GB2290001A - Rangefinder - Google Patents

Abstract

A camera rangefinder operates by the triangulation principle and comprises a light beam transmitter 1, 10 and sensor 3. The sensor output is integrated 60. The distance to an object is calculated when the sensor output exceeds a threshold, but if this threshold is not reached after a predetermined time the distance is calculated anyway. This allows for the possibility of very remote targets. <IMAGE>

Description

DISTANCE MEASURING APPARATUS The present invention relates to a distance measuring apparatus. In particular, the present invention relates to a distance measuring apparatus for use in a camera, and to a camera incorporating such a distance measuring apparatus.

Previously various different distance measuring apparatus of light projecting and light receiving type using integrating circuits have been proposed. These apparatus operate a light projecting circuit and calculate a distance to an object by counting the number of light projections and by measuring the time taken for the integration voltage to exceed a predetermined voltage.

However, since the aforementioned distance measuring apparatus cannot anticipate the time necessary for the integral voltage to exceed a predetermined voltage, the distance measuring process may take too long when the object to be photographed is very far away or when the reflected light is very low.

This may result in a shutter chance being missed, which is extremely disadvantageous for photographing a moving object or for continuously photographing an object.

According to the present invention, there is provided a distance measuring apparatus for measuring the distance to an object comprising: radiation emitting means for projecting radiation onto an object; radiation receiving means for receiving radiation reflected by said object and converting the received radiation to an electrical signal; circuit means for obtaining an integrated output signal from the electrical signal of said amplifying circuit; means for determining the duration of radiation emission; and calculation means for calculating a distance signal indicative of the distance to the object, wherein the calculation means terminates the radiation emission when the output signal from said circuit means exceeds a predetermined reference signal or when the duration of radiation emission exceeds a predetermined value, and the calculation means calculates the distance signal from the integrated output signal and the duration of radiation emission when the radiation emission is terminated.

The apparatus of the present invention simultaneously detects the integral voltage and counts the number of light projections during light projection to an object whose distance is to be measured. The apparatus of the present invention terminates the distance measurement when either the integral voltage or the number of light projections reaches a predetermined value and a distance signal is obtained from the integral voltage and counting frequency at that time.

For a better understanding of the present invention, and to show how it may be brought into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a structural view showing an embodiment of the present invention.

Figure 2 is an operational diagram illustrating the integration operation in the embodiment of the present invention.

Figure 3 is an operational diagram illustrating a series of operations at the time of gain determination in the embodiment of the present invention.

Figure 4 is an operational diagram illustrating a method for calculating the count numbers Nf and Nn in the embodiment of the present invention.

Figure 5 is an operational diagram illustrating a series of operations at the time of distance measurement in the embodiment of the present invention.

Figure 6 shows a table in a ROM 82 for determining a distance from the value X in the embodiment of the present invention.

Figure 7 is a flowchart showing the operation of an embodiment of the present invention.

Figure 8 is a flowchart showing a portion of a subroutine where the gains of the amplifying circuit 40 and the amplifying circuit 50 in the flowchart of Figure 7 are determined.

Figure 9 is a flowchart showing a portion of a subroutine where the distance is measured by the first current-voltage converting circuit in the flowchart of Figure 7.

Figure 10 is a flowchart showing a portion of a subroutine where the distance is measured by the second current-voltage converting circuit 30 in the flowchart of Figure 7.

The construction of a camera distance measuring apparatus of one embodiment of the present invention will now be explained in conjunction with Figure 1. A light projection circuit 10 is a driving circuit for driving a near infrared ray emitting device 14 (referred to as an IRED hereinafter) which circuit comprises a transistor 11, resistors 12 and 13 and an IRED 14. When a calculating circuit 80 (hereinafter referred to as a CPU) generates a light projection signal, the IRED 14 emits light. The emitted light passes through the light projection lens 1 to be partially reflected by an object to be photographed (not shown). Part of the reflected light passes through the light receiving lens 2 to be projected on the PSD 3. The IRED 14 may be pulse driven.

