THCS Generalized Airplane Control System Design

20 Pages·2013·0.56 MB·English

by  Antonius, Adrianus Lambregts

Proceedings of the EuroGNC 2013, 2nd CEAS Specialist Conference on FrAT3.2
Guidance, Navigation & Control, Delft University of Technology, Delft,
The Netherlands, April 10-12, 2013
THCS Generalized Airplane Control System Design

Antonius A. Lambregts*

Abstract The Total Heading Control System (THCS) was developed in the late eighties to overcome well
known safety/design deficiencies of traditional SISO-based Flight Guidance and Control (FG&C) systems.
THCS uses generalized MIMO based airplane control strategies to functionally integrate all desired lateral
directional automatic and augmented manual control modes and achieve consistently high performance.
The counterpart to the THCS is the Total Energy Control System (TECS), which functionally integrate all
desired vertical automatic and augmented manual modes for vertical flight path and airspeed control.
Recent TECS developments are described in a companion paper. This paper documents further insights
gained over the past years on THCS design details for achieving precision control decoupling, integration
of augmented manual control modes and innerloop design using airplane dynamic model inversion. Also a
TECS/THCS-based Mode Control Panel concept and a Primary Flight Displays concept that incorporates
the TECS/THCS control and guidance strategies are discussed.

1 Introduction

Automatic Flight Guidance and Control (FG&C) systems have evolved into highly capable systems. These
systems have contributed immensely to the improvement of aviation safety. Unfortunately, these systems
still use traditional SISO control strategies that do not provide full 6 degrees of freedom airplane control.
Therefore, airplanes equipped with these systems are still vulnerable to Loss of Control (LOC).
Furthermore, these systems have become exceedingly complex, due to an excessive number of modes,
mode overlap and mode idiosyncrasies, making it a challenge for the flight crew to avoid mistakes using
these systems that can jeopardize operational safety. Most of the FG&C system modes are considered
“non-flight critical”. This means that the flight crew is assumed to recognize and safely manage any failure
of function of such modes. However, too often this assumption has proven to be unwarranted. As a result
there have been too many automation related incidents and accidents, due to stall, roll divergence after an
engine failure, icing etc. The current generation of FG&C systems do not take full advantage of modern
MIMO control strategies to functional integrate all modes, eliminate well known safety deficiencies (e.g.
by incorporating full flight envelope protection) and provide simpler, more efficient and less costly designs.
The THCS concepts described in this paper were developed to overcome the noted deficiencies.

TECS and THCS Development
FG&C system design and safety deficiencies were well recognized as long ago as the late seventies. In the
early eighties NASA initiated research to address these deficiencies. This work resulted in the Total Energy
Control System (TECS), which uses a generalized MIMO-based energy control strategy to functionally
integrate all vertical flight path and speed control modes. The approach provides inherent envelope
protection and avoids open ended SISO mode operations, thereby largely eliminating LOC safety risks.
System complexities are reduced sharply by eliminating mode overlap, simplifying mode processing and
providing more intuitive Man Machine Interfaces (MMI). Design generalization makes the system directly
reusable, thereby reducing development costs for new applications. The system was successfully
implemented and flight tested on the NASA B737 in 1985. The counterpart to TECS is the Total Heading
Control System (THCS) which integrates all lateral directional control modes. Its design objectives and
strategies are analogous to TECS. It was developed in the late eighties on the Condor High Altitude Long
Endurance autonomous UAV program. TECS and THCS were successfully applied on the Condor and
flight tested to demonstrate autonomous control capability under all operational and variety of failure
conditions.
The original THCS concept is described in [2]. This paper describes THCS design updates since the early
nineties, including

