Laser Measuring System for Incremental Assemblies

This invention generally relates to laser measuring systems and more particularly to a laser system for automating the measurement of incremental assemblies.

As a specific example of an incremental assembly which can be accurately measured by the system according to the invention, a wired grid frame assembly composed of small diameter wires which is the basic component of a spark chamber will be considered. Spark chambers are used for measuring the energy and direction of ionizing radiation and may comprise forty to fifty wired grid frames forming a cubic grid at the bottom of which is a scintillation crystal that absorbs the radiation and emits secondary radiation for detection by photo-multiplier tubes. Each frame has a square opening which may measure, for example, 81 by 81 cm with two planes of 992 parallel and evenly spaced wires on two mutually perpendicular planes, the two planes being separated by 4 mm. Each wire on the frame is associated with circuitry which records the current generated by the ionizing radiation. The wire wound on the frame assembly is made from 0.076 mm (or 0.003 inch) diameter Be-Cu wire stock.

Prior to the present invention, the grid wires on the frame assemblies were measured manually with a standard optical theodolite instrument. This procedure is very tedious for the inspector, and visual fatigue becomes a prime factor on the quality of inspection. Using this procedure, approximately fifty man-hours were required to inspect one frame assembly.

It is therefore an object of the invention to provide a system for automating the measurement of incremental assemblies.

It is another object of this invention to provide a highly accurate laser measuring system for incremental assemblies such as wire wrapped frames and the like.

It is a further object of the invention to provide a system for measuring and recording distances in incremental assemblies in a fast and accurate manner.

The foregoing and other objects are attained by using a laser, an interferometer and retroreflector to precisely measure distance. A light source and a photodetector are located adjacent the incremental assembly and mounted on a movable carriage. The interferometer is also mounted on the movable carriage, while the laser and retroreflector are positioned at either end of the carriage track. The carriage is moved along one edge of the incremental assembly between the retroreflector and the laser, and as the carriage is moved, the light from the light source to the photodetector is interrupted. This produces a trigger command to a control unit which in turn causes a distance measurement to be made. A printout is provided for each sampling trigger command to list such items as ideal position, actual position and amount of error.

The specific nature of the invention, as well as other objects, aspects, uses and advantages thereof, will clearly appear from the following description with reference to the accompanying drawings in which:

FIG. 1 is a block diagram of the laser measuring system according to the invention;

FIG. 2 is a detailed block diagram showing the photodetector and light source assembly; and

FIG. 3 is a schematic and block diagram of the trigger command and control unit of the system shown in FIG. 1.

Referring now to the drawings, and more particularly to FIG. 1, a laser head 10 produces a coherent beam of light which is directed toward an interferometer 11 and a retroreflector 12 lying on a common line. The laser head 10 may be, for example, a helium-neon gas laser. The laser head 10 and the retroreflector 12 are fixedly mounted on pedestals at either end of a carriage track (not shown) on which there is mounted a motor driven carriage 13. The interferometer 11 is mounted on and movable with the carriage between the retroreflector 12 and the laser head 10. It will be appreciated that, while the figure shows the laser beam as having a two-dimensional path with two right angle bends, the laser beam in fact has a linear path. When there is relative movement between the interferometer 11 and the retroreflector 12, the return light to the interferometer will be Doppler shifted. This return light is directed to Doppler detectors and amplified to provide a Doppler signal for the laser display unit 14. The reference signal from the laser head 10 and the Doppler signal from the interferometer 11 are converted to logic pulses by the laser display 14, and these logic pulses are applied to the calculator 15. As a specific example of components which may be used in the laser distance measuring system, the laser head 10 may be a Hewlett Packard model 5500C, the interferometer may be a Hewlett Packard model 10565B, the retroreflector 12 may be a Hewlett Packard 10550B, the laser display 14 may be a Hewlett Packard model 5505A, and the calculator may be a Hewlett Packard model 9815A. Since these components are all commercially available components and their operations are well documented by the manufacturer, no further discussion will be given of the details of the construction and operation of the laser distance measuring system used in the practice of the invention.

