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Third-Party Research & Development Temperature Behavior of Hallpot® Angle Sensors and Sin-Cos Hallpot® Resolvers John R. Potts, Senior Scientist/Engineering Manager BRIEF Elweco Inc. (Painesville, OH) manufactures and markets rotary transducers based on the Hall Effect. Essentially, two basic types of calibrated transducers are available: (1) directly coupled Hallpot® Angle Sensors, for precise system control and/or measurements of angular displacements generally over restricted ranges (+/- 30 degrees) and (2) directly coupled Sin-Cos Hallpot® Resolvers, for precise system control and/or measurements of angular displacements generally over unrestricted ranges (+/- 360 degrees or more). This paper explains why changes in ambient temperature have small effect on the characteristic Hallpot® Angle Sensors and virtually no effect on Sin-Cos Hallpot® Resolvers. Techniques for extracting information from Sin-Cos Hallpot® Resolvers are discussed in Extracting Static and Dynamic Information From Sin-Cos Hallpot® Resolvers: The ATAN2 Function. Techniques for extracting information from Hallpot® Angle Sensors are discussed in Extracting Static and Dynamic Information From Hallpot® Angle Sensors:Linear & Sinusoidal Sensors. Third-Party Research & Development fabricates and markets diagnostic systems based on RVDT and HallPot® rotary sensors. IDRA® Ballistometer System, which is used in cosmetic claims testing, uses a HallPot® Angle sensor and dynamically assesses and is applied to monitor changes in several mechanical properties of skin and other soft materials. Third-Party R&D also provides custom hardware and software engineering development support services for HallPot® sensors. HOW AMBIENT TEMPERATURE CHANGES AFFECT Hallpot® OUTPUT RESPONSES A Hallpot® sensor output can be expressed as the sum of two contributing factors: an offset term and a span term: HallPot® Sensor Output Voltage (millivolts)= Offset Voltage (millivolts)+ Span Voltage(millivolts) [1] The internal reference for a HallPot® Sensor is precisely +5,000 millivolts, which is generally a maximum bound on the sensor output voltage. However, the output of a Linear Hallpot® Angle Sensor is an exception since its value can reach the supply voltage, Vs, as shown in Figure 1 of Extracting Static and Dynamic Information From Hallpot® Angle Sensors:Linear & Sinusoidal Sensors. The magnitude of the Offset Voltage is ideally +2,500 millivolts. This offset has its origin soley in electronic circuitry, where it is used at a fundamental level to eliminate the need for a dual power supply. The change in the magnitude of this offset voltage as a result of ambient temperature fluctuation varies somewhat from sensor to sensor, although, the actual impact of temperature fluctuation is generally quite small, i.e., a one degree Celsius change in ambient temperature is estimated to change the magnitude of the offset voltage by only +/-0.02 %. The span term reflects the overall Hallpot® sensitivity to orientation and magnetic flux changes of an internal magnet, as a result of shaft rotation. A Span Voltage is calculated from these two sensor components: Span(millivolts)= Hallpot® Sensitivity (millivolts/Gauss) * Max Flux Density (Gauss)* Sin (theta) [2] The change in the magnitude of HallPot® Sensitivity as a result of ambient temperature fluctuation varies somewhat from sensor to sensor, however, the actual impact of temperature fluctuation is relatively small, i.e., a one degree Celsius change in ambient temperature is estimated to change the magnitude of the HallPot® Sensitivity +0.03 to +0.04%. The estimated change in the magnitude of Magnetic Flux Density of the internal magnet for a one degree Celsius change in ambient temperature is -0.03 % to -0.04%. These two components tend to compensate each other and reduce even further the effect of temperature fluctuation on Span. For isothermal operating conditions at any particular fixed temperature, within the normal operational range (-40 to 125 degrees C), thermal effects are eliminated by simply doing a calibration, i.e., the HallPot® Sensor operates at the same temperature at which it was calibrated. For non-isothermal operation and relatively small fluctuation in temperature following a calibration, any residual thermal effects can be ignored without introducing significant errors. Temperature coefficients for a HallPot® Sensor are generally small: a 100 degree change in ambient temperature may result in an error output voltage of only a few percent or less. VIRTUAL ELIMINATION OF TEMPERATURE EFFECTS ON Hallpot® SENSOR Sin-Cos Hallpot® Resolvers potentially offer highest accuracy and precision under isothermal and/or non-isothermal operating conditions. Resolvers generate two output signals, each of which has a similar, and very small temperature dependency. These signals are processed as ratios and thereby effectively cancel any residual thermal effects (see Extracting Static and Dynamic Information From Sin-COs Hallpot® Resolvers: The ATAN2 Function.).
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