AA290: Problems in Aero/AstroFinal Report

Suleman KaziDepartment of Electrical Engineering

Stanford UniversityEmail: sbkazi@stanford.edu

Abstract—This report contains a summary of the work doneat the free flying robotics lab as part of the AA290 (Problemsin Aero/Astro) course taken under the supervision of ProfessorMarco Pavone of the Department of Aeronautics and Astronau-tics. It goes through the tasks performed through the durationof the course including automating part of the work flow,redesigning an on board manipulator, extensive testing of the freeflying robots and a demonstration of the robots’ path planningcapabilities at the Center for Automotive Research at Stanford’sannual meeting.

I. INTRODUCTION TO THE FREE FLYERS

The free flying robot laboratory is located in the Durandbuilding. University policy required the completion of thefollowing safety courses before access to the laboratory couldbe granted:

1) General Safety, Injury Prevention, and EmergencyPreparedness

2) Chemical Safety for Laboratories3) Compressed Gas Safety

After completion of the preliminary requirements, a study ofthe development already done on the free flyers was startedbeginning with an overview of the system setup. A basicdescription of the system is as follows: The lab has threefree flyers named Huey, Dewey and Louie. Three smaller freefloating bodies, April, May and June are used to simulateobstacles (e.g. space debris). All of these use compressedair to ”float” on a cushion of air on a smooth granite tablethus emulating a zero gravity environment in the 2D plane ofthe table. The free flyers are equipped with thrusters and amomentum wheel. These are controlled by an on-board ARMmbed [1] microcontroller, and a laptop which runs the pathplanning and control algorithms. An OptiTrack [2] motioncapture system is used to relay information about the positionof the free flyers and obstacles over a wifi network to the on-board computers. OptiTrack uses infrared cameras oriented tolook at the table to detect infrared reflecting markers placedon the bodies. A desktop PC named Sputnik acts as the centralconsole from where all the free flyers can be accessed via wifi.

II. AUTOMATING THE BOOT SEQUENCE

The first task performed was to automate the software bootsequence that was manually carried out on Sputnik each timethe free flyers had to be run. This sequence consists of thefollowing series of actions: First Sputnik must be connected tothe FreeFlyers wifi network. Then Motive (the software for theOptiTrack tracking system) must be started up. A pre-saved

Fig. 1: Screenshot of the Autobot program showing GUI and connectionoptions

project is loaded which contains the camera calibration anddefines the names and identities of each of the free flyers andobstacles. Motive then starts broadcasting the positions of thefree flyers through the wireless network. Once that is done,Xming (an X Window System Server for Windows) is started.Then PuTTY is used to SSH into the laptops on board the freeflyers, the NetBeans IDE containing the free flyers project isstarted and only after all these steps are done can the freeflyers be ready to run.

A program was developed called ”Autobot” which auto-matically performs all the software sequence described aboveand essentialy transforms the whole sequence into a singleclick operations. The details of Autobot are described in theaccompanying software description, but a brief overview ispresented here.

Autobot was developed using the QT [3] cross-platofrmapplication framework. It uses QT’s networking classes toconnect to the FreeFlyers Wifi network. Opening up therequired software is done using the Windows API, the APIis also used to get a low level hook to the system’s keyboardwhich is then used to perform the sequence of actions thatthe user would have to otherwise perform. Fig. 1 shows ascreenshot of Autobot. As can be seen, the program gives theuser the option to connect to either one or several of the freeflyers and specify which steps of the software boot sequencethe user wants it to perform.

III. MANIPULATOR REDESIGN

The free flyer Louie has a fixed manipulator mounted onit which can enable it to dock with the other free flyers. Atfirst the feasibility of replacing the fixed manipulator with a

Fig. 2: The redesigned manipulator

6-DOF robotic arm was considered. Such an arm could enableberthing in which the robotic arm would autonomously grabhold of the free flyer being docked to during the final stagesof the docking process. However this idea was later discardedafter consultation with other lab members since the roboticarms available in the lab were significantly heavy and couldweigh down the free flyers.

The gripping action of the manipulator was achieved bya pneumatic solenoid valve [4]. However the internal air sealof the valve would fail after just a few uses. The valve wasreplaced with another one but that too soon failed. This waspossibly due to the fact that these valves were obtained froma dated robotic arm and would fail when pressurized.

An alternative mechanism to the pneumatic valve wasdevised and the design of the manipulator was changed overtwo iterations so that the gripper at its end could be operatedby a standard 5V analog servo motor. A voltage regulator wasadded to the existing electronics on board Louie to providethe servo with power from the batteries. The code of thembed microcontroller was also changed to accommodate thenew gripper mechanism. Calibration of the servo was neededin order to enable successful operation of the gripper. Thiswas accomplished by iteratively widening the width of thePWM pulse being sent to the servo and observing wherethe maximum opening of the manipulator was achieved. Thisposition was then set in the code for the microcontroller.Testing of the robustness of the new mechanism was carriedout by writing an infinite loop which continuously opened andclosed the manipulator claw and running it for one hour.