* FAA Chief Scientific and Technical Adviser for Advanced Control
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 Roll/Yaw control innerloop design, using Model Inversion, feedback normalization and feedback
concatenation
 development of a simple and effective design methodology for the augmented manual control
modes, to meet traditional handling qualities requirements
 development of integrated vertical and control NLF authority allocation and envelope protection
 various approaches to providing integrated airspeed, normal load factor and roll angle envelope
protection
This paper also discusses a TECS/THCS-based Mode Control Panel concept and a Primary Flight Displays
concept that incorporates the TECS/THCS control and guidance strategies. A companion paper [3]
describes TECS design updates. Another companion paper [4] provides more details on Flight Envelope
protection strategies.
Design objectives. The THCS design objectives include:
 use of one pilot-like MIMO-based control strategy for all automatic and manual control modes
 full authority integrated lateral directional control, including automatic roll and yaw retrim, to
prevent LOC due to engine out roll-yaw divergence
 generalized functionally integrated design, consistency of operation between modes
 elimination of Yaw Damper, Turn Coordinator and Thrust Asymmetry Compensator
 Decoupled Mode Command responses, reduced controller activity
 reduced design complexity by using shared modular building blocks, elimination of mode
overlap and simpler mode processing
 simpler, more intuitive Mode Control Panel (MCP), clearer Flight Mode Annunciation (FMA),
 large cost reductions by generalized/reusable design, minimal application specific development,
reduction in laboratory and flight testing and shorter application development cycle.

2 THCS – Architecture and Conceptual Design

Core Controller Design
Original Core Controller. The original THCS Core Controller is shown in Figure 2.1. The control
strategy of the THCS is analogous to the TECS. A number of variations of this architecture are possible to

suit the application need or design preference. One variation will be discussed below.

AAiirrppllaannee iinnddeeppeennddeenntt ddeessiiggnn AAiirrppllaannee ttaaiilloorreedd ddeessiiggnn
 ˆˆ  pp PPooss AAiilleerroonn::
RRoollll RRiigghhtt
cc +++___ KK +++___ ++++ VVttggrruuee KKSSRRII ++__ KK ++__ KKpp AAAccctttuuuaaatttooorrr aa

  ff ((qq))
cc  cc
iill
cc  rr ff ((qq))
cc

cc +++___ KK +++___ ___+++ KKSSYYII ++__ KKrr ---111 AAAccctttuuuaaatttooorrr rr
PPooss RRuuddddeerr
 ˆˆ rr YYaaww lleefftt

Figure 2.1 THCS Core controller – basic functional architecture

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The outerloop horizontal path mode (lateral path, heading or track angle) error is normalized into a standard
yaw rate command (c ) (or an equivalent roll angle command()c ), and the directional control mode
ol ol
(sideslip control) error is normalized into the standard sideslip rate command ( ) and these signals are
c
used as the command input the THCS Core Controller. In the THCS Core Controller (Figure 2.1), the sum
of the error signals ()  is formed and used to develop the roll control effector command, while the
 
difference between these error signals ()  is used to develop the yaw control effector command.
 
To achieve coordinated turns without inducing a sideslip during lateral maneuvering (perfect roll/yaw
decoupling), the dynamic response of (  ) and ()  for a lateral maneuver command must
   
be designed to be identical. Then also the dynamics of ( ) and ()  will be the same,
   
assuming K  K . For a weather-cock type yaw oscillation about the airplane’s z-axis, with negligible
 
side acceleration, the response is characterized by . Such a motion should be heavily damped by
the rudder control, without a considerable roll control response. In principle this requires this K  K
 
and removal of the cross coupling effects. The THCS Core Controller roll control channel develops an
innerloop roll angle command () and features conventional roll-error and roll rate feedback control
c
il
loop. The THCS Core Controller yaw control channel develops an innerloop yaw rate command
{r (g /V ). } that is coordinated with () and uses body axis yaw rate to form a yaw-rate
c true c c
il il
error control loop. Therefore the numerical value of K and K should be the same. A gain factor
RI YI
Vg / is included in the forward signal path of the roll control channel, to maintain the proper
true
ˆ
kinematic relationship between , , cil and rc at all flight conditions. The feedback signals 
ˆ
and are derived from free running complementary filters. The processing for the individual lateral
outerloops control modes is analogous to the processing for TECS outerloop modes. For automatic mode
operations the outerloop  is normally zero. The  can be used to command a sideslip to minimize the
c c
drag during engine out operation and to provide a decrab function as part of the automatic landing control
mode. The decrab function may be designed to align the airplane heading with the runway heading just
prior to touchdown, without causing the airplane to drift sideways.

