WO2008005931A2

WO2008005931A2 - Acoustic based positioning system and method

Info

Publication number: WO2008005931A2
Application number: PCT/US2007/072663
Authority: WO
Inventors: Robert C. Ashenfelter; Walter J. Naumann; Larry Wade
Original assignee: Nac Service, Inc.
Priority date: 2006-06-30
Filing date: 2007-07-02
Publication date: 2008-01-10
Also published as: WO2008005931A3

Abstract

Described herein is a system (100) for measuring a position of a model vehicle. The system (100) includes an acoustic transmitter (102) located onboard the vehicle (110) that broadcasts an acoustic signal that is received by a plurality of acoustic receivers (104) at fixed positions. The durations of time required for the acoustic signal to be received by the various ones of the acoustic receivers (104) are measured and used to determine corresponding distances between each of the acoustic receivers and the acoustic transmitter (102). These determined distances are then used to arrive at a position for the acoustic transmitter (102) and the associated vehicle (110) based upon the known positions of the plurality of acoustic receivers (104). In some implementations, the acoustic signal used is an ultrasonic signal.

Description

ACOUSTIC BASED POSITIONING SYSTEM AND METHOD

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention is directed generally to positioning systems.

Description of the Related Art

Many positioning systems use triangulation to compute position based upon multiple transmitters and a single user, such as found with the Global Positioning System (GPS), which uses radio frequency transit times. Conventional vehicle positioning systems include model vehicle position measuring systems, such as for model railroading that make use of periodically placed track segments or track side devices. Such conventional model railroad positioning systems include those that use blocks of track that are electrically isolated from one another to indicate a train's position on a given one of the isolated blocks of track based upon detected electrical current being received through the given isolated track.

Other conventional approaches provide detection of a train's presence through sensors placed along a model railroad track based upon optical, acoustic, magnetic, radio frequency, tactile, or other methods. For instance, a model train's position can be identified through an optical bar code, radio frequency tag, or other information containing device placed directly upon the train to be read by a trackside sensor as the train is positioned in close proximity to the trackside sensor. Unfortunately, conventional vehicle positioning systems, such as for model railroad vehicles, can lack adequate precision or may supply adequate precision only at high cost or with an impractical amount of equipment involved. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Figure 1 is a schematic top plan view of an acoustic based positioning system.

Figure 2 is a block diagram of a version of the processor of the acoustic based positioning system of Figure 1.

Figure 3 is a perspective view of an implementation of the acoustic based positioning system including speakers.

Figure 4 is a schematic top plan view of an implementation of the acoustic based positioning system with multiple vehicles on adjacent track.

Figure 5 is a schematic side-elevational view of an implementation of the acoustic based positioning system with multiple vehicles at various elevations.

Figure 6 is a schematic top plan view of an implementation of the acoustic based positioning system with multiple vehicles with maintained spacing.

Figure 7 is a schematic side-elevational view of an implementation of the acoustic based positioning system accounting for vehicle elevation change.

Figure 8 is a schematic side-elevational view of an implementation of the acoustic based positioning system showing control of associated fixtures.

Figure 9 is a schematic top plan view of an implementation of the acoustic based positioning system with an additional acoustic transmitter.

Figure 10 is a block diagram of an implementation of an acoustic receiver of the acoustic based positioning system.

Figure 11 is a block diagram of an implementation of an acoustic transmitter of the acoustic based positioning system.

DETAILED DESCRIPTION OF THE INVENTION

As described herein a system measures position of a model vehicle. The system includes an acoustic transmitter located onboard the vehicle that broadcasts an acoustic signal that is received by a plurality of acoustic receivers at fixed positions. The durations of time required for the acoustic signal to be received by the various ones of the acoustic receivers are measured and used to determine corresponding distances between each of the acoustic receivers and the acoustic transmitter. These determined distances are then used to arrive at a position for the acoustic transmitter and the associated vehicle based upon the known positions of

. 7 _ the plurality of acoustic receivers. In some implementations, the acoustic signal used is an ultrasonic signal.

Use of an acoustic signal provides opportunity for relatively high precision at relatively low cost compared to systems such as those relying on radio frequency signals, such as the Global Positioning System (GPS). For instance, an acoustic wave speed can be on the order of 300 meters per second versus 300,000,000 meters per second for a radio frequency signal.

Also unlike the GPS, implementations of the present system can use one transmitter located at the position to be measured (the vehicle) and multiple receivers located at stationary predetermined positions. Implementations use small bandwidth for communication to an acoustic transmitter located on a vehicle (such as a model railroad train) and require no computation at the acoustic transmitter site, which aids in reduction of size and cost of equipment located on the vehicle. Use of stationary acoustic receivers is furthermore conducive to conveying data to a stationary measurement processor for computation and display.

While vehicles of various sizes can be used, an exemplary system includes a model railroad train including a locomotive running on an electrically conductive track. The track is described herein as a communication media. For implementations with vehicles that do not use electric tracks, other forms of communication media can be used such as radio frequency signals.

Implementations can include further aspects. Measurement of acoustic receiver positions can be assisted by the acoustic receivers themselves thereby facilitating installation. Periodic automatic calibration for sound speed variations and component drift can be done to increase measurement accuracy. Positions of multiple vehicles can be measured through polling by address over a second communication media such as electric track, radio or other another acoustic frequency. An impulse driven acoustic transmitter and broadband acoustic receivers can provide a simple, accurate time of flight measurement. A second, narrow-band, receive-signal processing enables frequency screening of broadband acoustic pulses thereby improving noise tolerance. In another implementation, times of zero crossings of the received signal discriminate against noise.

As shown in Figure 1 , a vehicle positioning system 100 includes an acoustic transmitter 102 and a plurality of acoustic receivers 104, shown in this case to have a total of four acoustic receivers. The acoustic receivers 104 are communicatively linked via signal wire 106 to a processor 108. The acoustic transmitter 102 is affixed to a vehicle 1 10 (such as a model railroad train) that is coupled to a track 1 12 (such as a model railroad track). In other implementations, the vehicle 1 10 can be other than a train. The acoustic transmitter 102 broadcasts an acoustic signal.

