Lab 3 - Unit Testing Sensors and Actuators

1  Overview and Objectives
2  Unit testing sensors
     2.1 Depth sensor
     2.2 Barometer sensor
     2.3 Attitude and heading reference system (AHRS)
     2.4 Global Positioning System (GPS)
     2.5 Self checkoff assessment
3  Unit testing actuators
     3.1 Control surfaces
     3.2 Main propulsion thruster
     3.3 Light Emitting Diode (LED) indicators
     3.4 Self checkoff assessment
4  Assessment

1 Overview and Objectives

Goal - learning what are the base sensors and actuators inside SeaBeaver II AUVs
Goal - mastering how to login to the vehicle's embedded computers
Goal - Learning how to unit test individual sensors and actuators to ensure they are in working order
Goal - Get an understanding on what each sensor measures

2 Unit testing sensors

As shown in Figure 2.1, the PocketBeagle serves as a hub that interfaces with the sensors and actuators of the vehicle. Throughout the AUV's life; that is, starting from the initial breadboard design phase to printed circuit board (PCB) testing, pre vehicle assembly, post vehicle assembly and during in-water operations; it is necessary to check if individual sensor and actuator components work as a single unit. To support that, we have developed a set of low-level programs to unit test individual components.

Figure 2.1: Sensors and actuators of SeaBeaver II AUVs are connected to the PocketBeagle

First you will need to SSH into the PocketBeagle. If you don't know how to do that, please refer to lab: How to login to a SeaBeaver II AUV.

2.1 Depth sensor [top]

The depth sensor of an AUV measures the vehicle's current depth, by measuring the external hydrostatic water pressure and converting it to depth, using the equation:

where P is the measured hydrostatic pressure, is the density of water, and g is the gravitational acceleration). In SeaBeaver II AUVs, we use a BlueRobotics Bar30 depth sensor: https://bluerobotics.com/store/sensors-cameras/sensors/bar30-sensor-r1/

This sensor is connected to the PocketBeagle via the Inter-Integrated Circuit (I2C) interface. When the depth sensor is plugged in to the daughter board, you can run the test_br_depth program to test if the depth sensor is connected to the PocketBeagle, and is in proper working order:

 $ test_br_depth

If the depth sensor is working as expected, you will see an output similar to this:

 debian@beaglebone:~$ test_br_depth 
 Acquired PROM contents
 crc_data: 28673, c1: 30169, c2: 29708, c3: 18186, c4: 18854, c5: 28659, c6: 26886
 Raw temp: 7386608, raw pressure: 4304880
 dT: 4716612, ti: 49904
 Depth: -0.09 m, Pressure: 1004.10 mbar, Temperature: 21.59 C
 Raw temp: 7387120, raw pressure: 4305392
 dT: 4716612, ti: 50416
 Depth: -0.06 m, Pressure: 1007.10 mbar, Temperature: 21.61 C
 Raw temp: 7387632, raw pressure: 4305392
 dT: 4716612, ti: 50928
 Depth: -0.06 m, Pressure: 1007.10 mbar, Temperature: 21.63 C
 Raw temp: 7387120, raw pressure: 4305392
 dT: 4716612, ti: 50416

Press ctrl+c to exit. If the depth sensor is not properly connected or faulty, you will see an output similar to this:

 debian@beaglebone:~$ test_br_depth 
 ERROR: in rc_i2c_send_bytes, system write returned -1, expected 1
 ERROR: Can not send reset

Troubleshooting:

Make sure that the depth sensor is plugged into the J4 connector of the daughter board.
If you have a different depth sensor that you know that works (perhaps a depth sensor that has been tested by a classmate), try it. This will help isolate the problem.
Talk to an instructor.

2.2 Barometer sensor [top]

SeaBeaver II AUVs come with an internal barometer that measures the pressure inside the pressure hull. As you will learn later in this class, observing internal pressure is important to check if the pressure hull has leaks (i.e. once you assembled the vehicle, you will pull the air inside the pressure hull out to create a vacuum. If the vacuum leaks out at a rate higher than what is acceptable, that means your pressure hull has a leak somewhere!).

