Robot Wars Home Page Southampton University
Robot Wars 1999/2000

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DEVELOPMENT
BASE
ARM INTEGRATION
CHASSIS LID
DRIVE INTEGRATION
GROUND CLEARANCE
MASS
CONCLUSION
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THE CHASSIS
DEVELOPMENT
Originally the robot was intended to compete in the heavyweight category, thus the chassis was expected to support a robot with mass not exceeding 79.4 kg. The material of choice for the chassis frame construction was steel. Steel is the most commonly used engineering material because of its high yield strength and relative cheapness. Steel can also absorb a great deal of impact energy before failure. Naturally the group was keen to produce the body panels, the main frame and the arm of the robot from steel. It was envisaged that the frame would be manufactured from steel "L" or "C" section bars, about 3 or 4mm thick. It was intended to manufacture the frame by conventional welding processes.

However after most other components of the robot had been ordered it became clear that it would be over-weight. The weight of the chassis therefore needed to be reduced. An immediate suggestion was to manufacture the chassis frame from aluminium bar instead of steel. Aluminium has a density about one-third of that of steel, but it is far more difficult to weld, has inferior mechanical properties and is more expensive, as well as requiring the use of bolts to join the pieces together.

After consultation with the EDMC a new and final approach to weight-saving was established. The chassis framework is made from 1.5mm thick steel plate. The steel plate was cut into strips that were bent along their length into L or C sections as appropriate. (See chapter 20, on the bending of L and C sections). This negated the need for any form of conventional metal bars in the chassis framework. The steel strips are thin enough to allow them to be spot-welded together. Spot-welding is a quick and simple process that can be carried out by relatively unskilled students. This decision saved a lot of time, weight and cost in the manufacturing of the chassis framework.

After further weight concerns regarding the entire robot had been addressed, the decision was taken to enter the robot in the super-heavyweight competition category. This required the chassis to support about 50% more weight than originally intended. As a response to this significant increase in loading, the front, rear and side panels of the chassis were constructed from steel plate of greater thickness (2mm). These plates were spot-welded to the frame as originally planned, and provide it with substantially greater rigidity in bending. The chassis construction to this stage is shown below.



BASE
The critical issue of the chassis base plate was a far different concern. An important consideration was the requirement of the base plate to be very rigid whilst supporting a great load. The robot runs on pulleys and belts that allow approximately 10mm ground clearance at the robot's centre, so the robot could not be allowed to sag. Furthermore, the bearings that support the pulleys needed to be precisely aligned and easily mounted to the chassis base. In addition, the internal components and the arm needed to be mounted easily and securely. Initial material suggestions such as polycarbonate sheet and steel plate were totally inappropriate due to poor bending performance. Medium Density Fibreboard (MDF) was identified as the most appropriate material. An 18mm thick sample was chosen for use as the chassis base. The material is both rigid and facilitates the use of screws and bolts. The base plate is attached to the robot's chassis frame by 26 self-taping screws.

ARM INTEGRATION
The chassis needed to hold the robot's arm rigidly in place. This was a very demanding requirement as the fully set-up arm weighed over 20kg. All of the mass acts so as to tip the front of the robot, although the centre of gravity is not very far in front of the base of the arm.

A decision had to be made on how the arm would be securely attached to the chassis whilst allowing the two items to be separated when desired. After consultation with the EDMC a strategy was developed: An open-top steel box would be constructed and bolted to the chassis base plate. The steel box has been made in the same manner as the chassis itself - i.e. 2mm thick steel plates spot-welded onto an L section edge framework of 1.5mm thick steel. The base of the arm (which was also made as a box-section) fits snugly into the steel box. A bolt, placed through both the arm and the steel box holds the two together. When tightened the bolt prevents the arm from moving within the box. Although the box was bolted to the chassis base, reinforcement was needed to be sure that the arm would not buckle under its own forces. Thus C section steel strips were spot-welded to the top of the edge framework behind, and in front of, the arm's base. The box and C section strip are shown below.


CHASSIS LID
Polycarbonate sheet (6mm thick) was selected for the top-plate of the chassis body. This material was chosen because of its excellent resistance to heat and impact. It is essential to the robot's maintenance that the top plate can be easily removed, the top plate was therefore attached to the chassis frame by Velcro strips. It was seen in testing that Velcro is incredibly strong, and the group was more concerned about the difficulty of removing the lid than the possibility of the joins failing. A C section steel strip provides structural reinforcement to the top of the frame, to help it withstand the forces transmitted to the base of the arm. The polycarbonate top-plate rests on this steel strip as well as the top of the edge framework.


DRIVE INTEGRATION
The drive train also needed to be accommodated within the chassis. Although the drive train is dealt with in a separate chapter of the report, there was close liaison in the design of these two parts. Their integration into the final robot needed to be smooth. Ample room was provided within the chassis for both the motors and the gearboxes. The design process was held up slightly by the late delivery of the motors, but this hurdle was overcome with relative ease.

The only significant problem with the integration of the chassis and the drive train involved the robot's ground clearance. Due to the small diameter of the belt pulleys, the bearings that support the freewheeling axles needed to be sunk into trenches milled into the MDF base-plate. The problem is explained below.


GROUND CLEARANCE
The robot is potentially vulnerable to being flipped. Therefore it rides low on the ground and has a ramp at the front, in order to prevent other robots getting underneath it. However, care had to be taken to ensure that no part of the underside of the robot touched the ground. The first measure taken was to ensure that the base of the chassis was as rigid as possible and would not sag under loading. Hence, the selection of 18mm thick MDF for the base plate construction. The problem then took the form outlined below:

The robot chassis is born on belt tracks. Each track held in tension by three pulleys, two of which are in contact with the ground, each with a radius of 45mm. Given that the axles which locate the pulleys had a radius of 8mm, there is 37mm height between the bottom of the axle and the floor.

The exact ground clearance was calculated by inserting dimensions into AutoCAD design software. At its minimum value the ground clearance was found to be 5.5mm, as shown in the figure below. At the centre of the robot, the ground clearance is around 10mm.



MASS
The mass of the fully constructed chassis is approximately 27kg, although this figure includes both the arm mounting box and the unstressed lid. It also includes the robot's armour plating. The chassis represents about 20% of the robot's mass.

The chassis was not intended to be as heavy as 27kg. However when it became clear that the robot would be over its original weight limit, a decision had to be made i.e. Would the chassis be reduced in weight in an attempt to come in under the original limit? Or would the chassis be reinforced in an attempt to cope with the extra load upon it? The second option was taken. Hence the front, back and side plates of the robot were made from 2mm thick steel rather than 1.5mm, and the weight of the chassis escalated accordingly.


CONCLUSION
The chassis proved to be a particularly challenging part of the robot. The design had to accommodate all other components, and therefore was the last part of the robot design to be finalised. This meant that any last-minute concerns over time, weight, and cost limits had knock-on effects for the chassis development. However, because the chassis was the base of the entire robot it was also the first part to be constructed. Hence there was little scope for error in the chassis design or construction, as they could have held up the entire robot's completion.

The chassis was designed and constructed in a manner that satisfies all the requirements placed on it. In particular, it was very simple to construct and allows excellent access for maintenance to internal components and the robot's arm. It was very cheap to produce because most of the materials and manufacturing expertise were provided without cost to the project group. The only failing of the chassis is its increase in mass, which was a necessity partly due to mass increases in other parts of the robot.

THE CHASSIS