In some implementations the acoustic transmitter 102 is a resonant piezoelectric crystal that is impulse driven to produce at decaying string of cycles. Some versions of the acoustic transmitter 102 broadcast at particular set intervals. For instance, the acoustic transmitter 102 can be a resonant transducer formed as a single pulse generator to produce an acoustic transmission characterized by a sharp rise to a peak followed by a decaying sinusoid at the resonant frequency of the resonant transducer. The sharp rise provides a readily identifiable signal for accurate timing and the resonant frequency provides a signature allowing discrimination against random noise.

Each of the four acoustic receivers 104 shown in Figure 1 detects the acoustic signal broadcasted, measures the value of the associated time of arrival of the acoustic signal at the acoustic receiver, and sends the value to the processor 108, which then determines the position coordinates of the acoustic transmitter 102. Some versions of the acoustic receivers 104 use broadband acoustic receivers that retain the sharp rise of the acoustic signal to facilitate accurate timing. The position coordinates are displayed on a personal computer screen or in another suitable manner.

In some implementations the acoustic receivers 104 can also use an additional acoustic receiver with a transducer having narrowband reception at the signal dominant frequency of the acoustic signal being sent by the acoustic transmitter 102. One example of such a narrowband acoustic receiver 104 has a transducer with resonant crystal. This narrowband form of the acoustic receiver 104 identifies sinusoidal frequency of the acoustic signal being sent by the acoustic transmitter 102 to reject random noise. This identification approach allows for a window of time to detect a sharp signal rise. If the acoustic receivers 104 do not include a narrowband acoustic receiver, a second processing with a narrowband filter identifying the sinusoidal character of the acoustic signal can be used to reject noise and allow for a detection time window for the sharp rise of the incoming acoustic signal. Implementations can also include acoustic receiver correlation of the received pulse-sinusoid signal to a stored copy of the transmitted pulse-sinusoid signal.

In other implementations, the acoustic transmitter 102 produces a unique, non-repeating wave shape. The acoustic receivers 104 each contain a wave pattern correlator to compare the wave shape of each of the acoustic signals received by the acoustic receiver with the wave shape stored in the wave pattern correlator to provide a precise time of arrival.

A version of the processor is shown in Figure 2 as having a vehicle measurement component 1 14, a sound generator 1 16, a storage 1 18, a display 120, and a controller 122, which has a digital command and control (DCC) portion 124, and an other portion 126. The sound generator 1 16 is coupled to acoustic speakers 130, such as those shown in Figure 3, to create an audible perception to a human listener, otherwise known as a virtual sound, in the vicinity of the acoustic transmitter 102, that a sound, such as a train engine sound, is originating from the area of the acoustic transmitter, such as from the vehicle affixed to the acoustic transmitter. The display 120 can include a computer monitor that displays position of the vehicle 1 10 on a map or other status panel and updated frequently.

In a situation where there is a plurality of the vehicles 110, such as shown in Figure 4, each of the acoustic transmitters 102 will be affixed to an individual one of the vehicles. The DCC portion 124 of the processor 108 is used to address one of the plurality of the vehicles 1 10 at a time, such as to provide locomotive identification if the vehicles are model railroad trains.

As shown in Figure 3, the acoustic receivers 104 can be positioned in a three dimensional orientation so that the vehicle position can be determined as a three dimensional position thereby allowing discrimination of trains on adjacent tracks (as shown in Figure 4) and on different levels (as shown in Figure 5). Train separation may be maintained, manually or automatically (as shown in Figure 6). Speed may be computed. Power can be computed from speed and elevation (E) change (as shown in Figure 7). Model railroad signal lights 132 or semaphores 134 may be controlled according to determined position (as shown in Figure 8). A three dimensional layout of the track 112 can be drawn automatically by running the locomotive vehicle 1 10 to every point on the track while determining position of the vehicle. Precision Enhancements

Additional precision is achieved in several ways. In some implementations, the receivers 104 can also receive acoustic signals from a stationary transmitter 136, as shown in Figure 9. Since the distance between each of the acoustic receivers 104 and the stationary transmitter 136 is known and fixed, any variability in the distance as determined by the system 100 due to such factors as temperature, humidity, and equipment fluctuations can be determined and accounted for in the vehicle position determination.

A primary noise source is acoustic including environmental noise, which is generally broadband in nature, and including reflected acoustic signals that act as noise. This acoustic noise can be addressed by using narrowband transmissions, for example, emitted by a resonant piezoelectric crystal as the acoustic transmitter 102, and received by narrowband form of the acoustic receivers to discriminate between narrowband acoustic signals and broadband acoustic noise. The waveform of the acoustic signal emitted from the acoustic transmitter 102 can also be cross-correlated with a known acoustic waveform to obtain accurate time arrival data in the presence of noise.

Further, in some implementations the acoustic transmitter 102 can be pulsed with a single high-energy pulse, which produces as exponentially decaying resonant acoustic wave. The first peak of the exponentially decaying resonant acoustic wave will be much larger than following peaks so will be relatively easier to be identified by the acoustic receivers 104.

A version of the acoustic receiver 104 is shown in Figure 10 as having a broadband microphone 138, a narrowband filter 140, a demodulator 142, and an acoustic transmitter 144. For this version of the acoustic receiver 104, the acoustic signal is received from the acoustic transmitter 102 as an ultrasonic transmit pulse by the broadband microphone 138 of the acoustic receiver thereby enhancing detection of the first resonant cycle of the received acoustic signal. The signal-to- noise-ratio, and therefore accuracy, can be large for this situation compared to conventional ultrasonic pulse-echo systems. Furthermore, narrowband filtering of the received acoustic signal by the narrowband filter 140 enables detection of the decaying string of cycles to distinguish the first resonant cycle from random noise further enhancing measurement accuracy. For further enhanced noise rejection, in some implementations, the acoustic transmitter 102 has a frequency modulator 146 along with a piezoelectric emitter 148 as shown in Figure 1 1. Ultrasonic signals generated by the piezoelectric emitter 148 are frequency modulated by the frequency modulator 146. Accordingly, the demodulator 142 of the version of the acoustic receiver 104 shown in Figure 10 demodulates the acoustic signal modulated by the frequency modulator 146 of the acoustic transmitter 102 shown in Figure 11. Some implementations of the demodulator 142 can use a digital signal controller such as the dsPIC 30F3012. In some implementations the FM demodulation is by timing of zero crossings of the signal. In some implementations the FM signal consists of just the decaying string of cycles with the first cycle being large. In this implementation, the frequency modulation is from zero frequency to the resonant frequency.

