We can imitate most functions in Walter’s famous tortoise. The pro-gram we will use simulates the neurons used in the original robot.
To fabricate the chassis, we need to do a little metalwork. Working metal is made a lot easier with the following tools:
䊐 Center punch. Used to make a dimple in sheet metal to facilitate drilling. Without the dimple, the drill is more likely to
“walk off” the drill mark. To use, hold the center punch in the
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center of the hole needing to be drilled. Hit the center punch with the hammer to make a dimple.
䊐 Hand shears. Used to cut sheet metal. I would advise the purchase of 14″metal shears. Use like scissors to cut metal.
Note: Metal is a lot harder to cut than paper.
䊐 Nibbler. Used to remove (nibble) small bits of metal from a sheet. Used to nibble cutouts and square holes in light-gauge sheet metal. Note: Radio Shack sells an inexpensive nibbler.
䊐 Vise. Used to hold metal for drilling and bending.
䊐 Drill 䊐 Hammer
Most hardware stores will carry these simple metalworking tools.
They will also carry the light-gauge sheet metal and aluminum bar needed to make the chassis.
I built my chassis out of 18″12″aluminum rectangle bar and 22-to 24-gauge stainless-steel sheet metal. Stainless steel is harder 22-to work with than cold-rolled steel (CRS), and if I had to do it over again, I would use aluminum or CRS.
Drive and steering motors
The drive motor is a 100:1 gearbox motor (see Fig. 8.1). I like this gearbox motor because it has a motor mounting bracket. For the steering motor I used a standard 42-ounce (oz) torque servo motor.
There are three pieces of sheet metal one needs to fabricate.
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8.1 100:1, 1.5- to-3.0 VDC gearbox motor170
The U channel (see Fig. 8.2) holds the front wheel and drive motor.
The U bracket is fabricated from 22-gauge, 1″ 5″ sheet metal.
Three holes in the center area are drilled to mount the servo horn from the servo motor. The center drill hole (18″) is larger than the two outer holes (116″). Remove the servo horn from the servo motor by unscrewing the center screw and pulling straight up on the horn.
Line up the servo horn on the bracket and mark the center and two outer holes. Drill the three holes. Mount the servo horn, using the center servo motor screw. For the outer holes use two 0-80 machine screws and nuts. Drill three 18″holes for mounting the L bracket to the side. Drill the two axle holes for the front wheel. Use 18″holes.
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8.2 U channel wheel mount detail171
Mount the metal in a vise and make the two 90 degree bends at the marked lines to form the metal into a U bracket.
Use the L bracket to mount the drive motor to the U channel (see Fig. 8.3). The L bracket is 1.5″ 3″. Use the gearbox motor to mark the mounting holes for the gearbox. Make sure the three holes in the L bracket for mounting to the U bracket match the mating holes in the U bracket.
Figure 8.4 is a diagram of the base with a diagram for the 42-oz servo motor. The base measures 3″5.5″. The base will hold the power supply and the electronics. Use the servo motor diagram to remove metal from the base.
First drill the four (18″) holes. Next use the drill to cut holes all along the rectangle inside the perimeter of the servo motor hole. Removing metal this way is much easier than trying to saw or nibble it away.
When you have removed as much material as possible this way, use the metal nibbler to finish the job. Before mounting the servo, file the edges of the hole. Drill the two back holes for the rear axle bracket.
The rear axle bracket is shown in Fig. 8.5. It is made from 18″
12″10″aluminum bar. Drill the four 18″holes in the aluminum
be-L motor mount bracket 90° bend
Holes for mounting gearbox motor
Holes to watch U bracket
3/4"
NOT TO SCALE 1:1 SCALE
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8.3 L bracket for mounting gearbox to U channel172
fore bending it into shape. For the rear axle, I used the wire from a metal coat hanger.
To continue we need to mount the front drive wheel to the gear-box motor. The rubber wheel used in this prototype is made to friction fit a 3-millimeter (mm) (0.118″) shaft. The shaft diameter of the 100:1 gearbox motor is about 2 mm (0.078″).
