design and construction of mobile surveillance robot

ALTERNATIVE DESIGNSSession 4CThere were three different designs considered for use: ahanging weight design [1], a “hamster ball” drivendesign[2], and a dual-tread design[3]. The capabilities andrequirements were considered along with several othercriteria when generating these design possibilities.The hanging weight design consisted of a central shaftwith a carriage hanging from the shaft as displayed in Figure2. The carriage would have the ability to rotate around theshaft which would allow for movement forward andbackwards. The carriage would also have a small weightinside which would shift side to side to allow the sphere totilt from side to side while moving forward or backward, andin so doing provide a turning capability.FIGURE 3HAMSTER BALL DRIVEN DESIGNThe dual-tread design would contain two wheels thathave the ability to turn in opposite directions to allow for azero turn radius. The design would not be perfectlyspherical, but would still retain the benefits of a sphericaldesign. A model of the dual-tread design is displayed inFigure 4.FIGURE 2HANGING WEIGHT DESIGNThe “hamster ball” design would operate similar to howa hamster is able to move around in a plastic ball. A robotwould be contained within a clear plastic sphere withcontrollable wheel(s) at the bottom that roll along the insideof the sphere. The wheel(s) would be able to rotate so thatthe sphere could travel in any direction. It would alsocontain stabilization arms that would keep the robotcentered, the wheel(s) in contact with the sphere, and wouldalso protect the components inside the robot if the spherewere to suffer a sudden impact. A sketch for the “hamsterball” design is displayed in Figure 3.FIGURE 4DUAL-TREAD DESIGNAll three of these designs were evaluated using adecision matrix that included several design criteriaimportant to the project. The results can be seen in Table 1.Pittsburgh, PA March 26 - 27, 2010ASEE North Central Sectional Conference4C-2

Session 4CTABLE 1DECISION MATRIXCriteriaWeightHanging Carriage Hamster Ball Dual-Tread SphereScore Total Score Total Score TotalStability 25 0.0 0.0 0.5 12.5 1.0 25.0Cost 10 1.0 10.0 0.3 3.3 0.0 0.0Durability 8 0.5 4.0 0.0 0.0 1.0 8.0Power Use 6 1.0 6.0 0.0 0.0 0.5 3.0Feasibility 10 1.0 10.0 0.0 0.0 1.0 10.0Mobility 15 0.0 0.0 0.8 11.3 1.0 15.0Ease of Assembly 8 1.0 8.0 0.3 2.7 0.0 0.0Size 5 0.0 0.0 0.7 3.3 1.0 5.0Complexity 5 1.0 5.0 0.0 0.0 0.3 1.7Clarity 8 0.3 2.7 1.0 8.0 0.0 0.0Total 100 45.7 41.1 67.7The design that was selected based on the specifiedcriteria was the dual-tread design. It was determined that thedual-tread model would be the most stable and the mostmobile due to its wheeled design. It should also be easier toinsert equipment inside of the dual-tread design, allowing fora smaller size which will benefit the mobility of the robotboth in-use and for transportation.PROPOSED SYSTEM DESIGNThe design chosen to fulfill the requirements is a dual-treadrobot with a capsule shape as shown in Figure 5.The middle cylindrical-section of the robot is stationarywhile the two wheels are domed on the outside and arecontrolled separately, allowing for a zero-turn radius. Themechanical, controls, data acquisition, and powersubsystems are contained within the middle section of therobot.The elements of the subsystems are mounted in such away that the center of gravity is well below the geometriccenter of the robot. The camera and transmitters are mountedcloser to the center of the section, on the shaft. This allowsfor a minimization of interference in the signals due to themotors and power systems. The power subsystem ismounted near the bottom of the containment section toprovide stability and to also prohibit interference from themechanical systems to the power performance.The different subsystems are hard-mounted to the shaftfor strength and stability. The central shaft remains relativelystationary as the wheels rotate around the shaft. The outershell of MSR is made of polycarbonate to increase strengthand durability, while also allowing for optical clarity.Polycarbonate also has a low electrical conductivity, whichis beneficial for the power system design.FIGURE 5DUAL-TREAD ROBOT DESIGNDESIGN OF MECHANICAL SUBSYSTEMThe drive train for the Mobile Surveillance Robot consists oftwo (2) electric motors, four (4) gears, a stationary shaft, andtwo (2) wheels with bearings. The motors drive the two setsof gears, which are attached to the wheels. The wheels arePittsburgh, PA March 26 - 27, 2010ASEE North Central Sectional Conference4C-3

