CH651387A5 - DISTANCE MEASURING DEVICE, IN PARTICULAR FOR PHOTOGRAPHIC CAMERAS. - Google Patents

Description

The invention relates to a distance measuring device, in particular for photographic cameras, with a transmitter for emitting a pulse to the object to be measured, a receiver for receiving the echo reflected by this object and a signal representing the distance from the time elapsing between emitting the pulse and recording the echo dissipative circuit.

In apparatuses in which ultrasound range finders are used, such as, for example, in photographic cameras according to the US patent application Ser.No. 3,371 of January 15, 1979 (hereinafter referred to as US-PA 3,371 / 79), a retroreflective or echo pulse, which was triggered by an ultrasound pulse emitted from the camera to determine the distance of a target object, is used in the rangefinder of the camera, to automatically focus the camera lens on the target object due to the distance-dependent duration of this pulse. Such range finders are exposed to interference signals and / or noise signals that are generated or recorded by the range finder. If an interference signal is received after the transmission of a measurement pulse but before the arrival of an echo pulse, it can result in an incorrect setting of the camera lens.

In order to reduce the sensitivity of such range finders to interference signals and / or electronic noise, all of them are described in US Pat. No. 3,371 / 79 cited above

Received signals above a predetermined threshold value are integrated by means of a capacitance. An object echo signal intended for further utilization is only generated in the range finder when the voltage across the integrating capacitance exceeds a predetermined threshold value.

The duration and shape of an echo signal coming from a target object now depends on various variables, such as the object distance, the shape of the target object, the path difference between different parts of the reflected signal, etc., which is why the described use of an integrator to reduce the sensitivity of the distance meter for interference signals can lead to distance measuring errors.

Interference signals can come from various sources. In the case of range finders of the type described above, for example, the ultrasonic energy is both transmitted and received by an electrostatic converter. Such a transducer has an oscillatable diaphragm which, on the one hand, is made to vibrate in the transmission mode by relatively strong high-frequency electrical signal pulses and converts it into ultrasound pulses, but, on the other hand, is vibrated in the receiving mode by relatively weak echo pulses of previously transmitted ultrasound pulses and converts them into electrical echo signals . Transducers of this type have common input and output terminals for transmit and receive operation, and therefore the receive circuit used to receive the echo signals must be masked out with respect to the strong transmit signals effective at the transducer terminals during transmit operation, i.e. be made ineffective by blocking on the input side. This suppression of the reception circuit is canceled again briefly after the end of a transmission pulse. With some transducers, after the vibrations caused by the transmission pulses have completely subsided and after the receiving circuit has been released for signal reception by the transducer terminals, the diaphragm again vibrates in a similar manner. These membrane beats form interference signals that interchange echo signals and can therefore lead to incorrect adjustment of the camera lens.

The range finder described in US Pat. No. 3,371 / 79 for a camera with automatic focusing contains a receiver with variable gain, the degree of gain of which is changed in a number of stages depending on the transit time of the measuring pulses. By increasing the gain with increasing transit time, the difference between the signal strengths of the echo signals coming from near and far target objects can be compensated. If the degree of amplification is changed step by step, electronic noise occurs briefly at the time of the change, which can likewise lead to a reception disturbance which causes an incorrect setting of the camera lens.

The amplitudes of all the above-mentioned interference and noise signals are time-dependent, and the signals are only of relatively short duration. For example, the interference signals resulting from membrane swings and the interference signals caused by a gradual change in the degree of amplification of the receiver have a duration of the order of magnitude of 200 | is.

The object of the present invention is to provide a distance measuring device which does not have the disadvantages mentioned above.

According to the invention, this object is achieved in the case of the distance measuring devices of the type mentioned at the beginning,

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as defined in the characterizing part of claim 1.

Examples of the invention will now be explained in more detail with reference to the drawings. Show it:

Figure 1 is a block diagram with those parts of an older rangefinder and working on the retroreflective principle, which are used to suppress interference and / or noise signals.

