Building the My First Robot Chassis
RBB-Bot: Assembling the Motors and Wheels
With the base and control panel prepared, turn now to constructing the motors and wheels. You'll need the following:
- (2) Tamiya #70093 3-speed crank axle gearbox motor kits
- (1) Tamiya #70096 off-road tires (pair)
These items are widely available through online retailers and some local hobby shops. See the Sources box for a selected list of Web retailers that carry these and other Tamiya parts.
Assembly instructions are provided with the motor gearboxes and wheels. You will need a #0 Philips screwdriver and small needle nose pliers. A pair of flush cutters—like the kind for snipping off the ends of wires when constructing circuit boards—is useful for separating the various plastic pieces.
The gearboxes can be constructed using one of three speed ratios: 7:1, 58:1, and 204:1. For Phase 1 of the RBB-Bot select the 204:1 ratio. This is the slowest speed, and the easiest to control. You can always disassemble the motor gearboxes and remake them at a faster speed (the 17:1 ratio is not useful for robot locomotion). Be sure to save all the extra parts in a reclosable plastic sandwich baggie so you can change out the gearing should you ever want to.
Each gearbox kit comes with a long hex-shaped drive shaft. You'll need to cut the shaft to length as shown. Do this before final assembly of the gearbox.
After each gearbox has been built (but before you have inserted the small DC motor into the gearbox) loosen the set screw that holds the drive shaft in place. Adjust the position of the shaft within the gearbox as shown in the illustration.
You want about 1" of the shaft protruding from the outside of the gearbox. Retighten—but don't over tighten!—the setscrew when the shaft is in the proper position.
(In order to access the set screw you may need to manually rotate the gearing to make the screw accessible. Each motor comes with its own miniature hex wrench for tightening the set screw. Be sure to save the wrench for use later.)
The wheels likewise come with their own assembly instructions. When mounting the rubber tires, orient the treads so the "V" pattern points to the same direction. Remember that the wheels are opposite one another, so the rubber tires need to be mounted on their wheel rims in mirror image.
Assembling the Base and Control Panel
With the motors and wheels made you can now assemble the RBB-Bot base and control panel. Refer to Figure 3 for where to mount the motors, skid, and cable clamp to the RBB-Bot base. The parts list calls for mounting the motors using 4-40 x 7/16" pan or flat head screws. The head of the screws are on the side of the gearbox; the nuts on the opposite side of the base. Don't use longer screws, as they'll stick out too much, and could drag against the floor.
When using a skid, the motors are on the "top" of the base; but remember that the RBB-Bot is invertible—meaning it can be operated motor-side up or down. In future installments I'll show a variation of the robot where the motors face down.
Figure 4 shows a side view of the base, indicating how to assemble the machine screw and two nuts to make the skid.
Likewise, refer to Figure 5 for how to mount the switches, corner standoffs, and cable clamp on the control panel. Figure 6 shows the construction of the corner standoffs, which serve as "feet" for the control panel. The switches attach to the control panel using their own threaded nuts. Use one or two 1" squares of Velcro to secure the battery holder to the underside of the panel.
Wiring the Motors and Control Panel
This phase of the RBB-Bot demonstrates the control of a robot using switch control. A set of two double-pole, double-throw (DPDT) switches operate the motors, and show how differential steering works. You can practice using the switches to see how different positions affect the motion of your robot.
For a full appreciation of how to control the RBB-Bot, choose miniature DPDT toggle switches that have a center-off position, as well as momentary "on" action. In electronics catalogs and online shopping carts, you'll often see these listed as:
where the (on) or (mom) in parenthesis means that the switch is spring loaded. Releasing the toggle returns the switch to its off position.
Each switch controls one motor. In the center-off position, the motor is unpowered, so it doesn't move. The motor turns in one direction when the switch is pushed toward either on position. With the switch pushed toward the other position, the motor turns in the opposite direction.
The figure to the left shows the concept of how a DPDT switch is wired in order to reverse the direction of a motor. Note that in one position of the switch, current from the battery flows through the motor one way; when the switch is in the other position, current from the battery flows through the motor in the opposite way. This is what causes the motor to reverse directions.
Refer to Figure 7 for how to wire the switches on the control panel to the motors on the RBB-Bot.
[Figure 7. Wiring diagram for the RBB-Bot battery holder, switches, and motors. The connection between the switches and motors is through an 8 to 10 foot length of 4-conductor modular telephone wire.]
1. Start first by connecting the battery holder to the switches. Each switch needs power from the batteries. Be sure not to cross up the red (positive) and black (negative) connections between the switches, or you might create a short circuit.
2. Wire the switches as shown in blue. The criss-cross of the wires is what makes the motor reversal work.
3. Wire the switches to the motors using an 8 or 10 foot length of 4-conductor modular telephone extension. Clip off the modular plugs on either end, and strip back about five inches of the outer insulating jacket to expose the wires inside. The wires are color coded—red, black, green, and yellow (or orange) are common. Solder the switch and motor connections as shown.
(On the control panel end, thread the wire through the extra hole before soldering to the switches. This provides added strain relief.)
Telephone wiring is made to be very flexible, which is why it's chosen here. Many kinds have a thin filament that when soldered burns away. It takes practice getting a good solder job with this stuff, so take your time and try again if your first attempt doesn't turn out. If you continue to have trouble, try a different brand of wiring, or substitute with four-strand ribbon cable. The ribbon cable isn't quite as flexible, but it will do for our purposes. Avoid the use of solid conductor wire.
Important!The terminals on virtually all DPDT switches provide for the wiring diagram as shown in Figure 14. The switch has six terminals, two rows of three terminals each. The "Common" connections of the switch are the two terminals in the middle.
If your switch came with a wiring diagram, check it to be sure the two center terminals are Common. If you have a multimeter, dial it to test continuity, and apply one test lead to a Common terminal, and the other lead to a terminal on either side. Toggle the switch one way, then the other. In one of the positions the meter should show continuity.
Keep the telephone wiring from pulling out by making a small coil around a 1/4" U-shaped plastic cable clamp. The clamp acts as a strain relief. Use a clamp on both the control panel and the robot base.
With the wires attached to the motors, fit the motors into their gearboxes. The solder terminals should be oriented so that they are not blocked by the body of the gearbox. The motors "click" into place. The design of the gearbox allows you to readily remove the motor, something we'll do in a later phase of the RBB-Bot.
The wiring of the finished RBB-Bot base and control panel is shown below.