ACS6116 Advanced Control
Assignment
Dr Paul Trodden
[email protected]
Room C10, AJB
P. Trodden
Spring 2019–2020 Assignment
ACS6116
Advanced Control
Assignment weighting
25% (of the total mark for ACS6116)
Assignment released
Monday 23rd March 2020 (Easter vacation week 1)
Assignment due
To be confirmed— by the end of the academic year
Penalties for late submission
Late submissions will incur the usual penalties of a 5% reduction in the mark for every working
day (or part thereof) that the assignment is late and a mark of zero for submission more than 5
working days late. For more information see http://www.shef.ac.uk/ssid/exams/policies.
Feedback
This will include the overall mark, individual component marks and comments on performance
on the assignment. The attached assessment criteria (at the back of this document) provides a
guide to what areas the feedback will be provided on. Note that marks may be subject to change
as a result of unfair means.
Unfair means
The assignment should be completed individually. You should not work together to complete
the assignment—it must be wholly your own work. References must be provided to any other
work that is used as part of this assignment. Any suspicions of the use of unfair means will be
investigated and may lead to penalties. See http://www.shef.ac.uk/ssid/exams/plagiarism
for more information.
Exenuating circumstances
If you have extenuating circumstances that cause you to be unable to submit this assignment
on time or that may have affected your performance, please complete and submit a special
circumstances form along with documentary evidence of the circumstances. See http://www.
sheffield.ac.uk/ssid/forms/circs, particularly noting point 6 (Medical Circumstances af-
fecting Examinations/Assessment).
Assignment briefing
This laboratory assignment will assess your fundamental understanding ofmodel predictive con-
trol and your ability to design MPC controllers and simulate and analyse MPC-controlled sys-
tems.
The assignment comprises an open-ended design and/or analysis exercise: you are asked to
choose one of the listed topics, and tackle the described problem. Each problem includes ele-
ments of design, simulation, and analysis.
Produce a report (limit: 4 pages in the provided template) containing your answer.
In order to create a level playing field between candidates’ submissions, you are asked to
prepare your submission using the document templates supplied on MOLE. This is a 10pt, two-
column format, which allows ample space for this assignment even with the 4-page limit. (Please
note that no appendices are necessary and even though you may wish to include them, they
probably will not be read.)
It is up to you how you tackle the problem and stucture your answer. However, it is suggested
that you look at (i) the help below and (ii) the attached assessment criteria (at the back of this
document) for guidance on what to include.
Assessment criteria
The assessment criteria for this exercise are derived from themodule learning outcomes, which
are:
1. Describe and explain the principles of more than one advanced control technique.
2. Analyse practical performance specifications and convert these into functional require-
ments on controllers.
3. Design, implement and evaluate an advanced control system against these requirements.
1
P. Trodden
Spring 2019–2020 Assignment
ACS6116
Advanced Control
4. Compare and contrast different advancedcontrol solutions to aparticular control problem
or application.
5. Describe the receding-horizon principle, and hence compare and contrast LQ-optimal con-
trol and MPC.
6. Construct a constrained finite-horizon optimal control problem — including constraint,
model and cost definition — re-formulate it as an optimization problem, and recall and
evaluate the analytical expression for the control law in the unconstrained case.
7. Analyse, design, implement and simulate MPC controllers with guaranteed properties, in-
cluding feasibility, stability and offset-free tracking.
In particular, learning outcomes 2, 3, 6 and 7 are relevant to this assessment, and the attached
assessment criteria — the marksheet that will be used to assess the assignment — are derived
from these. The marksheet indicates the criteria that will be used in assessing your answer, and
also the expectation for each criterion in order to achieve a mark within the specified ranges. It
is suggested that you study this marksheet before completing the assignment.
These assessment criteria are deliberately broad, in order to accommodate the three quite
different topics available. Some topics may require more emphasis on certain criteria than oth-
ers; however, no student will be disadvantaged by topic choice.
Please note that a 4-page limit, using the supplied template, applies to your report, and you
should consider carefully how you can effectively meet the assessment criteria within this limit.
Guidance
• This assignment briefing, lecture slides, and the laboratory exercise document provide the
main information that is required to complete this assignment. You may wish, however, to
consult the literature relevant to your problem (especially for Topics 2 and 3) and review
it in your report.
• Basic M programming is required, including the use of functions and loops; however,
in tackling the assignment you may use the MPC-specific M functions (used in the
laboratory exercises) available on theMOLE page for ACS6116, plus any code you developed
during the laboratory exercises.
• The non-assessed exercises which you completed in the laboratory are good preparation
for this assignment. However, the laboratory exercises were well structured, whereas this
assignment is open-ended: you need to decide what is the most appropriate approach to
solve this assignment, and also how to present your results.
• Regarding the report, you are recommended to consult the attached assessment criteria
for guidance on what to include and to what level of detail. In particular, the assessment
criteria suggest that your report might need to include, among other things,
– Details of the optimal control problem / MPC formulation you used, including the
correct identification and implementation of constraints.
– A description and explanation of how the controller was designed and tuned in or-
der to meet the specification, including the selection of all parameters, with correct
explanations and justifications.
– Clear reporting and discussion of results (including clear, labelled plots), and critical
evaluation of the controller. (Think about more than just, for example, “Did my con-
troller meet the spec.?” — what are the strengths and weaknesses of your solution?
What could be improved?)
– Some analysis, evaluation and/or qualification of stability and feasibility — does your
design come with stability and feasibility guarantees? If so, what are these, and how
are they achieved? What else can you say or show?
This is not an exhaustive list, and what you should include will vary depending on the topic
you choose. However, a suggested outline for any report is
1. Abstract
2
P. Trodden
Spring 2019–2020 Assignment
ACS6116
Advanced Control
2. Introduction
3. Problem statement
4. Design
5. Results
6. Analysis and discussion
7. Conclusion
This might not be the ideal structure for your report, however, and you may wish to com-
bineor change someof these sections, depending on the topic you choose and theprogress
you make.
You do not need to include the code that you write, but you may do so (e.g. snippets of
code) if you think it adds value to your report.
Please note that, in order to achieve the highest marks, you will need to go beyond simply
implementing the methods that you have learned in the lectures and practised in the lab.
That is
• Should you need clarification or have questions on any part of the assignment then please
just ask! (Talk to me in class, email ([email protected]) or come to my office (C10,
Amy Johnson Building)).
Submit your report via MOLE/Turnitin by 23:59:59 on Tuesday 28th April 2020
3
P. Trodden
Spring 2019–2020 Assignment
ACS6116
Advanced Control
Topic 1: Frequency control in a power system
The operation of an isolated power system under primary frequency control is modelled by the
following block diagram.
1
sTg + 1
Governor
1
sTt + 1
Turbine
1
Ms+ D
Power
system
50
1
R
Regulator
pv pmpref ! f+

