WO2014207296A1

WO2014207296A1 - A method and device for transmitting' electrical power and data

Info

Publication number: WO2014207296A1
Application number: PCT/FI2014/000013
Authority: WO
Inventors: Pekka Nikander; Teemu HAKALA
Original assignee: Ell-I Open Source Co-Operative
Priority date: 2013-06-25
Filing date: 2014-06-24
Publication date: 2014-12-31

Abstract

The invention provides methods, systems and devices for carrying both electrical power and data communication over a transmission medium such as a power cable comprising at least three conductors. The inventive method comprises at least the steps of communicating data using differential transmission with a pair of conductors of a power cable, where at least one of the conductors of said power cable have a cross-sectional area of at least a 0.75 mm2, communicating data using a one-sender-at-a-time data communication protocol over said pairof conductors, and transmitting a DC current using said pair of conductors as a first conducting line of a DC current transmission circuit, and using at least a third conductor of said power cable as a second conducting line of the DC current transmission circuit.

Description

A method and device for transmitting^' electrical power and data

BACKGROUND OF THE INVENTION

Field of the Invention

This document relates to systems and techniques for transmitting electrical power and digital data over a transmission medium.

Description of Related Art

Today almost all commonly used solutions for transmitting electrical power are based on the alternating current system, where the voltage and current alternate between a positive and negative maximum, for example 110 or 230 volts RMS and 20 amperes, at a fixed frequency, for example 50 or 60 alternations per second (Hertz). In such a system, it is quite hard to transmit data along the same cable, as the various so-called Data-over-Powerline and Ethernet-over-Powerline solutions clearly indicate. In an alternating current system, the alteration of the relatively high voltage and current induce an alternating electrical and magnetic field around the transmission wires. These induced fields cause secondary voltages and currents in nearby conductive substances, including the wires originating the fields and other wires that run along the originating wires. In such an environment, any data signals carried together with the alternating voltage or current may get heavily distorted, for example, due to the electric and magnetic fields generated by the alternating voltages and currents in the originating wires and any other wires running along. To avoid these distortions, Ethernet-over-Powerline solutions typically modulate a carrier frequency signal which is then injected in the power cable on top of the power carrying voltage. This requires the use of a complicated modulator at the transmitting end and a complicated demodulator at the receiving end, increasing the complexity of the system. In an Ethernet-based twisted pair data communication system, data is transmitted over two or more twisted pairs of conducting wires. For example, a 10 megabit per second Ethernet system may drive a square wave Manchester encoded differential signal over the wire pair. In such a system, the data signal may be represented as a small voltage difference between the wires, for example of ±2.5 volts, and an associated small current flowing in different directions along the wires, for example 50 milliamperes. In an Ethernet system, there may be two or more such twisted pairs, typically each pair having a different twisting rate.

In a typical Power-over-Ethernet system, electrical power is transmitted as a direct current along two or more twisted pairs of conducting wires. In a so-called Mode A Power-over-Ethernet solution, the same twisted pair wires may be used to conduct both electrical power and digital data. In such a system, two twisted pairs may be placed at differing direct current voltage potentials, for example, so that one pair has the nominal voltage potential of zero volts and the other the nominal voltage potential of -48 volts. The Ethernet data is typically combined to and separated from the twisted wire pairs using a centre-tapped isolation transformer. The combining transformer may induce a Manchester or otherwise coded data signal over the twisted wire pair, and the separating transformer may separate the data signal.

Power-over-Ethernet systems suffer from the problem that the power carrying capacity of an Ethernet cable is limited to a few tens of watts. This is sufficient for providing power for many kinds of devices such as network devices or monitoring cameras. However, for many purposes in building automation, such as for controlling and providing power for lighting, this is not sufficient. Power- over-Ethernet systems also require the use of Ethernet cables, for example so- called Category 3, 5, 6, or 7 cables, which in addition to limited power carrying capacity, are required to have a specific impedance and other electrical properties. In general, a transmission medium designed for transmitting electrical power may not be well suited for high-speed data transmission. For example, a cable designed for transmitting electrical power may use an insulation material that causes a high signal loss for high-frequency signals, it may have an unknown or varying high-frequency impedance, and it may be otherwise designed so that high-frequency signals may not be easily conveyed through such a cable.

Furthermore, when such a transmission medium is used for transmitting typical alternating current power, at for example 110 or 230 volts, the alternating current induces electrical and magnetic fields that result in high and varying noise in any high-speed data communication carried over the same or nearby cables.