Position of the acoustic receivers 104 can be automatically and periodically calibrated through use of the acoustic transmitter 144 found in the version of the acoustic receiver shown in Figure 10. With all of the acoustic receivers 104 having the acoustic transmitter 144, the acoustic receivers can periodically and automatically calibrate each of their positions. Each of the acoustic receivers 104 can take a turn broadcasting an acoustic signal with its acoustic transmitter 144 to the other acoustic receivers whereby the other acoustic receivers would measure travel time of the acoustic signal and report this back to the processor 108. The acoustic transmitter 102 is shown in Figure 1 1 as also having a receiver 150 (either an acoustic, electrical, or electromagnetic) that can be used in some implementations for synchronization between the acoustic receivers 104 and the acoustic transmitter as described further below.

Synchronization and Ambiguity of Position

As a summary, Table 1 highlights exemplary configurations for various implementations of the system 100. Detail includes quantity of dimensions measured, whether synchronization (described further below) between the acoustic receivers 104 and the acoustic transmitter 102 is implemented, quantity of transmitters used, and quantity of receivers used. Further comments regard averaging and ambiguity. A "yes" under the "averaging" column indicates when averaging can increase measurement accuracy. A "yes" under the ambiguity column indicates when ambiguity exists between two-position solution sets consequently requiring additional information.

Dimensions Synchronization Transmitters Receivers Averaging Ambiguity

1 yes 1 1 no yes

1 yes 1 >1 yes no

2 yes 1 2 no yes

2 yes 1 3 yes no

2 yes 1 >3 yes no

3 yes 1 3 yes yes

3 yes 1 >3 yes no

1 yes >1 1 yes no

2 yes 2 1 no yes

2 yes >2 1 yes no

3 yes 3 1 yes yes

3 yes >3 1 yes no

1 no 1 2 no no

1 no 1 >2 yes no

2 no 1 3 no yes

2 no 1 >3 yes no

3 no 1 4 no yes

3 no 1 >4 yes no

1 no 2 1 yes yes

1 no >2 1 yes no

2 no 3 1 no yes

2 no >3 1 yes no

3 no 4 1 no yes

3 no >4 1 yes no

Table 1.

As noted in the table above, some configurations of the receivers 104 and dimensions measured result in two solutions for the calculated position that the vehicle 110 is determined to occupy whereas in reality only one of these positions is the actual position of the vehicle. Since the vehicle 1 10 really only exists in one position, only one of the two solutions, in correct and the other solution is incorrect. With the two solutions, a degree of ambiguity exists since without further information, one cannot be certain which of the two solutions is the position of the vehicle 1 10.

Consider the example of two of the acoustic receivers 104 that are synchronized with one of the acoustic transmitters 102 to measure position of the vehicle 110 in two dimensions. In this example, to be synchronized means that the acoustic receivers 104 are each sent an electric or electromagnetic signal as a synchronization mark (much faster than acoustic) the same time that the acoustic signal is transmitted from the acoustic transmitter 102 to the acoustic receiver to determine travel time of the acoustic signal based upon the measured time of arrival of the acoustic signal at the acoustic receiver. The first acoustic transmitter 102 is determined to be somewhere on a circle of a radius equal to a distance computed by the transit time of the acoustic signal from the acoustic transmitter 102 to the first acoustic receiver multiplied by the speed of sound. The time of arrival to the second of the pair of the acoustic receivers 104 establishes that the acoustic transmitter 102 is on a different circle in an analogous way.

These two circles must in general overlap and in particular intersect at two points, which causes the ambiguity. One of the two points of intersection of the two circles is the actual position of the acoustic transmitter 102 and the other point of intersection is not. This ambiguity may be resolved in more than one way. One way is to employ a third of the acoustic receivers 104 so that the acoustic transmitter 102 is known to be on a third circle intersecting the first two circles. Since there is only one point that all three circles will intersect, within the accuracy of the measurements, this point of intersection will be the actual location of the acoustic transmitter 102.

For economy, the ambiguity with two of the acoustic receivers 104 may also be resolved by orientation of the acoustic receivers 104 and the one or more acoustic transmitters 102. In the above example this may be achieved by placing the two acoustic receivers such that the acoustic transmitter 102 is always on one side relative to the two acoustic receivers 104. One way to do this is to place the two acoustic receivers 104 along a wall adjacent to the measurement area. As a consequence, one solution will be in the measurement area, the other will be behind the wall and therefore impossible so that the ambiguity is resolved.

Another method of ambiguity resolution is with an initial knowledge of the location of the acoustic transmitter 102. With this approach, measurement coordinates are compared to a previously known position of the acoustic transmitter 102 and the closest one of the positions determined for the new position of the acoustic transmitter is selected as the actual position.

With three dimensions, if three of the acoustic receivers 104 are used, each will measure when the acoustic signal arrives at the acoustic receiver from the acoustic transmitter 102 resulting in two mathematical solutions for the position of the acoustic transmitter without additional information to determine the actual position of the acoustic transmitter from the two solutions resulting in another ambiguity. Analogous to the two-dimensional case with two of the acoustic receivers 104 described above, this ambiguity resulting from the three-dimensional case with three of the acoustic receivers 104 may be resolved using an additional one of the acoustic receivers 104 or by additional information.

