Optics: measuring and testing – Position or displacement – Position transverse to viewing axis
Reexamination Certificate
2000-11-20
2003-01-28
Pham, Hoa Q. (Department: 2677)
Optics: measuring and testing
Position or displacement
Position transverse to viewing axis
C250S231180, C033S707000
Reexamination Certificate
active
06512589
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention is in the field of measurement transmitters, in particular optical measurement transmitters for angles and/or distances.
Transmitter systems, which determine the absolute distance or angular displacement between a sensing head and a relatively shifted scale embodiment and which process measurement values with signal evaluation electronics for a further use, are known on the market. The scale embodiment has in this case a number of digital or analog signal tracks applied thereon.
In recent times, systems are increasingly described in the literature which have a serial absolute code, that is to say a “bit pattern” is applied in a track. These bit patterns are for the most part encoded using pseudo-random codes (PRCs) and are provided for evaluation with a basic interval of 1 bit, for example in optical systems having a number of equidistant photoreceivers (for example with 9 photodiodes in the case of 9 bits). In this case, however, for a further fine resolution of the absolute distance or angle information, an incremental track (basic resolution as absolute value) applied in parallel is also considered, with the generation of a SIN/COS signal, which can be interpolated appropriately (AD conversion). Thus a higher signal resolution is achieved by essentially using two tracks. Such a measurement transmitter is described in Published, Non-Prosecuted German Patent Application No. DE 195 18 714.8, for example.
Irrespective of the difficult signal processing both of the absolute values and of the SIN/COS signals in the distance transmitter and angle transmitter itself, it is not possible to achieve desired high resolutions. As the speeds or numbers of revolutions increase further in the respective technological applications (for example in the case of technologically advanced servo drives) currently possible signal processing speeds in electronic modules emerge as a limiting factor (for example in application-specific integrated circuits (ASICs), microprocessors and so on). A further limiting factor in advanced applications is the signal transmission over relatively long lines of, for example, a few meters in length. In the case of rotary encoders, for example, with 6,000 revolutions/min and 12-bit resolution, the resulting processing speeds are approximately 5 MHz.
If it is desired to evaluate the absolute value in the case of a serial code (see Non-Prosecuted German Patent Application No. DE-195 18 714.8) of for example at least 12 bits in an expedient way, without increasing the outlay on electronics and costs in the case of an ASIC developed for this purpose to an unacceptable extent, the obvious course is to clock out the
12
diode values (12 bits) serially. However, it becomes immediately apparent that the reading time=clock time×bit resolution=(1/clock frequency)×bit resolution will then be increased, and the continuous signal evaluation in real time is a limiting factor for many technological applications.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method and a device for position measurement which overcome the above-mentioned disadvantages of the heretofore-known methods and devices of this general type and which allow to increase the reading times or evaluation times approximately to the maximum clock time itself.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for measuring a position of a sensing head with respect to a scale embodiment, the method includes the steps of:
providing a sensing head including at least one sensor;
providing a scale embodiment including a linear code;
obtaining, with at least a given resolution, an absolute value of an initial position of the scale embodiment with respect to the sensing head;
moving the sensing head and the scale embodiment relative to one another for deriving sensor signals of a relative movement from the linear code; and
measuring, with the given resolution, a position of the scale embodiment with respect to the sensing head by obtaining information from a first number of the sensor signals of the relative movement, the first number of the sensor signals of the relative movement being smaller than a second number of sensor signals required for a position measurement with the given resolution at a relative speed of zero of the scale embodiment with respect to the sensing head.
In other words, in order to measure the position of a sensing head, which contains at least one sensor, with respect to a scale embodiment which contains a linear code, in such a way that a predefined resolution is achieved, an absolute value of an initial position at at least the predefined resolution is obtained initially. During the measurement, the sensing head and the scale embodiment are moved relative to each other and fewer sensor signals are used than would be needed at a standstill, information needed to achieve the predefined resolution being obtained from sensor signals of the relative movement, which are derived from the linear code.
The invention is based on the idea of a “data compression” (with a certain analogy to the methods known in image processing). Data which change and are relevant are—when possible and practical—registered or recorded, transmitted and evaluated. Data which do not change and are redundant are ignored. This reduces unnecessary redundancy or avoids it altogether. Thus it is possible to reduce the quantity of data to be transmitted and processed.
The method according to the invention and the measurement transmitter according to the invention are particularly well suited for optical measurement systems; however, they can also be transferred to other measurement systems, for example to magnetic, electric and/or electrostatic systems. In order to explain the method and the measurement transmitter in terms of their basic principle and their action, an optical system will be explained below in an exemplary manner. In these exemplary embodiments, photodiodes are used for the detection of light; of course, other types of photodetectors or photoreceivers can also be used.
The invention makes use of the fact that the linear code is moved bit by bit past an observer, for example past an individual photodiode. If the code formation law is then known and the relative movement between the sensing head and the scale embodiment sweeps over or exceeds at least the step width of the bit resolution for absolute values, then the positional location can be determined from this point on. Thus, for example in the case of an absolute transmitter with a 12-bit PRC (pseudo-random code), at a standstill, 12 photodiodes are needed to determine the absolute position. If the sensing system is then moved in a relative manner by at least these 12-bit positions (=step width of the resolution) then from this point on, through the use of the inventive idea, the evaluation of the 12 photodiodes needed at a standstill can be dispensed with and the evaluation system can be reduced virtually down to a 1-bit photodiode.
This has significant advantages, since in this way only one photodiode is sufficient for determining the absolute position, and therefore the limiting speed of the transmitter system can be brought up to the clock frequency to be processed. In the example presently being explained, with a 12-bit absolute code, the limiting speed can then be increased by the factor
12
. This turns out to be even higher in practice in the case of integrated photodiodes, since special measures which increase the switching frequency can easily be made for a photodiode. The absolute values can therefore then be determined at high speeds and under real-time conditions, so that the limiting rotational speed of about 1000 rev/min, which currently limits the measured-value transmission in 12-bit absolute rotary encoders, can be shifted upwards significantly. For high-resolution servo systems, this is the sought-after solution in application use. It is therefore also poss
Greenberg Laurence A.
HERA Rotterdam B.V
Mayback Gregory L.
Pham Hoa Q.
Stemer Werner H.
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