EP0070305A1 - Apparatus for measuring angular displacement - Google Patents

Abstract

An angular displacement is measured in perpendicular planes between a fixed point and a mobile point whose distance or angular displacement relative to the fixed point must be measured by precise tracking of the movement of a reference point (75) on a rotary element (31, 32) which is driven at constant speed by means of detector elements (37, 40) corresponding to the location of the fixed point (75) and the mobile point (76). In each plane, the ratio of the duration of displacement from the fixed point to the point (duration 'A') to the duration of a complete revolution between the fixed point (duration 'B') is converted into an angular unit of measurement which can be used to activate a display (102) or to be recorded. A series of measurements can be taken and the average of these measurements can be used to minimize errors or erroneous measurements.

Description

APPARATUS FOR MEASURING ANGULAR DISPLACEMENT

Description of the Invention Background and Field of the Invention This invention relates to the art of measuring angular displacements, and more particularly relates to a novel and improved method and means for measuring horizontal and vertical angular displacements in a single instrument so as to permit conversion of such measurements into a number of readings which can be averaged and displayed. Angular displacement measuring apparatus have been in widespread use for a number of years. Various instruments have been devised which are capable of measuring horizontal as well as vertical angular displacements and are adaptable for use in numerous disciplines, such as, compasses, transits, navigational aids, geophysical instruments.

In the past, encoders which relied upon a single code bearing track have tended to be excessively large or too exacting in smaller sizes but in any case have placed definite limits on the number of code bits which can be represented and on the resultant accuracy of the readings taken from such devices.

It is therefore proposed to overcome the above and other drawbacks and limitations in angular displacement measuring apparatus by the utilization of a method and apparatus which is greatly simplified, accurate and versatile to use.

Summary of the Invention It is therefore an object of the present invention to provide for a novel and improved apparatus for accurate and reliable measurement of angular displacements in a simplified, accurate manner and which will insure fast, accurate reading and direct conversion of measurements into a visual display substantially free of error or ambiguity.

A further object of the present invention is to provide for a novel and improved method and apparatus for measuring angular displacement which avoids the use of complex code tracks while permitting accurate binary counting with a direct and immediate readout of the information which is compact, portable, lightweight and economical to manufacture.

In accordance with the present invention, angular displacement can be measured both in horizontal vertical planes by advancing a movable track along a closed path of travel and at a constant rate of speed, aligning a movable sensor with a point or target to be measured, comparing the time required for a reference point on the track to advance successively a full revolution between a fixed reference sensor with the time required to advance between the fixed reference sensor and the movable sensor, then converting the ratio of the second time to the first time into an angular unit or other distance value.

Preferably, the movable track is in the form of a pair of disks mounted for rotation at a constant rate of speed on a common drive shaft, each disk provided with a reference point, one reference point being aligned for movement across the fixed reference sensor and the other reference point being aligned for movement across the movable sensor. In this way, the movable sensor is freely rotatable without interference from the fixed reference or in other words can be rotated through greater than 360". The reference points preferably take the form of apertures of limited size which extend through the periphery of each disk, and the sensors are each defined by light sensing devices which are responsive to passage of the reference aperture thereacross to generate the signal. Individual counters are coordinated through common counter control logic to time the ratio of the time intervals and transmit same to a data processor for conversion of that ratio into a digital value representative of the angular displacement between the fixed point and position to be measured.

Various configurations, such as, a slit in a rotating disk or drum, a hole in a rotating shaft, or a rotating prism may be employed to generate the necessary signals. A number of readings may be taken at each position to be measured and averaged to generate an active measurement while discarding any measurements which vary more than a set amount from a previous measurement. The digital values into which the ratios are converted may be easily displayed or transferred to other recording or measuring devices.

