” module in the “Pixhawk Target Blocks” toolbox, which is in the “Library Browser” of Simulink, and drag it to the new created Simulink file. The RC transmitter module “input_rc” in Fig. 3.55 is also dragged into the Simulink as the input signals to control the LED light. A Simulink example file completing the whole process of configuration is available in “e02.PSPOfficialExpspx4demo_input_rc.slx”.

Fig. 3.55 LED output interface model for PSP toolbox

(2). The RBG_LED module can be used to control the mode and color of the LED light on Pixhawk. Its mode enumeration variable “RGBLED_MODE_ENUM” includes the following seven options

• SL MODE OFF
• SL MODE ON
• SL MODE DISABLED
• SL MODE BLINK SLOW
• SL MODE BLINK NORMAL
• SL MODE BLINK FAST
• SL MODE BREATHE.

The color variable “RGBLED_COLOR_ENUM” has the following options

• SL COLOR OFF
• SL COLOR RED
• SL COLOR GREEN
• SL COLOR BLUE
• SL COLOR YELLOW
• SL COLOR PURPLE
• SL COLOR AMBER
• SL COLOR CYAN
• SL COLOR WHITE.

The above variables have been registered as MATLAB global parameters when installing the PSP toolbox so that they can be directly applied. For example, an LED light with blue color and fast blinking mode can be obtained by sending the variable “RGBLED_MODE_ENUM.SL_MODE_BLINK_FAST” with the “Constant” module to the “Mode” port of the LED module (the upper input port of the LED module), and sending the variable “RGBLED_COLOR_ENUM.SL_COLOR_BLUE” with the “Constant” module to the “Color” port (the lower input port of the LED module).

(3). Controller design. According to the introduction to the RC system provided in the previous chapter, the data range of the PWM signals received by Pixhawk is 1100–1900. As shown in Fig. 3.56, we will use two channels of the RC transmitter in this experiment to control the mode and color of the LED light. The controller design procedure is described next.

Fig. 3.56 LED controller with an RC transmitter

1). Use the CH3 channel of the RC transmitter (see the throttle channel in Fig. 3.54) to change the blink mode of the LED light. When the throttle stick is turned to the upper side, i.e., when the PWM value of the CH3 channel is higher than 1500 microseconds (abbreviated as CH3>1500), then the “Mode” port receives an “RGBLED_MODE_ENUM.SL_MODE_BLINK_FAST” variable corresponding to a fast blinking mode; when CH3≤1500, the “Mode” port receives the “RGBLED_MODE_ENUM.SL_MODE_BLINK_NORMAL” variable corresponding to a slow blinking mode.

2). Use the CH4 channel of the RC transmitter to change the color of the LED light. When CH4>1500, the “Color” port receives an “RGBLED_COLOR_ENUM.COLOR_RED” variable corresponding to red color; when CH4≤1500, the “Color” port receives an “RGBLED_COLOR_ENUM. COLOR_BLUE” variable corresponding to blue color.

6.1.3. Controller Code Generation and Firmware Uploading

(1). For MATLAB 2017b–2019a, as shown in Fig. 3.57, click the “Simulation”—“Model Configuration Parameters” option on the Simulink menu bar to enter the Simulink setting dialog; for MATLAB 2019b and above, click the “Settings” button. The obtained Simulink setting window is presented in Fig. 3.58.

Fig. 3.57 “Model Configuration parameters” option on Simulink

Fig. 3.58 Simulink setting dialog

(2). Select the target hardware. As shown in Fig. 3.59, set the option “Hardware Implementation”—“Hardware Board” to “Pixhawk PX4”.

Fig. 3.59 Setting target hardware to Pixhawk PX4

(3). Compile the model. As shown in Fig. 3.60, compile the designed controller into the PX4 firmware file by clicking the “Build” button on the Simulink toolbar. The code generation and firmware compiling process can be observed in the “Diagnostic Viewer” window of Simulink.

Fig. 3.60 Button for compiling code in Simulink

For MATLAB 2017b–2019a, it can be opened by clicking on the “View diagnostics” button in the status bar below Simulink (see the lower box in Fig. 3.61) or by clicking the “View” - “Diagnostic Viewer” (see Fig. 3.61) button on the Simulink menu. For MATLAB 2019b and above, click the “Diagnostics” button to open the “Diagnostic Viewer” window.

Fig. 3.61 “Diagnostics” option in different Simulink versions

As shown in Fig. 3.62, the code and firmware are compiled successfully when the “Build process completed successfully” message appears in the “Diagnostic Viewer” window. A code generation report shown in Fig. 3.63 can also be obtained. At this point, the corresponding C/C++ code files have been generated in the folder “Firmwaresrcmodulespx4_simulink_app”, and the “make px4fmu-v3_default” command has been called to complete the firmware compilation process.

