Transcript Calibration of Electrical Fast Transient/ Burst Generators

Calibration of Stopwatch
by Using High Speed Video Recording
and an In-house Designed Synchronous Counter
Speaker: C.M.Tsui
The Government of the Hong Kong Special Administrative Region
Standards and Calibration Laboratory
36/F Immigration Tower, 7 Gloucester Road, Wanchai, Hong Kong
Phone: (852) 2829 4850, Fax: (852) 2824 1302, Email: [email protected]
Authors: C.M.Tsui, Y.K.Yan, H.M. Chan
The Government of the Hong Kong Special Administrative Region
Standards and Calibration Laboratory
Methods for Calibrating Stopwatch
Stopwatch can be calibrated using three methods :
 The direct comparison method
 The totalize method
 The time base method
The time base method has the smallest measurement
uncertainty. The direct comparison method has the largest.
The Direct Comparison Method
The time interval measured by the stopwatch is compared
to that of a traceable time interval reference which
usually is an audio signal broadcast by the official
timekeeper of a region. An example is the WWVH in the
USA.
The Time Base Method
 The frequency of the time base of the stopwatch,
which is likely to be a quartz oscillator, is pickup by
either ultrasonic acoustic sensor or inductive sensor and
measured directly.
 This method is very fast. The measurement can be
completed in a few seconds.
 Since only the time base is tested, the functionality of
the stopwatch is not checked. In addition, the
relationship between the time interval readings of the
stopwatch and its time base is not known.
The Totalize Method
 A laboratory time interval reference is set up as
follows.
Traceable
frequency
standard
Signal
generator
Universal
counter
 At beginning of measurement, the stopwatch is started
and the universal counter’s gate is opened at the same
time. After some time, the stopwatch is stopped and the
counter’s gate closed.
 The time interval measured by the stopwatch and the
counter is compared.
Photo Totalize Method (1)
 A high speed camera is used to take photo of the
stopwatch and a universal counter when they are both
counting. After a suitable time has elapsed, another
photo is taken.
Traceable
frequency
standard
Signal
generator
Universal
counter
Stopwatch
Camera
Photo Totalize Method (2)
stopwatch
counter
2.567
Photo 1
1.3
counter
stopwatch
8120.511
8119.4
Photo 2
 The elapsed time measured by the stopwatch and the
universal counter can then be obtained from these two
photos and compared. The relative correction is
(8120.511 2.567)  (8119.4  1.3)

 1.92105
(8120.511 2.567)
 The measurement uncertainty is dependent upon the
display resolution of the stopwatch.
Video Totalize Method (1)
 We has modified this method by utilizing video
recordings instead of photographs to obtain readings
of the stopwatch.
 The instant when the reading of the stopwatch display
changes is found by examining the recorded video
frame-by-frame.
 The measurement uncertainty is limited by the frame
rate of the video recording rather than the display
resolution of the stopwatch.
Video Totalize Method (2)
 Digital cameras that can record high speed video at
240 fps or 420 fps are commercially available. Using
such cameras, the measurement uncertainty can be
reduced significantly.
 At high frame rates, the display of a normal universal
counter cannot keep up and does not update at
uniform intervals.
 To overcome these problems, we has designed and built
a synchronous counter that allows its count to be easily
read from a recorded video with a resolution as small as
1 millisecond
SCL Caesium Beam
Frequency Standard
A signal generator feeds a 1 kHz
clock signal to the synchronous
counter. This 1 kHz clock is phase
locked with the laboratory caesium
frequency standard.
10 MHz Reference Clock
Signal Generator
1 kHz Clock
dicator
LSD Indicator
In-house
Designed
Synchronous Counter
9
0
1
2
8
7
3
6
5
4
Stopwatch
under test
A high speed digital camera
CASIO EX-FH100 is used to
record a 10 second video
clip at 240 fps or 420 fps for
the stopwatch and the
synchronous counter when
they are both counting.
High Speed
Video Camera
Figure 1
The Calibration Method
 The recorded video is viewed frame-by-frame to
search for a frame at which the display of the stopwatch
starts to change to a new value.
 The readings of the stopwatch Tuut_start and that for the
synchronous counter Tref_start are then taken from the
frame immediately preceding this frame.
 The least-significant-digit (LSD) of the synchronous
counter is indicated by a circular LED display. The value
corresponds to the LED which lights up at the most
clockwise position.
