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Cool things I have made
Just some short descriptions - no project data supplied
Roman Black - started 12 Feb 2008
RC Servo tester - exerciser
Early 2007, web Feb 2008.
This gadget drives any RC Servo and allows the servo to be tested,
positioned or exercised (run in). Sometimes I work with RC Servos for
robotics and this unit has been very handy!
Made from junk box parts, the only new part I think was the neat
plastic zippy box with clear lid! Uses PIC 16F628 and a 16x2 line LCD.
The 6 button assembly was from an old TV set I think.
DC power is from my bench supply so I can use its volt and amp meters
when testing the servos.
The unit has its own +5v regulator built in for the PIC and LCD.
Everything was thrown together with hot-melt glue as I was in a hurry!
It has a few modes, manual and auto sweep modes. In auto sweep the
unit sweeps the servo full scale every second, ie from 1.0 mS to 2.0 mS
pulse width. This is handy for running in mechanisms and heat testing etc.
In the manual modes the servo can be moved in very small amounts or
larger amounts depending which button is pressed.
The unit has accurate timing (driven from 8MHz xtal), and at all times
it displays the servo pulse width in actual milliseconds and
microseconds. Smallest adjustment is 5 microseconds. This allows
the servo to be positioned to an exact angle, then the mS value
(ie 1.245 mS) read from the display and that can be used later in
robotics software for that same servo etc. It also lets me test servos
and rotate them well past the "proper" limits to find their exact
travel-limits.
Yes I know that some of the Servo companies sell similar servo testers
to this, but they are quite pricy! This one was built for a few bucks
to suit my exact needs, and I can reprogram the PIC at any time to add
new modes and features.
Precision temperature controlled leveling hotplate
Mar 2007, web Feb 2008.
This is an electric hotplate, about 400 x 300 mm size. It is made from
12mm (1/2") thick aluminium plate which has been surface-ground to a very
flat surface. It can be "levelled" by adjustment of 2 of the 3 feet,
which are stainless bolts and can simply be rotated to adjust the height.
I positioned the feet in a right-angle triangle so one bolt changes "roll"
and one changes "pitch".
It has 1 button and 3 leds (see below). By pressing the button the PIC micro
(inside the black zippy box) sets the hotplate to one of these 6 heat
settings;
35'C, 40'C, 45'C, 50'C, 55'C, 60'C.
These temperature settings are very useful for hardening and curing
molds and epoxies etc, equally useful is the precision flat-ground
levelled surface.
Clever stuff; To get the thing to maintain a very exact temperature
even with its large thermal mass, I developed a "forced time-constant"
closed-loop temperature control algorithm. The PIC micro using this
algorithm keeps the hotplate within 0.2'C of the desired temperature
which is a big improvement over the large thermal ripple and laggy
response caused by a normal closed-loop temp control.
With the bottom cover removed you can see the transformerless design.
The 240v 50Hz AC mains is rectified
and the resultant 100Hz 300v DC feeds the 4 main flat pack transistors. These
act as transistors (ie controllers) and they are ALSO the heating elements!
They are 1500v 5A TV h-out transistors with internal protection,
so they are much more rugged than required, to give long life. They are
also a fully insulated package type, because the metal hotplate is
grounded! All the wiring and parts are secured with screws and
high-temp silicone so nothing comes loose.
The PIC micro (inside the black control box) is powered via 2x 10 watt
resistors into a 16v 5 watt zener diode and a big filter cap. This gives
16v DC at about 25mA, reasonably regulated.
There is a +5v (low quiescent current) secondary regulator inside the
box that gives a very stable +5v dc for the PIC, leds and temp sensor.
The PIC is a little 8-pin 12F675 running 4MHz on internal RC osc. The
temp sensor is a DickSmith 100k NTC Thermistor, clamped and epoxied to
the hotplate next to one of the 4 big power transistors. My special
algorithm allows the use of a "crude" thermistor and still gives very
good temperature control.
The hotplate heaters run at a forced max of 40w each (160w total). They run
in constant-current mode with +4v on their base and an emitter resistor.
So each heater produces the same amount of heat. I positioned the 4 so
the heat is equally distributed over the entire hotplate.
Another large transistor acts as a darlington common driver and gets the
4 drive currents from the mains, the pic only needs a couple of mA to
operate all 4 heaters.
In use the heaters are ON about 1/4 of the time at 35'C and
about 2/3 of the time at 60'C, 160w was just right for good temp
regulation and gives a reasonably fast warm up of 5 to 10 minutes.
- end -
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