Abstract:
The invention is a remote controlled vehicle adapted for navigating inside HVAC supply trunks. It is equipped with a moveable camera and a powered tool for snagging a string or parachute propelled into the trunk by other methods. A command box is provided to view the image from the camera and control the vehicle&#39;s various functions. The installation technician inserts the vehicle into the trunk through an access hole and uses the command box to navigate the vehicle inside a HVAC trunk and locate and secure the string to the vehicle. The technician then controls the vehicle to pull the string back to the access or the technician manually pulls the vehicle back to the access by its tether.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     This invention relates generally to HVAC zone control systems for retrofit, and specifically to a remote controlled vehicle to assist in threading string, air tubes, and wires through concealed HVAC duct systems. 
     2. Background Art 
     Most zone control systems for HVAC systems use electromechanical dampers to selectively control the airflow through portion of the trunk and duct system. Installation of these zone systems requires access to the ducts at multiple locations so that the dampers can be installed. Although the duct is accessible for damper installation, there may be no easily accessible path to run control wires from the damper to the control system because portions of the duct may be enclosed in walls, floors, or ceilings. However the duct system does provide a clear path provided the zone control equipment is located near the HVAC equipment. The existing ductwork can be used as a conduit for running the control wires, but this requires a practical method for threading the wire from the damper to the HVAC equipment. 
     U.S. Pat. No. 6,786,473 issued Sep. 7, 2004 to Alles, U.S. Pat. No. 6,893,889 issued Jan. 10, 2004 to Alles, U.S. Pat. No. 6,997,390 issued Feb. 14, 2006 to Alles, U.S. Pat. No. 7,062,830 issued Jun. 20, 2006 to Alles, U.S. Pat. No. 7,162,884 issued Jan. 16, 2007 to Alles, U.S. Pat. No. 7,188,779 issued Mar. 13, 2007 to Alles, and U.S. Pat. No. 7,392,661 issued Jul. 1, 2008 to Alles, describes various aspects of a HVAC zone climate control system that uses inflatable bladders. The present invention is by the same inventor and is designed to assist in the installation of this system. 
     The system invented by Alles has multiple inflatable bladders installed in the supply ducts such that the airflow to each vent can be separately controlled by inflating or deflating the bladder in its supply duct. Each bladder is connected to an air tube that is routed through the duct and trunk system back to a set of centrally located computer controlled air valves that can separately inflate or deflate each bladder. Based on temperature readings from each room and the desired temperatures set for each room, the system controls the heating, cooling, and circulation equipment and inflates or deflates the bladders so that the conditioned air is directed where needed to maintain the set temperatures in each room. 
     U.S. Pat. No. 7,062,830 issued Jun. 20, 2006 to Alles describes a method of installing the air tubes. This method uses air flow from the vent toward the HVAC equipment to pull a parachute and thin string from the vent to the HVAC equipment. At the HVAC equipment, an air tube is connected to a string and the string is pulled toward the vent until the air tube reaches the vent. This method requires all vents but one be blocked so that all of the airflow generated by a blower at the HVAC system comes from one vent. This method works well for many duct systems and specific duct paths. However, this method does not work well for some duct systems and specific duct paths. 
     Excessive duct leakage can prevent this method from working. With all vents sealed but one, all of the airflow generated by the blower should flow through the one open vent. However, the airflow can also come for all of the leaks in the duct system. If the leakage is excessive, there is insufficient airflow at the vent to inflate and pull the parachute. 
     Small supply ducts at the vent in the range of 4″ to 6″ in diameter can prevent this method from working even with strong airflow. In a small vent, a large portion of the parachute is in contact with the walls of the duct creating a large drag, and screws or sharp edges are likely to snag the parachute. In addition, the airflow in the small cross-section area produces only a small force on the parachute. Increasing the air flow to increase the pulling force also increases the drag since parts of the parachute are pushed harder against the duct walls. The combination of high drag and small force makes it difficult for the parachute to pass through the duct. 
     If a smaller parachute is used for smaller ducts, it is often easier for the parachute to pass through the duct. However, the small duct eventually connects to a larger duct or main supply trunk. As the duct cross-section increases, the air velocity decrease and the small parachute can not product enough force to pull the string to the HVAC equipment. 