Synchronized Receivers and Transmitters

Those of the acoustic receivers 104 that are not synchronized with the acoustic transmitter 102 each measure times of arrival at the acoustic receiver of the transmitted signal, but cannot determine actual transit time for the acoustic signal between the acoustic transmitter and the acoustic receiver. Consequently, the configuration needs an additional one of the acoustic receivers 104 and different computations compared to synchronized cases.

Consider the example of two dimensions without synchronization, which requires three receivers. The time of the acoustic signal arrival at the first of the acoustic receivers 104 is not adequate alone to provide position information. The difference in arrival times of the acoustic signal received by the first and second of the acoustic receivers 104 multiplied by the speed of sound establishes the difference between the distance between the acoustic transmitter 102 and the first of the acoustic receivers 104 and the distance between the acoustic transmitter and the second of the acoustic receivers. This difference in these two distances define a hyperbola of possible positions for the acoustic transmitter 102. In a similar fashion, the difference in arrival times for the acoustic signal from the acoustic transmitter to the third of the acoustic receivers 102 and the first of the acoustic receivers 102 establishes a second hyperbola that intersects the first hyperbola generally at two points and thus having an ambiguity. In this case, a third hyperbola can be constructed from the different in arrival times of the acoustic signal from the acoustic transmitter 102 to second of the acoustic receivers 104 and to the third of the acoustic receivers that resolves the ambiguity. The actual location coordinates for the acoustic transmitter 102 are found at the intersection of all three hyperbolas.

Others of the acoustic transmitter 102 and multiple sets of the acoustic receiver 104 without synchronization measure position of the acoustic transmitter in analogous ways. Two of the acoustic receivers 104 can measure the position of one of the acoustic transmitter 102 in one dimension. The difference in arrival times to the first and second of the two acoustic receivers 104 multiplied by the speed of sound is the difference in the distance between the acoustic transmitter 102 and the first of the two acoustic receivers and the distance between the acoustic transmitter and the second of the two acoustic receivers. Position of the acoustic transmitter 102 is computed as follows.

Constants:

L is the distance between receivers

Relationships: L = X1 + X2 D = X1 - X2 L₊D = 2^*X1

Measured:

D is the difference in receiver to transmitter distances

Computed position:

X1 = the distance from the transmitter to receiver 1. X1 = (L+D)/2

X2 = the distance from the transmitter to receiver 2. X2 = L - X1

An example application for this one-dimensional measurement is a train on a known track.

With the above example, if synchronization were done between the acoustic transmitter 102 and one of the acoustic receivers 104 with an electrical signal or an electromagnetic signal for the purpose of enabling the acoustic receivers to directly measure the time difference between transmit and receive, this synchronization could reduce the number of the acoustic receivers required from two acoustic receivers to one acoustic receiver for the one-dimensional case. The X coordinate is now equal to the distance measured with a fixed offset of the distance from the coordinate origin to one of the acoustic receivers 104. From the equations above, the distance X = D.

To take into consideration the vertical height, H, that the acoustic receiver 104 is positioned above the acoustic transmitter 102 the following enhancement to the above formula can be used:

X = (D^Λ2 -H^Λ2)^Λ0.5

A first alternative implementation involving synchronization involves a version of the acoustic transmitter 102 as shown in Figure 1 1 that has the receiver 150 as an acoustic receiver. In this implementation, to determine position of the acoustic transmitter 102, a first one of the acoustic receivers 104 (used as a synchronization receiver/transmitter) is configured as a version of the acoustic receiver shown in Figure 10 and uses its transmitter 144 to broadcast a first signal, which is an acoustic signal at generally a first frequency, which is received by the receiver 150 of the acoustic transmitter 102. The first acoustic receiver 104 also uses its transmitter 144 to simultaneously transmit a second signal (in this depicted implementation an electric signal) to the other of the acoustic receivers 104 that use the second signal as a synchronization mark of when the first signal was transmitted by the first acoustic receiver 104.

Once the first signal is received by the receiver 150 of the acoustic transmitter 102, the acoustic transmitter broadcasts a third signal, which is an acoustic signal that is generally of a different frequency than the first signal, which is also an acoustic signal. The third signal is then received by the acoustic receivers 104, which measure the time of arrival of the third signal with respect to the initiation of the first signal. The time of transit of the first signal from the first of the acoustic receivers 104 to the acoustic transmitter 102 can be approximated as half the time between the time when the first of the acoustic receivers initiates the first signal to the acoustic transmitter to the time when the first of the acoustic receivers receives the third signal from the acoustic transmitter 104. Calculations are made precise by calibration of the fixed latencies. Once determined, the time of transit of the first signal from the first of the acoustic receivers 104 to the acoustic transmitter 102 can be used to correct the time of arrival of the third signal to the other of the acoustic receivers 104. The position of the acoustic transmitter 102 is then computed as described above. The acoustic frequencies of the first signal and the third signal are in general at sufficiently different frequencies to prevent interference with one another.

A second alternative involving synchronization uses a digital model railroad control system such as conforming to the NMRA DCC, National Model Railroad Association (NMRA), Digital Command and Control (DCC) Recommended Practice. For instance, a first DCC command decoder can be used solely by the acoustic transmitter 102 and a second DCC command decoder can be shared by the acoustic receivers 104. A DCC controller can send the synchronization information through DCC command signals to be decoded with a small latency difference by the first and second DCC command decoders. The disadvantage of synchronization is the addition of the synchronization complexity. Advantages are the reduction in quantity of the acoustic receivers 104 required and reduction in noise sensitivity by looking only at a narrow period of time, as may be seen from Table 1.

Bandwidth

Narrowband systems are frequently used in ultrasonic measurements and usually employ resonant piezoelectric crystal transducers for transmission of the acoustic signal by the acoustic transmitter 102 and reception of the acoustic signal by the acoustic receivers 104. These are relatively efficient and the resonance inherently favors a wave train of substantially one frequency. The single frequency rejects much broad-spectrum noise for greater reliability. The high efficiency of resonant transducers allows longer range. And it allows the use of pulse echo where the sound echoes off a target to be measured. This technique requires twice the distance of propagation, reducing the maximum range. Conversely, separation of the acoustic transmitters 102 and the acoustic receivers 104 requires half the propagation distance as pulse echo and therefore has a shorter maximum range. This in turn allows improved noise rejection at a given range.

