Introduction to an automatic function tester for the control panel of a wall mounted air conditioner

At present, the online testing of relevant manufacturers at home and abroad has been relatively mature, with advanced software and hardware supporting facilities and most of them being domestically produced. The degree of automation of their online testing is estimated to be almost the same as that of foreign countries. Despite this, the automation process of functional testing has been progressing quite slowly. Although many domestic manufacturers have been researching automated instruments for functional testing, and a small number of products have been put on the market, from the current application situation, the actual application effect seems to be unsatisfactory, far from meeting the customer's expectations of greatly improving production efficiency. This is why, until now, various processing plants have only equipped their production lines with automatic online testers but not automatic functional testers.

In the production process of the control board for wall-mounted air conditioners, the control board must undergo successful online testing (ICT) and functional testing (FT) before it goes offline, otherwise the product produced will be a defective product. Online testing mainly measures the electrical values ​​of components such as R, L, and C of the control board to determine whether there are reverse plug-ins, missing parts, wrong plug-ins, etc.; functional testing mainly detects whether the overall operating performance of the computer board meets the standards and whether each module works normally. Only control boards that have successfully passed the above two tests are qualified products.



It is for this purpose that this article introduces the specific implementation of an automatic functional tester for the control board of a wall-mounted air conditioner.

Communication method between computer and tester

The computer controls the entire process of automatic testing of the instrument. All of this must be realized by supporting computer testing software. The related supporting testing software used by this equipment is developed in VisualBasic language, and can display the test status of four instruments at the same time with four screens. The software can realize the functions of statistics, classification and query of test data, and the final test results can be exported as EXCEL format files.

The computer and each tester use standard RS-232 full-duplex asynchronous serial communication with a rate of 9600b/s, 1 start bit, 1 stop bit, and odd parity. The communication protocol uses the following format:

header + length + function + data (N bytes) + checksum byte

In order to fully ensure that the test data is correctly transmitted between the computer and the tester without conflict, when the host computer software and the tester start running, the master-slave mode with the host computer as the host and the tester as the slave is first adopted. At this time, the host computer has the highest control authority, allowing both parties to obtain the correct handshake signal to establish a smooth communication link and prepare for the correct transmission of test data. Once the handshake between the two parties is successful, they enter another master-slave mode with the host computer as the slave and the tester as the host, and the highest control authority is transferred to the lower computer. In this mode, the tester uploads the obtained test data to the microcomputer through the serial port, and the microcomputer saves and displays the data[5].

When the computer and the tester receive the instructions and data sent by each other, they first calculate the checksum. This protocol stipulates that the last byte of the received and sent data packets is the checksum byte, which is equal to the XOR sum of all the previous bytes. The specific calculation method is as follows:

header byte ^ length byte ^ function byte ^ data byte 1... ^ data byte N = checksum

Signal detection and remote control simulation

Voltage detection

The AC signal output by the control board has a high voltage and must not be measured directly by an instrument. Therefore, in order to protect the instrument, the AC signal must be photoelectrically isolated before entering the instrument, as shown in Figure 2. The 220V AC first passes through a 100k/3W large resistor to reduce the voltage and limit the current, and then enters the optical coupler. In this way, the other end of the optical coupler will have a pulse output with an amplitude of 5V and a frequency of 50HZ. This signal then enters the measured microcontroller, and the pulse is counted through the program to obtain the existence of this strong electric signal, thereby measuring whether the output function of this computer board is intact.



The 5V and 12V voltage signals on the control board also need to be output. The process is that the DC voltage signal is inverted by the transistor and directly enters the test terminal of the microcontroller as shown in Figure 3. The DC voltage signal enters through the diode D504, is inverted by the transistor, and is taken out from the resistor R504 and enters the test pin of the microcontroller. Adding a transistor in the middle can prevent interference signals from entering the microcontroller, thereby enhancing the anti-interference ability.

[page]

Pulse detection

There are two methods to measure pulse signals, one is the period measurement method, and the other is the frequency measurement method. The period method is suitable for measuring low-frequency signals, and the frequency method is suitable for measuring high-frequency signals.

(1) Detection of the number of buzzer beeps

Since the buzzer beep pulse output by the air conditioning control board is only output when it beeps, and because the actual test is the number of buzzer beeps, rather than the period or frequency of its beep pulse, no matter which of the above methods is used for testing, it is not appropriate. Here, the method of purely counting the buzzer beep pulse is used to obtain the number of buzzes. From the oscilloscope measurement, it can be seen that the buzzer beep pulse period is 0.25ms, the length of each beep is 100ms, the count of one beep is 100/0.25=400, and the time interval between two or four beeps is also 100ms. It is also known that the buzzer will only sound 2 or 4 times after the compensation command is sent to the air conditioning control board, so the counting can be started at this time, so that the count value after sending the compensation command may be about 800 or 1600, and the number of buzzer sounds can be determined accordingly.

(2) Detection of stepper motor drive pulses

The stepper motor drive pulse is a group of pulses with special rules. The air conditioning control board uses a 4-phase stepper motor with an 8-beat drive mode (A-AB-B-BC-C-CD-D-DA), and its drive signal is shown in Figure 4. The number of level changes in each phase per unit time is the same. By comparing the count values ​​of the number of level changes of each phase per unit time, the stepper motor short circuit, wrong phase, and missing phase can be detected.

(3) Speed ​​measurement

The speed of the PG motor on the air conditioner control panel can be obtained by measuring the pulse cycle method, because the maximum speed of this motor is generally not higher than 2000r/min. Based on this, the maximum frequency of the feedback pulse of its speed is equal to 33Hz × 3, which is about 100Hz (the PG motor outputs 3 pulses for each revolution). For such a low pulse, the error produced by the measurement cycle method is smaller than that of the measurement frequency method.



R is the speed value, TPG is the time for the PG motor to rotate one circle, 200ms is the time base value for the timer to count 5 revolutions of the PG motor, and TCN is the number of counts. (200ms × TCN) is the time taken by the PG motor to rotate 5 circles. The reason for first calculating the time for the PG motor to rotate 5 circles is to improve the measurement accuracy and prevent the speed value measured each time from deviating too much.

Remote control simulation

Modern air conditioner control panels all have remote control functions, which are indispensable. To test the air conditioner control panel, the tester must send simulated remote control commands to it to enable it to perform actions such as power on, power off, heating, and cooling. Various remote control commands are actually a series of pulses encoded according to a certain protocol. Generally, this protocol is a special protocol for the remote control to send data to the air conditioning control board. This pulse command can be generated by the tester according to the programmer's wishes by relying on the timer in the single-chip microcomputer. The generated signal has the advantages of neat waveform, regularity, and no burrs, which is very suitable for simulating some special low-frequency communication signals.

When the timer of the single-chip microcomputer simulates the signal sent by the remote control, the timing must be accurate. The error between the pulse width generated and the protocol required should not exceed 1ms, otherwise it will cause the signal simulation to fail, so that the computer board cannot receive the correct remote control command. This error is very secretive and difficult to capture. Sometimes developers may suspect that there is a problem with their program. After searching for a long time, they finally find that it is because their timing accuracy does not meet the design requirements. Hardware

Implementation

As shown in Figure 6, the simulated remote control signal enters from the resistor R8 connected to the base of the transistor Q2, is output from the collector of Q2, and is directly connected to the remote control receiving end of the air conditioning control board. Here, the transistor Q2 plays the role of strengthening the driving ability.

[page]

Software

Programming The data sent by the tester according to the remote control protocol is essentially nothing more than a pulse signal with a constant period used to represent binary 0 and 1. It is just that this signal has a fixed format agreed upon by humans in advance, so that it has the ability to transmit binary data.

Once the two parties in communication (this remote control transmission is essentially a simplex communication) know the content of the communication protocol and the waveform representation method of the fixed format of binary data 0 and 1, they can realize the communication between the two parties through software programming. The following is a C code for generating binary data 1 according to the waveform format above. The code is written in the 51 development software WAVE environment:

#include
#define MS2 2000
#define MS9 9000
sbit RemoteSend = P1 ^ 0;
void InitTimer0(void);

void main(void)
{
InitTimer0(); //Call the initialization timer
function
while (1);
}
void InitTimer0(void)
{
TH0 = (65536 - MS2) / 256; //
Initialize to 2MS
TL0 = (65536 - MS2) % 256;
RemoteSend = 1; //Output 2ms high first
TMOD = 0X01; //TC0 initialized to
16-bit timer
ET0 = 1; //TC0 overflow interrupt enable
EA = 1; //Global interrupt enable
TF0 = 0; //TC0 interrupt flag cleared to 0
TR0 = 1; //TC0 starts
}
void Timer0Service(void) interrupt
1 using 0 //Interrupt service program
{
TH0 = (65536 - MS9) / 256; //Initialize
to 2MS
TL0 = (65536 - MS9) % 256;
RemoteSend = 0; //Output becomes low
}

Conclusion

The developed functional automatic test protocol has been applied in the production process of the control board, improving the efficiency of detection and realizing the classification and summary of different functional defects, providing a basis for improving and improving the product quality of the control board.