The above and other objects, advantages and features of the present invention will become better understood from the following detailed description when taken together with the accompanying drawings, in which: Brief Description of the Drawings Figure 1 is a side view in elevation of a preferred form of angular displacement measuring apparatus. Figure 2 is a front elevational view of the preferred form of apparatus with portions broken away and in section to schematically illustrate the vertical and horizontal measuring systems;

Figure 3 is an enlarged view partially in section of the horizontal measuring system;

Figure 4 is a cross-sectional view taken about lines 4-4 of Figure 3;

Figure 5 is a schematic block diagram of the preferred form of counting circuit; and Figure 6 illustrates the counter control logic circuit in the counting circuit shown in Figure 5. Detailed Description of the Preferred Embodiment

Referring in detail to the drawings, there is illustrated in Figures 1 and 2 a theodolite 10 is comprised of an upper housing 12 or U-shaped upper end portion having a pair of spaced legs 15 and 16 to support support arms or shafts 17 and 18 for a common telescopic sight 20. Each support arm 17 and 18 is journaled in a bearing support 22 by leg 16 so that the telescope 20 is free to be rotated about a horizontal axis of rotation through the support arms 17 and 18.

The upper housing 12 is journaled on a stationary motor housing 14 and the arrangement of housings 12 and 14 generally corresponds to that of the Pentax Model No. Th10, manufactured by Asahi Optical Co., Ltd. of Tokyo, Japan so that the upper housing 12 is freely rotatable about a vertical axis of rotation through the center of the motor housing.

A first angular displacement apparatus 30 has a movable track in the form of closely-spaced rotatable disks 31 and 32 of corresponding size and configuration mounted for synchronous rotation on a common drive shaft

34 projecting upwardly from a constant speed drive motor

35 contained within the motor housing 14. A movable sensor 37 has a radial arm 36 which is keyed for rotation with a shaft 38 above the rotatable disk 31 which depends downwardly from a movable table 39 in the upper houisng so that the sensor 37 will follow the turning or rotational movement of the sight 20 when the sight 20 is aligned with an object or point whose angular displacement is to be measured. In turn, a reference sensor 40 is mounted on a fixed table 42 in surrounding relation to the motor 35.

A vertical angular displacement measuring apparatus 50 is secured within the leg 16 and corresponds to the apparatus 30, and like parts are correspondingly numbered. In order to follow movement of the sight 20 in a vertical plane; i.e., about a horizontal axis, the movable sensor 37 is provided with a radially inwardly directed arm 36 which is affixed to the end of the shaft 38 as an extension of arm 18 to be movable with the support arm 18 and sight 20. The reference sensor 40 is attached to a stationary bracket 54 within the leg 16, and the motor 35 projects outwardly from the leg 16 with its drive shaft 34 aligned on the axis of the arm 18.

The angular displacement apparatus 30 and 50 are illustrated in more detail in Figures 3 and 4. The rotatable disks 31 and 32 are interconnected in closely-spaced coaxially aligned relation to one another by an annular ring 60, and the lower or innermost disk 32 is fixed by a collar 61 to the upper end of the drive shaft 34, this shaft 34 extending upwardly through an opening in the stationary housing 14, as shown in Figures 1 and 2. The movable table 39 is mounted in the housing 12, and the outer base portion 64 of the housing is journaled on a bearing support 65 in the upper surface of the stationary housing 14, and the shaft 38 is fixed in a sleeve 66 in the movable table assembly so as to be freely movable with the housing 12 independently of the fixed reference table 42.

