Wallrus: Mechanical Design

The mechanical aspect of this project is divided into two primary components. The first is the carriage, suspended by the pulley belts, that hold the spray can and the actuator. Separate from this is the housing for the motor and pulley, which is positioned on a stand in order to raise it to the desired height.

Carriage Design

Here’s the carriage with a can of spray paint installed. Electronic actuation of the solenoids pulls down on the metal bar atop the can’s nozzle.

The team determined several requirements that needed to be fulfilled by the final carriage design in order for it to be successful. A low weight would minimize the strain put on the motor. Balance would also be important, as drift associated with extra swinging would prevent accurate plotting. A design goal of the team had also been to make the project modular; one aspect of this was to make the carriage assembly compatible with multiple sizes of spray cans.

With these goals in mind, the team determined that the best design for the carriage would be one of a lightweight plastic (specifically Delrin) frame that supported two solenoids that act as the actuators for the can. While the frame is solid, unnecessary material has been removed in order to achieve an even lighter carriage. The rigidity is not affected by these cuts, and the ends of the belts are kept at a consistent distance from one another. Balance, as least left to right balance, is achieved by making the left and right sides of the carriage mirror each other. Additionally, the lever that activates the spray can pulls down with (theoretically) no torque. The modularity of the frame is incorporated by suspending the spray can with Velcro straps that conform to the radius and height of the can.

In testing the carriage, some problems were found with the lever slipping from its desired position. In order to prevent this, a counterbore was cut to provide a place for the spray nozzle to rest. This lever is made of thin aluminum, partially for a clean counterbore and partially to minimize flex during actuation.

Motor-Pulley Housing Design

In designing the housing and its supports, the team decided that the point of a graffiti robot should not be to tag the same area over and over. Thus, we wanted it to be able to be moved to different locations. After considering a hooked design and a clamped bar design, we settled on a support for the housing that is essentially a multi-leveled pedestal.

On each level of the stand are two bolt holes; this gives us three heights to place the housing, and thus adjust the height and size of the available drawing area. Because the stands are movable, the width of the drawing area is limited only by the length of the belts. The stands were chosen to be made of wood for its general stability, low price, and ease of construction.

The carriage is suspended from the housings by way of a two toothed belts. These belts fit with a toothed pulley in order to achieve position control with minimized drift. Knowing the number of teeth that have progressed gives the length of the belt, and, combined with the stepper motor’s discrete steps, this ensures a highly accurate measure of position. Limit switches, activated by a metal piece fitted to the end of the belts, serves to initialize the belt lengths, giving a known starting position for each.

The motor-pulley housing assembly

One of the motor-pulley housing assemblies. Note the toothed pulley, which matches the tooth pattern of the belts.

The housings for the motors and pulleys serve one main purpose; they must keep each motor and pulley at positions relative to one another so that their axes are concentric. Because the pulleys are much larger than the motors, this requires that the pulley is either sunk down or the motor is raised. Since the housing is on top of a stand, raising the motor made more sense than cutting into the wood in order to sink the pulleys. The housings raise the motors onto shelves supported by feet, and the motors are bolted to a plate perpendicular to the shelves to keep them parallel with the pulleys.

The pulleys also need to be kept a certain distance from the motor in order for the belt to feed properly through the hole positioned next to the limit switch. This is done by set screwing the pulley to a coupler, which is also attached to the motor shaft. On either side of each pulley is a thin plate, designed to keep the belt from slipping off of the pulley. A tensioner wheel also helps in this regard, pushing from above to keep the belt from skipping any teeth. The couplers and tensioner wheels all use ball bearings to lower the stress on their respective axes.

The final design choice for this housing was the material. With the idea that it might become necessary to adjust portions of the housing (e.g. set screws) after assembly, the housing was made to be as open and easily constructed/deconstructed as possible. Because weight was not a factor for the housing, aluminum was used. Tapped holes for screws allowed repeated construction and deconstruction with no ill effects on the housing. The aluminum also provided a rigid structure that would not allow the pulley and motor to slip relative to one another.