Timing accuracy in also influenced by transducer bandwidth. Time of arrival is most accurate when the measured pulse has a short rise time. Then errors in detection amplitude and amplitude noise have a lower effect of timing accuracy. If a resonant transducer is energized by a burst of cycles at its resonant frequency, the ultrasonic output amplitude rises gradually over many cycles so selecting any particular cycle is difficult.

In general line of sight access from the acoustic transmitter 102 to a sufficient quantity of the acoustic receivers 104 is required. This requirement is a factor in the choice of quantity and position of the acoustic receivers 104. An algorithm to select the optimum set of the acoustic receivers 104 starts with acceptance of the acoustic receivers with the earliest arrival time as being the closest.

Selection rules include criteria of acceptable sets of the acoustic receivers 104 such as follows. Avoid obstructions that attenuate amplitude and force the acoustic signal to be longer than a straight-line path for the earliest arrival of the acoustic signal to the acoustic receivers 104. Scenery (such as with model railroad layouts) is allowed in the path of the acoustic signal to the extent it is sufficiently transparent. User tests will establish this. Objects that are opaque to ultrasound, such as model buildings and model mountains must be avoided in positioning of the acoustic receivers 104.

Avoidance of much interference can be done by positioning the acoustic receivers 104 sufficiently above obstructions and using numerous and possibly redundant individual or groups of the acoustic receivers. A model railroad tunnel or other hidden area is accommodated with a separate set of the acoustic receivers 104 in the tunnel or other hidden area. Separation of trains on multiple levels also requires line of sight from the acoustic transmitters 102 and the acoustic receivers 104 involved, which influences quantity and placement of the acoustic receivers. In summary, a simple tabletop model railroad will require only one set of the acoustic receivers 104. Larger and more complex model railroad layouts will require more of the acoustic receivers 104.

Sensitivity to Noise

The accuracy and reliability of the position measurements is a function of ultrasonic noise, which originates from many different types of sources such as follows for the case of a model railroad: mechanisms peripheral to the model railroad, vehicle mechanisms (such as the model railroad train itself), vocal sounds from users, street noise, all noise from outside the area of interest, human activity (such as walking, clearing throats and noses, etc.), desired sound sources such as model railroad sounds that are either actual or simulated.

Measurement Frequency

More measurements provide greater time resolution of the position measurements. Time available must be divided among the one or more acoustic transmitters 102 utilized in a given reception area. Time of transit for 25 feet is about 25 milliseconds. Additional time may be required for decay of reflections although utilization of first arrival data minimizes this restriction. For example a suitable repetition rate for many implementations can be 5 Hz. If 50 locomotives are desired all may be measured at 0.1 Hz. Or if the repetition rate is 20 Hz and 2 locomotives are desired, measurement rate can be 10 Hz.

Using More Receivers to Increase Accuracy Increasing the quantity of the acoustic receivers 104 beyond the minimum described above can provide improved accuracy in three ways. First, the most accurate set of the acoustic receivers 104 can be selected, based on previous test statistics. Second, multiple results can be averaged to reduce error. Third, the most reliable set can be selected based on residuals in the software.

Errors

Errors in the position measurement may occur from several sources. The following depicted example of sensitivity to errors is for illustrative purposes. A demanding exemplary application for the system 100 can be an N or Z scale model railroad, with parallel tracks 3 cm apart. The tolerance for each measurement can be +/- one half the track spacing, in this case, +/- 1.5 cm to distinguish between trains on the two or more tracks. This tolerance should be achieved to a high degree of certainty.

Error sources and a corresponding error budget is depicted in Table 2. The confidence of the total is 0.9999.

Error source centimeters of error allowed

Electrical timing 0.05

Decoder latency 0.05 Detection of a fixed point on the waveform 0.81 Knowledge of the position of the receivers 0.54 Knowledge of the position of the track 0.54

Speed of sound 0.54

Velocity of the air 0.27

Drift in component properties 0.27

Error amplification from geometry 0.70

Other 0.27

Total (rss, root sum square) 1.5

Table 2. Referring to the error sources listed in Table 2, electrical time is easy to control to the value specified. Decoder latency variation is also controllable to the value specified. Detection of a fixed point on the waveform requires procedures. The wavelength of the 40 kHz frequency chosen is 0.9 centimeters. The error budget is +/- 0.4 cm, a fraction of a wavelength. The position of the acoustic receivers 104, including orthogonally to a chosen coordinate system, would be difficult to measure manually.

The capability of the acoustic receivers 104 to also transmit enables self-determination of their positions. This calibration may be accomplished in controlled ambient conditions and averaged over numerous trials to reduce error. Position of the track 1 12 is determined by the measurement system 100 as the vehicle 110 (such as a train) is driven over all points to be stored such as in storage 1 18 to be displayed such as in map form on display 120. This process can be repeated and data averaged if added accuracy is desired.

The errors due to an inaccurate value for the speed of sound caused by such factors as changes in ambient conditions can be significant. To achieve the error limits specified above, the ambient conditions would have to be very stable. The speed of sound varies 0.15 % per degree C with temperature. At a distance of 3 m, a change in temperature of 1.0 degree C corresponds to a distance error of 0.44 cm and the effects of changes of humidity and air pressure are relatively smaller. Also at a distance of 3 m, a change in air velocity of 0.3 m/sec corresponds to a distance error of 0.26 cm.

