Intelligent digital measurement of inductance

In practical applications, it is often necessary to determine the inductance of a winding coil. Measuring with a Q meter requires calibration and tuning, which is very inconvenient. Using intelligent bridges, phase-sensitive detection and other methods, the measurement is complex and has low accuracy. With the development of science and technology, intelligent digital measurement of instruments has become a trend. This article introduces an intelligent digital measurement technology for inductance that converts inductance into time intervals and uses a counter and LCD for digital display.

   1 SolutionIWMS_AD_BEGIN IWMS_AD_END

   A three-point sine wave oscillator solution, the measurement expression is:

   

    When C is determined, as long as the oscillation frequency f is measured, the measured inductance can be calculated. This is an inverse square proportional relationship. If it is not corrected or calculated nonlinearly, it will be difficult to scale and read.

　　Another consideration is the hysteresis comparator square wave oscillator solution. The measured inductance Lx is converted into an equivalent capacitance Cx using an active gyrator, and then Cx is connected to the negative feedback channel of the hysteresis comparator to form a square wave oscillator. The square wave oscillator period is proportional to Cx, that is, the square wave period is proportional to the measured inductance Lx, and the relationship is linear. The block diagram is shown in Figure 1.

    This solution is simple and easy to implement, has good linearity, and is convenient for digital display.

           

       2 Circuit

　　This scheme is generally composed of two parts. The first part is the L/T conversion circuit. The second part is the counting, decoding and display circuit. Here we mainly introduce the first part of the circuit.

       2.1 Hysteresis comparator square wave oscillator

　　As shown in Figure 2, if CB is connected to the negative feedback channel of the hysteresis comparator, the signal to be compared is obtained from the output end of the comparator through the negative feedback channel. The reference voltage for comparison is zero, and the jump point voltage value of the circuit state conversion is determined by the output voltage U0 through the voltage divider composed of R2 and R3. The generated square wave positive period 

     

　　
　　From formula (1), we know that T∝CB. If CB∝Lx can be made, then T∝Lx can complete the digital measurement of inductance. The active gyrator can complete CB∝Lx, that is, L/C linear conversion. 

            

       2.2 Active gyrator

　　As shown in Figure 3. As long as the open-loop gain of the two operational amplifiers is close to infinity, the characteristics of the active gyrator can be characterized by its admittance matrix, that is, 
 

                          

　　If a potential Lx is connected between point D and the ground, a capacitor Cx will be equivalent between point B of the active gyrator and the ground, and its value is: 

    

　　
       3 Conclusion

　　As long as the amplifier, resistor and other related devices are reasonably selected, a certain degree of measurement accuracy can be guaranteed. It has the characteristics of simple circuit and low cost, and has a certain effect after practical application.