DE19722524A1 - Optical frequency sensor for measuring speed of objects - Google Patents

Abstract

The optical frequency sensor measures the relative speeds of a line grating and an optical scanning head, to obtain the proportional frequency. A correlator lens (3) is used in the scanning head, with at least one measuring grating on which the line grating is projected, and is adjusted to an odd harmonic of the line grating. The harmonic factor, k, is achieved by using a smaller constant for the measuring grating and by optically enlarging the line grating. Two or four black and white reference gratings may be provided, with k=3,5,7 or higher odd multiple spatial frequencies of the line grating projected on them. The reference gratings are displaced by (180 deg ,90 deg ) with two or four photocells to produce phase signals of the corresponding harmonic.

Description

State of the art

Optical frequency transmitters have the task of making the speed rotate or linearly moving body and measure as a proportional frequency to spend. By counting the periods, the distance covered can also be An angle or path are determined, which usually requires a second To generate alternating voltage of the same frequency but out of phase from this to determine the direction of movement (and when moving backwards count down.)

In the best known type, the incremental encoder, there is one on the body Line division attached, which - mostly optically - is scanned. Often, however not necessarily, the optical scanner has a gap instead also a line division, a "reference division" that is the same "Spatial frequency", i. H. has spatial division constant. The photocell will be like this the average of several lines of division. Except for those excellent by 90 °, phase-shifted voltage for direction detection you also need reference grids and photocells for each 180 ° shifted signals to equal light and in differential amplifiers Filter out interference frequency signals.

For measuring sections on which grid divisions cannot be attached, e.g. B. roads or running tape material, there are also donors who only in Probe reference divisions have: the optical correlators. You use the The fact that the statistical irregular fluctuations in brightness of the Surfaces when scanning through a slit similar to noise Result in voltage that contains all frequencies. The reference grids in the scanner are mapped onto the surfaces, so to speak, "correlate" with those there existing same spatial frequency and result in the filtered time frequency proportional to speed. Of course, their amplitude is only a very small fraction of the overall brightness, which is a much larger one Gain and better frequency filtering are required than with incremental encoders. The phase position of the statistically obtained frequency is also ongoing Subject to fluctuations. In the known, only for statistical Structures certain correlators are used as prism gratings Measuring grating uses the 0 ° and 180 ° signals through a field lens concentrated in different directions on one photocell each. So the image strips mix on the same image section, and their equals Light and external frequency components can be more effective in the difference ver are eliminated more than is possible with separate reference grids is. Light is used twice as much as black and white grids.  

The resolution of incremental encoders must above all during the measurement slow movements are great, so the long periods for regulators do not cause inadmissible dead times. You can by frequency Ver multiplication can be increased: with rectangular 0 ° / 90 ° signals by Mixing up to 4 times, with sinusoidal signals by interpolation to higher multiples. However, the latter requires a lot of effort and increased Precision.

According to the invention, the disadvantages of both methods in claim 1 Task to achieve very high resolutions through their combination overcome. For this combination, new correlator systems are used in A4 to A9 suggested that even in the original scope unmarked objects are advantageous.

In Fig. 1, the previously used, known type of the correlator is shown: The moving in the direction of arrow 2 unmarked object 1 is imaged by the lens 3 onto a transparent prism grating 4. Only a small section of this is shown greatly enlarged. The prism shape alternately deflects a slit-shaped area of the image on one side, the next on the other side, so that the field lens 5 images one area on photocell 6 and the other on photocell 7 . Their signals are fed to a differential amplifier 8 , which suppresses all constant light and external frequency components and amplifies only the signals associated with the correlating spatial frequency. Because of the interlacing of the 0 ° and 180 ° signals in the same image section, this works particularly well, better than with separate grids in different sections.

In claim 3, the combination of this known correlator with a line division on the object is claimed ( FIG. 1, but with a line grid instead of the unmarked object 1 ), the additional advantage being the considerably larger amplitude of the odd harmonics and a fixed phase position with respect to the fundamental frequency .

Because of the large amplitude, the harmonic cannot be too high use cheaper black and white reference grids, claim 2. The This is the only difference between the encoder and the conventional one Incremental encoders with reference grids that this one around the 3, 5, 7.. . times have higher spatial frequency than the object grid.  

In FIG. 2, 10 is a variation of the method claimed in claim 4 prism grid 4, which eliminates the need for a second Korrelatoroptik for generating a 90 ° voltage. The 4 different prism angles of the hipped roof-shaped prism surfaces 11 , 12 , 13 , 14 steer the light beams from 4 adjacent narrow image zones in 4 different directions. From a field lens (not shown), such as 5 in Fig. 1, they are bundled on 4 photocells. The signals of the 0 ° and 180 ° photocells are switched to one, those of the 90 ° and 270 ° photocells to a second differential amplifier, which output the phase-shifted square-wave voltages.