Fig. 3.62 Diagnostic viewer showing “build process completed successfully”

Fig. 3.63 Generated report after compiling

(4). Upload the firmware. Use the one-key upload function provided by the PSP toolbox to upload the PX4 firmware file. The specific steps are presented as follows.

1). Connect the MicroUSB port of the Pixhawk hardware with the computer by using a USB cable.

2). For MATLAB 2017b–2019a, as shown in Fig. 3.64, click the “Code”-“PX4 PSP: Upload code to Px4FMU” in the Simulink menu bar; for MATLAB 2019b and above, input the “PX4Upload” command in the MATLAB “Command Window” according to Fig. 3.40b to upload the firmware.

Fig. 3.64 Simulink firmware upload menu

3). As shown in Fig. 3.65, Simulink will automatically recognize the Pixhawk autopilot and start to upload and deploy the PX4 firmware file. The uploading and deploying process is successfully completed when the progress bar reaches 100%. Note that Pixhawk may need to be re-plugged in some cases to start the upload progress.

Fig. 3.65 Firmware is successfully uploaded

6.1.4. Experimental Result

By default, when the RC transmitter does nothing, the LED light is slowly blinking in blue. As shown in Fig. 3.66, do the following steps to verify the experimental results.

(1). When the left-hand stick of the RC transmitter shown in Fig. 3.54 is placed in the upper-right position (CH3>1500 and CH4>1500), the LED light on Pixhawk is quickly blinking in blue.

(2). When the throttle stick of the RC transmitter is placed in the upper-left position (CH3>1500 and CH4<1500), the LED is quickly blinking in red.

(3). When the throttle stick of the RC transmitter is placed in the lower-left position (CH3<1500 and CH4<1500), the LED is slowly blinking in red.

(4). When the throttle stick of the RC transmitter is placed in the lower-right position (CH3<1500 and CH4>1500), the LED is slowly blinking in blue.

Fig. 3.66 LED experimental results (the left LED is blue, and the right is red)

6.2. Experimental Procedure of Attitude Control Experiment

This section uses a well-designed attitude control system as an example to introduce the basic operation process of all the controller design experiments. This example is certainly complicated than the previous LED light control experiment.

6.2.1. Simulink-Based Algorithm Design and SIL Simulation

(1). Step 1 : controller design

Create a new Simulink file and design a multicopter attitude controller in it. For simplicity, an example of a well-designed attitude controller is available in “e03.DesignExpExp_AttitudeController.slx”. Open it, and the controller details are presented in Fig. 3.67. The design requirements for the controller are in the following.

Fig. 3.67 Attitude controller example

1). Input data

• CH1-CH5 channel signals of the RC transmitter (see Fig. 3.67), which correspond to the “ch1”-“ch5” input ports in Fig. 3.67. The actual data approximately range from 1100 to 1900, so calibration or dead zones are required in processing the RC data.
• Multicopter angular velocity (corresponding to the “p”, “q”, “r” input ports in Fig. 3.67, unit: rad/s). The above three inputs represent the velocity rotating around the x-axis of the body, the velocity rotating around the y-axis of the body, and the velocity rotating along the z-axis of the body.
• Multicopter Euler angles (unit: rad). Here, the roll angle and the pitch angle (corresponding to the “roll” and “pitch” input ports in Fig. 3.67) are mainly considered, and the yaw control is temporarily not considered in this experiment.

2). Output data

• PWM control signals of four motors (corresponding to the “PWM” output port in Fig. 3.67). The data range is 1000–2000.
• Identifier for the armed state (corresponding to the “ARM_Control” output port in Fig. 3.67). The data type is Boolean.

3). Expected effects

• The throttle stick (CH3 on the RC transmitter) controls multicopters to perform the upward-and-downward movement.
• Push up the pitch stick (CH2 < 1500) to control the multicopter flying forward.
• Move the roll stick leftward (CH1 < 1500) to control the multicopter flying leftward.
• Pull back (or down) the three-position switch (CH5 > 1500) to disarm the multicopter.

(2). Step 2 : generate the controller subsystem

Select all the components in Fig. 3.67 with the mouse (or simultaneously press the keys CTRL + A), and right-click the mouse, choosing “Create Subsystem From Selection” to pack the controller to a subsystem in Simulink. Right-click the obtained subsystem and click “Mask”—“Create Mask” to open the mask setting box in Fig. 3.68. Then, enter text “image(‘./icon/Pixhawk.png’);” in the “Icon drawing commands” input box in Fig. 3.68. Finally, click the “OK” button and adjust the positions of the input and output ports of the obtained subsystem to get a subsystem as presented in Fig. 3.69.