In general, in an environment where a transmission medium has not been designed for high-speed data transmission, the high-frequency signal loss and the high-frequency impedance of the transmission medium may be unknown, vary from one particular installation to another, or simply have a different value than one expected by any common data transmission method. For example, the high-frequency impedance of commonly used power transmission cables may be typically unknown and may vary from an installation to another, typically in the range of 80 to 200 Ohms. As another example, the high-frequency impedance of two conductors in a power rail may be several hundreds of Ohms.

SUMMARY OF THE INVENTION

This specification describes systems and methods that may be used to transmit data and electrical power over a single cable, rail, or bus, avoiding some of the problems associated with prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention will be described in detail below, by way of example only, with reference to the accompanying drawings, of which Figure 1 illustrates a power and data transmission system according to an advantageous embodiment of the invention, Figure 2 illustrates a method according to a first further aspect of the invention, and

Figure 3 illustrates a device according to a second further aspect of the invention.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS In the following, we describe an advantageous embodiment of the invention providing a system for transferring electrical power and data between two endpoints, with reference to Figure 1.

In this embodiment, the system may comprise a communications data signal transmitter 102, a DC power source 101 , a transmission amplifier 103, a source load 04, a first isolation circuit 105, a sensing circuit 106, a data signal and power combination circuit 107, a first impedance matching circuit 108, a transmission medium 109, a second impedance matching circuit 110, a separation circuit 111 , a sensing matching circuit 112, a second isolation circuit 113, a DC power load 117, a terminating load 114, a receiver amplifier 115, and a communications data signal receiver 16.

In some embodiments, some of the components may be combined into a single component. For example, an isolation circuit (105,113), a signal

combination/separation circuit (107,111), and an impedance matching circuit (108, 10) may be implemented with an impedance matching transformer. Some of the components may be omitted, for example if a source load impedance matches with the impedance of a transmission medium, an impedance matching circuit at the signal source side of a transmission medium may be left out. The transmission medium 109 can be an electrical power transmission line such as an electrical power cable, rail or bus. The data signal transmitter 102 and data signal receiver 1 6 can be for example circuits typically used for driving Ethernet signalling over Ethernet cables.

These elements shown in Figure 1 are described in more detail later in this specification.

In a further advantageous embodiment of the invention, the end-to-end data and power transmission system consists of an Ethernet standard compliant data signal transmitter, a 100 Ohm resistive source load, a transmission amplifier, an isolation circuit, a Power-over- Ethernet compliant sensing circuit, a 48V DC power source, a combination circuit, an impedance matching circuit, a three or four wire power transmission cable or rail used as a transmission medium, an impedance matching circuit, a separation circuit, a Power-over- Ethernet compliant sensing matching circuit, 48V DC power load, a 100 Ohm resistive terminating load, and an Ethernet compliant data signal receiver.

In a still further advantageous embodiment of the invention, the isolation circuits, combination or separation circuits, and impedance matching circuit may be implemented with isolation transformers. An isolation transformer can advantageously be built to be able to combine or separate large direct currents with a data signal. This can be achieved by using a sufficiently thick wire, with the cross area of for example 0.75 mm² or larger, for the secondary winding of the isolation transformer. For example, the combined or separated direct currents may be in the order of tens of amperes.

In an even further advantageous embodiment of the invention, the direct current voltage may be 380 volts, or any reasonable voltage, for example, between 12 volts and 2000 volts. In an even further advantageous embodiment of the invention, the data signal transmitted or received may be based on the RS-422 or RS-485 standards.

Embodiments with impedance matching

In an environment where the high-frequency impedance of the transmission medium is not known beforehand or where the impedance may vary, it may be beneficial to use a dynamically adjustable impedance matching circuit.

In an advantageous embodiment of the invention, impedance of the

transmission medium is measured, and the transmitting and/or receiving end attempts to match its impedance to that of the transmission medium. This functionality can be implemented for example by using an impedance

measurement circuit at the transmitting and/or receiving end, and adjusting an impedance matching element on the basis of the measurement results.

In another advantageous embodiment of the invention, the transmitting and/or receiving end has a circuit that is able to dynamically try different impedance matching ratios. The system can measure for example the communication error rate, reflected energy, or other quantity that is able to quantify the goodness of the applied impedance match. The system may dynamically search for an optimal impedance that yields least error rate or reflected energy, or the minimum or maximum of some measure that results or correlates with good communication quality.

