Proceedings of the 6th European Embedded Design in Education and Research, 2014
MULTICHANNEL AIRBORNE ULTRASONIC RANGING SYSTEM BASED ON THE PICCOLO C2000 MCU C. Cambini, L. Giuseppi, M. Calzolai, P. Giannelli, and L. Capineri Department of Information Engineering,University of Florence Italy Via S. Marta 3,50139,Firenze,Italy phone: + (39) 055 4796373,fax: + (39)0554796497, email:
[email protected] web: http://www.uscndlab.dinfo.unifi.it
retaInIng high performances with different target reflectivity (shape,acoustic impedance,dimensions).
ABSTRACT
The combination of different characteristics of airborne ultrasonic transducers for ranging applications opens up
The use of microcontrollers for developing single channel transducer front end have been proposed by the authors for different applications [7], [8]. In this work the resources offered by commercial microcontrollers have been exploited to design a versatile multichannellmultisensor system. Our system was developed using a TI C2000 Experimenter Kit equipped with a Piccolo TMS320F28069 MCU and custom transducer analog front-ends.
new applications for autonomous navigation systems in robotics.
The features of a Piccolo C2000 MCV are
suitable for the design of a ranging system with four channels. Each channel has different operating range and accuracy depending on the transducer characteristics and the implemented ranging technique. This paper presents the system architecture and the experimental results obtained with transducers operating at 40kHz, 100kHz and 300kHz.
For the design and the implementation of the ultrasound airborne system is important to define the following steps:
1. INTRODUCTION
Airborne ultrasonic ranging systems have many applications in the automotive and robotics industries,often requiring fast response time and high accuracy (e.g. for actuator control purposes). However, the performances of such systems are usually limited by the transducers themselves, as their characteristics are the result of severe trade-offs [1]. The state of the art of signal processing architecture and airborne ultrasonic front end can be found in two recent publications [2], [3] where CMUT devices are employed, while the performances of Time of Flight (TOF) estimation techniques are presented in [4]. Our system is designed with different kinds of airborne ultrasonic transducers that are chosen to cover short, mid and long ranges and different view angles with accuracy compatible with state of the art while minimizing the number of transducers.
Transducers selection Electronic front-end design Microcontroller selection Firmware implementation Finally the ranging obtained with three different transducers operating at 40kHz, 100kHz and 300kHz are presented and the experimental results are discussed in the conclusions. 2. SYSTEM ARCHITECTURE
The presented system uses three different transducers: two commercial piezo-ceramics from Murata (MA40S5 and MA300Dl) and one custom hemi-cylindrical PVDF transducer operating at 100kHz [7]. Table 1 lists the relevant characteristics of each transducer model.
Power consumption of the presented system is limited thanks to the optimization of C2000 microcontrollers and the custom analog front end. FPGA-based approaches are capable of more complex signal processing strategies but are more expensive and require larger circuit boards [4], [5].
The hardware architecture, shown in Figure 1, includes four independent ultrasonic front-ends, each composed of three parts:
Lastly, the presented system can operate with up to four transducers in parallel that can be selected with signals having different frequencies and bandwidths in order to implement more sophisticated algorithms for object detection and localization [6]. The combination of different transducer technologies and signal processing algorithms led to the implementation of a robust ranging system capable of measuring distances over a long range while
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•
The TX PULSER is a ZXMHC1OA07N8 integrated H-bridge from Diodes, Inc. operating at 24V. The drive signal is generated by the PWM peripherals of the MCU. This circuit can also operate as a half bridge when single-ended transducers are used.
•
The RX AMPLIFIER is a high-gain, low-noise instrumentation amplifier based on the TI
OPA4228 quad operational amplifier. The frequency response of each channel was specifically tailored to its transducer, as detailed in Table 2. •
The DUPLEXER circuit, which decouples the transmitter and receiver stages, includes an LC matching network tuned to optimize the input impedance of each transducer.