IV. TESTING THE FREE FLYERS

After getting the manipulator functional the next step wasto test out the free flyers and see if there were any weak linksin their operation. The free flyers were operated repeatedly tocheck for points of failure.

After just a few days of testing, the mbed microcontrolleron one free flyer became non-operational and in turn causeda voltage surge which burnt out the motor controller for theflywheel as well. Upon investigation it was discovered thatthe voltage regulator on the mbed which was stepping downthe battery voltage (12V) directly to the mbed’s operationalvoltage (3.3V) was heating up which caused it to fail over time.Although the mbed documentation [1] rates it as being ableto handle this level of input, the manufacturer recommendsdriving at a lower input voltage especially, as in our case where

Fig. 3: A frame of the realtime visualized data

a number of operations are being performed such I/O throughthe serial port, communication with the motor controller anddriving the transistors which operate the thrusters. New partswere ordered and the microcontroller and motor controllerwere both replaced.

A solution to this problem would be to lower the voltageprovided to the mbed in steps, in future versions of the PCB.A voltage regulator in series with the battery which lowersdown the voltage of the battery to 5V before supplying it tothe mbed would accomplish this.

V. REALTIME DATA VISUALIZATION

One often faced problem was that in case of failure of thefree flyers to reach their goal there was no way of knowingwhether there was a hardware failure, a problem with thetracking system or simply no possible solution of reachingthe target. In order to prevent this problem and to easilyvisualize the solutions being generated by the software it wasnecessary to develop realtime visualization which would showthe waypoints and trees being generated by the path planningalgorithm as well as the trajectory of the free flyer over time.

A system had previously been developed by membersof the Autonomous Systems Laboratory (ASL) which wouldvisualize the path plan but it was not able to visualize thedata concurrently while the free flyer was running. This workprovided a good starting point for developing realtime visual-ization. The visualization program in MATLAB was modifiedto enable it to plot the free flyer data. This work was thencompleted by other members of ASL.

Since the visualization was being calculated directly onthe onboard laptop for the free flyer, this needed to be dis-played on Sputnik. This necessitated the change of the remotenetworking protocol from SSH which was previously beingused to (Virtual Network Computing) VNC. VNC is a RemoteFrame Buffer protocol (RFB) and enables remote control ofanother computer with graphical screen sharing. This was dueto the fact that SSH did not permit a high enough bandwidthto smoothly stream the visualization data. A VNC server wasset up on the free flyer laptop which enabled the MATLABgenerated figures to be viewed on Sputnik.

VI. OPTITRACK RECALIBRATION

The Optitrack system tracks the positions of the free flyersup to millimetre level accuracy. However it was observed thatsometimes after restarting Motive the settings of the free flyer

project would be lost or reset and at times the Optitrack systemwould completely fail to track some objects or misrepresentthe orientation of some of the free flyers.

This problem was thought to be due to corruption of theproject files and camera calibration data due to a systemcrash while Motive was running. Hence it was decided torecalibrate all the cameras and redefine all the free flyers insideMotive. After recalibration the issues with the settings beinglost and calibrations being reset was resolved. However it wasnoticed that the cameras would still sometimes lose track ofthe obstacles during test runs. This problem was solved byincreasing the number of infrared reflecting trackers on theobstacles.

VII. FINAL DEMONSTRATION

A demonstration of the free flyers project was carriedout during the Center for Automotive Research at Stanford(CARS) annual meeting on December 5th 2014. Representa-tives from CARS affiliates were given an overview and demon-stration of the working free flyers. The demonstration involvedrunning the free flyer and showing it avoiding obstacles inorder to reach the goal location. Moving obstacles were also

demonstrated and the reaction of the free flyers to externaldisturbances was shown by letting participants interactivelytry to push the free flyer away from it’s planned path.

ACKNOWLEDGMENT

The valuable knowledge and experience gained duringthis study could not have been possible without the help ofProfessor Marco Pavone, and members of the AutonomousSystems Laboratory especially Joseph Starek, Federico Rossiand Edward Schmerling whose support was a great source ofguidance.

REFERENCES

[1] NXP Semiconductors. (2009, Feb.). 32-bit ARM Cortex-M3 microcon-troller [Online]. Available: http://goo.gl/sehKjO [Accessed December 9,2014]

[2] Natural Point. (2010, April.). ”NaturalPoint Tracking Tools Users Man-ual” [Online]. Available: http://goo.gl/lweQSZ [Accessed December 9,2014]

[3] Qt Project. [Online]. Available: http://qt-project.org [Accessed December9, 2014]

[4] Koganei. (2001, Feb.). ”Solenoid Valves 040 Series” [Online]. Available:http://goo.gl/Tlr8Ye [Accessed December 9, 2014]