Propeller clock displayer balancing
The propeller displayer was the main display screen for the whole system. It utilizes persistence of vision theory to display the illusion of stable patterns floating in the air, thus it requires demanding rotation balance skills, which does not fall within our familiar expertise.
The protoboard system was even not balanced when we set up the initial system and began the rotation test. We first changed the Li-ion battery position as initially we put the battery on one side of the rotating board. As the battery is definitely the heaviest part on the board, we put it to the center hoping to improve. But as the battery module itself was not symmetric, we still cannot balance the system. We searched several solutions on Internet, and removed our power supply system for the propeller clock off-board (explained in power redesign module), then the board itself can be balanced.
Then the led strip was added, the rotating clock could not maintain balance again. One possible reason may be that the LED strip is too soft to keep in position when the motor was rotating at a high speed(around 1800 rpm). As for this factor, we thought of using structure wood to support the led strip. Another possible reason for the unbalance introduced by the led strip is that the LED strip was mounted only on one side, we need to add one similar structure to the other side to balance during rotating. In the end, we found a more stiff material to hold the LED strip and use four wires to hold the whole system, structured like a bridge. We also added some coins to balance the weight and adjust carefully the distance. Finally, the whole system was successfully balanced with all components.
Charging system re-design
The redesign of the charging system has gone through a Li-ion battery to a slip ring, then to inductive wireless charging module. At first, a rechargeable Li-ion battery will be used to charge the propeller clock module. Following our initial charging system design, we successfully recharge the Li-ion battery using the lab power supply equipment, and modified the Li-ion battery boost module, added a switch to control the power and added a charging circuit, in the end, we successfully have 5V output. But as the cumbersomeness of the battery made balancing rotating board so difficult, we removed it and tried slip ring to power the rotary part.
We made a slip ring to replace the Li-ion battery with the following steps:
- Cut copper and aluminum slice to make a metal roll.
- Cut the AA battery to get a perfect metal roll.
- Cut the copper to get a perfect electrode.
- Design its mechanical structure to make it more reliable.
- Testing its reliability on high speed.
The finished slip ring worked mostly OK except the motor became hot after some time when rotating. We suspect it’s mainly because of the insulation(the tape and glue) when fixing the slip ring on the motor. Also the slip ring will increase the resistance of the rotation part, thus the rotating board will become somewhat unstable.To combat this problem, we experimented inductive charging module to power the propeller displayer system, and tested the set with oscilloscope and assembled it on the system. This is a lightweight and stable solution that we finally choose to use for our project.
Bluetooth to uart communication module design
The Bluetooth to UART communication module is an essential part of the system communication and controlling. We initially used the module HC-06 to test the communication between the two microcontrollers, but we cannot get the desired reply from the serial to the Raspberry Pi monitor. To debug, we first checked all the hardware and made sure that the circuit is correctly connected. Then, we checked the connection between our computer Bluetooth connection and the module. The HC-06 module could connect to the computer correctly. We then did research on the internet and found similar bugs related to the HC-06 module, so we switched to HC-05 module instead. Using HC-05 module, the testing program could be uploaded successfully. We were also able to achieve the bi-directional communication between Raspberry Pi 3 and Arduino pro mini: the command could be sent to Arduino from Raspberry Pi 3; Raspberry Pi 3 could receive and display the reply information from Arduino.Finally, we redesigned the connector between the Bluetooth module and the Arduino pro mini to make it more compact and easier to balance on the rotating board.
Initially when Raspberry Pi 3 communicated the with Arduino pro mini, the reply from the Arduino was not correct. After some research we figured out that a time lapse need to be added between sending the command and receiving the reply. Otherwise, the reply will not be prepared, then the receiving buffer would not have the full reply. That’s the reason that the reply displayed on the screen was not correct.
At first, the character displaying effect on the propeller displayer is similar with the picture shown below(left): the shortest distance between the led light spot was separated around 4-5 centimeters. To improve the displaying effect, we reduced the delay time and finally delete the delay time between the two status(light on and off) entirely, but it didn’t work.
So, we suspected that the effect of the fastest updating rate may just be like this. We then searched the document of the dotstar led strip on internet and found that the updating rate could be super fast: for Arduino, the PWM rating could be 8MHZ, which is the clock rate. If that is true, we were supposed to get a better POV effect than the displayed effect. We then figured it might be the software problem. As we were using the Adafruit library previously, twe tried the Fastled library to see the display effect. It worked! The displaying effect with the new library is much better, shown in the picture below(middle).
With the new library, we could display the characters on screen better, but we want to display messages more precise and mimic the real-world display. So we tried to change the software SPI to hardware SPI by changing the connection within the system and got an even better display effect as shown in the picture below(right).
Based on the third solution, we successfully displayed the character “ECE5725” on our system.