System and method for power line carrier communication using high frequency tone bursts

A communication system and method is utilized to communicate data over an AC power line. Tone burst are superimposed on an AC power signal at predetermined voltage reference levels or at predetermined phase angles to represent bit values. These bit values are represented by either the presence or the absence of the tone burst on the AC power signal. In this manner, control information can be communicated to an apparatus, such as a ballast for a gas discharge lamp.

Not Applicable

Not Applicable

BACKGROUND OF THE INVENTION

The present invention relates generally to systems and methods for communicating data across a power distribution network. More particularly, this invention relates to a system and method for communicating data using an AC power signal to a device connected to the power line for energy management and/or control of the device.

Power line carrier (PLC) communication systems are frequently used to send data and control signals between devices connected to a power distribution network. Some conventional PLC systems communicate data by generating and then detecting disturbances in the 60 Hz AC signal that is used to deliver power to the load device. In many such prior art PLC systems, the signal disturbance is generated by using a PLC transmitter to periodically create a “short circuit” condition across the AC power line using a gated electronic switch, such as a triac. The short circuit condition is typically generated at or near a zero crossing of the AC signal. A receiver associated with the load device detects the disturbances (e.g., a “notch”) in the AC signal and decodes various sequences or patterns of such disturbances as device control signals. When the transmitter in such prior art systems is in series with the AC power line and the load device, such systems can transmit data at only a 60 Hz data rate because the disturbance can only be introduced on the positive-to negative half-cycle of the AC power signal. Also, many of the gated switches used in these prior art systems cannot be turned off by their gate signal. Thus, if the switch is turned on just after a zero crossing, the AC line will be shorted through the switch. This may damage the switch and/or disable the power line by tripping an over-current device attached to the circuit. Also, conventional PLC systems using a gated switch configuration are less efficient and must use larger and more expensive components to handle the switching losses inherent with such systems.

What is needed, then, is a PLC communication system that is easy to use on existing power distribution networks, is energy efficient, and that is smaller in size and lower in cost as compared to conventional gated switch “notch” systems.

BRIEF SUMMARY OF THE INVENTION

To reduce the costs and the size of a PLC communication system over an AC power line, the system and method of the present invention superimposes a tone burst into the AC power signal rather than a notch. A tone generator circuit, for example an oscillator, is connected to the AC power line to introduce this tone burst into the power signal. The presence of a tone burst on the AC power signal can indicate that a first bit value is being transmitted, such as a one. The absence of a tone burst on the AC power signal can indicate a second bit value is being transmitted, such as a zero.

At the receiving device, a tone detection circuit is utilized to detect the presence of the tone burst on an AC power signal. The presence or absence of the tone burst can be decoded as a data packet of information. In a preferred embodiment, the tone burst is synchronized on the AC power signal at a particular reference voltage or at a predetermined phase angle. This reference voltage or phase angle may correspond to the zero crossing of the AC power signal.

To detect the tone bursts, the tone detector circuit has a filter circuit for filtering the tone burst out of the AC power signal. A tone burst detector circuit is then connected to the filter circuit to generate an indicator signal when the tone burst is present on the AC power signal. This indicator signal is transmitted to a pulse generating circuit that transmits pulses. When an indicator signal is present, the length of the pulse is modified so that a device receiving the pulses can discriminate whether a first bit value or a second bit value has been transmitted on the AC power signal.

In one embodiment of the invention, the PLC system is used to send control signals, such as lamp dimming signals, to an electronic ballast that is connected to a gas discharge lamp.

Accordingly, one of the objects of the present invention is to reduce the size and cost of the components in a PLC communication system utilizing an AC power line to communicate information.

Another object of the present invention is to utilize a tone burst instead of a notch superimposed on the AC power signal to communicate information to a device connected to the AC power line.

Yet another object of the present invention is to communicate information over an AC power line without creating a short circuit condition on the AC power line.

Still another object of the present invention is to create a system that is less likely to interfere with existing power line carrier systems.

