RL oscillation circuit for inductance measurement

Abstract

The application relates to an RL oscillation circuit using an inductor coil as a timing element. The control circuit, by changing the current in the inductor coil between an upper threshold current and a lower threshold current, generates a pulse signal having a cycle and an inductance value being directly proportional. The whole circuit is simple and easy to integrate.

Description

An RL oscillating circuit for measuring inductance Technical Field

[0001] The present invention relates to an oscillating circuit using an inductor coil as a timing element. Background Art

[0002] Currently, the LC oscillation circuit is commonly used to measure inductance coils. Since the inductor L and the capacitor C are both energy storage components, the LC oscillation circuit has poor controllability. When the Q value of the oscillation circuit is relatively low, the circuit is not easy to oscillate. At the same time, the inductance of the inductor is in a square relationship with the frequency, and the linearity is poor.

[0003] Summary of the Invention

[0004] Technical problems to be solved

[0005] To address the shortcomings of the existing technology, the present invention proposes an RL oscillation circuit using an inductor coil L as a timing energy storage element. The inductance of the inductor is measured by alternately controlling the increase and decrease of the current flowing through the inductor. The alternating process will generate a periodic pulse signal, and the period of the pulse signal is proportional to the inductance of the inductor and has a linear relationship.

[0006] Technical Solution

[0007] An RL oscillator circuit for measuring inductance is characterized by comprising a current sampling resistor connected in series with the inductance to be measured, a voltage comparison circuit connected to the current sampling resistor and having an upper threshold voltage and a lower threshold voltage, wherein the voltage comparison circuit controls a charging circuit for charging the inductor and a discharging circuit for discharging the inductor.

[0008] When the voltage drop across the current sampling resistor caused by the current in the inductor is less than the lower threshold voltage, the voltage comparison circuit opens the charging circuit and closes the discharging circuit, and the current in the inductor begins to increase.

[0009] When the current in the inductor increases to the point where the voltage drop across the current sampling resistor is greater than the upper threshold voltage, the voltage comparison circuit controls the charging circuit to stop working and the discharge circuit to start working, and the current in the inductor begins to decrease.

[0010] When the current in the inductor decreases to the point where the voltage drop across the current sampling resistor is less than the lower threshold voltage, the charging circuit starts working again and the discharging circuit stops working.

[0011] By repeating the above process periodically, a pulse signal output will be generated, and the period of the output pulse is proportional to the inductance of the inductor. When the voltage drop of the current value in the inductor on the current sampling resistor is between the upper and lower threshold voltages, the control circuit state remains unchanged.

[0012] The RL oscillator circuit includes a positive feedback circuit controlled by the output state of the voltage comparison circuit, which is used to generate an upper threshold voltage and a lower threshold voltage.

[0013] The RL oscillating circuit includes a voltage comparison circuit connected to an upper threshold voltage, a voltage comparison circuit connected to a lower threshold voltage, and an RS trigger for maintaining a current state.

[0014] The charging circuit is composed of a voltage source or a resistor voltage divider network controlled by a semiconductor switch, which is used to determine the steady-state current in the inductor and the charging time constant of the RL loop during the charging process.

[0015] The discharge circuit is a semiconductor switch. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG1 is a circuit diagram of an embodiment of the present invention

[0017] FIG2 is a circuit diagram of another embodiment of the present invention

[0018] In Figure 1, LX-inductor R1-current sampling resistor IC-voltage comparator T1-PMOS field effect transistor T2-NMOS field effect transistor WY-parallel voltage regulator circuit chip R2, R3, R4, R5-resistor POUT-pulse signal output terminal

[0019] One end of the inductor LX is connected to the current sampling resistor R1 to measure the current flowing through the inductor LX, and is also connected to an input end of the voltage comparator IC.

[0020] The PMOS transistor T1, used to charge the inductor LX, and the NMOS transistor T2, used to discharge the inductor LX, are used to expand the output capability of the voltage comparator. Since these two transistors are connected in series with the inductor LX during operation, their on-resistance affects the time constant of the RL loop. If the internal resistance of the voltage comparator output is low, T1 or T2 can be omitted, and the inductor LX can be charged and discharged directly by the voltage comparator output.

[0021] Since the operating voltage of the circuit is much greater than the input voltage of the voltage comparator IC, if the inductor LX is charged directly from the operating voltage, the steady-state current of the inductor LX will be relatively large, and the inductor LX will be charged to the upper current threshold in a short time, making the duty cycle of the output pulse very small. Therefore, using a current-limiting resistor R5 and a parallel voltage regulator chip WY to form a parallel voltage-regulated power supply with relatively small internal resistance to charge the inductor LX can extend the charging time.

[0022] Resistors R2 and R3 add a fixed bias voltage to the non-inverting input terminal of the voltage comparator, and resistor R4 is connected to the non-inverting output terminal of the control circuit.

[0023] When the output of the control circuit is at a low level, at this time T1 is cut off, T2 is turned on, and R4 is connected to the low level, generating a lower threshold voltage at the non-inverting input terminal of the voltage comparator IC.

[0024] When the output of the control circuit is at a high level, at this time T1 is turned on, T2 is cut off, and R4 is connected to the high level, generating an upper threshold voltage at the non-inverting input terminal of the voltage comparator IC.

[0025] The upper threshold voltage value is greater than the lower threshold voltage value; when the inductance of the inductor LX is in the microhenry order, it is recommended that the upper threshold voltage value be in the range of dozens to hundreds of millivolts, and the upper threshold voltage is recommended to be 2 times the lower threshold voltage, and Vz is a shunt voltage regulator circuit of 1.2 volts.

[0026] Circuit working process

[0027] When the circuit is powered on, since the current I in the inductor is 0, the signal voltage V2 at the inverting input terminal of the control circuit is V2 = I * R1 = 0, T1 is turned on, T2 is cut off, and the output of the control circuit is at a high level.

[0028] At this time, the voltage at the non-inverting input terminal of the control circuit is V1 = R2 * VCC / (R3 / / R4 + R2) = Vth, the voltage at the right end of the inductor LX is Vz, and the current I flowing into the inductor LX starts to increase.

[0029] When the current I increases to I * R1 > Vth, the control circuit starts to flip, T1 is cut off, T2 is turned on, and the output of the control circuit is at a low level.

[0030] At this time, the voltage at the non-inverting input terminal of the control circuit is V1 = R2 / / (R4 + R5) * VCC / (R2 / / (R4 + R5) + R3) = Vtl, the voltage Vz at the right end of the inductor LX is 0, the current I flowing into the inductor LX starts to decrease, and the inductor LX discharges through the current sampling resistor R1 and T2.

[0031] When the current I in the inductor decreases to I * R1 < Vtl, the control circuit flips back to the output high level again, T1 is turned on, T2 is cut off, the voltage at the non-inverting input terminal of the control circuit becomes V1 = Vth, the voltage at the right end of the inductor LX is Vz, and the current I flowing into the inductor coil LX starts to increase.

[0032] Repeating the above process, a pulse signal whose pulse period is proportional to the inductance of the inductor LX is output. When the current in the inductor LX is between the upper and lower thresholds Vth > I * R1 > Vtl, the control circuit maintains its original state.

[0033] Calculating the period of the RL oscillation circuit using the three-element method

[0034] During charging, the charging loop resistance is composed of the current sampling resistor R1, the DC resistance of the inductor LX, and the internal resistance RT1 of the PMOS field-effect transistor T1 in series. The loop resistance during charging is Rc = R1 + Rt1 + RLX, the charging time constant τc = LX / Rc, and when t = ∞, the steady-state value of the charging current is lc = Vz / Rc.

[0035] The current l in the inductor LX is charged from the lower threshold value Itl = Vtl / R1 to the upper threshold value Ith = Vth / R1. The charging time tc = τc*Ln(Ic-Itl) / (Ic-Ith), (the natural logarithm of Ln).

[0036] During discharge, the discharge loop resistance is the series connection of the current sampling resistor R1, the DC resistance of the inductor LX, and the internal resistance RT2 of the NMOS field-effect transistor T2. The discharge loop resistance Rf = R1 + Rt2 + RLX, the discharge time constant τf = LX / Rf, and the steady-state value lc of the charging current is 0 when t = ∞.

[0037] The current l in the inductor LX discharges from the upper threshold value Ith=Vth / R1 to the lower threshold value Itl=Vtl / R1, and the time taken for the discharge is tc=τf*Ln(Ith / Itl).

[0038] The period of the output pulse signal POUT is T=Tc+Tf

[0039] In Figure 2, LX-inductor R1-current sampling resistors IC1, IC2-voltage comparator IC3-RS trigger T1-PMOS field effect transistor T2-NMOS field effect transistor WY-parallel voltage regulator circuit chip R2, R3, R4, R5-resistors POUT-pulse signal output terminal

[0040] In FIG. 2 , IC1 is an upper threshold voltage comparator, IC2 is a lower threshold voltage comparator, IC3 is an RS trigger, and R2, R3, and R4 are used to generate voltage divider resistors for the upper and lower threshold voltages.

[0041] Unlike the embodiment of FIG1 , which uses voltage positive feedback to keep the state of the control circuit unchanged when the current in the inductor is between the upper and lower thresholds, the embodiment of FIG2 uses an RS trigger to keep the state of the control circuit unchanged when the current in the inductor is between the upper and lower thresholds.

[0042] Figure 2 Working process of the circuit

[0043] When the current in the inductor is I and the voltage drop across the current sampling resistor R1 is less than the lower threshold voltage of the voltage comparator lC2, the input terminal S of the RS flip-flop lC3 connected to the output terminal of the voltage comparator lC1 is "0", and the input terminal R connected to the output terminal of the voltage comparator lC2 is "1". The RS flip-flop lC3 is cleared to "0", the output terminal Q is "0", the output of the control circuit is high, T1 is turned on, T2 is turned off, the inductor LX is charged by the voltage Vz, and the current begins to increase.

[0044] When the current I in the inductor increases to the point where the voltage drop across the current sampling resistor R1 exceeds the upper threshold voltage of the voltage comparator lC1, the input terminal S of the RS flip-flop lC3 connected to the output terminal of the voltage comparator lC1 is "1", the input terminal R connected to the output terminal of the voltage comparator lC2 is "0", the RS flip-flop lC3 is set to "1", the output terminal Q is "1", the output of the control circuit is low, T1 is turned off, T2 is turned on, the voltage Vz at the right end of the inductor LX = 0, the current I flowing into the inductor coil LX begins to decrease, and the inductor LX discharges through the current sampling resistors R1 and T2.

[0045] When the current I starts to decrease until the voltage drop across the current sampling resistor R1 in the inductor coil is less than the lower threshold voltage of the voltage comparator IC2, the RS trigger IC3 is cleared to "0" again, the control circuit output is high, and the current in the inductor LX begins to increase again.

[0046] When the current in the inductor LX is between the upper and lower thresholds, the RS trigger maintains its original state.

Claims

1. An RL oscillator circuit for measuring inductance, characterized in that: The device is composed of a current sampling resistor connected in series with the inductor to be measured, a voltage comparison circuit connected to the current sampling resistor and having an upper threshold voltage and a lower threshold voltage, and a charging circuit and a discharging circuit controlled by the voltage comparison circuit.

2. The RL oscillator circuit according to claim 1, characterized in that: The invention comprises a positive feedback circuit controlled by the output state of the voltage comparison circuit, and is used for generating an upper threshold voltage and a lower threshold voltage.

3. The RL oscillator circuit according to claim 1, characterized in that: The invention comprises a voltage comparison circuit connected to an upper threshold, a voltage comparison circuit connected to a lower threshold, and an RS trigger connected to the outputs of the two voltage comparison circuits for maintaining the current state.

4. The RL oscillator circuit according to claim 1, characterized in that: The charging circuit is a voltage source or a resistor divider network controlled by a semiconductor switch.

5. The RL oscillator circuit according to claim 1, characterized in that The discharge circuit is a semiconductor switch.

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