Servo - May 2009

is a standard HS-322 or HS-325 servomotor.

Start construction by mounting the right and left drive servomotors to the bottom rear of the chassis. Each servomotor is mounted using two 90 degree servomotor brackets along with 6-32 machine screws, split lock washers, and nuts. Next, mount the front caster style wheel (see Figure 4). The caster style wheel uses a 1/4-20 screw through the chassis into a 1/4-20 coupler to which the caster screws into from the other side of the coupler.

Mount the up and down servomotor to the top front of the chassis using two 90 degree servomotor brackets, 6-32 machine screws, lock washers, and nuts. A small 90 degree clip is mounted behind the servomotor using a 6-32 screw, washer, and nut (see Figure 5).

Get the top servomotor bracket and attach a round servomotor horn to it using four #4 sheet metal screws (see Figure 6).

Place the plastic CMU camera bracket on the top servomotor bracket and secure the bracket using two 6-32 machine screws and nuts. Then, attach the CMU camera to the plastic bracket using two 4-40 screws and nuts (see Figure 7).

Rotate the top servomotor to its center position. Then, attach the CMU camera assembly to the servomotor. The servomotor horn will fit into the shaft of the servomotor. The back hole of the top servomotor bracket should line up with the hole on the 90 degree clip (see Figure 8).

If you want to make your own servomotor wheels as shown in the photographs, you can the follow the wheel construction details in the sidebar. If you have suitable servomotor wheels approximately 2.0” in diameter, you can use them.

FIGURE 3. 90 degree servo mounting brackets.

OBJECT TRACKING

FIGURE 4. Chassis with drive servomotors and caster.

Circuit Assembly

FIGURE 5. Up-down servomotor with clip.

The schematic is shown in Figure 9. The circuit can be hardwired onto a breadboard, or a printed circuit board (PCB) can be purchased from Images Scientific. The LCD is a two line by 16 character display. You can use an LCD module with a Hitachi 44780 controller or equivalent. These LCDs usually have a 14 or 16 pin header.

We need to set one jumper on the camera and connect four wires from our circuit to the CMU camera.

Look at Figure 10. Place a jumper on position 2. This sets the camera baud rate to 38,400. The power is supplied to the two header positions shown in Figure 11, as well as the header positions for the serial transmit and receive lines.

Power Supply

For power, I used a six AA battery pack holder with rechargeable NiCd batteries. The six batteries supply approximately 7.2 volts fully charged. The CMU camera is powered directly from the unregulated voltage from this battery pack.

The battery pack is secured to the undercarriage of the platform using a molded piece of plastic as shown in Figure 11. The battery pack fits inside the plastic and is secured to the platform using two 6-32 screws and nuts.

The entire circuit including the CMU camera has three power switches. I would recommend placing the CMU camera power switch in the ON position and using the main circuit board power switch to control power. The LCD also has a power switch. The LCD display is used in the Test Mode and only in the beginning of the Run mode. The power switch allows you to turn off the LCD to conserve power.