Fabrication and Assembly
The linkage mechanism is fabricated out of laser cut 1/8" acrylic and pressfitting bearings into the linkages. We connected the linkages to one another by cutting the 6 mm shafts to size and putting them through the bearings of each linkage. We used 1/8" acrylic as the linkages would have less friction as they crossed one another and the 1/8" acrylic was lighter than the 1/4" acrylic. The base plate was also laser cut out of 1/8" acrylic and had 2 parts as seen in the figures below. The top base plate directly interfaced with the linkage system as the driver linkage was connected to the motor via a 5 mm motor encoder and another linkage was attached to the base plate via 6 mm motor encoders attached to the base plate and linkage and a 6 mm shaft. The linkage system is essentially cantilivered from the base plate. The top base plate was utilized to not only interface with the linkage system, but also keep other components in place, such as the motor, breadboard, arduino and battery. The bottom base plate was necessary as the 6 mm motor encoder screwed into the bottom of the top base plate and the bottom plate allowed for the the mechanism to sit flatly on the table.
Figure 1: Full Assembly of Final Design
Figure 2: Bottom Base Plate
Figure 3: Top Base Plate
Electronics and Circuitry
Our system is driven by a 42mm NEMA 17 stepper motor powered by a 24V/2A supply. The motor is controlled by an A4988 stepper driver that is powered by the same 24V supply with a decoupling capacitor to reduce noise in the circuit. To actuate our motor we use a push button connected to GPIO2 of the Arduino Uno that is powered by a 9V battery. The button uses a pulldown resistor to prevent an accidental actuation as a safety precaution. Additionally, the RST (reset) and SLP (sleep) ports on the A4988 stepper driver are connected to each other to create a closed loop because we are not using them. The DIR (direction) pin is connected to port 8 of the Arduino and it controls whether the motor spins clockwise or counter-clockwise. The STEP pin is connected to port 9 of the Arduino and it controls the number of steps the motor takes.
Software Development
We chose to use the AccelStepper.h library to control the NEMA 17 stepper motor. The goal for our mechanism is to achieve a swinging motion that spans around 90 degrees, stops abruptly, and then returns to its original folded position. As a result, our code tells the motor to take 60 steps which equates to 108 degrees of rotation.