A light receiving circuit comprises a first current-voltage converting circuit 20, a second current-voltage converting circuit 30 together with a semiconductor position detector 3 (hereinafter referred to as a PSD). When an optical signal is incident on the PSD 3, the PSD 3 generates a current corresponding to the intensity and position of the incident light, and supplies this current to the current-voltage converting circuits 20 and 30. The first currentvoltage converting circuit 20 is a circuit for generating a voltage proportional to an input current and comprises an amplifier 21 and a feedback resistor 22, whereas the second current-voltage converting circuit 30 is a circuit for generating a voltage proportional to a signal current. The second currentvoltage converting circuit 30 comprises an amplifier 31 and a feedback resistor 32. The circuit 30 has the same construction as the current-voltage converting circuit 20.

The switch 4 supplies either one of the outputs of the current-voltage converting circuits 20 and 30 to the amplifying circuit 40. The switch 4 is controlled by the CPU 80. When the distance measured is a far distance, the switch 4 is switched to the first current-voltage converting circuit 20 side. When the distance measured is a near distance, the switch 4 is switched to the second current-voltage converting circuit 30 side.

The first and the second amplifying circuits 40 and 50 are amplifying circuits having alterable gains.

Since these amplifying circuits have the same construction, explanation will be provided with respect to only the first amplifying circuit 40. A capacitor 5 is connected to the input of the amplifying circuit 40 to block any direct current component of the input signal. The amplifying circuit 40 comprises an amplifier 41, three feedback resistors 43, 44 and 45, and switches 46 and 47 which either turn on or off the three feedback resistors, and amplifies the input signal by a given constant gain. The switch 46 and the switch 47 are controlled by the CPU 80. The closing of switch 46 short circuits the feedback resistor 45 while the closing of switch 47 short circuits the feedback resistor 44 and the feedback resistor 45. The gain of the amplifier 40 is therefore varied by stages corresponding to the states of these switches. The signal current is consequently converted to a voltage in accordance with the modified gain, with the result that the voltage is supplied to the next circuit. The second amplifying circuit 50 is operated in exactly the same manner. The CPU 80 operates the switch 56 and the switch 57 to set an appropriate gain so that a signal generated by the amplifying circuit 40 is amplified.

The output signal of the amplifying circuit 50 is supplied to the integrating circuit 60 via the switch 7.

The integrating circuit 60 comprises an amplifier 61, an input resistor 62, an integrating capacitor 63, a switch 64 and a voltage follower 65. The circuit 60 serves as a circuit for integrating the input signal.

When the switch 64 is turned on, any electrical charge across the integrating capacitor 63 is discharged. The integral voltage Vi between terminals of the integrating capacitor 63 is supplied to an ADC 70 via the voltage follower 65. The ADC 70 converts the integral voltage Vi into a digital value to be supplied to the CPU 80.

A readable and writable volatile memory 81 (hereinafter referred to as a RAM) is used for temporary storage of the calculation and count value of the CPU 80 as well as a flag. A readable non-volatile memory 82 (hereinafter referred to as a ROM) is used for the storage of program and data of the CPU 80.

Next, an operation of the circuit according to an embodiment of the present invention will be explained.

When the operation process enters a distance measurement routine, the CPU 80 firstly turns on the power source of all the circuits in Figure 1. Then, the content of the RAM 81 is cleared to determine the optimal gain of the amplifying circuit 40 and the amplifying circuit 50. When it has been judged that the object to be photographed is very close or the reflected light from the object to be photographed is very bright in the gain determination operation, a near flag Fn in the RAM 81 is set. Consequently, it is determined that the distance is near without measuring the distance and the value X is set to 1. Then the value Xf which is measured by the first current-voltage converting circuit 20 and is given in the following equation (1) is stored in the most appropriate address.

Xf = Nf-V3/Vf (1) Then the distance is measured with the second current-voltage converting circuit 30, and the value Xn which is given the following equation (2) is stored in an appropriate address of the RAM 81.

Xn = Nn V3/Vn (2) When it has been determined that the object to be photographed is very far or the reflected light from the object to be photographed is very low during the first and the second distance measurements, an infinite distance flag Ff is set. Consequently, it is determined that the distance is infinite and the value X is set to 0.5. When the infinite distance flag Ff is set, the CPU judges that the distance to the object to be photographed is very far. When the near flag Fn is set, the CPU judges that the distance to the object to be photographed is very short. When the distance measurement operation is terminated, if neither the infinite distance flag Ff nor the near flag Fn is set, the value X given in the following equation (3) using the count numbers Nf and Nn held in the RAM 81 is calculated.

