Making Shoebox Rovers for Racing

I met the toy Sojourner rover at the Oregon Star Party (OSP) in 2004 were I was asked to judge OSP's Rover Races (I was demonstrating a wireless rover and was the president of the Boise Robotics Group - The BoRG). After watching contestants steer their rovers by kicking them, I resolved to design a better rover. I've finished designing a simple and affordable rover and this article will teach you how to built the rover. It's my hope that this rover can become the standard for rover races at star parties, Space Day, and Astronomy Day.

Wanna drag?

The shoebox rover uses commonly available materials and requires simple machining. The largest tool you'll need is a drill press. A Dremel or other small handheld electric drill is perfect for cutting holes in the shoebox. The only uncommon tool is a tubing cutter, which you can purchase at your local hobby store.

A Dremel with small drill bit and a tubing cutter

The shoebox rover is built in the following steps.

1. Convert two battery-powered screwdrivers into the rover drive
2. Assemble and install the axles
3. Build the APXS (Alpha Proton X-ray Spectrometer)
4. Make a set of six wheels
5. Wire the rover for power
6. Add the photovoltaic array

Your junk box may already have many of the items needed to build the shoebox rover.

You can use Sintra in place of the hard board. Sintra is a foamed PVC and is very easy to work with. I used Sintra (since I had it available) in place of hard board for my shoebox rovers. Sintra is available from plastic suppliers and some on-line robot stores.

First, make sure the screwdriver is set to POWER, and not MANUAL. In MANUAL, the drive train is locked and the motor doesn't turn.

• Pull the battery holder out of the screwdriver handle
• Look inside the handle and get a feel for where the motor and its metal contacts are located
• Use an Exacto saw or hack saw to cut the screwdriver in half, at a point just before the metal contacts in the motor
• Remove the U-shaped pin holding the gear train housing to the motor housing

• Pull the gear train housing from the screwdriver case

• Open the screwdriver case lengthwise, along its seam
• Lift the motor out of the case
• Cut away the metal and plastic switch from the back of motor
  

  The motor after removal from screwdriver case

  • Solder nine foot long wires to the two brass contacts on the back of the motor
  • Place the motor back into the case
  • Close the case
  • Snap the gear train housing back on the screwdriver (you may have to rotate the motor drive gear to get everything to line back up)
  • Replace the U-shaped pin
  • Use masking tape to label each motor wire as either left or right motor (this is going to be useful when you wire up the PS-1 controller)
  

  The screwdriver after modification

  Now that the screwdriver case has been modified, you need to add the axle.

  • Cover one half of a one inch long headless 1/4-20 setscrew in heat shrink
  • Shrink the tubing
  • Pound the covered half of the setscrew into screwdriver chuck

  Note: If you have access to welding facilities, you can weld the setscrew into the screwdriver chuck, instead of hammering it in.

  Making Rover Axles

  In this step you'll cut three pairs of holes into the shoebox.

  There are three holes on the left and right sides of the rover body. The front and back holes are 1/4 inches in diameter and let the front and rear axles stick out. The center hole is 1-1/2 inches in diameter and lets the nose of the screwdriver stick out. I found it easier to use a Dremel and small drill bit to cut out the holes in the shoebox.

  The front and rear axles are 1-1/2 inches from the edge of the shoebox. The drive motors are mounted in the center of the rover and sit on the bottom of the shoe box. In the diagram below, I have illustrated the position of the front and rear axles and the hole for the screwdriver.

  

  You'll notice on the shoebox (and in my diagram), that there is a curved crease in the side of the shoebox. I don't know why this is here, but it sure help to properly position the axle holes. The center of the three-inch wheel axle is slightly higher above the bottom of the shoebox than the center of the four-inch drive motors. This lets the rover rock back and forth slightly during a turn. Without this ability, the front and rear wheels create excessive drag when the rover turns. So don't make the three wheel axles the same height.

