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How to design the test machine interface

1 Design ideas To complete the above functions, the change of the load to be measured should first be converted into a change of the electrical quantity, that is, the acquisition of the signal. After the electrical quantity is obtained, it usually needs to be amplified, and then the analog quantity is dispersed into a digital quantity in an analog-to-digital (A / D) conversion circuit. These digital quantities are transferred to a computer through a specific interface circuit, and a software program is used to control the sampling interval, the total sampling time, etc., and draw the corresponding animation curves and physical simulations.
2 Design process and working principle
2.1 Sensor Design The main function of the components in this section is to complete the conversion from non-electrical quantity to electric quantity. In addition to requiring certain accuracy in the design, the structure should be as simple as possible. It is found through analysis that the size of the load in the hydraulic universal testing machine can be expressed by the oil pressure of the hydraulic oil. Therefore, the pressure can be used to convert the oil pressure into the change in the resistance of the strain gauge, and the change in resistance to the change in voltage or current through the Wheatstone bridge, and then can be amplified and filtered to output. This solution needs to lead a way out of the working cylinder or drill the cylinder to install the oil pressure sensor. Another solution is to find a suitable position to install a tensile (compression) force sensor based on the structure of the universal testing machine, which is directly sensitive to the load applied to the test piece. This requires disconnecting a force applying member to install a tension (compression) force sensor. The above two schemes have to change the original machine structure, the reconstruction process is complicated, and the harvesting date of the bridge output: the electric signal with the price of 1 plus price needs to be processed by amplification, filtering, etc. The corresponding circuit is more complicated and the accuracy is not easy to guarantee. In order to find a simpler and more reliable method of force measurement, Jinan New Times Trial Instrument Co., Ltd. conducted an internal structure analysis of the test machine. The study found that the magnitude of the load applied to the test piece was proportional to the angle of the pendulum swinging inside the machine, pushing the pendulum to swing along the axis. That is, the displacement of the rod assembly movement has a linear relationship with the load. Using the structural characteristics of the machine, the problem of measuring the size of the load is transformed into the problem of measuring the displacement of the rod assembly. Considering the low frequency of movement in this part, a wire-wound high-precision linear resistance converter is used to measure the displacement. In this way, the structure and working state of the original machine and equipment are not changed, and the output value is large enough, and A / D conversion can be performed directly without amplification processing, so that the circuit is simple and the error is small.
2.2 A / D converter selection and peripheral circuit design This part of the circuit is to discrete the analog voltage signal that has been obtained previously into a digital signal for easy computer identification.
2.2.1 Selection and characteristics of A / D converter A / D conversion chip has many types, different performances and great price differences. According to its conversion principle, it can be divided into binary slope method, integration method, parallel comparison method, voltage-frequency conversion method and successive comparison method. Due to the small number of data channels to be measured in this experiment, the conversion rate is not high, and considering the simple peripheral circuits, good reliability, low power requirements, and economical factors, the ADC0809 chip was selected. It is a CMOs data acquisition device with 28-pin dual columns In-line package, it not only includes an 8-bit successive approximation converter, but also provides an 8-channel analog multiplexer and joint addressing logic. Its main features are: ① using a single 5V power supply, the working clock range is IOKH2-1MH2 (typical value is 640KHz); ② resolution is 8-bit binary code, precision is 7-bit, zero deviation and full-scale error are less than 0.51 SB, not Need to be calibrated; ③ With 8-channel door lock switch control, it can directly access 8 single-ended analog quantities. The internal structure of the chip is shown in Figure 1, which uses a sequential comparison method. This comparison method has many applications, and many A / D conversion chips are made on this principle. It uses a series of reference voltages (such as the code used when weighing the balance) and the voltage to be converted (weighing object) to determine whether each bit of the converted number is 1 or 0, and determines the order. From the high position to the low position, this is like weighing the balance, adding the yards one by one, from large to small, until the added yard is equal to the weight of the weight, and the yard weight is the weight.
2.2.2 Peripheral circuit design ADC0809 pin functions and processing are described as follows:
Channel analog input, through which three address decoding ADDA, A DDS, A DDC to select which way. AD DA, A DDB, A DDC: Analog channel selection address signal. ADDA is low and ADDC is high. For example, the 0 channel is gated, that is, U, o works. By analogy, since only one channel needs to be converted in this experiment, there is no need for channel address gating, but all three addresses are grounded, that is, it is only valid to input from Up. CL K: The clock input signal terminal, because this chip does not have an internal clock, so the external clock is added. According to its frequency requirements, the oscillator is designed as shown in Figure 2. Oscillator is used to generate a square wave pulse signal with the same frequency as the quartz crystal. The frequency is 32KHz, which meets the CLK clock requirements. EO C: Conversion end signal, indicating the end of A / D conversion. Send this signal to the computer to inform it that it can read data. Because the frequency of the input signal is low, no sample-and-hold circuit is provided.
2.3 Interfaces and Program-Controlled Computers There are various communication methods to receive digital signals, which can be done through the computer's motherboard expansion slot, or a parallel interface and a serial interface. Although the expansion signals of the motherboard are relatively comprehensive, it is easier to design the board, but the versatility is poor, and the host box needs to be opened to install the board. Although the application interface can perform long-distance communication, it is slow and requires a separate communication adapter. Parallel interfaces are often used to connect extraction devices (such as printers). But the software can control the computer to pick up data. This experiment uses C language programming, the port control source program is as follows:
2.4 Circuit Thousands of interference control circuit may have many interferences, such as power supply, ground noise, electromagnetic interference, etc., if not handled properly, it will inevitably affect normal work. The processing method is as follows: ① The electric power is taken from the computer and the working voltage is 5V. Considering that the voltage accuracy in the computer is not high, the LM7805 three-terminal regulator will be used to stabilize the 12V voltage. The accuracy can meet the requirements. In order to reduce power supply noise, a large-capacity electrolytic capacitor is added to the front end of the LM7805 for low-frequency wave construction to reduce the pulsation component. A high-frequency filter capacitor is connected between the power supply and the ground of each chip, which greatly reduces the interference caused by the peak current caused by the instantaneous on and off of the current load. ② For ground line interference control, the multi-point grounding method is mainly used. ③ Separate the power line from the signal line, shield the data line, and minimize the length of the data line to keep the electromagnetic interference within a small range.
Commonly used computer data acquisition systems often use motherboard expansion slots or serial interfaces. Through experiments, it is found that using a computer's parallel port for data acquisition is completely feasible, and has other advantages that other ports cannot match, namely simple lines, easy design, low interference, high transmission rate, and good plantability. This is of great significance for the study of computer control.

Updated: 2019-12-4
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