JPS596452B2 - Proximity switch oscillation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an oscillation circuit for a high frequency oscillation/loss detection type proximity sensor.

The high-frequency oscillation/loss detection type proximity sensor takes advantage of the fact that the high-frequency loss of the detection coil increases when a metal object approaches the detection coil, and forms an oscillation circuit that includes this detection coil. The oscillation width is attenuated or the oscillation is stopped as the oscillation width approaches.

In order to prevent chattering as a proximity switch, improve noise resistance, and ensure reliable operation, it is effective to provide hysteresis at the oscillation start point and oscillation stop point of the oscillation circuit to create a so-called hard oscillation state.

An object of the present invention is to provide an oscillation circuit with a single proximity switch suitable for realizing the oscillation circuit in the hard oscillation state using an IC circuit.

An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 1, the transistor Tr1 is connected as an emitter follower, and a resistor Re is connected to the emitter.

The collectors of transistors T, 2 are connected to the collectors of transistors Tr2 and Tr3.
The bases are commonly connected and the emitters are commonly connected to form a current mirror circuit.

A parallel resonant circuit (tank circuit) consisting of a detection coil L and a capacitor C is connected to the collector of this transistor Tr3.

Further, the emitter of the transistor Tr4 is connected to this parallel resonant circuit.

The collector and base of this transistor Tr4 are connected in common, and are connected to the base of the transistor Tr1 and to the collector of the transistor Tr5.

This transistor Tr4 constitutes a level shift circuit that raises the voltage of the parallel resonant circuit to a certain value.

The transistor Tr5 and the transistor Tr6 are configured as a current mirror circuit, and a current of Vcc -VBE/Ra flows through the emitter junction of the transistor Tr6 (VBE is the base-emitter voltage of the transistor Tr6). The same current flows to the collector of the transistor Tr5 due to the function of
1. It is sent as a bias current to transistor Tr4.

That is, this transistor Tr5, transistor T,
6 serves as a current source for supplying bias currents to the transistors Tr4 and Tr1.

Next, the operation will be explained.

If a voltage VT is generated in the parallel resonant circuit of the detection coil L and the capacitor C, the emitter voltage of the transistor Trl will be approximately equal to vT.

Note that the transistor Tr4 is inserted as a level shift circuit to compensate for the base-emitter voltage VBE (approximately 0.6V) of the transistor Tr□.

In other words, the base voltage of the transistor T4 is always higher than the voltage of the parallel resonant circuit by the base-emitter voltage VBE (0.6V) of the transistor Tr4.
The emitter voltage of the transistor Tri is always equal to the voltage of the parallel resonant circuit.

Therefore, the collector side D of the transistor Tr□ is 11=
VT/R8 (R8: value of resistor R8) A current 1 will flow.

This current 1 is the transistor Tr2. A current 1F, expressed as IF=1=VT/R8, is fed back through a current mirror circuit made up of Tr3.

The reason why the transistor Tr0 is connected as an emitter follower is as follows.

The role of the transistor Tr□ is to convert the voltage VT generated in the LC parallel resonant circuit into a current proportional to it.
Furthermore, it must have high input impedance so as not to affect the LC parallel resonant circuit.

By connecting the transistor Tr1 as an emitter follower, a current, I, flows through the emitter of the transistor Tr1.

is 8-vT/R8, which is proportional to VT, and transistor Tr1
Since the collector current i11 of is approximately equal to this emitter current, the collector current 2 of the transistor Tr0 is converted into a current proportional to VT as described above.

Furthermore, since the transistor Tr1 has an emitter-follower connection, its human impedance is high (and suitable for the above role).

Now, if we ignore the base input current of transistor Tr1, the circuit side conductance G seen from both ends of the parallel resonant circuit
OBC is expressed as Gosc = IF / VT = ' /Re.

That is, the base potential of the transistor Tri is higher than the parallel resonant circuit voltage vT by the VBE of the transistor Tr4 (
, since one end of the resonant circuit is connected to Ov, if the voltage vT generated in the resonant circuit becomes a sine wave centered around Ov as shown by the solid line in FIG. The VBE is higher than the 4th
It will look like the dotted line in the figure.

