The vehicle equipment used in Shanghai Metro Line 1 was imported from Germany in the early 1990s, and its technology was roughly the level of the late 1980s. The VV230/180-2 air compressor unit test device was also imported from Germany that year. The test process is as follows: when the pressure of the high-pressure storage cylinder is 8 bar and 10 bar, respectively, the operation is 0.5 h, and the temperature of the 5 temperature measurement points of the air compressor cannot exceed 200 °C. During manual operation, the manual control valve controls the pressure of the high pressure storage cylinder to be (8 ± 0.1) bar and (10 ± 0.1) bar. The original test process is completely manual operation, and the test accuracy and maintenance quality are difficult to guarantee; The noise is very large and unbearable. To this end, the development plan of the automatic test device for the air compressor unit was proposed. For the time being, no matter whether it is the subway, the railway trunk line, or the air compressor professional manufacturer test station, the automatic air compressor test device has not been used.

2 overall plan

According to the requirements of the original air compressor unit test program of the imported car, the air compressor should be subjected to the temperature change in each air passage of the air compressor and the current, voltage, power and speed of the air compressor motor under a given working condition. Test to assess the operating status of the air compressor unit.

In order to reduce the labor intensity, improve the test accuracy, and avoid the influence of human factors, this program adopts a fully automatic test form. It is regulated by the PID gas pressure control device to control the working condition of the compressor (mainly pressure) to stabilize it under a given working condition; various types of data acquisition devices convert various measured signals into a certain form of electricity. The signal is summed up to the data conversion transmission controller, and then transmitted to the single-chip microcomputer; after a certain program processing in the single-chip microcomputer, it is sent to the PC through the communication isolation device; the change of the test data is processed by the PC. The screen of the PC is displayed in the form of a table or a curve, and each test result is archived or printed. For faults or data overruns during the test, the sound and light alarms or forced shutdowns are performed hierarchically.

3 program content research

3.1 Current, voltage and electric power testing

It can be known from the compressor motor nameplate that the rated voltage of the motor is DC 1500V (+300V, -200V) and the rated power is 12.2kW. The motor is a high voltage DC motor. Therefore, this scheme adopts high-voltage power isolation conversion device, which can accurately measure DC voltage below DC2500V. The short-time over-voltage can reach 4000V, and the input-output isolation voltage is 3000V. The current measurement adopts DC power isolation converter rated at 20A, and the overload current can be Up to 10 times the rated current. After the voltage converter accurately measures the voltage and current value, the corresponding electric power is calculated by the PC.

3.2 Compressor gas path temperature test

In order to maintain the relative independence of the device and the original test device, the solution is also equipped with a temperature sensor. The external dimensions are the same as the original temperature sensor. After calibration, the corresponding temperature transmitter is manufactured to form a temperature detecting device. The temperature range of the design requirements is -50 ~ 250 °C. Therefore, it is proposed to use the platinum resistance temperature sensor HEL-700 with good linearity and precision. The resistance-temperature characteristic curve is approximately linear, and its function is: RT=R0 (1+AT+BT 2 -100T 3 +CT 4) where: RT is the resistance at T ° C; R0 is the resistance at 0 ° C; R0, A, B, C are known coefficients. Since the signal collected by the sensor is a non-standard signal, the temperature transmitter considers converting it into a standard signal (0 to 5V) by using a constant current source and amplifying a subsequent circuit.

3.3 Control and Detection of Compressor Pressure The gas pressure control of the compressor is performed by a PID gas pressure control device and a pressure regulating actuator. The PID gas pressure control device gives the pressure feedback signal detected by the PC and the pressure sensor according to the pressure sent by the PC. After the PID is comprehensively adjusted, the pressure adjustment signal is output, and the pressure adjustment actuator performs pressure regulation to achieve the purpose of stabilizing the pressure. The pressure regulating actuator is completed by controlling the opening degree of the regulating valve by the stepping motor and the mechanical transmission mechanism.

Since pressure detection is an important part of pressure control, its test accuracy is related to the performance index of the whole machine. Therefore, it is necessary to select a pressure sensor with high precision, high reliability and wide temperature application range. Honeywell's high-precision 40PC series pressure sensors meet these requirements. The pressure sensor converts the pressure signals of different measuring points and transmits them to the PC for processing, display, control, and the like.