The system may contain a calibration circuit, which may consists of two subcircuits, one at the transmitter end and another at the receiver end. The system may further use a transmission medium for passing signals from the receiver end calibration subcircuit to the transmitter end calibration subcircuit. The used transmission medium may be the same transmission medium used for transmitter-to-receiver communication, e.g. the same wire pair, or it may be another transmission medium, e.g. another wire pair in the same cable, or a combination of such transmission medium. The receiver-end calibration subcircuit may use a low data rate method for sending signals to the

transmitter-end calibration subcircuit. For example, the receiver-end subcircuit may apply a variable resistive load between two wires so that the transmitter- end subcircuit may sense it. The transmitter-end subcircuit may use a sensing circuit to sense such a variable resistive load between the wires.

The calibration circuit may implement an amplification or impedance calibration function. For impedance measurement, a calibration subcircuit may transmit a measurement signal to the transmission medium. A calibration subcircuit may measure the reflected signal.

For example, the transmitter-end calibration subcircuit may apply a variable; amplification gain and source impedance on the transmission medium and;i transmit a test or measurement signal. The receiver-end calibration subcircuit may report the received signal strength, by passing signals to the transmitter- end calibration subcircuit.

Impedance matching can be implemented for example using fully differential operational amplifier circuits, as is known to a man skilled in the art. Impedance matching can also be implemented using a variable source load circuit.

A dynamically adjustable impedance matching circuit may adjust the impedance matching ratio of a circuit, or it may adjust the source or load impedance of a circuit. For example, a dynamically adjustable impedance matching transformer may have an adjustable impedance matching ratio; for example, such a transformer may have the winding ratio varying from 0.5:1 to 2:1. As another example, an amplifier circuit may have an adjustable source impedance, an adjustable load impedance, or both.

An impedance matching circuit may for example consist of a current controlled current conveyer circuit. The current conveyer circuit may be a CCCII circuit. The current conveyer circuit may contain large-current bipolar or field effect transistors which may be able to pass large currents. The current conveyer circuit may contain an internal isolation structure that may be able to isolates a nominal DC voltage applied on one side of the circuit from the DC voltage applied on the other side of the circuit. A dynamically adjustable impedance matching circuit may have a transformer whose transformation ratio may be dynamically adjusted. Such a transformer may have multiple middle taps on one or more of its windings. In a preferred implementation, a large current center-tapped winding of the transformer consists of two equal windings where the DC current flows in opposite directions and the low current winding of the transformer consists of a winding that has a plurality of middle taps. A relatively low number of windings may be used. For example, a large current winding may consists of two windings with 10+10 rounds and a low current winding may consists of first 10 windings without any middle taps and then additional 20 windings with middle taps, for example, once each winding, twice each winding, or once every two windings.

According to an advantageous embodiment of the invention, an impedance matching circuit is realized using inductive elements. The impedance matching circuit may consist of an L-bridge, T-bridge, π-bridge, or some other filter circuit. One or more of the windings in one or more coils in an impedance matching circuit may be wound of a wire that is able to carry large currents. The

impedance matching ratio of such a circuit may be adjustable, for example, by having an electrically adjustable capacitor or for example a coil with multiple middle taps in the circuit, each middle tap representing a different impedance matching ratio.

An impedance matching circuit may contain an impedance matching

transformer. An isolation circuit may contain an isolation transformer. A combined impedance matching and isolation circuit may contain a transformer that works simultaneously as an isolation transformer and as an impedance matching transformer.

In the following, the term transformer is used to denote any of an impedance matching transformer, an isolation transformer, or a transformer which acts simultaneously as an impedance matching circuit and as an isolation circuit. In an embodiment, such a transformer may also provide other functions or act as additional circuits. One or more of the windings in a transformer may be made of a thick wire. The thick wire may be able to carry a large direct current (DC), for example, 20 amperes. The large DC current may flow through the transformer in two windings. Such two windings may be winded in opposite directions. The opposite current directions may cause the magnetic fields induced by the DC current to cancel each other. The cancellation may prevent the transformer core from saturating.

In the following, in a transformer with windings made of thin wire and thick wire, we call the thin-wire windings as "low current" windings and thick-wire windings as "large current" windings.

In a transformer, windings may be arranged so that any alternating current applied on the low-current windings gets induced to the the large-current windings, and vice versa. The induction may work even when a large-current winding or windings are simultaneously passing a direct current. The magnitude of the direct current over the thick wire winding may be much larger than the magnitude of the alternating current. For example, the direct current may be 20 amperes and the alternating current may be 50 milliamperes. A transformer may be winded around a toroid core. The toroid core may be selected so that the relative permeability of the toroid remains relatively constant and/or rational throughout the main frequencies the data

communication signal is coded into. For example, a toroid core may be selected so that its permeability does not change much or become irrational in the frequency band of 10-35 MHz. A toroid core made of Fair-Rite or other vendor Material 67 or 68 may be used.

