GB2195181A - Ultrasonic distance measuring device - Google Patents

Abstract

There is provided a non-contact distance measuring device in which a sound wave (106) is transmitted towards a target object (107) with the time taken for the second wave (106) to travel to and from the target object (107) being used to determine the distance. The distance is determined by processing means which include a variable gain amplifier the gain of which is increased automatically with propagation time of the sound wave. Provision of the variable gain amplifier results in the degradation of the sound wave during travel to and from the target object (107) being compensated. <IMAGE>

Description

SPECIFICATION Non-contact distance measuring device The present invention relates to a device for non-contact distance measurement.

It is well known to measure the distance to an object by determining the propogation time of a waveform reflected from the object.

Whereas electromagnetic waves are conventionally used for measurements in air and sound waves are used for measurements in a liquid, it has already been proposed that ultrasonic waves be used for distance measurements in air. This is suitable for relatively short range measurements, say up to 20 meters, and is particularly suitable for implementation in a small hand-held device. Such a device is described in German Patent No. 2 515 087.

The device described in the German Patent includes the desirable features of providing for adjustment of the frequency of the transmitted ultrasonic waves and provides a fixed compensation for the distance of the ultrasonic transducers from a measurement reference point, which is necessarily taken as a point on the device casing. However, it has been found that the performance of the device can be improved and the present invention is directed to such improvements.

According to a first aspect of the present invention there is provided a non-contact distance measuring device comprising transmitter means for transmitting a sound wave towards a target object, receiver means for receiving at least a portion of the sound wave as reflected by the target object and processing means which monitor the propogation time of the sound wave so as to determine the distance of the target object, the processing means including a variable gain amplifier the gain of which is increased automatically with propogation time of the sound wave whereby degradation of the sound wave during travel to and from the target object is compensated.

The device of the present invention thus has the major advantage that the received sound wave has effectively a constant amplitude for a given target object independent of the distance of the target object, subject to the maximum range of the device. This significantly improves the effeciency and reliability of the device.

Preferably, the device of the present invention includes means which inhibit the receiver means for a relatively short period subsequent to operation of the transmitter means. This prevents erroneous operation of the device as a result of reflections from nearby objects such as the operator's body.

Advantageously, the output from the variable gain amplifier is processed by a bandwidth limiting circuit and this significantly reduces any "noise" in the output from the amplifier.

An additional feature of the present invention is the optional inclusion of gain control means which compensate for variations in the efficiency of the transmitter and receiver means. Preferably, the gain control means also compensate for variations in performance of other components of the device.

Beneficially, the device of the present invention includes variable compensation means providing the facility to compensate for variation in the distance between the device and the reference position from which the distance to the target object is to be measured. This should be contrasted with the corresponding compensation provided in the device described in the German Patent in which the reference position must be fixed relative to the device.

The present invention enables greater flexibility in practical operation of the device.

The distance measuring device described in German Patent 2 515 087 includes the provision within the casing of the device of an electronic calculator. The calculator has the ability to store the results of distance measurements. An optional feature of the present invention is the provision of the same facility with the improvement of separate storage of two or three subsequent distance measurements and automatic calculation of the product of the two or three subsequent measurements. This facility provides the automatic calculation of the area and/or volume enclosed by the objects from which the subsequent distance measurements are taken.

In the most preferred arrangement of the present invention, the majority of the components of the device are implemented in a single integrated circuit.

Other distinguishing features of the preferred embodiment of the present invention will become apparent from the following detailed description.

An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of an embodiment of the invention; Figure 2 is a functional block diagram of the embodiment shown in Figure 1; Figure 3 is a detailed circuit diagram of the embodiment shown in Figure 1; Figure 4 is a timing diagram showing the relationship between signals generated in the circuit of Figure 3; Figure 5 illustrates a modification of the embodiment shown in Figure 1; and Figure 6 illustrates a further modification of the embodiment shown in figure 1.