3.4 Motor speed test

The motor speed is detected by a reflective infrared light detecting head and a digital pulse circuit. The principle is shown in Figure 2. During the detection, a reflective strip with a certain contrast with the rotating body is adhered to the rotating body of the motor, and the rotating speed of the motor can be measured by placing the infrared detecting head in a certain area of ​​the reflective strip. The detected data information is transmitted to the PC via a counter and a timer, and processed.

3.5 data conversion

Data conversion includes two parts: A/D and D/A conversion. The MAX197, a data acquisition chip with 8 inputs, single supply and 12-bit resolution, can meet the A/D conversion requirements. All of its logic inputs and outputs are compatible with TTL/CMOS levels. However, the reference voltage accuracy of the chip itself is low, which affects the accuracy of data conversion, and can be improved by using an external reference voltage. The chip converts the analog signal collected by the above sensor into a digital signal and transmits it to the single chip microcomputer for communication between the single chip computer and the PC for subsequent processing. The function of D/A conversion is to convert the processed digital quantity into analog quantity. It is used to transmit the parameters required by the PID control system to the pressure regulating actuator to achieve the purpose of controlling the pressure of the high pressure storage cylinder so that the actual pressure and the given pressure remain within the allowable error range.

3.6 communication device

The communication device is used to complete communication between the single chip and the PC. This device is intended to use serial communication. After the baud rate is set and a certain program is processed, the MCU transmits the digital quantity obtained by the A/D conversion to the PC, and causes the computer to perform post-processing such as display. The problem that serial communication may encounter is that the transmission is not synchronized, and the control protocol can be used to solve <4>.

The physical standard for serial communication is the RS-232 standard. The specified signal level and polarity are not compatible with TTL/CMOS levels and need to be converted. The TTL/CMOS level is typically converted to an RS-232 level using the integrated chip MCI1488, and the RS-232 level is converted to a TTL/CMOS level using the chip MCI 1489. In the RS-232 standard, the length of the cable should not exceed 30 cm. The device of this project complies with this standard.

3.7 Display of measured parameters of compressor unit

The PC part is programmed with Visual Basic. The data detected by each measuring device is finally collected in the PC, and the measured data is sorted and counted by the PC, and then displayed on the display screen of the PC in a table, curve or dynamic form; Display statistical rules, such as displaying the minimum value of a certain measurement point, the average number, etc.; the test report can also be printed in the form of a copy, and the final result and each data overrun can be reliably saved. The PC part requires the human-machine interface operation as convenient as possible, can be displayed in real time, and timely and effectively deal with the faults that occur.

3.8 Protection of automatic test equipment

In order for the automatic test rig to operate safely and reliably, it must have an alarm and forced shutdown function. To this end, the device is intended to use two levels of alarm: when the pressure exceeds the first set pressure value, the sound and light alarm, the pressure has not caused serious damage to the compressor, so it can be properly handled by the staff; when the pressure exceeds The second set pressure value (this pressure may cause serious harm to the compressor) is forced to shut down, and the alarm is given to the staff to facilitate maintenance inspections to prevent serious accidents.

4 key issues in program research

Compressor pressure control is a difficult point of this topic, and it will be a problem that will be solved in the process of manufacturing and debugging of air compressor automatic test equipment in the future. The pressure in the cylinder of the air compressor is accumulated by the cylinder of the air compressor. It is affected by the efficiency of the air compressor, the performance of the air compressor motor, the power of the air compressor motor, the storage cylinder and the pipeline structure, and the air outlet. The influence of many factors such as the degree of opening of the regulating valve. It is like a capacitor charging and discharging process, which is affected by many factors such as power supply characteristics, charging circuit impedance, discharge circuit and impedance. When the capacitor charging and discharging process is carried out at the same time, in order to maintain the constant voltage of the capacitor, the change of the parameters or state of any component in the charging and discharging circuit and the power supply will cause the response of the control loop, thus maintaining the capacitor voltage within a certain error range. The control loop design is a key issue.

In contrast, the constant pressure control of the air compressor cylinder is much more difficult. First of all, the characteristics of many of the above factors are unknown or unclear in advance, even if the characteristics or parameters are not clear through possible tests, which brings great difficulty to the design and commissioning of the control device. Secondly, the air compressor unit is large. Inertial system, to achieve the control accuracy of 8 ± 0.1 bar and 10 ± 0.1 bar required by the test, the design and commissioning of the control device is not easy. Currently, the gas pressure control device of the compressor is intended to be completed by a PID pressure regulating actuator.

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