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An Improved Radio-Controlled Sailboat
A detailed description of a radio-controlled yacht. The experimenter
may wish to similarly control other devices.
Robert H. Packard
The remote control of apparatus by means of radio is becoming
an increasingly popular hobby with many experimenters, and one of
its most fascinating applications is the control of model sailboats.
It is a mystifying and thrilling experience to watch a small sailboat
some distance off shore going through all the maneuvers of a full-sized
yacht - running before the wind, coming about, tacking and gybing
- just as if there were a Lilliputian crew aboard. And to the skipper
on shore who is responsible for these uncanny actions, the thrill
is an even greater one!
Such a model can be constructed quite inexpensively and without
a technical knowledge of radio. The following description is intended
to be a supplement to a less detailed account published in the July,
1934, issue of Radio-Craft and gives the method of constructing
the various components of the controlling mechanism.
Briefly, the theory of control is as follows: radio impulses
sent out from the transmitter on shore are picked up by the receiver
aboard the boat and cause a relay to close at every impulse. This
relay actuates a selector switch which in turn governs the operation
of electrical devices which move the rudder and adjust the sails.
The number of impulses or "dots" which are sent determines which
device will respond.
Fig. 1, below. The circuit of the transmitter at A, and the receiver,
B.
The Transmitter
The transmitter, shown in Fig. 1A, is a one-tube, unmodulated,
"modified, tuned-grid, tuned-plate" oscillator, operating at 78
meters. It is fundamentally conventional with only minor additions
for convenience and compactness. The trimmer condensers are used
to enable the transmitter frequency to be initially adjusted to
within the tuning range of the receiver. The fine control for occasional
tuning during operation is done with the 2-plate midget condenser,
C6. Coils L1 and L2 are wound on tube-base forms and L1 may be moved
with respect to L2 to vary antenna coupling. A 2.5-V. lamp in the
antenna circuit is used to check oscillation by throwing the switch
Sw.1, putting the lamp in series with L1 and C1. The antenna, a
quarter-wave Marconi type, consisting of 58 ft. of No. 24 enameled
wire, is shown in Fig. A, together with some string and a 3-foot
ground wire on a reel at the right-hand end of the transmitter.
The "B" supply is composed of 2 portable type, 45-V. batteries
and one small 22 1/2-V. battery. In normal operation the switch
Sw.2 connects to the 90-V. point but when the boat is very near
the transmitter, the voltage has to be reduced to 67.5 V. to prevent
"locking-in" of the receiver with the transmitter. The 22 1/2-V.
battery is used to temporarily increase the power in case the boat
should get out of range at the 90-V. power. The filament supply
is a home-made, 3-lb., 6-V. storage battery made with standard-size
plates cut into quarters and placed in a cut-down motorcycle battery
case with 1 positive and 2 negative plates per cell (see Fig. 3).
Control signals are sent by pressing the pushbutton Sw.3 which is
at the end of a 6-ft. flexible cord plugging into the Jack, J1.
The complete transmitter is built into a 5 x 8 1/2 x 10 1/2 in.
wooden box and weighs only 15 lbs.
Fig 2A and B, above; Fig. 2C, below. Details of the selector
unit.
Fig. 3, right. The parts in the hull.
The Receiver
The receiver is a standard three-tube regenerative set with a
sensitive relay in place of phones at the output, as shown in Fig.
1B. An R.F. signal from the transmitter, picked up by the ship's
antenna, produces an A.F. beat-note in the oscillating detector.
This is amplified by two transformer-coupled tubes, the second of
which acts also as a rectifier for the signal because it is biased
to plate-current cut-off by about 9 V. of "C" battery. Thus, when
a signal comes through, the plate current of the last tube rises
from zero to a few milliamperes and operates the sensitive relay.
This relay which trips on about 1 ma. was made by winding 2 coils
of 12,000 turns each with No. 36 enameled wire and slipping them
over a U-shaped core made from transformer laminations. The armature
is a piece of soft iron 2 x 1/2 x 1/16-in. with a vertical bearing
at one end. It is counterweighted with a lump of lead and is mounted
in the boat so that the tip of the arm moves fore and aft to minimize
effects of rocking and rolling of the ship which might tend to close
the contacts. The receiver is built on a 5 x 6-in. Bakelite panel
with the transformers and "C" batteries mounted below to keep the
center of gravity as low as possible. It rests on the bottom of
the boat near the bow and is held in position by 6 binding posts
fastened to the sides of the boat which also make the 6 connections
to the external circuit. marked ANT. GND, "B+45," "B+90," "A+2,"
and R in Fig. 1B. Thus the set may be quickly removed from the boat
through a 5 x 6-in. hatch for servicing.