The emitter potential of the transistor Tr1 is the same as that of the transistor Tr1.
Since the base voltage of transistor Tr□ is lower than the base voltage of transistor Tr,1, transistor Tr,1 and transistor Tr4
As long as the VBE of the transistors Tr
The emitter voltage of 1 is approximately equal to vT.

Since the emitter current of the transistor Tr0 flows only in the positive direction, as a result, the emitter voltage of the transistor Tri has a waveform equal to the upper half cycle of VT, and the waveform of the collector current 11 of the transistor Tr0 also becomes equal to this.

The collector current 1 of the transistor Tr1 is the same as the collector current 1 of the transistor Tr2. The current is fed back to the parallel resonant circuit through the current mirror circuit of Tr3.

Therefore, this feedback current 1F is obtained by half-wave rectification of vT as shown in FIG. 4B.

However, the effective component that excites the parallel resonant circuit is the component equal to the resonant frequency of this parallel resonant circuit, that is, the fundamental wave component I when the IF of the above half-wave rectified waveform is Fourier expanded.
Therefore, the waveform of this fundamental wave component IFI is a sine wave having the same period and phase as the original waveform, as shown in FIG. 4C.

On the other hand, the conductance on the detection coil L side is represented by the resonance point conductance G coi 1 of the parallel resonant circuit, and the oscillation condition is Gcoil + Gosc≦0.

Here, the fact that the oscillation circuit is in a hard oscillation state means that the conductance G□sc on the circuit side tends to increase with the amplitude vT.

This will be explained with reference to FIG. FIG. 2 shows the characteristics of conductance Gcoil and Gosc with respect to amplitude vT.

GcoilO is generally a straight line because it does not depend on the amplitude vT.

Therefore, in order for the oscillation circuit to enter the hard oscillation state, G o
As shown in FIG. 2, it is necessary for sc to have a tendency to increase slightly as the amplitude vT increases, and to eventually become saturated.

G coi 1 is low when metal etc. are not close,
It increases as you get closer.

Now, when there is no metal etc. approaching, G coil is a in Figure 2.
Assuming that, the oscillation width vT is stable at Po.

When a metal or the like approaches, G co i l increases to b, and the oscillation width becomes P2.

If the metal etc. approaches any further and Gcoil increases, there will be no stable point and oscillation will stop.

To make it oscillate again, Gcoil must be lowered to C.

This is the starting point of oscillation. When Gcoil is lowered to C, the oscillation width rapidly grows and stabilizes at P3.

That is, the state of oscillation is determined by the magnitude relationship between the negative conductance Gosc on the circuit side and the conductance Gcoil on the coil side.

A, Gosc) Gcoil...(In the state of excess energy, the oscillation becomes thicker.) Gosc = Gcoil...(In the state of equilibrium energy, the oscillation amplitude maintains that state and becomes stable.) Gosc (Gcoil...・(In a state of insufficient energy, the oscillation becomes smaller and finally stops.) If the conductance G osc on the circuit side is non-linear as shown in Figure 2, the conductance G coil of the coil
may intersect at two points.

For example, in the case of b, XGosc and Gcoil are P4, Pl
If they intersect at two points, the oscillation amplitude is originally P
If it is less than 4, oscillation will not grow and will stop.

If P4 is exceeded due to some factor (such as noise), the oscillation grows and reaches Pl, where it becomes stable.

If the oscillation amplitude becomes thicker than Pl, it will become state C and the oscillation will be attenuated.

On the other hand, if it tries to become smaller than Pl, it will be in the state of A and the oscillation will grow and it will always be pushed back to the 21 point.

In the case of C, P5 is near the amplitude O, and oscillation starts with a small amplitude, grows, and stabilizes at P3.

In the embodiment of FIG. 1, transistors Tr2. A resistor R is connected between the common emitter and the common base of the current mirror circuit formed by Tr3.

Set the value of this resistor R to infinity (no resistor connected), 1
By changing Ω to 00, Ω to 68, Ω to 39, the third
The characteristics of current (1) and current (2) F as shown in the figure are obtained.