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Figure 2- Electronic system: (aJ Analog front end boards for the three different types of airborne transducers; (b) Signal conditioning; (c) Tl C2000 Piccolo board; (d) Power supplies.
Sin Ie channel analo front-end
ADC
Signal conditioning
2.1. Microcontroller unit
MCU
The MCU selected for this work belongs to the TI F2806x Piccolo family and represents an efficient hybrid solution between a standard MCU and a DSP. MCUs now have enough computational resources for low frequency (e.g. airborne) ultrasound signal processing and adequate ADC characteristics (sample rate, resolution, linearity), I/O, communication and PWM on chip resources. The DSP features of C2000 MCUs are useful for the implementation of cross-correlation techniques and the on-chip modules (e.g. ePWM, DMA) allow synchronous operation of all the channels. MCUs are flexible because the firmware can be easily developed and upgraded. FPGA are more suitable for heavy computational requirements (high frame rate and arrays with high number of elements). The reduced cost, size and power consumption are advantages of MCUs with respect to multichannel (> 16 transducer elements) ultrasonic systems normally managed by a digital acquisition board (DAQ) with FPGA.
PWM
C2000 Piccolo TMS320F28069
Custom converters Figure 1- Hardware block scheme of the system. Power supplies are derived from USB.
Additional signal conditioning stages were included to interface the receiver stage to the ADC and the comparators integrated within the MCU. Two switching DCIDC converters were added to the Experimenter Kit prototyping area to generate the supply rails needed by the analog circuitry (-5V for the amplifiers and 24V for the H-bridge). The whole system is bus powered through USB and draws approximately 330mA during normal operation. The realized prototype of the electronic system is shown in Figure 2.
3. RANGING METHODS
Several techniques exist that are suitable for airborne ultrasound ranging. Our system was designed to operate with TOF measurements that rely on threshold and cross correlation echo detection [9]. These two methods provide different performances in terms of accuracy, repeatability, signal-to-noise ratio (SNR) and computational burden. In [9] is reported a comparison between these two methods assuming a received SNR of 35dB: the cross-correlation technique at these frequencies shows a sub-millimeter repeatability with a micron accuracy, compared to a millimeter repeatability and millimeter accuracy for the threshold technique.
Table 1- Airborne ultrasonic transducers. Model
Center Frequency
Range
PVDF
40kHz
100kHz
25crn-85crn
MA300Dl
300kHz
3,5cm - 6,5cm
MA40S5
6crn-25crn
Table 2- Receiver amplifier parameters. Transducer
MA40S5 PVDF
MA300Dl
Gain
Bandwidth (@-3dB)
50dB
22kHz to 156kHz
21kHz to 125kHz
4. IMPLEMENTATION ON MeV
33dB
130kHz to 970kHz
With reference to Figure 3, the four channels are synchronized by the external pulse repetition rate (PRP) signal, generated by the timer module ePWMl; therefore the PRP represents the measurement rate. The three control
60dB
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signals for the analog front-end necessary to drive the corresponding TX PULSERs are generated by three PWM modules, and synchronized by ePWMl. The latter also generates the blanking signal for each receiving channel. The start time and duration of the blanking signals depend on the transducer ringing response that hides the received echo signal from targets near the transducers. Then for each type of transducer is necessary to program the adequate blanking signal. An interrupt routine for the ePWMI module manages the two previous tasks. We can observe in Figure 3, that only three transducers are used in transmission because the fourth (MA40S5) channel operates only in the receiving mode as it is used for the position estimation with another 40kHz transducer in pulse echo mode.
The whole duration of the range function is 1.9ms, of which 68711s are necessary for computation of the FFT and IFFT and the residual time for the execution of other auxiliary code. The algorithm processing time has been measured and compared with benchmark functions available from TI. We observe that the achieved output rate is adequate for some autonomous navigation systems (e.g. wheelchairs).
4.1. Implementation of threshold technique
Short range
higher sampling frequency of the on chip ADC guarantees the best performances of the TOF estimates in terms of accuracy and offset.
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