Compound Control of Electromagnetic Linear Actuator Based on Fuzzy Switching

TELKOMNIKA, Vol.15, No.1, March 2017, pp. 273~282
ISSN: 1693-6930, accredited A by DIKTI, Decree No: 58/DIKTI/Kep/2013
DOI: 10.12928/TELKOMNIKA.v15i1.4685  273
Received September 5, 2016; Revised November 19, 2016; Accepted December 11, 2016
Compound Control of Electromagnetic Linear Actuator
Based on Fuzzy Switching
Zhu Jianhui, Wang Cheng*, Dai Jianguo
School of Traffic Engineering, Huaiyin Institute of Technology, Huai’an 223003, Jiangsu, China
*Corresponding author, e-mail: wangcheng@hyit.edu.cn
Abstract
Due to the motion control system of electromagnetic linear actuator (EMLA) is a nonlinear system
with poor controllability; single control strategy has been difficult to meet the requirements of its control. A
compound control strategy based on inverse system control (ISC) and proportional-integral (PI) is
designed in this paper. Switching between two algorithms, which is based on the fuzzy rules, prevents the
control algorithm to jitter and jump. System model is built under Matlab/Simulink to do simulation analysis.
The designed controller is integrated into the system simulation model and the system software of digital
signal processor (DSP) controller. Simulation and test results show that the compound control strategy
using fuzzy switching rules achieves the smooth transition of two control algorithms, and the goal of any
position location, and continuous adjustment in 0~4mm lift. Positioning accuracy is up to ± 0.02mm, while
the response time is less than 10ms.
Keywords: electromagnetic linear actuator, fuzzy switching, inverse system, PI
Copyright © 2017 Universitas Ahmad Dahlan. All rights reserved.
1. Introduction
With linear position servo’s characteristics of high performance, high response and high
precision, EMLA has been widely used in many fields, such as direct drive type servo valve [1],
electromagnetic gas distribution mechanism [2], and automatic transmission (AMT) etc. While
get applied at the same time, it has also put forward higher requirement to its control strategy.
On the one hand, it can meet the performance index of control accuracy as far as possible in
the steady state response; On the other hand, response should be fast enough to shorten the
adjust time of reaching the target position as far as possible in the transient response.
Regarding motion control requirement of high performance, In literature [3], Liu and
Chang uses inverse system control algorithm, and also applies to electromagnetic valve
actuator. It obtains a high response but also has a good seating control at the same time. In
order to improve its anti-interference ability and system robustness, An auto disturbance
rejection control algorithm is used which based on extended state observer in [4]. In addition,
the fuzzy control of direct torque control of induction motor studied in depth [5]. It has
outstanding contribution to the improvement and progress of system performance. Using a
single control strategy has a good control performance on the one hand, but there is still room
for improvement in adaptability [6]. But the compound control strategy is adopted, when
switching the algorithm, due to the selection of switching position, it will cause system jitter, also
easily causes instability. But if using fuzzy rule to switch of compound algorithm, it can achieve
a smooth transition between different algorithms. So multi model control has attracted much
attention. Issam Salhi [7] and M.Kalyan Chakravarthi [8] etc. conduct deep research on multi
model PI controller. Baozhu Jia has come up with a fuzzy switched PID (FS-PID), which
combines the advantages of fuzzy control and PID control [9]. Fuzzy PID is used to increase the
transient response speed, while traditional PID to improve the tracking error of steady state.
Ahmed Rubaai applies the fuzzy switch position controller to the traditional bang-bang control
[10] which improves the control precision of system, especially in the case of interference, which
can still be able to ensure superior control performance.
In this paper, the advantages of two control strategies are combined, and a compound
control strategy based on inverse system control and PI control is proposed. Based on the fuzzy
switching rules, it achieves the smooth switching and complementary advantages of the two

