Patent Application
20090295638
This application claims the priority of European Patent Application, Serial No. EP 08009649, filed May 27, 2008, pursuant to 35 U.S.C. 119(a)-(d), the content of which is incorporated herein by reference in its entirety as if fully set forth herein.
The present invention relates to a measurement signal evaluation of a position measuring device.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
In technical systems, in particular in automation technology, it is often necessary to determine the time difference between the occurrence of two events in a very accurate manner for a wide variety of tasks. A periodic square-wave pulse signal which is generated by a clock generator is usually used for this purpose. In commercially available systems, the time difference between the two events is determined from the number of square-wave pulses between the two events.
The resolution of the time difference between two events is thus limited by the frequency at which the square-wave pulses are generated by the clock generator. The higher the intended temporal resolution of the determined time difference, the higher the selected frequency of the square-wave pulse signal must be. If a very high resolution is intended to be achieved, the frequency of the square-wave pulse signal must accordingly be selected to be high, that is to say the square-wave pulses follow one another at very short intervals of time. However, square-wave pulse signals at a high frequency produce electromagnetic interference which may result in malfunctions, in particular in electrical components arranged in the vicinity.
Conventional clock frequencies which are used, for example, are in a range of 100 MHz or more. In this context, it is conceivable to evaluate a plurality of clock signals, which are delayed with respect to one another, using a plurality of logic gates in order to increase the temporal resolution. However, a very large number of delay elements are required in this case.
Furthermore, the time difference between two events can be measured by sampling a linearly rising voltage, for example by sampling the voltage across a capacitor which is charged using a current source. However, such a time measurement is relatively inaccurate.
In commercially available position measuring devices for measuring a position, for example, of a movable machine element, for example a motor shaft, two signal transmitters generally scan a movable material measure and, in a manner corresponding to the graduation arranged on the material measure, generate periodic signals which are used to determine the current position of the machine element with the aid of a measurement signal evaluation means. An example of such a position measuring device is described in German Pat. No. DE 10 2004 038 621 B3, whereby a position is determined from two sinusoidal signals, which have a phase offset with respect to one another, using a measurement signal evaluation means. Two signal transmitters scan the material measure graduation and output the two sinusoidal measurement signals which have a phase offset with respect to one another. The two measurement signals are then sampled in a fixed time frame using two analog/digital converters. A downstream measurement signal evaluation means determines the position signal from the two measurement signals which have been digitized in this manner. The measurement signal evaluation means subjects the measurement signals to analog/digital conversion on the input side and outputs the position signal determined by it on the output side.
In general, commercially available position measuring devices generally have a measurement signal evaluation means which determines a position signal, i.e. a position, from two or more measurement signals.
German patent document DE 27 29 697 A1 discloses a measurement signal evaluation means of a position measuring device to determine a position signal from two measurement signals. In this case, in order to increase the accuracy, such commercially available measuring devices often also include, for example, special methods for compensating for and/or correcting offset, amplitude, phase and harmonic errors in the measurement signals in order to increase the accuracy of the position determined.
Common to conventional measurement signal evaluation means of position measuring devices is the determination of a corresponding position, i.e. a corresponding position signal, from relatively low-frequency measurement signals with a very high level of accuracy.
It would be desirable and advantageous to address prior art shortcomings and to measure a time difference between two events with a high temporal resolution, electromagnetic interference caused by the time measurement being reduced.
According to one aspect of the present invention, a method for determining a time difference between a first event and a second event includes the steps of determining a first position value from a first periodic signal that is triggered when the first event occurs, determining a second position value from a second periodic signal that is triggered when the second event occurs, and determining the time difference from a difference between the first and second position values.
According to another aspect of the present invention, a position measuring system includes a measurement signal evaluation means for determining a time difference between a first event and a second event, the measurement signal evaluation means configured to determine a first position value from a first periodic signal that is triggered when the first event occurs, determine a second position value from a second periodic signal that is triggered when the second event occurs, and determine the time difference from a difference between the first and second position values.
According to yet another aspect of the present invention, q position measuring system includes a first signal generator signal producing a first periodic signal, a first A/D converter receiving the first periodic signal, a second signal generator signal producing a second periodic signal, a second A/D converter receiving the second periodic signal, an OR gate receiving a first trigger signal produced by an occurrence of a first event, and a second trigger signal and second event respectively, and transmitting the first and second trigger signals to corresponding trigger inputs of the first and second A/D converters and producing first and second triggered output signals that correspond to the periodic signals at a time of the first and second event, and an evaluation unit receiving at a first input the first triggered output signals corresponding to values of the first periodic signal for the first event and the second event, and receiving at a second input the second triggered output signals corresponding to values of the second periodic signal for the first event and the second event, the evaluation unit having an output that generates position values to be used for determining the time difference from a position difference between the first and second position values.
