REMOTE CONTROL
BY BOB YOUNG
Large servo amplifiers for
model yachts & machinery
This month, we will move on to some special
applications of large servo amplifier
technology. These are used for the remote
control of machinery and heavy vehicles,
although their principles of operation are
similar to those used in model aircraft.
Many areas of model R/C require
mechanical outputs which are quite
different from the normal servo which
delivers a rotary output restricted to
approximately 100 degrees of movement and 1-3kg of thrust. These outputs include large servos for quarter
scale aircraft (or bigger), robotic puppets and sail winches, plus speed con-
trols for electric propulsion and remote switches.
In one special case, where I radiocontrolled a full size Volkswagen 1600
TLE, I used servos which delivered
30kg of thrust. Thrust in this case is
defined as the actual force measured
on a spring balance from the output
arm used (I find this a little more
Fig.1: this servo is designed specifically to serve as a winch for model yachts &
features a multi-turn output for hauling in the sheets.
descriptive than the usual kg/cm). The
servo motors in this last case were
Valiant windscreen wiper motors
which had to be forced cooled from a
separate blower. As Hages would say,
"now that's a servo".
It was in the Volkswagen job that I
discovered the problems ofunderpowered servos in which compensation is
made by recourse to lower gear ratios.
The result is a servo of adequate thrust,
but uncomfortably long transit times.
In a vehicle designed to travel at speed,
long transit times on the steering and
brakes make for some hair-raising experiences.
I remember having a chuckle when
the Apollo astronauts complained
about the difficulties they had in steering their moon buggy. They had a
seven second transit time on the steering from memory, on a vehicle designed for about 10km/h top speed.
The script on the Volkswagen commercial called for a top speed of
80km/h so that it could overtake the
· camera car and pull away into the
distance. Because of the tight time
allowed for the job , I found to my
horror that I had to use a gearbox
which resulted in a 4-second transit
time on both the brakes and steering.
The difficulties in steering at · this
speed were indescribable.
On one occasion, I cut in too early
on the camera car after overtaking it
and the driver braked too heavily. As
a result I fell off my seat, dropped the
transmitter and had to go looking for
it. It the meantime, the Volkswagen,
given its freedom, had taken off across
the nearest paddock, dragging a barbed
wire fence behind it. Volkswagen had
that car back on the road the next
morning, comp lete with a new set of
AUGUST 1991
33
sistors. If these are poorly selected, it
can result in downgraded servo power
because some of the power is lost as
heat.
The photo of Fig.2 shows what is
essentially the same servo, this time
fitted with a double drum output. This
is very useful in model yachts, as
usually there are two sails which require trimming: the jib (or little sail
close to the pointy end) and the mainsail (or large sail at the blunt end).
One usually tries to sail the yacht
with the pointy end heading more or
less into wind. The degree to which
the yacht can point into wind is a
measure of its overall efficiency. To
achieve this requires good winches of
great power and accuracy.
Circuit description
Fig.2: this servo is basically the same as the one shown in Fig.1 but features a
double drum output. This is very useful in model yachts, as usually there are
two sails which require trimming: the jib and the mainsail.
passenger side body panels, and the
commercial went to air in due course.
Such are the joys of commercial TV
work.
Winch servos
The photo of Fig.1 shows a special
type of servo designed specifically as
a winch for model yachts. Note the
unusually large size of this servo,
which is not under weight restraint to
the same degree as an aircraft servo.
The main criteria for this type of winch
is output power and multiple turns
on the output drum.
The multiple turn output is required
because the drum is used to haul in
the sheets (sheets being the ropes,
just to confuse non-yachties). The
number of turns is usually about 8-12,
depending upon the yacht size and
servo power.
As a matter of interest, we used to
test our winches by lifting a bucket
off the ground. If the winch passed
the bucket test, we felt it was OK. No
big deal did I hear you say? Perhaps I
should have mentioned that the
bucket contained two house bricks,
each weighing 4kg. This was the thrust
necessary for an "A" class yacht working in heavy weather.
There are also some very large
servos designed for large model aircraft and these may be readily converted into a "BAR" type winch. By
using a large output arm , the amount
34
SILICON CHIP
of throw is sufficient to provide the
necessary travel to position the sails
in the correct location. This overcomes
the problem of the sheets becoming
tangled or slipping off the drum.
Another consideration in setting up
a winch is the transit time or time
taken to haul in the full length of the
sheet under heavy conditions. There
is little point in using the drum as the
major source of the mechanical advantage by the simple expedient of
using a smaller diameter drum, because this only increases the time required to complete the increased
number of revolutions. The speed of
manoeuvring when rounding a buoy
is dependent upon the speed with
which the sails can be retrimmed and
long transit times can be costly in
terms of race times.
Thus, we soon arrive at the conclusion that servo power is the all important factor in winch design and servo
power begins with motor size, particularly armature diameter and winding resistance. In fact , the motor more
or less dictates the servo size and the
servo is thus designed around the
servo motor.
