JPH0425218A - High frequency oscillation type proximity switch - Google Patents

Abstract

PURPOSE:To prevent malfunction at the time of supplying a DC magnetic field to a detection section by connecting a capacitor in parallel with a detection range adjustment resistor. CONSTITUTION:A frequency characteristic adjustment capacitor C is connected in parallel with a detection range adjustment resistor R. In this case, when a DC magnetic field is supplied to a ferrite core FC of a detection section 2a, the conductance of an oscillation coil HC is increased, resulting that the parallel impedance of the detection range adjustment resistor R and the frequency characteristic adjustment capacitor C is decreased, that is, the negative conductance of an oscillation circuit main body 2b is increased. Thus, malfunction of the switch due to the application of a DC magnetic field is prevented.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to a high frequency oscillation type proximity switch.

(Prior Art) FIG. 4 is a schematic circuit block diagram of a conventional proximity switch of this type.

In the figure, 2 is an oscillation circuit, and 4 is a switch output processing circuit. The oscillation circuit 2 is of a Hartley type, and includes a detection section 2a and an oscillation circuit main body 2b. The detection unit 2a is formed by attaching an oscillation coil HC to a ferrite core PC. The oscillation circuit main body 2b mainly constitutes an amplifier section as another circuit section of the oscillation circuit 2, and has a detection distance adjustment resistor R. The resistance value of this detection distance adjustment resistor R is adjusted in order to adjust the amount of positive feedback of the oscillation output to the amplifier section and determine the magnitude of the amplitude of the oscillation output of the oscillation circuit 2. Through adjustment, the magnitude of the leakage magnetic field of the oscillation coil HC, that is, the detection distance of the metal object to be detected can be adjusted.

When the metal object to be detected does not approach the leakage magnetic field of the oscillation coil HC in the detection unit 2a, there is no change in the leakage magnetic field, and the oscillation circuit 2 detects the leakage magnetic field of the oscillation coil HC at that time.
An oscillation condition based on the conductance of the oscillation circuit main body 2b and the negative conductance of the oscillation circuit main body 2b is established, and an oscillation state is established. On the other hand, when the metal approaches the leakage magnetic field of the oscillation coil HC, eddy current loss occurs in the metal due to the leakage magnetic field and the conductance of the oscillation coil HC increases, causing the oscillation condition of the oscillation circuit 2 to fail. The oscillation stops.

The switch output processing circuit 4 is composed of a detection circuit 4a, a Schmitt trigger circuit 4b, and an output circuit 4c, and the oscillation frequency output from the oscillation circuit 2 is detected by the detection circuit 4a, and the oscillation circuit 2 A Schmitt trigger circuit 4 with a certain threshold value detects the detection output when the oscillation is in the oscillation state and the detection output when the oscillation is stopped.
b, and this discrimination output is sent from the output circuit 4c as a switch-off signal (a signal when the oscillation circuit 2 is in a continuous oscillation state because the metal is in a non-approaching state) or a switch-on signal (a signal when the oscillation circuit 2 is in a continuous oscillation state because the metal is in a non-approaching state). The signal is output as a signal when the oscillation circuit 2 is in an oscillation stopped state.

The above operation will be explained in more detail.

First, assuming that the inductance of the oscillation coil HC itself is Ll, and the apparent magnetic permeability of the ferrite core PC is μapp, the overall inductance L of the detection section 2a is expressed by the following equation (2).

L-μal)l)L+... ■The oscillation frequency r of the oscillation circuit 2 is calculated from the above formula (2) by the following formula (2), assuming that the oscillation capacitance of the oscillation circuit 2 is C.

f = 1/2π, Masashita 1/2π stone i11] = ......■ Also, the conductance g (l) of the oscillation coil HC is expressed by the following formula (2).

g&=1/2πfLQ =1/2πf Ba ppL+Q −■ However, Q is the selectivity of the oscillation circuit 2.

Further, assuming that the negative conductance of the oscillation circuit 2 is gi, the oscillation circuit 2 oscillates when the following oscillation conditional expression (2) is satisfied.

gC+(-gi)≦0 . . . In such a relational expression, the cases in which the metal is in a non-approaching state and in an approaching state to the oscillation coil HC will be explained with reference to FIG. 5. In FIG. 5, the horizontal axis shows the oscillation frequency f of the oscillation circuit 2, and the vertical axis shows the conductance gQ of the oscillation coil HC.

First, when metal is not close to the oscillation coil HC, the frequency characteristic of the conductance gQ of the oscillation coil HC is (I). In this case, when the oscillation frequency of the oscillation circuit 2 is fl, the conductance gC of the oscillation coil HC is smaller in absolute value than the negative conductance -gl, as shown in FIG. The oscillation condition is satisfied, and the oscillation circuit 2 is in an oscillation state. the result,
The switch output processing circuit 4 outputs a switch-off signal indicating that metal is not in close proximity.