The present invention resolves prior art problems by determining the time difference through determination of the positional difference between the first position value and the second position value and weighting the positional difference with a proportionality factor. This ensures simple determination of the time difference. It also proves to be advantageous if the periodic signal is generated using a signal generator assigned to the periodic signal or the periodic signals are generated using signal generators assigned to the periodic signals. This makes it possible to generate the periodic signal or the periodic signals in a particularly simple manner.
It also proves to be advantageous if the periodic signal or the periodic signals is/are sampled solely when the first and second events occur in order to generate input values for the measurement signal evaluation means. This makes it possible to use the measurement signal evaluation means in a simple manner.
It also proves to be advantageous if the time difference determined in this manner is used to determine the position of a movable magnet in a magnetostrictive position measuring device. A high temporal resolution is required, in particular, in magnetostrictive position measuring devices.
Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:
Throughout all the figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the figures are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted.
According to the present invention, a measurement signal evaluation means is used to determine the time difference between a first event and a second event.
The two signals x1(t) and x2(t) are supplied, as an input variable, to the respectively associated analog/digital converters 11 and 12. When a first event A occurs and when a second event B occurs, a gate pulse i is respectively generated at the output of a logic OR gate 13, which pulse causes the two analog/digital converters 11 and 12 to respectively sample the analog signals x1(t) and x2(t) once. The occurrence of the first event A and of the second event B is supplied in this case to the OR gate 13 in the form of a respective trigger pulse as an input variable. The two analog/digital converters 11 and 12 thus generate the signal values x1(ta) and x2(ta) sampled at the time ta at which the first event A occurs and the signal values x1(tb) and x2(tb) sampled at the time tb at which the second event B occurs. The signal values x1(ta) and x2(ta) and the signal values x1(tb) and x2(tb) are then supplied, as input values, to a measurement signal evaluation means 3 which is known from the prior art and carries out the measurement signal evaluation already described.
On the output side, the measurement signal evaluation unit 3 determines a position value fA from the two signal values x1(ta) and x2(ta) and a position value fB from the two signal values x1(tb) and x2(tb). In this case, the measurement signal evaluation which is carried out in the measurement signal evaluation unit 3 corresponds, for example, to the measurement signal evaluation disclosed in German patent document DE 10 2004 038 621 B3, with the difference that in accordance with the invention, the two signal transmitters, as described in German patent document DE 10 2004 038 621 B3 and the material measure are replaced with the two first and second signal generators 1 and 2 indicated in
As illustrated in
Δφ=φB−fA
within the scope of the exemplary embodiment.
On the output side, the positional difference determination unit 4 outputs the positional difference Δφ determined in this manner to a multiplier 5 within the scope of the exemplary embodiment, said multiplier weighting the positional difference Δφ by multiplying it by a proportionality factor C within the scope of the exemplary embodiment. The proportionality factor C depends on the frequency f of the periodic signals x1(t) and x2(t) and, provided that the two signals x1(t) and x2(t) are at the same frequency f and are sinusoidal and are temporally offset with respect to one another, results, for example, as:
With the aid of the weighting which is carried out in this manner in the form of a multiplication by the proportionality factor C, the positional difference Δφ determined is converted into a corresponding time difference T and is output at the output of the multiplier 5. The time difference T determined in this manner can then be processed further in any desired manner according to the respective technical requirements.
For the sake of completeness,
φ=ωt
within the scope of the exemplary embodiment, rather than against time t. In this case, the two periodic signals x1(t) and x2(t) are given by
x
1(t)=D sin(ω·t), x2(t)=D cos(ω·t)
D: amplitude.
When the event A occurs at the time tA, the two signal values
x
1(tA)=D sin(ω·tA) and
x
2(tA)=D cos(ω·tA)
are detected and, when the event B occurs at the time tB, the two signal values
x
1(tB)=D sin(ω·tB) and
x
2(tB)=D cos(ω·tB)
are detected.
The following is first of all calculated from the signal values detected on the basis of the event A:
f′
A
=a tan 2(x1(tA), x2(tA))=arg(x2(tA)+jx1(tA)),
where a tan 2(b, a) and arg(a+jb) denote the argument of the complex number a+j b, with −p=arg(a+jb)=p (that is to say a+jb=|a+jb|ej arg(a+jb)).
The following is then calculated:
f
A=mod(f′A,p/2), where mod( ) is the modulo function.
In a corresponding manner, the following is calculated from the signal values detected on the basis of the event B:
f′
B
=a tan 2(x1(tB), x2(tB))=arg(x2(tB)+jx1(tB)).
The number q of zero crossings of the signals x1(t) and x2(t) between the events A and B is also determined. q is then increased by 1 if the first zero crossing counted
was a positive zero crossing of x1(t) in the case −p=f′A<−p/2,
was a negative zero crossing of x2(t) in the case −p/2=f′A<0,
was a negative zero crossing of x1(t) in the case 0=f′A<p/2, or
was a positive zero crossing of x2(t) in the case p/2=f′A<P;
in contrast, q is decreased by 1 if the first zero crossing counted
was a negative zero crossing of x1(t) in the case −p=f′A<−p/2,
was a positive zero crossing of x2(t) in the case −p/2=f′A<0,
was a positive zero crossing of x1(t) in the case 0=f′A<p/2, or
was a negative zero crossing of x2(t) in the case p/2=f′A<p.
