Download Measurement and Calibrate for Analog Quantity

2016 International Conference on Artificial Intelligence and Computer Science (AICS 2016)
ISBN: 978-1-60595-411-0
Measurement and Calibrate for Analog Quantity Based on the
Control of Mobile Frequency Convertor
Bo SUN, Wei-sheng GUO and Gang CHEN*
School of Automation and Electrical Engineering,
Shenyang Ligong University, Shenyang, China 110159
Email: [email protected]
*Corresponding author
Keywords: Mobile frequency convertor, Embedded system, Distributed control system, Analog
quantity measure.
Abstract. There were too many required measurement analog quantities in the control system of
mobile frequency convertor, design the extended circuit of multi-channel analog quantity
measurement can complete the job accurately and promptly to this problem. Use remote-calibrate
method, written analog quantity measure program of the embedded system, accomplish the
measurement job of multiple channel analog quantity. The process of calibrate was fast, simple and
preciseness, it can be applied to calibrate and check of measure analog quantity in the industry field.
It’s easy to master for the relative engineering and technical personnel.
Introduction
In order to improve productivity, quality and reduce the manufacture costs, it is a general request to
realize automation and intelligent of electrical equipment in the production of modern industry [1, 2].
The electrical devices typically select embedded and distributed control system design scheme to
solve the complex control problems of industry field [3, 4]. The embedded analog measurement
technology was one of core skills of distributed control system [5, 6, 7].
Measurement Circuit of Analog Quantity of the Control System
Control system of mobile frequency convertor was a power generation and distributed system, which
can provide power for the field electrical device. According to the consumption of load, control four
motors electricity supply alone or parallel operation output electricity to the load. The system need to
measure the voltage and current exactly, control the relays automatic switching, realize overload
protection so as to guarantee the mobile frequency convertor operation normally and power supply
safely.
The required measurement analog quantities in the field were voltage signal, its range from 0v to
5v, select the MPU (Micro Processing Unit) XC2365 as the control core, configure peripheral circuit
consist of the control terminal, form the control system of mobile frequency convertor finally. Due to
control terminal need to measure 24 channels analog signals, the 10bit A/D converter of the
microprocessor less than it, only have 15 channels, in order to ensure the precision, decrease the
development costs, the researchers design multi-channel measurement circuits of the analog.
The Control System Structure of Mobile Frequency Convertor
The frame of embedded distributed control system as shown in fig. 1, it was composed of a main
control board, a main control panel, two slave control boards and two slave control panels through
MAX487 connect each other form the control system network. The control boards complete the
control and measure function of switch value, the function of analog quantity measurement with
actuators. The main control board can measure 8 analog channels at the same time; the two slave
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control boards need to extra design multi-channel analog detect circuit measure 24 channels analog
quantities respectively. The slave boards specially designed the multi-channel circuit for measure.
In Fig. 1, the supervision computer connect each microcontroller with MAX487 too, use dotted
line substitute the field-bus in the picture, if the connected control panel or board was free, the dotted
line can be removed. The distributed control system was able to realize remote calibrate to the analog
detect which came from the control boards; monitor the states of whole input and output switch value,
the measure value of analog quantity. Besides, this system also can test the quality of circuit boards in
the operation process of the control system; monitor the mobile frequency convertor online.
Figure 1. Control system frame.
The Single Circuit of Measure Analog Quantity
One of the analog quantity measure circuits was shown in Fig. 2, it was composed of resistant R1, R2,
R3, and two voltage followers. In this section, capacitor C1 and C2 filter the interference signal, diode
D as the protection element to ensure the voltage didn’t over range. The relationship between output
voltage Vo and the input VI as follows equation (1).
VO 
R3
VI
R1 +R2  R3
(1)
The relationship between protect voltage VD and input voltage VI was shown in equation (2).
VD 
R2  R3
VI
R1 +R2  R3
(2)
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Figure 2. The single measure circuits of analog quantity.
Assign the three resistant R1, R2, R3 different value; keep the value of Vo in the linear area of
operational amplifier composed of the voltage follower when the input voltage below 5v, made Vo
according to the linear change of VI .The light emitting diode breakdown when VD is higher than 5v.
Consequently, the amplitude of Vo was limited under 5v, guarantee the microprocessor operation
normally.
The Extended Multi-channel Circuit of Measure Analog Quantity
Due to the required measurement analog of the slave controller was too much, select the analog
switch chip HC4051 to finish the detect job, form the multi-channel extended circuit, as follow in
figure. 2. Use three switch value output terminals of MPU to drive the high-speed latch HC373,
according to the built-in address selection interface A, B and C equate different values to choose the
analog channel, transfer the signal to the A/D module which attach to the single chip finish the 8
channels analog quantities convert to digital signal in chronological order. The relationship between
values of pin A, B, C and selected channel was shown in Tab. 1. Design the extended circuit measure
24 channels analog signals just need MPU XC2365’s three A/D pins, the rest were used for switch
value input terminals.