2A shows a typical signal at the input and output terminals of the electrostatic converter in the range finder according to FIG. 1, which are common for transmitting and receiving operation;

2B shows the time course of the voltage at the integrator of the interference signal suppression device according to FIG. 1;

2C shows the time course of the true object echo signal in the output of the device according to FIG. 1;

3 shows a diagram of the voltage at the integrator and the signal influencing this voltage, which is kept on a larger scale, similar to the representations in FIGS. 2A and 2B; the

4A and 4B show the course of an object echo signal, which comes from a relatively distant or from a relatively close object, after amplification;

5 shows a diagram on a larger scale of two echo signals of substantially different signal strength;

6 shows a block diagram of the range finder with an advantageous embodiment of the device for suppressing interference signals.

FIG. 7 is a logic flow diagram which explains the detection of true echo signals with the circuit according to FIG. 6, and

8 shows a diagram of the signal curve in the area of a true echo signal and the associated voltage curve at the integrator.

The preferred embodiment of the present rangefinder will be described with reference to the ultrasonic rangefinder according to the previously mentioned US-PA 3 371/79, and to facilitate this description in Fig. 1 the device for suppressing interference signals provided in this rangefinder shown in the block diagram.

1 shows a clock generator 12 which defines a time base for the entire measuring system and can be connected via a manually operated switch 16 to a supply battery indicated only by a pole 14. When the switch 16 is closed, a high-frequency generator is excited in the clock generator 12, the output signal of which is lowered by frequency division and then serves as a time base or clock signal for all functions of the range finder which are carried out in a predetermined time relationship.

By closing the switch 16, other components of the measuring system are also connected to the battery pole 14 via a line 11. When the clock generator 12 is switched on by closing the switch 16, its output signal acts on a transmission and fade-out stage 18, which in turn sends a signal to the terminals of an electrostatic converter 20 via a line 22, as a result of which an ultrasonic pulse is generated against a target object to be measured judges.

The receiving part of the range finder is connected to the converter 20 via the transmitting and masking stage 18, a full-wave rectifier (not shown) and a line 24. Since the input terminals of transducer 20 for transmit mode are identical to its output terminals for receive mode, it is important that the transmit signals are prevented from entering line 24 so that reception interference is avoided and transmit and echo signals cannot be confused in the receiver.

This is achieved by temporarily fading out or blocking the line 24 each time a transmission pulse is transmitted to the terminals of the converter 20 in the transmission and fading stage 18, this fading out during the entire transmission time of the transmission pulse and because of the subsequent decay processes beyond that.

If the line 24 is not blocked, it receives an electrical signal via the io transmission and masking stage 18 and the full-wave rectifier, not already mentioned, each time the converter 20 receives an echo from a previously transmitted ultrasound pulse. When the received signal received by line 24 reaches or exceeds a predetermined amount determined by a threshold detector 26, a gate 28 is opened by this detector, whereby a capacitor 30 serving as an integrator is connected to a constant current source 32. As soon as the voltage across the integrating capacitance 20 30 reaches or exceeds a predetermined amount, which is determined by a threshold value detector 34, a switching stage 36, e.g. can be designed as a Schmitt trigger, conductive and delivers an object echo signal 38 at its output.

25 The object echo signal 38 is then combined with further signals from the range finder in order to determine the object distance sought, but this process is no longer essential for the functioning of the device described above for suppressing 30 interference signals. Details of the parts of the range finder which determine the object distance are described in the already mentioned US Pat. No. 3,371 / 79.

To better understand the mode of operation of the older device for suppressing interference signals shown in FIG. 1, reference should now be made to the typical transmission and reception signal curve shown in FIG. 2A in the range finder and to the corresponding curve shown in FIG. 2B of the voltage at the integrator of this device, from which the graded object echo signal 38 according to FIG. 2C is ultimately derived.

FIG. 2A is an oscillogram of a sequence of transmit and receive signals that have occurred and amplified at the terminals of the electrostatic converter 20 in FIG. 1.

