User's Guide
Introduction
Features
Requirements
Controller
Chassis
Assembly
Testing
Troubleshooting
Programming
Contests
Extending
Appendix
Hints
Polarized
Resistors
Capacitors
Servos
Batteries
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Building the Chassis
The chassis is the mechanical base for the Mark III - it is where the motors are mounted, it holds the electronics and sensors in place, and it provides a stable foundation for additions. The chassis consists of a base plate, a scoop, four standoffs, a 4AA cell battery holder, a 9V battery snap, and assorted fasteners to hold everything together.The "Rev2" chassis design has shipped with all Mark III Kits sold since September 2002. The photographs in this section still show the "Rev1" design. However, the PDF drawings of the chassis and scoop are accurate and should be consulted if there is any confusion as to how the pieces fit together.
Parts List and Assembly Drawings
Before you begin to build the Chassis, remove all the parts from the bag they came in and match them up against the following parts list:
| Quantity | Description |
|---|---|
| 1 | 9V Battery Snap |
| 1 | 4 AA Battery Holder |
| 1 | Chassis Body |
| 1 | Chassis Scoop |
| 8 | 4-40 .375" phillips pan head sheet metal screw |
| 4 | Standoff 1.375 inch |
| 5 | 4-40 .375" phillips pan head machine screw |
| 5 | 4-40 Hex Nut |
| 2 | #4 Internal Tooth Lock Washer |
| 4 | #4 Nylon Washer |
| 4 | 6-32 .500" phillips pan head machine screw |
| 4 | 6-32 Hex Nut |
| 2 | 2" Velcro Hook Strip |
| 2 | 2" Velcro Loop Strip |
Note that the 6-32 screws are larger than the 4-40 screws, and the sheet metal screws have pointed tips while the machine screws have square tips.
Full-scale drawings of the Chassis and Scoop are available to show the dimensions and relative positioning of these parts in the Mark III robot:
The first step in assembling the chassis is to find the base plate. It is a powder-coated aluminum rectangle, with a large tab bent up in the rear and a small tab bent down in front. Four holes provide the attachment points for the standoffs/servo mounts. A slot on the front end is used to pass the wires from the underside of the chassis to the electronics.
Standoffs to chassis
After you have identified proper orientation of the base plate, it is time to attach the standoffs. The standoffs are four pieces of 1.375" long, 7/32" square aluminum tubing, filled with an ABS plastic core.
The standoffs serve double duty: first, as mounting posts for the
servo motors, second, as attachment points for the controller board.
The standoffs are not symmetric - the hole through the side of the
standoffs is closer to one end than the other. Be sure to orient
each standoff so that this hole is closer to the base plate end
of the standoff.
Standoffs should be oriented so the hole is facing the
side edge of chassis.
Hole should be nearer to the bottom than the top.
Fasten the standoffs to the chassis with the four #4-40 3/8" sheet
metal screws. The screws go into the holes in the end of the
standoffs, and create their own threads as they are screwed in.
Defer final tightening until all components are in place.
Servos to standoffs
Before you perfom this step, you need to modify your servo motors for continuous rotation. The procedure for doing this is shown in the Appendix. Don't attach your servo motors to your chassis until this modification is complete, otherwise you'll just have to take everything apart again.
Use the four #6-32 machine screws and four #6 hex nuts
to attach the servos to the standoffs. Make sure the
head of the screw is facing the outside edge of the base plate
so you have access to it if it needs tightening.
The servos should be positioned
so that the axle is closer to the rear standoff than the front.
This puts most of the weight of the servo in front of the axle.
If you want to use the rubber grommets that come with the servos
to provide cushioning, mount the grommets on the outside of the standoffs
between the standoff and the servo flange. Grommets
push the servos toward the outside of the robot, making
your robot wider. Be sure to stay within size limits!
Line sensors to scoop
Now find the scoop. The scoop is the smaller piece of powder-coated aluminum. It has three holes along its length for attaching the line sensors. Line sensors are in a package with a hole in the middle. Attach to scoop with a #4-40 3/8" machine screw and #4 hex nut. Screw head should be facing the front of the robot, with excess screw and nut on the underside. Don't tighten too much, or you may crack the plastic of the sensor case.
Attach the sensors
loosely at first, don't tighten until after you have everything
together. You will need to adjust the sensor height at a later
time - slide the sensor up or down along the mounting slot
in order to raise or lower the sensor. Optimal sensing distance
is about 0.2" from the floor.
Scoop to chassis
After you have the sensors mounted on the scoop, you may attach the scoop to the chassis. Use two #4-40 3/8" machine screws.
The screw head should be facing the front of the robot, with the
excess screw underneath the robot.
Thread the wire from the center sensor through the slot before you
attach the screws.
You will also need the two #4 lock washers here - the lock washers
keep the screws from rotating when the proximity detectors are mounted.
Put the washers on the ends of the screws then fasten the screws in
place with the remaning two #4 hex nuts. Again, don't do
your final tightening - we will have to remove these screws in
a little while to mount our infrared proximity detectors.
Battery pack/Velcro
Two 2" strips of adhesive-backed Velcro are included in the kit for attaching the battery pack to the underside of the chassis.
You may stick two strips of either the "hooks" or the "loops" to the
back of the battery pack. Whichever you choose, make sure you don't mix
them; you should have either two hook strips or two loop strips - not one of
each.
After you have attached the Velcro to the battery pack,
fasten the unused strips to the strips you have just mounted,
remove the adhesive, and carefully press the battery pack to the
underside of the chassis.
You should make sure that the batteries are oriented
side-to-side, so that the full width of the chassis is taken up by
the battery pack. Also try to position the battery pack as far
forward as possible, without interfering with the line sensor wires
running through the slot in the chassis. Positioning the pack forwards
improves the weight distribution of the Mark III.