Table 1. the relationship between A, B, C and selected channel.
The address select pin of HC4051
A
B
C
INH
0
0
0
0
0
1
0
0
The
selected
channel
X0
X1
0
0
1
1
1
1
1
1
0
0
1
1
0
1
0
1
0
1
0
0
0
0
0
0
X2
X3
X4
X5
X6
X7
It takes some time for the channel shift of chip HC4051, other hardware circuit operation also
generate time delay, when the microprocessor send instruction to the latch HC373 select channel, the
timer of single chip will delay few microseconds, wait for the voltage of selected analog channel
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became stable, then begin A/D conversion, the delay time of the control system of mobile frequency
convertor was 10ms.
The Calibrate Method of Measure Analog Quantity
Calibrate was method that develop relevant norm to transform the pending measured quantity into
digital quantity; the output signal was easy to process by MPU. The process of calibration just like a
ruler, researchers uses it to detect the analog.
Method of Two-point Calibrate
As in figure 2 and equation (1) shows, Vo and VI satisfying the linear relationship. Base on this, VI
with the A/D conversion value Lzy of Vo also satisfy the linear relationship too. Their linear
relationship can be inferred on the basis of Law that two points determine a line, then formulate
measure standard and detect the analog.
Figure 3. The linear relation diagram of A/D.
The linear relationship diagram between VI and Lzy as shown in Fig. 3. When VI take different
values Uzmax and Uzmin, the relevant A/D conversion value Lzy of MPU XC2365 equate logic full value
Lzmax and logic zero value Lzmin .The relational expression as equation (3).
When VI  U z max , then Lzy  Lz max
(3)
When VI  U z min , then Lzy  Lz min
Download the program which write by C language to the MPU, send the value of Lzmax and Lzmin to
the calibrate array of Flash for storage, it reminds worker the calibrate process of current analog
quantity measure was over.
Measure Analog Quantity
In the initialization process of microprocessor, if logic full value Lzmax and logic zero value Lzmin was
read out from the calibrate array, the next step detect the analog was begin, transfer the input voltage
VI to the relevant logic value Lzy , define the logic slope as the follow equation (4).
KL 
Lzy  Lz min
(4)
Lz max  Lz min
The measure value Uzy of analog quantity VI was calculated by equation (5) as follows.
U zy  K L  (U z max  U z min )  U z min
(5)
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The calculate equation of relative error δ between measure value Uzy and the required analog VI as
follows equation (6).

U zy  VI
VI
100%
(6)
The Remote Calibrate of Measurement Analog Quantity
Remote calibrate was a method that the monitoring computer exchange information with the control
terminal, issuing the instruction of calibration to microprocessor, and achieve it by particular slave
control board.
The monitoring computer serial communication with the main control board and the slave control
board under the control of computer program, the format of data frame was shown in table 2. In order
to avoid wrong actions, the analog quantity realize remote calibrate need three rigorous requirements.
1) Connect reference voltage Uzmax or Uzmin at the input of the measurement analog circuit, Uzmax
match the calibrate of logic full value Lzmax, and Uzmin correspond to the calibrate of logic zero value
Lzmin .
2) The calibrate demand was sent by the main control board or the slave control board, if the
specific digital input terminal which link the pull-up resistors was pushed down to low level. The
measurement analog in this paper select the method above.
3) When the monitoring computer send out data to the control boards in the format of Tab. 2, point
out the logic full value calibrate or the logic zero value at the same time.
Table 2. The format of data frame.
Header
5bytes
Direction
1byte
Instruction
2bytes
Check
1byte
The main control board and slave board realize two-points calibrate of the analog and answer the
monitoring computer when the above conditions were satisfied.
The Experiment of Measure Analog Quantity of the Mobile Frequency Convertor
Researcher design a upper computer software on the monitoring computer with visual basic language,
communicate with the each control box. Supervise and control the system, achieve the remote
calibrate of measurement analog quantity.
When calibration begins, set the reference voltage Uzmax and Uzmin equate 5v and 2.5v respectively,
use the method in this paper, if the precision of A/D conversion was 10bit, the relative error of main
control board should below 0.4%, the slave below 0.5%.
Conclusions
Study and summarize the remote calibrate of measurement analog quantity via design the embedded
and distributed system of the mobile frequency convertor, the conclusions as follows.
1) Use the extended circuit of multi-channel measure analog quantity can save the microprocessor
resources, cut down the design cost of control system. Obtain better detect precision of analog
quantity, if the program wrote more reasonable after practice.
2) The two-point calibrate emphasize the process of calibration, when the hardware circuit of
measure analog quantity was finished, without modification. This method also can apply in other
embedded system to measure the analog.
3) The process of remote calibrate was fast, simple and accurate, it can be applied to calibrate and
check of measure analog quantity in the industry field. It’s easy to master for the relative engineering
and technical personnel, made the equipment maintenance convenient.
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