Actuation - Motors
Locus Pocus Stepper Motor - 28BYJ-48 5V Stepper Motor and ULN2003 Driver Board
The clockwork mechanism needs to be actuated. In order to move the clock hands, the shaft for each hand needs to be rotated, and the rotation is driven by motors. An important part of the overall design was the selection of the type of motor(s) that would be used.
The main constraints on motor selection were that the hands need to be able to (1) hold specific positions, and (2) rotate a full 360 degrees (and perhaps beyond). So, the project needed a motor that would support this kind of rotation.
Research
I considered a variety of options in selecting the motor.
Types of Motor
In order to meet the 360 rotation requirement, there are two primary options: stepper motor and continuous rotation servo.
- Common positional servos will hold position, but are not suitable, as the rotation range is typically limited to 180 degrees
- Continuous rotation servos allow for 360 rotation, but are controlled by speed and direction. They can hold position while driven, so could be an option.
- Stepper motors move an incremental step at a time and hold position, so are a reasonable option
For holding specific positions, another potiential consideration is whether the motor itself can provide positional feedback. That is, a closed-loop system, so the motor itself has a means to know its actual position. Motors with build-in encoders can support better positional accuracy and reduce the need for external calibration.
My Earlier Fab Academy Assignments
We tried 2 different kinds of stepper motors as part of developing a drawing machine in the Mechanical Design, Machine Design week.
The initial exploration - while we waited on some parts - was for a basic 3D printed drawing machine. This used smaller 28BYJ-48 5V stepper motors. For the primary Inkuity drawing machine, we used Adafruit NEMA-17 size stepper motors.
Having previous experience with these stepper motors and associated motor controllers made these strong candidates for the project.
What Others Have Done Beforehand
I also reviewed the motors used in some of the previous related projects.
Motor Selection for Locus Pocus
My initial thought was to use the same kind of stepper motors that we used for the Inkuity drawing machine and had available - Adafruit NEMA-17 size stepper motors. However, considering the overall design, it seemed like these: (1) were a bit of overkill in terms of the force needed to turn lightweight clock hands, and (2) would make the clockwork mechanism to be fairly heavy, which could present issues for system integration.
On review, I selected the smaller 28BYJ-48 5V stepper motors for use in the project. They were more lightweight, I had previous experience with them, and this kind of motor had been used successfully in several previous projects. These motors come with a ULN2003 motor driver board.
28BYJ-48 5V Stepper Motor and ULN2003 Driver Board
In working with the 28BYJ-48 stepper motors, I found the reference of Control 28BYJ-48 Stepper Motor with ULN2003 Driver & Arduino to be very helpful.