The trimmer condenser C2 is used to initially tune the receiver
to the transmitter and is also useful in adjusting the receiver
to maximum sensitivity. To do this a milliammeter is plugged into
the home-made jack J1 to indicate plate or relay current, and with
the transmitter on, C2 and L2 are varied until the maximum current
is obtained. The coils L1 and L2 are wound on tube-base forms, the
tickler L2 being mounted on screws so that its distance from L1
may be varied.
The 9-V. "C" bias for the last tube is obtained from six small
flashlight batteries which are taped to the underside of the receiver.
The "B" supply consists of two portable 45-V. batteries and the
"A" supply is taken from a 6-V. storage battery (identical with
the one used in the transmitter) through the resistor, R2, which
drops the voltage to slightly over 2 V. A switch, Sw.2, is mounted
on the deck of the boat to enable the entire controlling mechanism
to be turned on or off. Also built into the deck is a pushbutton,
Sw.1, which closes the same circuit as the sensitive relay does,
and which was found very useful for checking operation of the selector
and machinery when the hatches were closed. Switch Sw.3 is merely
used to prevent operation of the selector while tuning the receiver.
The Selector
The selector may well be called the heart of the control system
and is perhaps the most difficult piece of apparatus to construct.
As seen in Fig. 2, there are two moving contactors or wipers which
advance together one step at a time along two rows of contacts.
These wipers are attached to a ratchet gear that is actuated by
the selector magnet so the motion is along the arc of a circle (shown
as a straight line in Fig. 1B for simplicity). By tracing the wiring
in the right-hand part of Fig. 1B (remembering that the motor will
not operate through the high-resistance path of a lamp), it will
be found that the motor operates, through the contact S, in one
direction when the wipers are at positions 1 and 3. and in the reverse
direction at positions 2 and 4. In positions 1 and 2, the gear-shift
magnet M1 is in series with the motor and so it closes causing the
rudder gears to operate. In positions 3 and 4 the magnet M2 closes,
operating the gears which wind up or unwind the strings attached
to the sails. Positions 5 and 6 may be used for any other controls
which may be desired such as an auxiliary propeller, or lights,
or a horn, etc.
The contact S which completes the return circuit from the wipers
to the negative side of the battery is open while the wipers are
advancing, but closes if a pause of more than 1/2-second is made
at any position. Thus the wipers are dead while they are sliding
over the contacts, a fact which not only prevents actuating some
machinery on the way to a higher position, but also eliminates sparking
and consequent pitting of the contact points. Once this contact
has closed, the next throw of the selector arm sends the wipers
back to the starting position and at the same time opens contact
S.
When the selector magnets are energized they attract the armature,
pushing. the arm A forward from the position shown in Fig. 2, so
that the "finger" E engages with the ratchet R and advances the
wipers W to the first position. At the same time the air dash-pot
V is pushed in slightly because of the arm X striking the washer
Z attached to the dash-pot. Now if the magnets are de-energized
the spring K causes the arm X to spring back, but due to the washer
Y attached to the dash-pot this return motion is slowed down so
that the arm A slowly retracts. Just before the contact S (which
also acts as back-stop) closes, a small projection on the edge of
arm A falls into a notch of the end of the lever L. Then contact
S closes, allowing current to flow through the wipers and the external
circuit connected to position 1.
The next time the magnets are energized the finger E is prevented
from engaging with ratchet R because as arm A advances it also has
to move to the right a little since it is hooked to the end of lever
L which turns in an arc. Thus the short branch of the arm strikes
the "tail" end of pawl T, releasing the ratchet and allowing the
wipers to snap back to the zero position. At the same time the tip
of the longer branch strikes the vertical slanting stop F and is
forced sharply to the right beneath guide D, allowing lever L to
become unhooked and to snap back to its original position, against
the stop G.
Now when the armature is released, arm A slowly returns and would
again become hooked to lever L if it were not for the fact that
a thin, flat spring M became engaged in a notch I when the tip of
the arm A was against the stop F, preventing the arm from returning
too far. This spring must be stiff enough so as not to buckle when
resisting the backward motion of the arm A, but flexible enough
to allow the arm to move to the left so that the finger E may engage
with the ratchet on the next impulse. The normal position of this
spring, which is assumed while the wipers are advancing, is shown
dotted.