To solve this size problem, I placed a 3″long length of 3-mm hollow metal tubing onto the shaft of the gearbox motor. I used a flat-head screwdriver and hammer to secure the 3-mm tubing to the 2-mm shaft. First place the motor’s shaft and tubing onto a hard (metal) surface that allows you to place force directly onto the shaft with-out causing any strain on the gears or motor. Next place the screw-driver head on the shaft-tubing assembly and hit it sharply with the
42-oz servo
Holes to match rear axle bracket
Sheet metal 3" x 5.5"
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8.4 Robot base showing cutout for 42-oz servo motor and holes for rear axle bracket173
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8.5Rear axle bracket detailWheel size
2" Wheel
1/8" hole for axle
1/8" holes 1/8" hole for axle
Axle height
35/8"23/4"
23/4" 3/4"
125°
Rear Axle Bracket SCALE 1:1
3/4"174
hammer. This force will cause the tubing to collapse onto the shaft making a strong friction fit. Strike the 3-mm tubing in one or two locations for insurance.
If one looks closely at the gearbox motor shaft, there is a keyway (flattened cutaway on the shaft) cut into the shaft. If you properly strike the tubing at that location to collapse the tubing into the keyway, you will create a very secure fitting between the motor shaft and tubing.
The drive wheel is mounted by pushing it onto the 3-mm tubing.
The friction fit of the wheel is strong enough to drive the robot without any slippage. If you wish to mount the wheel permanently (something I have not done) to the shaft, try mixing slow-setting epoxy glue and coating the 3-mm shaft with it before mounting the wheel onto it.
Counterbalance
When the gearbox motor is mounted on the U channel, the weight of the gearbox motor on one side makes the assembly unbalanced.
To balance the U channel, I placed 3 to 4 oz of lead on the opposite side. I have 18″ thick lead sheets lying around that I use to store radioactive isotopes. Cutting and drilling the lead is easy. You can mount any heavy object onto the shaft as a counterweight (like fender washers).
Shell
The original tortoise robot had a transparent plastic shell. The shell was connected to a bump switch that caused the robot to go into “avoid” mode when activated. I looked at, tried, and rejected a number of different shells. Finally I was left with no choice other than to fabricate my own shell.
Rather than fabricate an entire shell, I made a bumper that encom-passes the robot. The bumper is fabricated from 18″12″32″ alu-minum bar (see Fig. 8.6). The alualu-minum bar is marked at the center.
Each bend required in the bumper is also marked in pencil. The ma-terial is placed in a vise at each pencil mark and bent to the angle re-quired. The two ends of the aluminum bar end up at the center back of the bumper. These two ends are joined together using a 18″12″1″ long piece of aluminum bar. A 18″hole is drilled on each end of the aluminum bar. Matching holes are drilled in the ends of the bumper. The bar is secured to the bumper using two 5-40 machine screws and nuts (see Fig. 8.7).
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The upper bracket used to connect the bumper to the robot is identical to the front end of the bumper (see Fig. 8.8). The upper bracket is made from 18″12″14.5″aluminum bar. As with the bumper, the center of the bar is marked and each bend required is also marked in pencil. The material is bent in the vise the same way as the bumper.
91/2"
43/4" 43/4"
41/2" 41/2"
31/2"
32" long
Aluminum bumper
5-40 nuts
5-40 machine screws
1"- long aluminum bar
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8.6Top dimensional view of bumper fabricated from 1/8″1/2″ 32″aluminum bar䊏
8.7 Cutaway close-up of aluminum bracket used to secure the open ends of the bumpers176
Finding the center of gravity
It is important to find the center-of-gravity line of the bumper, because this will mark the optimum location where the upper bracket should be attached. Rest the bumper on a length of alu-minum bar. Move the bumper back and forth until it balances evenly on the aluminum bar. Mark the centerline positions on each side of the bumper. Drill a 18″ hole on each side. Drill matching holes on the ends of the upper bracket. Then secure the upper bracket to the bumper using 5-40 machine screws and nuts.
Attaching bumper to robot base
The bumper is attached to the robot body by the upper bracket.
Drill three 18″holes in the top of the upper bracket. One 18″hole is in the center and the two other holes are 118″ away from the center hole (see Fig. 8.9). Three matching holes are drilled in the robot base behind the servo motor. The holes should be placed so that the bumper (once secured to the base) has adequate clear-ance (18″to14″) from the back wheels. The matching center hole on the base must be offset by moving the drilled hole forward on the base by about 14″.
