Mine Explosion-proof based on Anti-interference Technology
Jihong Guo, Yanyu Li
Mine Explosion-proof based on Anti-interference Technology
1
Jihong Guo, 2Yanyu Li
Department of Electrical Engineering, Jiyuan Vocational and Technical
College,Henan Jiyuan 459000, China ,[email protected]
2, Yanyu Li
Department of Mechanical and Electrical Engineering, Jiyuan Vocational and
Technical College,Henan Jiyuan 459000, China ,[email protected]
1, Jihong Guo
Abstract
In QJR-400 mine flameproof and intrinsically safety type, when AC motor soft starting controller is
at run, there are all kinds of interference problems. In view of these interference, In the hardware
design, measures such as photoelectric isolation, anti-interference regulated power supply, reasonable
grounding measure can effectively avoid interferences; In the software design, digital filtering
program can effectively eliminate the interference of random signals and pulse signals; Manual reset,
"WATCHDOG" and software traps and other measures are taken for interferences of the MCU. The
experimental results show that these measures can effectively inhibit the external interference.
Keywords: Anti-interference, Hardware, Software, MCU
1. Introduction
As the core of QJR-400 explosion-proof and intrinsically safe type AC soft-start controller of
mine-used motors, the industrial grade MCU (with 10-bit A/D PIC16F877A) has many advantages,
such as trigger control with thyristor as the main power element, two-way communication, faults′ static
and dynamic self-diagnosis, load monitoring dynamic display, LCD Chinese display. Digital soft-start
controlling of high-voltage AC motor is realized in explosion-proof condition. Figure 1 shows its block
diagram. Space of explosion-proof box is so limited that a powerful interference source is formed when
high-power components of the light or heavy current, microelectronic chip, high voltage electric field,
as well as alternating magnetic field of high-current driving are intertwined. Transient disorder
interference always causes instability to many links, such as the on-off consistency of thyristor, the
reliability of the trigger, the accuracy of logic and the display of system physical value. Therefore,
anti-interference problems are the most important one to be solved in every aspect of the design [1].
Figure 1. Structure diagram of QJR-400 soft-start controller
Interferences generally impact on three parts of the MCU system. The first is the input channel.
Interference makes analog signal distortional and digital signal in error. The response of the MCU
International Journal of Digital Content Technology and its Applications(JDCTA)
Volume7,Number6,March 2013
doi:10.4156/jdcta.vol7.issue6.137
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Jihong Guo, Yanyu Li
system based on this input information must be a wrong one. The second is the output channel. It
makes the output signal disordered and not react the real output value of microcomputer system, which
will make actuator malfunction and lead to serious consequences. The third is three buses of MCU
system. The digital signal of three buses will be disorder to incorrect results under interferences.
Therefore, powerful anti-interference measures must be taken in both hardware and software aspects to
improve the working reliability of the computer monitoring system.
2. Hardware design of Anti-interference
Hardware anti-interference is high efficient and real-time, however increases investment and the
volume of the equipment. Software anti-interference has characteristics of low investment, slow speed
and low work efficiency. The organic combination of hardware and software makes a high efficient and
cheap anti-interference system.
Series mode interference and common mode interference are two kinds of hardware interference
signal. Corresponding measures are used respectively for these two types of interference.
2.1. Anti-interference measures on series mode interference
Series mode interference is usually superimposed on the input signal, or break into the system
through the supply line. Therefore, the anti-interference circuit is set on the only way of interferences.
Specific measures are as follows.
2.1.1. Using photocoupler
The primary side of the optocoupler is light - emitting diodes, and the secondary side is a
phototransistor. Signals are transmitted by light and there is completely no electrical connection
between primary and secondary sides. In Fig.2, a widely used interface device in computer monitoring
system is introduced. It isolates external circuit and MCU controlling system in the electrical way [2].
S+12V
R3
1K
U2
R4
1K
OPTOISO1
C2
0.1uF
SGND
Figure 2. Photocoupler
In input and output channels, the use of optical device carries out information transmission, the
single chip microcomputer system from the electrically will be isolated with a variety of sensors,
actuators and switches, and the vast majority of external equipment interference will be blocked out,
then all kinds of digital signals can use optical transmission. For the analog signal, it can be used to
linear optocoupler transmission.
2.1.2. Anti-interference regulated power supply
Interference is easy to occur in the supply line of power supply to the MCU control system.