DETAILED DESCRIPTION OF THE INVENTION

Referring now toFIGS. 1 and 2, a Power Line Carrier (PLC) communication system1that communicates data10on an AC power signal12transmitted along an AC power line14is shown. In order to communicate data10over the AC power line14, the communication system1periodically injects high frequency tone bursts28on the AC power signal12. As shown inFIG. 2, these tone bursts28are injected at predetermined phase angle locations along the periodic cycle of the AC power signal12. As will be explained in more detail below, the presence or absence of these tone bursts28at these predetermined locations along the periodic cycle of the AC power signal12are utilized to determine the data bit values being transmitted along the AC power line14. There may be a single predetermined location along the periodic cycle of the AC power signal12that is relevant to data transmission and detection or there may be several. In a preferred embodiment, there are two relevant predetermined locations along the cycle of the AC power signal12: the positive-to-negative zero crossings and the negative-to-positive zero crossing for the AC power signal. A device connected to the AC power line14will look to these relevant predetermined locations to detect and decode the data10transmitted on the AC power signal12at a data bit rate of 120 Hz.

As shown inFIG. 1, a tone generator (gated oscillator) circuit16is connected to the AC power line14to generate the tone bursts28. The tone generator circuit16is preferably electrically coupled to the AC power line14via coupled inductive components17, such as a transformer. This arrangement helps to filter out noise and other electromagnetic interference to the AC power line14. A power amplifier19can be used to amplify the tone burst signals28before they are coupled to the AC power line14. The inductive components17are preferably connected across the line (L) and neutral (N) legs of the AC power line14.

Preferably, the AC power line14will include a conventional transient suppression circuit21and a low pass filter23to prevent high frequency signals from being transmitted upstream to the AC power grid.

As shown inFIG. 2, the tone generator circuit16creates a tone burst28that is injected on the AC power signal12. The presence or absence of a tone burst28on the AC power signal12indicates to a receiving device whether a first bit value22or a second bit value26is being transmitted. In this example, the first bit value22is the bit value of a one and corresponds to a tone burst28being present on the AC power signal12. The second bit value26is a zero and corresponds to the absence of a tone burst on the AC power signal12.

The tone bursts28ordinarily are generated to have significantly higher frequency and lower amplitude compared to the AC power signal12. In a preferred embodiment, the tone burst has a frequency of 9.8 kHz. Using a 9.8 kHz tone burst has several advantages. First, a tone burst at this frequency reduces cross-talk to adjacent tandem power circuits with common neutrals. Second, when this system is utilized with gas discharge lamp ballast, a 9.8 kHz signal is below the self-resonant frequency of a typical EMI filter for the ballast. Also, a 9.8 kHz tone burst is more compatible with conventional power line carrier system hardware.

Referring toFIG. 3, a gated tone generator circuit16A is shown. The tone generator circuit16A is designed to work with (be enabled by) a pulsed drive signal18(FIG. 6). The tone generator circuit16A receives the drive (enable) pulse signal18from drive circuit32(FIG. 1). This causes the Butterworth filter oscillator circuit16B to generate a tone burst which is subsequently amplified by power amplifier circuit19. The tone burst28is then coupled to the AC power line14and superimposed on the AC power signal12. Preferably, the pulsed drive signal18is synchronized to generate tone bursts only at the predetermined locations along the cycle of the AC power signal. This can be done by using a conventional zero crossing detector circuit29(FIG. 1) that is electrically coupled to the AC power line and to the drive circuit32to provide the synchronization function. One example of a conventional zero crossing detector circuit that can be used is shown inFIG. 4(b) and described with reference to the system detector circuit. In a preferred embodiment as shown onFIG. 1, the zero crossing detector29is coupled to the AC power line14through a conventional full-wave bridge rectifier circuit31, and to the drive circuit32through an opto-isolator circuit33. An isolated power supply35is also coupled to the bridge rectifier circuit31to provide operating power to the system components.