In FIG. 3, 21 lines shown, 22 permeable gaps of the object grid in the plane of a photocell array according to claim 5 with a section 23 to 38 of the individual cells. Since 2 adjacent sub-cells each represent a period of the harmonic wave and that in FIG. 3 example shown to result in the 5th harmonic, an object-lattice period is mapped to 10 sub-cells. The signals of the odd-numbered cells 23 , 25 to 37 are amplified in the amplifiers of the series 39 , those of the even-numbered ones in amplifiers of the series 40 to such an extent that the brightness values are "digitized", that is to say only as "light / dark", based on one Threshold to be spent. The outputs of all amplifiers 39 and 40 are connected to an equal-sized summing resistor, series 41 and 42 . The other ends of the resistors of each row are connected to one another and those of the row 41 (odd-numbered) with the inverting, that of the row 42 (even-numbered) with the non-inverting input of the differential amplifier (comparator) 45 . If all the amplifiers of the rows 39 and 40 invert, then those of the photocells darkened (by the lines 21 ) are 23 to 27 and 33 to 37 . . . . high, that of the (in the spaces 22 ) illuminated photocells 28 to 32 , 38 . . .low. In the first grid period, cells 23 to 32 , 3high + 2low (= 1 high) outputs are given to the inverting, 2high + 3low (= 1low) outputs to the non-inverting input of amplifier 45 , whose reference voltage is in the middle between the "high" and "low" voltage must be in order for the logic zero signals to be calculated analogously as -1. . This is repeated in all n periods covered by the measurement. So a 2n fold low will switch the output to low.

Let us imagine a movement of the grid image 21 , 22 , 21 , 22 . . . down one photocell width: cells 24 to 28 = high, 29 to 33 = low. Thus there is now 2 high and 3 low on input 43 , 3 high and 2 low on + input 44 , a double high in effect, 2n times high of n periods.

A further movement by another photocell width allows amplifier 45 to switch back to low. A square wave corresponding to the 5th harmonic of the grating frequency is thus generated. The same with an upward movement of the grid image.

Are shown in FIG. 4, according to the A.6, the bars and spaces of the object grid 21, 22, 21,. . . mapped to 10 sub-cells of the array. (k = 5, 4 k = 20 cells per period). Each cell holds 90 ° of the harmonic. Every 1st, 5th, 9th, i.e. cell 51 . . ., is connected via amplifier and resistor to input 56 , every 3rd, 7th, 11th, i.e. cell 53 . .q. . connected via amplifier and resistor to the + input 57 of the 0 ° amplifier 58 , every 2nd, 6th, 10th (52....) also to the -input 59 , every 4th, 8th, 12th, . . . ( 54 ...) Is also connected to the + input 60 of the 90 ° amplifier 61 , which output the phase-shifted voltages.

To explain the A.7 for the simultaneous generation of different frequencies, e.g. B. the 5th harmonic and the fundamental are drawn in Fig. 5 per period 21 + 22 ten adjacent photocell amplifiers (without numbering), to the outputs of which two summing resistors are connected. All of the upper resistors of the row 62 are, as in FIG. 3, connected alternately to the two inputs of the amplifier 63 , so that the latter generates (in the example again) five times the harmonic. Of the lower summing resistors 62 , five adjacent ones are connected to one another and alternately connected to the + and - inputs of the amplifier 64 . As a result, it outputs five times the period, i.e. the fundamental wave.

The idea of the A 8 to utilize the signals of the partial photo cells with 3 or more summing resistors for 3 or more output voltages is such a simple extension of the A 7 and FIG. 5 that a graphic representation is unnecessary. This task arises e.g. B. in two-dimensional arrays. With them the signals of the row pixels of the same number of all columns are connected to one another and then alternately fed to an amplifier, as in FIG. 3, for a harmonic voltage, or two amplifiers, as in FIG. 4, for two shifted harmonic voltages. In addition, a fundamental wave could also be obtained, as in FIG. 5.

With a further summing resistance per pixel, all the pixels in a row summarized and alternately connected to another amplifier, the then outputs frequencies, the speeds, or, in the case of grids, measure additional signal across the lines.  

In A.9 an attempt is made to use arrays of CCD imagers in this application exploit the described purpose. CCD cameras are in large numbers on the Market, so there is hope, especially with parts from this large series to be able to work inexpensively. They have the disadvantage of being in one sampling cycle to work within which the object grid must not move any further, than half a harmonic. On the other hand, the improved Resolution required especially for slow movements. The exposure with a light emitting diode would, if this limit range is approached, as a flash from Z. B. 1 µsec, controlled by the scanning end of the CCD sensor. Example of a speed measurement: The 5th harmonic of a 1000 line per The circumference of the grid is 1/5000 of the circumference, at 40 mm diameter approx. 25 µm Half period 12.5 µm. The long-known linear array Tc 102 with 128 pixels can be scanned at 10 MHz and then requires approx 20 µsec. 1 revolution with 10,000 half-periods of 20 µsec each would be 200 msec for the fastest rotation or 5 rotations / sec = 300 / min. So there is a technically interesting area.