In SeaBeaver II, we have a MPL3115A2 barometric pressure sensor (https://cdn-shop.adafruit.com/datasheets/1893_datasheet.pdf) connected to the PocketBeagle via I2C interface. You can unit test the internal barometer using the test_barometer program:

 $ test_barometer

If the barometer functions as expected, you will see an output similar to this:

 debian@beaglebone:~$ test_barometer 
  Pressure: 100.227. Alt: 92. Temparature (C): 28.375
  Pressure: 100.228. Alt: 397537. Temparature (C): 28.375
  Pressure: 100.232. Alt: 397537. Temparature (C): 28.375
  Pressure: 100.228. Alt: 397538. Temparature (C): 28.375

Press ctrl+c to exit. If you get an error, talk to an instructor to debug.

2.3 Attitude and heading reference system (AHRS) [top]

The attitude and heading reference system (AHRS) is essentially an Inertial Measurement Unit (IMU) with a fusion algorithm that outputs the roll, pitch and heading angle of the vehicle. These are crucial parameters for underwater navigation.

In Seabeaver II, we use a XSens MTi-3 Micro-Electro-Mechanical systems (MEMS) AHRS

https://www.xsens.com/hubfs/Downloads/Leaflets/MTi-3.pdf

The IMU is connected to the PocketBeagle using a universal asynchronous receiver/transmitter (UART) interface. You can unit test this sensor using the test_xsens_mti3 program.

 $ test_xsens_mti3

If the sensor is connected and is in the working order, you will be asked to check if the latitude and longitude currently written in the sensor's non-volatile memory is correct (as you will learn later in this class, the MTi-3 AHRS corrects for the magnetic declination; therefore, it is important that it knows our approximate operational area):

 debian@beaglebone:~$ test_xsens_mti3 
 Testing the Xsens MTi-3 IMU
 First scanning for USB devices...
 No USB device found. Scanning: /dev/ttyO4 at baud rate: 115200
 Found a device with ID: 038898CE @ port: /dev/ttyO4, baudrate: 115200
 Opening port...
 Device: MTi-3-8A7G6, with ID: 038898CE opened.
 Putting device into configuration mode...
 Configuring the device...

  Current Latitude: 52.2405 | Longtitude: 6.83822 | Altitude: 1.29442e-312
 Do you want to change the position for magnetic declination? 
 Enter Y to change Lat/Long. Any other input to keep them.

In this example, latitude and longitude does not represent Cambridge, and this will most likely be the case for the AHRS in your brand new training-kit. Press Y and hit return:

  Current Latitude: 52.2405 | Longtitude: 6.83822 | Altitude: 1.29442e-312
 Do you want to change the position for magnetic declination? 
 Enter Y to change Lat/Long. Any other input to keep them. 
 y
 Enter the new Latitude..

Then you will be asked to enter your latitude. The latitude of the MIT sailing pavilion is 42.3584, let's enter that.

  Current Latitude: 52.2405 | Longtitude: 6.83822 | Altitude: 1.29442e-312
 Do you want to change the position for magnetic declination? 
 Enter Y to change Lat/Long. Any other input to keep them. 
 y
 Enter the new Latitude.. 
 42.3584                                  
 Enter the new Longitude..

Then you will be asked to enter your longitude. The longitude of the MIT sailing pavilion is -71.08745, let's enter that. Then the sensor will start outputting raw data -- you will see an output similar to this:

  New Latitude: 42.3584 | Longtitude: -71.0875 | Altitude: 9.54898e-313
 Putting device into measurement mode...

 Reading data...
 Acc X:1.06, Acc Y:-0.15, Acc Z:9.70 |Gyr X:0.01, Gyr Y:0.01, Gyr Z:-0.00 
 |Mag X:0.09, Mag Y:0.36, Mag Z:-0.56 |Roll:-0.91, Pitch:-6.29, Yaw:37.35, Hdg:232.65

Turn the training-kit around and observe how the raw acceleration, gyroscope and magnetometer readings; as well as fused roll, pitch and heading angles vary. Press ctrl+c to exit.

2.4 Global Positioning System (GPS) [top]

While GPS does not work underwater, AUVs use GPS to initialize the position while on the water surface. SeaBeaver II vehicles use an Adafruit Ultimate GPS Breakout V3 as the GPS module (https://cdn-learn.adafruit.com/downloads/pdf/adafruit-ultimate-gps.pdf). This module is connected to the PocketBeagle via the UART interface.

You can unit test the GPS by using the linux minicom tool (https://linux.die.net/man/1/minicom). Minicom will display the NMEA strings that the GPS outputs.

