Image 1: Design Ideas
Image 2: CAD Model of Final Prototype
Summary of Modifications
Motor Selection
We decided to use the 2 Propeller Continuous Rotation Servos and 1 Futaba Standard Servo to actuate our mechanism. We chose these motors because they were relatively lightweight, and we had free access to them through the UT Austin Advanced Mechatronics lab. The continuous rotational servos have a stall torque of approximately .294 Nm, and the standard servo has a stall .431 Nm. We showed our mechanism to our TA, and after observing how little friction and inertia we had to overcome, we qualitatively decided that the motors should have enough torque to drive their portion of the mechanism.
Motor Control
We controlled each of our three motors using a Propeller Activity Board. This board contains five sets of three-pin headers that can be connected directly to small servomotors. The 5V voltage regulator that control’s the devices maximum output voltage and current can deliver up to 1.5A with a 6V power supply. To put this in perspective, the Arduino Uno has a D/C current limit of 20mA for each I/O pin.
Additionally, the Propeller Activity Board contains a microcontroller that is capable of 8-core multicore processing. This ability would allow us to control each motor independently of one another, allowing for the possibility of incorporating very fast feedback loops if we were to use a timing method for synchronization or different PID loops.
After reviewing these specifications, we decided to use the Propeller Activity Board because it would not require us to use an external motor driver to power our motors, and it would potentially allow for tight control loops.
We programmed the Propeller Activity Board in C, and used the Standard Servo library to control each of our motors (a copy of our code is shown in the appendix). We specified an input speed for each of our continuous rotation motors on a scale of -100 to 100. 100 corresponded to an a full speed CCW rotation, and -100 corresponded to a full speed CW rotation. On the other hand, we specified different set points for the angular position of our standard servo. Additionally, we specified the max rotational speed the standard servo could move at based on how fast we wanted the mechanism to rock back and forward.
In order to recreate this product, it is very important that the continuous rotation servos are calibrated correctly if using the open loop control described above. Small differences in the friction in the motor, the friction of a mechanism wing, and the rotational inertia of the mechanism wing can make each motor rotate at different speeds even when given the same set point velocity.
If given more time to work on this project, we would have incorporated three potentiometers into our design that would be used to regulate the rotational speed of each motor. The user can uses the output voltage from each potentiometer to change the reference speed for the continuous rotation motors and set the max rotational speed for the standard servo. This would effectively create a form of closed loop control using a proportional controller, where the error is completed visually from the user.