**3.3 Improvement of Extension from SISO to MIMO in RFPT-based Systems**

**4.1.1 The RFPT-based MRAC Controller for a 2 DoF TORA System**

The “TORA” (Translational Oscillations with an Eccentric Rotational Proof Mass Actuator) system was
considered as a simpliﬁed model of a dual-spin spacecraft with mass imbalance in the literature. It
served as a “benchmark problem” for controller design in [66]. In [67] it was controlled by a cascade
and a passivity based controller. In [68] for example the “Tensor Product Form” of the system model
was applied to develop a model-based controller. For our purposes its 2 DoF variant was considered
that consisted of a cart, a pendulum (practically a beam) and a dial that can be rotated around an axle
attached to the end of the beam. Its equations of motion are given in (4.1). This seemingly 3 DoF
system was made 2 DoF indirectly driven one by setting*Q*_{3}≡0and trying to control the motion of
axles*q*2[rad]and*q*3[m]by properly setting*Q*1[N×*m]*and*Q*2[N×*m]*[A. 6].

with the dynamic parameters as follows: *m*= 20 [kg](the mass of the dial),*M*= 30 [kg](the mass of
the body of the cart,*L*= 2 [m](the length of the beam), andΘ= 20 [kg×*m*^{2}](the momentum of inertia
of the dial with respect to its own mass center point). Utilizing that*Q*_{3} ≡0the last equation of (4.1)
determines*q*¨1in (4.2) [A. 6]

¨

*q*_{1}=*mLsin(q*1) ˙*q*12−(m+*M) ¨q*3

*mLcos(q*_{1}) (4.2)

that can be substituted into the ﬁrst two equations of (4.1) to calculate the necessary*Q*_{1}and*Q*_{2}torque
components in (4.3).

that is considerably different to that of the directly driven Classical Mechanical systems: certain
ele-ments of the “inertia matrix” and the additional terms are singular and may contain inﬁnite eleele-ments if
cos(q_{1}) = 0. The aim of the MRAC controller in our case was to simplify the dynamics given in (4.3)*for*
*an external control loop*as a 2 DoF*reference model*described by (4.4) [A. 6].

Θ_{ref}*q*¨_{2}=*Q*_{2}*,*−(

*M**ref* +*m**ref*

)*q*¨_{3}=*Q*_{1}*.* (4.4)

with the parameters*m** _{ref}* = 15 [kg],

*M*

*= 25 [kg], andΘ*

_{ref}*= 16 [kg×*

_{ref}*m*

^{2}]for nominal motion that avoids the conﬁgurations in whichcos(q1)approaches 0. The details of how to develop MRAC con-trollers by the use of RFPT were described in details e.g. in [38]. In the sequel we give only simulation results that exemplify how can the “precursor” oscillations be utilized for guaranteeing stable tuning for

*A*

*[A. 6]. Λ= 10/s*

_{c}*kinematic tracking*was prescribed for

*i*= 2,3. The actively driven axle

*q*1 started from the

“not dangerous”*q*_{1} = 0initial position. A digital controller was assumed to yield constant torque for
cycle time of∆t = 1*ms. In the* *adaptive case*the adaptive control parameters were set as follows:

{K*c* = −10^{4}*, B**c* = 1} * or*{K

*c*= 10

^{4}

*, B*

*c*=−1} for yielding the appropriate precursor phenomenon, and

*A*

_{c}*= 10*

_{ini}^{−7}were chosen. For tuning

*A*

*an*

_{c}*asymmetric rule*was applied that cautiously increases its value if no “precursor oscillations” are observed but causes fast decrease if the oscillations appear as

*A*˙* _{c}*=

{ *v*_{+}*if* *F*ˆ* _{n}*−

*F*

*≥0and*

_{thr}*A*

*≤*

_{c}*A*

_{c}

_{ini}−c*v**v*_{+}*if* *F*ˆ* _{n}*−

*F*

_{thr}*<*0or

*A*

*≥*

_{c}*A*

_{c}*(4.5) with*

_{max}*v*

_{+}= 10

^{−}

^{4},

*c*

*= 3,*

_{v}*F*

*= 10*

_{thr}^{−}

^{4}, and

*A*

_{c}*= 10A*

_{max}

_{c}*. The*

_{ini}*F*ˆ

*values were calculated by a “forgetting*

_{n}*integral” that served as a*

*model-independent observer*in the following manner: the

*scalar products*

*F(t*

*) := [*

_{n}*Q(t⃗*

*)−*

_{n}*Q(t⃗*

*−∆t)]*

_{n}*[*

^{T}*Q(t⃗*

*−∆t)−*

_{n}*Q(t⃗*

*−2∆t)]in general can be used for*

_{n}*monitoring the precursor*

*of chattering*since its positive value pertains to deﬁnite

*modiﬁcation of the control force*“approximately in the same direction” while negative value reveals a signiﬁcant fluctuation in the direction of the force between the subsequent control cycles. (The scalar product

*Q(t⃗*

*)*

_{n}

^{T}*Q(t⃗*

*−∆t)could reveal only a rough chattering.) The forgetting integral with the output*

_{n}*F*ˆ

*n*:= (1−

*β)*∑

_{n}*s=0**β*^{s}*F**n*−*s*with*β*= 0.1and threshold
value*F** _{thr}*is able to ﬁlter out single changes in the direction of the forces that normally occur only in
certain points of a smooth variation [A. 6].