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TELKOMNIKA Vol. 15, No. 1, March 2017 : 273 – 282
274
controllers. The designed controller is applied to the motion control system of EMLA, in order to
meet the performance requirements of system response and accuracy.
2. System Scheme and Principle
EMLA is a kind of element which can directly convert electrical energy into mechanical
energy of linear motion form without the need of any intermediate conversion device, also called
"linear servo motor". It can be used as the executive component of all kinds of automatic control
system, and has a characteristic of compact structure, high power density, large thrust, and high
control precision etc.
The structure of motion control system for EMLA is shown in Figure 1. It is mainly
composed of system controller, system actuator, system sensor, and display part of system
output. The controller uses high performance controller of TMS320F2812. System sensor
includes current sensor and displacement sensor. And the display part of output mainly contains
host computer PC and related software.
PWM
ADC
DSP—LAN
DSP
TMS320F2812
Control Board
Power Driver
Current Sensor
Displacement Sensor
PC
System Sensor
System
Controller
Actuator
System Output and
Display
EMLA
Output
Figure 1. Structure of Motion Control System of EMLA
Its working principle is system controller mainly completes signal acquisition and control
signal output of each sensor, and also communication of host computer at the same time.
Through the corresponding software design, it achieves a variety of control algorithms functions.
The small power signal output by controller is amplified by power drive module, and controls
high power signal. EMLA, which used as a controlled object of system, can realize the double
closed loop control lift through control of current and the position of actuator. The host computer
part completes output and display, and also storage and processing of final data output.
ISC
PI Control
ISC
Hold Open Hold Close Hold
Target Xr1
Closed
Position Xr
Switching
Position xr
Switching
Position
xr1
0 Open Time
T0
Close Time
T1
Next Cycle
PI Control
Figure 2. The Lift Control of EMLA

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275
In order to realize the compound control of EMLA, the lift control of EMLA is divided into
three stages: opening, closing and keeping. Control algorithm has two kinds: inverse system
control and PI control which is shown as Figure 2.
Upon receipt of the opening instruction, firstly EMLA cancels holding force, and enters
opening stage from holding position, till up to the specified target lift, and keeps this position
after reaching the scheduled target. Upon receipt of the closed command, it executes shut down
procedure, enters closing phase, and remains closed after fully closed until the next work cycle
is turned on.
In opening phase, before the target lift smaller than Xr1, it uses the control parameters
of inverse system. When it is bigger than switch position xr1 until reaching the target lift Xr1, PI
control is used. In closing phase, upon the receipt of closing instruction, and before reaching the
switch position xr, the inverse system control is used. When it is smaller than the switch position
xr, but reaching the closed position Xr, PI control is used. Using fuzzy switching rules to
automatically realize switching between position Xr1 and xr.
3. System Model and Compound Controller Design
3.1. System Mathematical Model
The motion control system for EMLA includes circuit subsystem, magnetic field
subsystem, and mechanical subsystem. Coupling relationship between different parts is shown
as Figure 3. When power is supplied to circuit subsystem from power system, subsystem
outputs current to magnetic field, an electromagnetic force generated by the interaction between
current and magnetic field is transferred to mechanical subsystem. Drive actuator straight part
overcomes external resistance to generate motion, and make motion speed transfer to circuit
subsystem for the calculation of back EMF. Finally, the control of actuator lift is realized.
Circuit Subsystem
（R、L）
Magnetic Subsystem
(Φ(t))
Mechanical Subsystem
(m、k)
i(t) Fm X(t)
dx(t)/dtE(emf)
e(t)
Electric Signal
Figure 3. Each Subsystem Coupling Between the Control System
Circuit subsystem converts the input voltage of EMLA into current. It can be equivalent
to a series connection of resistors and inductors.
in
di
U = Ri+ L +e
dt
(1)
In the equation, the counter electromotive force E can be expressed as
ee= BlNv= K v , inU is supply voltage, i is the current through coil, R and L is the coil
resistance and inductance respectively, e is the back EMF generated by cutting magnetic lines
when coil moves in the air gap field, eK is the constant of counter electromotive force, v is the
coil velocity.
Magnetic subsystem is the interaction between current and magnetic field generated by
the circuit subsystem, and generates electromagnetic force, and then realizes the linear motion
of moving parts. So the driving force of movement comes from the Lorentz force of self loading
coil in the air gap magnetic field. According to the Lorentz force of electric coil in magnetic field,
the electromagnetic force equation of EMLA is:
m b mF = k B lNi= k i (2)