There is one general and two specific restraints for using sound waves in air in order to measure distance. One is that any person near the measuring device will hear the transmitted sound, which may be very annoy ing if repeated measurements are made. The other two restraints are that to transmit efficiently, reflect and receive a sound wave, the transmitter/receiver and target object must have dimensions comparable to the wavelength of the sound used. Ultrasonic sound waves are not audible to the human ear and their short wavelength allows a hand-held device to be used to measure the distance to relatively small target objects. Such a device has many applications apart from the simple display of measured distance, one example of which is distance measurement in an autofocus camera.

As shown in Figure 1, the distance measuring device comprises a housing 101 in which is provided an operating button 102, a transmitter 103, a receiver 104, a display 105 and a scale change button 110. Transmitter 103 transmits an ultrasonic sound wave 109 which is reflected from a target object 108 and received by the receiver 104.

Line 107 indicates the position from which actual distance measurement occurs and line 106 indicates a position from which distance measurement may be required. Line 106 may or may not coincide with the end of the housing 101 and the separation of lines 106 and 107 is compensated for in the distance measurement.

The basic principle of operation of the present invention is measurement of the time taken for a train of ultrasonic pulses to travel between the transmitter 103, the target object 108 and the receiver 104. Obviously, this involves the use of a timing unit. But by appropriate selection of the frequency of the timing unit, a simple display of a count taken directly from the timing unit can be used directly to indicate the distance of the target object in corresponding units of distance.

In addition to the components illustrated in Figure 1, the distance measuring device includes a custom integrated circuit which implements the circuit shown in Figures 2 and 3, with the exception of the liquid crystal display 105.

The various functional units of the illustrated embodiment will now be described.

Transmitter 103 converts voltage pulses generated by the integrated circuit into ultrasonic sound pulses which are directed towards the target object. At least a portion of the ultrasonic sound pulses reflected by the target object are received by the receiver 104 which produces a corresponding electrical output signal. This electrical signal is processed by the integrated circuit so as to produce drive signals which cause liquid crystal display 105 to indicate the distance to the target object. The transmitter 103 operates in a series resonant mode, that is with a frequency at which the series reactance is a minimum. In contrast, with a receiver 104 operates in an anti-resonant mode, that is the frequency at which the series reactance is a maximum. It is possible to use a single transducer to implement both the transmitter and the receiver.In this case the frequency is adjusted so as to provide the optimum compromise between the frequency requirements of the transmission and reception functions.

The basic timing function of the measuring device is established by an oscillator. The frequency of the oscillator may be varied in particular to allow for changes of scale and to compensate for variation of the speed of sound.

The speed of sound in dry air at zero degrees celcius and 760 millimeters of mercury with 0.03 mole-percent content of carbon dioxide is 33,145 + 5 cm/second. The speed of sound will vary with changes in density, pressure, temperature, molecular weight, specific heat, coefficients of viscosity and other factors. The major factor influencing the velocity of sound in air is the ambient temperature and the correction factor is proportional to the square root of the ratio of the temperatures expressed in degrees kelvin. If the temperature range is confined within five to thirty five degrees celsius, the speed of sound can be considered to increase linearly with respect to temperature.

As the ultrasonic signal must travel the distance between the measuring unit and the target object twice, the time base clock or timing unit used to determine the distance operates at half of the speed of sound. The ultrasonic ranging unit uses an oscillator with a nominal frequency of 17169 Hz at 20 degrees centigrade and has a temperature compensation of 0.18% per degree for measurements in "m/cm".

For measurements in "Ft/in" units the oscillator frequency is 6759.5 Hz with the same correction for temperature. As the velocity of sound will vary slightly for many environmental factors, for example, the atmospheric pressure and humidity the user can calibrate the ultrasonic ranging unit by adjusting it to give the same distance reading as that made by other means, such as a flexible steel tape.

Operation of the integrated circuit is described below with reference to Figure 3 of the accompanying drawings. However, functional attributes of the integrated circuit will first be described with reference to Figure 2.

A single integrated circuit is used to implement all of the functions within the boundary indicated in figure 2.

The integrated circuit includes a reset circuit 201, the function of which is to provide a delay when the device is switched on so as to enable the integrated circuit to stabilise and obtain an initialised state. A resistor and capacitor (R/C) external to the integrated circuit are used by circuit 201 to generate the delay. The reset circuit 201 may be operated additionally once the device is in use, in order to initiate repetitive distance measurements.