-Filter. A raw aerodynamic  signal can be derived from a -vane or hemispherical differential
pressure sensor, calibrated for the local potential flow distortion around the airplane nose structure where it
is mounted. It also may need to be calibrated to remove the angular yaw acceleration effect due to the
location forward of the airplane center of gravity. Furthermore, such a calibrated aerodynamic -signal is
ˆ
too noisy under conditions of atmospheric turbulence. Therefore a-filter is used, as shown in figure 2.2.
This filter uses an inertial sideslip rate signal developed from the airplane side force equation:

I (vI /VI){(Fyaerprop /m)g.cos.sin}/VI r p.tanI (2.1)
(A g.cos.sin)/V r p.tan
y I I
Here V is the inertial velocity vector, but for most applications a groundspeed or a filtered true airspeed
I
ˆ
can be used without a significant impact on performance. The  signal is used in the THCS Core
Controller for airplane Dynamic Model Inversion and sideslip angle feedback. [The TECS Core uses a
similarly filtered Angle of Attack signal (ˆ ).]

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The ˆ ,ˆ filter gains are determined to achieve the preferred compromise between control activity and
effectiveness in suppressing induced pitch, roll, yaw, acceleration responses and path deviation in
turbulence.
...
^^^

KKK
222
222
 ++ KK11 +++ 11 ^^
--__ SS ++++++ +++ SS
..
 ^^

III
11
ˆ
Figure 2.2 -Filter

Automatic Roll/Yaw trim. The processing of (or in the design variation discussed below)
and  signals through integral control signal paths also provides for trimming of the roll and yaw control

effectors, to null out any rolling and/or yawing moment imbalance due to thrust asymmetry (e.g. engine
failure), fuel load imbalance or lift asymmetry due to wing icing or damage. So, depending on which lateral
control mode is engaged, the roll channel will trim to null its outerloop mode error, e.g. roll angle for the
augmented manual mode, or when an automatic mode is engaged it will trim that mode error to zero, thus
eliminating the need for manual roll and yaw trim functions. The yaw channel will trim the rudder to
establish  .
c

Provisions Roll and Yaw Control Effector Command Saturation. The classical way to
limit the control effector command to prevent integrator windup, is to continually calculate and apply the
instantaneous integrator limit. The instantaneous integrator limit is computed by subtracting from the
actual control effector limit the incremental control effector command contribution by all the proportional
signal paths downstream of the integrator output. Alternatively, the integrator may be moved to end of the
control effector command processing path, where the output to the integrator can simply be limited to the
control effector limit. The input to such an integrator represents the control effector command rate, which
can also be limited to the actual effector rate capability. If the integrator is placed at the end of the
command processing chain, a differentiator must be placed in all the original proportional signal path that
contribute to the control effector command. In the later TECS and THCS designs the latter approach is
used, because it is simpler. With this design approach care must be taken to avoid propagation step
commands (originating from mode commands) through the original proportional signal paths to the control
effector, because a step change in signal amplitude cannot be reproduced when passed through a
differentiator/integrator combination. In the TECS/THCS design step commands to the control effectors
are avoided.

Outerloop Modes Design

The most basic outerloop mode is the Heading Angle control mode, which is included in the THCS Core
controller architectures, shown in Figure 2.1 and 3.1. The Track Angle mode is entirely analogous to the
heading mode, requiring in principle only the substitution of Track Angle for Heading Angle. The
Localizer and LNAV modes require the closure of an additional control loop for controlling the cross track
error. In the Condor UAV application and in the TECS/THCS real time system demonstration systems this
additional feedback loop is concatenated around the heading control. In effect the cross track error signal is
ˆ
multiplied by the normalization gain K and divided by V to form the heading angle command, in a
y true
manner analogous the feedback error normalization used in the Glide Slope and Altitude Acquisition
modes. The detailed mode processing is fairly standard, so for brevity reasons further details will be
skipped in this paper.
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3 THCS – Design Updates
THCS Core Controller architecture updates

The THCS architecture of figure 2.1 is not directly compatible with the Augmented Manual Lateral
Directional control mode requirement for controlling roll angle by a roll control inceptor and sideslip angle
ˆ
by a yaw control inceptor. The architecture can be made compatible by replacing the  feedback with an
equivalent roll angle feedback using the relationship for a level coordinated turn: (g/Vtrue).tan
or for relatively small angles  (g/Vtrue)., resulting in the architecture shown in Figure 3.1.