X = Xf/ (Xf + Xn) (3) When the value X is determined, the CPU obtains the distance to the object to be photographed by referring to an address of the ROM 82 which is readily determined by means of the determined value X as shown in Figure 6. Lastly, after the motor 83 is controlled and the lens mirror cylinder 84 is driven to a focusing position, the power source of the distance measuring circuit is turned off, thereby finishing this routine.

Next, a gain determination operation of the first amplifying circuit 40 and the second amplifying circuit 50 will be explained in detail using Figure 2.

Firstly, the CPU 80 switches the switch 4 to the current-voltage converting circuit 20 side. Then the CPU 80 turns on the switch 64 to discharge an electrical charge accumulated in the integrating capacitor 63 (line a of Figure 2). After sufficiently discharging the electrical charge, the CPU 80 turns off the switch 64 (line b of Figure 2) and generates a clear signal to clear the count number Nf to o (line c of Figure 2). Then the CPU 80 operates the light projection circuit 10 to generate a light projection signal EM, thereby driving the IRED 14 for initiating light projection (line d of Figure 2). To secure a rise time of each amplifier and to decrease the effect of power variation at the initiation of light projection, the CPU 80 turns on the switch 7 after a time T1 following light projection to perform integration for time T2 (line e of Figure 2). After this is finished the CPU 80 terminates light projection, switches off the switch 7 (line f of Figure 2), and waits for time T3, thereby generating a countup signal CU to add 1 to the number of times Nf (line g of Figure 2).

After repeating the aforementioned operation a predetermined number of times (for example, ten times), the CPU 80 supplies the voltage between terminals of the integrating capacitor 63, i.e. the integral voltage V1, to the ADC 70. Then, the ADC 70 converts the result of the output into a digital signal to be supplied to the CPU 80. If the output of the ADC 70 is larger than the voltage V1 (line h of Figure 2), the CPU 80 turns on the switch 46 (line i of Figure 2), and if the output of the ADC 70 is not larger than the voltage V1, it is judged that an optimal gain has been attained. Thereafter, in the same manner, the integration operation and the comparison calculation are repeated and if the output of the ADC 70 is larger than the voltage V1, the switches 56, 47 and 57 are subsequently turned on in that order. If the output of the ADC 70 is larger than the voltage V1 when all the switches are turned on, the near flag Fn is set. This means the gain of the entire amplifying circuit is set.

Figure 3 shows an exemplary case in which the optimal gain is obtained in the fourth repetition of the gain determination operation when the switches 46, 56 and 47 have been turned on.

Next, distance measurement using the first current-voltage converting circuit 20 will be explained in detail based on Figure 4. Firstly, the CPU 80 turns on the switch 4 to the side of the current-voltage converting circuit 20 (line a of Figure 4). Then the CPU 80 turns on the switch 64 to discharge an electric charge accumulated in the integrating capacitor 63 and then turns off the switch 64 (line b of Figure 4).

This sets the potential difference across the integrating capacitor 63 to 0. Then the CPU 80 generates a clear signal CR to clear the count number Nf to 0. After that, the CPU 80 operates the light projection circuit 10 to generate a light projection signal EM so that the IRED 14 is driven to initiate light projection (line d of Figure 4). To secure a rise time for each amplifier and to decrease the effect of power source variation during the initiation of light projection, the integration circuit is operated only for time T2 after a time T1 following light projection (line e of Figure 4). When the operation of the integrating circuit is terminated, light projection and integration are suspended (line f of Figure 4) and placed on standby for time T3. Then the CPU 80 generates a count-up signal CU to add 1 to the count number Nf (line g of Figure 4). Subsequently, if the number of times Nf has not reached the count number Nfm (for example, 300 times) and the integral voltage Vi has not reached the voltage V3, the CPU 80 increments the count number Nf while repeating the operations of lines d-g of Figure 4. If the integral voltage Vi does not reach the voltage V2 even when the count number Nf reaches the count number Nfm, an infinite distance flag Fg in the RAM 81 is set, whereas if it does attain the voltage V2 the value Xf is calculated and the operation is terminated. Even when the count number Nf has not reached the count number Nfm, the CPU 80 calculates the value Xf and terminates the operation if the integral voltage Vi reaches a voltage V3.

