This document provides a description of patent GB780010 (A) for improvements to electrical follow-up systems. The system uses a magnetic amplifier to amplify an error signal modulated at a sub-carrier frequency, which helps eliminate effects of datum instability in the amplifier. It then applies the amplified signal to a motor to reduce the error signal to zero and correctly indicate the measured value, such as liquid level.

1. * GB780010 (A)
Description: GB780010 (A) ? 1957-07-31
Improvements in or relating to electrical follow-up systems
Description of GB780010 (A)
PATENT SPECIFICATION
Inventor: FREDERICK ROGEt MILSOM Date of filing Complete Specification
June 24, 1955.
Application Date June 24, 1954.
Complete Specification PublishedJuly 31, 1957.
78O0,010 No. 18591/54.
Index at acceptance:-Classes 37, A9C1, A19(K: Lli: M); and 40(4), F9J.
International Classification:-GO1r. H03f., COMPLETE SPECIFICATION
0 Improvements in or relating to Electrical Follow-Up Systems We, S.
SMITH & SONS (ENGLAND) LIMITED, a British company, of Cricklewood
Works, Crickewood, London, N.W.2, do hereby declare the 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 state.
ment: The present invention relates to systems (hereinafter referred
to as "electrical AC.
follow-up systems ") in which an A.C. carrier is modulated in
accordance with an error to give an error signal which is applied to
an amplifier and the output from the amplifier is applied to a motor
which operates means to reduce the error signal to zero. It is
frequently desirable to use a magnetic amplifier, i.e. an amplifier
utilising saturable reactors or transductors, in such systems, but
this has proved difficult in the past when the magnitude of the error
signal involved is very low, owing mainly to the effect of datum
instability in the magnetic amplifier, which, when the usual technique
of demodulating the error signal and using it to control the magnetic
condition of a transductor core is used, results in random
inaccuracies.
It is the object of the present invention to provide an electrical
A.C. follow-up system in which the amplification of the error signal

2. is effected entirely by means of a magnetic amplifier, and in which
harmful effects of datum instability in the amplifiers are largely
eliminated.
According to the present invention we provide an electrical A.C.
follow-up system as above defined comprising a first phase-sensitive
demodulator to one input of which the error signal is applied, a
reference phase signal being applied to the other input thereof, one
of the inputs to the first demodulator being modulated at a
sub-carrier frequency, low compared with the carrier frequency so
giving as an output from the first demodulator a signal at the
sub-carrier frequency modulated in accordance with the error, a
magnetic amplifier to the input of which said sub-carrier error [Price
3s. 6d.] signal is applied, the output therefrom consisting of a
further carrier modulated by the subcarrier error signal and means to
render 50 ineffective components of the magnetic amplifier output
modulated at frequencies substantially below the sub-carrier
frequency.
Components of the magnetic amplifier output modulated at frequencies
substantially 55 below the sub-carrier frequency may conveniently be
rendered ineffective by applying that output to one input of a second
phase-sensitive demodulating device and applying to the other input
the further carrier modulated by 60 the sub carrier alone. The output
of the second phase-sensitive modulating devices is applied to the
motor which is of a type appropriate to the output signal. A two phase
induction motor may itself conveniently constitute the demodulating
device, the amplifier output being applied to one phase winding of the
motor and the further carrier, with its phase shifted by 90 and
modulated with the sub-carrier, being applied to the other. 70 An
embodiment of the invention, in which it is applied to a liquid
contents gauge of the kind in which the variation in capacitance of a
plurality of capacitors distributed in a space to be wholly or
partially occupied by a liquid 75 is utilised to provide an indication
of liquid volume will now be described with reference to the
accompanying drawing.
Power for the device is supplied from threephase 400 c/s A.C. lines
indicated at 1, 2, 3. 80 The primaries of first and second power
supply transformers 4 and 5 are arranged in a conventional Scott
connection across lines 1, 2, 3 so that the voltages developed at
their respective secondaries whose terminals are denoted f f 85 and g
g respectively are in phase quadrature.
A third power transformer 6 has its primary connected across lines 1
and 2, its secondary terminals being denoted a, a; b, b; c, c; d, d;
e, e. 90 The secondary terminals (g, g) of transformer 4 are connected
to an oscillator 7 which is of any convenient form and produces an