     In some duct networks with long duct runs with many turns, the resistance between the string and the duct walls become excessive as the length of the string being pulled increases. The force generated by the parachute is not sufficient to overcome the string pulling friction. 
     Patent application 12240570 discloses a method that overcomes some of these limitations. It discloses methods for propelling a string through a small duct to a larger trunk and separate methods for retrieving the string in the trunk and pulling it to an access cut into the trunk near the HVAC equipment. 
     A specially adapted remote controlled vehicle can be used to capture and retrieve a string in a trunk. Small remote controlled vehicles are produced in various sizes and styles for the toy and hobbyist market. Their design and function are understood by those skilled in the art. However, they are not adapted for use in HVAC trunks and for the purpose of capturing a string or parachute. 
     U.S. Pat. No. 5,020,188 issued Jun. 4, 1991 and U.S. Pat. No. 5,072,487 issued Dec. 17, 1991 to Walton discloses a vehicle adapted for traveling inside HVAC ducts and spraying liquids to clean the ducts. It was guided by the duct wall and had no provisions for remote steering. It did not provide video camera and display for showing the inside of the ducts as it traveled. 
     U.S. Pat. No. 5,317,782 issued Jun. 7, 1994 to Matsuura discloses a remote controlled tracked vehicle adapted for traveling inside HVAC duct and cleaning ducts. It included a video camera fixed to the body of the vehicle and a remote display for viewing the image. It also included a swiveling air jet for blowing debris from the duct wall. The vehicle followed the walls of the duct and provided no method for remote controlled steering. 
     U.S. Pat. No. 5,377,381 issued Jan. 3, 1995 to Wilson describes a vehicle adapted for traveling inside HVAC ducts and cleaning the ducts. It had specialized tools for spraying and brushing. It did not have the ability make controlled turns since it was designed to be guided by the duct walls. It did not provide video camera and display for showing the inside of the ducts as it traveled. 
     U.S. Pat. No. 5,528,789 issued Jun. 25, 1996 to Rostamo discloses a remote controlled tracked vehicle adapted for cleaning ducts. The vehicle could be steered remotely and could be maneuvered independent of the duct walls. It included a video camera fixed to the body of the vehicle with a lighting system so the inside of the ducts could be viewed on a remote display. It also included a rotating brush powered by air pressure that could be raised and lowered by remote control. 
     The remote controlled vehicles of the previous art for use in HVAC duct were adapted for cleaning. Thus they were relatively large to support the weight and stress caused by the cleaning apparatus and process. They required a compressed air source to power the cleaning apparatus. They were too large to fit in many trunks routinely used in residential HVAC systems. They did not have a moveable tool adapted to capture string or a moveable video camera adapted to searching for string. 
     OBJECTS OF THIS INVENTION 
     An object of this invention is to provide a remote controlled vehicle to assist in threading a string through an HVAC duct system from a vent to the HVAC equipment where a small duct supplies the vent and the small duct is connected to a large supply trunk connected to the HVAC supply plenum. 
     Another object is to provide a remote controlled vehicle to assist in threading string in a HVAC duct system that is smaller, less expensive, and more functional than the prier art. 
     Another object is to provide a remote controlled vehicle to assist in threading string such that the installation labor is less and more predictable for a wider variety of duct systems than the methods of the prier art. 
     SUMMARY 
     The invention is a tethered remote controlled vehicle adapted for navigating and maneuvering inside HVAC supply trunks. It is equipped with a moveable camera and a powered tool for snagging a string or parachute propelled into the trunk by other methods. A command box is provided to view the image from the camera and control the vehicle&#39;s various functions. The installation technician inserts the vehicle into the trunk from an access hole and uses the command box to navigate and maneuver the vehicle inside a HVAC trunk and locate and secure the string to the vehicle. The technician then controls the vehicle to pull the string back to the access or the technician can manually pull the vehicle back to the access by its tether. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be understood more fully from the detailed description given below and from the accompanying drawings of embodiments of the invention which, however, should not be taken to limit the invention to the specific embodiments described, but are for explanation and understanding only. 
         FIG. 1  is a perspective view of a HVAC system with tools for threading a string. 
         FIG. 2  is a perspective view of the vehicle with its cover removed. 
         FIG. 3  is a perspective view of the vehicle top with circuit board attached. 
