Patent Application
20110098939
Process analytic sensors are generally configured to couple to a given process, such as an oil refining process or a pharmaceutical manufacturing process, and provide an analytical output relative to the process. Examples of such analytical outputs include, but not limited to: measurement of pH; measurement of oxidation reduction potential; selective ion measurement; and measurement of dissolved gases such as dissolved oxygen. These analytical measurements can then be provided to a control system such that process control can be effected and/or adjusted based upon the analytic measurement. Such sensors are generally continuously, or substantially continuously, exposed to the process medium.
The environments within which process analytic sensors operate are sometimes volatile or even explosive. In order to ensure that sensors and associated electronic equipment do not generate sources of ignition within such volatile environments, energy storage and/or discharge rates are generally limited. Intrinsic safety requirements set forth specifications which ensure that compliant electrical devices will not generate sources of ignition within volatile or explosive process environments. Intrinsic safety requirements are intended to guarantee that instrument operation or failure cannot cause ignition if the instrument is properly installed in an environment that contains explosive gases. This is accomplished by limiting the maximum energy stored in the process analytic device in a worst case failure situation. Excessive energy discharge may lead to sparking or excessive heat which could ignite an explosive environment in which the process analytic device is operating.
Examples of intrinsic safety requirements include European, CENELEC Standards, EN500014 and 50020, Factory Mutual Standard, FM3610, the Canadian Standard Association, the British Approval Service for Electrical Equipment Inflammable Atmospheres, the Japanese Industrial Standard, and the Standards Association of Australia.
In order to ensure stringent compliance with automation industry safety protocols and specifications, only equipment certified by an independent agency can be used in such locations. Since process analytic sensors and equipment is often used in such volatile environments, it is highly desirable for such devices to be designed to meet intrinsic safety requirements, or at least provide an option of intrinsic safety compliance.
Process analytic sensors are currently undergoing a significant shift in technology. Previously, an analog process analytic sensor, such as a pH sensor, would be mated to an analyzer and then a series of calibration steps would be performed to essentially calibrate the sensor/analyzer assembly. If the pH sensor were then moved to a different analyzer, the entire process would need to be repeated. While such process analytic sensors were of the analog nature, some did include analog preamplifier circuitry in order to provide a robust signal to the analyzer. The recent innovation stems from the utilization of digital electronics within the sensor itself. These new “smart” process analytic sensors are able to communicate digitally with the analyzer. However, in order to facilitate industry acceptance of such sensors, the sensors themselves should still be able to operate on power budgets and signaling levels of previous analog-based sensors. This creates a difficult tension between intrinsic safety requirements, industry-accepted power budgets, and the array of new features provided by digital circuitry within the sensor itself. Achieving a useful balance between these various design considerations would provide a smart process analytic sensor that would meet with industry approval more readily.
A process analytic sensor is provided. The process analytic sensor includes a process analytic sensing element that is coupleable to a process. The process analytic sensing element has an electrical characteristic that varies with an analytical aspect of the process. A microcontroller is disposed within the process analytic sensor and is coupled to the process analytic sensing element to sense the electrical characteristic and provide an analytical signal based on the sensed characteristic. The microcontroller is operable on as little as 0.5 milliamps and includes electrically erasable programmable read only memory (EEPROM) that can be written while the microcontroller operates on as little as 0.5 milliamps.
The provision of digital circuitry within a process analytic sensor provides a number of advantages. For example, process analytic sensor calibration information that would typically be required to be generated each time a sensor is paired with an analyzer can simply be loaded into the process analytic sensor by the manufacturer. Accordingly, then the process analytic sensor can simply upload or otherwise transmit its calibration information to any analyzer to which it is coupled. In this manner, significant calibration setup time is reduced. Further still, should a user wish to perform an additional calibration when the process analytic sensor is coupled to a first analyzer, that calibration information can be stored or otherwise saved within the process analytic sensor itself such that the information can be transmitted or provided to a second analyzer if the sensor is later coupled to the second analyzer. Further still, user and/or application-specific data for the sensor can be saved within the sensor itself thereby facilitating user setup. Finally, the provision of digital electronics within sensor 14 allows sensor 14 to perform diagnostic operations and potentially communicate diagnostic information back to the analyzer. Thus, the potential need for recalibration and/or maintenance can be determined by the process analytic sensor itself and such information can be communicated to the analyzer as an alert or other suitable indication. Accordingly, the provision of digital electronics, and specifically a microcontroller, within process analytic sensor 14 provides myriad new features and advantageous over traditional analog-base process analytic sensors.
Process analytic sensor 14 includes process analytic sensor microcontroller 22 coupled to microcontroller 20 via cable 16. Microcontroller 20 is preferably a low-power CMOS 8-bit microcontroller based on the AVR enhanced RISC architecture. Microcontroller 22 is configured to operate on an extremely low power budget. For example, microcontroller 22 operates on as little as 0.5 milliamps and includes circuitry that helps achieve compliance with intrinsic safety requirements. For example, the total capacitance of all capacitors within process analytic sensor 14, in the illustrated embodiment, sum to no more than 0.255 μF. In one embodiment, micro controller 22 is sold by Atmel Corporation under the trade designation ATtiny84. One design challenge for process analytic sensor 14 is the operation as a two-wire instrument with the significant power constrains (0.5 milliamps). One particular operation of microcontroller 22 that is challenging is the writing of data to the electronic erasable programmable read only memory (EEPROM) within microcontroller 22. While reading data can be accomplished within a 0.5 milliamp reading process, the writing of data to the EEPROM requires a current that is approximately 20 times higher than that available from the 0.5 milliamp supply. This happens due to the fact that EEPROM uses higher energy in the writing of the data process. If there is an attempt to write data to the EEPROM without power limitation considerations, this can create significant problems for process analytic sensor 14 ranging from potential reset of the sensor 14 to an entire shutdown or failure of sensor 14.
In accordance with an embodiment of the present invention, writing to EEPROM within microcontroller 22 is done within a 0.5 milliamp current budget. The data to be written to EEPROM is divided into small packets, such as single bytes, and the energy necessary to write each packet is stored in local capacitance within microcontroller 22. The writing pauses after each packet long enough to recharge the local capacitance for the next packet. Packets are placed in mapped EEPROM the same way as if the writing would be done in continuous mode.
Microcontroller 22 is coupled to process analytic sensor element 24 which has an electrical characteristic that varies with the process analytic variable of interest. In the embodiment illustrated in
Sensors, such as process analytic sensor 14, that include digital circuitry help eliminate the need for field calibration since the as-tested calibration data is embedded in the sensor's memory. Analyzer 12 then reads this calibration information automatically, providing immediate live process measurements. This saves significant resources and is believed to provide significant advantages to end users. The capability to read the embedded calibration information can be provided in various analyzers. One process analytic sensor that includes such digital circuitry is sold by Emerson Process Management under the trade designation PERpH-X® pH sensor.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
The present application is based on and claims the benefit of U.S. provisional patent application Ser. No. 61/255,183, filed Oct. 27, 2009, the content of which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61255183 | Oct 2009 | US |
