Power, Energy, and Control - Research Article

Advances in Mechanical Engineering

2020, Vol. 12(10) 1–14
Ó The Author(s) 2020
Reliable speed control of a permanent DOI: 10.1177/1687814020970311
journals.sagepub.com/home/ade
magnet DC motor using fault-tolerant
H-bridge

Soban Ahmed, Arslan Ahmed Amin , Zaeema Wajid and Faizan Ahmad

Abstract
Reliable, smooth, and fault free speed control of a Permanent Magnet (PM) DC motor using an H-bridge is an important
need for many industrial applications such as robotics, automotive, and process industry to improve the overall efficiency
and productivity. The reliability of H-bridge depends on the semiconductor switches used. The faults in these compo-
nents can lead to a complete failure of the system. This paper presents a dual redundancy-based fault-tolerant system
with a Fault Detection and Isolation (FDI) unit that can detect, isolate, and replace the faulty switch with the standby to
prevent the unwanted shut down of the system and support the process continuity thereby increasing reliability.
MATLAB/Simulink environment was used for simulation experiments and the results demonstrate the stable operation
of the motor in the events of faults while maintaining its speed. The presented work establishes that the dual
redundancy-based fault-tolerant H-bridge with the FDI unit is a highly reliable solution for the speed control of a DC
motor.

Keywords
Redundant switches, fault-tolerant control, H-bridge, DC motor speed, IGBTs

Date received: 19 June 2020; accepted: 2 October 2020

Handling Editor: James Baldwin

Introduction further categorized into active and passive. The active

analytical redundancy consists of fault detection, its iso-
Fault-tolerant control
lation, and reconfiguration of a controller but that
Fault-Tolerant Control (FTC) strategies are utilized to makes it a complex, high computational and slow in
improve the reliability of the machines by preventing performance, however, it has the advantage of incor-
events failure due to faults. A fault is commonly known porating a wide range of faults. The passive analytical
as a deviation in the output of a system from its desired redundancy is a relatively simple and faster method as
output while failure is known as a complete shutdown it is based on a robust controller design that can deal
of a system. The main role of an FTC is to prevent
faults in critical systems that can result in its failure.
Department of Electrical Engineering, FAST National University of
The systems used for critical missions such as Computer and Emerging Sciences Chiniot Faisalabad Campus, Chiniot,
unmanned aerial vehicles, aircraft, and nuclear facilities Punjab, Pakistan
cannot tolerate failure, hence, to improve the safety and
reliability of such systems FTC is used. Redundancy is Corresponding author:
Arslan Ahmed Amin, Department of Electrical Engineering, FAST
one of the methods used to design an FTC system and National University of Computer and Emerging Sciences Chiniot
it falls into two main categories: analytical redundancy Faisalabad Campus, Chiniot, Punjab, Pakistan.
and hardware redundancy. Analytical redundancy is Email: arslan_engineer61@yahoo.com

Creative Commons CC BY: This article is distributed under the terms of the Creative Commons Attribution 4.0 License
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without further permission provided the original work is attributed as specified on the SAGE and Open Access pages
(https://us.sagepub.com/en-us/nam/open-access-at-sage).
2 Advances in Mechanical Engineering

