Remote indicator for displaying transmitted data by angular displacement

A remote indicator displays data contained in a transmitted signal by angular displacement of a rotor. In essence, the indicator comprises a rotatably mounted magnetic rotor, a plurality of controllable magnetic field generators disposed circumferentially around the rotor, and appropriate selection circuitry. A sector selection circuit responsive to the transmitted data selectively enables at proper polarity a plurality of generators appropriate for controlling the rotor within the sector representative of the data, and an angle selection circuit responsive to the transmitted data alternately drives selected generators by electrical pulses time modulated to drive the rotor within the sector to the angular position representative of the transmitted data. Distortions due to residual magnetism are substantially eliminated by the use of AC pulsing, and non-linear salient pole effects are eliminated through the use of look-up table techniques.

FIELD OF THE INVENTION 
This invention relates to remote indicators for displaying transmitted 
data; and, in particular, to a remote indicator for displaying data by 
angular displacement of an indicating rotor. 
BACKGROUND OF THE INVENTION 
Remote indicators are useful in a wide variety of monitoring applications 
where it is desired to angularly display measured data at one or more 
locations remote from the point of measurement. The data can be inherently 
angular in nature, such as compass bearings, or it can be data, such as 
pressure, which is related to angular display only by calibration on a 
circular dial. On a large ship, for example, it may be desirable to 
transmit and display at several locations, the compass heading determined 
to a single properly-compensated, highly accurate compass; and where a 
ship tows a barge or another ship, it is highly desirable to transmit to 
the towing ship, the leading of the barge or ship being towed. 
Typical prior art remote indicators utilize stepping motors with 
step-driven coils to displace a magnetic indicating rotor. These devices, 
however, suffer from a number of disadvantages. One such disadvantage is 
that their accuracy is typically limited by the resolution of the stepping 
motor. Thus an indicator employing a 7.5.degree. stepping motor is 
typically limited in accuracy to .+-.3.75.degree.. Although gearing can be 
added to reduce the step increments, this technique is considerably less 
reliable than the direct drive approach utilized in this invention. Higher 
resolution stepping motors are available, but their cost, complexity and 
size rise rapidly with increasing resolution. 
A second difficulty arises because of residual magnetism. Because the 
magnetic fields do not completely dissipate upon the withdrawal of 
current, the motor does not accurately respond to a fast changing signal, 
introducing errors in accuracy. 
Accordingly, it is desirable to provide an economical remote indicator 
capable of providing more accurate angular display of transmitted data. 
SUMMARY OF THE INVENTION 
In accordance with the invention, a remote indicator for angularly 
displaying data contained in a transmitted signal comprises a rotatably 
mounted magnetic rotor, a plurality of controllable magnetic field 
generators disposed circumferentially around the rotor, and appropriate 
selection circuitry. Specifically, sector selection circuitry responsive 
to the transmitted data selectively enables at proper polarity generators 
appropriate for controlling the rotor within the sector representative of 
the data, and angular selection circuitry responsive to the transmitted 
data sequentially drives the selected generators by electrical pulses time 
modulated to drive the rotor within the sector to an angular position 
representative of the transmitted data. Distortions due to residual 
magnetism are substantially eliminated by the use of AC pulsing, and 
non-linear salient pole effects are eliminated through the use of look-up 
table techniques.

DETAILED DESCRIPTION 
Referring to the drawings, FIG. 1 is a schematic diagram of a preferred 
embodiment of a remote indicator in accordance with the invention. The 
indicator comprises in essence, a rotatably mounted magnetic indicating 
rotor 10, a plurality of controllable magnetic field generators 11A, 11B, 
11C and 11D disposed circumferentially about the rotor and selection 
circuitry 12. 
The indicating rotor 10, here a single magnet, conveniently comprises an 
elongated bar magnet having a north pole N and south pole S. The rotor can 
optionally be affixed to a circular indicating disc if desired (not 
shown). 
The controllable magnetic field generating means preferably comprise four 
conductive coils disposed about the ends of two magnetically permeable 
cores, the core ends and coils wrapped thereon being distributed about the 
circumference of the rotor at substantially 90.degree. intervals. 