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8.8 Side dimensional view of upper bracket fabricated from 1/8″1/2″141/2″aluminum bar
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8.9 Side dimensional view for hole placement in top of the upper bracket177
The bracket is secured to the base using two 1″long 6-32 machine screws; four 6-32 nuts; and two 1″long, 2-pound (lb) compression springs, with a 18″ center diameter (see Fig. 8.10). The tension and resiliency of the bumper can be adjusted by tightening or re-leasing the upper 6-32 machine screw nuts. Once assembled, the bumper will tilt back and close the bumper switch when the robot (bumper) encounters (pushes against) an obstacle.
Bumper switch
The bumper switch makes use of the center holes. Looking back at Fig. 8.10, the center hole is fitted with a 6-32 machine screw held on by a standard (zinc-plated) nut, followed by a brass nut. The brass nut has a wire soldered to it. The purpose of this little assembly is just to attach a wire to the bracket-bumper assembly. Brass nuts are used because it is possible to solder wires to brass to make elec-trical connections. This is in contrast to the standard zinc-plated steel nuts that are very difficult (impossible) to solder.
The second half of the tile switch is comprised of a 1″6-32 plastic machine screw and three 6-32 machine screw nuts, one of which must be brass with a wire soldered to it (see Fig. 8.11). Figure 8.12 is a close-up of the finished tilt switch. The assembly is adjusted so that the brass nut on the top of the 6-32 machine screw lies just underneath the upper aluminum bracket without touching. When the upper bracket tilts forward, contact is made between the alu-minum bracket and brass nut, which is read as a switch closure.
Photoresistor
The cadmium sulfide (CdS) photoresistors used in my prototype have a dark resistance of about 100K ohms and a light resistance of 10K ohms. The top of the 100:1 gearbox motor bracket is a perfect
6-32
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8.10 Side view of upper bracket detailing the mounting of the upper bracket to the robot base using machine screws and compression springs. Also details bracket half of the tilt switch178
shelf for mounting the photoresistor (see Fig. 8.13). I used a small piece of plastic to mount the photoresistors at a 45 degree angle up with an opaque vane mounted in between the photoresistors (see Fig. 8.14). Mounting the photoresistors on the drive wheel assembly keeps the sensors pointing in the same direction as the drive wheel.
This replicates the function of the original tortoise robots.
Using two CdS photosensors in this configuration alleviates much of the computation needed to track a light source. This is the same
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8.12 Close-up photograph, detailing tilt switch and spring mounting of upper bracketBrass 6-32 nut
Base
Plastic 6-32 machine screw 6-32 nuts
Wire
Robot base Upper brackets
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8.11 Side dimensional detail (robot base side of the tilt switch) of plastic screw with top brass nut179
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8.13 Close-up photograph detailing front drive wheel, showing coun-terweight, drive wheel, gearbox motor, and light sensor with shroud operation as described in Chap. 6 for the light tracker circuit.The operation of the sensor array is shown in Fig. 8.15. When both sensors are equally illuminated, their respective resistances are approximately the same. As long as each sensor is within ±10 points of the other, the PIC program will see them as equal and doesn’t move the servo motor (steering). When either sensor falls in the shadow of the main light source, the resistance variance between the sensors increases beyond the ±10-point range. The PIC microcontroller activates the servo motor to bring both sensors back under even illumination. In doing so, this steers the robot straight to the light source. If the sensors detect too great of a light intensity, the robot will go into avoid mode.
Schematic
The schematic for the robot is shown in Fig. 8.16. Intelligence for the robot is provided by two PIC16F84 microcontrollers. The steering servo motor control signal is provided by RB3 off the PIC microcontroller number 2. The 100:1 gearbox motor is attached to an H-bridge consisting of components Q1 to Q4, D1 to D4, and R1 to R4. The H-bridge is controlled by the PIC microcontrollers RB1 and RB2 input/output (I/O) lines. Sensor readings of the CdS cell are read off pin RB4. RB5 reads the tilt switch to check if the robot has encountered an obstacle. I assembled the entire circuit on two