Figure 1. AFTCS architecture.1

with the only considered uncertainties and faults in a Hardware redundancy. In the hardware redundancy,
system, thus, it cannot accommodate a wide range of along with the main component another secondary
faults.1 component is installed that can perform the same func-
tionality, making it one of the most reliable ways of
increasing the reliability of the system. Whenever the
Active Fault-Tolerant Control System. The main component
main component fails in such an FTC system, the sec-
of an Active Fault-Tolerant Control System (AFTCS)
ondary component overtakes to perform the same func-
is the Fault Detection and Isolation (FDI) unit whose
tionality, thus avoiding the damage and failure of the
responsibility relies on the detection of faults occurring
system.2 The basic dual redundancy system is shown in
in actuators and sensors as shown in Figure 1. The FDI
Figure 3. The reliability of the overall system increases
unit is implemented by designing an observer model
as the system will only fail if both components fail to
that generates the estimated values which are used in
perform otherwise the system will keep on performing
case a component incurs fault. After detecting and iso-
its normal operation.
lating a fault, controller reconfiguration is performed
by the FDI by utilizing the estimated parameter values
by the observer to adopt new conditions. AFTCS has Speed control of DC motor
the advantage of including a wide range of faults and
Pulse Width Modulation (PWM) is a popular technique
it’s an online fault detection-based method but that
used to control the speed of a DC motor by controlling
makes it a complex, high computational, and slow in
the duty cycle of the driving voltage of the switches.
performance.1
The speed of the motor can be easily controlled by vary-
ing the duty cycle of the PWM signal and is directly
Passive Fault-Tolerant Control System. The Passive Fault- proportional to the duty cycle that is greater the duty
Tolerant Control System (PFTCS) is much simpler cycle, the greater will be the speed of the motor. Figure
than the AFTCS as shown in Figure 2. It does not 4 shows the waveforms of 0%, 20%, 50%, 80%, and
require an FDI unit and during the design stage, all 100% duty cycle signal applied to the switches to con-
possible faults are assumed. Since the controller design trol the speed.3
of PFTCS is offline based and has a static structure, The H-bridges are commonly used for speed control
therefore, its functionality structure remains the same of DC motors in both forward and reverse directions.
after faults. The main advantage of PFTCS is the fast- The construction of H-bridges is mainly based on the
est response capability because of the simpler structure. power semiconductor. The reliability of the switches is
As the PFTCS controller is designed for only consid- of great importance as these switches in some cases can
ered uncertainties and faults, therefore, accommoda- turn out to be faulty for example; if a switch burns out
tion of complex faults risks its functionality.1 due to the passage of higher current that can lead to a
Ahmed et al. 3

Figure 2. PFTCS architecture.1

Ton
Duty cycle = 3 100 ð1Þ
Ton + Toff

Where Ton is the time as soon as the signal is in high

(on) state and Toff is the time as soon as the signal is in
the low (off) state.
Output current having a resonant load of power con-
verters is described as

diðtÞ RiðtÞ q ðt Þ Vs
1 + + = ð2Þ
Figure 3. Dual redundant system. dt L CL L
Where iðtÞ is the power converter’s output current, qðtÞ
is the charge of the capacitor, and Vs is the supply
voltage.5
When a PWM signal is applied to SW1 and logic
high to SW4, then the IGBTs would make a full path
from Vs to the GND and this makes the motor rotate
in the forward direction. By changing the duty cycle of
PWM applied to SW1, the speed of the motor can be
varied. Likewise, when the PWM signal is applied to
SW3 and logic high to SW2, then the IGBTs would
make a full path from Vs to the GND and this makes
the motor rotate in the reverse direction. By using the
PWM signal, we are essentially switching the motor on
and off at a certain rate to control the speed.

Figure 4. PWM waveforms at different duty cycles.

Literature review
Hossein Iman in his work6 explained how to bypass,
protect, and diagnose the fault that appears on an H-
catastrophic failure of the complete system. Therefore, bridge if they are placed in cascade. The author pro-
to prevent catastrophic failure of the system, fault- posed a redundancy strategy to make the system fault-
tolerant strategies are gaining popularity and are being tolerant. The major problem in cascade systems is that
utilized in a wide range of applications including power as the number of power switches increases, it increases
converters.4 the reliability issues in cascaded H-bridge converters.
The H-bridge power converter circuit topology is The proposed strategy showed the system detected the
shown in Figure 5. The power switches mainly use the faulty switches, bypassed the faulty switches, and reor-
insulated gate bipolar transistors (IGBTs) in H-bridge ganized the cascaded H-bridge converter to improve the
for power switching. The pair of four IGBTs could be overall performance. In paper,7 the author proposed an
utilized for controlling the speed of a DC motor by isolation and elimination technique to make the system
varying the input signal of the switches. The mathemat- fault-tolerant. The proposed strategy uses several relays
ical representation is given as:3 to perform isolation with which the overall system
4 Advances in Mechanical Engineering

Figure 5. DC motor speed control using H-Bridge.

