Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
1

Development of Fault-Tolerant Control for Unmanned
Vehicles Using Redundancy Actuators

KAZUMICHI ODA*1

TAKEHIRO ADACHI*2

ICHIRO AWAYA*1

Unmanned vehicles are required to have resiliency that enables them to autonomously
continue their operation even when actuator failures occur. This report proposes a fault-tolerantcontrol for unmanned vehicles using redundantly arranged actuators to achieve the resiliency.
Specifically, we have developed a fault-tolerant control function that allows unmanned vehicles tocontinue their travel in the event of an actuator failure. This function was realized with a failure
detector developed to detect actuator failures by comparison of the rudder angles estimated foreach actuator, as well as a fault decoupling controller developed to remove the effect of the failure
based on the detected actuator failure information. Furthermore, we conducted a simulativeevaluation to verify the effectiveness of the developed fault-tolerant control.

|1. Introduction
Against the background where manpower saving and replacement of human work inhazardous environments are demanded, many unmanned vehicles such as automated guided
vehicles (AGVs), unmanned aerial vehicles (UAVs), and autonomous underwater vehicles (AUVs)
have been developed in recent years. These unmanned vehicles operate in a place without humanintervention. Considering such characteristic, they must be resilient enough to continue their travelautonomously even in the event of actuator failures (1).
One of the methods to achieve the resiliency is thought to be fault-tolerant control, whichuses redundant actuators arranged in such a way that multiple actuators affect a single action so thateven if an actuator fails, the control can be maintained by using other actuators. For example, it has
been reported that the AUV "Yumeiruka" equipped with four redundant rudders can maintain theminimum action controllability required for its operation even when some of the rudders fail due tothe robust control designed for normal rudder operation, although the control performance
deteriorates (2). In addition, for UAVs, robust model predictive control, which takes into account theeffects of failures in advance, has been provided so as to enable hovering without loss of stability
even when some of the propeller motors fail (3).
On the other hand, methods for detecting actuator failures and proactively using the detected
abnormalities to deal with failures have also been studied. For example, the control mixer
method (4)(5)(6), which calculates the input distribution by solving simultaneous equations so that theactuator output becomes equal before and after a failure, as well as a method that has multiple
controllers designed for predefined failure modes in advance and switches them appropriately whena failure occurs (7) have been proposed. However, when a failure occurs that cannot be detected bysensors, for example, in the case the link mechanism connected to the motor sticks due to external
force, it is necessary to build a separate failure detector that detects the failure using signals sensed
from the controlled object.
In this report, we introduce a fault-tolerant control that takes advantage of the redundantactuators installed in unmanned vehicles. Specifically, we have developed a fault-tolerant control*1CIS Department, Digital Innovation Headquarters, Mitsubishi Heavy Industries, Ltd.

*2

Chief Staff Manager, CIS Department, Digital Innovation Headquarters, Mitsubishi Heavy Industries, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)2(8)function that allows unmanned vehicles to continue their travel in the event of an actuator failure .
To realize this function, we constructed a failure detector that detects an actuator failure by
estimating the state of each actuator and comparing the estimated rudder angle values, and afault-tolerant controller that removes the effect of the failure based on the detected actuator failure
information (8).

|2. Unmanned vehicle with redundant actuators
Figure 1 and Figure 2 show an AUV equipped with four rudders as an example of an
unmanned vehicle to be controlled by the function described in this report. The vehicle travels in
the , , and directions by moving forward while having an attitude motion (roll, pitch, andyaw), which are rotational motion around the three axes of , ,, respectively, of the vehicle
coordinate system. For example, to move upward in the depth direction, the vehicle moves forwardwith its pitch angle directed upward. The equation (1) represents the simultaneous differential
equations for the attitude motion of an unmanned vehicle at a certain equilibrium point.
ΔΔ
Δ
Δ
Δ+
(1)
Δ =Δ +Δ
Δ
Δ
Δ
Δ
where, , andare the coefficients of the equation of motion and Δ is the amount ofchange from the equilibrium point. , ∅ are the attitude angles shown in Figure 1 and Figure 2,, , and are the attitude angular velocities, andis the rudder angle of rudder #i operated bythe controller.
The unmanned vehicle to be controlled in this study has redundant actuators, and the numberof actuators that can be used for attitude motion of the three axes is four or more.

Figure1 X-Z plane view of unmannedvehicle (AUV)

Figure 2 X-Y plane view of unmanned
vehicle (AUV)|3. Development of fault-tolerant control
Figure 3 shows a block diagram of the developed fault-tolerant control function. The
fault-tolerant control function consists of a failure detector that estimates the location andmagnitude of actuator failures and a fault-tolerant controller that uses the fault state quantity output
from the failure detector to cancel the effects of failures.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
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Figure 3