Located on the carriage 13 is a light source and photodetector 16 shown in more detail in FIG. 2. In the preferred embodiment, the light source 17 and photodetector 18 are located on either side of the wires 19 of the frame assembly. The light source 17 may be a General Electric model 222 light source, and the photodetector 18 may be a Fairchild model FPT 110 photodetector. The photodetector 18 is masked to provide a 0.051 mm aperture which is smaller than the 0.076 mm diameter of the Be-Cu wire used in the spark chamber frame of the specific example. As the light source 17 moves past a wire 19, the light to the photodetector 18 is blocked by the wire, causing a voltage drop on the phototransistor of the photodetector. This voltage drop generates a trigger pulse in the trigger command and control unit 20. The trigger command and control unit triggers the command circuit of the laser display 14, and the external print command and external pause command signals are generated by the laser display 14. The external pause command is used to register the distance between wires to the output stage of calculator 15. The subsequent external print command initiates a printout cycle to the calculator 15. The printer 21 prints out the wire number, the ideal position, actual position, and amount of error for each wire as illustrated in Table 1 below:

Where an error exceeds the tolerance, the wire is flagged as illustrated in Table 2 below:

In addition to the foregoing, the distance of travel corresponding to each sampling can also be automatically printed out by the printer 21. A specific example of a printer which may be used is the Teletype Corp. model 33 printer.

The trigger command and control unit 20 is shown in more detail in FIG. 3. The phototransistor Q1 of the photodetector has its collector connected to a positive five volt regulated supply and its emitter connected to the non-inverting input of signal amplifier 22 through series resistor R2. A bias voltage is supplied to the inverting input of amplifier 22 by an adjustable potentiometer R3 connected across the five volt regulated supply and a resistor R4. A feedback resistor R5 is connected between the output and the inverting input of amplifier 22, while a parallel RC network consisting of resistor R1 and capacitor C1 is connected between the emitter of transistor Q1 and common. The output of amplifier 22 is connected through series resistor R10 to the inverting input of an amplifier 23 connected as a Schmitt trigger. The threshold voltage for the Schmitt trigger is established by a voltage divider consisting of resistors R6 and R7 connected to the five volt regulated supply. A capacitor C2 is connected across resistor R7, and the junction of resistors R6 and R7 is connected to the non-inverting input of the Schmitt trigger 23 through a series resistor R8. A feedback resistor R9 is connected between the output and the non-inverting input of the Schmitt trigger. The output of the Schmitt trigger is supplied to a pulse shaping circuit 24 which provides an output trigger pulse to the laser display 14. The pulse shaping circuit 24 may be a monostable multivibrator which is provided with a pulse width determining RC circuit comprising capacitor C3 and resistor R11. By connecting potentiometer R12 in series with resistor R11 and the five volt regulated supply, the pulse width may be adjusted.

The emitter of transistor Q1 is also connected to the non-inverting input of amplifier 25 which has its output directly connected to its inverting input. Therefore, amplifier 25 acts as a voltage follower. The output of amplifier 25 is connected through series resistor R14 to the inverting input of amplifier 26. Negative feedback for amplifier 26 is provided by resistor R15. A bias voltage is supplied to the non-inverting input of amplifier 24 by means of potentiometer R17 and series resistor R16. The output of amplifier 26 is connected through series adjustable resistor R18 and resistor R19 to the positive terminal of a milliammeter 27. The voltage follower 25 and inverting amplifier 26 are used to peak the meter 27 as soon as an output is provided by transistor Q1 to indicate the wire position.

The light source 17 is supplied with current from the five volt regulated supply through the series connection of an intensity indicator lamp 28, variable resistor R13 and resistor R20. The five volt regulated supply 29 is itself supplied from a 115 VAC source. The motor 30 and its associated clutch for the carriage 13 are also supplied from the 115 VAC source.

In one working embodiment of the invention, the following values for the various circuit components were used. The listing of these values is by way of example only and is not meant to limit the scope of the invention in any manner.

While the preferred embodiment has been described in terms of one specific example, it is possible to make various modifications in the system without departing from the spirit of the invention. For example, it is possible to locate the light source and photodetector on the same side of the wire plane by using reflected light. Such a modification would of course require modification of the trigger command and control unit circuit, but such modification is within the skill of the art and will not be further described. Further, as shown in FIG. 1, an automatic compensator 31 may be provided for the laser display 14 to increase the accuracy of the measurements. The automatic compensator automatically and continuously corrects the laser display for air conditions and, when desired, material temperatures. The compensator has an air temperature sensor, humidity sensor, and a barometer. The compensator may be a Hewlett Packard model 5510A. Other modifications, adaptions and uses of the invention will readily suggest themselves to those skilled in the art.