performs normally even during a fault however, the deviation of speed error. The comparison of PI and
overall output voltage level decreases with the introduc- Fuzzy is presented in this paper and the simulation
tion of a fault. results are used to observe the behavior of both. The
In Ref.8 author analyzes the fault-tolerant dual mul- study13 proposed consistent fault to diagnosis and
tiphase motor control system. The series-connected fault-tolerant control strategy of sensor faults in the
phenomena were proposed by the author and a stan- cascaded H-bridge rectifier. The presented work can
dard H-bridge topology. The work reported in Ref.9 achieve the exact position of the faulty sensor in less
presented cascaded H-bridge inverter for large scale time. The studies14,15 show the dual hardware redun-
motor driving. The fault-tolerant control is proposed dancy for engine sensors application in which the pri-
for equalization of power that means when the imbal- mary component performs all the normal operations
ance is introduced in power modules, they can operate until it fails and then back-up components come into
normally even in case of failure. In Ref.10 authors com- operation to perform the tasks. With this approach, a
pare the series and H-bridge methods for fault-tolerant single point of failure can be eliminated due to a single
of motor drives. The aerospace applications use H- sensor or actuator and the overall failure of the system
bridge because it lowers the weight and cost of the can be avoided that would have been caused by a single
system. component.
The work by Jaladi Suresh and Shaik Shareef11 The previous work addresses the H-bridge for con-
focuses to improve the fault-tolerant control perfor- trolling the motor’s speed, but the elimination of faults
mance of the cascaded H-bridge inverter by lowering and maintaining them was not quiet introduced because
the CM voltage. For this purpose, first, the algorithm of the complexity of the method. Fault-tolerance is an
is built to find the best fault with maximum voltage important area of research in the current period because
and then the modified technique is presented to calcu- one can overcome the huge damage. If there is a model
late the voltages under faults. The effectiveness of the present, which makes the system work even in case of
work compares with the old ones in different scenarios. faults can save from a disaster. While there has been a
In Ref.12 the authors proposed a dual-channel switched lot of research conducted in the fault tolerance domain,
reluctance motor. This technique is composed of two however, no work has been done related to fault toler-
models. The first model operates like 3-phase SRM is ance in H-bridge DC motor speed control up to over
abounding by H-bridge inverters. The second model is best knowledge.
composed of coupled SRMs that are providing current. A fault-tolerant strategy is required that can identify
Both used to eliminate the faults and realize the fault- and protect the power switches from becoming faulty
tolerant operations. The simulation results show the and saving the complete system shutdown. The pre-
advantages of SRM and couples SRMs. In paper11 the sented fault protection strategy of our system will be
fault-tolerant control based on space vector Pulse able to diagnose the fault by detecting the faulty
Width Modulation (PWM) is presented by maintaining switches. The faulty switches will be detected by the
the magnetomotive force of motor rotating and fixed presented solution and will be able to eliminate the
and fault-tolerant based on the current hysteresis band effect of the particular switch with the help of the
pulse width modulation is to keep the torque same bypassing technique. The secondary component will
before and after the fault. Both strategies are verified take over to avoid the severe damage and it will allow
by the experiment. the normal operation to continue.
The paper12 presented a fuzzy logic control for the The existing model does not have the feature to
effective performance of induction motor drive. The avoid faults on its own therefore fault-tolerant strate-
fuzzy logic consists of two inputs speed error and gies can be utilized to convert the existing model to a
Ahmed et al. 5