A reference point compensation circuit 202 is provided for in the integrated circuit. It will be apparent that the distance to a target object must be measured with respect to a particular reference point. The reference point may be located on the measurement device, as in the case of the device of the German patent, or alternatively may be a specific physical location with respect to the target object. The distance measuring device of the illustrated embodiment of the present invention allows for an adjustable compensation in accordance with the distance of the transducers 103, 104 from the reference point. This compensation is achieved by commencing timing of the wave propogation prior to actual transmission of the ultrasonic pulses. That is, circuit 202 delays transmission of the ultrasonic pulses by transmitter 103.In order to achieve this delay, circuit 202 makes use of a resistor/capacitor circuit (R/C) external to the integrated circuit.

Provision is made for displacement of the reference point from the transducers 103 and 104 within a range of O to 25 centimeters.

The length of the pulse train transmitted by transmitter 103 is variable and may be set to suit the particular conditions in which the measuring device is being used. This is achieved by a delay circuit 203 which uses a resistor/capacitor (R/C) circuit external to the integrated circuit. The length of the pulse train is thus adjusted by adjusting the value of the (R/C) circuit components. Typically, the pulse train may consist of between 2 and 16 pulses.

The transmitter 103 and receiver 104 comprise ultrasonic transducers, 217 and 218 respectively, which are driven by a single oscillator 205. A fifty percent duty cycle for the drive voltage of the transmitter transducer 217 is achieved by operating the oscillator 205 at twice the desired frequency with subsequent division of the output by two. The divider is contained within circuit 205. Circuit 205 makes use of a resistor/capacitor circuit (R/C) external to the integrated circuit and the external circuit includes a variable resistor. The integrated circuit is designed for use with ultrasonic transducers in an operating range of 22 kHz to 220 kHz. In practice, the low voltage output from the transducer oscillator and divider 205 is increased in amplitude to about 50 Vpp by a transducer drive unit 206 which interfaces the integrated circuit to the transmitter transducer 217.

When ultrasonic pulses 106 are transmitted by transmitter 103, the sound waves spread along a spherical wave front and this reduces the intensity in an inverse square ratio in relation to the distance travelled, as is well known. Additionally, there will be attenuation and absorption of the ultrasonic pulses. Specifically, absorption is determined by the frequency of transmission and many environmen- tal factors, the most significant of which are dust and humidity. In the preferred embodiment of the invention, provision is made for absorption of upto 90 percent of the transmitted ultrasonic signal, at the maximum operational distance. A certain amount of scattering of the ultrasonic pulses will also occur. However, this is relatively insignificant compared with the inverse square reduction in intensity and the absorption by dust and humidity.

It is a distinguishing feature of the present invention that such degradation of the ultrasonic signal is compensated for. Such compensation is achieved by a variable gain preamplifier 207 which receives directly the output from receiver 104. The gain of the preamplifier 207 increases from 0 dB to 55 dB during the time taken for ultrasonic pulses to traverse the maximum range of the measuring device. Typically this maximum range will be 20 meters. The bandwidth of the preamplifier 207 is from 16 kHz to 300 kHz and this is consistent with the above quoted operating range of transducers 217, 218 to be used in this embodiment.

The gain of the preamplifier 207 is controlled by an input control voltage. This control voltage is varied with time by a network of resistors and capacitors, (R/C) external to the integrated circuit and operating via gain control circuit 210.

Although not contained within the integrated circuit, a bandwidth limiting circuit and a gain setting circuit may be included in the measuring device. Output from the preamplifier 207 is passed through the bandwidth limiting circuit which is implemented as a tuned circuit.

The effect of the bandwidth limiting circuit is to reduce any noise or interference in the output of the preamplifier 207. The gain setting circuit is an optional feature provided to compensate for variations in the conversion efficiency of the ultrasonic transducers 217, 218.

It is also possible to arrange for the gain setting circuit to compensate for variations in performance of the integrated circuit.