A steam turbine produces mechanical power, pm, which is subsequently converted to electri-
cal power at a nominal frequency (f = 50Hz) via a synchronous generator connected to the grid.
Changes in power demand (from consumers/loads) and other uncertainties cause deviations in
frequency,f = ff. The control objective is to maintain these frequency deviations,f, close
to zero. To aid this, a governor controls the steam flow input to the turbine in response to the
error between the reference power pref and the regulated frequency deviation !=R, where
R > 0 is the regulation factor.
The primary frequency control loop present in the system does not, unfortunately, offer ade-
quate control. Therefore, the aim is to design a secondary frequency control loop that will adjust
the reference power pref in response to frequency deviations in order to improve transient
performance. To this end, a continuous-time state-space model of the system is given as:24 _!_pm
_pv
35 =
24 D=M 1=M 00 1=Tt 1=Tt
1=(RTg) 0 1=Tg
3524 !pm
pv
35+
24 00
1=Tg
35pref
f =

50 0 0
24 !pm
pv
35
In this model, the input, u = pref, is the change in reference power to the turbine governor
(in per unit (p.u.) – that is, normalized with respect to a base value), and the output, y = f, is
the frequency deviation (Hz). The states are the (deviations from operating points in) angular
frequency, !, mechanical output power of the steam turbine, pm, and output power refer-
ence from the turbine governor,pv. For the particular power system under consideration, the
model parameters are
M = 10;D = 0:8;R = 0:1;Tt = 0:5;Tg = 0:2
Your task is to design, implement and tune an MPC controller for this system in order to
meet the specification on the following page. You may assume that the state is available for mea-
surements. To obtain the discrete-time prediction model for controller, use a sampling time of
0:1 seconds and zero-order hold sampling (i.e. sysd = c2d(sysc,0.1) in M).
Specification
The controlled power system shall
• have guaranteed stability
• at all times, satisfy the constraints jprefj 0:5 and jfj 0:5
• have as large an operating region as possible
• from any initial state in the operating region, have the frequency settle to jfj
0:01 Hz within 2 seconds
(40 marks)
4
P. Trodden
Spring 2019–2020 Assignment
ACS6116
Advanced Control
Topic 2: Rocket landing control
The SpaceX company achieved the first successful propulsive vertical landing of an orbital-class
rocket stage in December 20151. The rocket in question, Falcon 9, is equipped with Merlin 1D
rocket engines, capable of vectored thrust, and grid fins which deploy from the stage-1 fuselage
following separation; these actuators allow sufficient controllability of the rocket to permit a
safe vertical landing. From a technical point of view, the successful landings were also enabled by
theoretical advances in how the kind of nonlinear optimal control problem associated with safe
rocket landing can be modelled and solved2.
A simplified model of the rocket landing problem—assuming that “nose-up” stabilization is
handled separately—views the rocket as a point mass, m, with position r =