One method of reducing errors is to keep the environment stable. Because of the difficulty in achieving this ambient stability a frequent calibration is preferred. One way of calibration for speed of sound is to measure the ambient parameters and calculate a correction. The preferred calibration method is to make use of the fact that the receiver locations have been self determined and are fixed. These positions can be re-measured periodically to calibrate for environmental variations. This recalibration using receiver locations also compensates for much of the drift of component properties of the system 100 and other errors. Since an exemplary computation of acoustic transmitter position from the measured distances to the acoustic receivers 104 is nonlinear, care must be taken to limit the placement of the acoustic receivers to minimize these computational errors. Placement

There is a lot of flexibility in the location of the acoustic receivers 104 and the acoustic transmitters 102 although it is not suggested that three or more acoustic receivers 104 be placed all in a line since positions in a line are not independent so would most likely not provide information for three dimensions. In general, geometry of receiver placement can influence accuracy of the position determination of the acoustic transmitter 102. Placement of the acoustic receivers 104 should avoid small angles between the acoustic transmitter 102 and the acoustic receivers 104 because small errors in time of arrival will result in large errors in computed position. The acoustic transmitter 102 and the acoustic receivers 104 should be close to each other to maximize signal strength and to maximize angles between the acoustic receivers and the acoustic transmitter. Ideally, the acoustic receivers 104 could surround the acoustic transmitter 102. One arrangement is to place the acoustic receivers 104 at the corners of a square all at approximately the same height above the movement range of the acoustic transmitter 102. Positions need not be precisely placed, just fixed, and accurately measured. Many different designs of amplifiers and transducers may be used. In some implementations separations of fifty feet or more can be achieved.

Measurement Waves

Audio frequencies may be used. These frequencies have a greater range but lower resolution. A disadvantage is that the human ear can hear them as noise. Ultrasonic frequencies are not audible which is an advantage in most applications. Any acoustic frequency may be used, so the following are provided as examples.

25kHZ This low ultrasonic frequency will provided low attenuation in the atmosphere, an advantage, but results in lower resolution which is a function of wave length.

4OkHZ This medium ultrasonic frequency may be the most used in the industry. It has intermediate resolution and range.

7OkHZ This higher ultrasonic frequency has higher resolution and shorter range. Use of Multiple Frequencies

Multiple sound frequencies may be used. The transmission time can be conveyed to the acoustic receiver 104 using a frequency different that the frequency used for time of flight and distance measurement. Multiple frequencies can be used for multiple simultaneous measurements in the same area. Multiple frequencies can be used for areas where receiver selection during computation at the same frequency is difficult. An example is at the entry to tunnels where the closest receiver may not be one that is desired in the tunnel.

Draw Track Layout

The track layout is drawn during a setup mode of operation by running a locomotive to all locations on the track. A map of the track is thereby produced for display of the measured vehicle position and for further computation.

Communication with Vehicle

The system 100 can comply with the National Model Railroad Association ,NMRA, Recommended Practice (RP) for bidirectional communication. The NMRA RPs for communication include a bidirectional option that allows the locomotive to report its position. The system 100 can comply in an innovative way by listening for a DCC request, such as from the DCC portion 124 of the controller 122, for position for a particular locomotive and creating a command on the user throttle bus to that locomotive decoder. This command is to activate the DCC function programmed for the system 100 to instruct the acoustic transmitter 102 to transmit the acoustic signal. The position of the acoustic transmitter once determined, is reported such as to the DCC portion 124 of the controller 122 using the locomotive address and the track bus. In this procedure, the locomotive decoder would either not have a bi-directional option in DCC or the bi-directional option would be set to ignore the position request. Consequently, the system 100 is compatible with the NMRA RPs, but does not require the bi-directional option.

The system 100 can conform to and use the (NMRA) Recommend Practice-9 ( RP-9) for communication with the vehicle 1 10. These RPs include the Digital Command and Control (DCC) system that can be found at the following site: http://www.nmra.org/standards/consist.htmlttrps. Other communication methods can also be used, for example radio. Sound

Sound can emanate from a virtual source on the vehicle 1 10. The sound can be full spectrum because it is projected by stationary speakers around the room. Doppler sound effects can be created so that the sound pitch drops as the vehicle passes by. This can be for a particular observer location, or using headphones it can be accurate for multiple observers. Sounds that change with power used by the vehicle to climb hills can be computed using the power data.

Steam locomotive chuff sounds can be repetition rate adjusted to conform to speed using determined speed data. The system 100 uses a virtual source system with speakers, such as the speakers 130, outside the vehicle 1 10 in stationary positions that project sound apparently from the vehicle. The peripheral speakers can be large allowing full-spectrum sound to apparently come from even small sized versions of the vehicle 1 10.

The virtual sound-source location can be offset from the measured location of the acoustic transmitter 102 in the vehicle 1 10. The virtual location can be computed from recent position measurements and the desired offset magnitude and direction. This is an advantage because an error in the sound source position can be observed and is objectionable. For example, available on-board sound generators for conventional systems are frequently placed in the tender of a steam locomotive when the sound should come from the cylinders. This detriment is eliminated in the system 100. An additional feature is to track the offset around curves. This is possible because the layout of the track has been drawn by the system 100 and may be used for the computation. In order to offset correctly when there is a track turnout, the turnout route can be measured by one of a variety of available convention detectors, for example a micro switch on the turnout, and provided for the computation.

The optional enhanced sound is produced by an available virtual- source system that creates sound apparently coming from a computed suitable position on the vehicle 110 using the peripheral speakers, such as the speakers 130. The virtual source is at the location computed from the measured position information.

Exemplary Implementation A model railroad in N scale of the Union Pacific at Sherman Hill Wyoming on June 20, 1949. It is contained in a 60 square meter room that is 15 meters long and from 2 to 4 meters wide. Tracks 112 are spaced as close as 3 centimeters apart. There are 20 trains as long as 100 cars long. The track 1 12 is from 1.5 to 1.9 meters above the floor and the acoustic receivers 104 are placed between 1.9 meters and 2.2 meters above the floor. The stationary acoustic receivers 104 are placed every 2 meters at the ceiling along both sides of the track bench. Separate acoustic receivers 104 are used in tunnel segments.

Locomotives and cabooses contain the acoustic transmitters 102 for location determination of the beginning and end of the train and for break-apart detection. These are triggered by signals from a personal computer that sends commands through a DCC (National Model Railroad Association, NMRA, Recommended Practice, RP, for Digital Command and Control, DCC) standard controller through the track 112 to decoders in the vehicles. The acoustic receivers 104 measure time of arrival from transmission. The acoustic receivers 104 acquire the transmission time using a separate stationary decoder.