         FIG. 4  is a perspective of the snag fixture. 
         FIG. 5  is a perspective view of the complete vehicle with the camera positioned for rear view. 
         FIG. 6  is a perspective view of the power system for the snag tool. 
         FIG. 7  is an exploded perspective view of the camera arm and snag arm. 
         FIG. 8  is a perspective view of the remote command box. 
         FIG. 9  is a block diagram of the command box and vehicle circuits. 
         FIG. 10  is a schematic diagram of the command box circuit. 
         FIG. 11  is a schematic diagram of the vehicle motor control circuit. 
         FIG. 12  is a flow chart of a portion of the command box logic. 
         FIG. 13  is a flow chart of a portion of the command box logic. 
         FIG. 14A  is a timing diagram of the control signal from the command box to the vehicle. 
         FIG. 14B  is a timing diagram of a control pulse showing its three states. 
         FIG. 15  is flow chart of the vehicle motor control logic. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a perspective view of a typical HVAC system found in residential dwellings. HVAC equipment  100  includes a fan for generating a flow of warmed or cooled air through a network of supply ducts that distribute the air through out the dwelling. The duct network includes a main trunk  101  connected to the supply plenum of the HVAC equipment  100 . Only a small section of the main trunk is shown. The open end  102  is connected to the remainder of the duct network. A smaller duct  104  connects to the main trunk at  107  and provides a path for airflow to vent  105 . There are one or more vents in each room of the dwelling. Each of the other vents is connected to a smaller duct that also connects to the main trunk. Dwellings typically have 10 to 30 vents; only one vent of many is shown in  FIG. 1 . Air is returned to the HVAC equipment through duct  103  which is connected to one or more large centrally located return vents in the dwelling. In many dwellings, the duct network is enclosed by walls, floors, and or ceilings. Easy access is only available at the vents and at the supply plenum. An access hole  106  cut in the supply plenum near the HVAC equipment provides access to the interior of the main trunk  101 . 
     A portion of the installation process requires threading a string from vent  105  through duct  104  and trunk  101  to access  106 . The threading is accomplished in two steps. First a small light object  120  connected to string  121  is propelled through the duct  104  using high velocity blower  110 . Typically the object  120  is a ball made from expanded polystyrene foam. This step propels the object  120  and string  121  through duct  104  through joint  107  into trunk  101 . A visual cutout  108  in trunk  101  provides a view inside the trunk. Object  130  and string  131  represent object  120  and string  121  after being propelled through duct  104 . 
     Remote controlled vehicle  200  is connected via tether  302  to the command box  800 . The vehicle  200 , tether  302 , and command box  800  are the subject of this invention. The installation technician inserts the vehicle into trunk  101  through access  106  and uses the command box to control the vehicle, navigating it through trunk  101  until it reaches object  130  near joint  107 . A video camera on the vehicle sends an image to the display  830  on the command box so the technician has a view of the inside of the duct. The technician commands the snag tool  238  to rotate while the vehicle is maneuvered near string  131 . After the snag tool captures the string, the technician can navigate the vehicle back to the access  106 , pulling the string along. Alternately the technician can use the tether  302  to pull the vehicle back to the access with the string. 
       FIG. 2  is a perspective diagram of the vehicle with the top cover removed. The overall size of the preferred embodiment enables it to navigate inside a 7″ round duct. The central structure of the vehicle is the U-shaped chassis  202  bent from sheet metal. The right side of the vehicle is propelled by the right gear motor  210  connected to drive wheel  212  which engages right track  214 . Idler wheel  216  is connected to chassis  202  and guides right track  214  along the right side of the chassis. The left side of the vehicle is propelled by the left gear motor  220  connected to drive wheel  222  which engages left track  224 . Idler wheel  226  is connected to chassis  202  and guides left track  224  along the left side of the chassis. Tracks are preferred over wheels because they maximize traction to the duct surface and provide high maneuverability. Several manufactures serving the hobby robot market provide suitable track and motor systems. For example, Solarbotics Ltd., 201 35.sup.th. Ave. NE, Calgary, AB T2E 2K5 supplies “Gear Motor 3” that is suitable for gear motors  210  and  220 . They also provide “Gear Motor Tread Cogs”, “Gear Motor Tread Links”, and “Gear Motor Tread Idlers” that are suitable for right track elements  212 ,  214 , and  216  respectively and for left track elements  222 ,  224 , and  226  respectively. 