If it is desired to reach position 2, it is only necessary to
give the selector two impulses in quick succession so that there
is not sufficient time between them to allow arm A to become booked
to the lever L. This time may be varied by adjusting the spring
K and the distance between washers Y and Z. Similarly any other
position may be reached by giving the correct number of impulses
and after a pause of about one-half second at any position, the
next impulse will send the wipers back to the neutral position:
Selector Construction
The selector magnets and armature are taken from a 20-ohm telegraph
sounder together with the sounder arm and its bearings. These are
attached to the lower part of a right-angled piece of aluminum,
B, 2 ins. wide as shown in Fig. 2, and a 2-in. square of bakelite,
C, is screwed to its top, forming the basis for construction of
the unit. A small strip of fibre is screwed to the top of a ratchet
gear, R, (taken from an old alarm clock) and two strips of spring
brass, with small pieces of silver, soldered to the ends, are screwed
to this fibre, forming the wipers W, as shown in the detail drawing
in Fig. 2. Current is taken from one wiper through a very flexible
flat lead N in a wide arc to one of the mounting screws. The other
wiper is "grounded" to the ratchet R and another flexible lead runs
from a hub on the top of the ratchet shaft to an upright attached
to the moving arm A. The bearing shaft is soldered to the center
of the ratchet and turns in holes in the base C and top contact
strip H. This strip and its companion strip about 1/8-in. below
it are made of 1/8-in. Bakelite and are fastened to the base C by
three screws, and are separated from each other and from the base
by six collars.
The twelve contacts which are cut from an old silver spoon and
filed into 1/8-in. discs are set into 1/8-in. holes drilled far
enough into the inner surfaces of the strips so that the discs set
in them, flush with the surface of the bakelite. They are held there
by drops of solder placed in similar holes drilled part way through
from the opposite side. The lead wires O are soldered to these points
as shown in the sectional view in Fig. 2 and pass through holes
in the base C down to two six-prong tube-bases which are screwed
to the bracket B and act as mechanical supports for the selector.
Not all these leads need to run to the sockets for, as seen in Fig.
1B, upper contacts 1 and 3 may be connected together and to lower
contacts 2 and 4, before leaving the selector.
A spiral clock spring J keeps tension on the string Q which is
wound around the hub of the ratchet R so that the wipers will return
to the first or "zero" position when the ratchet, is released. This
hub is filed to an approximate spiral so that its radius decreases
as the string winds up, a fact which keeps the torque constant even
though the spring tension increases. Thus the magnet does not have
to work any harder to advance to the higher contacts than to the
lower ones, a great aid to the efficiency of the selector. The ratchet
is prevented from slipping backwards at each step by the pawl T
mounted on the base C beneath the lower row of contacts as shown
dotted in the top view and is held against the ratchet by the flat
spring U. The selector arm A is composed of two pieces cut from
sheet brass and soldered together at right-angles and is bolted
loosely to the driving arm X so that considerable side motion is
possible. A short piece of 1/16-in. steel wire E is bent and filed
to fit the ratchet teeth and soldered to the longer branch of the
arm A to serve as a "finger" to advance the ratchet R. The shorter
branch is twisted at right-angles near its end and bent so that
it just clears the "tail" end of the pawl T while the finger E is
advancing the ratchet. The tip of the longer branch rests on a small
flat piece of brass screwed to the base C and is prevented from
lifting by wire D.
The flat spring P is bolted to the arm X so that it bears against
the vertical surface of the arm A, thus tending to hold the arm
and finger E against the ratchet. The air dash-pot, V, which consists
of a smoothly bored tube closed at one end sliding over a close-fitting
solid piston is attached to the bracket B; a short wire with two
washers Y and Z soldered to it is fastened to the moving cylinder
and passes through a hole in the arm X.
The Boat and Controlling Machinery
It is not within the scope of this article to describe, except
very briefly, the construction of the hull of the boat and the reader
will do well to use his own ideas on the subject. The important
points are to have enough displacement to carry the necessary weight,
enough sail to move this weight, and enough keel to balance this
sail. For a 4-ft. model the hull must be quite "tubbv," the rigging
much over-sized, and the keel deep and heavy, unless the weight
of the apparatus is considerably reduced, which is quite possible.