In an advantageous embodiment, a transformer may be constructed so that there are two large current windings winded an equal number of windings. The windings may be winded in parallel. Two of the winding ends may be connected to each other so that the windings form a single winding with a center tap. A large DC current may be passed from the center tap to the two other ends of the winding or vice versa. The may be one or more other windings. There may be a third winding with that has an equal number of windings as there are in the center-tap winding comprised of the two large current windings. The third winding may be a low current winding. The third winding may have a center tap.

For example, a transformer may be constructed as follows. Two pieces of AWG15 wire are aligned in parallel, and winded 20 rounds around a Material 68 toroid core. At one end of the windings, the two parallel AWG15 wire ends are soldered together and form a center tap. At the other end of the windings, the two other AWG 5 wire ends form the ends of the large-current center tapped winding. The wires and the toroid is insulated with a sufficiently thick isolation insulator. On the top of the insulator, two pieces of AWG24 wire are winded in parallel an equal number of rounds. Preferably, the windings are placed in between the AWG15 wire windings. The ends of the windings are used similar to the ends of the AWG15 wires. A thereby constructed transformer may be used as an isolation transformer. It may have favorable parasitic properties. It may function linearly at frequencies used by Ethernet data communication.

In an advantageous embodiment of the invention, a transmission medium 109 may consist of at least three wires or other conductors, where at least two of the conductors are at carrying a DC current at one nominal DC voltage and at least one in another nominal DC voltage. In this embodiment, a data communication signal is carried as a differential voltage applied over the two wires at a first DC voltage. For example, if two wires are carrying a direct current at a first nominal DC voltage of 48 volts, the system may modulate a Manchester coded square wave data signal (a baseband signal) at ±2.5 volts on the "top" of the 48 volts base voltage.

In a further advantageous embodiment of the invention, two pairs of conductors are used for carrying a DC current; one pair carrying a DC current at one nominal DC voltage and another pair at^" another nominal DC voltage. The system may carry a first data communication signal as a differential voltage applied over the first pair and a second data communication signal as a differential voltage applied over the second pair. Different data communication signals may be carried at different directions. For example, said first data signal may carry data from a first station to a second station while said second data signal may carry data from the second station to the first station. Distinct signals flowing in same or different directions may be carried at the same time (fully or partially overlapping) or at different times (at non-overlapping times). The timing may be controlled through Ethernet signaling. The timing may follow half duplex Ethernet IEEE 802.3-2008 standard.

A system where several one data signals may be carried at the same time may suffer from signal crosstalk. Signal crosstalk may be reduced by applying different twisting rates on the pairs carrying the data signals. The effects of crosstalk may be mitigated by carrying only one signal at a time, for example, by apply a half duplex data communication convention. A half duplex

convention may be according to the Ethernet standard. In one preferred implementation, a transmission medium comprises of a power transmission cable consisting of two or more wire pairs, each wire pair designed to have a specific high-frequency impedance; for example, the high-frequency impedance may be designed to be 100 Ohms resistive load. In the following, a first further aspect of the invention is described with reference to Figure 2. This first further aspect of the invention provides a method for transmitting electrical power and communicating data. This method comprises at least the steps of - communicating 210 data using differential transmission with a first pair of conductors of a power cable, where at least one of said conductors of said power cable has a cross-sectional area of at least a 0.75 mm², - communicating 220 data in only one direction over said pair of conductors at each point of time,

- transmitting 230 a DC current using said pair of conductors as a first conducting line of a DC current transmission circuit, and using at least one further conductor of said power cable as a second conducting line of the DC current transmission circuit.

In a further advantageous embodiment according to this first further aspect of the invention, the step 220 of communicating data in only one direction at each point of time is performed using a half duplex communication protocol.

In a further advantageous embodiment according to this first further aspect of the invention, a second pair of conductors of said power cable is used for communicating data, and said second pair of conductors are used as the second conducting line of the DC current transmission circuit.

In a further advantageous embodiment according to this first further aspect of the invention, the method further comprises at least the steps of

- measuring 240 an electrical property of at least one of said conductors, and

- adjusting 250 at least one of an output impedance, an input impedance, a transmission strength or a receiving amplification gain of a signal transmission circuit at least in part on the basis of said measurement.

In various embodiments of the invention, adjustment of a signal transmission circuit for improving the transmission of a signal can be made at the transmitting end, at the receiving end, or both. It may be advantageous to adjust the impedance at the receiving and/or transmitting end to match the impedance of the transmission medium. It may also be advantageous to adjust the

transmission and/or reception gain. In a further advantageous embodiment according to this first further aspect of the invention, said half duplex data communication protocol is the Ethernet IEEE 802.3 protocol used in half duplex mode. In various embodiments of the invention, the half duplex data communication protocol can be IEEE 802.3-2008 or an updated variant thereof.

