Tethered Power for Drones

An overview of wired power systems for drones

Quadrotors have become very popular in the past decade due to the massive development in the open-source community and a plethora of available hardware components. Their applications range from photography, cargo delivery, precision agriculture, disaster management to recreational hobby flying and drone racing.


The Roadblock - Uninterrupted Power for Quadrotors


Most quadrotors used for research are run in indoor environments. In order to keep them highly responsive and agile, we often end up making them power inefficient. In this article, we will explore the approaches to tether your quadrotor/UAV for indoor (and outdoor) use. This would be most helpful for researchers or budding roboticists who need to deal with the minimized flight time imposed by the Li-Po battery. This approach could then be incorporated into their existing setups.


Power Statistics


Before trying to answer how much power does a quadrotor consume, let us try to find out why it happens.Consider a case when the quadrotor is completely still in space (ideal hovering). Now if we compare this state with that of a stationary land/water vehicle, we can point out the difference instantly:


Since the quadrotor is not in contact with the ground, it has to continuously apply a force in a direction opposite to acceleration due to gravity (g) to maintain equilibrium. In case of ideal hovering, the thrust forces F1=F2=F3=F4. This leads to continuous power consumption by the motors to produce the required thrust.On the other hand, the self-weight of the ground vehicle is balanced by a contact reaction force or normal force (or upthrust in case of water vehicles). The ground/water vehicle does not need energy to maintain its equilibrium because it is in constant surface contact.

Now, this requirement of energy brings us to the following questions-


How much energy does my quadrotor need for hovering?

or more importantly —



How much flight time does my present experimental UAV platform support for a given Li-Po battery?

To answer this, let us consider a common quadrotor configuration with the following components-

4 x BLDC Motor ~ 250–300 gm

4 x ESC ~ 100 gm

1 x Frame ~ 600 gm

1x Flight Controller ~ 250 gm

4 x Propellers ~ 30–40 gm

1 x Battery ~ 500–700gm

TOTAL MASS: 1600–1900 gm


Now, let us take a close look at the thrust data for the BLDC motor operating at 14.8 V.


Thrust-Power Data for the BLDC motor (borrowed from hobbypartz)


If you do the calculation for your own UAV platform, you will find out that your quadrotor can hover only for about 15 minutes.To get a flight time of around 90–120 minutes for the system shown above, we need to recharge the battery at least 4 times. After the battery discharges, we need to land the vehicle, swap the batteries and go back to our experiments.

Honestly, this does not seem too impractical, does it? Certainly, not. However, in this process, we will have to keep a check on all of our batteries and charge them from time to time. Also, even with multiple batteries, we WON’T get an uninterrupted flight for even a couple of hours. So what could be a viable solution here? The answer - A Tethered Power System.


Tethered Power System : An Overview


A ‘tethered’ power system simply means a wired power system. Tethering aerial systems is not a new concept, it has been around for a few years now. It finds applications in advertising, construction, mapping and surveillance. Such a power system ensures virtually unlimited flight time making it a viable solution for certain applications mentioned above. It also provides a highly secure, wired data transmission to the ground. The hovering altitude of such aerial systems could be anywhere from 200m-2000m depending on the application.


RangeT tethered system


Commercial Outdoor Tethered Systems

In commercial tethered systems, the application mostly demands the quadrotor to carry an equipment (say a high resolution camera) along with an on-board Power Supply Unit (PSU) and other miscellaneous payloads. The PSU is necessary to convert the transmitted AC voltage to suitable DC voltage which could then be used by the drone.


Power Conversion in a Tethered Drone System

The AC voltage is stepped up to minimize the voltage drop along the length of tethering wire. Since, power is given by:


P = Vrms * Irms,

If we increase the the voltage, the current value will drop for a constant power requirement. Hence, the voltage drop along the tether wire will be minimized (read the details discussed below for indoor tethering).

The only drawback is that you need a big quadrotor which can easily lift the PSU weighing about a couple of kilograms. Hence, you will see such a drone only for outdoor applications, where you have the margin to increase the propeller size and overall dimension.


Tethering for Indoor Use

DISCLAIMER : Experimenting with high power devices poses a risk of accidents. Please ensure necessary preventive measures are taken before starting.

Since tethering with PSU onboard is not feasible for smaller, indoor drones, I am sharing the approach to design a tethered system while keeping the PSU on the ground. The major challenges that need to be addressed are as follows:

1. Selecting a proper PSU

An aerial system carrying all payloads which include sensors,cameras and other miscellaneous components is bound to demand high electrical energy for hovering and moving around. The power required could be anywhere from 400W-800W depending on your system. So, we need to select a regulated PSU capable of handling large currents. An authentic PSU which comes with a lot safety features like overload protection, overheating shutdown etc. could serve well in this application.


2. Wire gauge and wire length

Well, this is something new when working with a quadrotor. Because we are supplying DC voltage along the wires, we are bound to observe substantial potential drop along the wire length. Thus, wire selection becomes an important parameter for setting up your system. Now, let us explore the fundamental reason for potential drop before we select our wire for tethering.

We know that resistance of a constant diameter wire (at constant temperature) can be defined as :-

R = ρ L/A

where, R - resistance (ohm) , ρ - resistivity (ohm-m) , L - wire length(m), A-cross-sectional area (sq.m)

Thus, increasing wire length causes an increase in resistance which thus, increases the voltage drop (Ohm’s Law) for a given current value. This drop is directly proportional to the current flowing through it since, the wire has a near constant resistance. In reality, an increase in current causes increased heating (Joule’s Law) which in turn raises the wire temperature. This increase in temperature will cause increase in resistivity and hence, the resistance. But, for all practical purposes, we could safely assume a constant resistance.As per the above equation -Thicker the wire, lesser the resistance. However, if you select a bigger diameter wire, it will increase the mass of the tethered wire and thus, increase the overall power demand of the system.

The selection of the tether wire is thus, a trade-off between its resistance and weight.

3. Fail-Safe Mechanism

Now that we have selected a proper PSU and a suitable wire, and hooked it up to power the drone, are we ready to fly? Probably, no.

What happens if you face a power outage ?

OR

What if our PSU cuts off due to overloading?

OR

What if the MCB trips due to some other machine in the lab?

We surely don’t want our quadrotor to crash because of such possibilities. Thus, a fail-safe mechanism is mandatory.

A simple yet effective arrangement like the one shown in the video ensures that the quadrotor stays airborne in power outages.


This ensures that your flight controller does not reboot and your UAV stays airborne. Now we can safely test our quadrotor indoors!

Conclusion

We have thus, discussed the two major methods of tethering a UAV. This simulation and numerical data is an outcome of research and testing of our team at RangeAero, where we are working to create next generation UAVs.


Abhilash Ingale

Structures & Power systems @ RangeAero

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