Abstract:
A balanced differential signal communication system having at least two data lines connecting multiple nodes in series, each node comprising a signal generator for applying signals to the data lines that produce a controllable differential voltage across the data lines; a rechargeable storage device for receiving electrical energy from the data lines to charge the storage device; at least one device coupled to the storage device for receiving electrical energy from the storage device; and a controllable converter coupling the data lines to the storage device for controlling the charging and discharging of the storage device with power captured from the data lines.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to supplying power to devices coupled to a differential signal communication system by harvesting electrical power from data lines of the communication system. 
       BACKGROUND 
       [0002]    Differential signal communication systems are in widespread use. For example, RS-485 is a well known serial digital communication system that uses balanced differential signals for communicating with computers and other devices. RS-485 allows multiple devices to communicate at half-duplex on a single pair of wires, plus a ground wire, over long distances. Both the length of the network and the number of nodes can easily be extended using a variety of different repeater products that are readily available. The properties of differential signals provide high noise immunity and long distance capabilities. 
         [0003]    RS-485 is the most versatile communication standard in the standard series defined by the EIA, as it performs well for connecting data terminal equipment (DTE) directly without the need of modems, for connecting several DTE&#39;s in a network structure, for communicating over long distances, and for communicating at fast communication rates. RS-485 is currently a widely used communication interface in data acquisition and control applications where multiple nodes communicate with each other. RS-485 signals are floating with each signal being transmitted over a S+ line and a S− line. The RS-485 receiver compares the voltage difference between the two lines, rather than the absolute voltage level on a single line. 
         [0004]    RS-485 interfaces are often preferred for data acquisition and control applications because RS-485 is capable of internetworking multiple transmitters and receivers in the same network. High-resistance RS-485 inputs allow a large number of nodes to be used, and RS-485 repeaters can be used to increase the number of nodes even more. 
       BRIEF SUMMARY 
       [0005]    The present disclosure provides a balanced differential signal communication system having at least two data lines connecting multiple nodes in series, each node comprising a signal generator for applying signals to the data lines that produce a controllable differential voltage across the data lines; a rechargeable storage device for receiving electrical energy from the data lines to charge the storage device; at least one device coupled to the storage device for receiving electrical energy from the storage device; and a controllable converter coupling the data lines to the storage device for controlling the charging of the storage device with power captured from the data lines. 
         [0006]    One implementation includes a microcontroller coupled to the controllable converter and producing a control signal that affects the amount of power captured from the data lines by the converter; a multiplexer coupled between the microcontroller and the converter and having a pair of input terminals for receiving a pair of input signals, an output terminal, and a control input for receiving a control signal from the microcontroller for selecting which input signals are included in the output signal produced at the output terminal, one of the inputs receiving the control signal produced by the microcontroller; and a source of a fixed reference voltage coupled to the other of the inputs to the multiplexer. The microcontroller may be programmed to send messages via the data lines to other nodes coupled to the data lines to cause the other nodes to respond to the messages via the data lines, and the converter captures power from the responses for recharging the storage device. 
         [0007]    One embodiment includes a sensor coupled to the data lines for sensing the voltage differential between the data lines and supplying a signal representing the voltage differential to the microcontroller, and a microcontroller is programmed to use the signal representing the voltage differential to determine a load that can be imposed on the data lines without interfering with communications on the data lines. The converter is preferably controllable to vary the load that the converter imposes on the data lines, so that the load does not interface with communicating via the data lines. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in which: 
           [0009]      FIG. 1  is a block diagram of an RS-485 communication system that includes multiple nodes and a power harvesting device for harvesting electrical power from the data lines of the bus. 
           [0010]      FIG. 2  is a block diagram of one of the nodes in the system of  FIG. 1 . 
           [0011]      FIG. 3  is a flow chart of a program executed by the microcontroller in the node of  FIG. 2  to control the harvesting of electrical power from the bus. 
           [0012]      FIG. 4  is a flow chart of a program executed by the microcontroller in the node of  FIG. 2  to control radio/bus messaging. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Although the invention will be described in connection with certain preferred embodiments, it will be understood that the invention is not limited to those particular embodiments. On the contrary, the invention is intended to cover all alternatives, modifications, and equivalent arrangements as may be included within the spirit and scope of the invention as defined by the appended claims. 
