Sensing in the Automotive Environment

Today's automobiles utilize an ever-increasing amount of electronics and control systems. As a result, designers face increasingly difficult challenges incorporating these electronics into the car. This is particularly true for sensors and other feedback electronics that are required to make cars safer, more fuel-efficient, and emissions friendly.

Electronic systems designers are always faced with the challenges of improving system resolution and signal quality in order to keep pace with newer controllers with greater speeds and I/O capabilities. As a result, design flexibility is a key component of any sensing technology for today's vehicle environments.

One of the more demanding conditions for electronics is the range of temperatures they can experience. From cold environments of -40 °C to temperatures in the engine compartment that can reach 150 °C, sensors are pushed to the limits of current materials. Future applications, like variable turbo chargers, will continue to drive even more demanding temperature extremes that can exceed 180 °C. This requires sensor designers to develop material sets and packaging schemes to accommodate these demands.

At the same time, this packaging must be capable of assuming a wide variety of formats depending upon the needs of the overall system. Traditional sensing technologies like potentiometers and Hall effect devices may be packaged in either linear or rotary fashions. However, each of these technologies has its own inherent advantages and disadvantages.

Potentiometric technology, although highly flexible in its ability to be designed into linear and rotary designs, is limited by the nature of its analog output signal. While this signal can be electronically converted to a digital format, this conversion requires the addition of costly components, which adds cost to the sensor and the resultant signal is still not a true high-resolution digital format.

With the increasing number of high-speed networks and communication buses being designed into modern vehicles, the requirement to specify an A/D converter for each potentiometer is a real disadvantage. Potentiometers are also a contact-sensing technology, making them susceptible to wear-out from normal use over time. Excess vibration can also shorten the potentiometer's lifespan. When the potentiometer's wear becomes pronounced, it can result in excessive noise in the signal from this type of sensor.

Hall effect technology, by its nature, produces a digital output signal that is capable of being communicated on a high-speed bus system. However, this technology is somewhat limited in its packaging flexibility for various reasons. Because Hall technology measures changes in magnetic Gauss levels, it requires precise bearing systems in order to maintain its integrity. These bearing systems also add some level of cost to the sensors. On the positive side, Hall effect sensors are non-contacting and, therefore, do not suffer the same life degradation as potentiometers. Typically, in order to control the Gauss fields affecting a Hall effect sensor, they are designed with relatively short ranges of motion. In general, Hall effect sensors are designed with less than 180° of rotational travel or less than 25 mm of linear travel.

Recently, new inductive sensing technologies have been developed that capitalize on the strengths of potentiometer and Hall effect technologies. TT Electronics OPTEK Technology has invested in developing its patented Autopad sensing technology for this purpose. Autopad is a non-contact inductive sensing system consisting of two PCBs with drive and sensing electronics at its core. However, unlike Hall sensors, Autopad is compliant to misalignment in the X, Y and Z axes meaning elimination of the tight bearing systems. At the same time, it is a true digital sensor capable of producing a 12-bit PWM signal that can communicate directly with high-speed controllers. Autopad is also capable of being designed into a multitude of physical configurations including 360° rotary and linear packages from 20 mm to 200 mm in length. This design flexibility makes Autopad a robust and cost effective sensing technology for many automotive applications.