News Column

Patent Issued for Circuit Monitoring Device

September 11, 2014

By a News Reporter-Staff News Editor at Computer Weekly News -- According to news reporting originating from Alexandria, Virginia, by VerticalNews journalists, a patent by the inventor Bullmore, Eric (Victoria, AU), filed on July 1, 2013, was published online on August 26, 2014.

The assignee for this patent, patent number 8816869, is Tessler Research Pty. Ltd. (Victoria, AU).

Reporters obtained the following quote from the background information supplied by the inventors: "Security management systems are typically employed in correctional facilities, such as prisons, as well as buildings intended for other purposes where restricted access is required. Some examples of such systems include those sold under the names Pagasus, Card key and Access. In general, these systems are proprietary, and components from one system will not work with components from another system. Additionally, any modifications to the hardware or software must generally be made by the original manufacturer.

"In a typical prior art security management system (SMS) a number of field devices, perhaps several hundred or even thousands, are wired back via various circuits to a centralised SMS control unit. Typical field devices include infra-red motion detectors, read switches on doors and windows, glass breakage tapes on windows, smoke or heat detectors and tamper switches. Each of these field devices includes a switchable element which is triggered when an abnormal or specified condition occurs, for example a read switch detects when a door is opened, an infra-red motion detector senses movement or a smoke detector senses smoke in the air. The switchable element may be a normally open contact (ie., it closes when triggered) or it may be a normally closed contact (ie., it opens when triggered).

"In general, a first resistive component is connected in series with the switchable element and a second resistive component, referred to herein as a field resistor, is connected in parallel with the switchable element. The field resistor is typically connected across the terminal block of the field device at the time of installation. If more than one field device is connected within a particular circuit, the switchable element of each of those devices is connected in parallel with the field resistor. In this configuration, the field resistor is usually connected across the switchable element of the last field device on a line extending from the SMS control unit.

"FIG. 1 shows a typical example of a single line circuit connected to a switchable element SW1 of a single field device. The circuit includes a first resistive component R1 in series with the switchable element SW1 and a second resistive component R2 (field resistor) in parallel with the switchable element SW1. Several field devices may be connected to this circuit and, in that event, the switchable elements of those field devices would be connected in parallel with the field resistor R2. In practice, the field resistor R2 would be connected to the field device farthest from the input terminals 1, 2 of the SMS control unit.

"On considering the circuit shown in FIG. 1, it will be appreciated that the line resistance measurable at input terminals 1, 2 of the SMS control unit will change when the switch SW1 closes. With the switch SW1 in the open position the line resistance will be R1 plus R2. With the switch SW1 in the closed position the line resistance will be R1 alone. The SMS control unit determines the status of the switch SW1 (opened or closed) by continuously measuring the circuit resistance of the line connected to its input terminals 1, 2.

"Each manufacturer of SMS equipment specifies a particular value of field resistor to be connected across the last field device in a line. Typical values may be 2 k.OMEGA., 4.7 k.OMEGA. or 10 k.OMEGA. The resistance of the cable itself is in general insignificant in comparison to the values of the resistive components R1 and R2 involved in the circuit. In many applications, the series resistor R1 is the same value as the field resistor R2. In any particular installation, wherein all lines are connected to a single SMS control unit, the field resistor R2 for each line of the system in the same value.

"The various field devices in a particular installation are often supplied by other manufacturers and those devices can generally be used with any SMS control unit. This is because the field devices merely contain a switching element and the field resistor is connected during installation of the system. In some cases however, the supplier of the SMS control unit may also supply field devices and, in those cases, the field resistor may be hard wired within the device, rather than being externally wired across the terminal block at the time of installation. In that event, the field devices can only be used with the same brand of SMS control unit.

"These factors cause a few problems when the owner of an SMS system needs to upgrade or modify its system. Because each line connected to the system includes a field resistor of a particular value, the owner is forced to return to the original supplier of the SMS in order to provide an upgrade. Alternatively, the system owner must rewire each of the lines connected to the system and replace the field resistor with a different value, as specified by the supplier of the new SMS control unit. Where the resistor is built into the field device it cannot be changed and the system owner is forced to also replace each of the devices if it wants to change to a different brand of SMS control unit.

"Typical SMS systems include an operator interface providing a graphical representation of the system being monitored and controlled. The software employed in the interface is proprietary and cannot be changed by the user. Any modification to the operator interface thus needs to be made by the original supplier and this makes the owner vulnerable to excessive ongoing maintenance costs by the supplier.