The multi-frequency transmission signal 40, which contains a plurality of frequencies 4S in the range from 50 to 60 kHz, is applied to the terminals of the converter 20, for example, for 1.1 ms. The vibrations of the diaphragm of the transducer 20 caused thereby subside completely after a further approximately 0.3 ms. The "beat phenomena" already explained in the 50 transducers occur approximately at the locations designated 42 and 44 after the useful vibrations of the transducer 20 have completely subsided. The first beat at 42 is below the threshold 46 set by the threshold detector 26, and therefore the voltage on the integrating 55 capacitor 30 remains at its original value zero. However, even if the beat 42 should exceed the threshold 46, only a very small proportion of it would charge the integrating capacitance 30 via the constant current source 32 because the input of the receiver of the 60 rangefinder is hidden for a total time of 1.6 ms. This masking makes the receiver insensitive to all signals that appear at the terminals of the converter 20 during the masking time. The second beat occurs at 44 and exceeds the threshold 46 of the detector 26 so that the integrating capacitance 30 is charged by the constant current source 32 during the time that the beat 44 exceeds the threshold 46. During this time, the tension on the integrating

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the capacitance 30, but reaches its maximum value 48 when the signal 68 reaches a threshold value 72 or even exceeds that provided for further signal processing, for example by the detector 26 in FIG. 1, a threshold value 50 which is determined by the threshold value detector 34 , the integrating capacity 30 is set while; it then drops linearly again over a period of time 74 evenly charged. The signal 70 decreases to the value zero without triggering an object echo signal 38 s whose amount is significantly greater than that of the signal 68. Other interfering signals, which, on the other hand, cause echoes from the axle to charge the integrating capacitance, from secondary loops of the directional diagram of the converter 30 for a longer period of time 76. This difference in the 20 detected objects is caused, which charging times lead to a difference in the resulting charger converter 20 are closer than the target object, apparent voltages, which subsequently measure the measured genes at 52, but do not exceed the threshold 46. The object distance desired from the strength of the received signals depends on an echo from the target object.

Received signal 54 finally exceeds that from the detector. After this introduction, the device shown in block diagram 26 in FIG. 6 for a considerable time can suppress interference in order to suppress interference within which the linearly increasing voltage on the in signals and electronic noise in the case of an ultrasound-tegrating capacitance 30 at 56 exceeding the threshold 50, a range finder is received. According to FIG. 6, one can, so that the detector 34 switches the switching stage 36 into the master clock 78 via a manually operated switch 82 to one of the states and in the output thereof a counter battery signal indicated only by a pole 80 turns on echo signal 38 appears. bar. After closing the switch 82, the integrating capacitance 30, as already explained, 78 excites a high-frequency oscillator in the clock generator, whose output signal always integrates when an amplified actual frequency or 20 is divided in frequency and then exceeds the threshold 46 as a time base or a certifiable echo signal . If, on the other hand, the magnitude of the apparent or actual echo functions in the rangefinder serves for all those related to one another in time. As a result of the closing signal falling below the threshold 46, the voltage across the switch 82 also decreases linearly to the value of zero via a line 81 to others of the integrating capacitance. The components of the rangefinder operating voltage set voltage curve at the integrating capacitor 30 and a 2s. After the clock generator 78 has been switched on, the time-stretched part of a received signal, which causes this sem an output signal to a transmit and fade-out stage 84, voltage curve, is shown in FIG. 3. While then a line 88 sends a transmission signal to the terminals for the time intervals 58 and 60, or the transmission and reception mode common terminals of an amplified reception signal exceeds a threshold value 62 and releases electrostatic converter 88 and applies this to the outside, thereby causing a linear voltage rise on the integration of an ultrasound pulse against a target object, the capacitance 30 is caused by the Kon charging the same, the distance of which is to be measured. The current source 32 from. At all other times, the receiving part of the range finder is with the converter 86, the voltage across the integrating capacitance either decreases via the transmitting and masking stage 84, one not shown, or it remains at the value zero. Because of the relatively ten full wave rectifier and a line 90 connected. As long as the decay time of the voltage at the integrating capacitor 35 already executes the threshold value 62 in connection with the older capacitance 30 shown in FIG. 1 (FIG. 1) after a noise signal has dropped by means for suppressing noise signals, this threshold value should be relatively high , it is essential that the transmission signals appearing at the terminals of the, in order to avoid the risk of superimposed charging of the integrated converter 86, from entering into capacitive capacitance due to a subsequent interference signal which prevents the line 90 from interfering with the Emp-could trigger a false useful echo signal, avoiding as small a portion of the range finder as possible.

to keep. A blanking circuit in the transmitter serves this purpose.

