U.S. patent number RE31,774 [Application Number 06/532,347] was granted by the patent office on 1984-12-18 for measuring system.
This patent grant is currently assigned to Leeds & Northrup Company. Invention is credited to David L. Fletcher, Walter O. Stadlin.
United States Patent |
RE31,774 |
Fletcher , et al. |
December 18, 1984 |
Measuring system
Abstract
There is provided a method for measuring a parameter whose value
is calculable as an integral with time of a function of at least
one variable so as to make possible the use of a microprocessor
when measurements of such variables are desired. The variable is
sampled periodically at a frequency asynchronous with the wave form
which the variable follows and then there is periodically
calculated the value of the function by summing the values for the
function determined by the samples taken during the time between
the calculations, thus determining the desired integral.
Inventors: |
Fletcher; David L. (North
Wales, PA), Stadlin; Walter O. (North Wales, PA) |
Assignee: |
Leeds & Northrup Company
(North Wales, PA)
|
Family
ID: |
26683816 |
Appl.
No.: |
06/532,347 |
Filed: |
September 15, 1983 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
012632 |
Feb 16, 1979 |
04240149 |
Dec 16, 1980 |
|
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Current U.S.
Class: |
702/60;
324/94 |
Current CPC
Class: |
G01R
21/133 (20130101); G01R 19/02 (20130101) |
Current International
Class: |
G01R
21/00 (20060101); G01R 21/133 (20060101); G01R
19/02 (20060101); G06F 015/20 () |
Field of
Search: |
;364/483 ;324/94 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Wise; Edward J.
Attorney, Agent or Firm: Miller, Jr.; William G. Huberfeld;
Harold
Claims
What is claimed is:
1. The method for measuring a parameter whose value is calculable
as the integral with time of a function .Iadd.other than the
identity function of a single variable .Iaddend.of at least one
variable which has a value following a periodic wave form,
comprising the steps of:
sampling the value of said variable periodically at a frequency
asynchronous with said wave form .Iadd.and not a harmonic or
subharmonic of the waveform.Iaddend.; and
periodically calculating the value of the integral of the function
in accordance with the sum of the values for the function
determined in accordance with the values of all of the samples
taken during the time between said calculations.
2. The method as set forth in claim 1 in which the parameter is RMS
voltage, the variable is the instantaneous voltage e for each
sample, and the function of the variable is e.sup.2.
3. The method set forth in claim 1 in which the parameter is RMS
current, the variable is the instantaneous current i for each
sample, and the function of the variable is i.sup.2.
4. The method set forth in claim 1 in which the parameter is
average power, the variables are e and i for each sample, and the
function of the variables is ei.
5. The method set forth in claim 1 in which the parameter is energy
for a period T, the variables are e and i for each sample, and the
function of those variables is ei.
Description
BACKGROUND OF THE INVENTION
This invention relates to a method for measuring a parameter whose
value is calculable as an integral with time of a function of at
least one variable. The method of this invention makes possible the
use of a microprocessor when measurements such as effective (RMS)
voltage, effective (RMS) current, average power, and energy are to
be made by calculation from the outputs of current and potential
transformers.
Use of a microprocessor for making the required measurements from
current and potential transformer outputs has the advantage over
previously known methods in that there is not required the use of
separate measuring devices for each of the measurements and the
microprocessor provides an output which can be transmitted to a
central computer for use.
SUMMARY OF THE INVENTION
The method of this invention comprises a first step of sampling the
value of the variable of which the parameter being measured is a
function. .Iadd.other than the identity function of a single
variable. .Iaddend.The sampling is done periodically at a frequency
asynchronous with the wave form of the variable .Iadd.and not a
harmonic or subharmonic of the waveform. .Iaddend.Periodically
there is then made a calculation of the value of the function by
summing the values for the function determined from the values for
the variable obtained for each sample taken during the time between
calculations.
BRIEF DESCRIPTION OF THE DRAWINGS
In the Drawings:
FIG. 1 is a wave form showing the sampling points for the
measurement.
FIG. 2 is a block diagram showing one form the microprocessor
configuration can take for carrying out the method of this
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The theoretical basis for this measuring method is found in the
fact that an exact integral equation can be approximated by a
finite summation equation as follows: ##EQU1## where y is the
parameter being measured and x is some function of the variable
which when integrated over the period T gives the value for the
parameter. N is the number of samples made between calculations
(during period T) and .DELTA.t.sub.n is the sample period. The
period between samples becomes .DELTA.t.sub.n .apprxeq.T/N.
Obviously the larger the number of samples and the smaller the
sample period the closer the finite sum approximates the
integral.
Where the variable or variables which determine the parameter have
values which follow a periodic wave form, a number of samples of
the value of those variables is made during each period of the wave
form with the frequency of the samples being asynchronous with the
frequency of the variable. Therefore, with respect to the variable
there is obtained a sliding sample so that over a period of time
all parts of the wave form of the variable are sampled. It is, of
course, important to use a large number of samples if the wave form
of the variable is not symmetrical about the average, whereas a
smaller number of samples suffices if the wave form is
symmetrical.
It is convenient to utilize a microprocessor for calculating the
required function of the variable to obtain a calculation of the
value for the parameter being measured. Thus, it is possible to
measure parameters of a 60 hertz power system by utilizing the
standard current and potential transformers to provide inputs to a
microprocessor through analog to digital converters so that the
inputs provide the data required to calculate parameters such as
RMS voltage, RMS current, average power, and energy.
If FIG. 1 shows the periodic variable, which may be current or
voltage as it varies with time, it will be evident that the first
series of samples A1-A5 are obtained from a part of the wave form
which is different from that of the samples B1-B5, both of which
are in turn different from the samples C1-C5. Thus, as each period
passes the point of sampling slips with respect to the wave form in
FIG. 1 so that over a number of cycles each part of the wave is
sampled.
If the frequency of the wave of FIG. 1 is 60 hertz, then the period
of an ideal sine wave would be 16.667 msec. and the sample period
could be 3.125 msec. for 5 samples per cycle. If, for example, an
RMS calculation of the voltage is desired it may be carried out as
follows: ##EQU2## Similarly, the equation for RMS current becomes:
##EQU3##
The variables e and i represent the instantaneous ac voltage and
current which are taken from potential and current transformers by
way of analog to digital converters. Thus in FIG. 2 the analog to
digital converters 12, 14, 16, and 18 provide voltage and current
inputs to the microprocessor 20.
The microprocessor 20 may advantageously have connected to it the
read only memory (ROM) 22, and the random access memory (RAM) 24 as
well as clock source and a power supply so the microprocessor can
perform the necessary calculations.
The results of the calculated values produced by the microprocessor
can be decoded directly by the decoder 28 and supplied to a digital
display 30 for displaying the value of the parameter. Also, as
shown in FIG. 2 the output of the microprocessor can be transmitted
by transmitter 32 to a central computer which may be processing the
data obtained.
Other measurements which can be made advantageously in accordance
with this novel method are average power and energy.
Average power is defined as follows: ##EQU4## Thus, a product of
two numbers can be evaluated by a table look-up approach with a
suitable shift instruction for the factor 1/4N.
Energy is defined as follows: ##EQU5##
This relationship represents the energy for a given time period (T)
which is directly related to the sample period and the number of
samples.
* * * * *