"In an attempt to remove this dependency on the original supplier, the present inventor has in the past developed a universal replacement for a proprietary SMS system using a standard programmable logic controller (PLC) and analog input cards. This provided a flexible solution which could be programmed to cater for a wide variety of field resistor values. Any PLC could be used to replace the proprietary system without having to change the field resistors, thus saving considerable installation time. The programming of the PLC is more time-consuming, because all processing is done within the central processor of the PLC and this needs to be programmed using conventional ladder logic, but overall installation time is reduced. The main problem with this approach in a commercial installation, however, is the high cost of analog input cards for commercially available PLCs. The cost of these cards makes this form of PLC-based SMS prohibitively expensive for large installations.

"There therefore remains a need for a flexible system which can reproduce the function of a security management system, or similar systems, or which can be used in conjunction with standard and commonly available hardware and software to provide the necessary functionality."

In addition to obtaining background information on this patent, VerticalNews editors also obtained the inventor's summary information for this patent: "The present invention accordingly provides a device for monitoring the status of a circuit based on a measurable parameter of the circuit, the device including: measurement means for measuring the parameter of the circuit; comparison means for comparing the measured parameter to at least one threshold value and for assigning a status based on the result of the comparison; and output means for presenting an indication of the assigned status.

"This device may be used to measure the electrical resistance of a circuit and, based on that measurement, provide the functionality of a traditional security management system.

"In one embodiment, the circuit is an electrical circuit containing at least one switchable element. This switchable element may be incorporated within a field device of the type described above. The circuit includes a first resistive component in series with the switchable element and a second resistive component in parallel with the switchable element such that the status of the switchable element is reflected in the circuit resistance.

"In one embodiment the threshold value is adjustable by a user. In this way, the device is able to cater for a wide variety of values of the first and second resistive components. This enables the device to be retrofitted to existing SMS systems, wherein the resistors may have been installed many years earlier and may not be readily accessible for replacement.

"Preferably, the comparison means includes a plurality of threshold values for assigning a corresponding plurality of status conditions. In one embodiment, the plurality of status conditions includes the following:

"short circuit,

"alarm 2,


"alarm 1, and

"open circuit.

"The device preferably also includes communication means for communicating the status to a monitoring system. The communication means preferably employs an open communication standard such as the DeviceNet.TM. open network standard developed by the Open DeviceNet Vendor Association Inc. DeviceNet.TM. is a low cost communications link used to connect industrial devices (such as limit switches, photo electric sensors, process sensors, panel displays and operator interfaces) to a network and eliminate expensive hard wiring. The direct connectivity provides improved communication between devices as well as important device-level diagnostics not easily accessible or available through hard wired I/O interfaces. DeviceNet.TM. is a simple, networking solution that reduces the cost and time to wire and install industrial automation devices, while providing interchangeability of 'like' components from multiple vendors. A description of the DeviceNet.TM. standard can be found in the July 2000 DeviceNet.TM. Product Catalogue by Open Vendor Association, Inc. This Produce Catalog is incorporated herein by cross-reference.

"Another aspect of the present invention provides a security management system incorporating a circuit monitoring device of the type described above. Such a system may utilise standard programmable logic controller hardware together with standard operator interface software to provide a fully functional security management system. The circuit monitoring device may be in the form of a separate module which is connected to the PLC using a communications module based on the DeviceNet.TM. standard, or other suitable open communication standard. Alternatively, the circuit monitoring device may be configured as a plug-in card which connects directly to the back plane of the PLC. In this form, different versions of the circuit monitoring device would need to be made to plug in to different brands of PLC. A separate DeviceNet.TM. module thus has the advantage that it can be used with any brand of PLC.

"A major advantage of the present invention is that it allows the retrofit of existing security management systems, fire systems and building management systems, while utilising the existing circuit wiring regardless of existing resistance values. Retrofits and new installations may use various PLCs and operator interfaces, and a variety of hardware and software, instead of being locked into proprietary hardware and software.

"As a further alternative, the circuit monitoring device may be built into a card which is adapted to plug directly into a personal computer or similar device."

For more information, see this patent: Bullmore, Eric. Circuit Monitoring Device. U.S. Patent Number 8816869, filed July 1, 2013, and published online on August 26, 2014. Patent URL:

Keywords for this news article include: Software, Tessler Research Pty. Ltd.

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Source: Computer Weekly News

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