Synchronous counter
reading is 442.069s.
(442.06 from the 7segment LED display.
The most clockwise LED
that lights up in the
circular indicator is “9”,
hence the LSD is 9) The
reading of the digital
watch is 11:09:54
Figure 2(a)
Synchronous counter
reading is 442.073 s.
The LSD of the digital
watch reading starts to
change from “4” to “5”.
The lowest segment of
the digit “5” is
noticeable in this picture.
If this is chosen as the
start time of
measurement, then
Tuut_start = 11:09:54 and
Tref_start = 442.069 s (i.e.
the last frame)
The synchronous counter
reading is 442.077 s. The
digital watch reading is
changing from 11:09:54 to
11:09:55. The change is
obvious in this picture.
The reading of the
synchronous counter is
442.081 s. The reading of
the digital watch is
changing from 11:09:54 to
11:09:55.
Figure 2(b)
Figure 3(a)
The synchronous
counter reading is
852.180 s. The
reading of the UUT is
00:02:00. The
second hand has
remained stationery
at this position for
over 20 frames.
The synchronous
counter reading is
852.185 s. It can be
seen that the second
hand of the
stopwatch has
moved a little bit.
Figure 3(b)
The Calibration Method
After a suitable time interval (say 6 or 7 hours), the above
process is repeated to obtain Tuut_stop and Tref_stop. The
relative correction is defined as
RelativeCorrection
Time elapsedreportedby laboratorystandard Time elapsedreported by UUT
Time elapsedreportedby laboratorystandard
RelativeCorrection
(Tref_stop  Tref_start )  (Tuut_stop  Tuut_start)
(Tref_stop  Tref_start )
High Speed Digital Camera (1)
A high speed digital camera CASIO EX-FH100 is used to
record video. It supports the following high speed video
modes.
Frame rate
Image size (pixels)
120 fps
640 x 480
240 fps
448 x 336
420 fps
224 x 168
1000 fps
224 x 64
High Speed Digital Camera (2)
 The image size for the 1000 fps mode is too small. The
shutter speed for the 120 fps mode was too slow (~10
ms)
 The 420 fps mode is suitable if the stopwatch has a
large and clear display. Otherwise the 240 fps mode is
recommended.
 At high frame rate, more light is required due to higher
shutter speed. The video should be captured in a welllit environment.
In-house Designed
Synchronous Counter (1)
 The display readings of many commercial universal
counters did not update synchronously with the
input clock signal.
 In Figure 4, the time between frames is about 2.38 ms.
If the display of the universal counter was truly
synchronous, the counter reading should increment by 2
or 3 with each new frame.
 However, we can only observe three readings during this
76.2 ms period: 11490, 11539 and 11588. The universal
counter seems to only update its display about once
every 49 ms
Figure 4. Display of a
commercial universal
counter captured by a
high speed digital
camera at 420 fps. The
universal counter is
driven by a 1 kHz clock
input.
Progress of
frame
sequence
In-house Designed
Synchronous Counter (2)
 There are severe limitations when the video totalize
method is employed with some commercial universal
counters.
 The synchronous counter designed by SCL was built with
logic circuits that ensure its 10-digit LED display will be
updated synchronously with the input clock.
 It is driven by a 1 kHz clock generated by a signal
generator phase locked to the laboratory cesium
frequency standard. It has a resolution of 1 ms.
Figure 5
In-house Designed
Synchronous Counter (3)
 At 420 fps and 240 fps, the shutter speeds were 2 ms
and 4 ms respectively. The LSD of the counter updates
every millisecond, it is not possible to read its value from
the 7-segment LED display in the recorded video.
 To overcome this problem, a special LSD indicator was
designed which consists of 10 LED arranged in a circle.
Each LED corresponds to a decimal value and will light
up individually for about 1 ms in every 10 ms period.
 When recorded at a shutter speed of 4 ms, 4 to 5
consecutive LEDs will be illuminated in the recorded
image. The LED which lights up at the most clockwise
position is the correct value of the LSD, and allows the
counter to be read with a resolution of 1 ms.
