Appliance Design - February 2008 - (Page 29) CONTROLS & SENSORS changes are cumbersome, OEMs can choose the option of a “smart” integrated system with onboard or remotely located microprocessors for flexible application configurability and functionality. There are different approaches in output and signal interface architectures; ITC takes particular advantage of modern electronics’ ability to measure time with great precision. In turn, this translates into greater sensor precision and, in many measurement applications, can provide a new set of capabilities not previously feasible in analog systems (using current cost metrics) with customers looking to improve the performance/cost impact. The implementation of a variable-frequency, digital, squarewave output is achieved by counting the edges of the square wave or using the square wave as a counter enabler in order to realize high accuracy measurements. The speed of the processor determines the accuracy. In terms of direct comparisons between digital and analog signals, inGen Direct can offer generally higher resolution, a reduction in errors, elimination of time delays, elimination of signal conditioning (A/D and D/A), better dynamic response, higher stability (where oscillation is separated from ground), fewer components (error terms engineered out), more immunity to interference, and easier transmission and signal processing. The translation of analog signals through signal conditioning electronics to interface with digital networks causes delays, compounds errors and increases costs. ITC’s Direct Digital Transfer Signal Methodology can provide a readonly output using a method that is similar to pulse-width-modulation without the control feature. This is pulse-width-measurement in a sense, where designers can opt for the digital–to–digital interface www.applianceDESIGN.com AD02082Athena.indd 1 Fig. 1. Digital signal transfer methodology. they want without the need for protocols, handshakes, and carrier signals present in competing analog, or even more complicated digital sensor outputs. ITC’s time-based system can provide measurement ranges that are designed in and span operating ranges anywhere between 10 kHz to 20 MHz, depending on specific application requirements, target materials, appropriate dynamic bandwidth, and microprocessor, with decentralized or centralized control requirements in mind. The mean count deviation per 6 DegC over the measurement range is 19.1 counts, thus resolution of 1 percent of the measurement range is attained. The refresh rate with these conditions must also be examined. For example, the transducer can experience a change of temperature of 600 Deg C in 1 second. The same 1 percent resolution across the bandwidth requires a measurement every 10 ms. The longest sensor measurement time interval at 20 kHz is 500 us. So it yields 20 measurements in the 10 ms window. Compare that to a typical, signal-conditioned temperature measurement scheme such as an RTD or thermocouple using an amplifier and A/D converter. The amplifier has gain error and temperature error. The A/D converter has input error (sample and hold, etc.) and resolution error (depending on whether it is 8, 12, 16 bits). The A/D also requires control signals from the controller to initiate a conversion. Then, at the end of a conversion, the controller must read the data. Many converters use a serial data bus, so the data rate is the baud rate times the number of bits. The controller now has a data word that represents a temperature measurement term with gain and A/D errors and time delays for conversion Temperature measurement The following demonstrates how an inGen Direct sensor can be used in a temperature measurement application. In this example, the temperature transducer is a reactive component in the frequency generating circuit. Change in temperature alters the reactance (capacitance/inductance) of the transducer, which changes the frequency of the sensor output. The inGen Direct sensor has an operating frequency range between 200 kHz to 220 kHz, for a bandwidth of 20 kHz over a measurement range of 600 DegC. The sensor is used in conjunction with a processor that has a 400 MHz counter. Resolution of 1 percent of measurement range is required per the application. Making a measurement over 10 sensor periods will yield 20,000 counts at 200 kHz and 18,180 counts at 220 kHz. Cost-Effective Control for OEM Applications Temp Flow Pressure New EMC New EMC Multi-Loop Multi-Loop PID Controller PID Controller Heat Cool Alarm Voltage Level Current � Four Independent or Combined Loops of Auto-Tune Control for: PID, On/Off, Cascade, and Feed Forward Small 4.5˝ x 7˝ Size for Subpanel Mounting Communication and Custom Display Options Less Cost than Four Separate Controllers with Similar Capabilities � � � Athena Controls, Inc. For more information contact mktg@athenacontrols.com For more Information Enter 110 applianceDESIGN February 2008 29 1/8/08 11:13:53 AM http://www.appliancedesign.com
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