The gain setting circuit is connected in series with the bandwidth limiting circuit and the output from the preamplifier 207 passes through these two circuits to a fixed gain amplifier 208. Particular attention is drawn to the fact that the input into the fixed gain amplifier 208 has a constant amplitude for a given target object at various distances. The input into the fixed gain amplifier 208 will vary from one target to another as a result of different reflection coefficients for difference target objects. But, for the same target, the input remains constant regardless of variations in distance, provided the maximum range is not exceeded.

Output from the fixed gain amplifier 208 is applied to a threshold detector 209. The threshold detector 209 provides an output signal when the signal from the fixed gain am plifier 209 exceeds a predetermined threshold voltage. A capacitor is connected to the threshold detector 209 so as to provide an integrating time constant and thereby improve the noise rejection of the threshold detector 209.

Operation of the threshold detector 209 is inhibited during transmission of the ultrasonic pulses and for a relatively short period there after. This ensures that the detector circuit 209 is not triggered by signals induced by the transducer 217 of the transmitter 103. Inhibit ing operation of the threshold detector 209 for a short period after pulse transmission also negates the effect of any spurious reflections, such as might be caused by the body of the operator of the measuring device. The inhibiting of detector 209 is achieved by a delay circuit 204 which makes use of a resistor/capacitor circuit (RtC) external to the integrated circuit. The inhibit time corresponds to the time taken for the ultrasonic pulses to travel 50 centimeters. Thus, the minimum distance measurable by the device is 25 centimeters.

The basic timing unit of the measuring device is an intrinsically stable oscillator 211 which is provided with an external temperature dependent resistor, thus enabling the frequency of the oscillator 211 to be made consistent with the velocity of sound in air over a limited temperature range. It is feasible for the oscillator 211 to be implemented so as to match the velocity of sound in water, whereby the measuring device can be used as a marine depth sounder. For less stringent applications, the external frequency compensation may be omitted. Oscillator 211 makes use of a network of resistors and a capacitor (R/C) external to the integrated circuit. The measuring device of the present embodiment can be used to display the object distance in either metric or imperial units. This is achieved by switch 110 which switches between various resistors in the said network.

The integrated circuit comprises a counter 212 which counts the cycles of the oscillator 211 and is implemented as a three and a half decade counter. When the distance measurement is in metric units, the most significant decade digit of the counter 212 has a maximum count of 2. For measurements in imperial units, the first two least significant decade digits have a maximum count of 11 and the most significant decade digit has a maximum count of 6. This change in operation of counter 212 is achieved by a control logic circuit 216.

The integrated circuit includes a display driver 213 for the liquid crystal display 214 (105). The display driver 213 generates pulsed drive voltages which operate the liquid crystal display 214. The display driver 213 also provides for leading zero blanking, so as to conform with the normal practice of omitt ing non-significant digits. The liquid crystal display 214 itself includes a mask so as to be capable of displaying four decimal digits and an indication of the distance measuring units being used.

The distance measuring device includes a voltage regulator. Power supply for the various circuit components is supplied by a battery under the control of operating button 102.

The voltage regulator generates the appropriate voltages for the various components and obviates changes in voltage with battery degredation and other forms of interference.

The integrated circuit contains of control logic which determines the sequence of operation of the measurement device. This will now be described with reference to the timing diagram shown in figure 4 of the accompanying drawings.

The sequence starts at time TO when the operate button is pressed and power is applied to the logic within the custom integrated circuit.

At time T1 the reset timing is finished and the distance clock counter starts counting the pulses from the distance clock. At this time the reference point compensation time starts.

At time T2 the reference point compensation time is complete and the measurement device starts transmitting ultrasonic sound pulses.

The transmit time is complete at T3 and the inhibit time and gain control time both commence at time T3. The inhibit time is complete at time T4 and the device now waits for a reflected ultrasonic sound pulse. During this time, from T3 to T5, the gain of the preamplifier 207 is increased to compensate for the decrease in received sound intensity.

If there is no target object with the maximum range or the reflected ultrasonic signal is too small to trigger the threshold detector, the distance clock counter 212 will reach its maximum count of 2000 in "m/cm" units or 6000 in "Ft/in" units and an "E" will be displayed at time T6.

