Jingwen Cai
System Engineering
ASCI 530 Unmanned
Aircraft System Assignment 2
Embry Riddle
Aeronautical University
Abstract
This paper is a response
to a system’s question in an assignment from the UAS class. It explains how much knowledge the writer
learned as a system engineer and how to apply the knowledge to solve an UAS overweight
problem during the design phase. This paper includes the writer’s considerations,
priorities and the thought about the future prospects for the “next generation,
enhanced” version of the system.
Key word:
System engineering, life
cycle process, process flow chart, allocation requirement, verification and
validation, integration, trade off study, Vee model.
A general definition of
what is a system and what a system engineer does is best explained in Systems Engineering Fundamentals, a book
developed by the System Management College from the Department of Defense. It states that systems engineering is
an interdisciplinary engineering management process that evolves and verifies
an integrated, life-cycle balanced set of system solutions that satisfy
customer needs.
A simplified process flow
chart showing the key functions between the system input and out. It shows that
the functional analysis and allocation, system balance, and requirement
analysis are integrated part of the system management.
Source: System
Engineering Fundamentals, p6. Depart of Defense System Management College.
As
the System Engineer for a UAS that is designed for precision crop-dusting, we
started the life cycle process with conceptual design to understand user
requirements, and develop a system concept and validation plan. Then we
developed system performance specification, functional specification, “build
to” technical manuals, allocated specification, and system validation
plan.
It
seems we have capable teams that carried out the detail design at each
sub-system, and our design is on schedule and below budget. However, during the review meeting per the
validation plan, we found that two subsystem has a design feature-the weight, over
allocated requirement.
Weight
is a key feature for a UAS system because it impact the payload, the wing load,
the thrust requirement, the landing and launching systems. So we allocated the
total weight into multiple lower level weight requirement for each subsystem. The allocation requirement is not ambiguous,
but clearly written in the flow chart, and in each subsystem’s design
expectation package.
If
these two over weighted subsystem don’t cut off their excess weight, then either
we have to reduce the fertilizer carrying and spraying system, or cut into the
fuel margin. Neither of these two choices is in favor of our system’s final
performance.
As
a system engineer, I am expected not to be bias or in favor of one over the
others, and I am also responsible for deliver this design project to meet
customer requirement. My consideration is that the voice of the customer is the
priority, because they pay the bills for this design and counted on us to
deliver. The good news is that I have some tools from my system engineering
training to help me to weed out the designs that are not helping us to meet
customer need.
First,
we have the flow chart and design requirement that have all the subsystems grouped
and their allocation specification documented in order to meet the total
performance. By checking the weight control parameter, which is key design
characteristic labeled on the flow chart, I brought the team together and
performed an analysis. I put red color dots on the subsystem that are over
weighted-the Guidance, Navigation & Control subsystem, and the Payload
delivery subsystem.
One
of the tools in the system engineer’s tool box is the “Vee” model. This model
highlights the need to define verification plans during requirements
development, the need for continuous validation with stakeholders. Our
stakeholders are marketing manager, subsystem designer, safety engineer,
product assurance and others. We kept asking ourselves if we made what we said
we were going to make, or are we giving the customer what they want or need?
So,
I bring our marketing manager to the meeting, and she explained that she
already talked to customer about the new design and promised that our specific
UAS will carry sufficient weight to spread the specified amount of fertilizer
over the specified area. Customer is
happy with this idea and looking forward to put more orders if we can
deliver. The marketing manager also
mentioned that there are two other competitors that are bidding for this project,
but they carry much less fertilizer, which means more trips for the customer to
spray the same size of the area, and cost more fuel money and time for the
customer.
With
that said, I showed the calculation of the weight of the fuel that is required
to fly this UAV to the specified area, and had airframe designer, the power
plant designer, and launching and retrieving subsystem. They chose the fixed wing horizontal take off
and launching system to have more aerodynamic lift during cruising while using
minimum fuels. They also chose the high efficiency power plant to save the
fuel. Instead of 4 piston engine, they
chose 2 piston engine to reduce the weight, but use higher Aspect Ratio wing
design. They also chose the carbon fiber
composite material for fuselage and wings with hollow honey cone design to
reduce the total weight of the airframe without losing the structure strength
and pay load capacity. These designer come
out with great ideas showing how much extra effort they put in to modify and
improve their subsystem to ensure the weight is within the limit.
Part
of the responsibility for a system engineer is to perform trade-off studies
pertaining to the best approach in responding to the functional requirements. First,
we identify the problem by diving in the over weighted subsystem, and found out
that they have attempted to save cost by purchasing off the shelf hardware,
rather than a custom design. The GPS system, the guidance and control system
from the off the shelf product are for larger airplanes and median altitude and
all weather condition, which is not a necessary for the crop dust purpose.
Then
we followed the trade-off process to have brain storming to list all the
alternatives that the team can think of, and evaluate the wide ranges of the
alternatives one by one.
Zooming
into the end user’s point of view, we found out that the environment that the
dust crop UAS will be working in is at very low attitude, just a few meters
above the crops. This ensures the mist from the spray equally spread out and
reach the crops before the mist bumped into each other and become drops. Also
this UAS will not be operated in windy days, nor in rainy days, because the
rain will wash the fertilizer away from the leaf and stem and flowers. All these factors helps to reduce the
requirement for airframe structure to contour the air turbulence and cold
temperature for high altitude. So the airframe subsystem may contribute some of
the weight allocations. That is a good news for other subsystems that need more
allocation for weight.
The
end user of the UAS doesn’t need the low-light-level cameras, or the thermos
imagers, because it will be operating in day time with good views from the
ground operator. Those images were intended for giving the image back to the
control station for analysis, and adjustment of the fly route based on the
image. Corp fertilizer will be sprayed within a known boundary and landscape,
it doesn’t need an image feedback to determine where to fly to. If the farmer doesn’t have the geographic
data for his crop land yet, he may remove the fertilizer tank and spray system,
and put a video camera on the UAV to do surveillance. Once the data is collected, it can be used
for several years as long as the land and area is not changed. The fertilizer spray mission will only carry
the fertilizer and fly through a fixed course year after years on the same
land, without a camera on the UAV. But
those cameras are part of the guidance system in the off-shelf product that
caused over weight. The design engineer
could remove those cameras to reduce the weight by modify the off the shelf
product toward our need. This takes advantage of low price of the off the shelf
product, but also help us to reduce the weight in guidance and navigation
subsystem.
The
guidance system from the off-self product has a GPS and gyroscope build in but
the crop dust UAV may not need it. It has known grid of the land scope for
which area to be sprayed, so a way-point navigation system is a choice. Input the instruction to preprogramed chip,
to command th4e UAV fly ton a selected bearing at a selected speed and altitude
until all way points are visited. This tailored design reduced the total weight
of the guidance and control system.
By
visiting some spray nozzle suppliers, and compared the weight and performance
of more than 20 types of the spraying systems, there is a light weighted high
pressure system enlightened the mechanical engineer, who comes out a new spray
nozzle that with metal piece only in the nozzle channel and spiral track, but
plastic with a hallow structure in the other part of the nozzle to reduce the
vibration during the spray. This design reduced the weight of off the shelf
spray system because plastic with honey cone design is much lighter.
The
final step of the trade-off study is eliminating alternatives which do not appear
to meet requirements, or eliminating alternatives with low probability of
successful implementation. This step is also called “weeding out” by some text
books. I used the quantified the criteria by assign a weighting percentage for
each alternative, and asked the team to give their number based on the
performance, weight, reliability, producibility, safety, affordability,
schedule. Then I multiply their number with my weighting percentage, and get a
weighted index for each alternatives we listed. The lower score will be
eliminated.
After
eliminating a few alternatives by using the weighted criteria index, we selected
the most balanced system and resolved the conflicts of the weight allocation
among the subsystems.
My
thought about the future prospects for the “next generation, enhanced” version
of the system is that reducing the payload for guidance, navigation and control
by design an integrated circuit chip that is weighted only 1% of the total
payload of the corp dusting UAS, and a more efficient and light weighted spray
nozzle that weight only 5% of the total payload. This will allocate majority of
the payload to the fuel and fertilizer that benefit the end user of this kind
of UAS. With today’s technology, the
circuit design is possible and production of this kind of chip is affordable if
the sales volume is large. I am positive
to get more business for this new design because end user will find out that
this UAS can spray the fertilizer with longer endurance flight (carrying more
fuels) and precision of where and when to spray how much.
Reference:
Austin,
R. Unmanned Aircraft Systems. Wiley. 6th edition. P128-p.170.
Borror,
C. The Certified Quality Engineer Handbook. 3rd edition. P.126.
Gilsman,
P. Systems Engineering and Safety: Building the Bridge. P 7.