more robust and reliable one. In this paper, our contri- The fault-tolerant approach to the H-bridge is dual
bution is to add the dual hardware redundancy-based hardware redundancy that will increase the reliability
fault-tolerant control system that will be able to rectify of the system. In dual hardware redundancy, there is a
the overall failure problem of the system when a pair primary and backup hardware component. The pri-
or multiple switches fail during the operation. The mary part performs all the normal operations until it
approach used will be able to detect the faulty switches, fails and backs up components that take the charge to
provide an alternative path in case of failure of the perform the normal tasks. With this approach, a single
switch by bypassing the faulty switches, and ensure the point of failure is eliminated, in which the overall fail-
protection of the power switches. ure of the system can be avoided that would have been
The presented fault protection strategy of our sys- caused by a single component. In fault-tolerant H-
tem has not been implemented before in H-bridge for bridge, eight transistors will be utilized in which four
the speed control of the DC motor from the best of our transistors will be acting as primary while the remain-
knowledge. The solution presented will be able to diag- ing four transistors will be used as backup hardware
nose the complete fault by detecting the defective components. During the normal operation, the primary
switches and eliminating its effect to improve the over- four transistors will be working two transistors at a
all performance. time to control the speed of a DC motor. However, if
Further contents of the paper are organized as fol- any, of the primary switches face a fault the redundant
lows. Section 2 presents the methodology of the system. hardware will take over to continue the normal opera-
Section 3 contains the important results and discussion tion to provide the output to control the speed of a DC
of the paper’s findings. In the end, section 4 contains motor.
the concluding remarks and possible future directions. The flowchart of the fault-tolerant system is shown
in Figure 6. The system is initialized with assumed para-
meters before simulation. The H-bridge consists of four
Research methodology transistors that work in a pair of two groups. First tran-
sistors 1 & 4 will be operated simultaneously that will
The fault-tolerant H-bridge for controlling the speed of provide a positive voltage to the DC motor causing it to
a DC motor is implemented in the MATLAB and rotate in a clockwise direction afterward transistors 2 &
Simulink platform. The IGBT based H-bridge model 3 will be operated simultaneously that will provide a
available in the MATLAB and Simulink environment negative voltage to the DC motor causing it to rotate in
is utilized as a base model for the speed control of DC a counter-clockwise direction. During the operation,
motor. For the implementation of fault tolerance, a the transistors will be checked to see if anyone of them
dual hardware redundancy approach is proposed and is faulty. If so, the backup transistors will be utilized
integrated with the MATLAB provided model.16 instead of the primary transistors to provide the vol-
In the H-bridge model, four IGBTs are utilized as a tages to the DC motor.
switch because of their good properties in power The block diagram of the overall system is shown in
switching applications. During the operation of H- Figure 7. The supply voltages are provided to both the
bridge, two transistors are operated at a time by firing H-bridge section and backup components. The primary
a voltage pulse on the gate that results in a positive vol- section H-bridge is working in a normal condition. The
tage and rotation of the motor in one direction while Fault Detection and Isolation (FDI) block make sure
when the remaining two transistors are operated a neg- that the DC motor receives the right voltages. If during
ative voltage appears on the motor causing it to rotate the normal operation a fault is detected, the fault detec-
in another direction. The pair of two freewheeling tor and isolator block will eliminate the faulty transistor
diodes are operated during the operation to bypass the by the backup transistor to ensure the normal operation
two counter-transistors from the opposite polarity.17 of the load.
When an IGBT is operated a forward voltage VF The assumptions of the work include zero time in
drop is developed across the collector-emitter VCE junc- the switching action of H-bridge IGBTs. The limita-
tion typically of 1 V in the range. A similar property is tions of the work include the increase in the physical
available in the MATLAB and Simulink model. For size, weight, complexity, and cost of the overall system
the simulation, the block available in this model does due to hardware redundancy. The proposed solution
not require the gate current control signal, however, it will fail if both redundant switches simultaneously fail
utilizes Simulink signal (1/0). The model for the DC and will cause the overall shutdown of the system.
motor has the predefined specifications (24V 5HP Moreover, ideal switches are considered in which they
1750 rpm) and a load similar to a fan is utilized by the have zero resistance in the closed state and infinite
Simulink model. When the duty cycle is changed from resistance in the open state. However, these posses a
0 to 100% the mean armature voltages are varied small resistance in the closed state and high finite resis-
between 0 to 240 V. tance in the open state.
6 Advances in Mechanical Engineering

Figure 6. Flowchart of dual redundant fault-tolerant H-bridge.

Figure 7. Block diagram of dual redundant fault-tolerant H-bridge.

Results and discussion H-bridge switches

To control the rpm of the DC motor, the power elec- Four IGBT switches with freewheeling diodes are uti-
tronic system in MATLAB and Simulink environment lized in the construction of this model. The IGBTs have
is implemented by utilizing the available model pro- a forward voltage drop VF of 1 volt while diodes have
vided in Figure 8. Further changes are introduced in a forward voltage of 0.8 volts. During the operation,
this model to achieve the desired outcome. To under- two pairs of IGBTs are switched simultaneously by the
stand this model, it is divided into four parts. control circuitry having the sequence as follows Q1 &
Ahmed et al. 7

Figure 8. Speed control of a DC motor using H-bridge model implemented in MATLAB.

