POWER FREQUENCY DETECTION SYSTEM

Abstract: A signal transmission system utilizes the building or house wiring as the data transmission line. A frequency detection system is required to detect a frequency of the power voltage. The frequency detection system includes a zero-cross level detection circuit for developing a detection output when the zero-cross is detected. A timer element is provided in the frequency detection system, which functions to neglect an output signal from the zero-cross level detection circuit for a preselected period of time after the last detection output has been developed from the zero-cross level detection circuit.

Description: BACKGROUND AND SUMMARY OF THE INVENTION The present invention relates to a detection system for detecting a frequency of a power supply voltage and, more particularly, to a power frequency detection circuit in a signal transmission system which utilizes the building or house wiring. A signal transmission system has been developed, which utilizes the building or house wiring as a data trans­ mission line. In such a signal transmission system, the power frequency is often used as a reference clock. To achieve an accurate operation, the power frequency component must be correctly detected without regard to the noises. Accordingly, an object of the present invention is to provide a detection system for detecting a frequency component of a power voltage. Another object of the present invention is to provide a power frequency detection system for use in a signal transmission system which utilizes the building or house wiring as a data transmission line. Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. To achieve the above objects, pursuant to an embodiment of the present invention, a zero-cross level detection circuit is provided for detecting the frequency component of the power voltage applied thereto. An inhibition system is provided for precluding the operation of the zero-cross level detection circuit for a preselected period of time after the zero-cross level detection circuit has developed the last detection output. The inhibition system minimizes an erroneous operation of the zero-cross level detection circuit, that is, the zero-cross level detection circuit will not recognize the noises as the power frequency component as long as the noises are introduced into the system while the zero-cross level detection circuit is placed in an in­ operative condition by the inhibition system. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood from the detailed description given hereinbelow and the ac­ companying drawings which are given by way of illustra­ tion only, and thus are not limitative of the present invention and wherein: FIGURE 1 is a schematic circuit diagram of a power frequency detection circuit of prior art; FIGURE 2 is a waveform chart for explaining an operational mode of the power frequency detection circuit of FIGURE 1; FIGURE 3 is a circuit diagram of an embodiment of a power frequency detection system of the present invention; FIGURE 4 is a circuit diagram of another embodiment of a power frequency detection system of the present invention; FIGURE 5 is a waveform chart for explaining operational modes of the power frequency detection systems of FIGURES 3 and 4; FIGURE 6 is a flow chart for explaining an operational mode of still another embodiment of a power frequency detection system of the present invention; FIGURE 7 is a circuit diagram of yet another embodiment of a power frequency detection system of the present invention; FIGURE 8 is a waveform chart for explaining an operational mode of the power frequency detection system of FIGURE 7; and FIGURE 9 is a flow chart for explaining an operational mode of the power frequency detection system of FIGURE 7. DESCRIPTION OF THE PREFERRED EMBODIMENTS A signal transmission system has been developed which utilizes the building or house wiring as a data trans­ mission line. In such a system, the power frequency is used as a base clock. To detect the power frequency component, a zero-cross level detection circuit is generally employed. FIGURE 1 shows a typical construction of the zero-cross level detection circuit which includes a transistor 10 and a diode 12. A system timing signal is obtained through the use of the leading edge or the trailing edge of a synchronization output derived from the zero-cross level detection circuit (see FIGURE 2). In such a detection system, when noises are superimposed on the power supply voltage, there is a possibility that an erroneous detection output 14 appears in the synchro­ nization output. Appearance of the erroneous detection output 14 depends on the noise level and the timing of the noises. Anyway, the erroneous detection output 14 may provide an incorrect operation of the signal trans­ mission system. The frequency of the occurrence of the erroneous detection output 14 is considerably lower than the power frequency. Furthermore, the power frequency is normally known. Therefore, if a system is constructed to neglect the power voltage variation for a preselected period of time after the last edge has been detected, the erroneous detection output 14 will be removed. FIGURE 3 shows an embodiment of a power frequency de­ tection system of the present invention. FIGURE 4 shows another embodiment of the power frequency detection system of the present invention. Two embodiments differ from each other at an edge detection section for detecting the zero-cross timing of the power supply voltage. Both embodiments include a monomultivibrator 20 (not retrig­ gerable). The monomultivibrator 20 must be triggered by edges of a same polarity every half cycle of the power supply voltage. In the embodiment of FIGURE 3, a detection output of a zero-cross level detection circuit, which includes the transistor 10 and the diode 12, is directly applied to one input terminal of an exclusive OR gate 22. The other input terminal of the exclusive OR gate 22 receives a delayed detection output of the zero-cross detection circuit via an integration circuit including a resistor 24 and a capacitor 26. An output signal of the exclusive OR gate 22 is applied to the monomulti­ vibrator 20. In the embodiment of FIGURE 4, a zero-cross level de­ tection circuit includes photo-couplers 30 and 32 which function, in combination, as a wired OR gate. A point 34 provides a detection output signal as shown in FIGURE 5. The thus obtained detection output signal is applied to the monomultivibrator 20. It will be clear from FIGURE 5 that the output signal of the exclusive OR gate 22 in FIGURE 3 or the output signal obtained at the point 34 in FIGURE 4 bears a high level when the power voltage polarity changes. That is, the output signals of the exclusive OR gate 22 and the photo-couplers 30 and 32 are the zero-cross detection signal. Once the monomultivibrator 20 is set, noises such as an erroneous detection signal 36 are neglected as long as the monomultivibrator 20 is in the set state. That is, an output signal of the monomultivibrator 20 is not influenced by the erroneous detection signal 36. To ensure an accurate operation of the power frequency detection systems of FIGURES 3 and 4, the pulse width of the monomultivibrator 20 is selected slightly shorter than one half of the period of the power supply voltage. In a preferred form, the pulse width of the monomulti­ vibrator 20 is selected at 7.5 milliseconds when the power supply voltage has a frequency of 60 Hz. A microcomputer is widely used in a control system of the power line signal transmission system. Generally, the microcomputer includes a programmable timer. By utilizing the programmable timer, a desired operation can be conducted as the circuits shown in FIGURES 3 and 4. FIGURE 6 shows an operational flow when the above-mentioned programmable timer is used to control the power frequency detection. First, the programmable timer is set to a desired value. Then, the data processing is conducted. After the preselected period of time has passed, the above-mentioned edge detection is carried out. When the edge is detected, the programmable timer is again set to perform the next edge detection. Even when the programmable timer is not included in the microcomputer, a similar control can be conducted. A one-chip microcomputer 40 generally includes an input/ output port of the suspected bidirectional type. One of the bits can be used to form an analog integration timer as shown in FIGURE 7. More specifically, a resistor 42 and a capacitor 44 are connected to a terminal 46 of the microcomputer 40. The zero-cross level detection circuit including the transistor 10 and diode 12 is connected to a terminal 48 of the microcomputer 40. FIGURE 8 shows signals occurring within the circuit of FIGURE 7. FIGURE 9 shows an operational flow of the circuit of FIGURE 7. First, the input/output bit is set to "1". Then, the data processing operation is conducted. Thereafter, a determination is conducted as to whether the introduced input/output bit (signal at the terminal 46) is "1". If an affirmative answer is obtained, the input/output bit is set to "0", and the edge detection operation is carried out. In this embodiment, the timer set period T is represented as follows. T = k C R where: C is the capacitance value of the capacitor 44, and R is the resistance value of the resistor 42. In the TTL type construction, V th is about 1.4 volts, and V s is 5 volts. Thus, k is 0.33. If the power supply voltage has the frequency of 60 Hz, the one half period is 8.3 milliseconds. Accordingly, if the timer set period T is desired to be set at a value slightly shorter than the one half period (8.5 milliseconds), for example, 7.5 milliseconds, the capacitance value C of the capacitor 44 should be selected between 6.9 microfarads and 6.8 microfarads because the resistor 42 is generally selected at 3.3 kiloohms due to the sink current in the input/ output port (generally, 1.6 milliamperes). The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications are intended to be included within the scope of the following claims. Reference is hereby directed to co-pending application no. 82307005.7 from which the present application is divided.

