The 1999 Solenoid Engine
Origins
This project was started off with the series of articles by
Gordon Read published in Model Engineer from Sept 1998. This in turn
was based on an article from ME published in 1903. Following
discussion on the modeleng-list it became obvious that the only way
to find out more about the operation of the device was to build one.
As a secondary object it should provide an operating exhibit for club
displays.
Design progress
As ever when starting to build a model I
looked around my workshop to find the easiest materials to convert
into parts. The flywheel (3.5" diam) came from a photocopier as did
the solenoid. The baseplate is probably from the same source. My aim
was to reduce the power requirement as much as possible so all rotary
bearings are ball bearings. More photocopier leftovers!
The solenoid had to be dismantled and
rebuilt. Originally it had a single 24V coil. I unwound the coil,
split the former into three and inserted two iron dividers then glued
it back together. The original wire was then rewound as three
separate coils. The frame of the solenoid was then attacked and the
iron core which part fills the central tube was drilled out. A hole
was drilled in the back of the frame of the same diameter as the
central tube. Early experiments showed that although the plunger and
interior of the coil former are covered with some low friction
coating they actually produced a great deal of friction. Hence the
use of black plastic end covers to act as guides for the piston rod.
The plunger was cut into two pieces of different lengths and both
were drilled through the centre to take the piston rod. In order to
avoid rubbing on the inside of the coil former I turned the pieces
down slightly while held between centres . Experiments were conducted
to find which of the two lengths worked best. It seemed the longer
was slightly better. It is about the same length as one of the coils,
15mm.
End view
Having converted the solenoid, and decided
on the length of the piston it was easy enough to work out the crank
throw and make the crank. The connecting rod was originally just a
piece of bent wire so that it could be modified quickly. Once I was
happy with it I fabricated a more solid looking item from plastic
sheet.
Initial testing was conducted using a
variable frequency 4 phase drive with each phase on for one eighth of
a revolution. the second and forth phase are both used to drive the
centre coil. The results were unsuccessful. The wheel could sometimes
be made to continue to rotate when started by hand but as soon as any
attempt was made to vary the frequency the coils pulsed out of sync
with the position of the piston. Despite the heavy flywheel the
result was often immediate slow down and jittering backwards and
forwards. Obviously the power was available but it needed better
control.
Mk 2
As luck would have it the flywheel was
originally a gear wheel and had 86 teeth on the periphery. Two
methods of sensing the teeth were tried. The first using a hall
effect sensor was not successful so a photodetector was cut in half
and mounted either side. With careful adjustment of the position a
reasonable pulse was produced which was amplified to produce a square
wave with a fast rise time. The detector can be seen on the baseplate
at the opposite end of the flywheel to the coils.
Back view
Ok so now I knew how far the flywheel had
rotated but I still didn't have any way of telling the absolute
position of the crank. A second photodetector was used to sense a
small hole in the flywheel. This sensor is mounted under the
baseplate. At this point it became obvious that a microcontroller was
needed to tie all the control and drive signals together. Enter a PIC
16F84 microcontroller. This had some spare input/output pins so a
display and keypad were added to make diagnostic work easier. This is
housed in the grey 'hut' in front of the coils. Writing the software
and debugging it probably took longer than constructing the model and
it is not finished yet. However enough worked to exhibit it in our
local Model Railway Exhibition at which the Stroud Model Engineers
had a stand.
Front view
Current Status
The unit has worked continuously for 7
hours on two days at a speed of around 200rpm. It can be controlled
from around 40 to 700 rpm by means of the keypad. It would be able to
go slower if the flywheel was balanced better. (note the 4mm hex head
screw adding weight to the crank) To start rotation only requires a
push into one coils operating position. The engine will then
gradually pick up speed. RPM is displayed on the 4 digit display in
the 'hut' window. Just like a steam engine the timing has to be set
up with the crank being set at a particular angle to the hole in the
flywheel. Unlike a steam engine you can set the position that each
coil fires at by using the keypad. This allows you to set the speed
and fine tune the timing.
Results are encouraging. The engine runs
off a 9V AC adaptor. All the electronics is supplied with a regulated
5V and only the solenoids run direct from the 9V. Current consumption
averages out at about 0.25A including the LED displays. The coils do
not even get noticeably warm. With more work on the magnetic circuit
this could be reduced still further or mechanical power takeoff could
be attempted. There is no doubt that incorporating the
microcontroller has converted a temperamental fixed speed unit into a
flexible and docile unit which could be developed
further.