Typically, the magnetically permeable cores are iron cores. The coils are 
conveniently interfaced with the selection circuitry by a plurality of 
electronically controllable switches 13A, 13B, 13C and 13D electrically 
connected between respective coils and a power source 14, such as a dry 
cell. Closure of switch 13A, for example, activates the iron core 
establishing opposite field polarities in opposing core ends 11A and 11C. 
The opening of switch 13A and closure of switch 13C results in the 
reversal of the flux direction in field generating core ends 11A and 11C. 
The selection circuitry 12 basically comprises logic circuitry for 
receiving a digital angular data input signal typically representing an 
angle between 0.degree. and 360.degree., activating a plurality of coils 
at proper polarity for driving the rotor to the appropriate sector 
(quadrant in this embodiment) and sequentially applying width-modulated 
electrical pulses to the selected coils for driving the rotor to the 
appropriate angle. Preliminary circuitry can optionally be added to 
convert analog input signals to digital signals and to calibrate 
non-angular data, such as pressure or temperature, to an angular 
representation, in accordance with techniques well known in the art. 
Preferably the selection circuitry comprises a microprocessor including 
logical decision means, memory means and register means such as an Intel 
8048 military specification microprocessor. 
The operation of the embodiment of FIG. 1 can be understood by reference to 
FIG. 2 which is a flow diagram showing the operation of the logical 
selection circuitry. 
The logical decision means, represented by decision boxes 20, 21 and 22, 
comprises logical circuit means for analyzing the angular input data and 
determining which of the four quadrants contain the designated angle. 
The memory means, represented by look-up tables 23, 24, 25 and 26, comprise 
means for storing for each of the four quadrants, command data for 
commanding activation of the magnetic field generating means appropriate 
for displacing the rotor into the selected quadrant. The command data can 
conveniently comprise a pair of binary words for commanding closure of 
selected ones of electronic switches 13A, 13B, 13C and 13D so as to obtain 
positive or negative polarity energization of generators 11A, 11B, 11C and 
11D, respectively. Storage register means 27 are conveniently provided for 
temporary storage of the selected command data. 
Typical binary values of energization of the field generators for different 
representative angles are given below: 
TABLE 1 
______________________________________ 
ANGLE 11A 11B 11C 11D 
______________________________________ 
0 1 0 1 0 
90.degree. 
0 1 0 1 
180.degree. 
-1 0 -1 0 
270.degree. 
0 -1 0 -1 
______________________________________ 
Arithmetic means, represented by subtraction boxes 28, 29 and 30, are 
provided for calculating the acute angle between the angular 
representation of the data and a pre-selected base angle for each 
quadrant. Thus using the minimum angle of each quadrant as a base angle, 
90.degree. is subtracted from data angles in the second quadrant, 
180.degree. from angles in the third quadrant and 270.degree. from angles 
in the fourth quadrant, respectively, to calculate the residual acute 
angle .theta.'. 
Additional memory means in the form of look-up table 31 is provided for 
storing for each one of a plurality of angular increments of .theta.' 
between 0.degree. and 90.degree., the switch closing time intervals for 
driving the rotor to that residual acute angle .theta.' within a quadrant. 
These timing intervals are conveniently empirically determined as relative 
percentages of a fixed timing interval to be applied to switches 13A, 13B, 
13C or 13D. 
Typical field energization periods (as decimal fractions of the fixed 
timing interval) and polarities for different representative acute angles 
are listed in Table 2, below: 
TABLE 2 
______________________________________ 
ANGLE 11A 11B 11C 11D 
______________________________________ 
45.degree. 
(0.5)(1) (0.5) (1) (0.5) (1) 
(0.5) (1) 
135.degree. 
(0.5) (-1) 
(0.5) (1) (0.5) (-1) 
(0.5) (1) 
225.degree. 
(0.5) (-1) 
(0.5) (-1) (0.5) (-1) 
(0.5) (-1) 
315.degree. 