The reference sensor 40 includes a support leg 70 terminating in an upper lateral extension 71 for a light-emitting diode 72 and a lower lateral extension 73 for a phototransistor 74. A reference aperture 75 extends through the thickness of the lower disk 32 and is aligned with the light-emitting diode 72 and phototransistor 74 so as to selectively pass light from the LED 72 through the disk to activate the phototransistor 74 each time that the aperture 75 moves across the LED 72. The movable sensor 37 is constructed in a corresponding manner to that of the fixed reference sensor and like parts are correspondingly enumerated. Similarly, an aperture 76 extends through the thickness of the upper disk 31 and is aligned to pass between the LED 72 and phototransistor 74 of the movable sensor. Electrical leads L₁ and L₂ extend from the movable sensor 37 and the fixed sensor 40, respectively, into a counter control circuit as illustrated in Figure 5 and which is mounted within the upper housing 12. In order to transmit electrical signals from the fixed reference sensor 40 into the movable housing 12, the leads L₂ from the fixed reference sensor 40 are directed into a slip ring 77 which is journaled on the shaft 38 and is in direct electrical contact with a second slip ring 78 keyed for rotation with the shaft 38. The second slip ring 78 includes leads L₂' which extend into the counter control circuit; however, since the movable sensor 37 is mounted for rotation with the upper housing, the leads L₁ may be taken directly off of the movable sensor and connected into the counter control circuit.

In the vertical angular displacement measuring apparatus 50, the same considerations are involved in the electrical connection of the sensors 37 and 40 to the counter control circuit. In the apparatus 50, however, it is necessary to interconnect the leads L₁ from the movable sensor 37 into a slip ring 77' which is journaled on the support, shaft 38 and is in direct electrical contact with a slip ring 78' journaled on the shaft 38 and fixed in relation to the housing 12. Leads L₁' extend from the slip ring 39 into the counter control circuit together with the leads L₂ from the upper reference sensor 40.

The counter control circuit shown in Figure 5 functions to establish the ratio between time intervals for rotation of a reference point on a disk to advance to a position to be measured, represented as time "A" in Figure 4, to the time interval required for a full revolution, represented as time "B". The counter control circuit is illustrated for one of the measurement apparatus 30 and 50 and the measurement of each is carried out in successive steps. Further, in conducting each measurement the telescopic sight 20 will be advanced to a position to be measured and left at that position for a sufficient period of time to permit a succession of readings to be taken by the counter control circuit. Once a succession of readings has been taken and averaged on the digital display, the telescopic sight 20 can then be advanced to the next position. To this end, the sight 20 is capable of rotation about a vertical axis passing through the center of the housing 12 in the reading of angular positions in a horizontal plane; and is independently movable about a horizontal axis through the support arms 17 and 18 for taking of measurements in a vertical plane. Thus, the reference sensors 37 and 40 shown in Figure 5 are equally representative of the sensors for either apparatus 30 or 50 and the signals from each can be applied to a common counter control circuit. Each signal from phototransistor 74 of the movable sensor 37 is applied through a pulse shaping circuit 37' to a counter control logic circuit 80, and the signal from the phototransistor 74 of the reference sensor 40 is applied through pulse shaping circuit 40' to the counter control logic circuit 81. Assuming that horizontal angular displacements are to be measured, the motor drive 35 is energized to rotate the disks 31 and 32 at a constant speed and, when the aperture 75 passes the reference sensor 40, a start signal is generated to activate both reference and movable counters 81 and 82 through NAND gates 84 and 85 to enter clock or count pulses from the oscillator or clock 86. When the aperture 76 on the upper spinning disk 31 reaches the movable sensor 37, a stop pulse is generated by the phototransistor 74 to disable the movable counter 82. The reference counter 81 will continue to count until it undergoes -a complete revolution, or 360°, from the initiation of the start signal. Thus, when aperture 75 reaches the reference sensor 40 at the end of the first revolution, it will generate a second signal which disables the reference counter 81. A "done" signal is sent by the reference counter in response to the second signal and is applied over line 88 to microprocessor 90. At this point, the movable and reference counter values are output over lines 91 and 92, respectively, to the byte multiplexors 93. When T_o occurs in the microprocessor 90 in response to receipt of a "done" signal over line 88, the multiplexors 93 are addressed by the microprocessor 90 through a signal applied over address line 94 to order out the data in succession from the multiplexors through a buss terminal 95 and buss lines 96 which are directed into port 1 of the processor 90. The processor 90 contains the necessary software and programming to divide the movable counter value received from the multiplexors 93 by the reference counter value and convert same to corresponding angular units, such as, radians, degrees or seconds by appropriate multiplication.