Typical amplified received signals, the echoes from the emitting and masking stage 84, which operate in the same way as the target object, have a profile according to the masking circuit in the stage 18 of FIG. 1.

4A or 4B. The receive signal 64 shown in FIG. 4A when line 90 is enabled by stage 84 and begins at a relatively high average and remains at "earlier" from transducer 86 upon receipt of an echo. 4B, with the received signal 66 according to FIG. 4B with an emitted ultrasound transmission pulse, an electrical signal begins with a relatively low mean value and is then supplied with an egg pulse, this is passed over via stage 84 to a relatively high mean value. Signal 64 is ty-synchronized via line 90 to an integrating Kapisch for an echo from a relatively close target object, capacitance in a sampling and evaluation stage 94, whereas signal 66 is typical for an echo from a re- Signal within time intervals that are in the relatively distant target object. The difference between the comparison with the total duration of the received signal and the relative amounts of these two echo or received signals can be too small (for example, every 25 | is during a received signal, the result is that the integrator 30 has a duration of 1.1 ms, depending on the signal curve ) is sampled on a random basis, the integration process begins sooner or later, as a result of which the capacitance in the scanning and evaluation stage 94 integrates variable, distance-dependent distance measuring 55 or sums the sampled signal voltage values. Of errors are introduced into the measuring system. These errors are derived from a continuous signal if the amount of low signal levels in the vicinity of the sum of the voltage values sampled by the detector 26 increases between the fixed threshold 46 of greater importance than for subsequent samples and if the rate of increase significantly higher signal levels . The cause of these errors can be explained for the total duration of a considerable part of the reception with reference to FIG. 5, in which the two received signals stretched over the same time line are equal to or greater than a predetermined minimum value. If the amount of the sum voltage on the integrie are shown. capacity at stage 94 always increases, which by

In FIG. 5, an AV amount determination stage 96 is ascertained from two reception signals of different strengths, and if len 68 and 70 of the same frequency are shown only once or more for a single period, the rate of increase of the sum voltage to explain why in a distance 65 ajs ejn is the specified minimum value, which is

If a knife is used with a wax rate determination stage 98 for suppressing interference signals, an echo signal which conducts the device according to FIG. 1 with a substantially AND gate 100 becomes conductive and applies one of the two different signal strengths to distance measurement errors. Inputs of a further AND gate 102. As soon as the sensor

fade and fade-out stage 84 releases line 90 in the manner already described, an interval timer 106 is started via line 104, which, for example, defines a time interval of 0.6 ms for the range finder described here. If the AND gate 100 remains conductive for the duration of 0.6 ms, a signal which appears in the output of the interval timer 106 0.6 ms after its start and which acts on the second input of the AND gate 102 turns this gate in transferred the conductive state. If the AND gate 102 is conductive, it causes the closing of a subsequent switching stage 108, which then delivers a true object echo signal 110 in its output. On the other hand, if the sampled voltage amounts between successive scans do not increase, which is determined by the stage 96, or if the rate of increase of the sampled voltage values is less than a predetermined minimum value, which is determined by the stage 98, one of the stages 96 and 98 becomes active NAND gate 112 is conductive and supplies a reset signal to the sampling and evaluation stage 94 and to the interval timer 106 via a line 114. This reset signal both reduces the voltage on the integrating capacitance in the sampling and evaluation stage 94 to the value zero and also resetting the timer 106 to the time zero, after which the next start signal for the interval timer must be waited for.

There now follows a more detailed description of the device for suppressing interference signals and electronic noise with reference to the logic flow diagram according to FIG. 7, the block diagram according to FIG. 6 and the representation of the signal curve and the corresponding voltage curve at the integrating capacitance in FIG.

8th.

On the upper time line in FIG. 8, an echo signal 54 coming from the target object with a duration of 1.1 ms can be seen in particular in the signal curve. Depending on the strength of the received signals, the respective voltage across the integrating capacitance of the sampling and evaluation stage 94 is shown above the lower time line in FIG. 8.

