Synchronized measurement device using local area network with ethernet messaging

A plurality of measurement devices have analog sensors that measure the dynamic signals of physical events, sample the data into digital format with time synchronized clocks and generate time stamped Ethernet messages that are sent to a remote host. The remote host has a master clock that evaluates decoded time stamped messages from the measurement devices and sends back a message with a time correction error signal relative to the master clock. This feedback signal is used by the measurement devices to correct a local clock for data sampling and new message generation. Eight wire cable and associated connectors are used to handle three channels of traffic, with four wires dedicated to Ethernet messages as one channel, another two wires dedicated to reset and other commands as a second channel and another two wires to transmit power from the host to the measurement devices as a third channel.

TECHNICAL FIELD

The invention relates to data transmission and collection from distributed analog sensors for dynamic signal measurement in a network.

BACKGROUND OF THE INVENTION

There are many applications where an array of measurement sensors is deployed to collect dynamic physical event data, such as seismic shock, vibration, temperature, strain or humidity in industrial manufacturing and testing processes, remote machinery condition monitoring, building structure monitoring, etc. A problem that arises in data collection from such events is that the data, typically dynamic analog data at various frequencies, is collected using expensive instrumentation devices. These instruments are connected to each other via various networks with phase delays, even though taken simultaneously due to differences in network path lengths or slight local clock differences or circuit latency. The kind of measurement system that cannot measure the dynamic data with synchronized clock will make the data acquisition meaningless. For example, if two channels of data are all acquired at 12:00 pm while one of them is marked as 12:01 pm, the time delay or error of the time clock will cause these two signals to be out of phase so that they cannot be compared and used together. Therefore for dynamic data measurement using multiple sensors, acquiring the data at the “same time”, meaning, with time synchronization technology, is critical.

U.S. Pat. No. 6,469,639 discloses a method of collecting data from a plurality of sensors, including shock and vibration sensors. The method includes converting the plurality of sensor analog signals into digital data, processing the digital data, generating a data communications protocol for communicating the digital data, and simultaneously and remotely detecting the generated communications data. A data communications processor controls power management of the data acquisition processing circuit.

U.S. Pat. No. 7,200,158 discloses a method of synchronizing data across a network with a device that recognizes the time synchronization packet and substitutes a real-time value from the master internal counter into the proper place in a data packet that is placed between an Ethernet Media Access Controller (MAC) and a Physical Interface Transceiver (PHY). A second device monitors the packet passing from the MAC to the PHY and determines when it is a time synchronization packet from the time master. Upon recognition of the proper packet, the second device simultaneously captures the master's time value and captures the value of a local real-time clock. The result of these captures are presented to the local host computer which controls the time base clock that increments the local real-time clock to either speed up or slow down this local clock, thereby synchronizing the local clock to the time master clock. The offset and skew of the local clock to the master clock is reduced to only the network latency plus variability due to network congestion.

European patent application 2288215 discloses a method for exchanging data between a plurality of sensors and/or actuators and an appropriate destination in a core network.

U.S. Published Application 2013/0246543 discloses networked sensor devices configured to obtain sensor readings from one or more sensors and then store the sensor readings in a server by way of a wireless or wired network using Ethernet protocol for communication of sensor information.

U.S. Published Application 20140088873 discloses use of Ethernet protocol for devices that integrate sensors for dissemination of lightning data over wired or wireless networks. The apparatus features a communication jack that houses a communication port, a sensor and a sensor data server. The server is coupled with both the port and the sensor and is configured to obtain sensor data from the sensor and provide access to the sensor data via the communication port. The system is a plurality of such communication jacks distributed over a geographic range.

Power over Ethernet (PoE), described at http://en.wikipedia.org/wiki/Power_over_Ethernet, is a power supply technology for Ethernet wiring where, in one mode called “Mode B”, spare lines of Cat 5 or Cat 6 cable are energized by line drivers capable of sourcing a few milliamperes of current that is sufficient to obtain all required electrical power for a remote device through the lines. In another mode called “Mode A”, power can be carried on the data lines. In either situation, a host can provide power to remote devices such as switches, IP phones and the like.

An object of the invention was to reduce packaging size for remote sensors of a sensor array, while increasing the flexibility of supporting large voltage drops in long cables, and reducing the cost.

Another object of the invention was to devise an array of high speed remote sensors that measure analog physical signals and can gather data from a single event or vibration source at a central server for recording or display, with synchronization to account for phase delays and the like among individual remote sensors.

SUMMARY OF INVENTION

The above object has been met with a network of measurement devices, arranged over an area, i.e. a local area network, and exhibiting phase delays, as well as circuit or transmission latency when sampling analog data from the same event, such as seismic data, vibration and shock, and other dynamic data that may be sampled in milliseconds or faster. The network features a host having a central processing unit (CPU), storage memory, a master clock, and an Ethernet network switch that sends and receives messages. The host has multiple connectors communicating with the CPU, a power supply with power management logic communicating power to power lines associated with each connector, an Ethernet transceiver associated with each connector for sending and receiving time synchronization messages from the network switch on message lines associated with each connector, and control lines associated with each connector to send commands from the CPU. Each connector is reserved for communication with one of the measurement devices. Ethernet communication of measurement data between a measurement device and the host is via a four-wire channel associated with a connector. Each connector employs an additional two channels, including a two-wire channel as a power channel and a two-wire order wire channel for commands, for a total of three dedicated channels per connector.

