PIERS Proceedings, Hangzhou, China, March 24-28, 2008
536
Research on a Maglev Ball Control System Based on DSP2812
Q. H. Guo, D. S. Li, Z. Y. Lu, W. Wang, and L. Z. Ye
College of Mechanical Engineering and Applied Electronics Technology
Beijing University of Technology, Beijing 100022, China
Abstract— As the basis of maglev technology, a single-freedom suspension ball control system
with hybrid electromagnets is introduced in this paper. According to the physical model of the
system we designed, the linear model and control strategy were established and analyzed. A fixedpoint digital signal processor TMS320F2812 is chosen as the controller of the system for its high
speed and high efficiency of calculation and control. A/D module and the EventManager(EV)
module are used in the system. PID algorithm that is applied frequently in industry is adopted
and DSP’s output signal—PWM signal is used to control the driver. Both the simulation by
MATLAB and actual control results show the magnet suspension system works well.
1. INTRODUCTION
The analysis and research of single Maglev(Magnetic Levitation) control system is the basis of
analyzing and controlling of the maglev systems such as maglev train, maglev bearing, etc.
The suspension systems can use passive, semi-active or active [1]. However, the passive suspension systems can not assure the desired performance from a modern suspension system and
the active systems are complex and expensive and are relatively high energy consumption. In this
paper, it proposes an hybrid suspension system which can realize the function and overcome the
defects of passive and active suspension systems.
As for the choice of controller, there are many methods [2] such as analog control system, the
singlechip controller, DSP control system, etc. Considering the analog controller has a disadvantage
that the hardware can not be easily changed and the parameter can’t be adjusted and the processing
speed of single chip controller often does not meet the needs of system, this paper introduces a
method which uses DSP instead of the traditional analog controller and singlechip to realize digital
control.
2. SUSPENSION SYSTEMS DESCRIPTION
The maglev ball control system is the platform of the research of maglev system. It contains maglev
ball, permanent magnet, electromagnet, electric eddy current displacement sensor, controller and
drive circuit, as Figure 1 shows. It uses DSP2812 as the controller and feedback regulation to
maintain the ball at a fixed distance from the electromagnet. To be specific, according to the offset
the sensor detected, the controller adjusts the duty-cycle of PWM output signal and further the
current in the electromagnet. When the ball gets too close to the electromagnet, the current in the
electromagnet will decrease, allowing the object to begin to fall away due to gravity. When the
objects get too far away from the electromagnet, the current in the electromagnet will increase,
pulling the object closer to the electromagnet. Basically, it will do whatever is needed with the
current in the electromagnet in order to keep the object at a fixed distance below the electromagnet.
3. SUSPENSION MODEL
The system model is built on the ball’s motion equation and the electrical characteristics of electromagnet. The ball’s motion equation is
m
d2 x(t)
= mg − F (i, x)
dt2
(1)
Suppose x is the distance between the electromagnet and the upside of the ball, F (i, x) is the
electromagnet force. According to Kirchhoff’s law of magnetic circuit and law of conservation of
energy, we got
µ ¶
µ0 AN 2 i 2
F (i, x) = −
(2)
2
x
Progress In Electromagnetics Research Symposium, Hangzhou, China, March 24-28, 2008
537
Electromagnet
Drive
Circuit
F
PWM1
Balance
Position
DSP
(TMS320F2812)
ADCIN3
Permanent
magnet
Suspend
Ball
Mg
Signal
Process
Circuit
Electric Eddy
Current
sensor
Figure 1: Maglev ball control system.
By Kirchhoff’s voltage law, the circuit relationship of the coil is
U (t) = Ri(t) + L
di(t)
dt
As the ball’s gravity is equal to the electromagnet force to it at the balance place, we got
µ ¶
µ0 AN 2 i0 2
mg = F (i0 , x0 ) = −
2
x0
(3)
(4)
Then we got the transfer function of the system according to Equations 1 to 4 above which is
G(s) =
s3
− k3
k2 /k1
− k1 s + k1 k3
s2
(5)
2
2ki0
2ki0
R
Here, k = − µ0 AN
, k1 = mx
3 , k2 = −
2
mx20 , k3 = − L .
0
As it shows above, there is a root on the right plane, so it is an instable system that we should
design a controller to make the system stable. We should take care that this motion equation can
not describe the motion exactly if the coefficient or the balance position of the system changes.
2
4. HARDWARE DESIGN OF THE DIGITAL CONTROLLER
An open loop control system is not stable. We need the ability to control quickly the current of
the electromagnet depending upon the position of the ball to the electromagnet. Thus we need
a sensor to monitor the position of the ball which is then processed by DSP controller to get a
feedback control value to regulate current in the electromagnet, and then control the current in the
electromagnet to keep the system dynamic stable.
Figure 2 shows the hardware structure of the digital control system. It is composed by sensor,
signal-processing circuit, AD converting circuit, DSP part, driving circuit and so on. We choose
TMS320F2812 [3] as the processor. TMS320F2812 is the newest member of the DSP platform.
The 32 × 32-bit MAC capabilities of the F2812 and its 32-bit processing capabilities, enable the
F2812 to handle efficiently higher numerical resolution problems that would otherwise demand a
more expensive advance floating-point processor solution. TMS320F2812 is the best choice to the
unsteady maglev system as its fast real time calculation speed.
The project uses F2812’s own 12-bit ADC module that makes the system simple and can intensify
anti-interference ability. We should attention that the input electrical level must be in a range 0∼3 V
PIERS Proceedings, Hangzhou, China, March 24-28, 2008
538
MOS Drive
Circuit
Electromagnet
Sensor
PID Control
Algorithm
PWM
Signal Process
A/D
TMS320F2812
Figure 2: Maglev ball control system hardware block diagram.
that the ADC module of DSP can accept. Thus we realize it by signal regulation circuit in which
the resistance voltage divider translates the sensor output to a suitable voltage.
The signal the sensor detected is filtered by DSP controller is calculated in PID control algorithm,
and output to drive circuit as a PWM wave [4] in different duty-cycle corresponded to the input
signal. PWM wave controls the MOSFET’s gate in power amplifier circuit and further controls the
current of electromagnet. IRF730 is adopted as the switching element in this amplifier circuit.
start
Start
Clear interrupt
flag
Clear SXM,
reset exterior
interrupt
PID subroutine
Calculate control
value
System
initialization
Close
interrupt
Update CMPR1
Initialize PIE
Initialize EV
manager
Initialize AD
converter
Open
interrupt
Set EVA flag
register
Signal
Sample
Initialize PIE table
Start timer
Set interrupt
entrance
Wait
interrupt
Open interrupt
Signal
filter
Return
Figure 3: Interruption sub-program.
Figure 4: Main program.
5. SOFTWARE DESIGN OF THE DIGITAL CONTROLLER
In the program, we use timer interruption to trigger signal acquisition, to be specific, in every timer
period, the program collects signal in the channel, completes algorithm computing and controlling
at the same time, then jump to waiting subprogram the rest time until the next timer interruption.
Every timer period accomplishes one control that changed the duty-cycle of PWM wave that drives
the power tube to regulate the current. The frequency of carrier wave is decided by the value in
timer 1 period register, whereas duty-cycle is determined by the value in compare register 1 of full
compare unit 1. In the paper, the frequency of carrier wave is set to 1 kHz, and the sample time is
set to 1 ms. As Timer 1 period interruption is time-fixed, the current regulation frequency is also
fixed.
The software contains initialization program, AD conversion program, digital filter, control
algorithm program and EVA’s PWM output code. Every function can be realized by corresponding
Progress In Electromagnetics Research Symposium, Hangzhou, China, March 24-28, 2008
539
program module. Figure 3 and Figure 4 shows the main program flowchart and interruption
subprogram flowchart.
6. CONTROL ALGORITHM
PID control strategy is one of the most mature method and widely applied in most industry
production, and has successfully applied in some domestic and foreign maglev control systems.
Its principle is simple, easily to realize since the control parameters are relatively independent.
The traditional PID control method [5] is classified to position-model and increment-model. PID
incremental control method is adopted in this paper as position-type tends to produce integral
saturation in calculation process. After analyzing and reasoning, the difference equation is
u(kT ) = u(kT −T )+Kp[e(kT )−e(kT −T )]+Ki∗ e(kT )+Kd[e(kT )−2e(kT −T )+e(kT −2T )] (6)
The option of the three parameters’ values such as proportion parameter Kp, integral time T i
and differential time T d are keys of control system. In the actual experiment, we tried many times
to determine them according to the influencing tendency of three parameters to the control process
and the regulation sequence: first proportion, then integral, and then differential. The final values
are Kp = 2.3, Ki = 6, Kd = 10.
7. CONCLUSIONS
To sum up, the hybrid suspension system based on DSP controller can regulates the force between
the electromagnet and the ball to make the ball stable, but the overshoot of the system is not small
enough and the robust ability is not well enough. All of these show that the model of the system is
not perfect and the control algorithm should be improved in future. Above all, the system meets the
needs in simulation and actual test. Further developments will increasingly see the improvement.
In addition, the technology will be applied to a four freedoms suspension system.
REFERENCES
1. Zhang, S. Y., “The digital control research of hybrid maglev system,” Journal of Northwest
University (Natural Science Edition), Vol. 33, No. 5, 613–615, 2003.
2. Liu, D. S., L. M. Yin, and L. J. She, “Modeling method and design principle of maglev ball
system based on DSP,” Electronic Engineer, Vol. 27, No. 11, 58–60, 2001.
3. Texas Instrument, TMS320F28xx Digital Signal Processors Data Manual[Z]., DSP.ti.com,
2003.
4. Xu, Y. H., “The application of DSP in current control of switching reluctance motor,” Electromechanical Technology, Vol. 31, No. 4, 42–43, 2002.
5. Zhang, J., G. H. Chen, etc, “Research on hybrid suspension system based on TMS3202812,”
Power Electronics, Vol. 40, No. 4, 14–16, 2006.