         [0014]    Turning to the drawings,  FIG. 1  is a functional block diagram of an RS-485 communication system that includes a differential bus  10  is formed by a pair of data lines S+ and S−, and multiple nodes formed by devices  11   a,    11   b . . .    11   n  coupled to the bus  10  at different points along the length of the bus. An RS-485 communication channel can be shared by multiple receivers and multiple senders. In the illustrative embodiment, he bus  10  is terminated by resistances R 1  and R 2  at opposite ends, and a bias voltage is supplied to one end of the bus from a source  12  through voltage divider formed by a pair of resistors R 3  and R 4  in combination with the terminating resistor R 1 . For high bit rates and long wiring runs, termination resistances are necessary on both ends of the bus  10  to eliminate reflections, but not at drop points along the bus. High bit rates are possible because the transition between logical 0 and logical 1 is only a few hundred millivolts, and currently available RS-485 drivers can achieve bit rates of at least 35 mbps. 
         [0015]    Although  FIG. 1  illustrates an RS-485 two-wire multi-drop bus, it will be understood that an RS-485 network can also be connected in a four-wire mode, using four data wires and an additional signal ground wire. In a four-wire network, one node is a master node and all others are slave nodes. The network is connected so that the master node communicates to all slave nodes, and all slave nodes communicate only with the master node. Since the slave nodes never listen to another slave response to the master, a slave node cannot reply incorrectly to another slave node. RS-422 systems are also use balanced differential signals, using a dedicated pair of wires for each signal, a transmit pair, a receive pair and an additional pair for each handshake/control signal used (if required). In a “two-wire” network the transmitter and receiver of each device are connected to a twisted pair. “Four-wire” networks have one master port with the transmitter connected to each of the “slave” receivers on one twisted pair. The “slave” transmitters are all connected to the “master” receiver on a second twisted pair. In either configuration, devices are addressable, allowing independent communications with each node. 
         [0016]      FIG. 2  is a more detailed illustration of one of the node devices  11 , which includes a communication and bus loading control  20  and an energy capture system  30 , both of which are coupled to the two data lines S+ and S− of the bus  10 . As is well known, the data lines S+ and S− are preferably in the form of a twisted pair, to provide noise immunity. The energy capture system  12  supplies electrical power to a power supply  40  that powers the communication and bus loading control  11 . 
         [0017]    Within the communication and bus loading control  20 , an RS-485 bus driver  21  sends signals to, and receives signals from, the data lines S+ and S− of the bus  10 . The driver  21  delivers received signals to a microcontroller  22 , and receives signals from the microcontroller  22  for delivery to the bus  10 . Only one device can drive the data lines at a time, so the drivers at the various nodes must be put into a high-impedance mode (tri-state) when they are not in use. An RS-485 driver is typically enabled and disabled by an RTS control signal from an asynchronous serial port. Setting the RTS line to a high (logic 1) state enables the driver, while setting the RTS line to a low (logic 0) state puts the driver into a tristate condition that in effect disconnects the driver from the bus, allowing other nodes to transmit over the same pair of data lines. The RS-485 driver typically returns to its high impedance tri-state within a few microseconds after data has been sent, so it is not necessary to have delays between data packets on the RS-485 bus. These tristate capabilities of RS-485 nodes allow a single pair of wires to share transmit and receive signals for half-duplex communications. 
         [0018]    The microcontroller  22  also receives the output of an operational amplifier  23  that has two inputs coupled to the two data lines S+ and S−, so that the output of the amplifier  23  represents the difference between the voltage levels on the two data lines S+ and S−. The microcontroller  22  also receives inputs from a real time clock  24  and a radio transceiver  25 , and delivers control signals to the energy capture system  30 , as will be discussed in more detail below. The transceiver  25  is typically a generic low power radio transceiver, such as a ZigBee radio, or the transceiver may be replaced with a signal/protocol converter. 