     The snag gear motor  230  provides the drive for the snag fixture  238 . A suitable gear motor is supplied by the aforementioned Solarbotics as “Gear Motor 6”. O-ring belt  232  transfers rotation from motor  230  to drive tube  234  and flexible shaft  236  connected to snag fixture  238 . The drive tube  234  allows the flexible shaft to slide in and out of the drive tube. End cap  235  on the drive tube  234  limits the travel of the flexible shaft so it can not be pulled out of the drive tube. The outer surface of the flexible shaft has a spiral wrap of wire that creates a fine-pitched shallow thread. This thread is used to create a force to move the flexible shaft as it is rotated. The rotation motion provided by motor  230  causes the snag fixture  238  to extend or retract depending on the direction rotation. 
     The camera gear motor  240  rotates the camera arm  242  and snag arm  244 . A suitable gear motor is supplied by the aforementioned Solarbotics as “Gear Motor 3”. Camera arm  242  supports camera  246  and LEDs (light emitting diodes)  248 . The camera arm has a range of rotation of about 170 degrees. Downward rotation is limited by camera arm  242  interfering with chassis  202 . Upward rotation is limited by camera  246  interfering with camera motor  240 . When fully rotated upward, the camera provides a reward view that is used when navigating the vehicle backwards. 
     Snag arm  244  controls the elevation of the flexible shaft  236 . The snag arm  244  is free to rotate about the axis of the drive shaft of camera motor  240 , independent of the camera arm. However, the stiffness of flexible shaft  236  limits the range of rotation of snag arm  244  to about 45 degrees above and below the axis of the drive tube  234 . Magnet  243  provides a “sticky-coupling” between camera arm  242  and snag arm  244 . The magnet couples the snag arm to the camera arm for limited up and down rotation of the camera arm. If the camera arm is rotated more than about 45 degrees upward, the magnet will release the snag arm. The camera arm can then rotate upward to its maximum rotation. The snag arm position is then determined by the stiffness of flexible shaft. As the camera arm is rotated fully down, the magnet again couples the camera arm and the snag arm. The downward rotation of the snag arm is limited by the flexible shaft pressing against the bottom duct surface. As the camera arm rotates fully down, the magnet slips so that the camera arm and snag arm become approximately aligned. This sticky-coupling enables the camera motor to control the elevation of both the camera and snag tool while allowing a larger range of rotation for the camera. 
       FIG. 3  is a perspective diagram of the vehicle top cover  300 . The vehicle PCB (printed circuit board)  301  contains the vehicle control circuits and is attached to cover  300 . PCB  300  has connector  303  for connecting to tether  302 . In the preferred embodiment the tether is standard 50 foot length of 8-conductor CAT-5 cable with factory installed connectors on both ends. These cables are available through multiple retail and wholesale stores and are typically used to make connections to an Ethernet. These cables are flexible, have a sufficient number of conductors and current carrying capacity, and are sufficient strong and durable for use in a HVAC duct system. The tether  302  is secured to end  350  of top  300  by strain relief  304 . The strain relief transfers pulling forces on tether  302  to top  300  without straining the tether connection with connector  303 . 
     The primary components of the vehicle control circuit are the microprocessor  310  and H-bridge motor drive ICs (integrated circuits)  311  for the right motor,  312  for left motor,  313  for camera motor, and  314  for snag motor. The PCB  301  has connection points for the vehicle components. These connections are made by soldering wires connected to the components to the connection points. Connection points  320  connect to LEDs  248  shown in  FIG. 1 . Connection points  322  connect to camera  246  shown in  FIG. 1 . Two of these connection points provide power and ground to the camera and the third connection point connects to the camera video output. Connection points  324  connect to right motor. Connection points  326  connect to the left motor. Connection points  328  connect to camera motor. Connection points  330  connect to snag motor. 
     Surface  351  of top  300  covers the top of chassis  202  of the vehicle shown in  FIG. 1 . Cut out area  352  provides clearance for the camera  246  and camera arm  242  to rotate upward until the camera touches the top of camera motor  240 . Clearance holes  360  are for screws that attach to the bottom of chassis  202 . Clearance holes  361  are for screws that attach to the side of chassis  202 . 
