SCARA: Mechatronics Competition

Competition 

The competition had two teams attaching steel clad wood cubes to a platform in five minutes. To accomplish this the robot must navigate autonomously to a block dispenser, determine if the block was red, blue, or yellow, and then place it in an appropriate location on the platform. 

Scores were determined by how many blocks were attached, and the pattern of blocks attached. All teams had a single Arduino Mega to control their robot once it received the wireless activation signal. 

SCARA Design

The initial plan was a SCARA on a simple platform that would catch the blocks on an extendable arm. This would minimize the travel distance and potential loss of position due to tracking error. Complex geometries were 3d printed, while the large flat panels and gears were laser cut. 

Refinement

The model was updated to include an internal bucket to catch the block and determine its color. The top panel was cut in half to allow access to the internals without full disassembly. Additional sensors were added to determine distance from walls on each side of the robot. 

The prototype of the body was further refined into a final CAD model. This allowed the robot to move more freely by rounding the corner against the wall of the test area. The half plate was kept and used to mount the battery underneath. This allowed quick replacement of the battery, and full access to all the internal circuitry for trouble shooting. 

Mechanism Analysis

To ensure the power draw was within the ratting of the battery, a mechanical analysis was done. This also determined the current draw limits for the stepper motor boards. Given the battery required the system was able to operate for over 30 minutes while maintaining its peak voltage. 

Electronics & Manufacture

The heart of the system was an Arduino Mega. To navigate the play area a line follower was mounted to the underside with standoffs. The power distribution board had 5 volts on the blue pins, ground on black, and 12 volts on red. All cabling was silicone insulated with colors matching its required voltage. 

A blade fuse was connected between the battery and the power distribution board to protect against shorts.

 

Controlling the robot required 5 motor drivers. An L298 driver board controlled the two driving wheel motors independently with support from a castor ball mounted under the arm's bearing housing. The arm itself required four stepper motors, and so four A4988 control boards were needed. 

The circuits holding the A4988's have a bootstrap capacitor to ensure no voltage spikes would damage the boards. Each was also shorted between the activation pins, and headers were soldered to allow the option for micro stepping. 

Current spikes were the next potential failure point. To ensure the boards would limit current to the AF988, each on board variable resistor was set to the desired maximum current of its stepper. 

After successfully navigating to the dispenser, the SCARA would bump a lever and receive a block. Manipulation of that block was done with an electromagnet controlled by a latching relay on the SCARA's arm. 

With the motor control boards in place, the the full operation of the arm was tested. 

Finally the system could be fully assembled. The distance sensors were installed on the front and back of the robot. The color sensor was installed in the bucket to be calibrated during the final test to ensure accurate reading with the lighting conditions at the competition. 

With the critical systems functioning a final test run of the robot was done to ensure everything was working. 

The process was not without challenges. Components would arrive faulty, and there was always risk of the robot being damaged during operation or transport. To ensure it was as repairable as possible, a full, clearly labeled pin and circuit diagram was made. 

 

The effort put into documenting the hardware saved the project. The Arduino shorted during testing and caused several pins to die. The problem wasn't apparent without full disassembly and testing of every component. A new Arduino was installed and work continued. Later, with less than a week before the competition, something else failed that caused the sensors to work improperly. It was quickly determined that a failure in the line follower array's PCB was the source of the issue. After its replacement, calibration of the system continued, and very little time was lost.