         [0019]    The energy capture system  30  includes a rectifier  31  having a pair of inputs coupled to the two data lines S+ and S−, producing a rectified output that is smoothed by a filter  32  and then supplied as the DC input to a controllable DC-to-DC converter  33 . The DC output of the converter  33  is supplied to an energy storage device  34 , such as a rechargeable battery or an ultra capacitor. The output of the storage device  34  is connected to the power supply  40  that supplies power to the communication and bus loading control  20 , including the microcontroller  22  and the radio  25 . When the storage device is a rechargeable battery, it may be installed pre-charged to reduce the initial charge and discovery time, or it may be initially charged by some other means such as a USB plug-in to a computer. 
         [0020]    To control the charging of the energy storage device  34 , the microcontroller produces control signals for a multiplexer  35  that also receives a reference voltage form a reference voltage source  36 . The multiplexer  35  selects one of its two inputs, from the microcontroller  22  and the reference voltage source  36 , for application to the converter  33 . The selection is controlled by a “select” signal generated on output line  22   a  from the microcontroller  22  and applied to the select input of the multiplexer  35 , which determines which of the two multiplexer inputs is supplied to the converter  33 . It will be understood that either a digital or analog multiplexer may be used, depending on the type of control desired for the converter  33 . This control of the converter  33  enables the system  30  to capture small amounts of energy from the biasing voltage on the bus  10 , or to capture larger amounts of energy from data signals on the bus when the data lines S+ and S− are driven by other devices on the bus. By controlling the converter, the system is able to vary the load it puts on the bus  10  to capture more or less power, while ensuring that the load does not interfere with communications. The converter may be controlled to change not only the power captured from the bus  10 , but also the impedance it presents to the bus to improve communication and/or the power harvesting capabilities. 
         [0021]    Upon connection to the bus  10 , the converter  33  and the storage device  34  capture a small amount of power from the biasing circuit for the bus  10 . The converter  33  varies its load to draw as much power as possible from the bus biasing circuit without exceeding the specification of the communications system or dropping the biasing voltage below the appropriate threshold for the applicable communications standard. Any communications that occur on the bus  10  allow the converter  33  to charge the storage device  34  faster than just the biasing voltage, and those communications can also be used to identify the type of network or protocol being used, as described in more detail below. 
         [0022]    After the storage device  34  has accumulated enough energy to enable the microcontroller  22  and the driver  21  to generate a request, they transmit a harmless request to the other nodes on the bus  10 , such as a Modbus read or identify request. The microcontroller may identify the desired type of communication automatically by monitoring traffic on the bus  10 , or the microcontroller may have been pre-configured for a certain setting. If there is no response to a request generated by the microcontroller  22  and the driver  21 , the converter  33  continues charging the storage device  34  from the bias voltage while the microcontroller  22  continues to generate different types of messages or targets different nodes until a response is received. 
         [0023]    After a response is received, the power-harvesting system has a better source of power and can begin issuing additional requests to detect other protocols or devices on the network. For example, certain nodes on the bus  10  may have stronger drivers, which can be detected by varying the load on the network while listening for a response, or certain nodes may respond faster or support other commands that produce longer responses and therefore supply more power for each request. The microcontroller may be programmed to look for the option that will provide the most power for charging the storage device  34 , both in terms of the device&#39;s driving capabilities and the ratio of power received to power required to transmit. 
         [0024]    In the illustrative example with the radio  25 , the microcontroller  22  can turn on the radio  25  and thus begin wireless communications after a stable source of energy has been obtained and sufficient energy has been stored. Other nodes on the bus  10  can be polled as needed to maintain the required power. 
         [0025]    When the illustrative device is acting as a bridge or converter between a radio network and a wired communications bus, incoming requests can be buffered while power is being harvested from the wired bus. Then a message can be transmitted from the wireless network, and any response will be forwarded back onto the radio network. The radio  25  or the entire device may be designed to go into a low power “sleep” state and only power up when traffic is received from the wired communications bus, or in response to a periodic radio timer. 