attached to the stationary central shaft through the use ofinternal bearings.The electric motors were selected to meet severalspecified criterion including maximum voltage, maximumpower, minimum torque, minimum rotational speeds, andoverall dimensions. The required torque was calculatedbased on a 10 lb design with traveling up a 5˚ ramp with anacceleration of 2 ft/s 2 . The required rotational speed wasbased on a desired speed of 5 ft/s. The desired length was themaximum length possible to mount the motors back to backin the center of the robot.Based on these criteria, a Portescap B-230013-12Abrushless DC motor was chosen for use in the MSR. Table 2summarizes the desired specifications and the actualspecifications of the motor chosen.CriterionTable 2: Motor Specifications [4]Desired MotorSpecification SpecificationMax. Voltage 12 V 12 VMax. Power 20 W 28.8 WMin. Torque 0.75 in-lb 1.0 in-lbMin. Rotational Speeds 480 RPM 2650 RPMLength < 1.75 in 1.44 inDiameter < 1.5 in 2.22 inThe diameter and power consumption of the selected motorwere larger than desired. However, due to a difficultymeeting all of the desired specifications, it was decided thatthis motor would be acceptable for the design.The gears were initially selected based on dimensionalrequirements of the motor so that the motor would beapproximately 3 inches below the center of the shaft. Thereason the motor was to be placed at this location was toprevent electromagnetic interference with data collection,transmission, and control signals from the motors. Asecondary design requirement was to have a large gear ratiobecause DC motors generally have a high rotational speedand a low torque which needed to be stepped down for theMSR’s desired speed. The two spur gears that were selectedhad a pressure angle of 20˚and a pitch of 16. The large gearis made of nylon and the pinion in made of steel. The largegear has a pitch diameter of 5 inches with a 0.5 inch bore.The small gear has a pitch diameter of 1 inch with a 0.25inch bore. The large gear was modified from the stockdesign. The bore was increased so that it has a clearance fitaround the bearing on the shaft, and four small holes weredrilled into the wheel to allow it to be attached to the largegear. Also, the hubs on all gears were removed because ofwidth constraints. Figure 6 shows how the motors and gearsfit in the MSR assembly.FIGURE 6MECHANICAL DRIVE SYSTEMSession 4CThe central shaft supports the two (2) hemispheres and thecentral cylinder. The wheels are attached to the shaft withbearings. The shaft also serves as a mount for the camera,motors, batteries, and other electronic equipment. The shaftis made from 0.75 inch diameter circular aluminum stockthat is approximately 12 inches long. It has six (6) stepchanges as shown in Figure 7 to facilitate the mounting ofthe bearings and the camera:FIGURE 7SHAFT DESIGNThe two (2) wheels were made from two 12 x 12 x0.375 inch polycarbonate plates. Polycarbonate was chosenbecause it is a durable plastic that is permeable to electricsignals. The plates were machined to 12 inch diameterwheels with cutouts (spokes) to reduce weight and provideaccess to the space outside the wheels. A finite elementanalysis was conducted using ANSYS Workbench ® thatverified that the wheels would still have a safety factorabove 10 when a 10 lb load was apply. The wheels also havea groove on the outer edge to allow a wide o-ring to beattached to provide traction and durability to the wheels.The outer race of the bearing was press-fit onto the 1.375inch bore of the wheel. The inner race of the wheel’sbearing was secured onto the 0.5 inch diameter section of thecentral shaft by threading the shaft using a nut to fix it inplace.Pittsburgh, PA March 26 - 27, 2010ASEE North Central Sectional Conference4C-4