The amount of power transmitted from the host power supply to each measurement device on the power channel must be sufficient to power the analog sensor, an A/D converter, logic circuits and a data transceiver. Although low power CMOS digital circuits are used where possible, adequate power for all components must be provided. Power demand is established by each measurement device and met by the host. Commands on the order wire channel can be on-off and reset commands, as well as other commands. While an Ethernet channel can be used to send and receive data, program code or even commands, there are cases when an Ethernet channel is not usable such as when TCP/IP protocol has not been established. Thus a separate order command channel is needed and can establish high reliability of controlling the remote measurement devices from the host.

Each measurement device has an analog sensor measuring dynamic data, such as shock or vibration, connected to an analog-to-digital converter producing digital data from the dynamic analog data. The process of converting analog data into digital is called sampling. How fast and when the sampling happens is governed by a sampling clock. The sampling clock of each analog-to-digital converter is derived from a local clock, i.e., the slave clock on each measurement device. It is the objective of this invention that all the sampling clocks on all measurement devices connected by the host are eventually synchronized. A typical sensor is a 3-axis accelerometer that transmits 3-path analog outputs to a 3-path analog to digital converter and then to time stamped message formation. The measurement device has a logic circuit with an embedded core that applies a time stamp from the slave clock to the digital data for transmission to the host as time stamped local data messages in Ethernet protocol. Transmission is via a data transceiver that transmits the time stamped messages to the host using the four-wire Ethernet channel and associated connector. The host then provides feedback to each measurement device using the same Ethernet channel carrying time synchronization correction messages for the slave clock relative to the master clock so that all sampling clocks are synchronized, and the time synchronization of measurement data at the host from the plurality of measurement devices is achieved by exchange of time stamped data messages and time synchronization correction messages.

DETAILED DESCRIPTION

With reference toFIG. 1an area network11features a host device13communicating with sensor measurement devices15,17,19and21. The host device13has a network switch23with 8-wire connectors25,27,29and31that may be standard RJ45 connectors. Each sensor measurement device, such as device15, communicates through an associated connector, such as connector25using 3 dedicated channels, including a power channel33, an Ethernet message channel35and an order wire channel37for commands. The three channels are accommodated on 8 wires for an 8-wire connector, for example of the RJ45 type. A typical allocation of wires is further discussed below with reference toFIG. 3.

The network switch23is a typical Ethernet message switch chip set that distributes Ethernet messages among the connectors25,27,29and31. The network switch is connected to a fanless computer board39that acts as a server. The board may be a PC104 system on a chip (SOIC) type of board with an Intel Atom processor or CPU, electronic memory, a hard drive for data storage, a graphics processor, and input-output circuitry. A slot for a wireless card41is needed to allow a remote display device43to be wirelessly connected to the host or to cloud storage45. A power manager module47is connected both to the switch23and to the computer board39. The power manager module receives DC power from a computer power supply, not shown, that produces several levels of rectified DC power from AC. The power manager module provides power to the host in the usual way and also provides calibrated amounts of power to each of the sensor measurement devices15,17,19,21to meet the power demands of each device, with power being communicated over a power channel33associated with a connector25. Power is fed to each power channel through the switch23on a continuous basis since measurement devices must feed power to sensors that can respond at any time to measurement phenomena.

With reference toFIG. 2, the host11has power manager module47transmitting electrical power over two wires that make up channel one,33, in a cable connected by a connector25to the measurement device15. Switch23of the host transmits Ethernet messages over four wires of channel two,35, of the same cable, while the CPU or SOIC39transmits commands over two wires of channel three,37, to the measurement device, also using the same cable. All channels, with a total of 8 wires, pass through connector25into the measurement device15.

Within the measurement device15, a power converter55receives power remotely from host11. Power can be transmitted up to 300 meters on a CAT 5 cable with significant voltage drop. The voltage drop will be well handled by the power regulation circuitry in each measurement device. The input voltage to the power converter is typically in the range of 5.5 volts to 15 volts. The power converter steps this voltage down to a regulated 5 volt output that is fed to a power management circuit57. Voltage from the power management circuit57is supplied to analog to digital circuits59that are connected to at least one sensor60, also receiving the same voltage from the power management circuit. Sensor data is taken in the analog to digital circuits59and packaged as Ethernet messages sent with a transceiver on channel two,35, to switch23of the host11. The SOIC39, using memory and I/O circuits46can issue stop, start or reset commands via channel three,37, essentially an order wire for sending commands from the host11to the measurement device15.