In the following, a second further aspect of the invention is described with reference to Figure 3. This second further aspect of the invention provides a device 300 for transferring electrical power and communicating data. In an advantageous embodiment according to the second further aspect of the invention, the device 300 comprises at least a transformer 305 having a primary winding 306 and a secondary winding 307, said secondary winding consisting at least of wire having a cross sectional area of at least 0.75 mm² and said secondary winding comprising at least one middle tap 308, a first interface 315 for connecting to a data communications network, a second interface 330 for connecting to a power source or a load, said second interface being connectable to at least two conductors, said second interface _. being arranged to connect one of said at least two conductors to a middle tap 308 of said secondary winding 307 of said transformer 305, a third interface 350 for connecting to at least three conductors for transferring electrical power and communicating data over said three conductors, said third interface being arranged to connect two of said at least three conductors to said secondary winding 307 of said transformer 305, a controller 360 for receiving data from said first interface and for transmitting data through said first interface, and a controller 370 for communicating data through said transformer and said third interface using a half duplex data communication protocol.

The controller 360 and controller 370 may be implemented in a single circuit, and may also be implemented using a single controller.

In a further advantageous embodiment according to the second further aspect of the invention, the device 300 further comprises at least a circuit 380, 381 , 382 for adjusting an output impedance of said third interface 350.

In the example of Figure 3, the circuit for adjusting an output impedance of said third interface is illustrated as comprising of switches 381 , 382 selecting between middle taps of the secondary winding of the transformer 305, the switches being controlled by a control circuit 380. However, this solution is only one example of various ways to implement an adjustable output impedance, as was discussed in more detail previously in this specification.

In a further advantageous embodiment according to the second further aspect of the invention said half duplex data communication protocol is the Ethernet IEEE 802.3 protocol used in half duplex mode.

In various embodiments of the invention, the half duplex data communication protocol can be IEEE 802.3-2008 or an updated variant thereof. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention. While a preferred embodiment of the invention has been described in detail, it should be apparent that many modifications and variations thereto are possible, all of which fall within the true spirit and scope of the invention.

Claims

1. A method for transmitting electrical power and communicating data, comprising at least the steps of communicating data using differential transmission with a first pair of conductors of a power cable, where at least one of said conductors of said power cable has a cross-sectional area of at least a 0.75 mm², communicating data in only one direction over said pair of conductors at each point of time, transmitting a DC current using said first pair of conductors as a first conducting line of a DC current transmission circuit, and using at least one further conductor of said power cable as a second conducting line of the DC current transmission circuit.

2. A method according to claim 1 , wherein the step of communicating data in only one direction at each point of time is performed using a half duplex communication protocol.

3. A method according to claim 1 , wherein a second pair of conductors of said power cable is used for communicating data, and said second pair of

conductors are used as the second conducting line of the DC current

transmission circuit.

4. A method according to claim 1 , further comprising at least the steps of measuring an electrical property of at least one of said conductors, and adjusting at least one of an output impedance, an input impedance, a

transmission strength, or a receiving amplification gain of a signal transmission circuit at least in part on the basis of said measurement.

5. A method according to claim 2, wherein said half duplex data communication protocol is the Ethernet IEEE 802.3 protocol used in half duplex mode.

6. A device for transferring electrical power and communicating data,

comprising at least a transformer having a primary winding and a secondary winding, said secondary winding consisting at least of wire having a cross sectional area of at least 0.75 mm² and said secondary winding comprising at least one middle tap, a first interface for connecting to a data communications network, a second interface for connecting to a power source or a load, said second interface being connectable to at least two conductors, said second interface being arranged to connect one of said at least two conductors to a middle tap of said secondary winding of said transformer, a third interface for connecting to at least three conductors for transferring electrical power and communicating data over said three conductors, said third interface being arranged to connect two of said at least three conductors to said secondary winding of said transformer, a controller for receiving data from said first interface and for transmitting data through said first interface, and a controller for communicating data through said transformer and said third interface using a half duplex data communication protocol.

7. A device according to claim 6, further comprising a circuit for adjusting at least one of an output impedance, an input impedance, a signal transmission strength, or a signal receiving amplification gain of said third interface.

8. A device according to claim 6, wherein said half duplex data communication protocol is the Ethernet IEEE 802.3 protocol used in half duplex mode.