VVttrruuee
gg
 
cc    pp
VVttrruuee ooll cc
gg
cc +++___ KK +++___ ++++++ SS11 +++___ KKRRII +++___ KK +++___ KKpp ppcc PPooss AAiilleerroonn::
 RRoollll RRiigghhtt
cc VVttggrruuee ccooss cciill ppcc 
aa
DDyynnaammiiccss cc
IInnvveerrssiioonn 
cc VVttggrruuee rrcc rrcc
rr PPooss RRuuddddeerr::
ccooll +++___ KKooll ___+++ SS11 ++++++ KKYYII +++__ KKrr cc YYaaww LLeefftt
ˆˆ
ˆˆ  rr
Figure 3.1 THCS Roll Attitude based Core controller

System Stability at High Roll Angles. During the early evaluation of the manual control mode
incorporated in the architecture of Figure 3.1 a roll/yaw system stability degradation observed at high roll
angles in the, e.g. for >45 degrees. A that time gains used were identical to the corresponding gains in
the architecture of figure 2.1. Analyses showed that the stability degradation at high bank angles was due to
a missing cos gain factor in outerloop roll angle error cross feed signal path to the yaw control channel.
This became clear by considering the relationship between yaw rate and the measured body axis yaw rate
r in a steady turn:
r coscossin (5.1)
In this expression the pitch rate term can be neglected and cos1 may be assumed. Therefore, without
the roll to yaw channel cross feed gain K cos, the resulting magnitude of r was excessive at high
RY c
bank angles, relative to the measured body axis yaw rate feedback. Addition of this cosgain factor
eliminated the stability degradation.

Engine out Control. During further evaluation of the system of Figure 3.1 it was found that the
performance for a worst case engine out condition (Max thrust Go Around ) was marginal and that the
performance for this condition could be improved significantly by restructuring the yaw channel -
innerloop to be analogous to the-innerloop of the roll channel. This restructured THCS architecture,
shown Figure 3.2, includes new gain K when compared to the architecture of Figure 3.1. and the gain

il
K has been moved to cross feed signal path from the yaw channel to the roll channel, in order to

ol
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maintain the same gain for the -error input to the roll channel as in Figure 3.1 Now, by selecting
K =K , K  K and K  K the resulting yaw channel response dynamics to  -command
YI RI il  r p col
will be identical to the roll channel response dynamics to a  -command, since the Dynamics Inversion
c
ol
results in fully decoupled roll and yaw axis dynamics. Both the -error outerloop integral control gain
and the-error proportional innerloop gains are considerably higher than the corresponding gains in
Figure 3.1, thereby achieving the desired engine out control performance. It also resulted in much tighter
sideslip control during turn entry.

Figure 3.2 Restructured THCS Roll Attitude based Core controller

In order to maintain coordination between the  of the roll channel and r of the yaw channel
c c
il
innerloop in response to a  , a new gain 1/K has been inserted in the cross feed path from the
c 
ol il
outerloop -error to the yaw channel, as shown Figure 3.1.

Decrab Control. For the earlier architecture of Figure 2.1 decrab capability was provided by a 
c
cross feed term added to the input of the roll control channel integrator and a  feed forward term added
c
to the rudder command. This approach required fairly complex  filters and gain schedules to achieve the
c
precise roll-yaw coordination to quickly establish the commanded sideslip angle, without causing the
airplane to drift sideways. For the updated design these complex ad hoc designed elements have been
eliminated and replaced by a new  signal cross feed to the outerloop roll angle command and feedback
c
summing junction, providing an incremental roll angle command   K . . The gain
c decrab c
ol
K  / is now computed by solving the lateral directional equations of motion in near real
decrab c c
ol
time for the steady state roll angle(pr 0), yaw and roll control surface deflections needed to achieve
zero side acceleration with  and zero acceleration along the y-body axis (no side drift). The
c
computation is shown in Figure 3.3.