       FIG. 4  is a perspective view of the snag fixture  238 . The fixture is cut from flat sheet metal and formed to fit around collar  400  and attached using solder or adhesive. Collar  400  attaches to flexible shaft  236  by set screw  401 . Points  402  are bent up from the plane of  238  by about 20 degrees. Points  404  are bent down from the plane of  238  by about 20 degrees. Rotating the flexible shaft clock wise (when view from the front) tends to cause causes the points to capture string or parachute material. The string or parachute wraps around  238  as it rotate, creating a strong connection between the snag fixture and the string or parachute material. 
       FIG. 5  is a perspective view from the rear of the vehicle  200  with the top  300  attached. Four sheet metal screws pass through holes  360  and  361  shown in  FIG. 3  and engage with the surfaces of chassis  202  shown in  FIG. 2 . Only screw  501  is visible in this view. Top surface  350  covers the back of the vehicle. Strain relief  304  secures tether  302  to the surface  350 . Surface  351  covers the top of the vehicle. The camera  246  is fully rotated upwards so that it provides a view toward the rear. Cut out  352  provides clearance for the camera and camera arm  242 . The elevation of the snag arm  244  is determined by the flexibility of the flexible shaft  236 , its length of extension, and the weight of snag fixture  238 . Visible components of the right side drive include drive wheel  212 , track  214 , and idle wheel  216 . Visible components of the left side drive include drive wheel  222  and track  224 . 
       FIG. 6  is a perspective view of the snag tool drive mechanism. Drive tube  234  is supported by bearing blocks  600  and  602  that allow the tube to freely turn. The bearing blocks are attached to chassis  202  shown in  FIG. 2  by screws  601  and  603 . Pulley  612  is attached to drive tube  234  by solder or adhesive. The interface between pulley  612  and bearing block  600  constrains drive tube  234  against pulling forces to the right. In the absence of a pulling force to the right, the drive tube is constrained by the force exerted by O-ring drive belt  232 . Snag motor  230  rotates pulley  610  which drives belt  232  and causes drive tube  234  to rotate. The rotation may be in either direction. Drive tube  234  has a view cutaway section between the bearing blocks so that the interior structure is visible. A square tube  620  is attached to the inside of drive tube  234 . Square tube  620  has a cutaway view so that drive block  622  can be seen. Drive block  622  is sized to slide freely inside square tube  620  and is attached to flexible shaft  236 . The right end of drive tube  234  is capped by plug  235  which has a round hole large enough to allow the flexible shaft to slide in or out. The hole in plug  235  is small enough to prevent drive block  622  from passing through. The drive plug  622  and flexible shaft  236  are free to slide inside the square tube from the cap  235  on the right to the end  624  of the drive tube. The flexible shaft and drive block can be inserted and removed through end  624 . When assembled, the right motor provides a stop that prevents the drive block  622  from disengaging from the square tube  620 . This drive mechanism couples the flexible shaft  236  to the rotation provided by snag motor  230  while allowing the flexible shaft and drive block  622  to slide nearly the length of the drive tube  234 . Pulling force on the flexible shaft when it at its extreme right position is transferred by drive block  622  to plug  235  to drive tube  234  to pulley  612  to bearing block  600  to the chassis  202 . 
       FIG. 7  is an exploded perspective view of the camera arm and snag arm assembly. Coupler  704  slides over the drive shaft  701  of camera motor  240 . Set screw  706  engages flat surface  702  to hold the coupler securely to the drive shaft  701 . Camera arm  242  is attached using solder or adhesive to coupler  704 . The camera arm has a tab  709  bent at 90 degrees attached to camera  246 . LEDs  248  are attached to the camera. Coupler  704  has a shaft  708  that fits inside collar  710  such that the collar  710  can freely rotate about the shaft  708 . Snag arms  244  and  732  are attached using solder or adhesive to collar  710  and collar  711 . Collar  710  is constrained by screw  712  threaded into a matching threaded hole in shaft  708 . After screw  712  is tightened, the assembled snag arm composed of collar  710 , arms  244  and  732  and collar  711  can rotate freely rotate on shaft  708 . 