Multiple vehicles 110 are addressed in sequence by their DCC address. The stationary decoder for the acoustic receivers 104 detects the time of all transmissions without regard to address. Data from each vehicle 1 10 is identified by time of transmission. Measurement rate is 0.5 Hz

A resonant 40kHz transmission piezoelectric transducer receives a single 100 volt pulse of one millisecond time-constant exponentially-decaying duration from a charged capacitor. This produces a wave train at 40 kHz that is exponentially decaying. The receiver transducer is broadband allowing reliable detection of the first cycle of the wave train. The received signal is also narrowband filtered and this signal is used to validate the broadband signal providing discrimination from noise. The acoustic receivers 104 are gated to the expected time of arrival range after the known transmission time further reducing noise sensitivity. The map of the track 1 12 is drawn automatically during setup by running one of the acoustic transmitters 102 to all points on the track. This map and the location of the trains 1 10 is displayed on multiple computer screens.

The acoustic receivers 104 are capable of also transmitting during initial setup and these signals are used to self locate the acoustic receivers in the same way that locomotives 1 10 are located in operation. The acoustic receivers 104 also transmit periodically during operation to compare the measured times to the times measured during set up. These data are used to calibrate for variations in sound speed in the air, velocity of the air and for drift of system component parameters.

Positions are displayed on multiple personal computer screens 120. Commercially available model train control software controls the trains 1 10. It can be run manually or automatically providing train separation. The computer also controls track switches and sets track side lights and semaphore signals.

Position information is conveyed to a Soundtraxx brand sound generation unit that provides over 100 different train sounds including horns, bells, steam chuff, brakes, doors, valves, generators spinning and conductor voices. Each of these is independently adjusted for amplitude and five-channel equalization. Computations of location, speed, and grade enable synchronization of sound type, amplitude and frequency.

Sound is produced in one of two ways. The first way is with a set of stationary speakers. The high frequency speakers are located separate from the position measurement receivers for optimization of both the position measurement and sound reproduction. Low-frequency, non-directional, sounds are produced by a 10 cubic meter Klipschorn woofer under the layout. Care must be exercised to avoid derailing the train with too much amplitude. Position information is given to a virtual source system that places the source of the sound apparently at the locomotive. The transmitters will be placed in a convenient location in or near the locomotive such as a tender or dedicated following box car. The virtual source position for the sound will be computed to be in the desired spots such as the locomotive cylinders. Sound is produced from appropriate speakers to provide the most amplitude from the nearest locomotives.

A second sound system concept uses multiple headphones for observer specific sound. Again a virtual source is created at the locomotive. Additional position transmitters, similar to those in the locomotives, in the head phone provide its location and orientation. Sound is again loudest for the nearest locomotives and Doppler frequency shifts are also introduced.

From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For instance, in most of these considerations, one transmitter and multiple receivers are assumed. Some implementations can be used with one receiver and multiple transmitters depending upon application logistics. Accordingly, the invention is not limited except as by the appended claims.

Claims

The invention claimed is

1. For a model railroad train, a system comprising: a first acoustic transmitter configured to generate a first acoustic signal with a first frequency, the first acoustic transmitter sized to be affixed to the model railroad train; a plurality of acoustic receivers each configured to receive the first acoustic signal; and a processor configured to determine position of the first acoustic transmitter based at least in part upon when the first acoustic signal arrives at each of the acoustic receivers.

2. The system of claim 1 wherein a first of the acoustic receivers includes a second acoustic transmitter configured to transmit a second acoustic signal to the first acoustic transmitter and a non-acoustic signal to the other of the plurality of acoustic receivers upon transmission of the second acoustic signal, the first acoustic transmitter configured to generate the first acoustic signal upon receipt of the second acoustic signal.

3. The system of claim 1 further including a decoder to receive a model railroad digital command and control signal for at least one of the acoustic receivers as a synchronization mark when the first acoustic transmitter generates the first acoustic signal.

4. The system of claim 1 wherein the processor is configured to determine position of the first acoustic transmitter based in part upon an initial location for the first acoustic transmitter.

5. The system of claim 1 wherein the processor is configured to determine position of the first acoustic transmitter based in part upon possible solutions found in a storage.

6. The system of claim 5 wherein the possible solutions are positions on a first side of a wall.

7. The system of claim 1 wherein the first acoustic transmitter includes a frequency modulator to modulate the first acoustic signal according to a first modulation and the acoustic receivers include a frequency demodulator to demodulate the first acoustic signal according to the first modulation.

8. The system of claim 1 further including a second acoustic transmitter configured to generate a second acoustic signal with a second frequency, wherein the plurality of acoustic receivers are each configured to receive the second acoustic signal and the processor is configured to determine position of the second acoustic transmitter based at least in part upon when the second acoustic signal arrives at each of the acoustic receivers.

9. The system of claim 1 wherein the first acoustic transmitter is a resonant piezoelectric crystal that is impulse driven.

10. The system of claim 1 wherein the first acoustic transmitter is a single pulse generator.

1 1. The system of claim 1 wherein the first acoustic transmitter has a transducer to produce the first acoustic signal having a non-repeating wave shape.

12. The system of claim 1 1 wherein each of the acoustic receivers store a wave pattern having the non-repeating wave shape to compare with the first acoustic signal received by the acoustic receivers to determine time of arrival of the first acoustic signal at the acoustic receiver.

13. The system of claim 1 wherein each of the acoustic receivers include a transducer with a narrowband reception at the dominant frequency of the first acoustic signal.

14. The system of claim 1 wherein each of the acoustic receivers include a narrowband filter to discriminate the first acoustic signal from noise.

15. The system of claim 1 wherein each of the acoustic receivers store a copy of an first acoustic signal to be correlated with the first acoustic signal received by the acoustic receiver.

16. The system of claim 1 wherein the acoustic receivers each have a broadband microphone configured to receive a broadband first acoustic signal.