  • Find a point 1-1/2 inches from the front and back of the shoebox and mark it with a felt tip marker
  • Drill a 1/4 inch diameter hole at the 1-1/2 inch point so that the top of the hole is flush with the curved crease in the shoebox
  • Locate the center of the shoebox and mark it with a felt tipped marker
  • Draw a 1-1/2 inch diameter circle here, with the bottom of the circle just above the bottom of the shoebox and the top of the circle just above the crease in the shoebox
  • Drill or cut out the 1-1/2 inch hole

  Reinforce the bottom of the shoebox before mounting the axles and motors.

  • Cut a 1/8 inch thick sheet of hard board 4-1/2 by 9-1/2 inches
  • Tape the reinforcement into the bottom hollow of the shoebox
  • Cover the shoebox in aluminum tape to give it a metallic look

  Note: You can tape the shoebox later, if you feel comfortable taking the rover apart after you've bolted the entire thing together (I did this several times when constructing my first rover).

  The motor case has a nice waist for mounting it to the bottom of the shoebox.

  • Place the motors inside the shoebox and stick their noses out
  • Position the screwdrivers as symmetrically as you can
  • Place a 1-1/4 inch galvanized pipe strap over the waist of the screwdriver body and mark the location of its two mounting holes
  • Remove the motors and straps and drill 1/8 inch diameter holes through the bottom of the shoebox at the locations marked

  Note: A 1/8 inch hole seems to work fine, but that may have something to do with me drilling the holes by hand.

  • Put the screwdrivers back in the shoebox and bolt them into place with the pipe straps and #4-32 hardware
  

  The motors strapped to the inside of the rover

  Note: I had to add two washers beneath each hole in the strap to fill the gap between the strap and bottom of the shoebox.

  Now its time to make the axles for the front and rear idler wheels

  • Cut two pieces of 1/2 by 3/4 inch pine trim five inches long
  • Cut two pieces of 9/32 inch aluminum tube nine inches long
  • Put the wood trim in the bottom of the shoebox and align them with the axle holes
  • Lay the trim so it's wider than it's taller
  • Slide the aluminum tubing over each of the one foot length of 1/4-20 threaded rod
  • Slide each axle through the axle holes
  • Slide the 1/2 by 3/4 inch trim beneath the axle - it should be a tight fit (if its too tight, then enlarge the holes in the rover body slightly)
  • Place a nylon wire strap over the center of the axle
  • Position the trim so you can drill through the hole in the nylon wire strap and into the pine trim and shoebox
  • Drill the hole
  • Bolt the nylon strap and axle into place with a #4-32 bolt
  

  The finished idler wheel axle seen from inside rover

  Note: The axle fit should be tight enough that a single strap is sufficient to hold everything in place. It that's not the case in your rover, then bolt two nylon wire straps to the axle and rover body.

  • Bolt #1/4-20 nuts onto the threaded rod to keep it from slipping from the aluminum tube
  • Now repeat this process for the other axle
  

  Finished idler wheel axle seen from outside rover

  Making the APXS

  Next you'll turn a broom handle into a precision scienjpgic instrument, the APXS. Well, maybe not. You'll need a band saw, small drill, soldering iron, hot glue.

  The APXS is the alpha proton x-ray spectrometer, a device that determines the presence of elements in a rock or dirt sample. Inside the APXS are several pieces of the isotope curium-244. Alpha particles emitted by curium-244 will do one of three things when they strike a sample, bounce off the atoms, create a mild nuclear reaction that emits protons, or excite atoms to emit x-rays. Each element has its own characteristic response to bombardment by alpha particles. The spectrometer function of the APXS determines the atoms present in a sample from the number, types, and energies of particles emitted from the sample. The rate of particle emission from the sample is not great, so the APXS often spends hours recording the results from the bombarded sample. But after doing so, researchers on Earth can determine the elements making up a sample. Basic geology then determines the types of minerals present.

  The APXS in this rover doesn't carry a radioisotope, but does carry a normally open roller lever switch. This APXS only detects when it makes contact with a sample, which is one function of the real APXS.