 $ minicom -D /dev/ttyO0 -b 9600 -w

If the GPS is connected to the PocketBeagle properly, and is it working order, you will see an output similar to this:

 Welcome to minicom 2.7.1

 OPTIONS: I18n 
 Compiled on May  6 2018, 10:36:56.
 Port /dev/ttyO0, 22:10:29

 Press CTRL-A Z for help on special keys

 $GPGGA,222359.000,4221.5879,N,07106.2830,W,1,06,1.24,3.5,M,-33.7,M,,*51
 $GPGSA,A,3,06,12,05,11,25,19,,,,,,,1.56,1.24,0.95*04
 $GPRMC,222359.000,A,4221.5879,N,07106.2830,W,0.40,139.56,300324,,,A*74
 $GPGGA,222400.000,4221.5880,N,07106.2831,W,1,06,1.24,3.5,M,-33.7,M,,*5D
 $GPGSA,A,3,06,12,05,11,25,19,,,,,,,1.56,1.24,0.95*04
 $GPRMC,222400.000,A,4221.5880,N,07106.2831,W,0.41,139.56,300324,,,A*79
 $GPGGA,222401.000,4221.5881,N,07106.2831,W,1,06,1.24,3.5,M,-33.7,M,,*5D
 $GPGSA,A,3,06,12,05,11,25,19,,,,,,,1.56,1.24,0.95*04
 $GPRMC,222401.000,A,4221.5881,N,07106.2831,W,0.45,139.56,300324,,,A*7D

Try to figure out what these data strings mean (hint: these are NMEA sentences). To exit from minicom, press ctrl+A and then X.

2.5 Self checkoff assessment [top]

Ensure that you have:

Have you confirmed that you can read depth data (i.e. if a depth sensor is plugged in)
Have you confirmed that you can read internal pressure data
Have you confirmed that you can read AHRS data
Have you confirmed that you can read GPS data

3 Unit testing actuators

Now let's focus on unit testing the actuators.

3.1 Control surfaces [top]

The servo motors that actuates the control surfaces of the SeaBeaver II AUVs are driven by Pulse-Width Modulation (PWM) commands provided by the PocketBeagle. The signal lines of the servo motors are connected to certain Programmable Real-time Unit (PRU) output pins of the PocketBeagle that are configured to generate necessary PWM signals. Within the range, each PWM command corresponds to a certain position/angle of the servo motor.

You can test the servo motors that run the control surfaces by running test_control_surfaces program:

 $ test_control_surfaces

If the servos are properly connected to the J6 pins of the daughter board, you wwill observe the servos moving, one at a time. Press ctrl+c to exit.

3.2 Main propulsion thruster [top]

WARNING: Always consider that the main thruster is live. Do NOT put fingers near the propeller while the thruster is plugged in!

SeaBeaver II AUVs use a Blue Robotics T200 thrusters (https://bluerobotics.com/store/thrusters/t100-t200-thrusters/t200-thruster-r2-rp/) as the main propulsion thruster. Similar to the servo motors, the main thruster is driven by PWM signals signals. However, in this case, PWM signals correspond to a speed value of the thruster. You can test the main thruster by running test_br_thruster program:

 $ test_br_thruster

Now you will be asked to provide a thruster percentage value. If you are testing the actual Blue Robotics T200 thruster, please do not provide a value beyond 50%. 20% is a good value to test the Blue Robotics T200 thrusters.

 debian@beaglebone:~$ test_br_thruster 
 Enter thruster percentage between 100 and -100. ctrl-c to exit: 20

If everything is in proper order, you will now see the propeller turning. {\color{red} Note: Please do not run the Blue Robotics T200 thruster in air for more than 10 seconds as it will over heat.} You may run the mockup main motor provided in your training-kit as long as you like.

3.3 Light Emitting Diode (LED) indicators [top]

While the LED indicators are not crucial for SeaBeaver II AUVs, they can make the vehicle operators' lives easier by indicating the current status of the vehicle, increasing the mission success rate. They are connected to the PocketBeagle using the general purpose input/output (GPIO) pins. You can test them by running the test_leds program:

 $ test_leds

If everything is working as expected, the LEDs should now start blinking, one at a time. Press ctrl+c to exit.

3.4 Self checkoff assessment [top]