17. The system of claim 1 wherein the model railroad train is couplable to a model railroad track, the first acoustic transmitter configured to be activated to generate the first acoustic signal by an electric signal transmitted through the model railroad track.

18. The system of claim 1 wherein each of the acoustic receivers includes an first acoustic transmitter to transmit a second acoustic signal to the others of the acoustic receivers to be used by the processor for calibration of position determination.

19. The system of claim 1 wherein the processor uses reception of the second acoustic signal by the acoustic receivers

20. The system of claim 1 wherein the first acoustic transmitter is configured to transmit a synchronization mark signal to each of the plurality of acoustic receivers to be used to determine transit times of the first acoustic signal from the first acoustic transmitter to each of the acoustic receivers.

21. The system of claim 20 wherein the synchronization mark signal is one of the following: a radio frequency signal and an electric signal.

22. The system of claim 1 wherein the first acoustic signal is an ultrasonic signal.

23. The system of claim 1 wherein the processor is configured to determine position based upon triangulation of distances associated with when the first acoustic signal arrives at each of the acoustic receivers.

24. The system of claim 1 further comprising a plurality of audio speakers coupled to the processor, wherein the processor includes a sound generator configured to generate sound according to the determined position of the first acoustic transmitter.

25. The system of claim 1 wherein the processor is configured to determine position of the first acoustic transmitter to include elevation of the first acoustic transmitter.

26. The system of claim 1 further comprising a plurality of audio speakers coupled to the processor, wherein the processor includes a sound generator configured to generate sound according in part to the determined change in elevation of the first acoustic transmitter over a period of time.

27. A system comprising: a first acoustic transmitter configured to generate a first acoustic signal with a first frequency; an acoustic receiver each configured to receive the first acoustic signal; and a processor configured to determine position of the first acoustic transmitter based at least in part upon when the first acoustic signal arrives at the acoustic receiver.

28. A system comprising: a first acoustic transmitter configured to generate an first acoustic signal with a first frequency; a plurality of acoustic receivers each configured to receive the first acoustic signal; and a processor configured to determine position of the first acoustic transmitter based at least in part upon when the first acoustic signal arrives at each of the acoustic receivers.

29. For a model railroad train, a method comprising: generating a first acoustic signal with a first frequency from an acoustic transmitter affixed to the model railroad train; receiving the first acoustic signal with a plurality of acoustic receivers located at different positions; and determining position of the first acoustic transmitter based at least in part upon when the first acoustic signal arrives at each of the acoustic receivers.

30. The method of claim 29 further including transmitting a second acoustic signal to the first acoustic transmitter from a first of the plurality of acoustic receivers and transmitting a non-acoustic signal from the first of the plurality of acoustic receivers to the other of the plurality of acoustic receivers upon transmission of the second acoustic signal, wherein the generating the first acoustic signal occurs upon receipt of the second acoustic signal by the first acoustic transmitter.

31. The method of claim 29 further including receiving a model railroad digital command and control signal at one of the acoustic receivers as a synchronization mark upon generation of the first acoustic signal.

32. The method of claim 29 wherein determining position of the first acoustic transmitter is based in part upon an initial location for the first acoustic transmitter.

33. The method of claim 29 wherein determining position of the first acoustic transmitter is based in part upon possible solutions found in a storage.

34. The method of claim 33 wherein the possible solutions are positions on a first side of a wall.

35. The method of claim 29 wherein generating the first acoustic signal includes frequency modulating the first acoustic signal according to a first modulation and wherein receiving the first acoustic signal includes frequency demodulating the first acoustic signal according to the first modulation.

36. The method of claim 29 further including generating a second acoustic signal with a second frequency from a second acoustic transmitter, receiving the second acoustic signal by the acoustic receivers, and determining position of the second acoustic transmitter based at least in part upon when the second acoustic signal arrives at each of the acoustic receivers.

37. The method of claim 29 wherein the generating the first acoustic signal uses a resonant piezoelectric crystal that is impulse driven.

38. The method of claim 29 wherein the generating the first acoustic signal uses a single pulse generator.

39. The method of claim 29 wherein the generating the first acoustic signal includes generating the first acoustic signal having a non-repeating wave shape.

40. The method of claim 39 further including storing a wave pattern having the non-repeating wave shape and comparing the first acoustic signal received by the acoustic receivers to determine time of arrival of the first acoustic signal at the acoustic receiver.

41. The method of claim 29 wherein receiving the first acoustic signal includes receiving with a narrowband reception at the dominant frequency of the first acoustic signal.

42. The method of claim 29 wherein receiving the first acoustic signal includes using a narrowband filter to discriminate the first acoustic signal from noise.

43. The method of claim 29 further including storing a copy of an first acoustic signal to be correlated with the first acoustic signal received by the acoustic receivers.

44. The method of claim 29 wherein the receiving the first acoustic signal includes using a broadband microphone.

45. The method of claim 29 wherein the model railroad train is couplable to a model railroad track and the generating the first acoustic signal includes generating the first acoustic signal upon transmission of an electric signal through the model railroad track.

46. The method of claim 29 further including transmitting a second acoustic signal from one of the acoustic receivers to the other of the acoustic receivers to calibrate determination of the position of the acoustic transmitter.

47. The method of claim 29 further including transmitting a synchronization mark signal from the acoustic transmitter to each of the plurality of acoustic receivers to determine transit times of the first acoustic signal from the first acoustic transmitter to each of the acoustic receivers.

48. The method of claim 47 wherein the synchronization mark signal is one of the following: a radio frequency signal and an electric signal.

50. The method of claim 48 wherein the first acoustic signal is an ultrasonic signal.

51. The method of claim 48 wherein the determining position is based upon triangulation of distances associated with when the first acoustic signal arrives at each of the acoustic receivers.

52. The method of claim 29 further including generating sound according to the determined position of the first acoustic transmitter.

53. The method of claim 29 wherein the determining the position includes determining elevation of the first acoustic transmitter.

54. The method of claim 29 further including generating sound according in part to the determined change in elevation of the first acoustic transmitter over a period of time.