Signal simulation apparatus and method

Abstract

The present invention relates to a method of simulating an initial component of a signal to approximate a component of a reference signal, the method characterized by the steps of: i. generating a source signal which includes at least one harmonic component, and ii. determining the average amplitude and duration of the source signal, and iii. referencing the amplitude of the reference signal component to be simulated, and iv. integrating the source signal over a period of time sufficient to produce a value for the signal component amplitude approximate to the reference signal component amplitude.

Claims

1. An imaging system, comprising: a range assessment system operable to assess a range of an object; a light source operable to illuminate the object with a source signal; and a signal simulation apparatus operable to provide modulation signals to the light source and the range assessment system, wherein the modulation signals are compensated to mitigate error arising from harmonics in the source signal, wherein the signal simulation apparatus is operable to vary a respective phase of the modulation signals and to vary a respective presentation time for each phase, and wherein the presentation time used for each particular phase increment is proportional to an absolute value of an amplitude change experienced over the particular phase increment.

2. A system as claimed in claim 1, wherein the range assessment system comprises an optical camera transducer.

3. A system as claimed in claim 2, wherein the range assessment system further comprises an image intensifier.

4. A system as claimed in claim 1, wherein the signal simulation apparatus is operable to adjust a phase of the modulation signals supplied to the light source to mitigate error arising from the harmonics.

5. An imaging method, comprising: providing modulation signals to a light source and a range assessment system; illuminating an object with a source signal generated by the light source in response to the modulation signals; operating the range assessment system in response to the modulation signals; and mitigating error arising from harmonics in the source signal by adjusting the modulation signals including varying a respective phase of the modulation signals and varying a respective presentation time for each phase, wherein the presentation time used for each particular phase increment is proportional to an absolute value of an amplitude change experienced over the particular phase increment.

6. A method as claimed in claim 5, wherein the source signal includes at least one harmonic component.

7. A method as claimed in claim 5, further comprising integrating the source signal over a period of time sufficient to produce a value for a signal component amplitude corresponding to a reference signal component amplitude.

8. A method as claimed in claim 5, further comprising compensating the modulation signals to phase shift the source signal.

9. A method as claimed in claim 5, further comprising phase shifting the source signal in accordance with a resolution.

10. A method of claim 5 including applying periodic phase changes to the source signal.

11. An imaging method, comprising: providing modulation signals to a light source and a range assessment system; illuminating an object with a source signal generated by the light source in response to the modulation signals; operating the range assessment system in response to the modulation signals; determining a respective presentation time by sampling a sine wave over a particular period, with sample resolution equal to an electronic phase step resolution, and setting the respective presentation time as the ratio of separation between resulting amplitude values; and mitigating error arising from harmonics in the source signal by adjusting the modulation signals including varying a respective phase of the modulation signals and varying the respective presentation time for each phase.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further aspects of the present invention will become apparent from the following description which is given by way of example only and with reference to the accompanying drawings in which:

(2) FIG. 1 shows an exemplary schematic diagram of a set of components employed to provide a range imaging system which can employ the present invention, and

(3) FIGS. 2A and 2B illustrate phase measurement errors introduced by the presence of harmonic components in the signals used to modulate the light source and camera of the system shown with respect to FIG. 1, and

(4) FIGS. 3A-3D illustrate a prior art sinusoidal signal generation technique which employs a group of square wave signal generation circuits which have their output summed instantaneously to provide an sinusoidal signal, and

(5) FIG. 4 shows a plot of a section reference signal to be simulated when used to calculate the presentation time of each regular phase increment employed in a preferred embodiment, and

(6) FIGS. 5A and 5B illustrate timing diagrams over an integration period of the phase adjusted source signal, and a simulation signal provided in the embodiment shown with respect to FIG. 4, and

(7) FIGS. 6A-6D illustrate experimental results of the reduction in measured phase error using the present invention, and

(8) FIGS. 7A and 7B illustrate two alternative source signals which can be employed with the present invention, and

(9) FIGS. 8A-8H illustrate the effect of employing the alternate source signals shown with respect to FIGS. 7A and 7B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(10) FIG. 1 shows an exemplary schematic diagram of a set of components which provide a range imaging system (generally illustrated by arrow 1) which can employ the present invention.