On the other hand, when metal approaches the oscillation coil HC, the conductance ge of the oscillation coil HC increases, and the conductance characteristic becomes (U). Here, as is clear from equation (2), since the oscillation frequency of the oscillation circuit 2 remains fl, the conductance of the oscillation coil HC increases by 620 minutes at the oscillation frequency fl, and as a result, the oscillation Conductance of coil HC (gQ
+ΔgC) becomes larger in absolute value than the negative conductance -+z2, and the oscillation condition of the above formula (■) is not satisfied, the oscillation of the oscillation circuit 2 stops, and the switch output processing circuit 4
will output a switch-on signal indicating that metal is in close proximity.

In this way, the conventional proximity switch can detect the proximity state of metal.

(Problem to be Solved by the Invention) By the way, in this proximity switch, the ferrite core FC of the detection unit 2a is not made of metal, which is the object to be detected.
For example, when a DC magnetic field from a magnet is applied, the apparent magnetic permeability μapp of the ferrite core FC gradually decreases to, for example, μapp' (μapp). Then, as is clear from the equation (2), the oscillation frequency f1 of the oscillation circuit 2 increases to "B" as expressed by the following equation (2).

f 1' = 1/2πF〒71”selfτ...■Also,
As is clear from equation (2), the conductance gQ of the oscillation coil HC also increases, and its characteristics become as shown in (I[[) in FIG. 5.

Then, the conductance gQ' of the oscillation coil HC at the oscillation frequency I+' is given by the following equation (2).

g (1''', 1/2πf 1' μa l
) I) 'L+Q...■ As a result, the conductance of the oscillating coil HC (gQ+Δg 12') becomes smaller than the negative conductance -ge due to the increase in the conductance of the oscillating coil (gQ') shown in Figure 5. also becomes large in absolute value, and the formula (■) that satisfies the oscillation condition no longer holds true, and the oscillation circuit 2
The oscillation will stop.

Such stoppage of oscillation is very inconvenient because it causes a malfunction in a proximity switch that detects the proximity of metal.

Therefore, an object of the present invention is to prevent malfunction even if a DC magnetic field is applied to the detection section.

(Means for Solving the Problem) In order to achieve such an object, the high frequency oscillation type proximity switch of the present invention has an oscillation circuit and a switch output processing circuit, and the oscillation circuit is connected to a detection section. and an oscillation circuit main body, the detection part is made up of an oscillation coil attached to a ferrite core, and the oscillation circuit main body has a detection distance adjustment resistor, and also has a detection distance adjustment resistor and a magnetic field leakage from the oscillation coil. When the metal is not close, the oscillation condition based on the conductance of the oscillation coil and the negative conductance of the oscillation circuit body is established and the oscillation state is in effect. On the other hand, when the metal is close to the leakage magnetic field. The oscillation condition is no longer satisfied due to an increase in the conductance of the oscillation coil, and the oscillation is stopped.
The detection distance adjustment resistor has a resistance value that can be adjusted to determine the magnitude of the negative conductance, and the switch output processing circuit outputs a switch-off signal in response to the output of the oscillation circuit in an oscillation state. and outputs a switch-on signal in response to the output of the oscillation circuit in a state where oscillation is stopped, and the oscillation circuit main body has a frequency characteristic adjustment capacitor, and the capacitor is connected to the oscillation circuit main body. It has a frequency characteristic and is characterized in that it is connected in parallel to the detection distance adjustment resistor.

(Function) When metal is not close to the leakage magnetic field of the oscillation coil,
An oscillation condition based on the conductance of the oscillation coil and the negative conductance of the oscillation circuit body is established, and the oscillation circuit is in an oscillation state. In response to the oscillation circuit output in this oscillation state, the switch output processing circuit outputs a switch-off signal.

Also, when metal approaches the leakage magnetic field of the oscillation coil,
The conductance of the oscillation coil increases, and as a result, the oscillation condition is no longer met, and the oscillation circuit enters an oscillation-stop state. In response to the oscillation circuit output in the oscillation stopped state, the switch output processing circuit outputs a switch-on signal.

On the other hand, when a DC magnetic field is applied to the ferrite core of the detection unit, the conductance of the oscillation coil increases, but as a result, the oscillation frequency of the oscillation circuit increases, and as a result, the detection distance adjustment resistor and frequency characteristic adjustment capacitor Since the parallel impedance decreases, that is, the negative conductance of the oscillation circuit body also increases, even if the conductance of the oscillation coil increases due to the application of the DC magnetic field, the conditional expression for oscillation is satisfied, resulting in As a result, the oscillation circuit can maintain the oscillation state. by this,
Even if a DC magnetic field is applied, the switch output processing circuit is prevented from outputting a switch-on signal, and malfunction of the switch due to the application of the DC magnetic field can be prevented.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic circuit block diagram of a proximity switch according to an embodiment of the present invention, and FIG. 2 is a circuit diagram of the oscillation circuit of FIG. 1. In these figures, parts corresponding to those in FIG. 4 are designated by the same reference numerals, and a detailed explanation of the parts corresponding to the same reference numerals will be omitted here.

The proximity switch in this embodiment has a detection distance adjustment resistor R
It is characterized in that a frequency characteristic adjustment capacitor C is connected in parallel to the frequency characteristic adjustment capacitor C.

