Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
2001-03-09
2003-07-08
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C702S041000, C702S145000, C073S116070, C073S862326, C324S160000
Reexamination Certificate
active
06591200
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the accurate measurement of angular rotation and, in particular, to the performance testing of rotating machines.
BACKGROUND OF THE INVENTION
Conventional approaches to the performance testing of electric motors generally measure steady-state performance of the motor after the elimination of transient effects. To this end, motor speed data is collected as a function of time and the data is filtered so as to remove fluctuations in order thereby to derive standard motor performance characteristics as set out in the appropriate IEEE standards, for example. Such characteristics invariably relate to the torque-speed performance of the motor at no load whereby the motor is accelerated from rest under no load conditions, and the torque is derived as a function of the time derivative of the speed curve in accordance with Newton's Second Law of Motion. Additionally, so-called “signature” tests as well as load tests are performed, although in all cases the effects of fluctuations are eliminated.
Signature testing is an extension of no-load testing that utilizes faster measuring techniques and processing in order to compare a specific motor's no-load performance with that of a pre-calibrated “master” motor which serves as a yardstick against which production line motors may be assessed. Load tests measure the motor's performance under operating conditions whereby a specific torque is applied to the running motor under test, and the resulting speed, current and power are measured.
Typically, the motor speed is measured using a tachometer coupled to the motor's axis. This allows acquisition of the motor speed in analog form and suffers from low resolution and severe noise contamination. For this reason, digital methods are preferred and significant effort has been expended during the last two or three decades to allow more accurate digital sampling of the rotational velocity of motor shafts and the like. Many such methods still employ what are essentially analog transducers to derive the speed signal and then digitize the speed signal using AID converters, so as to allow subsequent processing to be performed digitally.
R. Szabados et al. describe such a technique in “
Measurement of the Torque
-
speed Characteristic of Induction Motors using an improved new digital approach
” appearing in Transaction on Energy Conversion, Vol. 5, No. Sep. 3, 1990. Their method uses a fast data acquisition system to sample the output of a d.c. tachometer as well as other relevant parameters such as line current and voltage. The measured data is then processed digitally to remove the noise, perform dynamic average filtering to eliminate extraneous coupling vibrations and to determine the relative torque profile from the time derivative of the speed curve using Newton's Law. Since the removal of noise also eliminates the fluctuations, it thus emerges that the elimination of fluctuations by filtering the raw speed data is an inherent feature of the method proposed in this article.
Indeed, it is further shown that the raw speed data is contaminated and that the first task of the data processing phase involves removing the extraneous signals without distorting the speed profile. A major contribution of the above-referenced paper resides in the improved filtering algorithms which are presented.
U.S. Pat. No. 5,218,860 (Storar) assigned to Automation Technology, Inc. discloses an alternative approach wherein, rather than measure the speed using analog transducers, a digital gray scale (incremental) encoder is used.
FIG. 1
shows pictorially a motor test bed
10
wherein a motor is mechanically coupled to a test system according to U.S. Pat. No. 5,218,860 via a test fixture consisting of a rotating shaft
12
supported on high-quality bearings
13
. Mounted on the shaft
12
are a flywheel
14
of known inertia and a high-resolution rotary digital encoder
15
. The flywheel
14
acts as an inertial load whereby torque may be determined according to the equation:
T
=
I
·
ⅆ
v
ⅆ
t
(
1
)
where: T=Torque,
I=Inertia of flywheel,
v=speed, and
t=time.
As explained in U.S. Pat. No. 5,218,860, the torque-speed characteristics are sampled at regular known time intervals during the time it takes for the motor to reach full speed from standstill. The measurement time interval is fixed by a crystal oscillator and is usually 16.67 ms, corresponding to the period of one 60-Hz power line cycle. The change in speed is determined by the rotary encoder which resolves as little as 0.0072° of angular displacement. Torque and speed are computed for each 16.67 ms period from the time power is applied until it reaches its maximum no-load speed. The inertia of the flywheel attached to the motor is so selected that the motor reaches full speed in about 4 seconds, this being the time it takes to sample some 240 torque and speed results, enough to describe the entire torque-speed curve from standstill to full speed.
The digital rotary encoder as described U.S. Pat. No. 5,218,860 offers a significant improvement over analog transducers, and allows the measurement of certain motor characteristics that were previously not easily obtained. However, the resolution of the device is still relatively poor since, in effect, a very large number of pulses are averaged during each sample time period. Specifically, it is stated in U.S. Pat. No. 5,218,860 that the incremental encoder produces 25,000 pulses during each full rotation of the motor shaft. Assuming an average motor speed of 10,000 rpm, this means that the number of pulses produced per 16.67 ms time period is nearly 70,000. The actual number of pulses is counted by a binary counter so as to provide an accurate indication of the angular speed of the motor. However, during a sampling period as large as 16.67 ms, the fluctuations are no longer measurable: thus allowing only the smoothed characteristics to be determined. Moreover, there would appear to be no particular advantage in employing a rotary encoder having such a high resolution, as well as cost, given that such a coarse sampling interval is employed. Theoretically, the resolution could be improved simply by sampling at smaller time periods. However, in practice it is difficult to achieve this accurately and inexpensively using current technology.
Moreover, the flywheel attached to the motor shaft loads the motor and, whilst this does not derogate from the static performance of the motor, it substantially eliminates the fluctuations to which the transient effects are subjected. Consequently, loading the motor as taught in U.S. Pat. No. 5,218,860 does not allow measurement of the dynamic performance of the motor.
The present inventor has found that the dynamic performance of the motor provides invaluable information about the motor, such that without a knowledge of the dynamic performance of the motor it is impossible to derive fundamental behavior of the motor. However, for the reasons set out above, the dynamic performance data is unattainable during a sample period as large as 16 ms, since during this time period most of the fluctuations on the transient part of the curve are lost. Even regardless of the actual magnitude of the sampling time period, and bearing in mind that some improvement can clearly be achieved by reducing the sampling time period conducive with prevailing technology and price constraints, any improvement is limited in scope. This follows from the fact that counting pulses during a fixed time period, however small, can never allow optimum results to be achieved. Thus, even if the sampling time period could be reduced indefinitely (which, of course, it cannot) it can never be reduced to less than the period of a single pulse since, in such case, no data would be obtained during the sample period. On the other hand, whilst increasing the sampling time period ensures that sample data will be obtained, it does so at the cost of producing multiple data per sample. This means that the resolution t
Cohen Eyal
Cohen Menachem
Barlow John
M.E.A. Motor Inspection Ltd.
Vo Hien
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