  • Cut a one inch diameter wooden dowel 15-1/2 inches long
  • Cut a 1/4 inch wide and 1/2 inch deep notch into one end of the dowel (this is now the front)
  • Wrap this end of the dowel in aluminum tape for the first three inches starting from the front
  • Solder two nine foot long wires to the contacts labeled COM and NO in the lever switch
  • Slide heat shrink over your soldered connection and shrink
  • Slide the lever switch into the cut slot of the dowel, carefully bend the wires as you do so
  • Use hot glue to permanently lock the switch body into the APXS
  Cut a hole, slightly larger than one inch across, in the center face of the rover body and above the ridge (see diagram below to see how the APXS mounts in the rover body)
  

  An x-ray's view of the side of the rover body. The APXS dowel extends about 2-1/2 inches beyond the front of the rover body

  • Slide the APXS wires through the hole before the APXS dowel
  • Drill a hole in the back of the rover body and into the center of the dowel
  • Screw in a small screw to hold the APXS in place
  • Use masking tape to label the ends of the wires, APXS
  • Leave the APXS wires for now, we'll do the electrical work later
  

  The business end of a completed APXS. I covered the very front of the APXS with a painted PVC tube for looks

  Making the Wheels

  In this step, you'll change six PVC (or polystyrene) caps into wheels. You'll need a centering jig and a drill press to make these changes. Two of the wheels are four inches in diameter and need a 1/4 inch hole drilled through the center. The other four wheels are three inches n diameter and have a 3/8 inch diameter hole drilled in their centers. After drilling the hole, you'll mount an axle through it. We'll start by making a jig to locate the center of each cap. Be sure you use pipe caps with flat, and not domed, tops, as the domed ones are harder to work with.

  

  This jig was made from modeling plywood and bass wood strips

  Follow these steps to make the wheel centering jig.

  • Cut a four inch square out of 1/16 inch thick modeling plywood (Cut it as square as possible)
  • Draw a line across one diagonal of the plywood (this will be the drawing slot)
  • Cut the drawing slot (centering it in the jig) two inches long and 1/16th inches wide
  • Cut two pieces of 1/4 by 1/4 inch basswood strips 3-1/2 inches long (these are the alignment strips)
  • Glue the alignment strips at a 90 degree angle by gluing them to two adjacent sides of the plywood

  To find the center of each cap, place the cap firmly against the alignment strips of the jig and drawing a line across the face of the wheel. Rotate the cap by a quarter of a turn and draw a second line across the face of the wheel. The center of the cap is where the lines intersect.

  • Find the center of the five remaining caps
  • Drill a 3/8 inch hole in the center of the three inch caps
  • Drill a 1/4 inch hole in the center of the four inch caps
  • Paint the caps with silver spray paint and let them dry.

  The two four-inch wheels are ready to be mounted to the drive motors.

  • Put a 1/4 inch fender washer on a screwdriver motor's setscrew
  • Put a four inch wheel on the setscrew (I had to spin mine, cutting shallow threads into the wheel in the process)
  • Slide a second 1/4 inch fender washer on the setscrew
  • Tighten a 1/4-20 nut on the setscrew (don't over-tighten the nut, or else you'll pull the setscrew out of the screwdriver chuck)
  • Repeat for the other wheel

  Now to make axles for the three inch wheels.

  • Insert a one-inch long lamp nipple into the hole of a three inch wheel
  • Put a 3/8 inch fender washer on both sides of the nipple
  • Lock the washer in place with a lock nut (tighten the lock nuts well)
  • See the diagram below if this is not clear
  

  A side view of the three inch wheel. Note that the fender washers are sandwiched between the cap and lock nut

  • Repeat these steps for the remaining three-inch wheels

  Now mount the three inch wheels to their axles.