Subsequently, q is increased by 1 again if the last zero crossing counted
was a negative zero crossing of x2(t) for −p=f′B<−p/2,
was a negative zero crossing of x1(t) for −p/2=f′B<0,
was a positive zero crossing of x2(t) for 0=f′B<p/2, or
was a positive zero crossing of x1(t) for p/2=f′B<P;
in contrast, q is decreased by 1 if the last zero crossing counted was a positive zero crossing of x2(t) for −p=f′B<−p/2,
was a positive zero crossing of x1(t) for −p/2=f′B<0,
was a negative zero crossing of x2(t) for 0=f′B<p/2, or
was a negative zero crossing of x1(t) for p/2=f′B<P
As a result, q−1 indicates the number of quadrants of the signals x1(t), x2(t) which have been completely passed through between the events A and B. The possible corrections by +1 or −1 according to the first and last zero crossings counted are necessary in this case because measurement inaccuracies may result in inconsistencies both between the first zero crossing counted and the quadrant determined for fA and between the last zero crossing counted and the quadrant determined for f′B.
Instead of the above-described determination of q, DE 27 29 697 A1 uses a direction discriminator and an electronic up/down counter since there the process does not start from a sin/cos transmitter which rotates at a constant angular velocity but rather must also take changes in the direction of rotation into account.
The following:
f
B
=q·p/2+mod(f′B,p/2)
is then calculated.
The positional difference then results as
Δφ=φB−fA
and is output at the output of the positional difference determination unit 4.
The time difference T then results as
where the above relationship is implemented in the multiplier 5.
The invention can be used in a particularly advantageous manner in position measuring systems for measuring a position, for example in magnetostrictive position measuring systems in which it is necessary to accurately determine the time difference between two events, which occur after one another in a short period of time, in order to determine a position.
In order to determine the position x of the permanent magnet 7 at the time tv (occurrence of event A), the time difference T is multiplied by a constant K using a multiplier 6, the constant K resulting from the speed of the structure-borne sound wave in the measuring rod 8 whilst disregarding the propagation time of the electrical starting pulses 9.
The position x of the permanent magnet 7 at the time tv thus results as
x=K·T.
The time difference T to be determined between the starting pulse, which represents the first event A according to
The interference caused by the necessary time measurement can thus be reduced by the invention.
However, it goes without saying that the invention can also be used in other types of position measuring systems which are based on the measurement of a propagation time, that is to say in other types of position measuring devices which are based on the measurement of a propagation time.
Magnetostrictive position measuring systems are disclosed, for example, in U.S. Pat. No. 5,334,933, U.S. Pat. No. 3,898,555 and European patent document EP 0 442 985 B1. Furthermore, magnetostrictive position measuring systems are disclosed in the book entitled “Lineare Weg-und Abstandssensoren” [Linear Displacement and Distance Sensors], 2004, ISBN 3-937889-07-8, pages 53 to 66. Magnetostrictive position measuring systems are also disclosed, for example, in a leaflet entitled “Magnetostriktion, Physikalische Grundlagen” [Magnetostriction, Physical Principles] from MTS Sensors which is disclosed on the web page www.mtssensor.de/fileadmin/medien/downloads/mts_messprinzip.pdf.
The signal x1(t) is supplied to the analog/digital converter 11 as an input variable. When a first event A occurs and when a second event B occurs, a gate pulse i is respectively generated at the output of the logic OR gate 13, which pulse is forwarded via the OR gate 14 and causes the analog/digital converter 11 to respectively sample the analog signal x1(t) once. The occurrence of the first event A and of the second event B is supplied in this case to the OR gate 13 in the form of a respective trigger pulse as an input variable.
The analog/digital converter 11 thus generates the signal value x1(ta) sampled at the time ta at which the first event A occurs and the signal value x1(tb) sampled at the time tb at which the second event B occurs. However, after each occurrence of an event, the time delay element 15 causes the analog/digital converter 11 to carry out a further sampling operation, which is temporally offset by the period of time TV, and thus to generate the signal values x1(ta+TV) and x1(tb+TV) on the output side. The switches 16 and 17 are used to output the signal values x1(ta), x1(ta+TV), x1(tb) and x1(tb+TV) to the measurement signal evaluation means 3 as input values, the switches 16 and 17 switching their input signal through to their output when a logic “1” occurs at their control input S.
In this case, the signal value x1(ta+TV) corresponds to the signal value x2(ta) according to
While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit and scope of the present invention. The embodiments were chosen and described in order to explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.
What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein:
| Number | Date | Country | Kind |
|---|---|---|---|
| EP 08 00 9649 | May 2008 | EP | regional |
