FR2458814A1 - Measuring system for slowly changing frequency - has programmable clock pulse generator, frequency adjusted higher than measured valve and gas pulse counter reset at each period - Google Patents

Abstract

The system contains a clock pulse generator (2) adjusted to an oscillation frequency higher than that to be measured. The generator pulses are counted, the counter (4) being next at an end of each period of the frequency to be measured. The latest count state is fed to a comparator (5) which programmes the clock pulse generator in such manner as to raise or lower its oscillation frequency. When the counter state, supplied to the comparator, is not identical with the preset frequency difference, in upwards or downwards sense, the comparator actuates the clock pulse generator accordingly. In adjusted state the clock pulse generator oscillation frequency is used as a value proportional to the measured value. Preferably the clock pulse generator, the counter, and the comparator are grouped into a control circuit.

Description

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  The present invention relates to a method and a device for measuring a frequency by digital servoing, the measured frequency

slowly changing over time.

  It is known to measure the frequency of the pulses by means of a counter counting the number of frequency tops for a given time. However this classic solution can not be both fast and accurate. Indeed, if you want precision, you have to count a large number of pulses so the measurement is slow. If you want to be fast, you can only count a small number of pulses so the measurement is imprecise. It should be noted that for a signal frequency f = 1 Hz for example, the response time of the measurement being related to the relative precision x of the measurement (for example x = 1%) by the relation 571 -x la classic measurement is done with a time of

  response greater than 100 seconds which is excessive.

  The device according to the present invention overcomes this disadvantage.

  In this one indeed, the precision can be as big as one wants

  while the response time of the measurement is reduced in significant proportions.

  The subject of the present invention is a method for measuring a frequency f that varies slowly over time, characterized in that a number N

  proportional to the value of the frequency f to be measured is obtained by

  a programmable clock at the frequency k "f, this servocontrol consisting in increasing the number N or decreasing it in function

  a comparison of F with a fixed reference k ".

  The present invention also relates to a device for measuring a frequency f with a slow variation over time, characterized in that

  programmable clock delivers pulses whose frequency is proportionally

  to a number N, which is taken for said measurement and which comes from a down-counter counting or counting the pulses of frequency f, according to a comparator, comparing a number F equal to the contents of a counter receiving the pulses. of the programmable clock at the time of the reset which is performed at the frequency f, at a reference number k ",

  provides logical states O or 1 depending on whether F <k "or F>, k".

  The invention will be better understood by studying the example given hereinafter with reference to the appended drawing in which FIG. 1 represents a schematic diagram of the measuring device and FIG.

  frequency measurement as a function of frequency.

  We see in Figure 1 a pulse among a pulse train at the frequency f. The pulses trigger a monostable 1 delivering short pulses at the frequency f. A programmable clock 2 receives from a

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  part a fixed frequency h, on the other hand a number N, coded in binary, variable, the clock 2 being programmed so that its output pulses are at the frequency Nh / k, k being a constant divisor dependent on the number digits of the number N, for example 12, or k = 212. The number N comes from a down-counter 3. It is this number N which is representative of the

  measured frequency f; it is proportional to him.

  The Nh / k pulses from the programmable clock 2 are sent to the input of a counter 4 whose reset is performed by the pulses f. The output of the counter 4 provides a number varying from 0 to F = Nh / kf which is applied to a comparator 5 to be compared with a constant number K. When the process is stabilized, we have the relation t

k "= F = Nh / kf, hence N = k" k f / h.

  For example if we want N to be equal to 100 f, k "k / h must be

  equal to 100 and k "should be equal to 24,414 (h for example being equal to 1 / MHz).

  The comparator 5 has two outputs, one for F <k "and the other for F> k". So when N> k "k f / h, when the monostable 1 is triggered, we have a 1 on the output F>, k". This output is applied to a

  NAND gate 6 at the same time as pulses f of monostable 1 at state 1.

  Only in the case where the two inputs of the door are 1,

  output of the NAND gate is O which results in a countdown of the counter-

  down counter 3 of a unit. The number N thus decreases by one unit each

pulse f, if F> k ".

  If F <k "the counting is not carried out because the output F 2 k" is in the logic state O, the NAND gate 7 receives this logic state O as well as the pulse f in the state 1 therefore delivers an output in state 1 which does not cause

  not counting down-counter 3

  Similarly, when 'N <k' kf / h at the moment when the monostable 1 is triggered, F k 'causes counting of the up-down counter 3 by one unit through the NAND gate 7. The number N thus increases. one unit at each pulse f

if F <k ".

  In our example, N = 100 for f = 1 Hz. It follows that the counter-

  the down-counter is filled with a number N = 100 initially after a duration of 100 seconds. In FIG. 2, it can be seen that when f = 1 Hz goes to f '= 1.02 Hz, the number N = 100 becomes, following two counts, N' = 101 and then N "= 102 for which N stabilizes. The response time is therefore equal to 2 seconds

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  since with the device of the invention, the response time is equal

  to N. As to the accuracy, is is equal to N is 1%.

  In operation, for small variations of the frequency f the response time is reduced in large proportions compared to the devices of the prior art. The applications of the measuring device concern, for example,

  the speed control of an electric motor.

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Claims (2)

  1 / A method for measuring a frequency f with a slow variation over time, characterized in that a number N proportional to the value of the frequency f to be measured is obtained by servocontrolling a programmable clock at the frequency kuf, this servocontrol of increasing the number N or decreasing it according to a comparison with a reference fixed k ".   2 / device for measuring a frequency f with a slow variation in time, characterized in that a programmable clock (2) delivers pulses whose frequency is proportional to a number N, which is taken for said measurement and which comes from a down-counter (3) counting or down-counting the pulses of frequency f, according to a comparator (5), comparing a number F, equal to the content of a counter receiving the pulses of the programmable clock, at the moment of its resetting which is performed at the frequency f, to a reference number k ", provides logical states O or 1 depending on whether F <k "or F ?? k".