Threshold value operation circuit, and, and gate circuit,...

Details Threshold value operation circuit, and, and gate circuit,... Threshold value operation circuit, and, and gate circuit,...

2000-05-01

2002-05-21

Ton, My-Trang Nu (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Specific signal discriminating without subsequent control

By amplitude

C327S502000

Reexamination Certificate

active

06392451

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a threshold value operation circuit which performs a threshold value operation on a level of an input signal, and generates an AC (alternating current) signal when at a set level (corresponding to logic value 1), and does not generate an AC signal when not at the set level (corresponding to logic value 0). In particular the invention relates to a threshold value operation circuit which can perform an analogue type threshold value operation, and which can generate an AC signal of stabilized frequency and duty ratio when an input signal is a set level, and to an AND gate circuit, a self hold circuit, and a start signal generating circuit to which this threshold hold value operation circuit is applied.
BACKGROUND ART
With signal processing in safety equipment and the like, where fail-safe characteristics are required, the presence of an AC signal is processed as a logic value 1 and the absence of an AC signal is processed as a logic value 0. Finally a threshold value operation is carried out on the AC signal and a judgement is made as to whether or not this is within a predetermined level, and a signal indicating safety or danger is output.
As one example of a conventional threshold value operation circuit applicable to such fail-safe signal processing, there is for example a fail-safe window comparator/AND gate. The circuit construction and operation of this is shown in for example Trans. IEE of Japan, Vol. 109-C, No. 9, September 1989 under the heading “A Structural Method for an Interlock System using a Fail-safe Logic Element having Window Characteristics”. Moreover, this is also shown in the specifications of U.S. Pat. No. 4,661,880, U.S. Pat. No. 5,027,114, and in the disclosure of Examined Japanese Patent Publication No. 1-23006.
Representative circuit examples thereof are shown in FIG.
31
(A) and FIG.
31
(B).
FIG.
31
(A) is a window comparator having a level detection function. When a level LI of an input I is in a range (r
1
+r
2
+r
3
)E/r
3
<L
1
<(r
4
+r
5
)E/r
5
(where E is the circuit power source voltage, and r
1
~r
5
are the resistance values of resistors R
1
~R
5
), the circuit self oscillates with respective transistors Q
1
~Q
3
repeating an on/off operation in a process of Q
2
: OFF→Q
3
: OFF→Q
1
: ON→Q
2
: ON→Q
3
: ON→Q
1
: OFF→Q
2
: OFF→ . . .
Furthermore, FIG.
31
(B) is a circuit having a level detection function and an AND function, with two window comparators as shown in FIG.
31
(A) connected in cascade. When the levels LI
1
, LI
2
of the inputs
11
,
12
are respectively in the range of (r
1
+r
2
+r
3
)E/r
3
<LI
1
<(r
4
+r
5
)E/r
5
, and (r
1
′+r
2
′+r
3
′)E/r
3
′<LI
2
<(r
4
′+r
5
′)E/r
5
′ (where E is the circuit power source voltage, and r
1
′~r
5
′ are the resistance values of resistors R
1
′~R
5
′), the circuit self oscillates , and if even one of L
1
and LI
2
goes outside of the abovementioned range, oscillation stops.
Furthermore, as another threshold value operation circuit, there is for example the one disclosed in Japanese Unexamined Patent Publication No. 8-162940, previously proposed by the present applicants. This is shown in FIG.
32
.
With the circuit of
FIG. 32
, if an output X
2
from an envelope detection circuit A based for example on an AC signal V
IN
exceeds a power source voltage Vcc, then due to a switching (on/off) of a photocoupler PC
1
which receives an AC signal from a separately provided AC source, a switching current intermittently flows in a photodiode of a photocoupler PC
2
via a current decreasing resistor R. Hence a phototransistor of the photocoupler PC
2
switches (on/off), and an AC signal is generated (output is logic value 1). When an AC signal V
IN
is not input to the envelope detection circuit A, the output X
2
from the envelope detection circuit A becomes approximately the power source voltage Vcc. Hence even if the photocoupler PC
1
switches, a switching current does not flow to the photocoupler PC
2
, and an AC signal is not generated (output is logic value 0). The envelope detection circuit B voltage doubler rectifies the AC signal due to the switching of the phototransistor of the photocoupler PC
2
, and outputs a DC (direct current) signal Y which is larger than the power source voltage Vcc.
However, the threshold value operation circuit which uses the window comparator of
FIG. 31
is a self-oscillation circuit, and the oscillation frequency and duty ratio are approximately determined by the on and off delay time of the respective transistors. Therefore due to changes in the characteristics of the transistors resulting from variations in transistor characteristics, temperature and humidity and the like, or due to the input signal level, the oscillation frequency and the duty ratio of the AC output change, so that there is the problem in that a stabilized AC output cannot be obtained. Moreover, this threshold value operation circuit uses processing outside of the power source limits (when an input signal of a predetermined level greater than the circuit power source voltage is input, an AC signal is generated), and must satisfy relationships where the input signal level and the threshold value of the window comparator both change in proportion to the circuit power source voltage. Therefore, there is the restriction that the input signal oscillation circuit and the window comparator must use the same power source. Hence the degree of freedom for circuit layout is low.
On the other hand, with the threshold value operation circuit of
FIG. 32
, since the construction is such that the AC signal from an external AC source is used to give oscillation, the oscillation frequency and the duty ratio are not susceptible to influence from the characteristics of the circuit elements or the input level, and hence a stabilized AC signal can be obtained. However, this circuit confines itself to judging as to whether or not the input signal is greater than the circuit power source voltage Vcc, in other words to digital judging for the presence or absence of the input signal, and does not have an analogue threshold value detection function for the input signal.
The present invention takes into the consideration the above situation, with the object of providing a threshold value operation circuit, which can perform an analogue threshold value operation, and which can generate an output AC signal of stabilized frequency and duty ratio, and for which the circuit layout degree of freedom is high. Moreover, it is an object to provide an AND gate circuit, a self hold circuit and a start signal generation circuit to which this threshold hold value operation circuit is applied.
DISCLOSURE OF THE INVENTION
Accordingly, with the present invention, there is provided a threshold value operation circuit which comprises; a modulation device for modulating an input signal by means of a switching signal from an AC (alternating current) source, and a level detection device for level detecting the input signal by comparing this with a previously set threshold value, and is constructed such that an input signal subject to threshold value operation is input to one device of the modulation device and level detection device, and an output from the one device is input to the other device, and which generates an AC signal when based on a level detection result of the level detection device, a level of the input signal subject to threshold value operation is a set level.
More specifically, the construction may be such that the input signal subject to threshold value operation is input to the modulation device, and a modulation signal modulated by the modulation device is input to the level detection device, and the level detection device generates an AC signal when the modulation signal is a set level. Furthermore, the construction may be such that the input signal subject to threshold value operation is input to the level det

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