Fig. 3.68 Mask setting dialog for Simulink subsystem

Fig. 3.69 Obtained attitude controller subsystem

(3). Step 3 : integrate the controller with the model

Open the Simulink file for SIL simulation, i.e., file “e01.SoftwareSimExpsCopterSim3DEnvironment.slx” used in the previous chapter; delete its original controller subsystem (remember to create a backup); then, copy the new controller subsystem obtained in Step 2 to replace it.

(4). Step 4 : connect and configure the inputs and outputs

As shown in Fig. 3.70, reconnect the controller to the multicopter model, where the output port “PosE” denotes the multicopter position vector in the earth frame, “AngEuler” denotes the Euler angle vector of the multicopter attitude, and “AngRateB” denotes the angular velocity vector of the multicopter. Given that the RC transmitter signals cannot be obtained in the SIL simulation phase, readers can use constant values to replace them or use the MATLAB functions to simulate the corresponding RC transmitter actions. The angular velocity input ports “p”, “q”, and “r” of the controller in Fig. 3.69 can be obtained from the “AngRateB” vector of the multicopter model; the angle “roll” and “pitch” can be obtained from the “AngEuler” vector. An example is also available in “e03.DesignExpsExp2_ControlSystemDemo.slx” whose controller and practical RC transmitter signals have been connected as presented in Fig. 3.70.

Fig. 3.70 Controller connected with multicopter model

(5). Step 5 : start a joint simulation

Click the FlightGear-F450 shortcut on the Windows desktop to open FlightGear, and click on the “Start Simulation” button on the Simulink UI to start the simulation. Then, it can be observed in FlightGear (see Fig. 3.71) that a quadcopter climbs up for some time and then rolls left and flies leftward, indicating that the controller has achieved the expected requirements.

Fig. 3.71 A quadcopter in FlightGear

6.2.2. Code Generation and Configuration

(6). Step 6 : configuration of the code generation environment

After finishing the SIL simulation in Simulink, copy the obtained controller subsystem to file “e03.DesignExpsExp3_BlankTemp.slx” (this file has been configured with all the settings required for code generation). Readers can also create a blank Simulink file and configure it according to Fig. 3.59.

(7). Step 7 : connect the controller to the PSP modules

Extract the corresponding I/O interfaces from the Simulink PSP module library (see Fig. 3.55) and connect it to the controller obtained in Step 6. As shown in Fig. 3.72, a complete example is available in “e03.DesignExpsExp4_AttitudeSystemCodeGen.slx”. Note that the motor control signals should be sent the uORB message of “actuator_outputs” to the “uORB Write” module instead of the PWM output module. This is because the controller is currently used for HIL simulation instead of actual flight tests.

Fig. 3.72 Attitude controller in Simulink for code generation

(8). Step 8 : generate code and compile the firmware

Click the “Build” button (see Fig. 3.37) on the Simulink toolbar to automatically generate code and PX4 firmware file. The result in Fig. 3.73 shows a successful compilation process.

Fig. 3.73 Simulink controller compiling successfully

(9). Step 9 : upload the firmware

Connect the computer to the Pixhawk autopilot with a USB cable, then use the “PX4Upload” function (see Fig. 3.40) to upload the firmware to the Pixhawk. A successful uploading result is shown in Fig. 3.74.

Fig. 3.74 Upload firmware successfully

6.2.3. HIL Simulation

(10). Step 10 : hardware system connection

As shown in Fig. 3.54, connect the RC receiver to Pixhawk with a three-color JR cable, then connect Pixhawk to the computer via a USB cable. At this point, readers can observe that the LED on Pixhawk lights up and blinks slowly, [2] the LED on the RC receiver is blue and white (this is for RadioLink and green light for Futaba receiver). Then, turn on the RC transmitter, readers can observe that the LED light on the Pixhawk blinks quickly for a few seconds, indicating that the RC transmitter data has been successfully received. If there is no change for the Pixhawk LED light, indicating that the connection between the RC transmitter and the receiver is not correct, readers should check and confirm the hardware connection.

(11). Step 11 : CopterSim configuration

Double-click the CopterSim shortcut on the Windows desktop to open it. There is no need to change the model parameters (or click “Model Parameters” - “Restore Default Parameters” - “Storage and Use Parameters” to restore aerial vehicle parameters to default values). Select the serial port of the Pixhawk autopilot in the “Select Pixhawk Com” drop-down box (usually in the format “*** FMU COM*”), click the “Start Simulation” button to start HIL simulation. As shown in Fig. 3.75, the message returned by the Pixhawk autopilot will be printed on the lower-left box of the CopterSim UI.