(11) As can be seen from FIG. 1 an FPGA (2) can be employed as a signal simulation apparatus to provide a modulation signal to both an image intensifier (3) associated with an optical camera transducer (4), in addition to a light source (5) used to illuminate an object (6) to have its range assessed. The resulting data is transmitted to and processed by a processing device (7). Further details with respect to these particular types of range imaging systems are also disclosed in PCT Publication No. WO2004/090568.

(12) As can be appreciated by those skilled in the art that FPGAs are readily available and may be obtained at comparatively low cost when compared with analogue signal generation circuitry, and in high-precision phase-locked particular pure sine wave generation circuits. An FPGA can readily generate square wave signals which have high order harmonic components, where the square wave signals are phased locked with respect to one another. Phase locking the modulation signals produced by the FPGA ensures that any phase differentials measured by the range imaging system accurately represent the range of the object illuminated.

(13) FIGS. 2a and 2b illustrate phase measurement errors introduced by the presence of harmonic signal components in the signals used to modulate the light source and image intensifier of the system shown with respect to FIG. 1.

(14) As can be seen from FIGS. 2a and 2b the use of square waves as modulation signals does introduce a regular source of error. One approach which could remove this error would be through the preparation of a calibration of the system for each modulation frequency employed.

(15) However, those skilled in the art would appreciate that the use of multiple calibrations introduces its own problems in terms of both preparing and maintaining calibrations for each and every modulation frequency to be employed.

(16) In order for harmonic signal components to affect the phase measurement, each harmonic must be present in both the light source waveform and image intensifier waveform. If one of those waveforms is effectively reduced to a sinusoid, then the harmonics not present in that waveform are eliminated, As such, the present invention may be used to remove or at least mitigate this source of error without requiring the use of calibrations.

(17) FIG. 3 illustrates a prior art sinusoidal signal generation technique which employs a group of square wave signal generation circuits. These signals are summed instantaneously to provide an output sinusoidal signal.

(18) FIG. 3d illustrates a sinusoidal simulation signal with a dotted outline of the actual sinusoidal signal simulated. The simulation signal is simulated through the instantaneous summation of the three phase separated square waves (FIGS. 3a, 3b and 3c) illustrated, A regular phase increment is applied between each square wave, while variations are seen in the maximum amplitude of the waves.

(19) As more signal generation circuits are made available the resolution of the resulting simulation signal is improved and further higher order signal harmonic components are removed. However, the use of additional signal generation circuits increases the manufacturing costs and system design complexity of any apparatus which is to employ this type of instantaneous summation. This process also generates an inherently analogue output signal, which negates the benefits of using a digital system.

(20) FIG. 4 shows a plot of section reference signal to be simulated used to calculate the presentation time of each regular phase increment employed in a preferred embodiment.

(21) The method of calculating presentation time illustrated samples a sine wave over the period 90 to 90, with sample resolution equal to the electronic phase step resolution to be used. The presentation time can therefore be taken as the ratio of separation between the resulting amplitude values.

(22) In the case shown, four samples are taken symmetrically about 0 with 45 separation (i.e. 67.5, 22.5, 22.5, 67.5) giving three ratios (1, 2, 1). It is also possible to take five samples over the same period (90, 45, 0, 45, 90), or four samples with different values (e.g. 80, 35, 10, 55). There are an infinite number of possible ratios, with the example values used (1, 2, 1) being one case only.

(23) It should be appreciated that a selection of the resolution of the samples taken is directly determined by the application in which the present invention is implemented. Factors such as the equipment available or required accuracy will influence the sample resolution which is selected to be implemented.

(24) FIG. 5a, 5b illustrates timing diagrams over an integration period of the phase adjusted source signal and a simulation of a signal provided in the embodiment discussed with respect to FIG. 4.

(25) In particular FIG. 5a illustrates the form and phase of the source signal applied over a variable presentation time. As can be seen from FIG. 5a presentation time varies in accordance with the scheme discussed and illustrated with respect to FIG. 4 for each of the phase increments applied to the source signal.