The disks 31 and 32 rotate at a constant rate of speed, but establish only the start and stop pulses through the reference sensor and movable sensor. However, the counters count at the rate of the system oscillator or clock 86. At the end of each count cycle or complete revolution of the spinning disks 31 and 32, a "reset" signal is generated by the processor 90 and applied over line 97 to clear both counters 81 and 82. This signal also resets the counter control circuit 80 and sets "done" line 88. Only then will the counters be cleared to respond to another start signal from the reference sensor. An "anti-race delay" signal is generated by the reference counter and applied over line 98 to the counter control logic circuit 80 a predetermined number of counts after the reference counter has been enabled by a start signal. The delay signal assures that the counters have advanced a predetermined number of counts before they can be stopped, such as, by any spurious noise in the reference sensor 40 as the aperture 76 passes through the reference sensor 40 to generate a start signal. For instance, assuming that a 20 megahertz oscillator is employed as the clock 86, the counters 81 and 82 each may take over one million counts per revolution of the spinning disks 31 and 32. Thus, the delay signal is generated in the reference counter, for example, on the order of 256 counts after the counter is enabled by the start signal so as to avoid any possibility that the counter may be disabled by an erroneously applied signal from the reference sensor in the initial stages of counting.

In the counter control circuit, the pulse shaping circuits 37' and 40' each may be a Schmitt trigger such as a TI 74LS14 chip which contains a series of six Schmitt triggers. For this application, only two of those Schmitt triggers are required for connection to the movable sensor 37 and reference sensor 40 in shaping the pulse from the phototransistor and converting its rise time to a signal acceptable to the counter control logic circuit 80. The gates 84 and 85 may be TI SN74LS00 quadruple, two input positive NAND gates. Each counter 81 and 82 is a three-stage, twenty-four bit, such as, three TI 74LS393 chips connected in series, the outputs of which are directed to the multiplexors 93. Here, the multiplexors 93 are connected in parallel and may consist of eight TI 74151 chips which are capable of accepting six groups of eight bits each from a counter so as to properly order in the data from the multiplexors through the buss terminal 95 to the processor 90. The processor 90 may be an Intel 8039 128 byte processor which has its output from port 2 applied to a 2K byte program memory. Communication lines 99 extend between port O of the processor 90 to the memory 100 and to a digital display 102. Referring to Figure 6, a preferred counter control circuit 80 has three flip flop latches 104, 105 and 106 in combination with AND gates 107, 108, 109, 110 and 111 in combination with the gates 84 and 85 to enable the reference and movable counter circuits 81 and 82. Typical latches may be defined by TI SN74LS74 dual D-type positive-edge triggered flip flops, and the gates are TI SN74LS00 quadruple 2-input positive NAND gates. Initially, a reset signal applied over line 97 resets the latches 104, 105 and 106. The Q output levels from latches 105 and 106 are low thus disabling gates 84 and 85 so that clock pulses are prevented from reaching the movable and reference counters 81 and 82. In the reset condition, the Q output of latch 104 is high providing a high signal to the D inputs of both latches 105 and 106. The first pulse from the reference sensor from pulse shaper 40' is applied to the clock inputs of latch 106 and latch 105 through gates 108 and 109 causing them to set, since the D inputs are high. When latches 105 and 106 are set, the Q outputs become high, enabling the gates 84 and 85 and allowing clock pulses from the clock 86 to pass through and increment the counters 81 and 82.

After a predetermined number of clock pulses have been incremented into the counters, the anti-race delay signal from the reference counter is applied over line 98 to generate a positive edge at the clock input of latch 104. Since the D input of latch 104 is permanently connected to a high level, the latch 104 will be set causing the Q output to become high and Q to go low so that the D inputs of latches 105 and 106 are low, and the signal path from the movable sensor 37 to the latch 105 clock input through gates 107 and 109 is enabled. When a pulse from the movable sensor 37 is received, it is transmitted through gates 107 and 109 to the clock input of latch 105, resetting latch 105 since the D input is low. The Q output of latch 105 becomes low, thereby disabling the gate 84 and stopping the movable counter 82. A pulse from the reference sensor 40 is prevented from reaching a clock input of latch 105 since the gate 108 is disabled; however, the pulse from the reference sensor 40 is applied to the clock input of latch 106 to reset that latch since the D input from the latch 104 is low. The Q output of latch 106 becomes low, thus disabling the gate 84 and stopping the reference counter 81.

The "done" signal is generated by the gates 110 and 111 and is high whenever the latch 104 is set and both latches 105 and 106 are reset. This occurs when a cycle has been completed and both counters 81 and 82 are stopped. Thus, the "done" signal applied over line 88 indicates that a complete revolution of the spinning disks has been completed and that valid counts are available both in the movable and reference counters.

Briefly reviewing the sequence of operations which occurs, whenever the power to the motor drive for one of the angular displacement measuring assemblies is on or a reset signal is applied, both the reference and movable counters 81 and 82 are cleared to zero, the "done" signal is reset and the counter control logic 80 is reset. Rotation of the spinning disks 31 and 32 will first generate a start signal when the first reference point or aperture 75 on the lower spinning disk passes the reference sensor to cause both counters 81 and 82 to be enabled and to count the rate of the system oscillator 86. The spinning disks 31 and 32 will rotate at a constant rate of speed and will continuously rotate as long as the power is turned on at that constant rate of speed. The stop signal generated by the movable sensor will cause the movable counter 82 to be disabled and to stop counting. Thereafter, once the disk has moved through a complete revolution, a second signal is generated by passage of the aperture 76 across the stationary or reference sensor and which signal will disable the reference counter and set the "done" signal. The counter values are then divided to provide a ratio of the time interval which has been converted into the desired angular unit as described. Although forming no part of the present invention as such, the microprocessor is capable in response to ordering in the counts from the movable counter and reference counter to compare a succession of readings in response to the highspeed rotation of the disks. For instance, the disks may be rotated at a speed in excess of 1,000 rpm using an AC hysterisis synchronous motor. At each position to be measured the disks 31 and 32 can be rotated through any desired number of revolutions to provide a succession of readings for the processor 90. There is a slight gap in time between consecutive readings in that the start signal enabling the counters 81 and 82 will be generated at most in response to every other revolution of the disks 31 and 32, since at the end of each revolution, a disabling signal is generated to disable the reference counter 81; and until a reset signal clears the counters, the counters cannot be enabled by the next start signal. Nevertheless, the processor may obtain a succession of readings within an extremely short period of time. Depending upon the accuracy desired, the processor can establish the desired accuracy of those readings and discard any bad or spurious data which is outside the tolerance or accuracy set within the programming. Similarly, the necessary acceptance criteria can be established within the program to insure that the motor is rotating at a constant rate of speed and that the oscillator 86 is operating at the requisite level.

It will be evident from the foregoing that the utilization of a pair of spinning disks affords utmost convenience in that the operator may continue to rotate the housing through greater than 360" about the vertical axis as well as about the horizontal axis in taking a series of different measurements.