The writer's hull is 48 x 12 x9 ins. deep and has 8 lbs. of lead
at the end of an 8-in. deep brass fin keel. The masts are 63 and
49 ins. above deck and the total sail area is 1,300 sq. ins.
Fig. A. A view of the self-control control transmitter, with
a wire. reel aerial on the side.
Figure 3 shows the location of apparatus within the hull. The
"B"-batteries and the storage battery are located amidships, the
selector and controlling mechanism near the stern, and the receiver
in the bow. The antenna runs out through a hole in the deck to the
tip of the bowsprit, thence to the tops of foremast and mainmast
(serving also as a stay) and down the latter to a dead end a few
inches above the deck.
The motor is mounted with its shaft and worm gear No. 1 vertical
as shown in Fig. 3. The rudder drive shaft has a fixed bearing at
its left end so that flat gear No. 4 is always engaged with worm
gear No.1, but its other bearing near magnet M1 is movable so that
worm gear No. 2 can move toward or away from flat gear No. 5 under
the action of M1. So when M1 closes and the motor runs, gear No.
5 together with the rudder turns in one direction or the other depending
on which way the motor is turning. When the rudder is hard over
against a stop, gear No. 5 keeps on turning but the rudder does
not due to a friction connection between the gear and the shaft.
Thus the rudder can be held hard over as long as desired. When the
motor stops and M1 releases, worm gear No. 2 backs off allowing
the rudder to spring back to its neutral position under the action
of a spring.
The sail drum shaft has a fixed bearing at its left end and a
moving one at its right end, and this means that flat gear No. 6
is not engaged with worm gear No. 1 except when the magnet M2 closes.
The drum itself was machined from a brass rod and has three sections.
Worm gear No. 3 drives flat gear No. 7 operating the limiting contacts
which stop the motor whenever the sails are all the way out or in.
Wire stays are used on the boat and serve the dual purpose of
strengthening the masts and carrying current to the two bulbs on
the mast-heads. These bulbs are necessary to tell from shore what
is happening aboard the ship. Referring to Fig. 1B, it is seen that
lamp K1 will light whenever the selector magnets are energized so
this lamp gives a check on the overall reception of the signals.
The other lamp, K2, lights while the selector wipers are advancing
and goes out as soon as contact S closes, indicating that the motor
is running and facilitating timing of the various controlling operations.
There is practically no limit to the variations and improvements
it is possible and advisable to make in the system of radio-control
just described.
List of Parts
Transmitter (Fig.1A)
One 150 mmf. fixed condenser, receiver type, C1;
Two 250 mmf. fixed condensers. receiver type, C2, C3;
Two approx. 50 mmf. trimmer cond., C4, C5;
One 2-plate midget variable condenser, receiver type, C6;
One 0.002-mf. bypass cond., receiver type, C7;
One 15 turn coil. No. 19 enameled wire, 1 in. form, L1;
One 10 turn coil, No. 13 enameled wire, 1 1/4 in. form. L2;
One 57 turn coil, No. 29 silk-covered wire, 1 in. form (grid
coil), L3;
One 10,000 ohm gridleak, 1 W., R1;
One 5 ohm filament rheostat, R2;
One type 71A tube, V1;
One open-circuit jack with filament control, J1;
One 2.5-V. flashlight bulb, K1;
One single-pole double-throw switch, Sw.1;
One sing le-pole triple-throw switch, Sw.2;
One single circuit pushbutton, Sw. 3;
Receiver (Fig. 1B)
One condenser made from two 1/2-in. square metal plates, about
1/8-in. apart, C1;
One Approx. 50-100 mmf. trimmer cond., C2;
One 100 mmf. fixed condenser, C3;
One 0.002-mf. bypass condenser, C4;
Two 1 mf. bypass condensers. C5, C6;
One 23 turn coil, No. 24 D.S.C. wire, 1 1/4 in. form, L1;
One 6 turn coil, No. 24 D.S.C. wire, 1 1/4 in. form, L2;
One 4 meg. gridleak, R1;
One 22 ohm fixed filament resistor, R2;
One audio transformer, 5/1 ratio, T1;
One audio transformer, 6/1 ratio, T2;
Three type 30 tubes, V1, V2, V3;
One closed-circuit jack, J1;
Two 5-volt flashlight bulbs, K1, K2;
One single-circuit pushbutton, Sw. 1;
One on-off switch, Sw.2.
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