Temperature relay is a kind of temperature sensitive element, its output state is completely determined by the required control temperature. The key process of temperature relay is the detection of temperature characteristics (action temperature when heating up and recovery temperature when cooling down). At present, the measurement of temperature relay also uses manual test, the tester constantly adjust the temperature, when the temperature meets the requirements, need to manually record each data, the operation steps are very complicated, the workload is also very large. In order to simplify test work, improve work efficiency and reliability of test data, this paper introduces a temperature relay test system based on Keithley 2700 data acquisition instrument and LabVIEW virtual instrument platform.

1 detection principle and system composition

1.1 detection principle

The temperature characteristic detection of temperature relay is essentially to determine whether the temperature at the moment when the temperature relay is open or closed meets the technical index. There are three testing methods for temperature relay, which are test block testing method, air testing method and liquid testing method. No matter which of the three methods is used to detect the temperature relay, it needs to determine the temperature characteristics of the temperature relay (heating off or cooling off) and monitor the temperature. Generally, the detection value of air method is close to that of liquid method, but there is about 3 ~ 5 ℃ difference between air method and test block method. Test block method and air method are usually used in the industry. In addition, the heating and cooling rate in product testing has a great impact on the test results, so the lifting speed must be selected according to the standard. The system adopts air measurement method.

1) Determination of temperature characteristics

In the traditional detection method, the LED is usually used to determine whether the temperature relay is open or closed, that is, if the diode is on, the temperature relay is closed, otherwise the temperature relay is open. When opening or closing, record the temperature manually. In this system, the keithley2700 data acquisition instrument circularly scans the resistance values of 40 channels. If the resistance value is 0, the temperature relay is determined to be closed; if the resistance value is ∞, the temperature relay is determined to be open. Automatically store the temperature record when the temperature relay of the corresponding channel of keithley2700 acts or returns to excel.

2) Temperature monitoring

The mercury thermometer is usually used to monitor the test block method, and the temperature sensor is usually used to monitor the temperature in the air method and liquid method. This system uses PT100 platinum resistance sensor, uses keithley2700 to scan PT100 resistance value circularly, and converts temperature through the following formula to realize real-time temperature detection.

In the actual test process, the formula needs to be modified in order to be closer to the real temperature value. In this system, the PT100 temperature error is less than 0.1 ℃ after calibration.

1.2 system composition

The temperature relay test system is mainly composed of computer, keithley2700, high and low temperature test box and temperature relay test circuit.

On the use of 2700 to automatically scan the resistance of each channel to determine whether the temperature relay is open or closed

1) Temperature relay test circuit

The circuit connects the temperature relay with 7700 switch module, so that the computer can control keithley2700 through LahVIEW to collect the temperature of the opening and closing points of the temperature relay. In scanning operation, a single 7700 provides 20 channels of double pole signal channel or 10 channels of 4-wire signal channel. In this circuit, channel 1 is connected with PT100 thermistor to collect the temperature of the incubator, and channel 2 to channel n are the temperature relay channels to be tested. In theory, keithley2700 can scan 40 channels and measure 39 temperature relays (channel 1 is used to measure temperature). Due to the limitation of conditions, the temperature relay acquisition board of this scheme only uses the first 10 channels of Keithley 2700, i.e. 1 channel for temperature measurement, 2 channels to 10 channels for temperature relay test.

2) Data transmission

LabVIEW and keithley2700 communicate through GPIB interface. USB is converted to serial port through ni488, and the computer communicates with keithley2700.

2 software design of test system

This system is an automatic test system of temperature relay developed on LabVIEW platform. The system has an intuitive operation interface, which can automatically collect and record data. Its work flow chart is shown in Figure 2.

On the use of 2700 to automatically scan the resistance of each channel to determine whether the temperature relay is open or closed

The LahVIEW operation interface of the scheme is shown in Figure 3.

On the use of 2700 to automatically scan the resistance of each channel to determine whether the temperature relay is open or closed

1) Device connection

Firstly, the data acquisition card 7700 is inserted into the keithley2700. Each keithley2700 can support more than two acquisition cards (40 channels in total). Connect the circuit board to the slot of the 7700 data acquisition card. The GPIB data cable is used to connect the computer and keithley2700 equipment. During the test, the GPIB address of keithley2700 should be consistent with addr: 06 of the software. The LahVIEW driver for keithley2700 is shown in Figure 4.

2) Real time temperature

The PT100 chip thermistor is connected to channel 1 of data acquisition card 7700. LabVIEW controls keithley2700 to scan the resistance value of channel 1 circularly

Then the real-time temperature can be calculated. In the actual measurement process, because of the resistance value of the wire, there are errors, so the formula needs to be corrected. The program diagram of LabVIEW is shown in Figure 5.

On the use of 2700 to automatically scan the resistance of each channel to determine whether the temperature relay is open or closed

3) Heating off

The resistance of the temperature relay is 0 when it is not disconnected during the heating process, and ∞ when it is disconnected. The resistance value of keithley2700 is judged in LabVIEW. If the resistance value of channel n is greater than 10, the temperature relay will be disconnected to record the current temperature (the temperature value is the temperature value of the secondary disconnection). When the resistance of all channels is greater than 10, the program will stop automatically. In order to realize loop judgment of resistance value in LabVIEW, loop nesting plus feedback can simplify programming. The program diagram of LabVIEW is shown in Figure 6.

On the use of 2700 to automatically scan the resistance of each channel to determine whether the temperature relay is open or closed

4) Cooling closure

If the resistance value of channel n is less than 10, the temperature relay will open and record the current temperature (the temperature value is the temperature value of the secondary closing). When the resistance of all channels is less than 10, the program will stop automatically. Secondary programming is similar to heating off.

5) Data processing

The test data are saved in the set spreadsheet. Open these files and you can see the measured data clearly. The macro can be called in Excel to judge whether the temperature relay is qualified or not.

3. Actual test and data analysis

This system uses 2700 to scan the resistance of each channel automatically to judge whether the temperature relay is open or closed. A 2700 can be inserted into two 7700, that is, 40 channels can be tested. Taking 80 temperature relays of a certain type (30 ± 5 ℃) as an example, 20 relays need to be tested 4 times each time, and the measured data are given in Table 1. The data will be saved in Excel.

It can be seen from the table that channel 8 and channel 16 exceed the standard and need to be eliminated for further verification. Other channels are qualified if they meet the technical requirements. It should be noted that channel 8 exceeds the standard, and the main error comes from the unevenness of each part in the heating process of high and low temperature box. Therefore, in the test process, the temperature of the temperature box needs to rise and fall slowly to reduce the error.

4 Conclusion

Temperature relay is very practical in many industries, the traditional manual testing and manual recording can not meet the higher production efficiency. The system can solve this problem well, meet the normal measurement requirements, and achieve the purpose of automatic testing. The system also has certain expansibility, for example, it can connect multiple keithley2700 to measure more temperature relays and greatly improve the efficiency of testing.

Editor in charge : pj

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