This is due to the following reason.

Transistor Tr2. In the current mirror circuit of Tr3, if we ignore the base current of transistor Tr3 (however, ■E8 ; emitter current IT of transistor Tr2 : emitter saturation current vo ; kT / q k : Hornmann constant T : absolute temperature q ; electron charge VBE ;) Base-emitter voltage IR of range metal Tr3: This is the current flowing through the resistor R, and if IT and IR are expressed in terms of the relationship of VBE, the result will be as shown in FIG.

In other words, since IT increases exponentially and IR increases linearly, in areas where VBE is small, the ratio of ■□ in ■1 is large, and as VBE increases, IT in 11 increases.
There is a tendency for the proportion of

In other words, the ratio of IT is small in areas where ■ is small, and the ratio of IT is large in areas where 11 is large.

IF is almost equal to IT, and as a result, the relationship between 11 and IF is as shown in Figure 3. In the area where 11 is small, the ratio of IF is small, and as 11 becomes large, IF tends to approach this. .

As can be seen from Figure 3, when the resistor R is not connected, the current IF is directly proportional to the current I0 at a ratio of approximately 1:1, but when the resistor R is connected, the ratio of the output to the input is in the region where the current 1 is small. becomes smaller.

That is, in a region where the oscillation width ⅠT is small, the feedback current IF is small and Gosc is small, and in a region where the oscillation width VT is large, the feedback current ⅦF is large and Gosc is also large.

Since Gosc has frequency characteristics, it tends to saturate and decrease in a region where the amplitude VT becomes thicker than a certain degree.

Therefore, it can be seen that the G osc of the circuit shown in FIG. 1 has almost the same characteristics as that shown in FIG. 2.

As described above with respect to the embodiments, according to the present invention, the resistor R is connected to the current mirror circuit constituting the current feedback circuit in order to provide nonlinearity to the input/output characteristics, so that the oscillation circuit is brought into a hard oscillation state. was completed.

Therefore, it is possible to prevent chattering and improve noise resistance.

Furthermore, according to the circuit of the present invention, the detection coil has two terminals, and one end of the detection coil can be shared with the Ov side of the power supply, making connection easy. It can be varied without changing the constant, and one end of this resistor R is also common to the power supply Ov side, and the oscillation frequency is approximately determined by the inductance of the detection coil and the capacitance of the capacitor connected in parallel to this detection coil. The structure such as the rotor can be extremely simplified.

Moreover, by converting it into an IC, the transistor Tr2,
The circuit of the present invention is suitable for IC implementation because it is easy to balance the characteristics of Tr3 and therefore a current mirror circuit can be easily constructed.

Further, the transistor Tr1 constitutes a simple emitter follower-connected amplifier circuit, and its operation does not depend much on variations or drifts in hFE, so the structure as a whole does not depend on variations in elements.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing one embodiment of the present invention, and Fig. 2 is a circuit diagram showing an embodiment of the present invention.
Width vT and conductance G to explain the operation in the figure
Figure 3 is a graph showing the relationship between current ■ and ■F when the value of resistor R is changed, and Figure 4 is a graph showing the relationship between current ■ and ■F when the value of resistor R is changed.
A waveform diagram showing the waveforms of each part in the figure, Figure 5 is a transistor T
It is a graph showing the voltage/current characteristics at r2. Tro... Transistor connected as an eminter follower, Tr2, Tr3... Transistor constituting a current mirror circuit, L... Detection coil.

Claims (1)

[Claims] 1. A first transistor connected as a mechanical follower;
A current mirror circuit consisting of a second and third transistor whose emitters and bases are commonly connected to obtain a current approximately equal to the current flowing through the collector of the first transistor, and a common emitter and a common base in this current mirror circuit. a parallel resonant circuit of a detection coil and a capacitor connected so that the output current of the current mirror circuit flows, and a parallel resonant circuit connected between the parallel resonant circuit and the base of the first transistor. This is a proximity switch oscillation circuit consisting of a level shift circuit with a bias current and a current source that supplies a bias current to the level shift circuit.