Sunday, March 26, 2017

Quadcopter selection and status update

The most notable update of the week is that I received my quadcopter in the mail. This is the quadcopter that I've initially chosen to build my wireless charger for, the SYMA X8C:

There were two broad criteria for my selection of this specific quadcopter:
1.) Cost: This is fairly self explanatory. As someone unfamiliar with quadcopters, I wanted to get a basic ready-to-fly no-frills quadcopter that was still reliable.
2.) Applicability for the given kit: Last time I took some key specifications for the wireless charging kit. Here are the two important numbers that drove my quadcopter selection
Power: 10W output
Weight: ~45 grams. I estimated this as 60 grams to include the weight of the board I'll be additing.

The power delivery of the kit is important in determining how quickly the receiver can recharge the battery of the quadcopter. A simple estimation for the best-case charging time is

Charge Time = Battery Capacity (Watt-Hours)/Wireless Kit Power (Watts)

Most quadcopter batteries specify a nominal voltage (volts) and capacity (amp-hours). We multiply this together to get the capacity.

The weight of the charger impacts the flight time of the copter. A rough estimate of the new flight time is:

Adjusted Flight Time = (Initial flight Time) x (Copter Weight) / (Copter + Charger Weight)

The rationale for this calculation is that the majority of the energy spent on a quadcopter is used just to keep it in the air, not even moving. 

We can combine these two numbers to get a single metric, flight/charge ratio:

Ratio = Adjusted Flight Time/ Charge Time

A larger ratio here is better.

The constructed table was created using data from the following site:

So... ultimately my ratio indicator was not actually that great, as it appears to indicate that the quadcopter should simply be as light as possible. In fact, the model I picked is actually has the worst ratio!

What ultimately drove my selection of the Syma X8C was the concern for carrying the added load of the charger as well as the power specification of 10W. For both the Nano and X5C, I was concerned that the weight of the charger block would weigh down the quadcopter too much. Conversely, I've read reviews citing the X8C's ability to carry additional compenents (like GoPros). Furthermore, the reality of battery charging is that it's not performed at constant power, as there are current and voltage limitations. The standard charger that comes with the X8C also charges at roughly 10W, and thus appeared as a good candidate. Once I've built and validated a charger for the X8C, I can try out substituting in smaller quadcopters.

Other Notes From The Week
- Thanks Wurth Electronik for continuing to send me free stuff! This week I received a t-shirt and testing tweezers. 

- I2C Testing of Extended Power Profile: I learned from testing the kit I recieved that the kit naturally operates with a 5V/5W output and requires some I2C communication to raise it's power output to 10W or 15W. I've done some initial testing using an arduino nano and have been able to engage EPP only for brief periods. I am working with WE's application engineers to get some firmware running that can easily enable the 10W output. 

- As a result, the project plan has slightly changed. Instead of building simply the analog charger interface, I'm planning to construct an Arduino clone that also has the battery charger on it. The arduino microcontroller wil communicate with the RX module to set the output power to 10W, and then engage the charger, as shown in the image below:
- Planned additional features include:
  • Toggle of 2S/1S battery packs
  • Charging current toggle
  • 2S balancing
  • USB access just like Arduino
  • Trickle Charging 
- Build schedule:
  1.  March to April 16: Circuit validation, layout, and PCB/BOM ordering. Specifically need to validate firmware to enable EPP using arduino interface. 
  2. April 16 to May 1: PCB Assembly and initial testing. Show that battery can be charged, charging rates. 
  3. May 1 to May 15: Packaging and mounting the charger onto drone, and drone testing. 
  4. May 15 to June 2: Video demo preparation. 
That's all for now, next posting will be about charger circuit design.

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