The servo illustrated in Fig.1 uses a
5-ohm 26mm motor and provides
plenty of grunt when fitted with an
amplifier capable of delivering the
required amount of current. Another
important consideration here is the
voltage drop across the output tran-
The diagram ofFig.3 shows the circuit of a typical winch amplifier, based
on the popular NE544 servo amplifier
IC. This is fitted in turn with a power
amplifier to provide the current required by a 5-ohm motor. The NE544
can drive a small motor direct provided it draws less than 300mA but
anything over this requires additional
amplification.
In the amplifier shown in Fig.3,
VRl is a 5kQ feedback pot which is
mechanically coupled to the output
shaft of the final gear on the servo
drive chain. R14 & R15 provide travel
adjustment and wiper centring. This
mechanical coupling can be in the
form of a reduction gear set to provide
the required number of turns on the
output drum. Fine adjustment of the
number of turns can then be obtained
electronically by varying the values
ofR14 & R15.
A similar result can be obtained by
adjusting Rl 0. However, this will also
shift the neutral point and there is no
easy way to readjust the neutral, although R14 & R15 can be used to
some effect for this purpose.
Input capacitor C7 provides DC isolation and C8 is the pulse stretcher.
Be sure that only a good quality barrier ceramic is used for C8. Do not use
TAG tantalum capacitors in this location. C4, C5 & C6 are all critical to
temperature and therefore use only
TAG tantalum capacitors here. Do not
use low voltage ceramics.
Transistors Q1-Q6 form a bridge
driver amplifier for the IC output.
Note that a separate battery may be
used to power this amplifier if re-
RX, MOTORV+
04
11
2TX753
10
C9
0.1
12
MOTOR
C7
1
IN~_ 4
IC1
NE544
Cl
.022
R4
150k
14
R2
1M
5
3
03
2TX653
R15
W 2.2k
owo
R12
150k
MOTORV-
Sk
R10
C5
.022
CB
0.22
VR1
C6
.022
R14
22k
W2.2k
W :WINCH
OW : DOUBLE WINCH
owo
Rxv~
Fig.3: this diagram shows a typical winch circuit, based on the popular NE544
servo amplifier IC. In this circuit, IC1 drives a bridge amplifier circuit (Q1-Q6)
to increase the current drive to the motor. The NE544 can also drive a small
motor direct, provided it draws less than 300mA.
quired. This is a good idea for, in
many cases, sheets can tangle, stall
the motor and thus flatten the battery.
If the receiver is running from this
battery, then control is lost and a long
swim is called for.
As an additional safety feature, a
fuse may be installed in this secondary battery to protect the power amplifier and servo motor. These things
sometimes take a long time to repair
and can end up being quite expensive. There is also an element of flexibility added to the design as a result
of using a separate battery, as the drive
voltage to the motor can be increased
without the need for voltage regulation on the servo amplifier.
Feedback resistor R2 provides the
damping required to prevent overshoot and oscillation around neutral
and is a critical adjustment. Too much
damping and the servo shuts down
too early. Also, the neutral is broadened and thus the servo is less accurate. The worst case here is that the
servo comes to rest before neutral and
leaves a residual current which may
be just below the motor start current.
This can be quite high in some motors and overheating of the output
transistors can occur. Conversely, too
little damping and the servo overshoots and oscillates before corning
to rest, again increasing current consumption and heating the transistors
and motor. In the worst case, the servo
never comes to rest and continues to
oscillate, which will quickly destroy
the output transistors and motor.
The ideal situation is called "deE).d
beat" and is difficult to obtain. The
------------------V+
11
CS
IN~l_ 4
01
BC327
U:1
N~S44
14
C4
0.1
C2
R3
150k
VR1
Cl
0.1
Rl
18k
0.1
CJ
0.1
C6
0.22
,.
Fig.4: this simple circuit can be used to drive a relay for those odd jobs that
require a "momentary on" contact but can also easily be modified for press-on
press-off operation.
most accurate situation is to allow the
servo in its unloaded condition to
overshoot once and then return to
rest. When the servo is loaded mechanically, this arrangement will perform very close to dead beat.
C9 is a suppression capacitor, to
eliminate motor noise. Sometimes a
resistor four to five times the motor
resistance is used in parallel with the
motor as well. The motor noise can be
quite troublesome and difficult to get
rid of with some brands of motor. In
this case, use a capacitor from each
brush terminal to the motor case.
Do not ground the case for sometimes the armature winding can short
out to the case and then bang goes the
servo amplifier.
As a last resort, RF chokes may be
fitted in series with each motor lead
but make sure they can carry the starting current of the motor.
Fig 4 shows the circuit of a simple
switch used to drive a relay for those
odd jobs around the model which call
for "momentary on" contact. Horns,
whistles, dropping bombs and waving pilots all call for this type of circuit. By replacing RLl with a latching
relay, a "press-on press-off" output
can easily be obtained. And although
I hav~ never tried it, I see no reason
why RLl cannot be replaced with a
bipolar relay and driven directly from
the NE544 bridge, giving a true toggle
switch output.
Once again the old faithful NE544
is called into service. Note that there
is no requirement for a gear driven
feedback pot here and so VRl is now
a standard trimpot.
SC
A UGUST 1991
35