     Flexible shaft  236  has an outer spiral winding of wire that forms a fine-pitched shallow thread. Sling  726  is made from knit fabric and interfaces with the flexible shaft. When a force is applied to the fabric to grip the flexible shaft, the fabric&#39;s thread loops grip the shallow threads so that rotating the flexible shaft exerts a force along the axis of the flexible shaft. Metal clamp  724  is shaped for a lose fit around the flexible shaft. The fabric sling  727  and flexible shaft  236  are placed inside clamp  724 . Screw  720  passes through holes  728  in the fabric sling and through clamp  724 . Nut  722  is used to adjust the force applied to the flexible shaft through the clamp and fabric. Nut  722  is adjusted to set the force of the fabric on the flexible shaft just strong enough to engage the threads on the flexible shaft. The force is set as weak as possible so that the flexible shaft is easy to rotate and can be pushed into or pulled out of the drive tube  234  by hand force. The flexible shaft extends forward when the snag motor  230  drives the flexible shaft  236  clockwise (when viewed from the front). 
       FIG. 8  is a perspective view of the command box  800 . The enclosure  802  provides the mounting surfaces for the controls and protection for the circuit components. Tether  302  and AC power cord  810  pass through the top side of enclosure  802 . Posts  804  and  806  and discs  805  and  807  are structures for storing tether  302  and power cord  810 . This is useful since the tether is typically 50 feet long. The tether storing structure is configured so that the tether can be wound in a figure-eight pattern which prevents twists as the tether is wound and unwound. Display  830  is a LCD (liquid crystal display) for viewing the image produced by camera  246 . 
     Switch  820  controls the rotation of the camera arm. The switch has three positions and a SPDT switch action. The switch is held by a spring action such that no connections are made when no force is applied to the switch. The service technician can raise or lower the camera by holding the switch up or down until the camera reaches the desired position. When the switch is released, the camera position is held. 
     Switch  822  controls the snag tool. The switch has three positions and a SPDT switch action. Once placed in any of the three positions, the switch holds that position. Normally the switch is in its center position and no connections are made. The technician moves the switch to its upward position to drive the snag tool clockwise to extend and capture. The technician moves the switch to its downward position to drive the snag tool counter clockwise to retract. The technician moves the switch to its center position to stop snag tool rotation. 
     Joystick  824  is used to navigate the vehicle. The joystick interfaces to four switches that represent the commands of forward, reverse, turn left, and turn right. The joystick has a spring action that centers it when no force is applied, so no switch contacts are closed. The technician can manipulate the joystick to produce eight combinations of switch closures and corresponding motor actions:
         1. Forward—both tracks drive forward   2. Reverse—both tracks drive reverse   3. Turn left—left track drives reverse and right track drives forward   4. Turn right—left track drives forward and right track drives reverse   5. Forward left—left track is off and right track drives forward   6. Forward right—left track drives forward and right track is off   7. Reverse left—left track is off and right track drives reverse   8. Reverse right—left track drives reverse and right track is off       

     The technician navigates the vehicle by manipulating the joystick  824  while watching the display  830 . Combinations  3  and  4  cause the vehicle to make pivot turns around its center. Combinations  5  through  8  cause the vehicle to make turns with a radius about equal to the length of the tracks. 
       FIG. 9  is a block diagram of the circuit components of command box  800  and the vehicle  200 . The display  830 , power supply  902  and power cord  810 , and remote control circuits  1000  are part of the command box  800 . The camera  246 , LEDs  248 , and control and motor circuit  1100  are part of the vehicle  200 . Element  904  is a connector on the command box for connecting to tether  302 . Element  303  is the connector on the vehicle PCB  301  shown in  FIG. 3 . Connectors  303  and  904  make connections to each of the eight wires in tether  302 . Wire  950  carries the command signal to the vehicle. Wire  951  carries the video signal from the camera  246  to the display  830 . A pair of wires carries power and ground for the camera and LEDs. Two pairs of wires carry power and ground for the motors and control. The separate power and ground supply for camera  246  and LEDs  248  isolates the video signal from noise induced by high current surges in the power and ground supply for the motors. 