Also as shown onFIG. 1, the drive circuit32can be a microprocessor having communication ports37to receive data10from an external source. For example, the communication ports37can be coupled to a remote dimming control circuit (not shown) to receive dimming commands for lamp loads connected downstream on the AC power line14. The drive circuit32can further have a current sense input41coupled to a current transformer39coupled to the AC power line. The current sense input41can be used for sensing the AC power line signal such as for synchronizing operation of the system to certain phase angles or other parameters of the waveform.

Referring again toFIGS. 2 and 7, the drive signal18is in a first state or form20when a first bit value22is being transmitted and is in a second state or form24when a second bit value26is being transmitted. In this example, the first state20is a pulse20A and the second state24A is the absence of a pulse. By sending the first state20to the tone generator circuit16, the tone generator circuit16is enabled or excited, thereby causing a tone burst28to be injected on the AC power signal12. However, it should be understood that the form of the drive signal18is not limited to a pulse. Any form can be utilized so long as this form can be translated by a tone generation circuit into a tone burst28.

As shown in the graph ofFIG. 2, the tone burst28is superimposed on the AC power signal12when a first bit value22, which in this case equals a one, is being transmitted on the AC power signal12. The tone bursts28are injected on the AC power signal12at certain predetermined phase locations. To accomplish this, the drive signal18may be synchronized to activate the tone generator circuit16when the AC power signal12approaches or is equal to a reference voltage34. In a preferred embodiment, the reference voltage is a zero crossing34A of the AC power signal12. However, the drive signal18may be synchronized to activate the tone generator circuit16at any contemplated reference voltage or set of reference voltages. These zero crossings34A and crossings at other desired reference voltages34will occur at predetermined phase angles36on the AC power signal12. Thus, any circuit attempting to extract information from the AC power signal may look to these reference voltages34,34A or to these predetermined phase angles36to determine what bit value is being transmitted.

For example, as shown inFIG. 2, when a data “zero” is being transmitted, no tone burst28will be present on the AC power signal12at the reference voltage34or at a predetermined phase angle36. However, when a “one” is being transmitted, a tone burst28is superimposed on the AC power signal12.

Normally, a drive circuit32is coupled to the tone generator circuit16to generate the drive signal18for activating the tone generator circuit16. In the embodiment shown inFIG. 1, the drive circuit32is configured to generate pulses20A. The drive circuit32may be a hardware device that receives the data10and encodes the data10into pulses20A (FIG. 7). However, the drive circuit32may also be a microprocessor which is programmed to receive the data10and encode the data into pulses20A.

As shown inFIGS. 4(a),4(b) and5, the communication system1can include a decoder circuit30associated with a receiving device. In the embodiment ofFIG. 5, the receiving device is an electronic ballast58that powers a gas discharge lamp56. The decoder circuit30detects the presence or absence of the tone bursts28to determine which bit value22,26is being transmitted. InFIGS. 4(a) and4(b), two different embodiments of a decoder circuit are shown. The decoder circuit ofFIG. 4(a) shows some of the basic components for implementing one embodiment of the invention. The decoder circuit ofFIG. 4(b) is a preferred embodiment.

To determine which bit value22or26is being transmitted, the decoder circuits inFIGS. 4(a) and4(b) include a filter circuit38for receiving and isolating tone bursts28from the AC power signal12.FIG. 4(b) shows the filter circuit38as an op-amp band pass filter to extract the tone bursts28. In a preferred embodiment, the filter38has a center frequency of approximately 9.8 kHz. Preferably, the filter circuit38is coupled to the AC line14through a buffer stage60. (FIG. 4(a))

A tone detection circuit40is connected to the filter circuit38. The detection circuit40generates an indicator signal42that indicates that a tone burst28is present on the AC power signal12. In a preferred embodiment, the tone detection circuit40includes a peak detector circuit46connected to an output47of the filter circuit38. The peak detector circuit46that conventionally operates to sense a peak output voltage from the filter circuit38. If no tone burst28is present on the AC power signal12, the voltage at the output of peak detector circuit46will be low. However, if a tone burst28is present on the AC power signal12, then the peak detector circuit46will store the peak voltage of the tone burst28. A comparator circuit48receives the output49from the peak detector circuit46and compares it to a reference voltage51.