rx ry rz
> 2 R3
and velocity v =

vx vy vz
> 2 R3. The coordinates (x; y; z) 2 R3 are defined with zmeasured
vertically upwards from ground (so z = 0 is sea level) and so that (x; y) is the lateral plane
parallel to ground. The net thrust vector emerging from the engine is f =

fx fy (fz mg)
>
—
where the z component is explicitly accounting for gravity—which causes acceleration of the
mass according to the (discretized) dynamics
r(k+ 1)
v(k+ 1)

=

I T I
0 I

r(k)
v(k)

+
1
m

(1=2)T2 I
T I

f;
where I is the 3 3 identity matrix, 0 is the 3 3 matrix of zeros, and T is the sampling time,
which you may assume to be 0:5 s.
The aim is to steer the rocket from an initial position r(0) and velocity v(0) to the target
ground position rt = 0 at rest (vt = 0). This should be done safely and at minimum fuel cost;
that is, a number of constraints should be met during the mission:
• The engines are capable of exerting a thrust satisfying:
– limits on vertical thrust: 0 fz=m 12N kg1.
– limits on lateral thrust:
jfxj fz tan and jfyj fz tan
where = 10 degrees is the maximum angle for thrust vectoring.
• The vertical speed shall not exceed 15ms1 in descent.
• The lateral speeds, jvxj and jvyj, shall not exceed 20ms1.
• In order to avoid premature ground collision, the positional trajectory shall respect a glide-
slope constraint
jrxj rztan and jryj
rz
tan
where is the glide-slope angle, and is 30 degrees.
Your task is to design, implement and tune an MPC controller for this system in order to
achieve safe landing from initial altitudes up to 500m and initial lateral distances up to 600m
from the target. You should investigate the feasibility of themission for a range of different initial
positions and also non-zero initial velocities, representing the real situation of the rocket already
having downward and lateral speed at the commencement of landing control. You should con-
sider that, although it is desired to minimize fuel, the mission should complete in finite time.
(40 marks)
1https://youtu.be/glEvogjdEVY
2Behçet Açıkmee, Lars Blackmore (2011), Lossless convexification of a class of optimal control problems with non-
convex control constraints, Automatica 47(2), 341–347. https://doi.org/10.1016/j.automatica.2010.10.037
5
P. Trodden
Spring 2019–2020 Assignment
ACS6116
Advanced Control
Topic 3: State constraints and stability
In a 1993 paper by Kenneth Muske and James Rawlings3, it was shown that the following system
x(k+ 1) =

4=3 2=3
1 0

x(k) +

1
0

u(k)
y(k) =
2=3 1 x(k)
is difficult to stabilize in the presence of a simple constraint on the output y(k).
Your task is to conduct a rigorous investigation into the stability of this system. The output
constraint that should be applied is
y(k) 1
and the desired tuning parameters for the MPC controller are Q = I, R = 1 and N = 5. A prob-
lematic initial state in this setting is
x(0) =

4 3
>
:
A non-exhaustive list of suggestions for what your investigation could include:
• Whether the system can be stabilized, by tuningQ, R and N, without using stabilizing termi-
nal ingredients (i.e., a stabilizing P and/or terminal set).
• Whether the use of stabilizing terminal ingredients can stabilize the system and, if so, what
those terminal ingredients should be.
• How sensitive the problem is to the magnitude of the output constraint; for example, if the
limit on y(k) is increased to 1+ , what value of is needed to ease the instability problem.
• Exploration of the region of feasibility (set of states for which the problem is feasible) and
region of attraction (set of states from which the system may be stabilized).
(40 marks)
3K. R. Muske and J. B. Rawlings (1993), Model predictive control with linear models, AIChe Journal 39(2), 262–287.
https://doi.org/10.1002/aic.690390208
6
P. Trodden
Spring 2019–2020 Assignment
ACS6116
Advanced Control
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7