The flow chart of the device for interference signal suppression shown in FIG. 7 begins with the resetting of the interval timer 106 (FIG. 6) to the point in time zero (ST = 0), corresponding to step 118 in the flow chart, and the resetting of a storage location, for example in a digital computer which has stored information about the magnitude of the sum voltage Vp at the integrating capacitance of the sampling and evaluation stage 94 during the previous sampling process to the voltage value zero (Vp = 0), corresponding to step 120. The stored sum voltage Vp (which at step 120 has taken the value zero) is continuously subtracted from the voltage Vc at the integrating capacitance (AV = Vc-Vp), in accordance with step 134. The storage location for Vp is set to the new value in accordance with step 136, which, as already mentioned, is equal to the amount of the sum voltage Vc at the integrating capacitance of the sampling and evaluation stage 94 (Vp = Vc). A V is now compared according to step 138 with a predetermined small value TOL (close to the value zero). If A V does not exceed the minimum value TOL (alternative 140), this means that Vc has not grown with sufficient speed compared to the previous scanning process and the received signal has therefore dropped to or near the background noise. True echo signals can show beat effects, but they never decay to the basic noise level during a duration of the echo of 0.6 ms. Therefore, if A V is less than TOL, there is no true echo signal and the voltage Vc at the integrating capacitance is changed according to step 128

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reduced to zero again. The next interval time start must then be waited for in accordance with step 130. On the other hand, if AV is greater than TOL (alternative 142), the duration of the sampled interval is added to the interval timer 106 (FIG. 6) according to step 144 by step 144, and it is determined with step 146 whether the elapsed time since the receiver was released Time exceeds the value of 0.6 ms. If this is not the case (alternative 148), then the next sampling interval must be waited for again according to step 150. However, if the elapsed time exceeds the value 0.6 ms (alternative 152), a true object echo signal is triggered in accordance with step 154.

FIG. 8 shows the part of the signal curve according to FIG. 1A containing the echo signal 54 and below it the voltage curve 156 corresponding to the echo signal at the integrating capacitance of the sampling and evaluation stage 94 according to FIG. 6. The charging and discharging of the integrating capacitance in stage 94 during previous sampling periods is illustrated in FIG. 8 by the voltage curve 158. If the voltage at the integrating capacitance increases, a distinction must be made as to whether it is an interference signal or a true echo signal. Interference signals usually do not last for 0.6 ms or more in measuring systems of the present type. Therefore, if the slope of the integrated signal in the time diagram goes against TOL / AT (where AT corresponds to a sampling time interval), or if the slope is less than a predetermined value (if AV is less than TOL), which corresponds to alternative 140 in FIG. 7 , the system is reset to the initial state (steps 118 and 120) as soon as the integrating capacity has been discharged (step 128) and the sampling time interval has elapsed (step 130).

Distance measuring systems of the type shown in FIG. 1, which belong to the prior art, use the shape or amplitude of the echo pulses to distinguish the echoes from the target object of interference signals. In contrast to these older systems, the device according to the invention can recognize a signal which results from the energy reflected by a selected target object, regardless of the shape of the reflected object, the signal strength and the pulse shape. The arrival time of the leading edge of a true echo signal can be determined within the limits of a sampling interval by determining the time that elapses between the first sampling interval that detects an actual or true echo from the target object and the generation of an actual or true object echo signal then subtracted from the time in which the actual or true object echo signal is generated.

The beat phenomenon 44 in FIG. 1A could be thought to be a true signal. However, since such beat phenomena usually do not last longer than 140 jxs and the duration of electrical pulse waves usually does not exceed 300 ns, the voltage change between adjacent sampling intervals will be smaller than the minimum value TOL even before the assumed interval time of 0.6 ms. As a result, such beat phenomena or electrical jags fail the test at step 138 in Fig. 7 and trigger the logical alternative path 140, reducing Vc to zero and waiting for a new scan. A major advantage of range finders according to the invention is thus their insensitivity to membrane swings and jagged interference signals of an electrical and mechanical nature. The generation of false object echo signals is prevented because interference signals of the type specified do not pass the 0.6 ms test. The accuracy of the range

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Knife is made more exclusive by the exclusive utilization of this with the older distance

Echo signals increased, which is the case taking into account their knives, whereby the accuracy of the range

Duration are identified, which improves the result of the distance measurement.