3.1

Block diagram of fault-tolerant controller

Development of failure detector

Figure 4 shows a block diagram of the failure detector. The failure detector estimates thelocation and magnitude of actuator failures from the inputs and outputs of the unmanned vehicle. It
is common to use an observer that can estimate the state using the results of sensing the attitude
motion of the unmanned vehicle and the motion model of the vehicle. However, when the actuators
are redundantly arranged as shown in Figure 1, the number of actuators to be estimated, 4, is larger
than the number of states, 3, so a single observer cannot accurately estimate the operation quantity.
Therefore, assuming that one actuator is normal, we constructed observers that estimate otheractuator operation quantities for each actuator. This allows the reduction in the number of actuator
operation quantities to be estimated by the observer, so that the number of actuator operationquantities to be estimated is the same as the number of states. For actuators assumed to be normal,
the input quantity from the controller is regarded as the actuator operation quantity.
As an example, equation (2) shows the system to be estimated assuming rudder #1 is normal.
In this equation,
represents the control input of rudder #i.
Δ
Δ
Δ

Δ
Δ
Δ

=+

Δ+

Δ
Δ+

Δ
Δ
Δ
(2)

In equation (2), the rank of the coefficients of the three items on the right-hand side
pertaining to rudders #2, #3, and #4 is 3, so the amount of rudder operation can be estimated using
the pseudo-inverse matrix.
As shown in Figure 4, the actuator that has failed is determined by comparing multiple
operation quantities estimated for the same actuator among the operation quantities estimated by
the configured observer. For example, the estimated values for rudder #1 are
estimatedassuming rudder #2 is normal,
estimated assuming rudder #3 is normal, andestimatedassuming rudder #4 is normal. If either of rudder #2, #3, or #4 fails, there will be a difference
between the control input and the rudder operation quantity, so the operation quantity estimatedassuming the failed rudder is normal will have a deviation from the other estimated values.
Therefore, the failure judgment logic for each actuator determines the actuator with thelargest deviation from the average value of the estimated operation quantity output from the threeobservers as a failure candidate. As an example, Figure 5 shows a flowchart of the failure
judgment logic for rudder #1.
Finally, actuator failure is determined with unanimity of the failure candidate flags output
from the three failure determination logics. The failure estimation value is the average value of theestimated operation quantities, taking into account the estimation error of the observers.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
4Figure 4

Block diagram of failure detector

Figure 5
Flowchart of failure judgment logic(Rudder #1 failure for example)

3.2

Development of fault-tolerant controller

The decoupling controller is based on a decoupling control law that cancels the effect of afailed actuator on the vehicle by correcting the command values of the remaining actuators basedon the estimated results of the failed actuator output from the failure detector.
For example, equation (3) shows the AUV state equation in the case of a rudder #1 failure.
The failed rudder becomes uncontrollable and acts as a disturbance force on each motion axis. The
number of controllable actuators becomes three, and the coefficients of the three items aredegenerated.
ΔΔ
Δ

Δ
Δ
Δ

=+

Δ+

Δ
Δ+

Δ
Δ
Δ
(3)

In order to decouple the failed actuator, by transforming equation (3) so that the accelerated
angular velocity of the axes of motion (roll angle, pitch angle, and yaw angle) becomes zero andsolving with respect to the normal actuator, the rudder angle correction amount can be obtained.

Mitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
5|4. Verification with simulation
4.1

Simulation condition

Using an AUV as an example, the effectiveness of the developed fault-tolerant control wasverified. As the simulation condition, a case in which a rudder fails while the AUV is travellingstraight ahead was considered. The assumed actuator failure used for this verification was thesticking of one of the X rudders of the AUV that occurred due to external forces while travellingstraight ahead.

4.2

Simulation result

Figure 6 shows the 3D vehicle track simulation result. The result indicates that when thefault-tolerant control was not applied (red line), the vehicle moved up significantly due to theactuator failure, resulting in the track that deviated significantly from the target track. When thefault-tolerant control was applied (blue line), the effect of the failure was suppressed and thevehicle was able to follow the target track even after the failure occurred.

Figure 6

Simulation result (3D vehicle track)

Figure 7 Simulation result (positionand attitude angle)

Figure 8

Simulation result (rudder angle)

Figure 7 shows the AUV positions and AUV attitude angles simulation results. Without thefault-tolerant control, the AUV deviated from the target track significantly in both the y-axis andz-axis directions due to the effects of the failure. On the other hand, it can be confirmed that
applying the fault-tolerant control reduced the deviation from the target.
Figure 8 shows the rudder operation quantity simulation results. It can be seen that theMitsubishi Heavy Industries Technical Review Vol. 60 No. 1 (March 2023)
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failure of rudder #1 was immediately detected by the failure detector, and the effect of the failure
was canceled by the normal rudders using the fault-tolerant controller.

|5. Conclusion
We have developed, for unmanned vehicles with redundant actuators, a fault detector which
consists of observers constructed for each actuator, and a fault-tolerant controller that reduces theeffect of an actuator failure by canceling the effect of the failed actuator with normal actuators.
As a result of the simulation of a rudder sticking failure using an AUV as an example, it wasconfirmed that the control deviation at the time of a failure could be reduced by applying thefault-tolerant control, and that the developed fault-tolerant control was effective against actuator
failures.
We believe that the result described in this report will contribute to improving the resiliency
of vehicle products (AGVs, UAVs, and AUVs) that have redundant actuators, and we will promote
its application to our products.

References
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(2)
(3)
(4)
(5)
(6)
(7)

(8)

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