An implementation of ultrasonic time-of
Transcrição
An implementation of ultrasonic time-of
Surveying technical An implementation of ultrasonic time-of-flight based localisation by Gideon Ferreira, CSIR NRE Mining The implementation of an underground localisation system, utilising ultrasonic and electromagnetic signals to facilitate time-of-flight (TOF) based trilateration. T he localisation system described is a 2D positioning system and mainly finds its application within excavations of the order of 30 m x 30 m x 1 m in tabular ore bodies. The vertical height in the application environment is limited and no significant value is added by determining this coordinate. The system consists of a number of beacons with known locations and a number of objects that need to localise themselves within a shared reference grid. An object can only localise itself within coverage of a sufficient number of beacons, in this case at least three. The system is required to resolve coordinates with 10 cm accuracy. Localisation is achieved by implementing a trilateration algorithm with an ordinary least squares (OLS) estimator. With this approach, the object only has to measure the distance between itself and the beacons, and know the associated beacons’ locations in order to localise itself. The OLS estimator is required (as opposed to analytical methods), since only approximate distances can be measured as a result of noise inherent in the system. Objects determine the distance to a given beacon by measuring the time-of-flight (TOF) of an ultrasonic signal emitted by a beacon. The TOF is related to the distance between the beacon and the object by the speed at which the wave travels. In order to measure TOF, the object has to know when the signal was transmitted. To accomplish this, a beacon periodically emits the ultrasonic signal, together with an electromagnetic signal at the same instant, so that the TOF upon reception by the object is given by the difference between the arrival times of the two signals. The technique assumes the TOF of the electromagnetic signal to be zero, allowing the electromagnetic signal to act as a synchronisation mechanism. Over time, an object acquires distance measurements from different beacons covering it and localises itself after the sufficient number of distances has been measured. Beacon implementation Beacons exist in a common environment and periodically transmit information that is utilised by any number of objects within coverage. This information includes the synchronised ultrasonic and electromagnetic signals used for ranging, a form of unique identification and the beacon’s own location. It should be possible for an object to uniquely identify a beacon in order to associate ranging results with the transmitting beacon. The location of a beacon is surveyed during installation and stored in memory on the beacon. Hardware and software description In the design, location and identification information are transmitted with a radio. The radio signal is used for a number of functions. Apart from transmitting a beacon’s location and identification, the start of the transmission serves as the synchronisation mechanism and during transmission the carrier is used to avoid transmission collisions. Fig. 1 illustrates the timing of the events during the transmit process. Fig. 1 and 2: Images of the beacon devices that make up the underground localisation system. These are screwed onto a roof bolt that is mounted in the hanging wall. 22 PositionIT - April/May 2009 SURVEYING technical programmable duration. Typically, the transmission has a duration of 16 cycles. The radio frequency carrier is only turned off after enough time has passed for each object to have received all the information and the ultrasonic pulse, ensuring collisions can be avoided during the complete ultrasonic and radio transmission cycle. This is achieved by taking into account that the greatest distance an ultrasonic pulse needs to travel is approximately 30 m, assuming the pulse takes longer to reach the objects than a complete data transfer over the radio link. The flow diagram in Fig. 4 summarises the described transmit process that is implemented on the main controller. Fig. 3: Beacon transmit cycle. Power consumption is improved by turning the ultrasonic transmitter off when it is not transmitting, to increase battery life and reduce noise emissions. Object implementation An object’s main tasks are to acquire information from a sufficient number of beacons and then to localise itself using this information. First of all, the object needs to identify a unique beacon and then determine the distance from itself to the beacon. This information is then stored and the process repeated for different beacons. After enough information has been acquired, the object will execute the estimation algorithm on the acquired information and return an estimated position. Hardware and software description The object also utilises a CC1100 radio transceiver and an ATmega128 microprocessor as the main system controller. A fundamental component of an object is the ultrasonic receiver system. transceiver generates an interrupt on the controller at the instant transmission begins, at which time an ultrasonic pulse is emitted. The receiver is implemented with a two-stage amplifier having a large input impedance and high gain, allowing an object to detect faint ultrasonic signals over a range of 30 m. The amplifier also offers some band-pass filtering to reduce noise. A piezoelectric transducer is used to generate the 40 kHz ultrasonic pulse. The transducer is driven with a 40 V peak-to-peak signal with the aid of a bridge driver and a boost converter. The main controller is responsible for generating the control sequence to emit the ultrasonic pulse, with software Fig. 5 shows the input stage of the receiver, implemented with a high input impedance bootstrapped non-inverting amplifier [1] having a gain of 58 dB. The frequency response is shown in Fig. 6. It has a rather wide bandwidth and mainly filters out lower