JPH0422048B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a circuit configuration of an oscillation circuit used in a high frequency oscillation type proximity switch.

Prior Art and Its Problems In high-frequency oscillation type proximity switches, oscillation circuits based on negative conductance are often used because the oscillation conditions are clear and the circuit design is easy. FIG. 1 shows an example of this oscillation circuit. In this figure, the detection coil 1 is connected to the capacitor 2.
are connected in parallel to form a resonant circuit, and a constant bias current is supplied from a constant current source 3 through a diode 4 to this resonant circuit. The base of the emitter follower transistor Tr1 is connected to the connection point between the constant current source 3 and the diode 4. The transistor Tr1 converts voltage changes in the resonant circuit into current changes, and has a semi-fixed emitter resistor Re connected to its emitter, and a well-known current mirror circuit 5 consisting of transistors Tr2 and Tr3 connected to its collector. Ru. The current mirror circuit 5 is connected so that the same current as the collector current flowing through the transistor Tr2 flows through the transistor Tr3, and the collector of the transistor Tr3 is connected to a parallel resonant circuit and positive feedback is provided by the current. Note that E is a DC voltage source that supplies a constant voltage to each part. Here, the voltage across the parallel resonant circuit is
If Vt, the emitter voltage of transistor Tr1 is also approximately equal to Vt, so transistor Tr1
The collector current of is Vt/Re, and the current positively fed back to the parallel resonant circuit via the current mirror circuit 5 is also Vt/Re. Therefore, the conductance Gi seen from the parallel resonant circuit to the circuit side is expressed by the following equation (1).

Gi=Output voltage/Input voltage=-Vt/Re/Vt=-1/Re...
…(1) Therefore, the emitter resistance Re of transistor Tr1
By changing the value of , the value of negative conductance can be adjusted. Here, when a metal object approaches the detection coil 1, the conductance Gr of the resonant circuit increases, and when it becomes larger than the conductance Gi on the circuit side (Gr≧Gi), the oscillation stops. Therefore, by adjusting the emitter resistance Re, it is possible to adjust the object detection distance.

By the way, in the case of proximity switches, in order to adapt to the conditions of the place where objects are to be detected and to be able to be installed in especially narrow installation places, a detection sensor including a detection coil, an oscillation circuit and a level discriminator are required. , a detection circuit section having an output circuit, etc. may be separated from each other. In such a separate proximity switch, although a large number of detection sensors are provided depending on the installation location, only a small number of detection circuit sections are often provided by appropriately switching the cables connected to the detection sensors. Therefore, it is necessary to adapt the oscillation circuit to the detection coil connected at that time and to adjust the emitter resistance Re so that the detection distance becomes a desired value.

However, with proximity switches that vary the detection distance over a wide range, there is a problem that the difference in conductance between detection coils becomes large, making it difficult to adjust the emitter resistance Re according to various detection coils. It was hot. In other words, the conductance Gi on the circuit side with respect to the emitter resistance Re is given by the formula (1)
Based on this, it is determined as shown in FIG. Therefore,
In order to increase the object detection distance, the conductance Gi is reduced to, for example, 100μ, and when adjusting 10% of it according to the detection coil, it is necessary to set the emitter resistance Re to 10KΩ and adjust it by 1KΩ. In addition, in order to reduce the detection distance of an object, the conductance Gi is increased to, for example, 1 m, and when adjusting 10% of it according to the detection coil, it is necessary to adjust the emitter resistance Re to 1KΩ and 100Ω. Therefore, it is actually extremely difficult to make such adjustments using a single variable resistor, and delicate adjustments are required.

OBJECT OF THE INVENTION The present invention solves these conventional problems, and aims to improve the emitter resistance of an emitter follower transistor in an oscillation circuit of a proximity switch.
An object of the present invention is to provide a proximity switch oscillation circuit that can easily adjust emitter resistance by changing the relational expression between Re and circuit conductance.

Structure and Effects of the Invention The present invention provides a negative conductance system that guides the induced voltage of a resonant circuit having a detection coil and a capacitor to the input terminal of a first emitter follower circuit, and positively feeds back the collector current to the resonant circuit via a current mirror circuit. A proximity switch oscillator circuit of the type, in which a second emitter follower circuit is provided in parallel with the first emitter follower circuit for the resonant circuit, and the collector of the second emitter follower circuit is connected to the output side of the current mirror circuit, The present invention is characterized in that the detection distance is adjusted by varying the emitter resistances of the first and second emitter follower circuits.

