Servo - June 2011

There are a number of similar L298 Arduino shields and stand-alone modules, and if you use one of these, just remember to adjust the I/O connections accordingly. On the DFRobot shield, the motors are connected to pins 4/5 and 6/7. Each motor uses two pins: one for direction and one for control.

Listing 1 is a demo Arduino sketch that cycles through the basic functions of DC motor control for a robot: moving it forward and backward; steering it left or right; and stopping.

The control pin for each motor can also be used with pulse width modulation (PWM) to set the speed of the motor. Listing 2 shows how this is done using the Arduino’s analog Write statement. To set the speed of the motor, you specify a number from 0 (off) to 255 (full on). These values control the duty cycle — the ratio between on and off times — of a fast series of pulses sent to the motor.

Not all motors respond to very low duty cycles. When used with the Tamiya worm gearmotors, values under about 96 have little or no effect. That’s why the for loop in Listing 2 starts at 96, and goes to 255.

Important note! In programming code, the physical direction of a DC motor is ambiguous. It all depends on how the motor is wired to the control circuitry. I intentionally wired both motors in the exact same way to the L298 shield which then required that the motors be commanded in opposite — one motor is set to LOW, while the other is set to HIGH. This propels the bot forward or backward. If a motor doesn’t turn in the direction you want it to, merely flip its wiring to the control circuit.

FIGURE 7. A much larger version of the Mini T-Bot, made with a pair of 12” T straps.

Using Larger Ts for Larger Bots

You’re not limited to just the 6” T strap. By using a bigger T, you can construct a larger (and heftier) robot. For your reference, Table 1 are the specifications of the most commonly available sizes of Simpson Strong-Tie strapping Ts and their weight in ounces.

Yikes! ... the 1212T strap weighs almost a pound, so you’ll need bigger motors (and batteries) to haul that kind of weight around.

The robot brute in Figure 7 uses a pair of 12” straps, separated by 5” long aluminum tubing used as risers. In this

FIGURE 8. Outline shapes of three common nail plates. Like T straps, these are found in the lumber section of local hardware or home improvement stores.

particular prototype, the motors were mounted at an angle, with the metal of the lower T bent at 45°. This was partly done to accommodate the motor itself, as its mounting holes were on the side opposite the axle and wheels. I also did it simply to be different.

Model

66T

128T

1212T

Material L

14 gauge galvanized 6”

14 gauge galvanized 12”

TABLE 1. *Width is the width of the strapping metal.

H W*

5” 1-1/2”

8” 2”

12” 2”

Weight

5 oz

11 oz

14 oz

Other Sheet Metal for Lumber Strapping

Don’t stop with just T-shaped straps for building robot bodies. The same lumber section of your local home improvement store has plenty of other choices. Some of it is specially formed and bent for things like hanging 2 x 4 joists in an attic or garage. These are somewhat less useful than flat metal.