At time T5 which can be anywhere from time T3 to T6 depending on the distance to the target, the signal from the receiver transducer triggers the threshold detector, subsequent to amplification of the signal, and thus causes the count stored by counter 212 to be displayed on the liquid crystal display.

More detailed operation of the circuitry will now be described with reference to figure 3 of the accompanying drawings. In this embodiment the circuit is defined in terms of Transistor Transistor Logic (TTL) but it will be readily understood that the circuit can equally well be implemented in other integrated logic forms.

Initial operation of the circuit is concerned with the reset function, that is during time TO to T1. When the operate switch (386) is pressed the voltage regulator (385) switches the regulated battery power to the component circuits of the custom integrated circuit. At this time capacitor (C302) is discharged and the input to the schmitt trigger (303) is a logic low. The output of the schmitt trigger (303) sets the D-type flip-flop (304) output to a logic high. This forces the output of the open collector inverter (305) to a logic low and holds capacitor (C306) discharged. In a similar fashion the output of the schmitt trigger (308) is a logic high which sets D-type flip flop (309) and forces the output of the open collector inverter (310) to a logic low, thus holding capacitor (C311) discharged.The schmitt trigger (313) logic high output will force the outputs of the open collector inverters (328 and 323) to a logic low and hold capacitors (C330 and C327) discharged. The schmitt trigger (331) will inhibit the level detector (365) and the network of resistors (R324, R325 and R326) will hold the preamplifier (355) at minimum gain.

The transmit oscillator (318) will be running but the divide by two D-type flip flop (317) will be held clear by the gates (314, 315 and 316). The output transistor (0375) will be held in a non-conducting state as will the external driver transistor (0378).

The distance clock counters (345, 346, 347 and 348) will not count as they are inhibited during this time by the output of D-type flip flop (304) through the inverter (373).

The cross coupled latch formed by the negated output OR gates (369 and 370) will be held clear, that is the signal ECNTL will be a logic low.

During this time (TO to T1) the amplifiers and level detector will stabilize. As capacitor (C302) is charged through resistor (R301) the voltage into the schmitt trigger (303) will increase until eventually the threshold voltage is exceeded and its output switches from a logic high to a logic low. The next positive clock into the D-type flip flop (304) will change its output from a logic high to a logic low. This event is defined as T1 ending the TO to T1 time wich is nominally 250 mS.

Reference point Compensation Time, T1 to T2, occurs next. The output of inverter (373) will change from a logic low to a logic high enabling the distance clock counters to count.

The output of open collector inverter (305) will be turned off enabling capacitor (C306) to be charged via resistor (R307). The voltage into the schmitt trigger (308) will increase until eventually the threshold voltage is exceeded and its output swithces from a logic high to a logic low. The next positive clock into the Dtype flip flop (309) will change its output from a logic high to a logic low. This event is defined as T3 and ends the time period T2 to T3 which is nominally 100uS.

The time period from T3 to T4 is associated with the inhibit function. The open collector inverter (328) output will be turned off allowing capacitor (C330) to charge via resistor (R329). The voltage into the schmitt trigger (331) will increase until eventually the threshold voltage is exceeded and its output switches from a logic high to a logic low.

When the inhibit input to the threshold detector (365) switches to a logic low the threshold detector (365) is ready to respond to the amplified ultrasonic sound signal reflected from the target object. This event is defined as T4 and ends the inhibit time T3 to T4 which is nominally 2.9 mS.

During time period T4 to T5 the device waits to receive a reflected signal. The transmit ultrasonic transducer (X375) generates pulses of ultrasonic sound that are reflected from the target object and return to the receive ultrasonic transducer (X352).

Transducer (X352) generates an electrical signal upon receipt of a reflected sound wave.

The electrical signal is applied to the input of the variable gain preamplifier (355).

As noted previously, the reflected ultrasonic signal decreases with increasing distance between the measuring device and the target object. The gain of the variable gain preamplifier (355) increases to compensate for this signal degredation and thus provides an output which is primarly dependant on the reflection coefficient of the target object.