DESIGN OF CONTROLS SUBSYSTEMThe design chosen to control the motors of the robot is thatsimilar to a remote control car. The system has one sixchannel 72MHz computer radio that transmits using pulseposition modulation. The radio uses a rechargeable batterypack which eliminates the need for replacement batteries. Asix channel FM receiver is used to receive the pulse positionmodulation signal from the radio and convert the signal to apulse width modulation. The signal is then sent to the twomotor controllers which determine the speed and direction ofthe motors. The additional channels could be used for futureapplications. The motor controllers that were selected arecapable of handling input voltage of up to 25 volts. Theyalso allow the motor controllers to be connected to acomputer by a USB device that allows for the accelerationand deceleration curves to be manipulated, providingoptimal performance and power consumption. The motorcontrollers used are also able to detect when the batteries arebelow the minimum voltage to operate and will shut down toprotect the batteries from damage. The control system ispowered by the same battery, and the motor controllersprovide power to the receiver which will eliminate the needfor extra wires. Figure 8 displays the wiring diagram for thecontrol system.FIGURE 8WIRING DIAGRAM OF CONTROL SYSTEM [ 5]A computer is used to interface with the RF transmitterthrough a National Instruments data acquisition device tosend control signals to the robot. Computer software wasdeveloped to generate the signal that is sent out by thetransmitter to the robot. The system allows for predefinedmovements to be programmed as well as adjust thesensitivity of the control system. An Xbox controller thencan be attached to the computer and used as an analog inputto the software so that the robot can be easily driven.Session 4CActions can also be assigned to the buttons on the keyboardand controller to perform specific actions.DESIGN OF DATA ACQUISITION SUBSYSTEMThe data acquisition consists of one wireless camera thatrequires 12 Volts and 0.5 Amps. The camera includes itsown transmitter that operates at 2.4GHz. The cameratransmits to a receiver that is connected to a display at thebase station.Future expansions of the system would allow thecamera feed to be displayed on a computer as well asrecorded, either onboard the robot or at the base station.Additional data sensors may be incorporated into the designsuch as a temperature sensor or a sensor that detects variousenvironmental conditions.DESIGN OF POWER SUBSYSTEMThe power system is designed to operate with four batteries.Each of the motors require 12 V and a maximum current of2.4 A. Two lithium polymer batteries are used to power thetwo motors. The chosen battery provides 14.8 V and 2.2 Ah,allowing for a 25% contingency in power flow management.A third battery is also connected to the motor subsystem inorder to allow for a longer life before charging. Lithiumpolymer was selected because a large enough voltage andcurrent can be provided while the battery itself is relativelysmall. The chosen camera also operates at 12 V, so the sametype of lithium polymer battery is used. While the camerahas a receiver built into it, the robot requires a receiver forthe controller. The acquired receiver does not need anadditional power source; therefore, no battery is needed.The decided optimal system design for the powermanagement requires an isolation of two subsystems. Thisallows for a minimal interaction of the electromagneticfields and ambiguous current flow. It also allows the designprocess to focus on the specifics of the current and voltagefor each of the elements in the overall object design of therobot.The first system is designed for the management of themotor control system. This system requires an integration ofthe controls systems and the power systems, as well as themechanical motors. The system is designed with three of thefour batteries in parallel. This is allowable because thespeed controllers can regulate the current supplied to themotor. A voltage regulator is also present to step down the14.8 V source to the required 12 V. The presence of asecond battery allows for the achievement of the optimalcurrent requirement for the desired ten minutes of peakoperation. The third battery is present in the case that one ofthe batteries fail as well as to provide more power to thesystem overall. The circuit used in this subsystem can beseen below in Figure 9.Pittsburgh, PA March 26 - 27, 2010ASEE North Central Sectional Conference4C-5

Session 4Ccircuits. This low battery indicator consists of twotransistors and an LED, which will turn on at a specifiedvoltage. The indicated voltage is set to allow enough powerto keep the speed controllers in operation and return theMSR to the user. The LED for the motor subsystem isvisible to the camera so the operator of the device knows tobring the MSR back for recharging and data collection.CONCLUSIONFIGURE 9MOTOR SUBSYSTEM CIRCUITThe physical design of the robot allows for the fourbatteries to be positioned in the lower half of the middlesection. This assists in the self-righting ability of the ball.Sufficient distance was maintained between the batteries andthe motors to prevent interference with electromagneticfields. This is important to the operational performance ofthe robot and prevents the lithium polymer batteries fromlosing the ability to recharge.The second system is designed to power the datacollection unit. Only one battery is needed for the powerrequirements over 10 minutes. A transistor acts as a switchto protect the batteries from using all of their charge. Thecircuit for the camera subsystem is shown below in Figure10.In this project, several alternative designs were consideredand compared based on their ability to meet the requirementsestablished by the design team. The requirements werecreated in order to assure that the robot was able to becontrolled wirelessly from another location without line ofsight, provide live video feedback, and have at least tenminutes of full operational capabilities. Other criteriaconsidered in the comparison of the design alternatives werestability, cost, durability, power consumption, feasibility,mobility, manufacturability, size, and surveillance clarity. Itwas concluded that a dual tread capsule shape design wouldbest fulfill these requirements established by the designteam. Once this design was chosen the eight-student designteam divided the design into four subsystems to better utilizethe specialties of each team member. The subsystemsconsisted of a mechanical subsystem, created by the fourmechanical engineering students on the design team, acontrol and a data acquisition subsystem, both created by thetwo computer engineering students on the design team, and apower subsystem created by the two electrical engineeringstudents.REFERENCES[1] Perterdewnl. "Balbot." YouTube. 20 June 2007. Web. 11 Oct. 2009..[2] Rusil. "Spherical Autonomic Robot S.A.R." YouTube. 28 June 2006.Web. 11 Oct. 2009. .FIGURE 10CAMERA SUBSYSTEM CIRCUITA manual three way switch is present in order toconnect to the separate charging circuits for each subsystem.This will isolate the rest of circuit during the chargingprocess. A low battery indicator is also present in both[3] Palmisano. "Cyclone RC Robot Kit." YouTube. 20 Apr. 2008. Web. 11Oct. 2009. .[4] "B-230013-12A Spec Doc." Portescap. A Danaher Motion Company.Web. 8 Feb. 2010. hhttp://www.portescap.com/BrushlessSlottedDC>.[5] Drivers Ed Guide Instruction Manual for All Castle Creations Car andTruck Brushless Power Systems. Castle Creations, 2009. Print.Pittsburgh, PA March 26 - 27, 2010ASEE North Central Sectional Conference4C-6