The comparator circuit48will output an indicator signal42at the comparator output53if the peak output voltage49is greater than or equal to the reference voltage51. The reference voltage51is selected so that when a tone burst28is present, the output49from the peak detector circuit46is greater than the reference voltage51. In this situation, the comparator circuit48will produce the indicator signal42. If the peak output voltage49is lower than the reference voltage51, no indicator signal42will be transmitted.

The indicator signal42is coupled to a pulse generator circuit44that generates pulses45. As shown inFIG. 6, the pulse generator circuit44modifies the length of one of the pulses45when the indicator signal42indicates that a tone burst28is present. Thus, in the example shown inFIG. 6, when a zero is being transmitted, the pulse generating circuit44has a normal length pulse. However, when a one is being transmitted, a longer pulse45is transmitted.

The pulses45are coupled to a receiving device so that the device can utilize the information to perform a particular function. The receiving device can include a microprocessor that interprets a sequence of ones and zeros as load control commands.

In a preferred embodiment, the pulse generator circuit44receives the AC power signal12in a rectified form. However, the pulse generator circuit may receive any type of signal associated with the AC power signal12including the AC power signal12itself. However, a full-wave rectified AC input signal55allows the pulse generator circuit44to generate pulses on or near both the positive-to-negative and negative-to-positive zero crossings of the AC power signal12.

As shown inFIGS. 4(a) and4(b), the pulse generator circuit44includes a peak detector circuit50. The peak detector circuit50receives the AC input signal55and produces an input signal reference voltage54proportional to a peak voltage for the AC input signal55. Thus, the peak detector circuit50measures the peak voltage of the AC input signal55and then uses a voltage divider54A to produce the input signal reference voltage54. In a preferred embodiment, the input signal reference voltage54is relatively close to the zero crossing of the AC input signal55. A comparator circuit52then receives and compares the input signal reference voltage54and the AC input signal55. The comparator circuit52outputs a pulse45when the AC input signal55is less than the input signal reference voltage54. When the input signal55is again exceeds the input signal reference voltage54, the transmission of the pulse45stops.

The comparator circuit52also receives the indicator signal42. When the indicator signal42is received by the comparator circuit52, the input signal reference voltage54is increased. Accordingly, the magnitude of AC input signal55will spend a longer amount of time below the magnitude of the input signal reference voltage54. This lengthens one of the pulses45. Thus, when a tone burst28is detected on the AC power signal12, a longer pulse45will be generated.

As shown inFIG. 6, the data10is represented by the length of the pulses45. A data “zero” corresponds to a normal pulse while a data “one” is represented by a longer pulse. However, it should be understood that the presence of the indicator signal42could be utilized to lower the reference voltage54, thereby shortening the length of the normal pulse, if the circuit that receives this information knows that a shorter pulse represents a change in the pulse length value.

As shown inFIG. 5, the pulses45can be sent to control a device connected to the AC power line14such as a gas discharge lamp56A. Lamp56A is powered by an electronic ballast58which can control the dimming of the lamp56. To send dimming control signals to the ballast58, the AC power line14is often utilized. The ballast58will incorporate a decoder circuit30which can be a microprocessor. This microprocessor receives the series of pulses45and is programmed to decode the pulses to extract the transmitted data packet10. In this manner, the desired dimming level of the lamp56can be controlled utilizing the power line14.

For example, the desired dim level may be encoded by transmitting a repeating pattern of evenly distributed ones and zeros, where the ratio of ones to zeros is proportional to the desired lamp brightness. Because the ones and zeros are evenly distributed, the decoding can start anywhere in the data pattern. The dimming level is decoded by counting the number of ones or zeros within a group of bits equaling the denominator. The even pattern distribution causes any successive group of bits to yield the same result.

Thus, although there have been described particular embodiments of the present invention of a new and useful System and Method for Power Line Carrier Communication Using High Frequency Tone Bursts, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.