The operation is determined by the negative conductance -gi of the oscillation circuit main body 2b.
To explain with reference to FIG. 3 which shows the frequency characteristics of the oscillation circuit body 2b (viewed from the A-A end), first,
When no metal comes close to the leakage magnetic field of the oscillation coil HC, the oscillation conditional expression (2) based on the conductance g of the oscillation coil HC (and the negative conductance -gi of the oscillation circuit main body 2b) is established, and the oscillation circuit 2 is in the oscillation letter request.

In response to the output of the oscillation circuit 2 in this oscillation state, the switch output processing circuit 4 outputs a switch-off signal.

Next, when a metal approaches the leakage magnetic field of the oscillation coil HC, the conductance of the oscillation coil HC increases by 6gg, and as a result, the oscillation conditional expression (2) does not hold, and the oscillation circuit 2 stops oscillating. Become. In response to the output of the oscillation circuit 2 in the oscillation stopped state, the switch output processing circuit 4 outputs a switch-on signal.

On the other hand, when a DC magnetic field is applied to the ferrite core FC of the detection unit 2a, the conductance of the oscillation coil HC will increase, but as a result, the oscillation frequency f of the oscillation circuit 2 will increase, and the detection distance will be adjusted. Since the magnitude of the parallel impedance Z between the resistor R and the frequency characteristic adjustment capacitor C decreases, that is, the negative conductance -gi of the oscillation circuit main body 2b also increases as shown in Fig. 3, the application of the DC magnetic field decreases. Therefore, even if the conductance of the oscillation coil HC increases, the oscillation conditional expression (2) is satisfied. As a result, the oscillation circuit 2 can maintain the oscillation state, and thereby, even if a DC magnetic field is applied, the switch output processing circuit 4 is prevented from outputting a switch-on signal, and the DC magnetic field is applied. It is possible to prevent malfunction of the switch due to

(Effects of the Invention) As is clear from the above explanation, according to the present invention,
When metal is not close to the leakage magnetic field of the oscillation coil,
The oscillation condition is satisfied by the conductance of the oscillation coil, and the oscillation circuit is put into an oscillation state.In response to the oscillation circuit output in this oscillation state, the switch output processing circuit outputs a switch-off signal, and the oscillation coil When a metal approaches the leakage magnetic field of the oscillation coil, the oscillation condition is not met due to an increase in the conductance of the oscillation coil, causing the oscillation circuit to stop oscillating, and in response to the oscillation circuit output in the oscillation stop state, the switch output processing circuit On the other hand, even if a DC magnetic field is applied to the ferrite core of the detection unit and the conductance of the oscillation coil increases, the negative conductance of the oscillation coil body also increases. Even if a magnetic field is applied, the conditional expression for oscillation of the oscillation circuit is satisfied,
Malfunction of the switch due to application of a DC magnetic field to the ferrite core can be reliably prevented.

[Brief explanation of the drawing]

FIG. 1 is a schematic circuit block diagram of a proximity switch according to an embodiment of the present invention, FIG. 2 is a circuit diagram of the oscillation circuit of FIG. 1, and FIG. 3 is a frequency characteristic diagram of negative conductance of the oscillation circuit main body. FIG. 4 is a schematic circuit block diagram of a conventional proximity switch, and FIG. 5 shows the metal approaching state and non-approaching state,
FIG. 3 is a diagram showing the frequency characteristics of the conductance of the oscillation coil corresponding to the application state of a DC magnetic field. 2... Oscillation circuit, 2a... Detection section, 2b... Oscillation circuit main body, R... Detection distance adjustment resistor, C... Frequency characteristic adjustment capacitor, FC... Ferrite core, HC
...Oscillation coil, 4...Switch output processing circuit.

Claims (1)

[Claims] (1) Oscillation circuit (2) and switch output processing circuit (
The oscillation circuit (2) has a detection section (2a) and an oscillation circuit main body (2b), and the detection section (2a) has a ferrite core (FC) and an oscillation coil (HC). ), and the oscillation circuit main body (2b) has a detection distance adjustment resistor (R), and when no metal comes close to the leakage magnetic field of the oscillation coil (HC), the oscillation circuit body (2b) The conductance of the oscillation coil (HC) and
While the oscillation condition based on the negative conductance of the oscillation circuit main body (2b) is satisfied and the oscillation state is in the oscillation state, when a metal approaches the leakage magnetic field, the oscillation coil (2b)
The oscillation condition is no longer satisfied due to the increase in the conductance of the HC), and the oscillation is stopped.The resistance value of the detection distance adjustment resistor (R) can be adjusted to determine the magnitude of the negative conductance. The switch output processing circuit (4) outputs a switch-off signal in response to the output of the oscillation circuit (2) in an oscillation state,
The oscillation circuit (2b) outputs a switch-on signal in response to the output of the oscillation circuit (2) in a stopped oscillation state, and the oscillation circuit main body (2b) has a frequency characteristic adjustment capacitor (C), and the capacitor (C) is a high frequency oscillation type proximity switch characterized in that the oscillation circuit main body (2b) has frequency characteristics and is connected in parallel to a detection distance adjustment resistor (R).