Servo - July 2016

This all happens hundreds of times a second!

If you have any aerospace engineer friends, you’ll hear them describe the current state of a vehicle by its state vector. This is just a vector of numbers describing the orientation of a vehicle in space and the state of its systems. If you don’t have any aerospace engineer friends, I highly recommend getting some.

Using double-stick tape, I placed the receiver in the airframe cavity. I connected power from the BEC to the middle (positive) and lower (negative) pins of the “bat” terminal, right below the bind terminal.

The wires from the RC lead of the flight controller connect to the signal (top)

line of channels 1-6 on the receiver. These are single leads since we just need signal as there is no servo to power.

These can really be connected in any order (we’ll fix it in software later), but I connected them to the channels in the same order they were on the connector to make any troubleshooting easier.

I decided to mount the flight controller on top of the drone. A model with side facing connections for the servo would easily fit in the airframe, but I’d like to be able to get to the USB port and other connectors quickly when experimenting.

The flight controller also has a couple of status LEDs that tell us if the system is armed; I like visual confirmation of

40 SERVO 07.2016

Figure 11. There is plenty of room inside the drone for all of the equipment! Check that everything is secure and insulated before you attempt to power-up the vehicle.

Figure 10. I drilled 1/2" holes to allow me to pass the ESC wires and the battery lead to the top deck of the drone. This also makes room for any future equipment to get power and signal wires.

You may wonder why I wanted to use a battery eliminator (i.e., power supply) for test powering the equipment instead of just connecting up the real battery. This is because large RC batteries contain an alarming amount of energy.

If you connect them to a system with a short or with faulty equipment, they can provide many amps, resulting in melted wires, destroyed equipment, and possibly even a dangerous fire. A good bench power supply with a current limit setting is always handy to have, but not totally necessary if you check for shorts with a meter and are very cautious during initial testing.

To demonstrate how much power is in a battery, let's take the total energy stored in a 3,000 mAh lithium polymer battery and make it relatable. If you get lost in the math, don’t worry. Hang in there.

First, we need to get battery capacity into energy. The first step is determining the average voltage during discharge. As batteries discharge, their voltage does not go down in a linear fashion (part of why battery life predictions on products can be off). That’s a whole other story, but we’ll assume that our three-series cell battery has an average voltage of 11.1V. Multiplying 11.1V by 3 Ah, we get a capacity of 33. 3 watt-hours.

Multiply this by 3,600 seconds/hour to get to 119,880 watt-seconds. Reaching way back to high school physics, we can remember that a watt-second is a Joule of energy. If you’re like me, 119,880 Joules doesn’t mean much.

Let’s convert that potential energy to something we can visualize. We know that potential energy of a mass is the mass times the acceleration of gravity times the height at which it sits. Assuming an average person mass of 80 kg (~175 lbs), let's see how high 11,880 Joules could lift us in an ideal system: