Instrument for measuring the speed in RPM of a rotating gear

A method of and an apparatus for converting a first series of pulses into a second series of pulses having a frequency proportional to the frequency of the first series. The method and apparatus are particularly useful for providing a series of equally spaced pulses representative of the speed of a rotating gear in revolutions per minute (RPM). A proximity detector provides a pulse as each tooth of the gear traverses the face of the proximity detector and a sample period is formed by the passing of several teeth. A series of pulses is counted by a counter for the length of the sample period. The number of pulses at the end of the sample period is stored in a latch. A programmed divider counts down from the count in the latch to zero at a reference frequency and an output pulse is provided each time the programmed divider reaches zero. The programmed divider is reset to the count in the latch upon the occurrence of each output pulse. In a specific application, the number of output pulses from the counter is made equal to the number of pulses that would be generated by a gear having sixty teeth and rotating at the same speed for convenient display of RPM on a digital counter with a one-second time base.

BACKGROUND OF THE INVENTION 
This invention relates to a method of and an apparatus for converting a 
first series of pulses into a second series of pulses having a frequency 
proportional to the first series and, more particularly, to an instrument 
for measuring the velocity of a rotating element and converting the series 
of pulses representative of the rotational velocity into a form which can 
be easily read by an RPM meter. 
Accurate measurement of the velocity of a rotating gear is desirable. When 
convenient, a sixty-tooth gear is included in or connected to the rotating 
system. A proximity detector sensing the teeth of the sixty-tooth gear is 
coupled to an electronic counter having a one-second time base. This 
provides a direct reading of RPM. However, if a sixty-tooth gear (or 
multiples thereof) is not available, a variable time base counter must be 
used to convert the number of pulses received from the gear into a form 
readable by the electronic counter having the one-second time base. 
Adjustment of the time base is required and the accuracy of the system 
depends upon the adjustment of the time base. Adjustable time base 
counters are expensive and are often not readily available. 
I have developed a method and apparatus for converting the pulses from a 
gear to the number of pulses which would be generated by a gear having 
sixty teeth and rotating at the same velocity as the gear being measured. 
SUMMARY OF THE INVENTION 
The present invention is directed to overcoming one or more of the problems 
as set forth above. 
A method of and an apparatus for converting a first series of pulses into a 
second series of pulses having a frequency proportional to the first 
series is provided. The method and apparatus are particularly useful in 
providing a series of equally spaced pulses representative of the velocity 
of a rotating gear having a known number of teeth. A proximity detector 
detects the passing of gear teeth to provide a series of pulses. A series 
of pulses originating from an oscillator is counted by a counter for the 
length of a sample period. The length of the sample period is determined 
by the passing of a preselected number of gear teeth past a proximity 
detector. At the end of the sample period, the number of pulses in the 
counter is stored in a latch. The count stored in the latch is entered 
into a programmed divider. The programmed divider counts down from the 
count in the latch to zero at a reference frequency. When the programmed 
divider reaches zero, an output pulse is provided. Output pulses occur as 
the programmed divider repeatedly counts the number stored in the latch, 
and the programmed divider is updated with more recent information from 
the latch at the occurrence of each output pulse. The output may be 
provided to an electronic counter having a one-second time base to provide 
a direct reading of RPM.

DESCRIPTION OF PREFERRED EMBODIMENT 
Referring to FIG. 1, the velocity of a rotating gear, 10 which may have any 
number of teeth, is to be measured and displayed as revolutions per minute 
on a digital counter 11 having a one-second time base. The circuit of FIG. 
1 converts pulses derived from gear 10 to a series of equally spaced 
pulses equivalent to those which would be generated by a "standard" gear 
having sixty teeth and rotating at the same speed. A sixth-tooth gear is 
considered standard, as the pulses derived from it when displayed on a 
digital counter having a one-second time base give a reading directly in 
revolutions per minute. If a digital counter with a time base other than 
one second is used, a standard gear 10 will have a different number of 
teeth. 
The passage of teeth 12 of gear 10 is detected by a proximity sensor 14 
producing a series of pulses having a repetition rate representing the 
rotational velocity of the gear. Briefly, in accordance with the invention 
a plurality of pulses from sensor 14 are counted to establish a sample 
period. The number of cycles of a reference signal which occur during the 
sample period is counted and used to generate the output signal. The pulse 
conversion system is set for the number of teeth on gear 10 by setting a 
numeric selector 16. The system is adjusted for the desired number of 
teeth per sample by selector 18. Both selectors 16 and 18 may be decimal 
thumb wheel switches. 
Oscillator 20 provides a reference frequency, F, to the divide-by-N counter 
22 by line 24. The divide-by-N counter 22 is a programmed divider which 
has a single output pulse on line 26 after the occurrence of N number of 
pulses on line 24. N is entered by thumb wheel switch unit 18. The counter 
counts down from N and an output pulse is provided on line 26 when the 
divide-by-N counter 22 reaches zero. The output pulse also resets the 
counter 22 to N by line 28 when it reaches zero and the next countdown 
from N is started. 
The output of the divide-by-N counter 22 has a frequency (F/N) and is 
multiplied by multiplier 30. The value set by multiplier 30 is equal to 
the proportion of T number of gear teeth 12 (as set by thumb wheel switch 
unit 16) divided by reference T.sub.o, the number of teeth of a reference 
gear (60, in the case of a system using a digital display with a 
one-second time base). Multiplication of the signal on line 26 by this 
proportion generates a signal at a frequency F/N.times.T/T.sub.o, which is 
connected by line 32 to counter 34. Counter 34, a 2.sup.16 bit up-counter, 
counts the pulses from multiplier 30 for a sample period, the length of 
which is related to the time it takes N gear teeth to pass detector 14. 
The sample period begins with the movement of a tooth 12 past the 
proximity detector 14 and ends when N teeth, as set by thumb wheel switch 
unit 18, have passed the proximity detector 14. Counter 34 counts at the 
frequency F/N.times.T/T.sub.o for a period of time required for N teeth of 
gear 10 to pass detector 14. 
The output of proximity detector 14 is provided to pulse shaper 36 by line 
38. Pulse shaper 36, a monostable multivibrator, assures that the pulses 
received from the proximity detector 14 are of even amplitude and width. 
The output of pulse shaper 36 is provided to delay circuit 40 (0.5.mu. 
second delay) by line 42 and the output of the delay 40 resets flip-flop 
44 by line 46. Flip-flop 44 provides a single Q output pulse on line 49 to 
counter 34 upon the reception of the first pulse from detector 14 to start 
the sample period. Subsequent pulses provided to the flip-flop 44 on line 
46 during the sample period do not affect the condition of flip-flop 44. 
The output of pulse shaper 36 also connects to divide-by-N circuit 48. 
Divide-by-N circuit 48, a programmed divider, counts down from N to zero. 
N, the number of teeth per sample, is entered by thumb wheel switch unit 
18. An output pulse is provided on line 50 to latch 52 through delay 54 
(0.5.mu. second delay) when divide-by-N circuit 48 reaches zero. The 
occurrence of the output pulse represents the end of the sample period. 
The pulse from divide-by-N circuit 48 causes latch 52 to store the number 
of pulses counted by counter 34 at the time the pulse is received at latch 
52. The occurrence of the output pulse at 50 also resets the divide-by-N 
circuit 48 to N. The number of pulses in the latch 52 at the end of the 
sample period is directly proportional to the length of the sample period. 
Also, the length of the sample period is a function of the selected number 
N and the speed of gear 12. The higher the number N, the longer the sample 
period. The faster the velocity of gear 12, the shorter the sample period. 
The size of counter 34 must be sufficient to preclude an overflow 
condition resulting from an extremely long sample period. 
The signal from delay 54 is also provided to delay 58 (2.mu. second delay). 
The output of delay 58 sets flip-flop 44 and the Q output goes low. This 
resets counter 34 to zero and holds the divide-by-N circuit 48 and counter 
34 in a reset condition until the next pulse is received from detector 14. 
This ensures that a full sample period will be measured instead of the 
sample period being occupied by the latch pulse or the reset pulse. 
The count stored in latch 52 at the end of the sample period is available 
to the programmed divider 60 by line 62. The programmed divider 60 counts 
down from the count in the latch to zero and is clocked at a rate F from 
oscillator 20 by line 64. An output pulse is provided on line 66 when the 
programmed divider 60 reaches zero. When an output pulse is provided on 
line 66, the programmed divider 60 is reset to the latest available count 
in latch 52. If the next sample period has not ended, the programmed 
divider 60 is provided with the same count as previously provided. At the 
end of each sample period, a new count is available to programmed divider 
60 from latch 52. Since the programmed divider 60 is being clocked by 
frequency F on line 64, F is removed from the equation to provide equally 
spaced output pulses on line 66 proportional to T/T.sub.o times the speed 
of the rotating gear 10. Output shaper 68, a monostable multivibrator, 
provides pulses on output line 70 to digital counter 11 which are of even 
amplitude and width. 
Referring to FIG. 2, multiplier 30 is shown. The input of the multiplier is 
a series of pulses at the frequency F/N. The series of pulses on line 26 
is divided by divide-by-T.sub.o (sixty) circuit 72 to provide F/NT.sub.O. 
The output of the divide-by-T.sub.o (sixty) circuit 72 is provided to 
phase lock loop 74 by line 76. The phase lock loop 74 multiplies the 
signal F/NT.sub.o times T number of gear teeth as set by thumb wheel 
switch unit 16. Programmed divider 78 counts down from T to zero and 
provides an output pulse on line 80 each time zero to reached. If the 
output is to be displayed as RPM on a one-second time base, the reference 
divisor T.sub.o is sixty. For other displays a different reference divisor 
will be used. 
The selection of N determines the number of gear teeth 12 which must 
traverse the proximity detector 14 for a sample period. The accuracy of 
the instrument depends upon the number of pulses counted per sample. The 
larger N, the more accurate the count received and stored by latch 52. 
Conversely, the resolution of the instrument depends upon the number of 
sample periods obtained by the instrument per revolution of gear 10. The 
smaller N, the shorter the sample period. More samples can be acquired per 
revolution of the gear 10 with higher resolution. Hence, by selecting the 
number of teeth per sample period, the operator is allowed to select the 
optimum sample period to maintain the desired accuracy of the instrument 
without sacrificing resolution. The chart below provides an example of the 
setting of the thumb wheel switch unit 18 for gears having the number of 
teeth shown. 
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TEST GEAR TEETH TEETH PER SAMPLE 
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181-200 10 
161-180 9 
141-160 8 
121-140 7 
101-120 6 
81-100 5 
61-80 4 
41-60 3 
21-40 2 
1-20 1 
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The proximity detector 14 may be of any type capable of detecting the 
presence of a metal tooth. In lieu of the proximity detector 14, an 
optical device may be employed if, for example, the velocity of a 
nonmetallic gear is to be measured.