Distance is measured in the second current-voltage converting circuit 30 in a similar manner. Firstly, the CPU 80 switches the switch 4 to the current-voltage converting circuit 30 side (line i of Figure 4). Then the CPU 80 turns on the switch 64 (line j of Figure 4) and after discharging the charge which has accumulated in the integrating capacitor 63, turns off the switch 64 (line k of Figure 4). Then the CPU 80 generates a clear signal CR to clear the count number Nn to 0 (line 1 of Figure 4). Subsequently, the count number Nn is incremented while repeating light projection (line m of Figure 4). Thus light projection continues until the count number Nn reaches the count number Nnm (for example, 700 times) or the integral voltage Vi reaches the voltage V3. The CPU either calculates the value Xn or sets the infinite distance flag Ff to terminate the operation.

Figure 5 is a flow chart showing the aforementioned operation from the determination of the gains of amplifying circuits 40 and 50 to the end of measuring distance by means of the second current voltage converting circuit 30.

The aforementioned passage describes the operation of the circuit according to an embodiment of the present invention. Figures 7 through 10 are flowcharts showing the aforementioned operation. Firstly, the main routine is explained based on Figure 7. When the operation process enters distance measurement, the CPU 80 turns on the entire distance measuring circuit (step 001) to set each of the switches (step 002).

Subsequently, the CPU 80 clears the content of the RAM 81 (step 003). Then the CPU 80 determines the gains of the amplifying circuit 40 and the amplifying circuit 50 (step 004), and confirms the state of the near flag Fn (step 005). When the near flag Fn is set (step 005), the CPU 80 sets the value X to 1 (corresponding to the nearest distance) and jumps to step 013. When the near flag Fn is not set, the distance is measured by means of the first current-voltage converting circuit 20 (step 005). Then the state of the infinite distance flag Ff is confirmed (step 008) and, if it is set, the CPU 80 sets the value X to 0.5 (corresponding to an infinite distance) (step 009) and jumps to step 013.

Then the distance is measured by means of the second current-voltage converting circuit 30 (step 010).

Subsequently, the state of the infinite distance flag Ff is confirmed (step 011) and, if it is set, the CPU 80 sets the value X to 0.5 (step 009) and jumps to step 013.

The CPU 80 calculates the value X from the value Xf and the value Xn obtained in the operation of the subroutine steps 007 and 010 (step 012). As a consequence, the CPU 80 determines the distance to the object to be photographed by referring to an address in the ROM 82 which can be readily determined (step 013).

After controlling the motor 83 and driving the lens cylinder 84 to a focusing position (step 014), the CPU 80 turns off the power source of the distance measuring circuit (step 015) and exits this routine.

Next, the operation within each of the subroutines will be explained. First, the subroutine for gain determination of the subsequent stage amplifying circuits (amplifying circuit 40 and amplifying circuit 50) will be explained in conjunction with Figure 8.

When the operation process enters a subroutine in which the gain of the post stage amplifying circuit is determined, the CPU 80 switches the switch 4 to the side of the current-voltage converting circuit 20 (step 101). Then the CPU 80 clears the count number Ns to 0 (step 102), turns on the switch 64 to discharge any electric charge accumulated in the integrating capacitor 63, and turns off the switch 64 (step 103).

The CPU 80 generates a clear signal CR to clear the count number Ne to 0 (step 104).

Subsequently, the CPU 80 generates a light projection signal EM and operates light projection circuit 10 to initiate light projection (step 105).

When the CPU 80 stands by for time T1 (step 106), the switch 7 is switched to perform an integration operation (step 107) while the CPU stands by for time T2. During this time, an electrical charge is accumulated in the integrating capacitor 63 (step 108).

Then the operation of the light projection circuit 10 is suspended to terminate the light projection operation. The CPU 80 turns off switch 7 to suspend the integration operation (step 109).

Subsequently, the CPU 80 generates a count-up signal CU to add 1 to the count number Ne (step 110).

When the count number Ne is less than the predetermined count number Ng, the CPU 80 jumps to step 105 (step 111). When the count number Nf reaches the count number Ng, the CPU 80 turns off the switch 7 to read the integral voltage Vi through the ADC 70 (step 112).