frequencies. Fig. 4: Beacon transmit process. The radio is designed around a CC 1100 transceiver, which is configured and controlled through its onboard serial peripheral interface (SPI). Before transmission, the transceiver is loaded with a packet and configured to only start transmitting after a clear channel assessment is completed to avoid collisions. An ATmega128 microprocessor is used as the main controller in the system. The PositionIT - April/May 2009 23 SURVEYING technical Fig. 5: Input stage. Fig. 7: Output stage. Fig. 6: Input stage frequency response. Fig. 8: Output stage frequency response. The output stage of the receiver is shown in Fig. 7 and is implemented with a multiple pole feedback filter. The filter is centered on 40 kHz with a bandwidth of 3 kHz allowing for transducer tolerances. The gain of the output stage is 38 dB and the frequency response of the filter can be seen in Fig. 8. simple fixed threshold detector is implemented with the analog comparator, which interrupts the controller upon detection. The detector can easily be expanded to dynamically adjust the threshold of the detector by taking the received signal strength indicator (RSSI), supplied by the radio transceiver, into account. In order for the object to measure the TOF of the ultrasonic pulse, the radio transceiver interrupts the main controller at the instant reception is started. The controller then starts a timer with microsecond resolution, pending the reception of the ultrasonic pulse. In the meanwhile, the packet transmitted from the beacon is being transferred into the receive FIFO of the radio transceiver, containing the beacon identification and location. The timer on the controller is immediately stopped when an ultrasonic pulse is detected by the analog comparator, resulting in the TOF measurement. The output of the ultrasonic receiver is rectified and applied to the controller’s onboard analog comparator. A 24 The TOF measurement is converted to distance using Eqn. 1: ݀ ൌ ቌ͵͵ͳǡ͵ඨͳ ܶ ቍ ൈ ܱܶܨ ʹ͵ǡͳͷ (1) In this equation the influence of temperature on the speed at which sound travels can be seen, where T is the ambient temperature in degrees Celsius. The speed of sound in 0°C dry air is approximately 331,3 m.s-1. The temperature is currently taken to be the average temperature of the operating environment. Other external effects such as humidity and pressure are calibrated once by adding a constant offset to the speed estimation of sound. Should conditions vary more than expected, the temperature sensor onboard the radio transceiver can be used to compensate for changes in temperature. The calculated distance is stored with the beacon information that is transferred from the radio transceiver. The process is repeated until the minimum number of required beacons has been detected, after which the OLS algorithm is executed. The algorithm returns the object’s estimated location, which is made available over a serial communication interface. The flow diagram in Fig. 9 summarises the receive process implemented on the main controller. PositionIT - April/May 2009 SURVEYING technical Fig. 10: Typical system configuration. Fig. 9: Object receive process. OLS estimator The estimator will be discussed with regard to the simulation results presented in this section. Fig. 10 illustrates the typical configuration of the system, with beacons installed on opposite sides of the excavation, approximately 30 m apart, and a number of objects inside the excavation. Fig. 11: OLS estimator performance. Let Bi = (xi,yi) denote the ith beacon with surveyed location and θ = (x,y) an unknown object’s location. The exact distance between the object and the beacon is denoted with di(θ). A method of linearisation is used by Navidi et al. [2], in which a reference point is introduced at the mean of the beacon locations. The distance between a beacon and the reference point is denoted by dir and the distance from the object to the reference point by dr(θ). The actual measurement made by an object for the distance between itself and the ith beacon is denoted by ri. The OLS estimator minimises the quantity given by Eqn. 2 [2]. ଶ ൣ݀ െ ݎଶ െ ݀ ሺߠሻଶ ൧ ୀଵ An earlier simulation result is shown in Fig. 11, in which the OLS estimator’s performance PositionIT - April/May 2009 (2) was investigated as a function of the object’s location within the excavation. Measurements were simulated with a uniform error distribution, using a standard deviation of 1 m and zero mean. The RMS error in the estimate is plotted for each object in Fig. 9 and was typically better than 70 cm. It was noted that location estimates for objects closer to the centre of the excavation were more accurate than estimates for objects close to the sides of the excavation, as expected. Conclusions The implementation of an underground localisation system was described in this article. The system comprises two types of elements, viz. beacons with surveyed locations and objects with unknown locations that need to be estimated. In order to accomplish this, objects implement a trilateration algorithm with an OLS estimator, which only requires an object to measure the distance between itself and the beacons, and know the associated beacons’ locations in order to localise itself. Beacons periodically transmit their locations over a radio channel synchronised with an ultrasonic pulse. Objects use the radio transmission and ultrasonic pulse to determine the distance between itself and the transmitting beacon by measuring the TOF of the ultrasonic pulse. A fundamental component of the object hardware is the receiver amplifier that enables the object to detect faint ultrasonic pulses over a range of approximately 30 m. References [1] P Horowitz and W Hill: The Art of Electronics, Cambridge University Press, pp. 1039, New York, 1989. [2] W Navidi, W S Murphy Jr. and W Hereman: Statistical methods in surveying by trilateration, Computational Statistics & Data Analysis, Vol. 27, pp. 209-227, 1998. Contact Gideon Ferreira, CSIR NRE Mining, Tel 011 358-0091, [email protected] 25