Therefore, the following measures are taken to eliminate the impact of interference.
①Isolation transformer and noise filter
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Controlling transformers of the system are isolation transformers, primary side of which is
symmetrical to eliminate the parasitic capacitance. The core and the shielding layer between the
primary and secondary windings are grounded together, which can reduce the distributed capacitance
between windings and improve the anti-interference ability of the high-frequency interference. A noise
filter is added to the AC side of the MCU system′ s power supply. Most of the "glitch" can be absorbed
in the power supply, so that relatively "pure" power supply can be provided for MCU system [3].
②Filter capacitor in parallel with rectification component
Rectifier element is nonlinear and there are many impacts in waveforms of voltage and current.
They both are sources of high-frequency interferences. Filter capacitor is connected to both ends of the
rectifier element in parallel, which can greatly weaken high-frequency interference.
③Using stabilizer circuit with high-quality
High-quality voltage stabilizer circuit can provide a DC voltage of small ripple. Its electrolytic
capacitor is made of two layers of film winding and equivalent to an inductor of high-frequency. The
filtering effect of high-frequency signal is not good. Aiming to obtain better filtering effect, ceramic
capacitor or polycarbonate capacitor (0.1uF) with good high-frequency characteristic and electrolytic
capacitor should be used in parallel in the filter circuit.
④Connecting decoupling capacitor
When DC power supply is connected to the MCU system, in order to improve the performance of
high-frequency anti-interference of LSI, 0.1uF high-frequency decoupling capacitor should be
connected to the circuit board. CPU, RAM with poor anti-noise and anti-interference ability must
connect decoupling capacitors. The pin of decoupling capacitor should be as short as possible. The pin
of decoupling capacitor is too long to make self-resonance and its high-frequency characteristic will be
poor [4].
2.1.3. Digital signal negative logic transmission
When high resistance line is affected by interference source, there will be more interference signal
than low resistance line. In the digital signal system, internal resistance is small for low level output
and large for high level output. Therefore low level is defined to be effective level and high level to be
ineffective. Malfunctions caused by interferences can be reduced and transmission of the digital signal
will be more reliable.
2.1.4. Transforming analog signal to frequency signal
The amplitude of analog signals is easy to be affected by interference, however not for frequency
signals. Analog voltage is converted into a square wave signal which is proportional to frequency by
V/F converter and the signal is sent to MCU. Corresponding analog value can be calculated when the
frequency is measured.
2.1.5. Hardware filter
In the low frequency signal transmitting circuit, it can access to some RC low-pass filter that can
greatly weaken the various types of high-frequency interference signal, however the hardware filter
disadvantage is too bulky and higher costs, at the same time the frequency is too low, it need to
cooperate with software digital filtering [5].
2.1.6. Overvoltage protection circuit
In order to prevent the introduction of high voltage injury system, it can use a certain overvoltage
protection in input or output channel, circuit is composed by current limiting resistor and voltage
regulator tube, a current-limiting resistor is selected to be appropriate, too much can cause signal
degradation, too small can not protect the role of the regulator. The choice of the regulator should be
appropriate, and its voltage value is a slightly higher transmission voltage signal, too low has limiting
amplitude function on the effective signal, so that the signal is distortion. For weak signal (0.2V), it
usually uses two anti-parallel diodes instead of the regulator, which can take the same over-voltage
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protection [6].
2.2. Anti-interference measures on common mode interference
There is common mode interference usually for digital signal. Anti-interference measures on
common mode interference of this system will be introduced as follows.
2.2.1. Good grounding system
Power supply of MCU system is independently +5V power and the ground of MCU system are
isolated from the ground of power supply system. The fundamental purpose of this grounding design is
to eliminate the noise voltage of circuits’ current in public ground line, and to avoid the influence of
electromagnetic fields and the ground potential, which means that it can not be a ground loop. In the
low-frequency circuit, the loop formed by grounding circuit effects interference greatly, therefore
one-point grounding is necessary [7].
2.2.2. Shielding
The entire circuit board of the main MCU controlling system is packaged by a metal box.
Connecting to the ground, the metal box plays the shielding role. It can eliminate and reduce
distributed capacitance between electrostatic field and signal lines, cut transmission of power lines,
inhibit interference voltage caused by electrostatic induction.