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Figure 3.3 Computation of K
decrab

Final THCS Core Controller Configuration. In the final THCS architecture reconfiguration,
shown in figure 3.4, the integrators are moved to the end of the control effector command processing string,
which is now part of the Dynamics Inversion processing block. As a result the proportional signal paths
now include a differentiation function. In this reconfigured architecture the output of the gain block K
YI
may be considered a  , just like the output of the gain block K in the roll channel may still be
cil RI
regarded as a  . The only reason why in Figure 3.4  and the yaw channel innerloop-feedback are
c c
il il
not combined in one summing junction is the new placement of the integrator and the end of the yaw
effector command processing chain, which now terminates in the Dynamics Inversion processing block.

 pp
VVttrruuee
gg
   KKRRII +++___ KK +++___ KKpp
VVttrruuee cc dduu//ddtt
gg ooll
PPooss AAiilleerroonn::
cc +++___ KK +++___ ++++++ KKRRII KK KKpp ++++++ RRoollll RRiigghhtt
dd((pp ))ddtt
gg ccooss  pp cc
cc VVttrruuee KKiill cciill cc aa
DDyynnaammiiccss cc
IInnvveerrssiioonn 
VVttrruuee KK  rr rrcc
gg ooll cciill cc dd((rr ))ddtt
cc
ccooll +++___ ___+++ KKYYII KKiill KKrr ++++++ PPooss RRuuddddeerr::
YYaaww LLeefftt
 dduu//ddtt
 KK KK
 +++ rr
iill
__
rr

Figure 3.4 Final THCS Roll Attitude based Core Controller Architecture
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Design of the THCS Core Controller feedback Gains

Applying Dynamics Inversion. The generalized design approach used for TECS Core Controller,
including feedback normalization/concatenation, airplane dynamic model inversion and loop gain
separation or pole placement, was also applied to the updated THCS Core Controller design. Only the Roll
and Yaw dynamics are inverted. A simplified Dynamics Inversion approach is represented in Figure 3.5.
This approach was used in combination with the architecture of Figure 2.1 with the output of the integrators
representing the ( )and r for the roll and yaw innerloop control. In the processing of Figure 3.5 only
c c
ol
the direct moment contributions due to airplane state feedback are inverted, under the assumption that the
inertial coupling moments are insignificant. Clearly, for high rates of maneuvering, such as in case fighter
planes this will not be a valid assumption. The inputs to the inversion process are a roll acceleration
command and a yaw acceleration command, represented by the outputs of the gain blocks K andK in
p r
Figure 2.1. The outputs of the Dynamics Inversion processing (Figure 3.5) are the roll and yaw control
effector commands. In this Inversion processing the actuator dynamics are not taken into account and for
that reason, as explained in [3], the gain of the most innerloop roll and yaw control (K andK ) are
p r
selected to accommodate the addition of actuator dynamics without significantly impacting the final system
stability and response. The results achieved using the inversion approach of Figure 3.3 were quite
satisfactory for all routine maneuvers, although the approach has not been tested for extremely high
airplane maneuver rates.

Figure 3.5 Simplified Roll Yaw Control Inversion

A slightly more elaborate Dynamics Inversion processing that includes inversion of the of the inertial
coupling moments is shown in Figure 3.6. It applies to the architecture of Figure 3.4 where the integrators
have been moved to the end of the control effector commands processing string. As a result the integrators
now appear as the last processing step to generate the control effector commands in Figure 3.6. This
approach has been used in all recent THCS work, including the simulation results shown below. The inputs
to the inversion process of Figure 6 are the rate of change of the roll acceleration and the rate of change of
the yaw acceleration command. Because the collective inversion term in Figure 3.6 associated with the
moment dependencies on the state derivatives will also be integrated in the final processing step, a
differentiation needs to be applied to. The inversion of the moments of inertia associated cross coupling
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terms are developed using as input the rate of change of the “pure decoupled “ rolling moment coefficient
command and the rate of change of the “pure decoupled “ yawing moment coefficient command.