       FIG. 10  is a schematic diagram of the circuit used to convert actions at the command box  800  into the control signal  950  sent to the vehicle. Microprocessor  1002  monitors the states switches  820 ,  822 , and joystick  824  using eight inputs and generates the control signal. Several semiconductor companies supply suitable microprocessors. The preferred embodiment uses device PIC12F629 supplied by Microchip Technology Inc., 2355 West Chandler Blvd., Chandler, Ariz. 85224-6199. Each of the eight inputs to the microprocessor is connected to a high value resistor which is in turn connected to the positive power supply. For example, resistor  1015  connected to input  1011  ensures a high level is read when switch  1010  is open. These resistors ensure that the inputs will be read as a high when the switches are open. Switches  1010 ,  1012 ,  1020 , and  1022  are part of joystick  824 . Pushing the joystick forward causes switch  1010  to close, connecting the forward input  1011  to ground. This overcomes the high signal supplied by resistor  1015  so input  1011  is at a low level. Pushing the joystick rearward causes switch  1012  to close, connecting the reverse input  1012  to ground. Switch  1020  controls the state of the turn left input  1021 . Switch  1022  controls the state of the turn right input  1023 . The state of camera switch  820  controls the camera up input  1031  and the camera down input  1032 . The state of snag switch  822  controls the snag out input  1041  and the snag in input  1042 . 
       FIG. 11  is a schematic diagram of the vehicle circuit that decodes the control signal  950 . Microprocessor  310  processes signal  950  and produces two output control signals for each of the four motors. Several semiconductor companies supply suitable microprocessors. The preferred embodiment uses device PIC 12F629 supplied by Microchip Technology Inc., 2355 West Chandler Blvd., Chandler, Ariz. 85224-6199. 
     Several semiconductor suppliers provide suitable H-bridge circuits for driving the motors. The preferred embodiment uses model BD6225 supplied by Rohm Co., LTD., 21, Saiin Mizosaki-cho, Ukyo-ku, Kyoto 615-8585, Japan (www.rohm.com). H-bridge IC  311  drives the right motor  210 . When outputs  1111  and  1112  are low, H-bridge  311  supplies no power to the right motor  210 . When output  1111  is high, H-bridge  311  drives motor  210  such that the right track moves forward. When output  1112  is high, H-bridge  311  drives motor  210  such that the right track moves in reverse. Signals  1111  and  1112  are never high at the same time. 
     H-bridge IC  312  drives the left motor  220 . When outputs  1121  and  1122  are low, H-bridge  312  supplies no power to the left motor  220 . When output  1121  is high, H-bridge  312  drives motor  220  such that the let track moves forward. When output  1122  is high, H-bridge  312  drives motor  220  such that the left track moves in reverse. Signals  1121  and  1122  are never high at the same time. 
     H-bridge IC  313  drives the camera motor  240 . When outputs  1131  and  1132  are low, H-bridge  313  supplies no power to the camera motor  240 . When output  1131  is high, H-bridge  313  drives motor  240  such that the camera rotates upward. When output  1132  is high, H-bridge  313  drives motor  240  such that the camera rotates downward. Signals  1131  and  1132  are never high at the same time. 
     H-bridge IC  314  drives the snag motor  230 . When outputs  1141  and  1142  are low, H-bridge  314  supplies no power to the snag motor  230 . When output  1141  is high, H-bridge  314  drives snag motor  230  such that the snag tool rotates counter clockwise and is retracted. When output  1142  is high, H-bridge  314  drives motor  230  such that the snag tool rotates clockwise, and extends to capture a string or parachute. Signals  1141  and  1142  are never high at the same time. 
       FIG. 12  is a flow chart of the logic used by microprocessor  1002 . Those ordinarily skilled in the art can translate such a flow chart into a program suitable for running on microprocessor  1002 . The flow chart is the logic that reads the four joystick switches and encodes commands for the right motor  210  and left motor  220 . Valid combinations of the four joystick switches  1010 ,  1012 ,  1020 , and  1022  can produce a total of nine command combinations. In the flow chart, the four switches are called “FORWARD”, REVERSE”, “LEFT”, and “RIGHT” and correspond respectively to signals  1011 ,  1013 ,  1021 , and  1023  in  FIG. 10 . Each decision in the flow chart is base in on the state of one of these switches. Each command combination is represented by a box that contains the drive commands for the right motor  210  and left motor  220 . For example, “LEFT FW” and “RIGHT RV” commands the left track  224  to drive forward and right track  214  to drive in reverse. This is the command for a pivot turn to the right. 