2.2.3. Balance symmetrical input
In the design of the signal source, it is usually all kinds of sensors, the signal source should be as far
as possible balance and symmetry; otherwise it is easy to generate additional differential mode
interference, so that the subsequent circuit is not easy to deal with.
2.2.4. Selection of high quality differential amplifier
High quality differential amplifier characteristics are high gain, low noise, low drift and wide band,
which can be composed of the operational amplifier to gain enough high common-mode rejection ratio.
3. Software design of Anti-interference
Creating a basic clean working environment for MCU system, the above mentioned hardware
anti-interference measures are very necessary. Therefore hardware anti-interference measures
cooperating with software anti-interference measures, the system can work more reliably. As the
second line of defense, software anti-interference measure is indispensable.
3.1. Software anti-interference measures on interferences of I/O channel
3.1.1. On-off signal input anti-interference
Interference signals are generally very narrow pulse, while the on-off signal has a longer efficient
time. According to this feature, the same on-off signal is sampled continuously and repeatedly, and
only the completely same sampled result in two or more times continuously should be considered valid.
If the signal is always fluctuating under many, the sampling should be stopped and an error warning
should be given [8].
3.1.2. On-off signal output anti-interference
When MCU is applied for industrial controlling, on-off signal output circuit is used to control relays,
contactors and other devices. Electric arc causes strong interference signals with those devices action.
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The signals will be fed back to output interface through the public line. Contents of the output register
may be changed and malfunction may occur. Outputting the same data to external load repeatedly is a
most effective software solution. If possible, the repetition period should be as short as possible. When
the external device had no chance to respond the interference signal, the correct output information
arrives, so malfunction can be prevented.
3.1.3. Analog signal input anti-interference
Signals such as current, voltage, electric leakage detected by system are analog signal. Those analog
signals can be accepted by MCU after A/D conversion. When interference signals have an effect on the
input channels of analog, A/D conversion results deviate from the true value. Sampling once, a more
credible conversion value can not be obtained. Known as digital filtering, the software process makes
no additional hardware cost and the filtering program can be used in many channels. There is a
disadvantage that the MCU running time is occupied. However this approach can meet the real-time
requirement for this system.
3.2. Digital filter treatment on analog signals
After analog signal must pass through the A/D conversion, it can be accepted by microcontroller,
and its interference will act on the analog signals, so that the conversion results deviate from the true
values; the interference divides into two kinds of periodic and non periodic, non periodic random
interference can use digital filter method to suppress, the commonly used filtering has:
3.2.1. Procedure judgment filter
Many physical changes all have a certain range, the adjacent two sampling values changes between
yn and yn-1 will certainly have a certain extent. According to the practical experience, procedure
judgment filtering determines the maximum deviation e between two adjacent sampling signals; if
the actual two adjacent sampling values go beyond the deviation value, it shows the occurrence of
interference, not bound by the data and the use of yn-1 instead of yn . If it is less than deviation value,
its signal is sampling value. It can be approximated to launch [9]:
yn  yn
yn  yn 1  e
(1)
yn  yn 1
yn  yn 1  e
(2)
3.2.2. Median filtering
For the measured parameters, to carry out continuous several sampling, and then the sampling value
is carried out sequence, selecting the sampling value of middle position as an effective value.
3.2.3. Arithmetic average filter
For the same sampling point, to carry out continuous sampling n times, with its arithmetic average
value as an effective sampling value, its mathematical expression is:
1 n
(3)
y   yi
n i 1
The algorithm is applied to suppress random interference, if the sampling times are bigger,
smoothing effect is better, however the sensitivity of the system will be decreased.
3.2.4. To the extreme average filter
If the system is subject to significant pulse interference, average filter cannot be eliminated,
however it will influence weakened. After it is sampled n times, the cumulative sum can find the
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maximum and minimum values that are subtracted from the accumulation sum, according to n-2
sampling value to averaging, it can efficiently obtain sample value [10].