3. output voltage in phase with that at g, g and 780,010 modulated at 10
c/s. The 10 c/s oscillation constitutes the sub-carrier. The output
from 7 is applied to one phase winding of two-phase motor 8, and also
to an adjustable errormeasuring device 9 producing an output signal in
accordance with discrepancy between the indicated volume of liquid and
the actual volume, as determined by the actual capacitance of the
plurality of capacitors. The precise form of device 9 is unimportant,
but it will usually be some kind of bridge network of which one part
is constituted by the capacitance of the capacitors referred to.
Another part is a variable impedance adjusted by motor 8 as indicated
by broken line 10, the amount of adjustment to bring the output from
device 9 to zero providing the required indication of volume.
The output from 9 is thus a 400 c/s signal modulated at 10 c/s and
modulated further in accordance with the discrepancy, and constitutes
the error signal. This output is applied to a first phase-sensitive
demodulator 11, together with the output from winding a a (whose phase
is appropriately shifted by capacitor 12) so that the output from
demodulator 11 has a component at 10 c/s modulated in accordance with
the discrepancy. The demodulator output is coupled through a
relatively large capacitor 13 to the input windings 14, of a first
magnetic amplifier stage 16. Stage 16 has two conventional sections,
17 and 18, supplied with power from terminals b, b and c, c
respectively. The output is generated across a winding 19 which
constitutes the input to a second amplifier stage 20 and consists of a
c/s signal modulated in accordance with the discrepancy. Winding 19 is
tuned to 10 c/s by capacitor 21. Second amplifier stage 20 is also
conventional and is supplied with power from terminals d, d. Its
output, consisting of a 400 c/s signal modulated at 10 c/s and also
modulated in accordance with discrepancy is applied to the primary of
a coupling and impedance matching transformer 22.
The secondary of transformer 22 is connected to a conventional bridge
rectifier 23 whose output is connected in series opposition with that
of a similar bridge rectifier 24, supplied with a constant voltage
from terminals d, d.
to the input windings 25, 26 of a third amplifier stage 27 through
capacitor 29 and also through a low pass filter network 28, across the
input windings 14, 15 of first stage 16 to provide negative feed-back.
Network 28 is such that substantial negative feed-back is provided
thereby at D.C. and frequencies well below 10 c/s but practically no
negative feedback is provided at 10 c/s. Thus the first and second
stages are kept, by the negative feedback within the linear parts of
their working characteristics despite changes in working conditicns,
and the mean value of the combined output of rectifiers 23, and 24
stays very close to zero. Transformer 22 alters the impedance level of

4. the signals so that capacitor 29 does not have to be excessively
large. Third amplifier stage 27 is again conventional and comprises
two sections 30, 31 in push-pull feeding an auto-transformer 32 which
is connected 70 to the second phase winding of motor 8. Stage 27 is
supplied with power from terminals 1, f.
Its output is thus a 400 c/s carrier in quadrature with the voltage
applied to the first motor winding modulated with the 10 c/s
sub-carriel 75 and also modulated in accordance with the discrepancy.
It should be mentioned that the couplings at the input to the first
stage and between the successive stages are each such as to produce 80
a 60 phase lag in the 10 c/s sub-carrier, a total lag of 180- thus
being introduced by the whole amplifier.
It should be seen that the presence of subcarrier modulation in the
amplifier output 85 causes the motor 8 to produce a fluctuating but
uni-directional troque, whereas any other modulation such as would be
caused by amplifier drift causes it to produce a fluctuating torque of
mean value zero. Thus the motor 8 90 will run in such a manner as to
reduce the discrepancy to zero, and indicate the quantity of liquid
correctly.
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