The coupling capacitor (C356) isolates the tuned auto-transformer (L357 and C358) which filters out noise and spurious responses from the receive ultrasonic transducer (X352) and variable gain preamplifier (355) signal. The variable potentiometer (R359) is used to adjust the input to the fixed gain amplifier (361).

This compensates for variations in the performance of the ultrasonic transducer (X379 and X352) and the custom integrated circuit.

The output from the fixed gain amplifier (361) is a direct current signal and this is isolated by a capacitor (C362). Hence, the signal level required to trigger the threshold detector (365) can be set by a resistive divider (R363 and R364).

When the input signal exceeds the threshold level of the threshold detector (365) its output switches from a logic low to a logic high This sets the cross coupled latch (369 and 370) through the gates (367 and 368) and terminates the receive function of the custom integrated circuit. Counting by the distance clock counters (345, 346, 347 and 348) is terminated at this stage by inhibiting the output of the gate (371). The current count is displayed as a result of signal EDSPL which enables the liquid crystal display driver (350).

If the threshold detector (365) output is not triggered from a logic low to a logic high and does not therefore terminate the receive function, the distance clock counters will count to 2000 in "m/cm" units or 6000 in "Ft/in" units and generate an overflow signal. This overflow signal will terminate the receive function through the negated output OR gate (367). The overflow signal will also cause the liquid crystal display (351) to display an "E" indicating that no reflected ultrasonic sound was received. The overflow function can be inhibited for further applications of the custom integrated circuit.

The current source (372) connected to the RCLKP output of the negated input AND gate (371), inhibited by the LRSTH output of the Dtype flip flop (304), may be used for further applications of the custom integrated circuit.

One such application is use of a resistive load with the current source (372) output so as to generate pulses which may be counted by a microprocessor and used to calculate the area or volume bounded by two or three successive readings.

Another application of the current source (3-72) output is used with a capacitive load which will develop an analogue voltage (in discreet steps) which is proportional to the distance between the transducers and the object.

Various additional and optional features may be included in the measuring device. One particularly advantageous addition is the provision of a facility for automatic calculation of the product of two or three subsequent distance measurement, as noted above. This facility enables automatic calculation of the area and/or volume encompassed by targets objects from which the respective measurements are made.

A modified measuring device incorporating this feature is illustrated in figure 5 of the accompanying drawings.

The case (501), operate switch (502), transmit ultrasonic transducer (503), receive ultrasonic transducer (504), liquid crystal display (505), baseline (506), reference line (507) target object (508), ultrasonic sound (509) and the range switch (510) have the same juxtaposition as the corresponding components of the embodiment shown in figure 1. The liquid crystal display (505) may require an extra digit above the basic model to show the area and volume results. The additional buttons (511 to 517) are positioned in accordance with the ergonomic requirements of the operator. Three buttons (511 to 513) are used to indicate which of the three dimensions, length, breadth or height are being meausred. These may be labelled X, Y and Z, thus making use of the nomenclature commonly used in three dimensional graphs.

One button (514) is used to enable distances outside the normal range of the device to be measured. This is achieved by making two subsequent measurements, perhaps in opposite directions from a central point, and adding the results.

Operation of the area key (515) requires that the operator also press two of the three dimension keys, to indicate which two dimensions are to be multiplied to give the area.

Operation of the volume key (516) results in calculation of the product of all three dimension measurements.

A clear key (517) is provided to erase individual readings by pushing the respective key, X, Y or Z (511, 512 or 513) after the clear key (517). All readings are erased from memory if key 517 is operated twice in succession.

As a further optional feature, the measuring device may be provided with a paraboloidal reflector, as shown in figure 6.

The ultrasonic sound transmitted by the measurement device spreads out as it progresses forward. Consequently, objects to the side of the path directly in front of the device, and beyond the basic inhibit distance mentioned above, will have incident ultrasonic sound which may be reflected. The receiver transducer may respond to such reflections wherey a reading may be obtained from undesired target objects. Figure 6 illustrates the use of a single ultrasonic transducer (603/604) for both the receive and transmit function with the transducer located at the focus of a paraboloidal reflector (611). This reflector will reduce the dispersion of the ultrasonic sound to about 5 degrees either side of the direct path. This significantly reduces the responses from spurious targets but does make the device harder to aim.To overcome this a solid state LASER (612) is used to illuminate the target and show the operator where the device is aimed.

