At present, the vast majority of testing equipment for automobile safety parts are imported from countries with developed automobile manufacturing industry, especially the functional testing equipment for ABS (anti lock braking system) sensor. Therefore, it is necessary to independently develop a fast, stable and universal testing equipment suitable for production environment, In order to meet the needs of each process that must be inspected in the production process.
This design uses Ni pci-6220 multifunctional data acquisition card and LabVIEW 7.1 development software, and develops a test system that meets the requirements according to the ABS sensor function test standard of Volkswagen in Germany. The system has the advantages of high speed, reliable operation, data acquisition, analysis and storage, and has been put into use in the production line.
Working principle of ABS
The most important function of ABS is not to shorten the braking distance, but to maintain the directional stability of the vehicle during braking. When ABS works, the friction between the wheel and the road belongs to rolling friction. It will make full use of the maximum adhesion between the wheel and the road for braking, so as to improve the braking acceleration and shorten the braking distance, but the most important thing is to ensure the directional stability of the vehicle. When ABS works, it is equivalent to braking at a high frequency. Therefore, in an emergency, if you press the brake pedal to the bottom, you will certainly feel the brake pedal trembling and hear the “clatter” sound from the brake master cylinder, which means that ABS is working normally. The brake master cylinder constantly adjusts the brake pressure to produce continuous feedback force to the brake pedal.
As shown in Figure 1, during the test, the ABS rotation system is driven by the drive system to rotate at a constant speed in any direction. For mk60 ABS sensor, it means that it rotates at a constant speed of 139.5r/m without braking force. As shown in Figure 2, apply 12VDC and 3.3vdc voltages to the sensor between contacts ① and ② (UB) respectively, and conduct two consecutive tests during rotation. Each test shall ensure a complete cycle of the test. When the ABS sensor senses the signal plate, it will produce the peak of the square wave, otherwise it will produce the trough. Use the high-precision resistor R of 115 Ω to obtain the voltage us. When UB is 12V, the frequency of square wave, the maximum and minimum values of current peak IH, the maximum and minimum values of current Valley IL and the duty cycle of signal are calculated according to the measured voltage value. As shown in Figure 3, when UB is 3.3V, the pole pair and pole distance gap are calculated according to the measured voltage value. If these values are within the allowable range, it can be determined that the ABS sensor is qualified.
Fig. 1 Schematic diagram of ABS sensor test system
Figure 2 ABS circuit diagram
Fig. 3 ABS square wave
In order to ensure a complete test cycle and judge whether the pole pair of ABS sensor is correct, a reference pulse generator must be provided. Therefore, a toothed disc with 45 teeth is made. Because the pole pair of workpiece is 43 in normal condition, the number of teeth of the toothed disc is close to it. The backlash ratio of the gear disc is 1:1 and rotates with the tested workpiece. The number of teeth of the reference gear disc is detected by the photoelectric switch. Only when the signals of 45 photoelectric switches are detected can ABS be tested for a complete week. The photoelectric switch generates 0 ~ 24V sequence pulse, but it is converted into 0 ~ 5V sequence pulse after photoelectric isolation of the interface board, hereinafter referred to as ref signal. When selecting this photoelectric switch, pay attention to that the response frequency should be greater than 45 × 139.5÷60=105Hz。
The Ni pci-6220 multi-functional acquisition card is used to connect the ABS signal and ref signal as voltage analog to channel 0 and channel 1 of the multi-functional acquisition card in differential input mode. In order to ensure the test accuracy, set the sampling rate of analog to 25ks / s and the buffer area of each channel to 500Ks to store the test data reliably, Set the number of read points per channel to 250s. The test condition judgment signal and test conclusion value are output to DIO as digital signals. In order to realize the switching of test voltage, analog input and digital input or output, an interface board is designed and developed, and its structural block diagram is shown in Figure 4. There are two DC / DC modules in the interface board, which are used to convert 24VDC into 5VDC and 12VDC respectively. 5VDC is used for DIO of multi-functional acquisition card. In order to protect Ni pci-6220 multi-functional acquisition card, bidirectional input photoelectric isolation and NPN / PNP input selection switch are used for digital input, and 24 digital quantities are divided into 16 inputs and 8 outputs; The output part uses Darlington drive module to drive the relay output. The required accuracy of the test voltage is 12 ± 0.1V and 3.3 ± 0.1V. Use two voltage regulators and potentiometers to ensure that the test voltage is adjusted to 12V and 3.3V. Yaskawa sgmgh series servo motor and SGDM series servo controller are selected to keep the speed constant. The 17 bit encoder is selected for the servo motor, and the internal speed control mode is applied to realize the test under constant speed without other control modules.