  • Slide a three-inch wheel on an axle
  • Put two 1/4-20 nuts on the axle after the wheel and tighten them against each other

  Note: Be sure the nuts are not tightened against the wheel's axle, the wheel needs to spin freely. The nuts just keep the wheel from falling off the rover axle. If you have trouble using two nuts, then try using one nut and a little Lock-tite.

  Wiring the Rover and PS-1 Controller

  Now it's time to wire the rover up. You'll add batteries, switches, and a PS-1 controller to the rover. You'll need a soldering iron and small drill to complete this step.

  First you'll mount two battery holders inside the shoebox. Then you'll wire in battery connections to the APXS switch and screwdriver motors. This is followed by stripping out the PS-1 controller and adding switches to it. You'll finish the electrical work by wiring the PS-1 controller to the rover cable. Here we go.

  There's room between the axles and the screwdrivers to mount a two "C" cell holder and the nine volt battery holder.

  • Unbolt the APXS dowel
  • Position the battery holders and mark the location of their bolt holes
  • Remove the holders and drill a 1/8 inch hole at the marked location
  • Replace the holders and bolt them into place with #4-32 hardware

  We'll wire up the APXS before the motors. But first, we need to discuss splicing wires together. You need heat shrink tubing for this work, do not use electrician's tape. Heat shrink tubing, as its name implies, shrinks to half its diameter when heated (use a hot air gun and not your soldering iron to shrink tubing). Here's how you'll splice two wires together.

  • Cut a one-inch long piece of tubing that canslide over the wire but is less than twice the diameter of the wire
  • Slide the tubing over one of the wires (push it away from the end of the wire so it doesn't shrink while you're soldering wires together)
  • Strip about 3/4 of an inch of insulation from the ends of the wires you're splicing together
  • Twist the wires around each other as illustrated below
  • Tin a hot soldering iron and place the iron beneath the twisted wires
  • Apply solder to the wires as they get hot
  • Cover the entire joint with a thin coat of solder
  • After the solder cools, slide the heat shrink over the soldered splice and shrink it
  

  How to make a splice

  Now let's get to work wiring up the APXS.

  • Mount the buzzer to the front of the shoebox, near the APXS dowel

  Note: I found I had better sound quality if I mounted the buzzer with only one bolt.

  • Drill a small hole through the shoebox and near the buzzer so the buzzer wires can enter the shoebox
  • Cut one of the APXS switch wires about six inches long
  • Splice this wire to the red wire of the buzzer
  • Splice the buzzer's black wire to the black wire of a nine volt battery snap
  • Splice the red wire of the nine volt battery snap to the rest of the first wire from the APXS switch
  

  The APXS Switch wiring before and after adding buzzer and battery

  Now splice wires to "C" cell battery holder.

  • Cut a red and black wire, nine feet long (the actual colors are not important, but red and black are standard for power and ground wires, respectively)
  • Splice the new wires to the wires of the battery holder, matching the colors of the "C" cell holder
  • Use masking tape and label the free ends of the red and black wires as C Cell, indicating that the other end is connected to the battery

  Now you'll create a cable to the PS-1 controller.

  • Drill a small hole through the back of the shoebox and just below the back end of the APXS (this is the cable opening)
  • Pass the wires from the motors, APXS, and battery holders through this hole
  • Bundle the wires inside the shoebox
  • Slide a 1/4 inch nylon wire strap over the bundle
  • Drill a 1/8 inch hole about 1/2 inch beneath the cable opening
  • Bolt the nylon wire strap (and wire bundle) through the hole and to the shoebox (this is a strain relief and prevents the wires from getting yanked out of the rover)
  • Bundle the cable together using nylon wire zip ties every six inches

  Now you'll add switches to the PS-1 controller

  • Open the PS-1 controller (there are several small black screws in the bottom)
  • Set aside the screws so you don't drop or lose them
  • Remove the printed circuit board from the inside of the PS-1 controller and toss it
  • Remove the locknuts from the two DPDT switches you purchased
  • Try mounting the switches into the top half of the PS-1 controller, one on each side (see photo)
  
  • Trim the plastic around the openings until the switches fit. Use an Exacto knife to do this.
  • Remove the DPDT switches from the PS-1 controller and set them aside

  Next step is to add the LED to the controller.