The basic technique of ultrasonic distance measurement in air is well known and has been used in many diverse products. However, the present invention is distinguished by its automatic increase of sound wave degradation compensation with propogation time. The preferred embodiment also includes the novel feature of using a single integrated circuit to implement all or some of the following functions; ; (a) adjustable reset timing, (b) adjustable reference point compensation, (c) adjustable number of transmit cycles, (d) adjustable transmit oscillator frequency with 50% duty cycle output, (e) adjustable gain ranging control, (f) variable gain preamplifier, (g) fixed gain amplifier, (h) fixed threshold level detector, (i) adjustable distance clock, (j) distance clock counter, (k) liquid crystal display driver, (I) control logic, (m) voltage regulator and power switch.

Additionally, the following features of the described embodiments are believed to be novel.

Reference point compensation by delaying the start of transmission from the start of counting of the distance clock.

Temperature dependant frequency of the distance clock to compensate for the changes in velocity of sound through air.

Measurement unit change from "m/cm" to "Ft/in" by changing the frequency of the distance clock and the distance clock counter dividers.

The liquid crystal display will show an "E" to the operator if the amplitude of the ultrasonic echo is below a given threshold, or if the target object is beyond the range of the device.

Display of the distance with all or some of the following attributes; (a) liquid crystal, (b) four digits where two of the digits are larger than the other two for the operator's convenience, (c) the units of measurements are displayed next to the digits; that is "m" or "Ft" next to the two large digits and "cm" or "in" next to the two smaller digits.

Inclusion of a special purpose calculator with all or some of the following fuctions (a) three measurement keys for the entering of length breadth and height, (b) an addition key so that all or some of the previous measurements may be entered in two or more steps, (c) a key to display the area formed by the product of any two of the previous entered dimensions, (d) a key to display the volume formed by the product of all three of the previous entered dimensions, (e) a key to clear any or all of the dimension entered for length breadth and depth.

Inclusion of a paraboloidal reflector to concentrate the ultrasonic sound into a narrow beam so that readings can be made with minimal interference from non-target objects.

Inclusion of a solid state LASER to illuminate the target object so that the operator can see to where the ultrasonic ranging will make it's measurement.

Claims (10)

1. A non-contact distance measuring device comprising transmitter means for transmitting a sound wave towards the target object, receiver means for receiving at least a portion of the sound wave as reflected by the target object and processing means which monitor the propogation time of the sound wave so as to determine the distance of the target object, the processing means including a variable gain amplifier the gain of which is increased auto matically with propogation time of the sound wave whereby degredation of the sound wave during travel to and from the target object is compensated.

2. A device as claimed in claim 1, further comprising inhibit means which inhibit the re ceiver means for a relatively short period sub sequent to operation of the transmitter means.

3. A device as claimed in claim 1 or 2, wherein the processing means includes a bandwidth limiting circuit which operates on the output from the variable gain amplifier.

4. A device as claimed in any preceding claim, further comprising gain control means which compensate for variations in the efficiency of the transmitter and/or receiver means.

5. A device as claimed in claim 4, wherein the gain control means also compensates for variations in performance of other components of the device.

6. A device as claimed in any preceding claim, further comprising variable compensation means for providing compensation for variation in the distance between the device and a reference position from which the distance to the target object is to be measured.

7. A device as claimed in any preceding claim, further comprising electronic calculator means including memory means for storing at least two subsequent distance measurements and including means for calculating automatically the product of the stored measurements.

8. A non-contact distance measuring device substantially as hereinbefore described with reference to figures 1-4 of the accompanying drawings.

9. A device as claimed in claim 8, including the modification illustrated in figure 5 of the accompanying drawings.

10. A device as claimed in claim 8, including the modification shown in Figure 6 of the accompanying drawings.