Fig. 3.75 Model simulator software configuration in CopterSim

(12). Step 12 : 3DDisplay configuration

Double-click the 3DDisplay shortcut on the desktop to open it. This software does not require any configuration; it passively receives the flight attitude and trajectory information of multicopters sent by CopterSim and displays it in real-time. The multicopter can be controlled by the RC transmitter to verify the designed attitude control algorithm. Move the throttle stick on the RC transmitter to the lower-right corner for three seconds to disarm the Pixhawk, and pull down (or back) the CH5 stick to the rearming position to disarm the designed controller. Then, the RC transmitter is able to control the multicopter to complete the corresponding action. As shown in Fig. 3.76, the multicopter attitude and position can be observed on the left side of the 3DDisplay interface. The real-time flight data are observed in the upper-right region, whereas the multicopter trajectory is observed in the lower-right region of the 3DDisplay UI.

Fig. 3.76 Multicopter 3DDisplay interface

6.2.4. Flight Test

(13). Step 13 : mount Pixhawk onto a multicopter airframe

The multicopter used in the outdoor flight tests is an F450 quadcopter (see Fig. 3.77). The parameters of the multicopter are accurately measured and identified by the system identification methods to ensure that the multicopter simulation model is consistent with the dynamics of the real multicopter system. For outdoor flight tests, the airframe of Pixhawk should be changed from “HIL Quadcopter X” to “DJI Flame Wheel F450” in QGC, which is presented in Fig. 3.78. All sensors should also be calibrated in QGC.

Fig. 3.77 F450 airframe and its components

Fig. 3.78 Flight test airframe in QGC

(14). Step 14 : modify the Simulink controller

Open the Simulink file in Fig. 3.72 and change the “uORB Write” module to the “PWM_out” module provided by the PSP toolbox, according to Fig. 3.79. An example with the modified motor output is presented in “e03.DesignExpsExp5_AttitudeSystemCodeGenRealFlight.slx”. Then, click the Simulink “compile” button to compile the controller into PX4 firmware and upload it to Pixhawk.

Fig. 3.79 Changing “uORB Write” module to PWM_output

(15). Step 15 : preparation for flight tests

Owing to the lack of complete failsafe logic for the generated control algorithm, safety should be fully considered to prevent accidents in outdoor flight tests. For example, the tests should be carried out in a relatively open area (such as a lawn) and on a nice day with good weather and low wind speed. When the above conditions are met, connect Pixhawk with the battery, and press the safety switch [3] on Pixhawk for more than three seconds. Then, control the multicopter with an RC transmitter to verify the actual performance of the controller.

(16). Step 16 : test results and analysis

Next, we will present the results from the HIL simulation and outdoor flight tests to verify the accuracy of the multicopter simulation model. Figure 3.80 presents the HIL simulation results when the sensor noise level in CopterSim is set to 1.0, where the solid line “PitchReal” represents the outdoor flight tests result, the dotted line “PitchSim” represents the simulation result, and the dashed line “PitchSP” represents the ideal expected value. It can be observed from Fig. 3.80 that the step response curves from the HIL simulation and the outdoor flight are close to each other in terms of dynamic processes and noise levels.

Fig. 3.80 HIL simulation result when noise level is set to 1.0

Figure 3.81 is the HIL simulation result when the noise level in CopterSim is set to 0. It can be seen that the response curve of the HIL simulation is very smooth with no noise disturbance, and the dynamic process and outdoor flight results are slightly different. Note that because the airframe “HIL Quadcopter X” is not exactly the same as the airframe “DJI Flame Wheel F450” used in outdoor flight, there are differences in the controller parameters. In addition, the data transmission speed of the Pixhawk serial port may fluctuate during the HIL simulation, thereby affectingthe real-time performance. Finally, the aerodynamics of the multicopter in the outdoor flight is very complicated, but a simplified aerodynamic model is used in the multicopter simulation model. Therefore, the error between their response curves is acceptable.

Fig. 3.81 HIL simulation result when noise level is set to 0

Notes

[1]	Higher Pixhawk hardware (e.g., Pixhawk 2/3/4/5) starts to discard LED module, so an external I2C LED module is required to observe the experiment result.

[2]	Higher Pixhawk hardware (e.g., Pixhawk 2/3/4/5) starts to discard LED module, so an external I2C LED module is required to observe the lighting effect.

[3]	https://docs.px4.io/master/en/config/airframe.html#safety_switch