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Steering angle sensor

FER Fahrzeugelektrik GmbH is working on a prototype steering angle sensor with an electronic-revolution counter. The measuring principle is based on the differential version of the induction theorem and is capable of recording absolute angles between 0° and 360° with an actual full-circle resolution of 12 bit (0.089°). The design consists of two ferrite casings which are firmly connected to the respective rotating shaft—as well as one measurement and one evaluation board.

The measurement board is located between the ferrite casings, which serve as the magnetic flow guiding device. Two stimulator coils, which are linked in sequence, are positioned in their center, on the upper and lower side. These feed the two ferrite half casings with an alternating current of 100 kHz. The magnetic alternating field which is permanently attached and led by the measurement finger, cuts through the measurement board, which is positioned vertically uses a printed circuit on its surface. Depending on the position of the two receiving ferrite casings, the coils are permeated by the alternating magnetic field. An induced voltage, which is dependent upon the angle of rotation, can be tapped at the coils. For purposes of an actual measuring value creation, these voltages are fed to an integrated, controlled circuit in a mixed mode ASIC.

For the improvement of reliability on the measuring circuit board, a second ASIC is used. Induced voltages are thus analyzed in a 180° phased difference to the other integrated circuit. With use of a higher controller unit, it is possible to compare both angle values. If the deviation is too much, the controller can switch off the whole unit.

A third IC, called a multiturn ASIC, is required to take the deviated zero impulses from the other two ASICs. The third IC counts the impulses depending on the sense of rotation. The resulting values are saved in a serial nonvolatile memory.

Allegro MicroSystems A3240 miniaturized chopped-stabilized Hall-effect sensor.

Hall-effect sensors

Allegro MicroSystems has developed a new family of high-temperature, precision, chopped stabilized Hall-effect switches and latches. The A3240 switch and the A3280 latch are temperature stable and stress resistant sensors designed to operate over extended temperature ranges to +150°C. The improved performance is made possible by a patented, chopper-stabilization technique that reduces offset drift caused by temperature and stress, and allows for sensitive switch points. The A3240 is a sensitive Hall-effect, unipolar switch which can be used for position sensing. A3280 is a sensitive Hall-effect latch designed for circuits that require sensitive switch points at large air gaps.

Melexis sensors.

Programmable sensors

Advantages in cost and function of sensor technology has been driven by the quest to reduce emissions, improve safety, achieve zero defect/quality, and reduce cost. The introduction of a mixed signal CMOS, programmable sensors and sensors systems by Melexis (Chad Brad and Chris Peter) is providing a path to meet these demands.

Signal conditioning will always be necessary for most sensor elements. Raw sensors exhibit some level of offset voltage, an output signal level when no stimulus is applied to the sensor. The span of a sensor, the output signal change from no stimulus to the maximum stimulus, typically needs to be amplified. A typical element may have a span of a few millivolts. This small amount of change may be difficult to detect and transmit over lengthy connections. Engine noise can easily drownout a small signal directly from the bridge with a long harness. By amplifying the signal to have a span of a few volts, the signal is much easier to covert to digital for use by a processor.

With the progression of the sensor and semiconductor technologies the integration of the sensor element and signal conditioning on one die is a logical step forward. With integrated signal conditioning electronics, programmability is required. There is no option of using a potentiometer or laser trimming a resistor. Most Hall-effect sensors are integrated, that is, the Hall plate and signal processing components are on the same chip. However, the signal to noise ratio of silicon Hall plates, prior to programmable linear development (MLX90215) forced higher precision systems to use a discrete Hall plate.

In pressure sensors, the converse has been true, as developers have previously assumed that the combination of integrated silicon electronics and micro-machining are incompatible and would result in unacceptable yield losses.

With the development of the first programmable linear Hall sensor IC, the MLX90215, researchers gained confidence to replace the Hall plate with an integrated micromachined pressure sensor. The first example of this is the Melexis MLX90218 programmable pressure sensor. This device includes the integration of a temperature sensor on the same die as the micromachined pressure sensor and the programmable signal conditioning electronics. The temperature sensors used for accurate temperature compensation of the pressure sensor. This eliminates the need for an additional temperature sensor. Also it puts the temperature sensing as close to the pressure sensor as possible.

The cost of integrated electronics will continue to decline and the level of integration will increase. This will enable realization of the IEEE 1451.2 specification in lower and lower cost sensors. The cost of implementing a programmable sensor has just begun to enable it to be used in the low cost high volume sensor applications. As the cost of a programmable integrated sensor or sensor interface declines so does the size. This reduction in size allows for more innovative solutions such as integrating a pressure sensor with programmable signal conditioning into a brake fluid solenoid valve.

As the IC supplier provides a smarter sensor, the module supplier must abdicate more of the electronic design to the IC designer. Additionally the sensor systems maker must be more involved with the IC and module design activities to maximize the capability of smart sensors in the total systems. This puts the module supplier in the middle as the coordinator who must understand IC, automotive, and module technologies as well as control the three way relationship.

The ideal programmable smart sensor would be one that is programmed with the desired output span and the electronics figures out the rest. The temperature coefficients, offset, and gain would be calculated automatically. The ideal sensor would further reduce manufacturing costs of the sensor assembly by reducing the programming time. Eventually this type of programmable sensor will be developed, either as an integrated device or as an interface circuit with a discrete sensor element. With the processing needed to make this possible, a microcontroller is required. Using a microcontroller will enable other features to be added at little additional cost.