VCC
9
3
LD
A
4
5
B
6
C
VCC
D
16
7
EN P
From
lower
digit
VCC
15
74HC162
10
EN T
RCO
8
2
1 kHz
Clock
CLK
CLR
1
To higher
digit
GND
QA
14
QB
QC
13 12
QD
11
To Reset
Button
VCC
VCC
1
2
DA
DB
DC
6
16
5
EL
4
7
BI
DD
VCC
8
GND
74HC4511B
3
LT
Og
Figure 6. Simplified circuit diagram for a
single digit of the synchronous counter.
Of
Oe
Od
Oc
Ob
Oa
14
15
9
10
11
12
13
17
18
1
2
3
15
16
7-segment display
14
VCC
9
LD
7
3
4
5
6
A
B
C
VCC
D
EN P
16
VCC
74HC162
1 kHz
Clock
10
EN T
2
CLK
15
RCO
CLR
1
QA
QB
14
13
QC
To higher
digit
8
GND
QD
12
11
To Reset
Button
VCC
1
A0
2
A1
3
A2
VCC
4
5
6
E1
E2
E3
Vcc
2
3
4
5
6
A0
A1
A2
E1
E2
E3
74LS138
8
74LS138
Y0
Figure 7. Circuit diagram for
the least-significant-digit (LSD)
indicator.
15
Y1
14
Y2
13
Y3
12
GND
Y4
11
Y5
10
Y6
9
Y7
7
16
1
16
Y0
15
Y1
14
Y2
13
Y3
Y4
12
VCC
11
Vcc
GND
Y5
10
Y6
9
Y7
7
8
Measurement Uncertainty Evaluation
The measurement model for the relative correction (RC)
of elapsed time counting of a stopwatch is shown below.
RC 
(1  C)  (Tref_stop  Tref_start )  (Tuut_stop  Tuut_start)
(1  C)  (Tref_stop  Tref_start )
Measurement Uncertainty Evaluation
C
Measurement
Remarks
Uncertainty Components
Relative correction of the The 1 kHz clock input is phase locked to the
1 kHz clock input to the
laboratory reference frequency. It is
synchronous counter
assumed to have a normal distribution with
zero mean. The standard measurement
uncertainty is taken as
:
5  86400
14
u (C)  4  10

(Tref_stop  Tref_start )
Tref_start ,
Tref_stop
Reading of synchronous
counter at start and stop
time of elapsed time
counting.
Due to limited resolution of the synchronous
counter. It is taken to have a rectangular
distribution with semi-range of ±1 ms.
Tuut_start ,
Tuut_stop
Reading of UUT at start
and stop time of elapsed
time counting.
Due to time interval between successive
frames of the video recording. It is taken to
have a rectangular distribution with semirange equals to the time interval between
two frames.
Measurement Uncertainty Evaluation
 There are two methods to derive the combined standard
measurement uncertainty for the final measurand
 GUM Uncertainty Framework (GUF)
 Monte Carlo Method (MCM)
 GUF is easier and faster to compute than MCM. However,
the domain of validity of GUF is narrower than MCM.
 Supplement 1 to the GUM describes a validation
procedure for GUF. GUF was found not valid for this
measurement model.
 The reason is that there are two dominant uncertainty
components u(Tuut_start) and u(Tuut_stop) with rectangular
probability distribution, violating the condition for valid
application of GUF.
Measurement Uncertainty Evaluation
 Assuming elapsed time of 7 hours between Tref_start and
Tref_stop, the estimated measurement uncertainties of the
relative correction RC for a 95% coverage interval,
calculated using MCM, are listed on next slide.
 For comparison, the estimated measurement
uncertainties for the photo totalise method are listed in
the same table. If commercial counter is used in the
photo totalise method, the counter reading taken from
the photo is assumed to have an accuracy of 0.1 s.
Measurement Uncertainty Evaluation
Estimated measurement uncertainty of
the relative correction RC for a 95% coverage interval
Display
resolution of
stopwatch
under test
Video totalize method used
Photo totalize method used
Video
recording at
240 fps
Video
recording at
420 fps
Using the
synchronous
counter
described in
this paper
1s
2.7 x 10-7
1.6 x 10-7
3.1 x 10-5
3.2 x 10-5
0.1 s
2.7 x 10-7
1.6 x 10-7
3.1 x 10-6
7.0 x 10-6
0.01 s
2.7 x 10-7
1.6 x 10-7
3.2 x 10-7
6.2 x 10-6
Using
commercial
counter