         [0026]      FIG. 3  is a flow chart of a power-up sequence for the power harvester illustrated in  FIG. 2 . Step  101  determines whether the energy storage device  34  has accumulated enough power to start the microcontroller  22 , and if the answer is negative, step  102  indicates that the DC-to-DC converter  33  uses the fixed reference voltage from source  36  to determine the loading on the bus  10 . That is, the converter  33  operates as a single unit load, capturing energy from the bias voltage and any messages on the data lines S+ and S− until enough power is accumulated in the storage device  34  to start up the microcontroller  22 . 
         [0027]    When the response at step  101  is affirmative, the microcontroller  22  is started at step  104  so that the balance of the steps in  FIG. 3  can be executed by the microcontroller. If at any time the power available from the storage device  34  drops below a preselected threshold, the microcontroller  22  is powered down and re-started when the accumulated power again rises above that threshold. After the microcontroller  22  has been started, the microcontroller  22  controls the variable load imposed on the bus  10  by the DC-to-DC converter  33 , via the control signal applied to the select input of the multiplexer  35  by the microcontroller. 
         [0028]    From step  104 , the system advances to step  105 , which determines whether there are any previously stored network settings. If step  105  yields an affirmative answer, the system proceeds to step  106  to load any settings for the power harvester settings and resume operation, including turning on the radio  25 . The system then advances to step  114 , which queries the network settings periodically and adjusts the power-harvesting parameters as needed to maintain a maximum power reserve. 
         [0029]    When step  105  yields a negative response, indicating that there are no previously stored network settings, the system advances to step  107  to turn on the bus driver  21 , to turn on the voltage differential-sensing amplifier  23 , and to listen for messages. The system then advances to step  108  to listen for messages. When a message is detected, step  108  produces an affirmative response, which advances the system to step  109  which analyzes the message to collect data from the message, such as network type, other devices on the bus and signal strength. The collected data is then analyzed at step  111  to determine desired statistics such as the identity of devices having the most transmit power, the longest message response, the maximum load the harvester can apply to the bus  10  without adversely affecting communications, etc. 
         [0030]    When no message is detected, step  108  yields a negative response, the system advances to step  110  to transmit a message via the bus driver  21  and then listen for a response. Any response is used to collect the same type of data collected at step  109 , namely, network type, other devices on the bus signal strength, etc. The collected data is then analyzed at step  111  to determine desired statistics such as the identity of devices having the most transmit power, the longest message response, the maximum load the harvester can apply to the bus  10  without adversely affecting communications, etc. 
         [0031]    From step  11 , the system advances to step  112  to adjust the loading on the bus  10  to provide maximum power to recharge the storage device  34 , and then those network settings are saved at step  113 . Then step  114  queries the network settings periodically and adjusts the power-harvesting parameters as needed to maintain a maximum power reserve. It will be understood that the loading on the bus is adjusted by the DC-to-DC converter  33  in response to the output signal received by the converter from the multiplexer  35 , which in turn is controlled by the control signal applied to the select input of the multiplexer  35  by the microcontroller  22 . 
         [0032]      FIG. 4  is a flow chart of a routine executed by the microcontroller  22  to control messaging operations via the radio  25  and the bus  10 . The radio  25 , which can wait to be triggered by another device or can query a central device on power up, serves to synchronize multiple power harvesters and to relay messages between the bus  10  and another location, either on request or as part of a scheduled operation. 
         [0033]    The routine in  FIG. 4  begins at step  201 , which determines whether a stable power supply is available, based on stored data relating to power supplied to the microcontroller from the power supply  40  over a selected time interval. This step repeats until it produces an affirmative response, which advances the system to step  202  to power up and configure the radio  25 . Step  203  then waits for data from the radio  25 , and advances to step  204  when such data is received. Step  204  determines whether the radio message is for the bus  10  or the power harvester. If the message is for the bus, the data is sent out on the bus  10  at step  205 , after being reformatted by the microcontroller  22  if necessary, and is then sent out on the radio  25  at step  206 . If step  204  determines that the radio message is for the power harvester, the routine advances to step  207  to analyze the message for information such as time synchronization, polling schedule, etc. Step  208  then applies any changes, sends an acknowledgment through the radio if required, starts scheduling polling, etc. 
         [0034]    While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations may be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.