The measurement of the voltage does not depend on the applied amplification or the sum of the properties which depends on the shape of the signals. s the following measuring device say the following:

In the typical retroreflective principle waveform shown in FIG. 2A, the true echo signal has a duration of 1.1-1.5 ms. For one, the leading edge of an echo signal can be clearly recognized

Echo signal of this duration has been determined empirically, and so distinguish this signal from interference signals. For the purpose of evaluating a signal section of 0.6 ms duration, identifying the leading edge of true echo signals is sufficient to recognize the signal as a true echo signal and to distinguish between true and false echo signals.

to be distinguished from interference signals. The duration of the total is all received signals in time intervals,

the total number of sampling intervals should be empirically the same as the relatively short compared to the total duration of the real, sufficiently determined partial signal duration. These time echo signals are repeatedly sampled for a duration of 0.6 ms, during which a received signal and the sampled signal values are summed, whereby it must continue to determine whether one of the test criteria of the echo recognition system determines whether the amount of the sampled signal value system to satisfy is not necessarily for increasing and whether the rate of increase of these signal values over a

all rangefinders that such a detection system maintains a predetermined minimum value. One for the removal

use the cheapest. The determination of a period of time to measure the true object echo to be used.

During which an echo signal continuously lasts is only generated in the rangefinder if it has to, but for the correct functioning of such a system, the sum of the sampled signal values of

essential. Process to scanning process increases and when the increment

When a true echo signal arrives, AV is greater than guessing over a period of time that is a significant portion TOL (alternative 142 in FIG. 7) and the signal takes longer than the duration of a true one coming from the target object to be 0.6 ms (alternative 152 in FIG . 7). These two criteria make up an echo signal, above which the minimum cause the system to have a true object echo signal worth 25.

to deliver (step 154) and a subsequent distance measurement signal according to the invention, which always corresponds to a runtime that is 0.6 ms shorter than the measuring device and is relatively insensitive to interference signals, object echo signal. This means a constant and / or electronic noise, mainly because of this, constant measurement errors of about 15 cm (0.5 feet), which is easily compensated for as a criterion for distinguishing between mechanical or electrical in the measuring device and apparent echo signals, not the signal amplitude, can be used to indicate the leading edge of an electrical signal, but rather the signal curve during a specific signal, albeit indirectly. takes time. There is a time delay

Because of its superior ability to suppress the echo signal edges selected for processing and the resulting interference signals, the device according to the invention can with an apparent transit time change, but which record all incoming signals for all target objects and not only those objects which are the same size and therefore easily switch over to electrical objects a prescribed threshold of how or mechanical compensation can be used.

C.

4 sheets of drawings

Claims (4)

651387 PATENT CLAIMS 1.Distance measuring device, in particular for photographic cameras, with a transmitter for emitting a pulse to the object to be measured, a receiver for receiving the echo reflected by this object and a circuit which derives the distance from the time elapsing between emitting the pulse and recording the echo , characterized in that this circuit (108) is preceded by a discrimination stage (94,96,106,100,102) which is connected downstream of the receiver (84, 86) and which checks at least the strength of each potential echo in successive time intervals within a predetermined period of time for the presence of a minimum value and integrating evaluation stage (94) contains (Fig. 6). 2. Device according to claim 1, characterized in that in the output of the scanning and integrating evaluation stage (94) a device (96) for checking whether the integrated signal is growing between the successive scanning processes, and a device (98) is also provided, which checks whether the growth rate of the integrated signal between successive sampling processes is greater than or equal to a predetermined minimum value. 3. Device according to claim 2, characterized in that a device (112, 114) for returning the integrated signal to an initial value is provided, which can be activated if the integrated signal does not increase during successive scanning processes or the rate of increase of the integrated signal is less than the predetermined one Minimum value is. 4. Device according to one of claims 1 to 3, characterized in that a device for determining the time of occurrence of the leading edge of a true echo is provided, which identifies the time after which the echo coming from the measurement object by the discrimination stage (94,96,106,100,102) is subtracting the predetermined period of time.