3.2.5. Weighted average filter
Arithmetic average filtering and negative average filter smoothing exist smoothness and sensitivity
contradiction, there are too small sampling number, poor smoothing effect, too many times, sensitivity
decreases and less sensitive to parameter changes, which can use weighted average filter, each of the
sampling values are respectively multiplied by the different weighted coefficients, and n weighted
average filtering algorithm is:
1 n 1
y   Cyn  i
(4)
n 0
In formula(4), C0,C1…and Cn-1 are all constants that should meet the following conditions:
C0  C1    Cn 1  1

C0  C1    Cn 1  0
(5)
3.2.6. First-order lag filter
First-order lag filter is also known as the low pass filter or the first-order inertial filter. First-order
lag filter algorithm belongs to the dynamic algorithm, and it can overcome the disadvantages of analog
filter. First-order lag filter algorithm is [11]:
yn  axn  (1  a ) yn 1
(6)
In formula (6), xn is unfiltered n  th sampling values, yn is n  th sampling value through the
filter output.
a

(7)
T 
In formula (7), τ is the filter time constant, T is sampling period; α is filtering smoothing
coefficient that is generally determined by the test.
4. Anti-interference measures on MCU
In the coal mine, the application environment of MCU is very bad, such as high voltage and current,
vacuum contactors frequently pulling in, so that strong interference signal appears in the stray current
of the ground. When the interference signal has an effect on the MCU through three buses, the MCU
will not be in the normal state and program will be in confusion. Anti-interference measures on MCU
aim discover interference, intercept out-of-control program and make the system back to the normal
state. Anti-interference measures on MCU will be introduced as follows.
4.1. Manual reset
The easiest way to deal with out-of-control MCU is resetting it. In Fig.3, reset circuit is connected to
the RESET end of MCU. The power up reset circuit is composed of capacitance C1 (10μF) and
grounding resistance R1 (1.2kΩ) and the program will be automatically executed from 0000H. The
manual reset circuit is composed of reset button res and resistance R2 (0.2kΩ). Both power up reset
circuit and manual reset circuit can provide high level larger than 10mv for the reset end, making the
system reset reliably [12-14].
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VCC
C1
rst
10u
RES
R2
0.2K
R1
1.2K
Figure 3. MCU reset circuit
4.2. Software trap setting
When accidents such as overflow or division by zero occur inside the computer, a specific program
will be automatically transferred, which is called the capture by means of setting traps. In the MCU
system, operation codes of LJMP0000 instruction are filled in all consecutive blank cells of the
program memory, which are 020000. When out-of-control program skips to these addresses, it will be
transferred to the entrance of main program. For example, LJMP0000 instruction 020000020000 is
continuously filled in blank memory which begins with 6000H. Wherever program skips to whichever
unit of these addresses, the out-of-control program is captured and directly transferred to the entrance
of main program or instructions following the blank area will be executed. In either case, the PC value
is to be adjusted and the program recovers to the initial state.
4.3. Program running monitoring system (WATCHDOG)
Manual reset forces the MCU on track, and the circuit is simple. However manual reset’s biggest
shortcoming is not timely. Usually people have no alternative to press the reset button after the system
is paralyzed. A program running monitoring system was setup to solve the problem. According to
statistics, 90% of crashes of the present computer monitoring system, data losing and dancing are
caused by power supply make-break, unstable instant power. Micro's latest product X5045 chip can be
a better solution to this problem. Watchdog timer, power supply voltage monitoring, serial E2PROM
are integrated within an 8-pin chip, which are the most commonly used functions of the computer
monitoring system. This combination significantly reduces space requirements of the circuit board and
simplifies the hardware design. System reliability can be improved, the cost and power consumption
can be reduced. Due to these advantages, X5045 has become the preferred device for the monitoring
system [15, 16].
4.4. Soft-start controller programming using X5045
4.4.1. Programmable watchdog timer
X5045 have a watchdog timer which can be set the time 200ms, 600ms, 1.4s or prohibition. In the
device, the X5045 timer is set into 200ms. In the cycle of main program, each cycle is about 2ms.
Setting a counter, a reset signal is given to X5045 after 100 cycles to make the re-count of watchdog
timer, which is called "feeding the dog".
/*reset DOG*/
void Rst_Wdog (void)
{
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CS=0;
CS=1;
}
4.4.2. Software piracy Prevention
An appropriate data was written in some unit of X5045 in advance, when the PIC16F877A main
program is power-on, and the data stored in the X5045 unit will be read. When the read data is different
from the data in PIC16F877A memory, the main program will not continue to run.
5. Conclusion
Anti-interference problems can be solved, and the control system can be run reliably and stably after
applying above mentioned methods and measures. Experiments show that those measures can
effectively inhibit external interference.
6. References
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