(26) The effective result of this approach is illustrated with respect to FIG. 5b which shows an effective simulation signal when compared with the fundamental frequency of reference sine wave in dotted outline. Those skilled in the art should obviously appreciate that an effective simulation signal is illustrated but this signal does not actually exist at any instantaneous point in time, but provides the same result due to its action over the integration time period involved.

(27) FIGS. 6a, 6b, 6c and 6d illustrate experimental results with regard to phase measurement error introduced by harmonic signal components in the signals used to modulate the light source (5) and image intensifier (3) of the system (1) shown in FIG. 1, and the effect of the present invention on same.

(28) FIG. 6a illustrates the measured phase error of the system (1) using a traditional homodyne (no phase increment) method. The principal systematic error is due to the 3rd and 5th order harmonic components of the signal.

(29) The linear variation from left to right is due to the heating of the particular experimental set up used to obtain these results during each acquisition sequence, as the extended period of operation required to obtain thermal equilibrium was not sustainable. This should be understood to be a limitation of the particular componetry, and not the invention itself.

(30) The reduction in measured phase error using the present invention may be seen in FIG. 6b. FIG. 6b illustrates the resulting error with a phase increment of 45 according to the method described with reference to FIG. 4. It may be seen that the error has been reduced by 8 times by the application of the present invention, due to the elimination of the error caused by the 3rd and 5th harmonics.

(31) FIGS. 6c and 6d illustrate the resulting error with phase increments of 22.5 and 3 respectively. In FIG. 6d the present invention has eliminated the error due to harmonics up to the 119th order harmonic, from which point the systematic error is negligible in comparison to other sources of systematic error.

(32) FIGS. 7a, 7b illustrate two alternative source signals which can be employed with the present invention, where the signal of FIG. 7a has a duty cycle of 50 percent and a signal of FIG. 7b has a duty cycle of 25 percent.

(33) As can be seen from FIG. 7a the dotted outline illustrates a reference sinusoidal signal with a fundamental frequency which is to be simulated. This may be contrasted with the square wave source signal shown with respect to FIG. 7b which has a duty cycle of 1:3 or 25 percent. As can be clearly seen in FIG. 7b, the amplitude of the simulated signal is significantly greater using a duly cycle reduction approach. Both waveforms have the same average power, while neither waveform contains error causing odd order harmonics

(34) FIGS. 8a-8h illustrate three alternate source signals along with their resultant magnitude of their fundamental frequency and harmonic components respectively before and after the present invention has been employed.

(35) FIG. 8a illustrates a sinusoidal modulation waveform in the Time Domain. FIG. 8b shows the magnitude of the components of the same waveform in the Frequency Domain. As the sine wave consists of only the fundamental frequency component, no harmonics and their associated error is present.

(36) FIG. 8c illustrates a square modulation waveform with a 50% duty cycle, having the same average power as the sine wave of FIG. 8a.

(37) FIG. 8d shows the magnitude of the components of the waveform of FIG. 8c in the Frequency Domain. The odd harmonic components may be seen.

(38) Applying the present invention, the harmonic components are eliminated, as seen in FIG. 8e. However, it should be noted that the magnitude of the fundamental frequency component has been reduced. With this reduction, the signal to noise ratio is decreased and the precision of the range measurement is subsequently impacted.

(39) FIG. 8f illustrates a square modulation waveform with a 25% duty cycle, again with the same average power as the waveforms in FIGS. 8a and 8c.

(40) In FIG. 8g the magnitude of the components of the same waveform may be seen in the Frequency Domain. Although the even harmonics are illustrated, they may be effectively ignored as they do not influence the range measurement with normal implementation of the present invention (capturing an even number of samples).

(41) When the present invention is employed, the odd harmonics are eliminated as illustrated in FIG. 8h. The magnitude of the fundamental frequency component is significantly greater than that of the waveforms shown in FIG. 8b or 8e.

(42) As a result the signal to noise ratio and hence the precision of the measurement is increased. When coupled with the increased accuracy due to reduction in systematic error due to harmonic components, the present invention provides higher measurement performance than existing systems which use either true sinusoidal or purely square modulation.

(43) Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof.