Abstract:
In order to provide a system for measuring relative velocity of a surface with the accuracy of a laser velocimeter but avoiding the problems associated with operating down to zero speed and with signal dropout during a run, a tachometer is coupled to measure the relative velocity and the output of the tachometer corrected using the output of the laser velocimeter thereby giving laser velocimeter accuracy over the major portion of the velocity range but still permitting operation down to zero speed and operation when signal dropout occurs.

Description:
This invention relates to laser velocimeters in general and more particularly to a laser velocimeter which is capable of operating down to zero speed and avoids the possibility of signal dropout during a run. 
     Various laser velocimeters have been developed in the past. Typical of these are the velocimeters disclosed in U.S. Pat. No. 3,432,237 and U.S. Pat. No. 3,525,569. In a laser velocimeter, a source of radiation such as a laser, directs a substantially monochromatic beam toward a reference surface. The reflected radiation is passed through an optical aperture or plurality of slits located near the source and received by a photomultiplier tube which has its anode connected to a frequency meter, the output of which is a function of the relative velocity between the radiation source and a reflecting surface. 
     Two limitations which are encountered in laser velocimeter applications are the incapability of operating down to zero speed and the possibility of signal dropout during a run. A further general problem is the delay incurred at the start of a run during which time the frequency tracker searches for the signal, i.e., the time to complete acquisition and begin tracking. That is to say, in a more complex system, rather than using a simple frequency meter, a frequency tracker which is adapted to lock onto the signal is utilized. As is well known, frequency trackers require certain acquisition time before they lock on and begin tracking. Furthermore, there are applications to industrial processes as well as to vehicle navigation in which, although true zero speed operation is not required, the measurements to be made involve length or distance. In such applications, good accuracy is impossible using a velocimeter alone. The same is true with respect to signal dropout which may not be possible to eliminate completely. Unquestionably, such signal dropout could result in a complete destruction of the accuracy of a distance measurement. 
     Thus, the need for an improved velocimeter which is capable of operating down to zero speed and which avoids problems associated with signal dropout during a run, becomes evident. 
     SUMMARY OF THE INVENTION 
     The present invention provides such a device. In essence, the system of the present invention accomplishes this by using the laser velocimeter as a velocity reference in a feedback loop around a tachometer connected mechanically to the moving strip, in the case of an industrial measurement or to the vehicle drive train, in the case of a vehicle. A tachometer operates down to zero speed without difficulty. However, it will not provide adequate accuracy by itself. Thus, in the system of the present invention, the laser velocimeter corrects the tachometer errors at all speeds in the range of the velocimeter. If dropout occurs, the tachometer continues to provide an output, using the last remembered correction from the velocimeter. An electrical analog of speed supplied by the tachometer is used directly or as a control for the local oscillator in the frequency tracker of the laser velocimeter. This places the tracking filter of the frequency tracker at or near the correct frequency at all times thereby eliminating the need for a search with its attendant time delay during acquisition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the basic configuration of the velocimeter of the present invention. 
     FIG. 2 is a block diagram illustrating the velocimeter with separate voltage to frequency converters. 
     FIG. 3 is a diagram similar to FIG. 3 showing operation with a multiplicative correction. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, a surface 11 exhibiting relative motion with respect to the velocimeter is illustrated. This surface could be the ground below a vehicle such as a jeep, for example, or could be a moving belt in an industrial process, for example. In either case, an accurate measurement of velocity of relative motion, along with possibly a measurement of distance, i.e., the integral of velocity, is required. In accordance with the present invention, there is a mechanical linkage 13 to a tachometer 15. In the case of a moving belt, the tachometer would be mechanically coupled to the belt. In the case of a vehicle, the tachometer will be coupled to the drive system of the vehicle. Also provided is a laser velocimeter sensor 17 which can be constructed in accordance with either or both of the aforementioned patents. The output of the tachometer 15 which is designated as a function G [v] is summed in a summing junction 19 with a signal designated e, to be described in more detail below. The output from this summing junction is the input to a voltage to frequency converter 21 which performs the function of a local oscillator for frequency tracking the frequency output of the laser velocimeter sensor 17. The voltage to frequency converter must be capable of operating down to zero frequency. A circuit suitable for this application is described in Engineering Design News, June 5, 1974, pp. 49 to 54. The output of the voltage to frequency converter 21, in addition to fulfilling its function as a local oscillator, also supplies a continuous frequency analog of speed, i.e., the function K [G (v)+e] where e is the correction to the tachometer output obtained from the laser velocimeter sensor 17. The output from the voltage to frequency converter 21 is fed to a mixer 23 where it is mixed with the output of the laser velcoimeter sensor. At operating speeds within the normal range of the laser velocimeter sensor 17, a signal f v  is available. It is mixed with the local oscillator signal f T  and then processed in a sine-cosine descriminator 25 in a manner commonly used in frequency tracking. A signal to noise ratio detector 27 monitors the quality of the signal holding the feedback path open by means of a switch 29 [shown as a mechanical switch but preferably a semi-conductor switch] when the signal to noise ratio is below a threshold and closing the loop when the signal to noise ratio is adequate. The output of the sine-cosine discriminator and integrator is an error signal, e, representing the difference between the frequency f v  and the frequency f T . This signal is fed back to the summing junction 19 and used to adjust the frequency of the output of the voltage to frequency converter 21 until f T  equals f v . 
     FIG. 2 illustrates an alternate embodiment of the present invention. The tachometer 15, laser velocimeter sensor 17, mixer 23, sine-cosine discriminator and integrator 25, signal to noise detector 27 and switch 29 are as in the previous embodiment. The primary difference in this embodiment is that separate voltage to frequency converters are utilized for providing the output frequency designated f&#39; T  and as the local oscillator. However, the two frequencies are locked together in frequency. The reason for using this embodiment is that presently available voltage to frequency converters which have a zero frequency capability are limited to a maximum frequency of about ten KHZ whereas the local oscillator frequency in standard velocimeter trackers must operate up to 800 KHZ. This requires separating the two functions if the normal operating frequency of the standard velocimeter is to be used. In this arrangement. the output of the tachometer is again provided to a summing junction 19 at the input to a voltage to frequency converter 21a. However, this voltage to frequency converter 21a only has a range of 20 to 800 KHZ. This summing junction is fed with the error signal from the sine-cosine discriminator and integrator 25 through the switch 29 and supplies its output back to the mixer 23 as before. Its output is then divided by 80 in a divider, e.g., a digital counter, 35 to give the frequency f LO/80 . This output is coupled into a single pole double throw switch 37, the output of which is the input to a frequency to voltage converter 39 which is then coupled through a phase detector 41 and a switch 45 shown as being mechanically coupled to the switch 29 and again responsive to the signal to noise detector. Once again, an electronic switch could also be used herein with the two switches electrically coupled. The output of the phase detector and low pass filter 41 is thus coupled through the switch 45 to a second summing junction 19a  receiving its other input from the tachometer 15. The switch 37, which again will preferably be a semi-conductor switch, along with the phase detector in the phase detector and low pass filter unit 41 are driven by a clock 47 operating at a relatively low frequency, 100 HZ, for example. Switching at the input and coupling through the converter 39 and phase detector 41 results in an output signal in analog form representative of the error between the input frequencies f&#39; T  and f LO/80 . This error signal designated e&#39; is summed with the output of the tachometer 15 at the summing junction 19a and thus corrects the output of the frequency of voltage converter 21 which has a range of from zero to ten KHZ and the frequency output f&#39; T  of which represents the sensed velocity. 
     FIG. 3 shows a modification of the circuit of FIG. 2. Rather than using an additive correction as in FIG. 2, it uses a multiplicative correction. The only differences in this circuit as compared to the circuit of FIG. 2. is that the summing junction 19a is replaced by a voltage controlled attenuator 49. The voltage controlled attenuator in normal operation obtains its input from the phase detector and low pass filter 41. However, upon operation of the signal to noise detector 27, it is switched to the output of a memory circuit 51. Memory circuit 51, which may be a sample and hold type circuit continuously samples the output of the phase detector and provides an output equal thereto. In the case of dropout during a run, the signal to noise detector 27 responds causing the switch 45a, which is now a single pole double throw switch, to connect the memory circuit 51 to supply its output to the voltage controlled attenuator 49. This results in a corrected output frequency f&#39; T  which is equal to AKG [v]. Again, because of the closed loop, the output of the phase detector and low pass filter 41 adjusts itself so as to provide a multiplicative correction to the voltage control attenuator which will result in the output of the voltage to frequency converter 21 equaling the divided output frequency of the voltage to frequency converter 21a. 
     The use of a multiplicative correction has a number of advantages. The additive correction of FIGS. 1 and 2 is a bias type error correction, whereas the multiplicative correction of FIG. 3 is a slope error correction. FIGS. 1 and 2 did not illustrate any provision for memory. Such a provision would be possible, for example, by providing a memory circuit like that of FIG. 3 at the output of the sine-cosine discriminator 25 coupled to switch 29 or in the case of FIG. 2 in the same location as in FIG. 3. However, it must be remembered that additive corrections vary with speed so that a fixed correction is valid only a particular speed. As the speed goes to zero, in fact, a last remembered additive correction would cause an ever increasing error. Such a remembered error would be useful in the case of velocimeter dropout, assuming the speed remained essentially constant. However, with dropout and a significant speed change, such an error correction would not be valid and could increase rather than decrease error. Thus, if such a correction is used with the embodiment of FIGS. 1 and 2, additional means must be provided to ensure that the remembered correction is only applied so long as there is a dropout and no significant speed change. This, of course, would require additional detection circuits to determine when and where not to use the correction. The multiplicative correction of FIG. 3, in contrast, provides a fixed value which is valid over a range of speeds. Using such a correction, as the speed drops down to zero, one would get a true zero output. In the case of dropout the last remembered multiplicative error correction to the tachometer very likely is beneficial to retain. However, it should be noted that mechanical slippage and tachometer non-linearity will limit the range of speed over which a fixed multiplicative correction remains accurate.