1. * GB780036 (A)
Description: GB780036 (A) ? 1957-07-31
Apparatus for measuring rotational speed
Description of GB780036 (A)
COMPLETE SPECIFICATION
Apparatus for Measuring Rotational Speed
We, D. NAPIER & SON LIMITED, a company registered under the laws of
Great Britain, of 211 Acton Vale, London, W.3, do hereby declare this
invention for which we pray that a patent may be granted to us, and
the method by which it is to be performed, to be particularly
described in and by the following statement:
This invention relates to apparatus for measuring rotational speeds
and has for an object the provision of a form of apparatus which will
measure with accuracy high rotational speeds, say of the order of 2500
to 30000 R.P.M. and upwards, with reasonable accuracy and will
indicate the rotational speed at any moment in a readily readable
form.
Apparatus for measuring rotational speeds according to the present
invention comprises a cathode ray oscillograph, an electro-magnetic
impulse generating means comprising a stationary part from which
impulses are fed to the X and Y plates of the cathode ray tube of the
oscilloscope through appropriate amplifying apparatus and a rotational
part arranged to be carried by the rotary member the speed of which is
to be measured, the arrangement being such that the impulses cause the
moving spot of the oscilloscope to move angularly about an axis at a
speed equal to the rotational speed of the rotary member, and timing
and control apparatus which repeatedly performs a cycle of operations
including switching in at a predetermined point in the imaginary
angular travel of the spot a timing circuit to cause the spot to
appear for a predetermined time interval and means for simultaneously
injecting into the amplifying apparatus a signal which causes the spot
to move progressively radially outwards from the axis referred to and
thus to follow a spiral path during the said predetermined time
interval whereby the length of the spiral trace thus produced is a

2. measure of the speed of the rotary member in terms of angular movement
during the predetermined time interval.
The term " imaginary angular travel " of the spot has been used to
mean the angular travel which would be talking place assuming the spot
were switched on at the moment and, for convenience hereinafter,
points in the path which the spot would follow if it were continuously
switched on will be referred to, to define such points even at times
when the spot is not in fact switched on.
Since with high rotational speeds, say of the order of 30000 r.p.m.
and a convenient predetermined time interval of say one thousand of a
minute, the trace would represent a spiral of about thirty turns, this
would be difficult and inconvenient to read. According to a further
and preferred feature of the invention therefore the timing and
control apparatus is arranged so that during each cycle it switches on
the spot for a predetermined relatively short time interval, of say
one ten thousandth part of a minute to provide a spiral trace of
relatively short readily recognised length, then switches off the spot
for a relatively long time interval of say one thousandth part of a
minute and at the end of such relatively long period switch the spot
on and off again with such rapidity as to produce substantially only a
fluorescent dot.
It will be seen that in this way the relatively short spiral trace
will give a readily read indication of the broad range within which
the speed lies, e.g., the number of thousandths of revolutions per
minute, while the position of- the dot gives an accurate indication of
approximately the precise speed within such broad range, e.g. the
hundreds and tens in the number of revolutions per minute.
The means for generating the electromagnet impulses conveniently
comprise a small magnet attached to the rotary member the speed of
which is to be measured and a fixed two coil assembly disposed where
it will be subject to the magnetic flux of the magnet in such a manner
that a two phase output is generated therein with the two phases
displaced from one another by 90" It can be shown that if such an
impulse generator is connected through suitable amplifying apparatus
to the
X and Y plates of the cathode ray tube of the oscilloscope without the
injection of the signal to cause the moving spot to move radially
outwards, the moving spot will follow a circular path about an axis
and perform one revolution per revolution of the rotary part carrying
the magnet.
The invention may be carried into practice in various ways but one
form of the invention is shown diagrammatically by way of example in
the drawings which accompanied the provisional specification filed in
respect of the present application in which:

3. Figure 1 is a diagrammatic illustration of the appaartus as a whole,
and
Figure 2 is a view showing the form of trace which would be obtained
with the apparatus shown in Figure 1.
In the arrangement shown in Figure 1 the apparatus comprises an
electric impulse generator comprising a magnet A attached to the
rotary member B the speed of which is to be measured, and two coils C
and Cl arranged so that a two phase current is generated with the two
phases displaced from, one another by 90 . The two phase current thus
generated is fed through an amplifier D to the X and Y plates E, and F
of the cathode ray tube of a cathode ray oscillograph indicated at G.
This arrangement will cause the moving spot of the cathode ray tube to
follow a circular path about an axis at a speed of rotation equal to
that of the member B.
Associated with the amplifier and cathode ray oscilloscope is timing
and control apparatus indicated at H, H? and including a switching
circuit H2 through which the moving spot of the cathode ray tube can
be switched on and off and a circuit H by which a signal can be
injected into the amplifier D such as to cause the moving spot to move
radially outwards.
The timing and control apparatus is so arranged that repeatedly at a
predetermined point representing zero in the rotation of the magnet A
and hence in the rotation of the spot, the spot is switched on and the
injection of a signal by the circuit He into the amplifier D is
initiated so that a spiral trace is formed on the cathode ray tube as
indicated at J in Figure 2. After a predetermined time interval of say
one ten-thousandth of a minute, the spot is switched off so that the
spiral trace terminates, as at J1, but the injection of the signal
through the circuit H continues so that the imaginary trace of the
moving spot is as indicated by the dotted line J2 in Figure 3. Then
after a predetermined interval of say one thousandth of a minute from
the moment when the spot was switched on the spot is switched on again
for an instant and immediately off again so as to produce a dot as
indicated at J2
It will be seen that the length of the spiral trace J thus indicates
that the speed is 11000 odd while the dot J gives the exact figure of
650 thus giving the accurate total speed as 11650 r.p.m.
The switching circuits for switching the moving spot on and off, that
is to say controlling the illumination of the cathode ray tube by the
electron beam might be modified type of Eccles Jordon circuit of known
type, the beam and timing circuits being switched on from the
amplifier while the switching off and the short final pulse are
effected by the timing circuits.
The timing circuits themselves might be in the form of a set of

4. electronic counters coupled to a high frequency crystal controlled
oscillator arranged so that after the switching on of the moving spot
had been indicated from the switching-circuit, the switching off
impulse would occur after a count equal to one ten thousandth of a
minute followed by a single pulse to produce the final spot after a
total count representing one thousandth of a minute.
The final pulse would also reset the counters to zero and reset the
switching circuits in readiness for the next cycle.
The screen persistence of the cathode ray tube used would preferably
be not less than one tenth of a second, the repetition rate being of
the order of sixteen per second. As indicated in Figure 2 a screen
would conveniently be fitted over the cathode ray tube graduated, in
the manner indicated, to permit ready reading of the speed indicated
by the spiral trace J and dot J9.
What we claim is:
1. Apparatus for measuring rotational speeds comprising a cathode ray
tube oscillograph, an electro-magnetic impulse-generating means
comprising a stationary part from which impulses are fed to the X and
Y plates of the cathode ray tube if the oscillograph through
appropriate amplifying apparatus, and a rotational part arranged to be
carried by the rotary member the speed of which is to be measured, the
arrangement being such that the impulses cause the moving spot of the
oscillograph to move angularly about an axis at a speed equal to the
rotational speed of the rotary member, and timing and control
apparatus arranged repeatedly to perform a cycle of operations
including switching in at a predetermined point in the imaginary
angular travel of the spot a timing circuit to cause the spot to
appear for a predetermined time interval, and means for simultaneously
injecting into the amplifying apparatus a signal which causes the spot
to move progressively radially outwards from the axis referred to and
thus to follow a spiral path during the said predetermined time
interval, whereby the length of the spiral trace thus produced is a
measure of the speed of the rotary member in terms of angular movement
during the predetermined time interval.
2. Apparatus for measuring rotational speeds as claimed in Claim 1, in
which the timing and control apparatus is so constructed and arranged
that during each cycle it will switch on the spot for a predetermined
relatively short time interval to provide a spiral trace of rela