Figure 4 structural block diagram of interface board
Fig. 5 is the flow chart of the test program, and Fig. 6 is the full diagram of the equipment. The specific test steps are as follows.
Fig. 5 flow chart of test procedure
Fig. 6 full view of equipment
1 start up and run the program. First initialize the program, including reading in parameter setting values, initializing arrays and clusters.
2 the test program scans the “test command” pulse signal every 50ms, and the pulse signal shall be maintained for at least 200ms. Once the “test command” pulse signal is scanned, the test is started.
3. The last test result should be cleared first.
4. Power on relays K1 and K13 on the interface board, connect ABS sensor, apply 12V voltage, conduct the first test, and calculate the test value.
5. The relay K13 on the interface board remains connected to the ABS sensor, K1 is disconnected, the voltage is switched to 3.3V, the second test is carried out, and the test value is calculated.
6 summarize the calculation results and draw a conclusion.
7. Output the calculation results and test conclusions, and send the I / O signals to PLC whether the test is completed and qualified or not; Display the calculation results and test conclusions on the main interface of the test software; Add the time, date and barcode to the calculation results and store them on the disk.
8. Wait for the feedback signal from the PLC. If the “result received” signal sent back by the PLC is received within 3S, return to step 2, wait for the “test command” pulse signal, and prepare for the next test; Otherwise, alarm prompt will be given.
Key points of program design
Taking the standard state machine of LabVIEW as the template, there are 19 frames in total. Four clusters of expressways are established as data, which are: ABS and ref sequence comparison, parameter setting, calculation results and measured data. A total of 15 subroutines are called to complete the functions of testing, parameter setting and hardware testing.
The display screen can be switched between “view data”, “first waveform”, “second waveform” and “parameter setting and hardware test”. The default screen is “view data”, which is also the main interface, as shown in Figure 7. The calculated value of the required test items, test results, test quantity, qualified quantity and qualified rate after the program is run are displayed on the default screen. During the test, the test process is displayed. The ABS signal and ref signal waveforms of two consecutive tests are displayed in the “first waveform” and “second waveform” screens. Fig. 8 is the first test waveform. In the “parameter setting and hardware test” screen, authorized users can set parameters. When the system needs to be debugged, the hardware test can be carried out. Data record files are generated every day with the date as the prefix of the file, such as “2006-6-3_abs”.
Figure 7 main interface
Fig. 8 first test waveform
The date, time and barcode of the tested part are added in front of the test data and stored in the corresponding disk. One line is added for each test and stored in the 80g C disk and D disk at the same time to ensure the safety of the data and facilitate data traceability. The data storage format is shown in Table 1. The record file can be viewed with Excel software.
Due to the fast acquisition speed, although it is theoretically considered that the rising and falling edges of the square wave signal jump, for example, from 0V to 5V, if the jump process is amplified many times, the intermediate value from 0V to 5V can be captured, which may be 2.7V, 3.5V, etc. It is a value in the jump process similar to this, that is, the peak or trough exceeds the limit. Through the analysis of the collected data, it is found that according to the required sampling rate and square wave frequency, at most one intermediate value can be generated in the jump process. In this way, such intermediate value can be identified and filtered in the program, so as to avoid the participation of false and unqualified data in the calculation. In addition, although macroscopically: if the ABS sensor is qualified and the reference signal has 45 teeth, ABS should have 43 teeth. However, through the analysis of the paper model, it is found that at the beginning of sampling, the relative states of ABS or reference signal (Ref) are different, and the calculated values are different. That is, if the pulse sequence judgment result is that REF precedes ABS, take ref as the benchmark, and when ref = 46, ABS = 43 is correct; If the judgment result of pulse sequence is that ABS precedes ref, take ABS as the benchmark. When ABS = 44, ref = 45 is correct; If the judgment result of pulse sequence is that ABS and ref arrive at the same time, based on ABS, when ABS = 43, ref = 45 is correct.
Practice has proved that the graphical programming of LabVIEW 7.1 is easy to read and understand. The rich routines in the software are very useful for beginners, and the practical decorations can make a beautiful and practical interface. Figure 9 is the LabVIEW block diagram program. At present, the ABS function test system has been put into use. The system has reliable technology, stable operation and can ensure the measurement accuracy. Compared with similar imported equipment, although the sampling rate of acquisition card is 250ks / s, the A / D conversion resolution of imported equipment is 12 bits, while the conversion resolution of Ni pci-6220 is 16 bits. In addition, the price of imported equipment is 3 or 4 times that of this system. The success of this system also saves users’ equipment investment.
Figure 9 LabVIEW block diagram program
Responsible editor: GT