Precision movement sensor - the circuit diagram

Radio, 1988, 09

This inductive device can be used as a very precision sensor of mechanical movements (see the circuit diagram in Fig. 1.). The circuit diagram comprises the oscillator, the inductive transducer and the voltage regulator.

The circuit diagram of the precision movement sensor

Fig. 1.
VD1..VD6 - D310 - old germanium USSR diodes; VT1 - KT502B; VT2,VT6 - KT315B; VT3,VT4 - KT316B; VT5 - KT361B;
C1 - 1μF; C2,C5 - 1.2nF; C3, C4 - 0.1μF;
R1 - 10k potentiometer; R2 - 1.8k; R3, R4 - 16k;
L1 - 500 turns, L2 - 500 turns of 36..37 AWG (0.12 mm) wire, wound on E-shaped ferrite cores of 4x4 mm;
PA1 - 30 μA ammeter with zero in the middle of the scale (model M4248).

The oscillator circuit based on the complementary transistor pair VT5 and VT6. Coils L1 and L2 of the sensor and capacitors C3 and C4 form a serial resonant tank that defines the oscillator frequency. Capacitors C2 and C5 is used in the feedback circuit. This connection method automatically provides the operation of the circuit in resonance because inductive resistance of the bridge is compensated by its capacitance resistance, so the resistance of any circuit network is almost equal to the resistance of inductance coils. Because the quality factor of circuit tank L1L2C3C4 is much more then 1, therefore the voltage across its load is sinusoidal (if the feedback network has an optimal gain).

The construction of the precision movement sensor

Fig. 2.

The Fig. 2. shows the construction of the inductive transducer. Coils L1 and L2 are winded on E-shaped ferrite cores 2. There is a gap between this cores for the anchor plate 1, made of a magnetic metal. The anchor plate is connected by the arm 3 to the moving part of an object.

Coils L1 and L2 have 500 turns of enameled copper wire, 36..37 AWG (0.12 mm), that is wound on E-shaped cores of 4x4 mm made of 2000NM ferrite.

Diodes VD1, VD2 and capacitors C2, C5 form two networks that restore DC part of the signal - it increases a voltage of starting pulses that makes easier starting of the oscillation of the circuit in case if power supply voltage is low. The circular detector network based on diodes VD3-VD6, it produces a difference DC voltage that is proportional to the movement of the anchor.

In experiments it was found that the sensitivity of the device remains almost the same if capacitors C3, C4 of the measurement bridge circuit have value from 0.01 μF to 0.18 μF. The resonant frequency will be changed automatically - it depends on parameters of serial LC networks.

Power supply voltage and ambient temperature are destabilization factors. To stabilize voltage, is used the voltage regulator, based on transistors VT1..VT4. The transistor VT4, connected as a diode, is used as a stable voltage source.

A differential amplifier is based on transistors VT2 and VT3, and it controls the transistor VT1. The output voltage can be adjusted by using the potentiometer R1. The output voltage should be set in range of 1.8...2.5 volts.

It was found that main sources of temperature errors are ammeter PA1 (model M4248 - its inside resistance changes 10 Ohms/°C ) and voltage regulator circuit.

Set of parameters of the precision movement sensor

Fig. 3, a.

Transfer curves of the precision movement sensor

Fig. 3, b.

The device consumes 4..7 mA, its sensitive is 3..6 mA per μm. Fig. 3, a, shows parameters of the precision movement sensor - output currents vs movement of the anchor (F = 5kHz, Usup = 1.5; 2; 2.5 V). As we can see, all 3 lines are intersected in a point aside of the zero point. In this case, if the anchor is located exactly between two cores, it means that output signal is not zero. The reason of it - both coils L1 and L2 are not identical - there is a slight difference between inductances of L1 and L2. To fix it, we can use a resistor of low value, connected in series with L1 or L2.

Fig. 3, b, shows transfer curves that determine the sensitivity of the device when the frequency is changing. The curve 1 was obtained with capacitive arms in the measurement bridge, the curve 2 - with inductive arms, and the curve 3 - with active arms. The curve 4 was obtained with capacitive arms, but in this case both coils L1 and L2 had twice number of turns - 1000 turns each.

As we can see from Fig. 3, b, circuits with capacitive arms are more efficient.

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