Mechanical Design

We mounted the 144 LED/meter dotStar strip on a 3 foot piece of wood. The LED stick was mounted onto a cardboard box that contained the Raspberry Pi, PIC 32, breadboard, and power supplies. The box sat on a wood frame that was connected to the motors and wheels, allowing the mechanism to move in one direction of any distance.

We also added five layers of tissue paper on the LED strip to help diffuse the light and create a more blended image without sacrificing light intensity.

Motor Control

The power enable pin controls when the motors are turned on and off. The two motors are connected in parallel while the motors are controlled by a PWM output with a 1 K Hz frequency. With a 40M Hz clock frequency, this means that the PWM output can have a duty-cycle ranging from 0 to 39999. Because DC motors are inductive loads, the release of the stored energy causes harmful voltage spikes that could damage the MCU. To protect the MCU, an optoisolator is used to electrically isolate the motor from the MCU. The optoisolator used is the 4N35. Additionally, a diode and capacitor were placed in parallel to the motor to moderate the motor voltage during the PWM transitions. A MOSFET was used to switch the motor on and off using the PWM signal and resistors (300 Ω, 1M Ω, and 10K Ω) were added to protect and optimize the circuit as shown below.

PIC32

On the PIC 32, we ran a uni-threaded program. The program contains a few initial setup calls to open timers and declare input pins, before than starting one thread. Although this program could have been made with no threading library, we decided to include the protothreads library due to our familiarity and experience with the library and the negligible disadvantages of unnecessarily using threading in this application.

Initializations

The program begins by opening Timer 2 with a generate period of 40,000, which corresponds to a frequency of about 1KHz. This timer is used to trigger an ISR which generates the PWM pulses necessary for precise motor control. After opening the timer, the program continues to open Output Compare 3, which is used to send the PWM signal to the motor control circuit. The output compare works off of a pwm_on_time, which corresponds to the amount of time to hold the PWM signal high, translating to a faster motor. The motors should be off initialing, so pwm_on_time is set to 0 at first. After opening the timer and output compare, the last thing to initialize is the SPI channel used to send data to the DotStar LED strip. We chose SPI channel 2 for the LED strip because the onboard TFT display on our PIC32 Big Board uses SPI channel 1, and we wanted to use this display for debugging purposes. After these three modules are open and configured, we are already to start our lone thread.

Thread

The thread starts by initializing the red, green, and blue (RGB) values of all pixels to 0, corresponding to a completely black or “off” strip. After this initialization, the thread goes into a while(1) loop where it uses function calls to read RGB data, set RGB data, and write RGB data. The thread accomplishes this through repetitive function calls to three functions: read_pixeldata and set_pixel_rgb, and write_pixels.

Results

The DotStar Light Painter was able to produce images effectively. However, because we had to reduce the color from 24 total RGB bits to 8 total RGB bits, some color accuracy is lost. Additionally, because we only had 144 LEDs per meter, resolution was also lost (depending upon the original resolution of the image). To increase the resolution, another LED strip could be added. Also, the LED strip could have performed at a higher speed if we had the Raspberry Pi send all the image data at once, however, since the Raspberry Pi sends the data column by column, there is a bit of delay. We did reduce the delay between each column sent from 0.5 seconds to .05 seconds, which shrunk the width of the image.

However, sending data column by column with our slow motors worked to our advantage. The slow speed of the motors was due to too much stress from the torque of the box and strip, so motors with a higher torque tolerance would also have allowed us to better tune the images. Lastly, our mechanical design could have been sturdier. As you can see in the images, the cart rocked side to side as it moved, creating warped images. This could have been prevented through a sturdier design and better materials.

Future Considerations

In future iterations of this project, the biggest thing we would like to improve is the mechanical design. Although our design was satisfactory, there were problems with the LED strip shaking while the motors were on, due to the high center of gravity. Additionally, stronger motors would be necessary, as we ended up blowing out our cheap DC motors from the load of the box and strip. Another interesting improvement would be using more LEDs to improve the resolution of the image, although then the machine would be over two meters tall, which could cause maneuverability issues indoors.