November 2014

Reference design for space-saving ultrasonic distance measurement systems

Keep your distance!

ELMOS Semiconductor has developed a reference design for a compact ultrasonic parking assistant jointly with TDK. The system warns the driver of any contact with obstacles during parking. In addition to the ELMOS chipset, an EPCOS transformer of the EP6 series assures that the ultrasonic signal is matched to the impedance of the transducer.

The core of the ultrasonic parking assistant (UPA) is the ELMOS E524.1x chip, which contains the entire electronics for generating the ultrasonic signal as well as evaluating the echo. This is performed by an A/D converter, an integrated microcontroller and an EEPROM. Implemented as a single-chip solution, the UPA module is particularly compact. The 20-pin package has a footprint of just 5 mm × 5 mm and an insertion height of 0.9 mm. The E524.1x series of chipsets are designed for operating voltages of between 7 V DC and 18 V DC and are able to drive transformers with currents of up to 496 mA. The UPA module communicates with the automobile power system via an additionally integrated LIN2.1 interface (local interconnect network). The UPA is designed for distance measurements in the range between 15 cm and 500 cm. The operating temperature range extends from −40 °C to +105 °C.

The new reference design employs a piezo-transducer to transmit and receive the ultrasonic signal. The frequency of the ultrasonic pulses can be programmed to between 30 kHz and 80 kHz. Digital filtering permits very precise evaluation of the received signals, thus making complex calibration and trimming of external components unnecessary. Temperature compensation is optionally available via the chip’s GPIO pins. As the key routines are already stored in a system ROM, the demo board is extremely simple to put into operation. Its scope of delivery includes a LIN-USB converter as well as the software, which can be installed on Windows PCs.

Thanks to the high integration density, this design requires relatively few external discrete components compared to previous solutions. The single-chip module thus offers not only compact dimensions but also greater reliability as well as procurement and logistics benefits. Figure 1 shows the basic design with its key components.

Figure 1: Circuit diagram of the reference design for measuring distance with ultrasound

As all required functions, including the bus connection, are implemented in a single chipset, external connections are reduced to a minimum.

Apart from the chipset, the most important component is the EPCOS transformer (TR1) with ordering code B78416A2232A003 (Figure 2). The transformer, which belongs to the EP6 series, is an SMD component with a ferrite core that matches the ultrasonic signal emitted by the chip to the transducer impedance. The outputs DRV1 and DRV2 are connected alternately to ground in order to generate a symmetrical signal. Accordingly, the primary winding of the transformer is implemented symmetrically with a center tap connected to a local energy reserve (RSUP_TD, CSUP_TD).

The transformation ratio of the EPCOS transformer is 1:1:8.82 and is thus optimally matched to the transducer impedance. The inductance of the secondary winding is 3 mH (52 kHz, 1 V, 25 °C) and the electrical properties of this transformer are specifically harmonized with the ELMOS chip. Thanks to its compact dimensions, the EPCOS EP6 platform has now become a worldwide standard in ultrasonic technology for distance measurement. The transformation ratio and impedances can be matched to all commonly available transducers.

Figure 2: EPCOS EP6 transformer for ultrasonic applications

The EPCOS transformer for ultrasonic applications has dimensions of only 7.6 mm × 8.8 mm × 7.1 mm and thus permits highly space-saving designs. Thanks to its encapsulation, this component exhibits high mechanical ruggedness and is suitable for use in automotive applications.

Simple noise suppression for the LIN bus

The ELMOS chip has a direct interface to the LIN bus and is compatible with the latest standard. LIN is designed specifically for the cost-effective communication of intelligent sensors and actuators in motor vehicles. The network is used wherever the bandwidth and versatility of a CAN or FlexRay is not required. The principle of LIN consists of a master-slave architecture with one master and up to 16 slaves. The maximum transmission rate of the asynchronous serial signal is 20 kbit/s; the voltage levels are 0.7 V DC and 12 V DC. Unlike CAN or FlexRay, LIN is not operated in differential mode. This means that there is no common-mode interference. Nevertheless, this bus system also requires sufficient noise suppression, which can be implemented cost-effectively with discrete inductors.

The TDK inductors of the NLV25 and NLCV25 series are suitable for this purpose. With dimensions of 2.5 mm × 2.0 mm × 1.8 mm, they are available in an inductance range from 0.1 µH to 100 µH and for temperatures from −40 °C to +105 °C. EPCOS inductors of the SIMID 1812 or 1210 series can also be used. They are available with inductance values from 0.0082 µH to 1000 µH and can withstand temperatures of up to +145 °C.

New TDK 3-line feedthrough filters were developed specifically for the noise suppression of the power supply leads. These rugged components are available in case sizes EIA 0603 (1.6 × 0.8 mm²) to EIA 1206 (3.2 × 1.6 mm²). They cover a capacitance range from 22 pF to 1 µF at rated voltages from 10 V to 100 V, are designed for temperatures of up to +125 °C, and offer a high current capability of up to 10 A.

The reference design described here is suitable not only to help vehicles to park safely, but can also be used in industrial applications for filling-level measurements of liquids, powders or granulates. The robotics domain also offers diverse applications for this single-chip circuit for distance measurement.

Figure 3: ELMOS demo board for distance measurement

The demo board includes a LIN-USB converter and software for Windows PCs.

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