  • Clip the leads of a LED to about 1/2 inch long
  • Clip the leads of the resistor to 1/2 inch long
  • Tin the leads of the resistor and LED
  • Hold a lead of the resistor against any lead of the LED
  • Heat the two leads with a soldering iron while holding the leads together

  Note: The solder on both leads will melt and fuse the leads together, but you must hold the leads still while doing this. we do it this way because it's easier than splicing in this case.

  • Remove the soldering iron and let the solder cool
  • Carefully bend the LED leads by 90 degrees (see the diagram below)
  
  • Look at the side of the LED case (and the diagram above), you'll notice one side has a flat surface shaved in it
  • Cover the black wire with a short length of heat shrink
  • Solder the black wire from the APXS to the lead on the flat side of the LED
  • Solder the red wire from the APXS to the other lead of the LED (no need to use heat shrink)

  As a test, snap a nine volt battery into the battery snap and press the APXS switch. The buzzer will buzz and the LED light up. If they don't, then reverse the battery and try again. If that works, then unsolder the red and black wires from the LED and switch them around. Otherwise, check one of the wires wasn't switched around. Now shrink the heat shrink over the red wire.

  • Push the LED into the ANALOG hole of the PS-1 controller
  • Lock the LED into place with hot glue
  • Cover the resistor and soldered connections with hot glue

  Now its time to wire up the motor switches. Each switch controls the direction of rotation of each motor. Look at the bottom of each switch and you'll see that there are three pairs of contacts. You'll connect the center contacts of each switch to a motor and the outside pairs of contacts will connect to the battery. By switching the polarity of each pair of contacts to the battery, you can reverse the motor's rotation by throwing the switch forwards or backwards. Are you ready to do a little soldering?

  

  The pin layout of the DPDT switch. The + and - contacts can be reversed, but still must mirror diagonally

  • Bare about 3/4 inch of insulation from the ends of the two wires from both motors
  • Slide a short piece of heat shrink over the wires
  • Solder the wires to the two center pins of the DPDT switches
  • Cover the soldered connection with heat shrink

  • Cut eight pieces of wire, about five inches long
  • Bare about 1/2 inch of insulation from one end of each wire
  • Solder one wire onto each of the remaining contacts of the DPDT switch
  • Cover the soldered connection with heat shrink
  • Bare about 1/2 inch of insulation from the remaining ends of the eight wires
  • Twist the ends of diagonally opposing wires together

  • Bare about one inch of insulation from the ends of the two wires from the "C" cell holder
  • Slide about two inches of heat shrink tubing on each wire
  • Solder three sets of wires together, one wire from the "C" cell holder, and one pair of twisted wires from each switch
  • Cover the connection with the heat shrink tubing
  • Now solder the last "C" cell holder wire to the remaining twisted pairs from each switch
  • Cover this soldered connection with heat shrink tubing

  Note: Don't worry about which pair of wire was solder to the battery wire, we'll straighten that out later.

  A schematic of what you have just done is below

  

  Test your connections by loading two "C" cells into the rover and throwing one of the switches. One drive wheel should begin rotating. Reverse the switch and the wheel should rotate the opposite direction. When the switch is placed in the neutral position, the wheel stops. Now confirm the other switch does the same thing to the other drive motor.

  Now it's time to mount the switches into the PS-1 controller. Since we didn't pay attention to which wires were connected to where on the switches, we don't know the proper orientation of the switches. But don't worry, we didn't pay attention to the wiring of the switches because it's more difficult than discovering their proper orientation after the fact.