Figure 3.6 Extended Lateral Directional Inversion

The still less significant inertial coupling terms associated with products of the angular rates have been
neglected. A final validation of the adequacy and correctness of the inversion process can be performed by
modifying selected or all coefficients and cross coupling moments of inertia involved in the airplane model
and in the inversion process and checking that the final augmented airplane dynamics remain unaffected for
all realistic airplane maneuvers.

THCS Core Controller roll channel gains. Inversion of the roll dynamics, removes all cross
axis coupling effects and leaves only the bare roll rate and roll angle integrators around which the new
proportional concatenated roll rate and roll attitude loops are closed with the gains K andK ,to create
p 
the desired innerloop roll dynamics. In addition, a roll error integral control loop is closed around the
innerloop roll dynamics to assure zero steady state roll command tracking error. The resulting THCS Core
Controller roll control channel, shown in Figure 3.7, is functionally the same as the roll control channel of
figure 3.1, 3.2, 3.3 .

KK
FFFFPP
KK
FFFFII AAuuggmmeenntteedd
cc AAuuttoo  RRoollll ddyynnaammiiccss
aauuttoo 11 MMooddeess ++ 11 +++ cciill KKppKK 
llccii KKssttiicckk SS cc ++-- SS +++ KKRRII SS22KKppSSKKppKK
ooll

Figure 3.7 THCS Core controller Roll Channel.

The basic transfer function for the system of figure 3.7 with the switch in the “Auto Modes” position is:

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   K K K
p  RI
   (3.1)
  S3 K S2 K K SK K K
 c  p p  p  RI
ol auto
The Dynamics Inversion process and the kinematic relationships built into the THCS Core controller take
care of all needed real world gain scheduling. As an example, we use pole placement to define the
dynamics of the basic [/ ] transfer function:
c auto
ol
 
 1
   (3.2)
  (.5S1)2(S1)
 c 
ol auto
The resulting gains areK 5 (rad/sec2)/(rad/sec), K 1.6(rad/sec)/rad and K .5 rad/rad. The
p  RI
[/ ]TF dynamics not only supports the closing the automatic control loops for the Heading, Track,
c
ol
the Localizer, or the LNAV mode, but was purposely designed to support augmented manual control
mode design, as is shown below.

THCS Core Controller yaw channel gains. The yaw channel of the reconfigured architecture of
Figure 3.2 or 3.4 is structured entirely analogous to the roll channel. Therefore, as discussed above, by
selecting K  K .5, K  K 1.6 and K  K 5, the resulting dynamics of the
YI RI   r p
il
[/ ] TF will be identical to the dynamics of the [/ ] TF , since the roll and yaw axes are fully
c c
ol ol
decoupled as the result of the Inversion of the original airplane dynamics. For a lateral maneuver, the
airplane will respond smoothly to a -command to establish  , while the cross feed from the
c c
ol ol
roll channel to the yaw channel commands the required yaw rate to ensure that the airplane will not
develop a significant side acceleration and sideslip. Likewise, for a decrab maneuver, the airplane
responds smoothly to a  -command to establish  , while commanding a precisely
c c
ol ol
coordinated roll angle through the cross feed   K . , to prevent the airplane from drifting
c decrab c
ol ol
sideways. The [/ ] response dynamics are quick enough to avoid the need for additional
c
ol
augmentation.

Performance in Turbulence and Windshear

Balancing the control command tracking performance against the control effector activity for operation in
turbulence and windshear conditions is a difficult problem for any flight control design. It is perhaps the
easiest to do so for the autoland mode, where Localizer tracking performance is of paramount importance
and all other performance metrics are secondary. Additional (secondary) performance objectives may
include minimizing RMS of lateral acceleration at the airplane center of gravity and in the back of the
passenger cabin, roll angle, yaw angle and control effector activity. Attempts to achieve additional
(secondary) performance objectives, often leads to design conflicts that can only be resolved by making
design compromises. For example, the specific feedback control architecture and the feedback signal
synthesis by the use of Complementary Filters (observers) can greatly affect the relative outcome of the
various performance measures. It is beyond the scope of this paper to address these issues in further detail.

4 THCS Automatic Modes Simulation Results

Final THCS Core Controller. The Core Controller architectures of Figure 3.2 and 3.4 represent the
highest level of development of the Total Heading Control Concept reached so far. However, during
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