Claims: 1. A detection system for detecting a frequency of a power voltage applied thereto comprising: a zero-cross level detection circuit for detecting the zero-cross of the power voltage and developing a detection signal when the zero-cross is detected; and inhibition means for precluding the generation of said detection signal for a preselected period of time after the last detection signal has been developed. 2. A detection system for detecting a frequency of a power voltage applied thereto comprising: a zero-cross level detection circuit for detecting the zero-cross of the power voltage and developing a first detection signal when the zero-cross is detected; timer means for counting a preselected period of time after the last first detection signal has been developed, and developing a control signal when said preselected period of time has passed; and control means for developing said first detection signal as a second detection signal when said control signal is developed from said timer means, said second detection signal being used as an output signal of the detection system. 3. A power frequency detection system for use in a power line signal transmission system, said power frequency detection system comprising: a zero-cross level detection circuit for detecting the zero-cross of the power voltage and developing a detection output when the zero-cross is detected; and a monomultivobrator connected to receive said detection output, said monomultivibrator has the pulse width of slightly shorter than a period of the power voltage, an output signal of the monomultivibrator being used as an output signal of the power frequency detection system. 4. The power frequency detection system of claim 3, further comprising a pulse shaping means disposed between said zero-cross level detection circuit and said monomultivibrator, said pulse shaping means functioning to shape said detection output from said zero-cross level detection circuit into pulses of a same polarity. 5. A system for detecting the frequency of an input signal by providing detection signals when the level of the input signal reaches a predetermined value, characterised by means for inhibiting the provision of said detection signals for a predetermined period after the provision of each detection signal.

Inventors: ISE, MASAHIRO, MACHINO, KATSUYUKI, MATSUBARA, TOSHIYUKI, TANAKA, HIDEHIKO, TERASAKA, TEIJI
Applicants: SHARP KABUSHIKI KAISHA
Application year: 1982
Main classification: G01R19
IPC codes: G01R19, G01R23