**4.1.1.2 Simulations for Big Negative***K**c***and***B**c*= 1

In Fig. 4.5 the movement of the axles in the adaptive case are described for a nominal trajectory
con-verging from zero to a third order spline function of time. In Fig. 4.6 the trajectory tracking errors are
described for*non-adaptive*and the*adaptive*controllers [A. 6].

Figure 4.5: Trajectory tracking of the*adaptive controller*[q_{2}[rad]: black,*q*_{3}[m]: green,*q*^{N om}_{2} [rad]: red,
*q*_{3}* ^{N om}*[m]: ochre lines] (LHS), and the motion of the actively driven axis [q

_{1}: [rad]vs. time in[s]units (RHS)] [A. 6]

Figure 4.6: Trajectory tracking errors of the*non-adaptive controller*(LHS) and the*adaptive one*(RHS)
[q_{2}[rad]: black,*q*_{3}[m]: green lines vs. time in[s]units] [A. 6]

Figure 4.7 reveals the control forces that have the following deﬁnitions: the “desired forces” with
super-script “Des” denote the force needs of the*reference model*calculated by the external control loop using
the*kinematically prescribed tracking error relaxation*and the*dynamic model of the reference system*
given in (4.4). The “exerted” forces are calculated by the internal loop of the MRAC controller that has
double responsibility: a) guaranteeing precise trajectory tracking, and b) generating the illusion to the
external loop that the reference model well describes the controlled system (the “MRAC illusion”). The

“recalculated forces” marked by the superscript “Rec” means*the force need of the reference model*at
the realized response of the controlled system in its actual state. It is evident that the controller well
generated the MRAC illusion since the “recalculated” forces are in the vicinity of the “desired ones” while
both of them considerably differ from the adaptively deformed, realized control forces actually exerted.

Figure 4.8 displays zoomed excerpts of the force diagram of the adaptive controller. In the diagram
little fluctuations in the control signal in certain segments can well be observed. These fluctuations are
related to the tuned value of*A**c*depicted in Fig. 4.9. By the use of the time axis of the diagrams it can
well be seen that the appearance of the the little fluctuations in the control signal are related to the too
great values of*A** _{c}*and that the fast decrease in

*A*

*makes them cease quickly [A. 6].*

_{c}Figure 4.7: The*generalized forces*for the*non-adaptive*(LHS) and the*adaptive*(RHS) cases versus time
in[s]units [Q^{Des}_{1} : black,*Q*^{Exerted}_{1} : red,*Q*_{1}* ^{Rec}*: brown, and

*Q*

^{Des}_{2}: green,

*Q*

^{Exerted}_{2}: ochre,

*Q*

^{Rec}_{2}: dark blue lines in[N×

*m]*units] [A. 6]

Figure 4.8: Detailed excerpts of the*generalized forces*of the*adaptive controller*versus time in[s]units
[Q^{Des}_{1} : black,*Q*^{Exerted}_{1} : red,*Q*_{1}* ^{Rec}*: brown, and

*Q*

_{2}

*: green,*

^{Des}*Q*

^{Exerted}_{2}: ochre,

*Q*

^{Rec}_{2}: dark blue lines in[N×m]

units] [A. 6]

Figure 4.9: Variation of*A**c*vs. time in[s]units (LHS) and the time-dependence of the forgetting integral
(RHS) [A. 6]

**4.1.1.3 Simulations for Big Positive***K*_{c}**and***B** _{c}*=−1

These alternative settings also produce observable and useful “precursor oscillations” for the same
nominal motion. Figure 4.10 reveals precise trajectory and smooth and precise phase trajectory
track-ing. According to Fig. 4.11 it can be stated that similar and efﬁcient parameter tuning happened as in
the case of the original settings{K*c*=−10^{4}*, B** _{c}*= 1}. In the diagram of the generalized forces (Fig. 4.12)
similar precursor oscillations can be observed, too [A. 6].

Figure 4.10: The phase trajectories [q_{2}[rad]: black,*q*_{3} [m]: green,*q*^{N om}_{2} [rad]: red,*q*^{N om}_{3} [m]: ochre
lines] (LHS) and the trajectory tracking errors of the*adaptive controller*using the alternative parameter
settings (RHS) [q_{2}[rad]: black,*q*_{3}[m]: green lines vs. time in[s]units] [A. 6]

Figure 4.11: Variation of*A** _{c}*vs. time in[s]units (LHS) and the time-dependence of the forgetting integral
(RHS) using the alternative parameter settings [A. 6]

Figure 4.12: The chart of the*generalized forces*of the*adaptive controller*at the alternative parameter
settings versus time in[s]units and its detailed excerpts [Q^{Des}_{1} : black,*Q*^{Exerted}_{1} : red,*Q*^{Rec}_{1} : brown, and
*Q*^{Des}_{2} : green,*Q*_{2}* ^{Exerted}*: ochre,

*Q*

^{Rec}_{2}: dark blue lines in[N×

*m]*units] [A. 6]