The reference apertures 75 and 76 are illustrated as displaced 180º to show the relationship of the apertures to their respective sensors 37 and 40 as they pass across the sensors. In practice, the reference apertures 75 and 76 may be in vertical alignment with one another so that it is not necessary to introduce a correction for such displacement into the processor 90. However, assuming that the apertures are angularly displaced with respect to one another, a constant corresponding to the angle of displacement between the apertures should be introduced into the computer program to compensate for such displacement. Similarly, the movable sensor is most desirably aligned physically with respect to the sight 20 in both the horizontal and vertical measuring apparatus 30 and 50. If however the movable sensor is displaced with respect to the sight 20, again a corresponding correction should be introduced into the program to compensate for such displacement. It is therefore to be understood from the foregoing that various modifications and changes may be made in the construction and arrangement of parts of the preferred form of the present invention without departing from the scope thereof as defined by the appended claims.

Claims

We claim:

1. Angular displacement measuring apparatus comprising: a reference sensor; a movable sensor; means for aligning said movable sensor with a position to be measured; a movable track defined by a pair of disks each having at least one reference point thereon, one of said disks aligned for movement of its reference point across said reference sensor, and the other of said disks aligned for movement of its reference point across said movable sensor, including means for advancing said movable track at a constant rate of speed; first sensing means associated with said reference sensor responsive to successive movements of one of said reference points to generate first and second signals; second sensing means associated with said movable sensor responsive to movement of the other of said reference points across said movable sensor to generate a third signal; first timing means associated with said first sensing means to measure the time interval between the generation of said first and second signals; and second timing means associated with said second sensing means to measure the time interval between the generation of said first and third signals whereby the ratio of the time interval between said first and third signals and said first and second signals is representative of the angular displacement between said reference sensor and said movable sensor.

2. Angular displacement measuring apparatus according to claim 1, said movable sensor rotatable through greater than 360º.

3. Angular displacement measuring apparatus according to claim 1, said first and second sensing means each defined by a light transmitting element and a lightsensitive element on opposite sides of the path of movement of said movable track, and said reference point defined by an aperture extending through said movable track.

4. Angular displacement measuring apparatus according to claim 1, including counter control means to simultaneously enable said counters for receipt of pulses from said clock means when said first signal is generated, said counter in said first counting means operative to generate a "done" signal in response to receiving said second signal.

5. Angular displacement measuring apparatus according to claim 4, processor means associated with said first and second timing means operative in response to receipt of a "done" signal from said counter in said first timing means to generate a reset signal which is operative to clear said counters and said counter control means.

6. Angular displacement measuring apparatus according to claim 5, said rotatable disks disposed in closely-spaced parallel relation to one another and mounted for rotation on a common axis of rotation, one of said disks aligned for movement of its reference aperture across said reference sensor, and the other of said disks aligned for movement of its reference aperture across said movable sensor.

7. In an angular displacement measuring apparatus wherein a rotatable housing is rotatable about a first axis and includes a sight which is mounted for rotation about a second axis perpendicular to said first axis, the combination therewith comprising: first and second angular displacement measuring means disposed in mutually perpendicular relation to one another for measurement of angular displacements in a plane perpendicular to each of said first and second axes, respectively, each said angular displacement means including a fixed reference sensor, a movable sensor having means for aligning said movable sensor with, a point to be measured with respect to its displacement from said reference sensor, a movable track having a reference aperture thereon aligned for movement successively across one of said reference sensor and said movable sensor including means for advancing said movable track at a constant rate of speed; and first sensing- means associated with the reference sensor of each said angular displacement means to generate first and second signals in response to successive movement of said associated reference aperture across said reference sensor, second sensing means associated with said movable sensor of each angular displacement means responsive to movement of the other of said reference apertures across said movable sensor to generate a third signal, and means for comparing the ratio of the time interval between said first and third signals with the time interval between said first and second signals.

8. In angular displacement measuring apparatus according to claim 7, each said movable track being a pair of rotatable disks arranged in closely-spaced parallel relation to one another along one of the axes of rotation of said housing and said sight.

9. In angular displacement measuring apparatus according to claim 8, said reference sensor and movable sensor for each of said angular displacement means being associated with a different disk with said movable sensor rotatable in a plane spaced from the plane in which said reference sensor is disposed.