The CPU 80 compares the integral voltage Vi with the voltage V1 (step 113), and the operation returns to the main routine when the integral voltage Vi is equal to the voltage V1 or less.

Where the integral voltage Vi is larger than the voltage I, the CPU 80 turns on the switch 46 (step 115) when the count number Ns is 0 (step 114); the switch 56 (step 117) when the count number Ns is 1 (step 116); the switch 7 (step 119) when the count number is 2 (step 118); and the switch 57 (step 121) when the count number is 3 (step 120). Thereafter, the CPU 80 adds 1 to the count number Ns (step 122) and returns to step Ic; If If the count number is none of the numbers 0 to 113 3, the CPU 80 sets the near flag Fn (step dot), passes through the subroutine, and returns to the main routine.

Subsequently, a subroutine in which the value Xf obtained by the first current-voltage converting circuit 20 is calculated will be explained in conjunction with Figure 9. When the operation process enters a subroutine in which the distance is measured by the current-voltage converting circuit 20, the CPU 80 turns on the switch 4 to the side of the first current-voltage converting circuit 20 (step 201). The CPU 80 generates a clear signal CR to clear the count number Nf to 0 (step 202). The CPU 80 turns on the switch 64 to discharge an electric charge accumulated in the integration capacitor 63 and then turns off the switch 64 (step 203).

Subsequently, the CPU 80 outputs a light projection signal SM to operate light projection circuit 10 for starting light projection (step 204).

After waiting only for time T1 (step 205), the CPU SO turns on the switch 7. While performing an integration operation (step 206), the CPU 80 waits only for time T2 (step 207). During this time, an electric charge is accumulated in the integration capacitor 63. Then the operation of light projection circuit 10 is stopped to terminate the light projection operation. Then the CPU 80 turns off the switch 7 to terminate the integration operation (step 208), and generates a count-up signal CU to add 1 to the count number Nf (step 209). Here, the count number Nf and the count number Nfm are compared with each other (step 210). In addition, the integral voltage vi is supplied to the ADC 70 and the ADC 70 converts the output integral voltage Vi of the integration circuit 60 to a digital signal and supplies the digital signal to the CPU 80.

If the count number Nf is larger than the count number Nfm, the integral voltage Vi is compared with the voltage V2 (step 211). If the integral voltage Vi is smaller than the voltage V2, an infinite distance flag Ff is set (step 212) but if the integral voltage Vi is either equal to or larger than the voltage V2, the CPU 80 calculates the value Xf in accordance with equation (1) (step 213), thereby returning to the main routine.

If the count number Nf is less than the count number Nfm, the CPU 80 compares the integral voltage Vi with V3 (step old). If the integral voltage Vi is equal to or less than the voltage V3, the CPU 80 jumps to step t but if the integral voltage Vi is larger than the voltage vf, the CPU 80 calculates the value Xf, in accordance with equation (1) (step 213) and returns to the main routine.

Next, a subroutine in which the second current voltage converting circuit 30 calculates the value Xn will be explained in conjunction with Figure 10. When the operation process enters the subroutine in which the current voltage converting circuit 30 measures the distance, the CPU 80 switches the switch 4 to the side of the second current-voltage converting circuit 30 (step 301). Then the CPU 80 generates a clear signal CR to clear the count number Nn to 0 (step 302), and turns on the switch 64 to discharge an electric charge accumulated in the integration capacitor 63. Then the CPU 80 turns off the switch 64 (step 303).

Subsequently, the CPU 80 generates a light projection signal EM to operate a light projection circuit 10, thereby starting light projection (step 304). Then the CPU 80 turns on the switch 7 after waiting for only time T1 (step 305). While performing the integration operation (step 306), the CPU 80 waits for time T2 (step 307), during which an electric charge is accumulated in the integration capacitor 63. Then the CPU 80 terminates the operation of light projection circuit 10, turns off the switch 7 to terminate the integration operation (step 308), and generates a count-up signal CU to add 1 to the count number Nn (step 309). Here, the count number Nn is compared with the count number Nnm (step 310). In addition, the integral voltage Vi is supplied to the ADC 70, and the ADC 70 converts the output integral voltage Vi of the integration circuit 60 to a digital signal and supplies the output digital signal to the CPU 80.

