Motors
The system contains two stepper motors : one driving the vertical axis, and one driving the horizontal axis. The decision to use stepper motors was
based on their discrete nature - they move a fixed number of describes per actuation. Based on the decision to control the system through software and
digital tools, the discrete nature of the stepper motor seemed to be a natural fit, rather than attempt to control a continous motor with a discrete system.
While it is possible, it was considered less-straightforward, leading to unnecessary complication.
Motor Drivers
Stepper motors require some form of driver - typically an H-bridge IC that generates the correctly modulated signal to energize each phase in the proper sequence.
Due to the team's lack of familiarity with this process, the decision was made to acquire stepper motor driver boards that accepted some from of command sequence and
generated the current and voltage waveforms required by the stepper motors. Based on this criteria, the Pololu A4988 breakout board/driver IC was selected for utlization.
The above diagram demonstrates the ease of the wiring diagram - the step and direction lines are sourced from the SAM3U-EK, and with the proper sequence, the motor driver
will produced the right waveforms to step the motor. The direction pins essentially controls polarization, and controls which direction the motor steps in, either set
to a logic high or logic low. Step is a falling-edge senstive line that, per pulse, steps the motor once, the exact distiance determined by the 3-pin MS bus. For
initial appications, both drivers were configured in full-step motor, though firmware could be written to allow finer correction.
Due to the high power nature of the motor driver and the desire to avoid damaging the SAM3U-EK, our application includes opitical isolation on both the step and direction
lines, so that any fault on the high-power side (as even though step and dir are logic level pins, they share the same ground with the 30V, 2.5A motor power supply) does
not destroy the microprocessor. The microprocessor, when it computes the necessary angle movement, convets that delta theta into an equivalent "step count", producing
a pulse train to the motor drivers that would correspond to a similar amount of angular rotation, accouting for the discrete nature of the stepper motor and the
gains that are present from the gear system.
The optical isolation for the system is generated by the H11L1 a 6 pin optical isolation device utilizing an LED that coincides with a Schmidt trigger.
The LED recieves a digital signal and lights up therfore triggering the Schmidt trigger that then outputs the same logic signal it recieved to the A4988.
H11L1 optically isolates the step and direction signals for the motors and therefore a total of 4 H11L1s was needed.
The stepper motors selected based on our design criteria are HT23-596's, operating in bipolar mode, with a measured torque output of 96 oz-in at the desired
operating current of .5A per phase (1A per motor).
The Pololu A4988 is a breakout board that mounts a A4988 motor driver, along with the necessary housekeeping that allows the user to simply provide power and control sequences,
greatly increasing the ease of use. Each board is rated 2A, but as they only have one H-bridge per IC, two were necessary to control two motors. However, given the
low cost of the board (20 US Dollars), it was an acceptable trade-off.
