A way to build a hot wire muscle. It can be assembled by a handy
amateur or could be mass produced at low costs. It was initially conceived
for RC command.
This is a drawing of the prototype I build:
It is made of four kind of pieces:
One big rectangle of printed circuit, with three solder plots.
One little rectangle of printed circuit, with three little holes.
Two pieces of fishing steel wire with a diameter of 0.24 mm (in red).
Three pieces of standard electric copper wire (in orange).
One very important thing is that the big rectangle exerts a strong pull
force on the two steel wires. You could play with those wires like with
the strings of a guitar.
The way it works is as follows:
If a battery of 4.5 V is connected between the upper left copper wire
and the right one, a current of about 1 A passes trough the upper steel
wire. It heats, expands. That makes the other steel wire pull on the lowest
hole and makes the little rectangle turn clockwise.
The same will happen if a current is sent trough the lower steel wire.
The little rectangle will then turn counterclockwise.
The advantages of this system are the following ones:
No hysteresis. It returns perfectly to the neutral position.
Good resistance to external forces. It is not easy to destroy this muscle
or upset it.
It can be manufactured with cheap materials and tools. The rectangles can
be made out of those preperforated bakelite plates for prototype circuits,
with a copper circle around each hole.
It has a neutral position and can deviate to the left or to the right.
It can be build out of materials resisting high temperatures and can work at high temperatures.
Its disadvantages are the following ones:
Compared to the force it exerts it consumes a lot of electric energy.
If it must keep a certain deviation then it must continuously be fed with
electricity. In order to be able to stop the current and let the muscle
remain at its position a blocking device must be added.
It is not counterreactive. You can make the muscle deviate more or less
by increasing or decreasing the current but you will not be sure of its
deviation if external forces are present. If you want to be sure of the
position a counterreaction system must be added.
A few tips:
The tree solder plots on the big plate may have difficulties to maintain
the steel wire. It will move inside the solder even once cooled down. So
you must make a little knot in the steel wire there where it will be plunged
inside the solder.
It may be a little difficult to pull on the wires while you are soldering
them on the big plate. (Don't forget they must be heavily stretched). Do
not hesitate to use a few decimeters of wire, latch them to pieces of wood,
use some lever devices... As a result of the pull force the big plate must
be slightly curvated. You may also block the big plate in a curvated position,
solder the wires, then make the plate free and let it pull the wires naturally.
Do not hesitate to try several times to do it right, pull on the wires
and melt the solder plots again and again.
The pull force on the wires must be as strong as possible. The stronger,
the more force the little plate will have while turning. Yet the pull force
should not be too strong and tear the wires apart.
The closer the three holes are from each other, the more the little plate
will deviate when a given current is sent trough one of the steel wires.
Yet it will deviate with less force.
The length of each steel wire should be choosed so they become really warm
when a current is send trough, but they should not become red. You can
use longer lengths in order to make the wires heat less yet make the little
plate deviate the same amount. Up to you.
The way the prototype is build it takes less than a second to turn to the
right or to the left. This is OK for a lot of applications. Should you
wish another speed then the following can be done:
Use a higher electric tension yet switch it on just a short time. That
will make the wire heat quickly and expand immediately. You may use a switching
power system to maintain the deviation, or a diode and a lower tension.
Put the wires inside water and use higher electric tensions. The movements
can that way become very fast, back and forth. A very thin steel wire inside
water could generate hearable sound waves when fed with a low frequency
electric signal (you may make a steel wire pull a loudspeaker cone).
Put a thermal insulation around the wires in order to allow them to heat
with less electric power. The movement will then become very slow in both
directions.
An industrially produced version of this system could look like this. A
little square tube with electric wires coming out of both ends and an axis
coming out of the sides. A strong spring must be build inside the tube
to pull continuously on the steel wires:
