Gas-Electric Combined Measurement Technology

Pneumatic measurement technology can be used for automatic measurement during mechanical processing, and can also measure the length, shape, and position of processed workpieces. It has the advantages of high measurement accuracy, high sensitivity, good reliability, long service life, convenient operation, non-contact and long-distance measurement, and easy automatic measurement control during processing. Pneumatic and electrical combined measurement technology combines pneumatic measurement technology with electronic amplification digital display technology, which has taken measurement accuracy and reliability a step further.

Application of buoy-type pneumatic measuring instrument

The most common form of pneumatic measuring instrument is the float pneumatic measuring instrument. This type of measuring instrument has a simple structure, is easy to use and maintain, and has a low price. The working principle of using a float pneumatic measuring instrument to measure the length of a workpiece is shown in Figure 1:

Clean compressed air enters from the direction of the arrow in the figure, and is introduced from the lower end of the tapered glass tube 2 through the stabilizer 1. Under the action of the airflow, the float 3 rises. The upper end of the tapered glass tube is connected to the measuring nozzle 8 through the zero valve 5. When the length L of the workpiece 7 to be measured changes, the gap will change, and the air flow through the nozzle will also change, causing the float position to change. The operator reads the change in the float on the scale, that is, the change in the workpiece size.

The zero position valve 5 can discharge the compressed air directly into the atmosphere. By adjusting the zero position valve 5, the flow rate through the tapered glass tube can be increased or decreased while the flow rate of compressed air flowing through the measuring nozzle remains unchanged, thereby changing the height position of the buoy and achieving the zero position adjustment of the buoy.

By adjusting the magnification valve 6, the amount of compressed air flowing through the tapered glass tube can be changed, thereby changing the magnification of the float-type pneumatic measuring instrument. From the working principle of the pneumatic measuring instrument, it can be seen that its measurement accuracy and measurement magnification are determined by the measuring instrument itself, and have nothing to do with the manufacturing accuracy of the measuring device (such as a workbench, base, etc.). Therefore, the design and manufacture of the measuring instrument device does not require high precision.

When the float type pneumatic measuring instrument is used for position dimension measurement, such as parallelism measurement, it is often necessary to use a pneumatic measuring instrument with two tubes for measurement. Its working principle is shown in Figure 2:

In order to measure the parallelism of the workpiece hole (diameter 20 ± d) to the reference plane A, the workpiece 1 is placed on the measuring table 3, so that the reference plane A fits the table plane, and the measuring head 2 is inserted into the hole to be measured. If the axis of the hole to be measured is not parallel to the reference plane A, the gap between the measuring nozzle of the pipe measuring and the measuring pipe 11 and the hole wall is different (as shown in the figure), and the measured values ​​are:

Non-parallelism error of the reference plane.

This measurement method is used to measure non-parallelism. When designing and manufacturing the measuring device, it is required that after the workpiece is placed on the measuring workbench, the measuring head should be able to be smoothly inserted into the measured hole of the workpiece; and the maximum gap between the nozzle and the workpiece hole wall must be within the working range of the measuring instrument. Since the double-tube pneumatic measuring instrument adopts the comparative measurement method, it is necessary to use a standard template or a calibration gauge to adjust the zero position when measuring the workpiece. Therefore, even if there is a non-parallelism error between the measuring head and the workbench positioning reference plane, this error can be eliminated by adjusting the zero position of the pneumatic measuring instrument float through the calibration gauge. Therefore, the design and manufacture of the measuring device is relatively easy.

When using a double-tube pneumatic measuring instrument to measure the position error of a workpiece (such as parallelism, verticality, etc.), the detection indication must be read and calculated. It is not intuitive and inconvenient. If used for automatic detection, the air pressure change signal or the position change information of the buoy sent by the pneumatic measuring instrument must be converted into a corresponding electrical signal. Therefore, a high-precision automatic digital display pneumatic and electrical combined measuring instrument has been developed in recent years, which is introduced as follows. [page]

The pneumatic-electric combined measuring instrument is a high-precision automatic testing instrument that applies pneumatic measurement methods. During testing, it automatically converts tiny changes in workpiece size into changes in air pressure, and then converts the changes in air pressure into changes in electrical quantity through a pneumatic-electric conversion device. After logical amplification and operation processing, the results are displayed digitally.

For example, it can be used to measure the parallelism between a 20 mm diameter hole and a 25 mm diameter shaft on the crank workpiece shown in Figure 3 (the allowable parallelism error is 100:0.02). As shown in Figure 4, the shaft is used as the measuring standard, the shaft on the workpiece 2 is installed into the positioning hole of the instrument body 3, and the double-tube probe 1 is inserted into the measured hole. If the axis of the measured hole is not parallel to the axis of the standard, the measurement indication of the double-tube probe is different. In order to convert these two different air pressure quantities into electrical signals, the gas-electric combination meter uses two gas-electric converters, which are respectively connected to the two probe air pipes. The structural working principle of the gas-electric converter is shown in Figure 5.

Gas-to-electric converter operation

Figure 6-block diagram of the working principle of the inductive micrometer. The gas-electric combination measuring instrument has many advantages. Its minimum size resolution is 0.1 micron, the signal stability is 2 microns/8 hours, the measurement performance is reliable, the operation is convenient, and the application range is wide. It is suitable for both mass production measurement and small production measurement. However, the price of the gas-electric combination measuring instrument is relatively expensive, so it is currently mainly used in batch production measurement. It can be believed that with the development of my country's electronic industry, electronic instrument manufacturing industry and testing and measuring tool industry, this gas-electric combination measuring instrument will be widely used. When the gas flows through the filter and the regulator to form a clean and stable compressed air flow, it is sent to the gas-electric converter (as shown by the arrow in Figure 5). The incoming air flow is divided into two paths. One path enters the inner cavity of the bellows 3 through the nozzle 1 and is discharged into the atmosphere through the exhaust nozzle 7; the other path enters the outer cavity of the bellows 3 through the intake nozzle 2 and is ejected from the measuring nozzle 6. As the measured size of the workpiece changes, the gap between the measuring nozzle and the workpiece also changes, thereby changing the originally set air pressure difference between the inner and outer cavities of the bellows 3, causing the bellows 3 to expand and contract along the axial direction, and driving the magnetic core 4 to move, causing the inductance in the induction coil 5 to change, and the change in inductance is sent to the inductive micrometer. Since the bellows will also expand and contract under the action of a slight change in air pressure, the measurement sensitivity and measurement accuracy are high.

In order to process the change of inductance through logical operation and display the processing result in digital form, the gas-electric combination meter simultaneously connects the electrical signal output ends of the two gas-electric converters to the input end of a digital display inductance micrometer for operation and processing, and the instrument displays the measurement result in digital form. The working principle block diagram of the inductance micrometer is shown in Figure 6.

The gas-electric combination measuring instrument has many advantages. Its minimum size resolution is 0.1 micron, signal stability is 2 microns/8 hours, measurement performance is reliable, operation is convenient, and application range is wide. It is suitable for both large-scale production measurement and small-scale production measurement. However, the price of the gas-electric combination measuring instrument is relatively expensive, so it is currently mainly used in batch production measurement. It is believed that with the development of my country's electronic industry, electronic instrument manufacturing industry and testing and measuring tool industry, this gas-electric combination measuring instrument will be widely used.