SISTEMA

Temperature Sensors
Resistance Temperature Detectors (RTDs)
Thermistors
IC Temperature Sensors


Drew Gilliam
GE/MfgE 330: Introduction to Mechatronics
03.19.2003




Introduction
There are a wide variety of temperature sensors on the market today, including Thermocouples, Resistance Temperature Detectors (RTDs), Thermistors, Infrared, and Semiconductor Sensors. This paper willdiscuss three of these alternatives: the RTD, thermistor, and semiconductor sensors.

As we are biased toward an application which can be integrated into the Mechatronics DSP, we will end the discussion with a specific semiconductor sensor, the LM75 offered by National Semiconductor (http://www.national.com/pf/LM/LM75.html). The LM75 has a variety of features, primarily the I2C serial interface,which would allow easy integration in the robot system.


For good descriptions of the sensors:

[1] National Semiconductor
National Temperature Sensor Handbook
http://luna.et-inf.fho-emden.de/datenblaetter/sensor/temphb.pdf

[2] Watlow Electric Manufacturing Company
Temperature Sensors, Educational Series Book 4, 1995http://www.watlow.com/literature/prodtechinfo/files/sensors/we4e_16.pdf

[3] Analog Devices, Walt Kester
Temperature Sensors
http://www.analog.com/UploadedFiles/Associated_Docs/245380809Power_sect6.pdf

[4] National Instruments
Temperature Sensor Vendors and Tutorials
http://www.ni.com/devzone/advisors/temperature.htm



Resistance Temperature Detector (RTD)
The RTD is a temperature sensing device whose resistance changes with temperature. Typicallybuilt from platinum, though devices made from nickel or copper are not uncommon, RTDs can take many different shapes (figure 1). To measure the resistance across an RTD, apply a constant current, measure the resulting voltage, and determine the RTD resistance. We then use a resistance vs. temperature plot to determine the temperature of the surrounding medium (figure 2). RTDs exhibit fairly linearresistance to temperature curves over their operating regions, and any nonlinearities are highly predictable and repeatable.

Figure 1
RTD element styles [2, p.17]


Figure 2
RTD Resistance vs. Temperature
upper curve is straight line reference [1, p.6]



The RTD requires external current excitation, as well as signal conditioning to account for lead wire effects andself-heating. Analog Devices supplies the ADT70, which provides both excitation and signal conditioning for a platinum RTD. The output of this device (5 mv/°C) is be fed through an analog to digital converter, to be converted by the DSP to temperature readings. Figure 3 on the next page supplies two circuit diagrams from the Analog Devices literature. For more information on the ADT70, seehttp://www.analog.com/Analog_Root/productPage/productHome/0%2C2121%2CADT70%2C00.html

RTD devices are available from numerous suppliers. To find suppliers who fit your needs, I suggest going to the Temperature Sensor listings from Sensors EZ Search (http://www.sensors-ez.com/sensors/0041546_0041557_1.html). While sensitive and accurate platinum sensors can be quite costly, more inexpensive sensors are availablethat would suit our applications.


Figure 3
ADC conversion, ADT70 [3, p.15]



Thermistor
Similar to the RTD, the thermistor is a temperature sensing device whose resistance changes with temperature. Thermistors, however, are made from semiconductor materials (figure 4). Resistance is determined in the same manner as the RTD, but thermistors exhibit a highly nonlinear resistance vs.temperature curve (figure 5). Thus, in the thermistor’s operating range we can see a large resistance change for a very small temperature change. This makes for a highly sensitive device, ideal for set-point applications.

Figure 4
Thermistor element styles [2, p.24]


Figure 5
Thermistor Resistance vs. Temperature [1, p.8]



Like the RTD, thermistors require external current excitation...