JPH0425217A - High frequency oscillation type proximity switch - Google Patents

Abstract

PURPOSE:To prevent malfunction of a switch by inhibiting the output of a switch-on signal to a switch output processing circuit when a malfunction prevention circuit detects the result that the increase in the oscillating frequency reaches a prescribed value. CONSTITUTION:A comparator circuit 6b discriminates whether or not an oscillating frequency rises in excess of a prescribed value by comparing the detection output of an F-V conversion circuit 6a with a reference value. Then an output inhibit circuit 6c outputs an output inhibit signal to an output circuit 4c in response to the output corresponding to the increase in the oscillating frequency of a comparator circuit 6b in excess of a prescribed value. Thus, the output circuit 4c inhibits the output of the switch-on signal in response to the entry of the output inhibit signal. Thus, malfunction by 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. 2 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 a Colpitts-type or Hartley-type oscillation circuit, 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 FC. The oscillation circuit main body 2b constitutes another circuit portion of the oscillation circuit 2.

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.
The oscillation condition is satisfied by the conductance of , and the 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, which increases the conductance of the oscillation coil HC, causing the oscillation circuit 2 to fail to meet the oscillation conditions. As a result, oscillation is stopped.

The switch output processing circuit 4 is composed of a detection circuit 4a, a Schmitt trigger circuit 4b, and an output circuit 4C.The detection circuit 4a detects the oscillation frequency output from the oscillation circuit 2, and the oscillation circuit 2 A Schmitt trigger circuit 4 with a certain threshold value detects the detection output when the oscillation is in progress and the detection output when the oscillation is sustained.
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, if the inductance of the oscillation coil HC itself is 11% and the apparent permeability of the ferrite core F'C is μapp, then the overall inductance of the detection section 2a is calculated by the following formula
Hail to you.

L-μappL+ . . . ■ The oscillation frequency f 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 = 1/2 πF lower = 1/2 πb T d ``Z...■ Also, the conductance gQ of the oscillation coil HC is expressed by the following formula (■).

sl! =1/2πfLQ =1/2yrf ua I) pL+Q −■ However, Q is the cue value 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 formula (0) holds.

g12+(-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. 3. In FIG. 3, the horizontal axis indicates the oscillation frequency of the oscillation circuit 2, and the vertical axis indicates 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. If the oscillation frequency of the oscillation circuit 2 in this case is 11, the conductance gQ of the oscillation coil HC is negative conductance - g, as shown in FIG.
The absolute value is smaller than i, the oscillation condition of the above formula (2) is satisfied, and the oscillation circuit 2 is in an oscillation state. As a 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 gQ of the oscillation coil HC increases, and the conductance characteristic becomes ■.

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 is Δge at the oscillation frequency fl.
As a result, the oscillation condition of equation (2) above is no longer satisfied, the oscillation circuit 2 stops oscillating, and the switch output processing circuit 4 outputs a switch-on signal indicating that metal is in close proximity. will be done.

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 direct current magnetic field such as 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 fl' as expressed by the following equation (2).

f 1' = 1/2tr 4app' L+c +++
(2) Furthermore, as is clear from the equation (2), the conductance gQ of the oscillation coil HC also increases, and its characteristics become as shown in (2) in FIG.

The conductance g (1') of the oscillation coil HC at the oscillation frequency fl' is given by the following equation (2).

g 12'-¥1/2πf 1'μapI)' L+
Q...■ As a result, due to the increased conductance Δgρ' of the oscillation coil HC shown in Fig. 3, the formula (■) that satisfies the oscillation condition no longer holds true, and the oscillation circuit 2 enters the oscillation stopped state. It ends up.

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 Problems) In order to achieve such an object, the high frequency oscillation type proximity switch of the present invention includes an oscillation circuit, a switch output processing circuit, and a malfunction prevention circuit. has a detection unit consisting of an oscillation coil attached to a ferrite core, and the oscillation condition is established by the conductance of the oscillation coil when no metal comes close to the leakage magnetic field of the oscillation coil, resulting in an oscillation state. On the other hand, the oscillation condition is not satisfied due to an increase in the conductance of the oscillation coil when a metal approaches the leakage magnetic field, and the oscillation is stopped, and the switch output processing circuit is in the oscillation state. A switch-off signal is output in response to the oscillation circuit output, and a switch-on signal is output in response to the oscillation circuit output in a stopped oscillation state,
The malfunction prevention circuit detects an increase in the oscillation frequency of the oscillation circuit, and outputs the switch-on signal to the switch output processing circuit based on the detection when the oscillation frequency increases to a predetermined value or more. It is characterized by the fact that it prohibits

(Function) 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 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.