  Notice that there are a pair of nuts and washers on each switch, located at the base of their lever arms

  
  • Using a pair of pliers, remove the washers and nuts (save a nut and lock washer)
  • Stand behind the rover (with the APXS pointed away from you)
  • Flip one of the drive control switches and observe which wheel is spinning and which direction the rover is trying to move
  • If the left wheel is spinning, then place that switch in the left hole of the PS-1 controller
  • If the rover is trying to drive forward on that side, then rotate the switch so the lever arm is in the forward direction
  • Place a regular 1/4" washer over the hole in the PS-1 controller and the lever switch
  • Place a lock washer over the lever switch
  • Screw down a nut over the switch and tighten it so the switch can't rotate
  • Test the second switch to determine if it needs to be turned around and then bolt it into place

  For the next test, set the rover on the floor and stand behind it. Throw both drive control switches forward and the rover should drive forward. Reverse both switch and the rover should drive backwards. Now throw one switch forward and the other backwards and the rover should turn in place. If the drive wheels slip, increase their traction with one of two #84 rubber bands (don’t put rubber bands around the four idler wheels).

  If you drive the rover long enough, your thumbs will begin to get sore. So cover the levers with thread protector. These are tiny little rubber boots that cover te threads of bolts. You'll find them with some switches sold at Radio Shack, if you can't find small enough ones at Ace.

  • Now bolt the PS-1 controller halves back together

  The Photovoltaic Array

  I used a sheet of black Sintra for my photovoltaic array. However, Sojourner's array was blue in color. Use either 1/8 inch thick Sintra or hard board to make your array.

  • Cut the array to the size you desire (mine was 12 inches wide and 14 inches long)
  • If made from hard board, sand the edges and paint it before proceeding
  • Use thin colored tape to create a grid across the face of the array (I used 1/4 wide silver tape)
  • Cover the edges of the array in aluminum tape, wrapping the tape over the edges
  • Flip the array on its back and position the upside-down shoebox lid on the top of the upside-down photovoltaic array
  • Bolt the array to the lid along the lower ridges of the lid

  Note: You'll notice the lid is molded with raised ridges. Don't place the bolts close to the edge of the lid, since the lid snaps around the shoebox edges. Bolting too close to the edge interferes with closing the lid.

  • Mark and drill holes for four #4-32 bolts that will mount the array to the lid
  • Bolt the array to the lid
  • Snap the lid over the shoebox and confirm that the lid still fits properly

  That completes the rover. You should be able to take it out on a spin for as long as the batteries hold up.

  Rover Races

  It only takes a single rover to hold a competition, but, the competition goes faster (and is more exciting) if there's a second rover. So if possible, built a second rover.

  Contestants start their rovers at the starting point and drive them through an obstacle laden course. Along the course are identified rocks that the rover operators are to place the APXS. Contact with the rock is not made until the APXS alarm and LED indicate it. Afterwards, contestants are required to drive back to the finish line. The contestant completing the course the fastest wins. A fun option is to use a cut sheet of plywood (representing the Mars Pathfinder) as the start and finish line.

  Camera Upgrade

  Micro-Mark sells a miniature wireless video system. The camera transmits a color video image that is displayed on a television. The camera is smaller than a one inch cube and runs from a 9V battery. The video receiver connects to the VIDEO IN of a television. The cost for the camera and receiver is $70. There's a second version with sound, but since there is little sound on Mars (and none on the Moon or asteroid), the camera version with sound is not needed.

  The camera is small enough to mount anywhere on the rover. Sojourner carried cameras in front and in the rear. The cost of the Micro-Mart camera is so low, that you could jusjpgy adding a camera to your rover and let the audience see the action from the rover's point of view.

  Photovoltaic Upgrade

  Surplus solar cells can be purchased cheaply. If you're familiar with using solar power, you may want to operate the rover motors from a real solar array on top of the rover, instead of batteries.

  The Ultimate Upgrade

  The ultimate upgrade is to run the rovers over radio control and only allow the operators to use the rover's video for guidance. Be prepared for longer competitions if you go this route. I'll see if I can find a suitably cheap radio controlled car to hack for this purpose.

  I hope you have fun building the shoebox rover. Please contact me if you have any questions or suggestions. I'll see you at the rover races!