Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By amplitude
Reexamination Certificate
2000-05-04
2001-07-03
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By amplitude
C327S072000, C327S077000
Reexamination Certificate
active
06255864
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and a circuit for monitoring a defined amplitude threshold value of alternating voltage signals, in particular alternating voltage input signals for electronic control units such as programmable logic controls or logic relays or alternating voltage input voltage for power supplies.
RELATED TECHNOLOGY
A circuit arrangement of this type is known from U.S. Pat. No. 3,836,854. It discloses circuitry for monitoring an amplitude threshold value of an alternating voltage signal, having a rectifier, a voltage divider, and a subsequent comparator, so that when an alternating voltage input signal that exceeds a fixed threshold value, a threshold-exceeded signal appears at the comparator output. Moreover, a means of controlling the threshold-exceeded signal is present, in the form of a D flip-flop, which works together with a multivibrator circuit such that, depending on the pulse width of the threshold-exceeded signal, a determination is made to whether the signal has actually risen above or fallen below the threshold value.
A comparator arrangement is known from U.S. Pat. No. 4,399,414 that operates as a low-noise pulse conditioner, whereby interference signals are suppressed with the help of zero-crossing detection and threshold value detection of an alternating voltage signal.
Distinguishing or detecting a certain limit or threshold value is indispensable to recognizing AC signals as input signals for controls of this type. Precise and rapid detection of this defined threshold value is particularly advantageous in various applications. A generally known principle for detecting when an alternating voltage input signal goes above or below a defined threshold value is illustrated in
FIG. 3
, the AC input voltage is converted to a DC input voltage and its level is then measured. A circuit consisting of a half-wave or full-wave rectifier, a voltage step-down circuit (voltage divider), a filter (capacitor), and a comparator is used. A disadvantage of this solution is that the required filtering with the RC filter section in the input circuit results in long turn-on and turn-off delays. For a universal 1151230 V (50 Hz) control device, the levels are determined in accordance with standard specification EN 61131: V
H
=79V
eff
and V=40V
eff
. Thus, the capacitor discharge during the time between two half-waves (20 ms for half-wave rectification, 10 ms for full-wave rectification) must be smaller than (V
H −V
L)
eff
*✓2=55 V. Due to the discharging time constant required for this, t=R*C, when the input switches from high signal to low signal at the maximum input voltage (U+10%) of 1.1*240V*✓2=373V, it takes a correspondingly long time for the voltage across the capacitor to reach the lower threshold of ✓2*40V. The opposite method, charging of the capacitor when the input goes from low to high signal, takes a similarly long time when the input voltage is minimal. In practice, these delays are at least 50-100 ms.
Moreover, in accordance with EN 61131 (6.3.7.3) alternating voltage power supplies for programmable logic controls and similar devices are subjected to an insensitivity test in which, when the input voltage drops below a certain threshold value, the control must assume a defined state (shutting off, for example). Conversely, when the supply voltage returns, the control must reliably turn on. For this purpose, reliable detection of the monitored threshold value of the input voltage for such power supplies is imperative.
With conventional power supplies, the power supply voltage is generally read and monitored after rectification and smoothing (see
FIG. 1
, points a and b) and the central unit is sent a signal as soon as it goes above or below a certain threshold voltage. This method is not sufficiently effective, particularly in power supplies for miniature controls and logic relays, where the available space and cost do not permit large smoothing capacitors. When there are considerable differences in current consumption due to differing numbers of outputs that are turned on and off (generally, relay outputs with correspondingly high current consumption), the problem arises in the conventional method that beginning at a certain load, no defined cut-on and cutoff point can be guaranteed. Thus, for example, when the supply voltage is turned on, when it rises above the turn-on threshold, and when outputs are turned on, the supply voltage smoothing is degraded in such a way that the voltage passes below the threshold value once again, the outputs are then cut off, the load drops, and the smoothing improves again, whereby the voltage rises above the threshold, the outputs are turned on again, etc. This cycle repeats itself until the supply voltage is at a sufficient distance from the corresponding threshold value or a smaller load is present.
SUMMARY OF THE INVENTION
It is an object of the provide invention to present a method and a circuit that can reliably and quickly detect when an alternating voltage signal passes above or below a defined amplitude threshold value.
According to the present invention, by detecting the zero-crossing of a reference signal that is preferably of the same frequency as the alternating voltage signal (AC signal) being monitored, the peak point of the signal half-wave to be monitored is detected, it is compared to a threshold value, and a logical signal is generated when it passes above or below this threshold value. Preferably, the signals obtained by monitoring the zero-crossing produce an interrupt for a microprocessor and are further processed by the latter. The subject matter of the present invention results in threshold value detection that permits a reliable determination concerning the amplitude of the AC signal that is to be monitored within a very short time. Here, the acquisition time is dependent only on the period, T, of the AC signal, whereby with half-wave rectification the acquisition time is equal to the period, T, and with full-wave rectification the acquisition time is equal to half the period, T/2.
Thus, when the subject matter of the present invention is used to monitor AC input signals to electronic devices, such as programmable logic controls, reading undesired signal states, such as “logical zero” caused by voltage loss, can be avoided, since early recognition of voltage loss produces a signal that is further processed so as to prevent reading of the signal input.
Moreover, in another application of the present invention, direct monitoring of the alternating supply voltage for power supplies guarantees reliable monitoring and defined turn-on and turn-off of the power supply voltage, regardless of the load on the output side.
REFERENCES:
patent: 3836854 (1974-09-01), Wehman
patent: 4245150 (1981-01-01), Driscoll et al.
patent: 4263520 (1981-04-01), Kajihata et al.
patent: 4399414 (1983-08-01), Bird
patent: 5019722 (1991-05-01), Hess et al.
patent: 5570052 (1996-10-01), Fonderie et al.
patent: 5675272 (1997-10-01), Chu
patent: 2671437 (1992-07-01), None
patent: 1 254 975 (1971-11-01), None
Kenyon & Kenyon
Moeller GmbH
Tran Toan
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