Further, when a metal is close to 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 satisfied, and the oscillation circuit enters the 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 oscillation frequency of the oscillation circuit increases, but when the malfunction prevention circuit detects that this oscillation frequency has increased to a predetermined value, a malfunction occurs. The prevention circuit prohibits output of the switch-on signal to the switch output processing circuit.

Therefore, in the proximity switch of the present invention, it is possible to prevent the switch from malfunctioning due to the direct current magnetic field being merely applied to the ferrite core without metal being close to each other.

(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. Parts corresponding to those in FIG. Detailed explanation will be omitted.

The proximity switch in this embodiment is characterized in that it includes a malfunction prevention circuit 6. That is, this malfunction prevention circuit 6 includes an F-V conversion circuit 6a, a comparison circuit 6b,
and an output inhibit circuit 6c.

The F-V conversion circuit 6a converts the oscillation frequency of the oscillation circuit 2 into a voltage, and outputs a detection output corresponding to an increase in the oscillation frequency. Comparison circuit 6b
determines whether the oscillation frequency has increased beyond a predetermined value by comparing the detection output of the F-V conversion circuit 6a with a reference value. The output prohibition circuit 6c is the comparison circuit 6b.
In response to an output corresponding to an increase in the oscillation frequency exceeding a predetermined value, an output prohibition signal is output to the output circuit 4c. The output circuit 4c is prohibited from outputting a switch-on signal in response to the input of this output prohibition signal.

Therefore, in the proximity switch of this embodiment, a direct current magnetic field like a magnet is applied to the ferrite core FC, and as a result, the magnetic permeability of the ferrite core FC decreases, and as a result, the oscillation condition of the oscillation circuit 2 is not satisfied, and the oscillation occurs. As a result, even if the switch output processing circuit 4 forms a switch-on signal, the switch-on signal can be prohibited from being output from the output circuit 4C, so malfunctions due to the application of a DC magnetic field can be prevented. can.

(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 the switch-off signal, and the oscillation circuit is turned off. When the oscillation condition is not met due to an increase in the conductance of the oscillation coil when a metal is close to the leakage magnetic field of the coil, the oscillation circuit is brought to a halt state, and a switch is activated in response to the output of the oscillation circuit in the halt state. A switch-on signal is output from the output processing circuit, and when a DC magnetic field is applied to the ferrite core of the detection section and the oscillation frequency of the oscillation circuit rises to a predetermined value, the malfunction prevention circuit turns on the switch. Since output of the switch-on state signal to the output processing circuit is prohibited, malfunctions caused by application of a DC magnetic field to the ferrite core can be reliably prevented.

[Brief explanation of drawings]

FIG. 1 is a schematic circuit block diagram of a proximity switch according to an embodiment of the present invention, FIG. 2 is a schematic circuit block diagram of a conventional proximity switch, and FIG. 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 unit, FC... Ferrite core, HC... Oscillation coil, 4... Switch output processing circuit, 6... Malfunction prevention circuit, 6a... F-
V conversion circuit.

Claims (1)

[Claims] (1) Oscillation circuit (2), switch output processing circuit (4),
and a malfunction prevention circuit (6), and an oscillation circuit (
2) The oscillation coil (HC) is attached to the ferrite core (FC).
It has a detection part (2a) attached to it, and
The oscillation condition is established by the conductance of the oscillation coil (HC) when no metal is close to the leakage magnetic field of the oscillation coil (HC), and the oscillation state is established. Due to an increase in the conductance of the oscillation coil (HC), the oscillation condition is no longer satisfied and the oscillation is stopped, and the switch output processing circuit (4) responds to the output of the oscillation circuit (2) in the oscillation state. The malfunction prevention circuit (6) outputs a switch-off signal in response to the output of the oscillation circuit (2) in the oscillation stopped state, and the malfunction prevention circuit (6) outputs a switch-off signal in the oscillation circuit (2). The device detects an increase in the oscillation frequency, and prohibits the switch output processing circuit (4) from outputting the switch-on signal based on the detection when the oscillation frequency increases to a predetermined value or more. A high frequency oscillation type proximity switch.