Horology: time measuring systems or devices – Plural timepiece system or system device
Reexamination Certificate
1998-10-16
2001-05-22
Miska, Vit (Department: 2859)
Horology: time measuring systems or devices
Plural timepiece system or system device
C368S047000, C370S507000, C375S354000, C375S358000
Reexamination Certificate
active
06236623
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a system and method for synchronizing clocks in a plurality of devices connected by a communication channel and is particularly suited for monitoring and/or accurately controlling the relative time relationship of events occurring proximate to one or more of the devices.
DESCRIPTION OF THE PRIOR ART
Event recorders have been used successfully for many years throughout industry, mainly for the purposes of scheduling maintenance actions and for post-incident analysis after an alarm or failure has occurred. For example, if a failure occurs on a power grid that caused portions of the grid to shut down, it would be useful to determine the sequence in which the shut downs occurred. Additionally, in the event of a catastrophic failure, e.g., an explosion in an oil refinery, the ability to determine the origin of the explosion by determining the sequence of events in the refinery could be extremely desirable. Event recorders work by time-stamping a signal when it arrives at the recorder's buffer. The time-stamping performed by such recorders relies upon an internal clock that is generally only accurate to within a millisecond. Typically, when multiple recorders are used, their internal clocks are not precisely synchronized even in systems where the recorders are in close proximity. Accordingly, determining the precise timing sequence of a group of events, e.g., a power shutdown or catastrophic failure, can be difficult due to the accumulated errors, i.e., from multiple clock and synchronization errors. Consequently, it is generally difficult to determine the event, and its corresponding location, which originated the sequence. In addition, if the recorders are widely distributed, the ability to synchronize the recorder's internal clocks typically becomes even more difficult and even less precise. Therefore, what is needed is a system that allows for precise synchronization between multiple event recorders and/or control devices that is essentially independent of the geographical distribution of such devices.
SUMMARY OF THE INVENTION
The present invention relates to a system and method for synchronizing clocks in a plurality of devices connected by a communication channel and is particularly suited for monitoring and/or accurately controlling the relative time relationship of events occurring proximate to one or more of the devices. Preferred embodiments of the present invention provide a master control device coupled to one or more slave control devices via a communication channel. The master control device, preferably microcomputer based, is a controller having an internal clock that is capable of periodic communication with the slave control devices to cause clocks within each slave control device to essentially synchronize with the master control device's internal clock. Furthermore, the master control device can command and/or monitor events at each of the slave control devices. The slave control devices, preferably microcomputer based, are controllers having internal clocks that are responsive to messages from the master control device. Additionally, the slave control devices include an interface to monitor local events, e.g., through a relay contact, and/or control external devices to cause local events to occur, e.g., through an output driver. Preferably, the communication channel is implemented according to an RS-485 interface. By periodically interrogating and monitoring the responses of each of the slave control devices, the master control device determines the transit time, i.e., communication, delays to each of the slave control devices. Operating in conjunction with this data, the master control device can then interpret and/or adjust any event times reported by the event recorders to improve the relative time accuracy of the event recorders as compared to a time maintained by the master control device. Consequently, embodiments of the present invention can achieve a relative time synchronization accuracy between master and slave times to within 100 microseconds and preferably 100 nanoseconds or less.
Accordingly, a system of the present invention is of particular use in determining the cause of a power shutdown or a catastrophic failure since the precision between event recorders permits more precise analysis of the actual order that events occurred. Additionally, slave control devices of the present invention can be constructed that allow for the precise synchronization of distributed control events.
A system for synchronizing clocks in a plurality of slave control devices interconnected via a communication channel might include a master control device, at least one slave control device, and a communication channel for providing a bidirectional communication path between the master control device and each of the slave control devices. In particular, the master control device comprises master clock circuitry for maintaining an essentially fixed frequency output and a master time, a communication controller for periodically sending an interrogation signal to each of the slave control devices and receiving an acknowledgment signal from each of the slave control devices in response thereto, each acknowledgment signal characterized by a time delay corresponding to each slave control device, and an offset register for storing one or more time delay values corresponding to each slave control device. Each slave control device comprises clock circuitry for maintaining a slave time, a communication controller for receiving the interrogation signal and sending the acknowledgment signal in response thereto, and an event controller for operating in association with the slave clock circuitry. The communication channel is used for sending and receiving the interrogation and the acknowledgment signals. Additionally, the slave communication controller is responsive to a synchronization signal sent from the master control device across the communication channel, used in conjunction with an associated time delay value in the offset register, to cause the slave time to essentially synchronize with the master time.
Such a system is particularly useful in precisely determining the time that events occur at one or more of the slave control devices when the master control device receives time-stamped event messages from the slave control devices. In a preferred embodiment, the offset register contains delay values for each of the slave control devices and adjusts the received time stamps when each event message is received. In an alternative embodiment, the delay value is sent from the master control device to the slave control device where it is retained in an offset memory. The slave control device then uses this value in synchronizing the slave time to the master time.
In a particularly preferred aspect of the present invention, the clock and the time of the master control device are periodically regulated by a GPS receiver which receives a globally precise timing signal, i.e., a timing signal that is precisely regulated to all places on the earth. Accordingly, multiple geographically-distributed systems of the present invention can be synchronized to a global reference, and thus to each other.
The invention will be best understood from the following description when read in conjunction with the accompanying drawings.
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Moore Leonard W.
Read James F.
Hogan & Hartson LLP
Miska Vit
Moore Industries
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