Q4 or Q2 & Q3. To supply the positive voltages to the Permanent magnet DC motor
DC motor to rotate it in the forward direction, Q1 and The DC motor used in the model is a preset model of
Q4 are supplied with the firing angles, and D2 & D3 permanent magnet DC motor having 5 HP 240 V
work as a freewheeling diode during the operation. 1750 rpm as given in Table 1. The motor simulates the
Moreover, to supply the negative supply voltages to the fan equivalent load type in which the square of the
DC motor to rotate it in the reverse direction, Q2 and speed is proportional to the load torque. Moreover, the
Q3 are supplied with the firing angles, and D1 & D4 mean armature voltages can be supplied from 0 to
act as a freewheeling diode. 240 V while the duty cycle is changed from 0 to 100%.
All of these settings are initialized in the pulse genera-
tor block in the Simulink model.
Control circuitry for switches
The IGBTs require a control gate signal to operate
therefore the gate is operated by providing a Simulink Measurement units
signal (1/0). The gate is turned on when 1 is applied Two scopes are utilized for measurement purposes.
while it is off when 0 is applied. The variation in the Scope 1 provides the information of the motor’s rpm,
duty cycle results in variation in the rpm of the DC armature current, and load torque while scope 2 pro-
motor. vides the information of IGBT and diode current.
8 Advances in Mechanical Engineering

Table 1. Rated motor parameters. reaches 0.5 s the voltages at armature suddenly reversed
to vary the direction of dc motor in an opposite side
Parameters Value that switches the rpm from + 1750 to –1750. Similarly,
Motor voltage 240 V the voltages across the motor also change its polarity
Motor current 71 A from + 240 V to –240 V.
Motor speed 1750 RPM During the operation, the current and voltage will be
Wound field 300 V passing through IGBTs illustrates the phenomenon as
Horse power 5 HP
presented in Figures 11 and 12.
The existing model does not have the feature to
avoid faults on its own, therefore, fault-tolerant strate-
gies can be utilized to convert the existing model to a
more robust and reliable one. Our contribution in this
model will be to add the hardware redundancy-based
fault-tolerant system that will be able to rectify the
overall failure issue of the system when a single switch
fails during the operation.
The proposed redundant, fault-tolerant system is
implemented on the H-bridge for reliable speed control
of a DC motor in Simulink is shown in Figure 13. The
FDI is implemented in this model for fault detection
and isolation shown in Figure 14. FDI consists of four
Figure 9. Motor voltages at 100% duty cycle. different inputs for each IGBT and produces two out-
puts for each pair of IGBT and redundant IGBT in the
H-bridge circuit.
The model is simulated by setting the duty cycle to In a normal scenario, when gate signals are applied
100% that provides 240 V to the motor and rotates it to each IGBT, they perform switching properly.
in a positive direction at max rpm of 1750 as shown in However, when an IGBT becomes faulty the behavior
Figures 9 and 10. Initially, the motor draws a high of the system completely deviates from its normal
inrush current when the motor is trying to attain the operation.
maximum speed but after some time its current starts In the presented model, the Fault Injection Unit
to decrease until it becomes constant because now the (FDI) is implemented to inject faults in any switch. The
speed of the motor also becomes constant. fault in a switch is represented by fail (0) while normal
To operate the motor in reverse direction armature (1) shows a healthy switch, as shown in Figure 15.
voltage control module can be utilized in which by Inside the FDI unit, there are four-fault detector blocks
varying the step size of the signal the direction can be whose function is to detect the fault in each IGBT
altered that is if step time is set to 0.5 s, as the time switch. Moreover, an average voltage function for