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276
In the equation, bk is structure coefficient, B is coil magnetic induction strength, l is
effective length per coil magnetic field, N is the number of coil turns, m bk =k B lN is the
constant of EMLA driving force.
EMLA has to overcome friction and inertia force during runtime. Inertia force is the
product of quality and acceleration of EMLA moving parts, in addition to load force here.
Therefore, the force balance equation of EMLA can be expressed as:
2
m v L2
d x dx
F = m +k + F
dt dt
(3)
In the equation, m is the quality of EMLA coil assembly, x is mover displacement, vk is
damping force coefficients of moving parts in a magnetic field, LF is load external disturbance
force.
Complex equation of (1), (2)and (3), the motion equations of EMLA is:
2
2
in
m b m
m v c L
di
U Ri L e
dt
F k B lNi k i
d x dx
F m k k x F
dt dt


  

 

    

(4)
3.2. Inverse System Design
The inverse system achieves transformation from nonlinear system to linear system by
means of a method of nonlinear feedback or dynamic compensation. By the definition of inverse
system, suppose nonlinear system is
1
 , the initial state of the system is 0 0( )x t x , the
operator that maps the system
1
 from the input to the output is 

, and  is values and
arbitrary continuous functions in a domain.  maps to u , that is
( )
y 
  , it is expressed as:
( )
y u y 
    
 
   (5)
Then
1
 is called  rank inverse system,  is original system. The inverse system
and original system is connected in series to realize decoupling process of nonlinear system.
System after reconstruction is called pseudo-linear system, and it can be controlled by the
theory of linear system. The input variables of control system for EMLA is voltage u and the
location for output is S , according to system voltage balance equation and differential
equations of motion, it takes the derivative of output, makes it explicitly contain voltage u , and
finally the expression of u can be induced as follows:
''' '' '
m
m m
mL BL
u S S k S IR
k k
    (6)
3.3. PI Controller Design
The essence of PI control algorithm is that it eliminates error on the basis of error, and
the corresponding of calculated output of incremental PI is the incremental position, but not the
actual position. So when the controller fails, position type PI makes a substantial change of
control amount thus causes a substantial change in actuator position. But due to incremental is
to control increment, it could still stay in the same position, and does not seriously affect system
operation. Incremental PI could be expressed as:

277
0 0
( ) ( ) ( 1)
( ( ) ( )) ( ( 1) ( ))
[ ( ) ( 1)] ( ( ))
k k
j j
U k U k U k
T T
Kp e k e j Kp e k e j
Ti Ti
Kp e k e k Ki e k
 
   
    
   
  (7)
In the equation, is sample number, is scaling factor, is integral coefficient,
Δ ( )is output control increment at sampling time, ( ) and ( −1) is output control increment
at sampling time of and −1, ( ) and ( − 1) is error value at sampling time of and -1.
Algorithm flow is shown in Figure 4 as follows:
Start
IS IT The Sampling
Time?
Calculate the Control Parameters Kp、Ki
Initial Values e(k)=e(k-1)=0
End
Sensor Sample Value x(k)
Calculate Deviation e(k)=r(k)-x(k)
Calculating a Control AmountΔU(k)
ΔU(k)=Kp[e(k)-e(k-1)]+ki(e(k))
OutputΔU(k)
Preparing for The Next Sampling Time
e(k-1)=e(k),e(k-2)=e(k-1)
Is it time to switch
position?
Y
N
Y
N
Inverse System Control
Figure 4. Flow Chart of Control Algorithm
3.4. Switching of Compound Control Algorithm
The motion control of EMLA uses two compound controls of inverse system control and
PI control, and the core of complex control is smooth switching between two methods. There
are generally two ways of switching. The first one is to set a threshold value in advance, and
automatic switching by the program. However, when using this switching mode, on the one
hand it is difficult to choose the right switch value, and also difficult to solve the contradiction
between system rapidity response and overshoot, on the other hand, it is difficult to guarantee
switching without disturbance from one control method to the other one, and thereby extends
system regulation time. The second one is that the switching mode based on fuzzy rules. The
first feature of this method is the control amount acting on controlled object is weighted mixed
output of two kinds of controllers, and combines the advantages of both controllers. Secondly, in
the control process, it can realize auto-adjust of two controls’ weight. Fuzzy switching principle
is shown as Figure 5(a), e is input and u is output. Membership function is shown as Figure
5(b). Fuzzy switching rule is as follows:
If e is “z(e)” then “u” is “uIS” ELSE “u” is “uPI ”
In the rule, uIS is input of inverse system control, and uPI is output of PI control, z(e) is
membership function under fuzzy switching rule. By changing the value of a, it obtains different
control intensity component.

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Inverse System
Controller
+
-
yf
X(t) Y(t)
Incremental PI
Controller
Fuzzy
Switching
E
M
L
A
e
uPI
uIS
u
-a a e
z
1
0
(a) (b)
Figure 5. Fuzzy Switching Rules
The value of a in Figure 5(b) is set according to system requirements, in order to get
different control intensity component. Its control algorithm is as follows:
= ( )PI z e (8)
1 ( )IS z e   (9)
PI PI IS IS
PI PI IS IS
PI IS
u u
u u u
 
 
 

  

(10)
In the equation, PI is output intensity factor of PI controller at the input bias of e, and
IS is output intensity factor of inverse system at the input bias of e, and uPI is output of PI
controller, uIS is output of inverse system control.
From the above analysis, only when system enters steady-state response, and error is
very small, PI controller plays a major role at this time. In the transient stage, the controller of
inverse system control plays a major role. Therefore compound controller keeps advantages of
PI and the traditional inverse system controller. It achieves a smooth transition between the two
control methods, and avoids the weakness of general complex control.
4. Simulation and Test
In order to verify the feasibility of algorithm, simulation model under Matlab/Simulink is
build, which makes a connection of the above obtained inverse system, PI, state observer, and
state controller. The block diagram of simulation model is shown as Figure 6, while given the
target position, it uses inverse system control at initial time. When approaching the target
position and meeting fuzzy switching conditions, it loads into the output UPI of PI control, and
forms control amount to EMLA. Through control algorithm of PI, and according to error size, it
constantly automatically adjusts volume control, and ultimately achieves a stable output.
Target
Position r IS
Controller
Incremental
PI Control
EMLA
Control Amount U
+ Output y
State Observer
State Feedback
Controller
.
.
.
.
.
I
Iˆyˆy
ˆy
ˆy
-
yf
yd
.
y
y
r
Fuzzy
Switching
UIS
UPI
Figure 6. Block Diagram of Controller Structure
4.1. Simulation Results
Through adding the designed controller into system simulation model and control
program of DSP, it could simulate and verify the effectiveness of control algorithms. The
simulation parameter settings are shown in the table. To verify the design requirements of valve

279
lift in any position, it makes divide at the setting lift of 0~4mm, and the initial process is divided
into 8 lifts, 0.5mm as a section. And then verify the smaller lift, such as 16 sections of lifts. Test
results only gives the simulation and test results of 8 lifts process. Other lifts can also achieve
this effect. It could continuously adjust up to 0~4mm lift, and adjustable program is not limited to
the illustrated figure, which is not to repeat here.
Table 1. Simulation Parameters
Parameter Value
Coil Resistance/Ω 4
Coil Inductance/mH 1.6
Moving Mass/Kg 0.036
Actuator Force Constant/(N/A) 11.3
Counter Electromotive Force Constant/(V/m/s) 11.3
Figure 7 are given simulation and experimental results in continuous lift. There is 8
setting positions, that is, 4.0mm, 3.5mm, 3.0mm, 2.5mm, 2.0mm, 1.5mm, 1.0mm, and 0.5mm.
The period of each work cycle is 400ms, and it keeps 200ms after opening, then it is closed.
0 200 400 600 800 1000 1200 1400
0
1
2
3
4
Lift/mm
Time/ms
4.0mm Simulation
4.0mm Experiment
3.5mm Simulation
3.5mm Experiment
3.0mm Simulation
3.0mm Experiment
2.5mm Simulation
2.5mm Experiment
2.0mm Simulation
2.0mm Experiment
1.5mm Simulation
1.5mm Experiment
1.0mm Simulation
1.0mm Experiment
0.5mm Simulation
0.5mm Experiment
Figure 7. Simulation and Experimental Results under Continuous Lift Process
As can be seen from Figure 7, in different target locations, experiment results and
simulation have a good consistency. It verifies algorithm’s suitability under the change of lift.
When the target lifts change, there is no need to re-change control parameters. As can be seen
from the results of comparison, and test results are shown that, the rise time of reaching steady
state is slower than simulation. In the actual control, in order to prevent overshoot, over the
target position, and bump after close, it has to reduce the scaling factor, and exchanges the
accuracy of steady state for sacrificing settling time. Experimental error statistics are shown in
Table 2. Results can be drawn that the same set of parameters can achieve different lift, and
steady-state error is in the range of ± 0.02mm. The feasibility of algorithm is verified. When the
switching position selects fuzzy switching, using complex control strategy of PI and inverse
system, compared with using only a single inverse system control method, it can automatically
adjust error at the steady state until steady-state error meets performance requirements.
To further verify the effectiveness of algorithm for linear electromagnetic actuator at
speed running state, the setting target lift is 4mm, and the setting cycle time is 40ms, simulation
and experiment comparison is shown in Figure 8.