If the count number Nn is larger than the count number Nnm, the integral voltage Vi is compared with the voltage V2 (step 311). If the integral voltage Vi is smaller than the voltage V2, the infinite distance flag Ff is set (step 312) but if the integral voltage Vi is either equal to or larger than the voltage v2, the CPU SO calculates the value Xn in accordance with equation (2) (step 313) and returns to the main routine.

If the count number Nn is less than count number Nnm, the CPU 80 compares the integral voltages Vi and V3 (step 314). If the integral voltage Vl is either equal to or less than the voltage V3, the CPU 80 jumps to step t but if the integral voltage is larger than the voltage V3, the CPU 80 calculates the value Xn (step 313) and returns to the main routine.

The aforementioned operation thus allows the measurement of the distance to the object to be photographed in the minimum time.

In the embodiment, the frequencies Nfm and Nnm and the voltage V3 are fixed, but the distance can be measured at a high speed by setting the frequencies Nfm and Nnm to smaller values or by setting the voltage V3 to a lower level. Consequently, the time is shortened from switch on to photographing so that the speed of continuous photography can be increased.

In the distance measuring apparatus for a camera according to the present invention, the integral voltage is detected and the number of light projections is counted at the same time. When either of the detection of the integral voltage or the counting of the number of times of light projection satisfies a predetermined condition, distance measurement is terminated, and a distance measurement signal is obtained from the integration voltage and count number obtained at that time, preventing the prolongation or distance measurement time to longer than required, making the present invention suitable for measuring distance during rapid photography.

In addition, since the integral voltage at which the distance measurement is terminated can be changed, appropriate photography can be performed depending on conditions through such processes as setting the integral voltage limit to a higher value in normal photography and setting the voltage limit to a lower level in continuous photography.

Claims (12)

1. A distance measuring apparatus for measuring the distance to an object comprising: radiation emitting means for projecting radiation onto an object; radiation receiving means for receiving radiation reflected by said object and converting the received radiation to an electrical signal; circuit means for obtaining an integrated output signal from the electrical signal of said amplifying circuit; means for determining the duration of radiation emission; and calculation means for calculating a distance signal indicative of the distance to the object, wherein the calculation means terminates the radiation emission when the output signal from said circuit means exceeds a predetermined reference signal or when the duration of radiation emission exceeds a predetermined value, and the calculation means calculates the distance signal from the integrated output signal and the duration of radiation emission when the radiation emission is terminated.

2. A distance measuring apparatus as claimed in claim 1, wherein the distance signal is calculated by X = N Vr vi where X = distance signal N = duration of light emission Vr = reference signal Vi = integrated output signal.

3. A distance measuring apparatus as claimed in any preceding claim, wherein if the calculation means determines that the duration of radiation emission exceeds the predetermined value, the calculation means compares the integrated output signal with an intermediate reference signal and determines that the object is far away if the integrated output signal is less than the intermediate reference signal.

4. A distance measuring apparatus as claimed in any preceding claim, wherein the radiation receiving means includes radiation reception means and first and second current-voltage conversion circuits for converting a current output by the radiation reception means to a voltage, and the calculating means calculates the distance to the object from the distance signals calculated for each of the first and second current-voltage converting circuits.

5. A distance measuring apparatus as claimed in claim 4 wherein the distance to the object is calculated using the formula: X= Xf Xf + Xn where Xf = distance signal calculated in respect of the first current-voltage converting circuit where Xn = distance signal calculated in respect of the second current-voltage converting circuit

6. A distance measuring apparatus as claimed in any preceding claim, wherein the radiation emission means is pulsed and the means for determining the duration of radiation emission comprises means for counting the number of pulses sent.

7. A distance measuring apparatus as claimed in any preceding claim, wherein the circuit means comprises an amplifying circuit and an integration circuit.

8. A distance measuring apparatus as claimed in claim 7, wherein the gain of the amplifying circuit may be set by the calculating means during an initial gain setting procedure.

9. A distance measuring apparatus as claimed in any preceding claim, wherein different reference signals and/or predetermined radiation duration values are selectable for different modes of operation of the distance measuring apparatus.

10. A distance measuring apparatus substantially as herein disclosed with reference to the accompanying drawings.

11. A distance measuring apparatus as claimed in any preceding claim for a camera.

12. A camera having a distance measuring device as claimed in claim 11.