2. Automotive sensors in frequency domain

This paper is focused on self resonant FDS. They are also called auto resonant or resonant sensors and the main advantages of resonant sensors over other kind of sensors are their stability, high resolution and quasi-digital output, among others [11].

As shown in Fig. 1, resonant sensors require a counter to measure frequency and, despite the fact that there many types of resonant sensors, this paper is focused on surface acoustic wave (SAW) sensors. These consist of two interdigitated electrode patterns (see Fig. 2), screen printed and fired onto an alumina substrate of thickness 630 µm [12].

The piezoelectric thick film (thickness 40µm) is deposited over the electrodes. The width and gap between the electrodes is 200 µm. The piezoceramic material is polarized between the ‘fingers’ of the electrode pattern.

One of the interdigitated transducers (IDTs) acts as an input device. A sinusoidal voltage is applied to the electrodes and, because of the piezoelectric effect; an elastic wave is generated within the film and travels towards the other IDT.

At the receiver, the acoustic wave is translated back into an electrical signal as a result of the direct piezoelectric effect. The wave velocity, v, is determinate by the acoustic properties of the medium. The wavelength, λ, is equal to the centre- to centre-distance between adjacent pairs of fingers. The typical frequency of the SAW devices is around 5 MHz. The information bearer in such sensors is primarily the time delay of the SAW or the central frequency of the SAW device [13].

Fig.1 Schematic representation for resonant sensors

Fig.2. A thick-film elastic wave sensor (SAW)

3. Applications of fds in automotive industry

Figure 3 shows some sensor applications in today’s cars, in which some of them are SAW sensors and different kinds of sensors variety working precisely in frequency domain. As automotive application, it is natural and evident that one of the most critical requirements to such sensors is the operating time, or speed of response, which is extremely important character for general operation speed of automobile’s onboard automatic control system. One of the examples are SAW torque sensors, as well as resistive strain gauges, measure the torque indirectly by detecting the strain or stress distribution generated by a torque acting on the shaft [14]. Fig. 4 shows a shaft with two mounted SAW torque sensors in its actual size.

Fig. 3. Sensors in nowadays cars.

Fig.4. SAW torque sensors mounted on a shaft

Wireless SAW pressure sensors are fabricated using a quartz diaphragm that bends under hydrostatic pressure. A reflective delay line is achieved by structuring one surface of the quartz diaphragm (see Fig. 5a). It can obtain a resolution of about 1% of the full range.

Figure 5 shows the physical principle of the pressure measurement in a car tire by a sensor and the typical time-domain pressure curve behavior.

Another way to process physical magnitudes trough electronics is the voltage to frequency conversion. Since most semiconductor pressure sensors [15] provide a voltage output, one must have a means of converting this voltage signal to a frequency that is proportional to the sensor output voltage.

Assuming the analog output voltage of the sensor is proportional to the applied pressure, the resultant frequency will be linearly related to the pressure being measured.

There are many different timing circuits that can perform voltage to frequency conversion. Most of the simple (relatively low number of components) circuits do not provide the accuracy or the stability needed for reliably encoding a signal quantity. Fortunately, in the market there are many voltage-to-frequency (V/F) converter IC’s commercially available that will satisfy this function.

Fig.5. a) Schematic drawing of a SAW pressure sensor; b) pressure measurement of a sensor in a car tire.

Also, it is essential to note that the capacity of information at the modulation voltage is 2πf₀ times worse than at the modulation frequency, where f₀ is the standard frequency value at normal operating conditions.

Another limitation of some V/F converters is the less than adequate switching transition times that affect the pulse or square-wave frequency signal.

The required switching speed will be determined by the hardware used to detect the switching edges. Some families of microcontrollers (see Fig.6) have input-capture functions that employ Schmitt trigger like inputs with hysteresis on the dedicated input pins.

In this case, slow rise and fall times will not cause an input capture pin to be in an indeterminate state during a transition. Thus, CMOS logic instability and significant timing errors will be prevented during slow transitions.

Since the output frequency of the sensor may be interfaced to other logic configurations, the main concern of the designer is to comply with a worst-case timing scenario [5].

For high-speed CMOS logic, the maximum rise and fall times are typically specified at several hundreds of nanoseconds. Thus, it is wise to speed up the switching edges at the output of the V/F converter. A single small-signal FET and a resistor are all that is required to obtain switching times below 100 ns.

The evaluation board shown in Fig. 6 is designed to transduce pressure, vacuum or differential pressure into a single-ended, ground referenced voltage that is then the input to a voltage-to-frequency converter. It nominally provides a 1 kHz output at zero pressure and 10 kHz at full scale pressure. Zero pressure calibration is made with a trimpot that is located on the lower half of the left side of the board; while the full scale output can be calibrated via another trimpot just above the offset adjust.

Fig.6 DEVB160 frequency output sensor EVB.