InFIG. 3, measurement device15is seen to have a sensor60, such as a 3-axis accelerometer on a chip, for example an ADXL326 chip that includes X,Y,Z axis accelerometers, amplifiers, demodulators and 3 output amplifiers. The use of accelerometers is indicated for shock and vibration measurements. Other sensors could be used for weather, seismic or volcanic, geophysical or manufacturing and testing data. Any kind of sensors that gather high speed analog signals could be used here. Analog sensor data from the accelerometers is transmitted to analog-to-digital converters that can be implemented with two AK5357 chips, with each chip handling 2 analog signals with only 3 of the 4 signal paths being needed. Digital output data from the converter needs to be packaged as Ethernet messages to be transmitted on 2 wires. This is done by an FPGA64that has an embedded microprocessor core66that forms the messages as will be explained below. Message formation is clocked by logic clock68running at a speed compatible with Ethernet transmission rates, such as 10baseT, 100baseT or 1000baseT running at speeds of 10 Mbits/sec., 100 Mbits/sec or 1000 Mbits/sec respectively. Logic gates61contain programming instructions for message formation and transmission with a bus connecting the gates61to core66. An optional memory card63can be used to store programming data that cannot be stored in the logic gates61. An optional serial peripheral interface flash memory65may be used to store programming data for the logic gates61that needs to be loaded serially into the logic gates. An amount of random access memory67is connected to logic gates61for storing intermediate results from message forming operations and reloading the intermediate results back into the logic gates61.

Digital data that has been packaged into 8-bit messages is transmitted on 2 lines to a PHY transceiver chip69which may be a LAN8710A transceiver by Microchip. A local 25 MHz oscillator71, connected to the PHY transceiver chip, or part of the chip, establishes the base frequency for data transmission after appropriate frequency division, depending on data transmission rate. This is the local frequency that will be corrected by feedback from the host. Message data input and output of the transceiver is sent through a transformer73for signal transmission purposes at appropriate low voltage signal levels.

Four wires, two transmit lines and two receive lines form an Ethernet message channel35connected to connector25. Two lines handle message traffic in one direction, while two wires handle simultaneous message traffic in the opposite direction. Typically, a first direction is for outgoing traffic, while a second direction is for incoming traffic. Two other lines form a power channel33feeding the DC input-DC output voltage step up chip75that is connected to a power distribution unit77that provides power to circuits of the measurement device as needed. Power is supplied by the distant host as required by measurement devices on the power channel associated with connector25. Two reset lines form a command channel or order wire channel37. Signals in this channel can trigger the reset switch79which restarts message formation by resetting the core66and the logic clock68.

The connector25is extended by a CAT 5 or 6 cable for up to 300 meters to a host without phase shift among bits in all of the channels. On the other hand, delay caused by the cable, or software latency, can cause data from several measurement devices that are measuring the same event to be skewed or out of synchronization. Since the data is transmitted in messages, the host computer provides time delay corrections from a master clock in return messages received in the Ethernet channel and sent to the core66via the transceiver69. The core adjusts the logic clock68so that new messages are synchronized.

InFIG. 4message synchronization is achieved in the core66that has incoming message capture buffers81and83where incoming time correction Ethernet messages from a remote host are received after passage through the PHY transceiver chip69. These messages are routed along parallel lines of a data bus to be decoded in the logic gates61using a time base clock appropriate for the message speed. The messages are converted to instructions sent to an instruction stack85for industry standard precision time protocol (PTP) a time correction algorithm executed by the core66for clock synchronization. Simultaneously, outgoing messages encoded in the logic gates61are packaged and routed via a transmit buffer87for transmission using the transceiver69.

FIG. 5shows another view of core66wherein a 3-axis sensor60generates 3 data components (X,Y,Z) via analog-to-digital converters62a,62band62c. As shown inFIG. 1, clock logic68is a sampling clock associated with the A/D converters. A timing signal for the sampling clock is derived from the local oscillator71that is associated with the PHY chip transceiver69. The oscillator timing, nominally 25 MHz, is transmitted into the PHY transceiver69from the local oscillator71. The timing signal is transmitted into core66via a buffer82that transmits the signal into a clock divider84to provide a clock signal that will set the message transmission rate selected by timer86. The rate signal, is applied to the PTP instruction stack85that works with the FPGA61to package messages from the data in the A/D converters62a,62band62c, with a time stamp corrected by incoming messages decoded in the FPGA. Time stamped messages are routed via PHY transceiver chip69to the remote host for data logging. All remote measurement devices should be sampling time corrected data.

In operation, measurement devices have analog sensors that measure an event and generate time stamped digital data by the analog-to-digital converter, and then send the digital data using an Ethernet transceiver to a remote host. The remote host has a master clock that evaluates decoded time stamped messages from the measurement devices and sends back a message with an error signal relative to the master clock. This feedback signal is used by the measurement devices to correct a local clock for new message generation with time synchronized measurement data.

The packaging of measurement devices can take various physical forms. They can be constructed by independent electronic components on a small printed circuit board, or compact many digital circuitry into an FPGA (Field Programmable Gated Array), or fully integrated into an ASIC (Application Specific Integrated Circuit) which has a size of less than a coin.