Testing a DC motor
It spins!
At the beginning of the week, Zach, Alfonso, Patricia, Aubrey, and I met up and characterized the current consumption of a small DC motor. Alfonso brought a board, and we attached a motor to it, connected it to a benchtop power supply, a big resistor (a high-wattage shunt resistor), and measured voltage change across the resistor with various amounts of power. We also hooked up an oscilloscope (just for fun) and observed the waves. Here are the results:
| supply voltage, V | resistor voltage drop, V | calculated current, A | motor power, W |
|---|---|---|---|
| 4.00 | 0.031 | 0.056 | 0.224 |
| 8.00 | 0.036 | 0.066 | 0.522 |
| 4.00 | 0.25 | 0.455 | 1.708 |
| 4.00 | 0.61 | 1.111 | 3.767 |
This is what we did to calculate the current through the circuit:
- The measured resistance across the big resistor was 0.549 ohms.
- We controlled the supply voltage using the benchtop power supply.
- For each test, we measured the voltage drop across the big resistor.
- With the voltage drop across the resistor and its resistance, we could calculate the current with Ohm's law (V=IR).
- To calculate the power, we first found the current through the entire circuit by dividing the supply voltage by the shunt resistance. Then, we found the motor voltage by finding the difference between the supply voltage and the voltage drop across the resistor. Finally, we got the power by multiplying the voltage of the motor by circuit current.
For the first two tests, we let the motor spin freely and just doubled the supply voltage. This caused the power to (approximately) double. In the third test, we loaded the motor slightly by pushing on it but still allowing some spin; in the fourth, we stalled it completely. In both cases, the power draw increases (very significantly, in the last case).
Making "my own" DC motor board.
Breaking the rules of the assignment (sorry)
Big disclaimer: I knew that the assignment (and Neil, in lecture) told us pretty clearly that we needed to draw our own board, and that could even entail re-drawing Neil's online example. However, I was still unable to get a single LED board to turn on or blink. I brought my attempted blinky boards to various people to get advice and help, but was unable to figure out what was going wrong. Therefore, I decided that if I could re-create Neil's board from the exact design online, I might be able to figure out if I was falling short on the design part or the fabrication (milling, soldering) part of my own board. So, this week would not be original work, but I hoped it would be a very helpful way to troubleshoot my previous weeks! Okay, justification over. Onto the work!
Milling
Spending more time with the trusty Roland machine.
Like previous weeks, I milled a board on the Roland, using Neil's designs for the traces and the outline. The first time, I had no issues with the traces, but the plate moved when I tried to mill the outline. Luckily, the endmill was fine, and I tried again after adding some more double-sided tape. The second try produced a fine-looking board.
Soldering, assembly
In which I regret my coffee consumption.
I grabbed the parts required for the board and soldered the thing together. My hands
were a little too shakey for my liking, since I'd had a few cups of coffee already,
but I decided that the board looked okay.
I then got a DC motor, a 9V battery, some wire, and connected the leads on the battery clip
to an adapter so that it could power the board. I then did the same for the motor, and soldered
those wires together for good measure.
Putting the code on the board
Historically, my least favorite step.
I was really dreading this part, because I had a lot of troubles with loading code onto boards in previous weeks. However, this went surprisingly smoothly. I used Zach's programmer to do this, since it both powers the board and loads the program. Thanks Zach!
Moment of truth and reflections.
It worked- what does that tell me?
I guess this should come as a surprise to absolutely no one, but Neil's board and code worked beautifully. I want to reiterate that I didn't doubt the accuracy / design, and that I decided to remake the board so that I would have something that should work perfectly if I assembled it correctly. Evidently, I assembled it correctly, because the motor spun (some slow spins, medium spins, and fast spins, according to the code). It was really delightful to see, and, despite not having designed it myself, I was so relieved to see that I could get somebody else's board to work in my hands. I'd been (illogically) getting worried that I was "just bad" at electronics and that I would somehow mess up anything I came in contact with, so this was a good way to snap out of it. From this, I learned that the problems with week 5 and 7 and 9 were probably less to do with my soldering skills but was most likely due to some error in the design that slipped by me (and everyone I went to) unnoticed. I'll hunt for that and post an update when I find it!