     The flow chart in  FIG. 12  includes a box called “ FIG. 13  FLOW CHART”. That logic is shown in  FIG. 13 . 
       FIG. 13  is a flow chart of the logic used by microprocessor  1002  to read the camera control switch  820  and snag control switch  822 . Each state of the camera control switch  820  is translated into three commands for the camera motor  240 . These commands are “CAMERA UP”, “CAMERA DOWN”, and “CAMERA OFF”. Each state of the snag control switch  822  is translated into three commands for the snag motor  230 . These commands are “SNAG IN”, “SNAG OUT”, and “SNAG OFF”. 
       FIG. 14A  is a timing diagram of the control signal  950  generated by microprocessor  1002 . The signal is a sequence of four pulses  1401 ,  1402 ,  1403 , and  1404  followed by a long period  1400  of low level signal. Each pulse encodes the commands for one of the four motors:  1401  for right motor  210 ,  1402  for left motor  220 ,  1403  for camera motor  240 , and  1404  for snag motor  230 . Each pulse can have one of three discrete durations illustrated by pulse  1404 . The short pulse  1404  corresponds to a command of snag motor off. The medium length pulse  1405  corresponds to the command of snag motor rotate counterclockwise to retract the snag tool. The long pulse  1406  corresponds to the command of snag motor rotate clockwise to extend snag tool. In the preferred embodiment, the short pulse duration is 1 ms, the medium duration is 1.5 ms, and the long duration is 2 ms. The separation between pulses is 2 ms and the long duration of the long low period is 10 ms. The command boxes in  FIG. 12  and  FIG. 13  control microprocessor output  950  such that the pulses have the proper durations and spaces as shown in  FIG. 14A . 
       FIG. 14B  is a timing diagram of a single command pulse. The diagram shows time period t 1  as the time between the leading edge  1408  of the pulse and the half way point between edge  1407  for a short pulse and edge  1405  for a medium pulse. The diagram shows t 2  as the time between the leading edge  1408  of the pulse and halfway point between edge  1405  of a medium pulse and edge  1406  of a long pulse. The pulse is decoded by first measuring its duration, and then comparing its duration to t 1  and t 2 . If the measured pulse duration is less than t 1 , then the pulse is determined to be a short pulse. If the measured pulse duration is greater than t 2 , then the pulse is determined to be a long pulse. If the measured pulse duration is more than t 1  and less than t 2 , then the pulse is determined to be a medium duration pulse. 
       FIG. 15  is a flow diagram of the logic in the microprocessor  310  used to decode the control signal  950 . Those ordinarily skilled in the art can translate such a flow chart into a program suitable for running on the microprocessor. Synchronization is accomplished by waiting for a low level signal that lasts longer than the time between rising edges of the pulses. The duration of each pulse  1401 ,  1402 ,  1403 , and  1404  is measured. The logic then compares the duration of each pulse to t 1  and t 2  to decode the command represented by each pulse. Then the corresponding output signals are set. The twelve boxes in the lower portion of  FIG. 15  represent all valid combinations of commands that can be made. For example, the box containing “RIGHT RV” sets signal  1112  a high level and signal  1111  to a low level. This causes the right motor  210  to drive track  214  in reverse. The box containing “RIGHT FW” sets signal  1111  a high level and signal  1112  to a low level. This causes the right motor  210  to drive track  214  forward. The box containing “RIGHT OFF” sets signal  1111  and signal  1112  to a low level. This causes the right motor  210  to be off. 
     Conclusion 
     From the forgoing description, it will be apparent that there has been provided an improved remote controlled vehicle to assist in threading a string from a vent to a central plenum of a HVAC system. Variation and modification of the described vehicle, tether, and command box will undoubtedly suggest themselves to those skilled in the art. Accordingly, the forgoing description should be taken as illustrative and not in a limiting sense. 
     The various features illustrated in the figures may be combined in many ways, and should not be interpreted as though limited to the specific embodiments in which they were explained and shown. Those skilled in the art having the benefit of this disclosure will appreciate that many other variations from the foregoing description and drawings may be made within the scope of the present invention. Indeed, the invention is not limited to the details described above. Rather, it is the following claims including any amendments thereto that define the scope of the invention.