Eye On Electronics

Motor, Mar 2008 by Dale, Mike

Temperature plays an important role in the decisions vehicle computer system must make. A growing variety of temperature-sensing devices are used to keep those systems informed.

It's easy enough to describe how automotive systems work in terms of mechanics and electronics. There is, however, another dimension worth thinking about. Automobiles also function on the basis of thermodyamies. Basic combustion is really a chemical process that releases heat. Lubricants are sensitive to viscosity, which is also temperature-related. Throughout a vehicle, temperature plays a role in the decisions its computer systems must make in order to ensure that everything functions properly.

Temperature measurement has become an increasingly important vehicle operating parameter. It's much more than just controlling the temperature of the coolant. Temperature measurement is about protecting electronic and electromechanical assemblies, improving HVAC control, making transmissions shift more smoothly and even ensuring the use of correct fueling and emissions strategies. To meet these needs, new types of temperature-measurement technologies are used in today's vehicles.

Temperature measurement is related to chemistry in that the density of air, the volatility of fuel, the speed of chemical reactions and the viscosity of oil and fuel are all a function of temperature. The function of solid-state devices such as power drivers and microprocessors is directly related to their temperature during both normal and abnormal operation.

Automotive temperature sensors fall into the categories of contact sensors and noncontact sensors. Contact sensors are those devices that come in physical contact with the item to be sensed. This includes not only liquid but air and solid objects such as hardware and cylinder heads.

Noncontact sensing is becoming increasingly important, especially in the area of HVAC control. Contact sensors can report on outside temperature, cabin temperature and duct temperature. In the end, though, it's the passengers' temperature that directly relates to their comfort. New-technology sensors released this spring by DENSO Corp. use infrared thermal sensing to measure the temperature of individual passengers. In this way, the zones can be adjusted to suit each person's comfort need.

The simplest form of heat detection is probably the thermistor. While the basic technology has been around since the 1940s, there are new uses, new packages and new functions for this device. Thermistors are semiconductor devices that exhibit a large change in resistance when the body of the device changes temperature. The temperature change in the thermistor is caused by direct contact with the solid, liquid or gas being sensed.

The most common thermistors are made of such metal oxides as manganese, nickel, cobalt, iron, copper or titanium. The process to make them involves mixing two or more of the oxides with binding agents, then shaping and firing them in a ceramic kiln. Typically, the active element winds up looking like a bead or a disk with two wires coming from it. Often the active element is sealed hermetically in glass, as this improves its accuracy. The finished thermistor could be about one-fourth the size of a pencil eraser, or even smaller. You've seen them most commonly as coolant sensors, but they take many other shapes as well. Often they're included in other sensor assemblies, such as MAP or MAF sensors, to give the temperature portion of the sensor output.

The actual resistivity of the device and its temperature coefficient characteristics depend on the ratio of the mixed oxides, the shape of the part, the gases in the sintering atmosphere and the temperature of the firing oven. With all these variables, a wide variety of part shapes and characteristics can be created for specific applications.

The most common type of thermistor used in vehicles is the NTC (negative temperature coefficient) type, so named because the resistance of the device falls as the surrounding temperature rises. NTCs are more popular because they have a linear, uniform rate of change. The ideal sensor should change its value just as much between 10° and 20°C as between 20° and 30°C. The better this linearity, the greater the accuracy of the sensor.

It's also useful when the sensor has a significant change value between one temperature and another. As an example, one sensors resistance will change from 10,000 ohms to 2500 ohms when the temperature changes by 75°C. Doing the math, it means that each degree of change produces a resistance change of 100 ohms. This is a large enough change for the electronics to easily interpret the device's resistance as an accurate measurement of temperature.

It's also possible to adjust the chemistry of a thermistor so the resistance rises as the temperature of the device rises. This is called a PTC (positive temperature coefficient) device. The main difference between PTCs and NTCs is that PTC thermistors do not have a linear change with temperature. When a predetermined temperature is reached, there's a dramatic change in resistance. This makes PTC thermistors ideal for thermal protection of the motors used in adjustable seating. They protect the motor by sensing when it gets too hot, then trigger the thermal overload feature that prevents further operation until the motor cools down.