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Talble 2. Error under Different Lifts
Lift
/mm
Maximum Value
/mm
Minimum Value
/mm
Error Range
/mm
Average of Sampling
Point /mm
0.5 0.52 0.49 -0.01-0.02 0.007
1.0 1.02 1.00 0-0.02 0.009
1.5 1.52 1.50 -0.01-0.02 0.0095
2.0 2.02 1.99 -0.01-0.02 0.003
2.5 2.51 2.49 -0.02-0.01 0.005
3.0 3.01 2.98 -0.02-0.01 -0.004
3.5 3.53 3.48 -0.02-0.02 -0.002
4.0 4.00 3.97 0-0.02 -0.01
0 10 20 30 40
-5
-4
-3
-2
-1
0
1
2
3
4
5
Simulation Lift
Experiment Lift
Simuliation Current
Experiment Current
Time/ms
Lift/mm
-10
-5
0
5
10
15
Current/A
Figure 8. The Position and Current of 4mm lift
As can be seen from Figure 8, in the high-speed running state, when using compound
control strategy, system can quickly reach the target position, response time is 3.7ms, test
matches simulation results.
4.2. Test Comparison under Different Control Strategies
In order to verify the validity of the designed control strategy, experimental designs
three control strategies for control effect of a lift. Target lift is set as 3mm, and control algorithm
uses inverse system, inverse system plus PI, and inverse system plus PI plus fuzz switching.
The test results are shown in Figure 9.
0 5 10 15 20 25 30 35 40
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
10
2.5
3.0
3.5
30 35 40
-0.1
0.0
0.1
0.2
Lift/mm
Time/ms
IS+PI
IS+PI+Fuzzy Switching
IS
Figure 9. Comparison of Different Control Strategies

281
It can be seen from the effect of these three control strategies, using single inverse
system control strategy, when parameter is not through repeated manual adjustment, but only in
accordance with fixed parameters, its output result of control exceeds the target lift, which is
because each parameter control selects other lift values, it could lead control result beyond
target position. But after increasing PI compensation, the control strategy based on erro of PI
could stabilize the final steady state value at target lift. As long as there is an error, it plays the
role. So PI can increase the accuracy of control, but the system is prone to overshoot. Through
dispatch the fuzz switching, it can achieve the target lift at steady state, but also can prevent
overshoot and oscillation.
From the control effect point of view, only using the control scheme of inverse system
control, response time is smaller than the other two control strategies. This is the advantage of
inverse system control, but the steady state accuracy is worse than the other two schemes. And
after increasing of PI, in order to increase system adjusting time, error must continue to reduce,
so it can achieve the steady-state accuracy of requirements at the expense of sacrificing
response time.
5. Conclusion
A combined control scheme of inverse system and PI is designed in this paper. It
combines fast response of inverse system control with precision advantages of PI control, and
realizes the smooth switching between two control algorithms based on the fuzzy switching rule.
It achieves the positioning goal of EMLA at any position in the lift of 0~4mm. Comparison of
three control schemes, inverse system, PI plus inverse, and fuzzy switching plus inverse system
shows that response time is fastest, and steady state error is also the largest by using single
control strategy of inverse system, but while using control method of inverse plus PI plus fuzzy
switching, it is better than the other two, especially in the control of steady state error. It could
reach ± 0.02mm. Although response time is slightly slow, it can still be able to meet the control
target less than 10ms.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant
No. 51505171), Natural Science Foundation of Colleges in Jiangsu Province (Grant No.
15KJD460003), and Science and Technology Support Programs of Huai’an (Grant No.
HAG2014026).
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Compound Control of Electromagnetic Linear Actuator Based on Fuzzy Switching

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