A possible evaluation of the CCD signals is shown in FIG. 6 as an example. The CCD sensor 70 is connected with the video signal output 71 to an amplifier 73 , in which the video signal is digitized, ie all pixels above a brightness threshold give a "1", below this threshold a "0". The digital output signal is at the input of a counter 75 . The internal switching frequency from pixel to pixel, at the output 72 of the sensor 70 , also drives a counter 74 , which only counts to 2, the "0" of which always clearly falls on the odd, the "1" on the even pixel numbers. This signal is on the up / down input of counter 75 , so that the ones of all odd pixels add up, which are subtracted from the even pixels. The counting result can easily be derived without an image: if, as in the example in FIG. 6, only one output frequency is to be generated (count to 2), then a line is depicted, as is a bright space between the object grid per k pixels, e.g. . B. at k = 5, the 5th harmonic, 5 pixels each. Alternately, 5 pixels are light and 5 are dark. The dark ones do not give a count at counter 75 . (In contrast to the analog evaluation in FIGS. 3 to 5, in which the "0" signals are counted as -1.) On the other hand, always 5 ones of the bright zones are counted, alternately added and subtracted. If an odd, adding pixel begins, the last pixel of this zone is also odd because of the always odd k - and vice versa. In between there is always one less with the opposite sign. Result +1 or -1, like the "border pixels".

With each subsequent light period, another 1 is added or subtracted. If the object grid has moved on by one pixel width, the sign changes. The number of grating periods shown is determined by the geometry of the grating, sensor and imaging scale. In the example of FIG. 6 with a line CCD of 128 pixels 12 periods. In the example above, this number also appears alternately positive and negative at the output of counter 75 : +12, -12. . . . This happens in the rhythm of the harmonic sought. The D / A converter 76 is shown as an example for conversion into an analog signal.

It is easy to imagine, instead of counting to 2 in counters 74 to 4 and shifting every first and third pulse, as in FIG. 6, to an output voltage and every second and fourth in a different counter to 90 ° Process voltage, as happened in A.6 analog. Likewise, the analog methods of claims 7 and 8 for generating further frequencies and for measuring perpendicular to the first direction of movement can be implemented with further counters, electronic switches and converters on the CCD technology. Of course, all signal assignments, additions and subtractions can also be carried out by a small micro -Processor to be performed, even in the case of the circuits according to claims 5 to 8 in the examples.

Claims (9)

1. Optical frequency generator, which is to measure the relative speed of a grating to that of an optical probe as a proportional frequency, characterized by a correlator optic as a probe, with at least one measuring grating set to an odd harmonic of the grating, on which the grating is imaged, whereby the harmonic factor k can be achieved both by a smaller measuring grating constant and by optical enlargement of the grating. 2. Opt. Frequency generator according to A1, characterized by two or four black and white reference grids of k = 3, 5, 7 or higher odd multiple spatial frequency of the grating shown on them, each by 180 ° or 90 ° offset, with 2 or 4 photo cells the phase signals of the generate the corresponding harmonic. 3. Opt. Frequency transmitter according to A1, characterized by a known transparent prism grating, which the nested 0 ° and 180 ° strips, (1/3 or 1/5 or 1/7...... Half a period of the grating shown wide ,) deflects differently, so that they are combined with a field lens on one photo cell ( Fig. 1) 4. Opt. Frequency transmitter according to A1, characterized by a transparent Prism grid, the 4 measuring beams with prisms flattened like a roof deflects differently, the four photocells with four phase signals offset by 90 ° produce. 5. Opt. Frequency generator according to A1, characterized by a photocell array that the function of the measuring grid and photocells takes over, the photocell-off gears individually amplified, so summarized by summing resistors be that the even-numbered the adding, the odd-numbered the Press the subtracting input of a differential amplifier at its Output the square wave you are looking for appears.   6. Opt. Frequency transmitter according to A1 and AS with array, for additional generation of a frequency shifted by 90 °, characterized by an imaging ratio of one object grid period to 4k partial photo cells (k = no. Of the harmonic wave), the signals of the 1st, 5th , 9.. . ., and the 3rd, 7th, 11th. . Photocells a differential amplifier like 45 in Fig. 3 (AS) for the 0 °, the 2nd, 6th, 10th,. . . and the 4th, 8th, 12th, photocells are fed to a second differential amplifier for the 90 ° voltage 7. Opt. Frequency generator according to A1 and A5 with array, for additional generation further smaller frequencies, preferably the fundamental, thereby characterized in that by means of additional summing resistors several Photocells as "1.", "2.", "3.", group for those described in claim 5 or 6 Function of the 1st, 2nd, 3rd, photocell combined and further up-down counters which are then fed with the smaller frequency simultaneously that of the harmonic. 8. Opt. Frequency transmitter according to AS to A7 with array, characterized by a two-dimensional, checkerboard-like photocell array that passes through Combine the columns as in AS to A7 of the photocells or photocell groups a measurement of the movement in one direction, by appropriate Summary of lines with simultaneous measurement of movement perpendicular to it or a signal applied perpendicular to the grid division allowed. 9. Opt. Frequency transmitter according to AS to A8 with array, characterized by a one- or two-dimensional array with CCD scanning, in which by CCD scanning signal controlled counters electronic switches or gates that Separate the video signal into the individual photocell signals, which according to A5 to A8 by summing and subtracting different pixel groups in advance Down counters of speed-proportional fluctuations of the Generate output count that is simply converted to analog voltages can be.