Figure 10. Motor speed and armature current at 100% duty cycle.
Ahmed et al. 9

When a fault is injected in a switch, the FDI unit by

using the fault detector signal detects the unusual beha-
vior of the switch and immediately isolates the faulty
switch by turning off its gate signal. At the same
moment, the redundant IGBT switch is providing the
gate signal to overtake the working of the faulty switch
and maintain the overall functionality of the system.
Figure 11. IGBT 1 voltage at 100% duty cycle.
The fault detector unit inside the FDI unit consists
of two signals, first is the constant zero signal block that
is representing that the redundant IGBT is turned off
while the primary IGBT is working correctly. Second is
the healthy signal that is used to turn on the primary
IGBT as shown in Figure 17. As the fault is injected the
fault control signal actuates the switch and isolates the
faulty switch by providing no gate signal to it while at
the same moment it applies the gate signal to redundant
IGBT.
The duty cycle is set to 100% and a fault is injected
by operating the manual switch as shown in Figure 18.
Figure 12. IGBT 1 current at 100% duty cycle. The voltages across the motor drop, but within a few
milliseconds the redundant IGBT takes over to prevent
the failure of the system.
calculating the average voltages across the motor is Similarly, the motor running at maximum rpm of
implemented as shown in Figure 16. 1750 deviates from its set point, but after a delay, it

Figure 13. Proposed dual redundant fault-tolerant H-bridge system.

10 Advances in Mechanical Engineering

Figure 14. Fault detection and isolation unit.

Figure 15. Dashboard of fault injection unit.

Ahmed et al. 11

Figure 16. Internal architecture of FDI unit.

regains its speed as shown in Figure 19. The high

inrush armature current of the motor also drops to zero
during the fault period but regains it when the redun-
dant IGBT takes over the functionality of the primary
IGBT.
The variations in voltage due to the fault across the
IGBT 1 redundant assembly are expressed in Figure 20.
The voltage of both redundant switches will be the
same, therefore, are shown in a single graph. The FDI
unit will be isolated the defect IGBT and replace it with
the redundant IGBT by supplying it with the gate
signal.
The current passing through the primary IGBT
becomes zero as the fault occurs as shown in Figure 21.
The FDI unit isolates the faulty IGBT and replaces it
Figure 17. Fault detector.

Figure 18. Motor voltage during fault.

12 Advances in Mechanical Engineering

Figure 19. Motor speed and armature current during fault.

Figure 20. Redundant IGBT 1 assembly voltage during fault.

Figure 21. Primary IGBT 1 current during fault.

with the redundant IGBT by supplying it with the gate current falls to zero, however, after a small delay the
signal as shown in Figure 22. FDI unit successfully detects the erroneous switch, iso-
The overall changes in the behavior of the current of lates, and replaces it with redundant IGBT and the sys-
redundant IGBT 1 assembly during fault are shown in tem resumes its normal performance.
Figure 23. In the normal operation, the motor draws a It can be seen from the simulation results that the
high inrush current but equally the fault is injected the proposed model with dual redundancy for actuators is
Ahmed et al. 13

Figure 22. Redundant IGBT 1 current during fault.

Figure 23. Redundant IGBT 1 assembly current during fault.

robust to faults and it can maintain the functionality of the speed control of a DC motor as it has given the
the system and avoid the single point of failure if any or ability to prevent the failure of the system.
all primary switches fail during the operation, therefore, Future work may include implementation of Triple
the presented fault-tolerant strategy is the best solution Modular redundancy (TMR) in H-bridge switches, pas-
for reliable speed control of DC motor. Moreover, such sive, and active fault-tolerant techniques with reliability
work is novel and not found in the literature so far, analysis. The TMR will further increase the reliability
therefore, we were not able to compare our results with of the system than dual redundancy but the cost of the
existing works. system will increase. However, in mission-critical appli-
cations, where reliability is more important than
cost, TMR can be implemented instead of dual
Conclusions redundancy.
This paper presented a dual redundancy-based fault-
tolerant system using MATLAB/Simulink environment Acknowledgement
that can detect, isolate, and substitute the faulty semi- The authors would like to thank to colleagues for suggestions
conductor switch to maintain the performance of the to improve the paper quality.
permanent magnet DC motor. The FDI unit was
implemented to detect the fault during the operation Declaration of conflicting interests
and remove it to prevent the single point failure. The
The author(s) declared no potential conflicts of interest with
simulation results showed that during a faulty condi-
respect to the research, authorship, and/or publication of this
tion the motor voltage, rpm, armature current, and article.
IGBT current deviate from their normal operation, but
as the redundant component takes over the parameters
become stable to prevent the failure of the overall sys- Funding
tem. The study showed that the dual redundancy-based The author(s) received no financial support for the research,
fault-tolerant H-bridge is a very reliable solution for authorship, and/or publication of this article.
14 Advances in Mechanical Engineering

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