According to the present invention having such features, it is extremely easy to adjust the emitter resistance of the oscillation circuit, and the conductance of the oscillation circuit can be freely adjusted in accordance with the detection coil. Therefore, even when a large number of sensors are used, only one detection circuit diagram is required, making it possible to construct an efficient detection system using proximity switches.

DESCRIPTION OF THE EMBODIMENTS FIG. 3 is a circuit diagram showing an embodiment of the proximity switch oscillator circuit according to the present invention. In this figure, the same parts as in the conventional example are indicated using the same reference numerals.
Now, a constant current is supplied from a constant current source 3 via a diode 4 to a parallel resonant circuit consisting of a detection coil 1 and a capacitor 2, and changes in the current are applied to an emitter follower transistor Tr1. Also, the transistor Tr1 is a transistor Tr1.
2, a current mirror circuit 5 consisting of Tr3 is connected,
It is also similar to the conventional example in that positive feedback is applied by the current. In the present invention, an emitter follower transistor Tr4 is provided in parallel with the transistor Tr1. The collector of the transistor Tr4 is connected to the collector of the transistor Tr3 of the current mirror circuit, and its emitter is connected to one end of the variable resistor R. The other end of variable resistor R is a transistor
The movable end is grounded to the emitter of Tr1. And the ground of the variable resistor R and the transistor
The resistance values between the emitters of Tr1 and Tr4 are respectively Re1 and
Let it be Re2.

According to the oscillation circuit having such a configuration, a value equal to the collector current of the transistor Tr1 flows out from the transistor Tr3, but part of it becomes the collector current of the transistor Tr4, and the remainder is fed back to the parallel resonant circuit. transistor here
A part of the output current of the current mirror circuit formed by transistor Tr3 will flow into Tr4, so
Positive feedback current decreases. Therefore the transistor
The transistor Tr3 operates as a positive feedback side transistor, and the transistor Tr4 operates as a negative feedback side transistor. Therefore, the conductance Gi seen from the parallel resonant circuit is expressed by the following equation (2).

Gi=Output voltage/Input voltage=-(Vt/Re1-Vt/Re2)/Vt=-(1/Re1-1/Re2) ...(2) Position of variable resistor R and conductance Gi based on this formula Figure 4 shows the relationship between As shown by the solid line in this figure, the value of the conductance of the emitter resistor R relative to the position of the movable contact is a curve that is a composite of two inversely proportional graphs, and when Re1=Re2, the conductance Gi becomes zero. In reality, oscillation does not occur unless the value of conductance Gi is positive, so the resistance value is adjusted within the range of Re1>Re2. With such a configuration, it becomes easy to adjust the detection position when the value of conductance is small.

In this embodiment, a variable resistor R is connected between the emitters of transistors Tr1 and Tr4, but since the resistance value is adjusted within the range of Re1>Re2,
It goes without saying that a fixed resistor may be connected to the emitter of the transistor Tr4.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing the oscillation circuit of a conventional proximity switch, and Figure 2 is the emitter resistance of this oscillation circuit.
FIG. 3 is a graph showing the relationship between Re and conductance, FIG. 3 is a circuit diagram showing an oscillation circuit of a proximity switch according to the present invention, and FIG. 4 is a graph showing the relationship between emitter resistance R and conductance of the oscillation circuit. 1...Detection coil, 2...Capacitor, 3...
Constant current circuit, Tr1 to Tr4...transistor,
R, Re... Emitter resistance.

Claims (1)

[Scope of Claims] 1. A negative conductance that guides the induced voltage of a resonant circuit having a detection coil and a capacitor to the input terminal of a first emitter follower circuit, and positively feeds back the collector current to the resonant circuit via a current mirror circuit. In the type of proximity switch oscillation circuit, a second emitter follower circuit is provided in parallel with the first emitter follower circuit for the resonant circuit, and a collector of the second emitter follower circuit is connected to the output side of the current mirror circuit. . A proximity switch oscillation circuit, characterized in that the detection distance is adjusted by varying emitter resistances of the first and second emitter follower circuits.