(0.5) (1) 
(0.5) (-1) (0.5) (1) 
(0.5) (-1) 
______________________________________ 
In operation, the microprocessor loads the timing interval for switches 13A 
or 13C into the timer. The signal from the timer closes switches 13A or 
13C, and the microprocessor goes into an idle mode while the timer counts 
down. When the timer times to zero, it generates an interrupt which loads 
the remainder of the fixed timing interval into a timer for switches 13B 
or 13D. Thus the switches sequentially energize the coils for maintaining 
the rotor within the selected quadrant. This sequential energization of 
switch pairs can be effected by using a set-reset flag and changing the 
flag with each timing-out interrupt. 
The coils should be sequentially activated at a rate much higher than the 
rotor can respond so that that the rotor acts as an integrator and 
oscillates within an intermediate position as if the coils were 
simultaneously energized. The pulse durations can be conveniently chosen 
so that the oscillations will be substantially unobservable to the human 
eye while being sufficient to overcome any static friction in the rotor 
mounting. Pulse repetition rates in excess of about 10 hertz demonstrate 
this quality. 
The advantage of using a look-up table with empirically determined pulse 
duration values is that accurate indicator positioning is thus obtained 
despite inherent non-linearities in the field strength versus displacement 
response of the coils. In a preferred embodiment, the pulse durations are 
empirically determined and stored for each quarter-degree interval. 
To obtain even higher levels of accuracy, the drive circuitry can be 
designed to effect a "positive zeroing" of coils when it is desired to 
shift a magnetic generator from a high value to a substantially zero 
value. In such shifts the pole tends to retain a residual magnetization 
which introduces errors in accuracy. This source of inaccurancy can be 
substantially eliminated by including an additional command in the look up 
table 31 for acute angles requiring a low level of energization for one or 
more coils. Instead of commanding a zero current, the command can apply a 
high frequency AC signal so as to produce cancelling effects on the rotor. 
The single magnet indicator of FIG. 1 is an absolute device in that the 
rotor has a unique magnetic orientation for every angle between 0.degree. 
and 360.degree.. In alternative forms of the invention employing 
multiple-magnet rotors, magnetic symmetries can be introduced which 
produce ambiguities, and in such instances the indicator should be 
modified to include some type of indexing means. 
FIG. 3 illustrates an alternative form of the invention employing a 
multiple magnet rotor and indexing means. Specifically, a four-pole rotor 
40 and indexing means in the form of an indexing magnet 41 are mounted in 
fixed relation to one another on rotatably mounted calibrated disc 42. The 
index is mounted at a displacement angle, B, here 15.degree.. An index 
sensing device, such as reed switch 43, is mounted adjacent the periphery 
of the disc in order to sense passage of the index point and generate a 
signal indicative of such passage. 
It will be readily appreciated that the rotor 40 possesses an axis of 
magnetic symmetry as indicated in the figure and that, in the absence of 
indexing means, the rotor would exhibit substantially the same magnetic 
configuration for rotational positions 180.degree. apart, producing a 
0.degree.-180.degree. ambiguity in the indicated value. 
As shown in FIG. 4, the electronic processing used in conjunction with the 
embodiment of FIG. 3 is specifically adapted to eliminate this 
0.degree.-180.degree. ambiguity. 
Using the processing shown in FIG. 4 as Subroutine 1, the microprocessor 
slews the rotor around until the index magnet passes the reed switch 
producing an index signal. 
When the index signal is detected, the microprocessor reads the angular 
input signal A and calculates the difference D=A-B. If D is zero, no 
change in position is required and the program takes a new reading. If D 
is not zero, the microprocessor inquires whether the absolute value of D 
is greater than 180.degree.. If it is not, the sign of D is retained. If 
it is, D is subtracted from 360.degree. and the sign is changed. In either 
case, the result is then fed into logic circuitry to move the rotor by the 
differential angle thus calculated, clockwise for positive values and 
counterclockwise for negative values. 
While the invention has been described in connection with a small number of 
specific embodiments, it is to be understood that these are merely 
illustrative of the many other specific embodiments which can also utilize 
the principles of the invention.