FEBRUARY 2006 £3.80 - Index of

HANDS-ON RC MODELLING
The environment
A small budget makes it easier to take risks. Development tools
are free, only the debugging/ programming interface needs to
be paid for. Softecmicrosystems (http://www.softecmicro.com)
proposes a debugging interface used to emulate and program
all of the devices of the whole ST7 flash family. Finally, it is
possible to obtain a ST7MC starter kit.(www.softecmicro.com/
products.html?type=detail&title=AK-ST7FMC)
includes a micro board, a high-voltage power stage, a brushless
motor and software, operating in Windows, to learn and
get practice on this type of application.
300 to 400 mV), while the MOSFET N
gates must be at almost zero voltage.
Next, we can apply a voltage of +V cc
on pins 2, 4 and 6 to put the MOSFET
P to ON (closed switch). Verify that we
have approx 300 to 400 mV voltage at
each gate. Disconnect the +V cc from
pins 2, 4 and 6 (MOSFET P OFF), and
short-circuit pins 1, 3 and 5 to ground.
Then verify that we have an approximate
voltage of +V cc less 300 to
400 mV on the gates of each N MOS-
FET. Everything is operational? Perfect!
You can then unsolder the pullup/down
resistors, and solder the two
boards. All that is left to do is add a
little bit of heat-conducting paste to
the MOSFET metal tabs, add a small
heatsink, and wrap it all in a piece of
heat shrink tubing; so well done you
could mistake it for a commercial product!.
One last important point; for those
who would like to make their own PCB
layout; it is absolutely necessary to
separate wiring for ground lines and
+V cc of power MOSFETs from the rest
of the circuit, in order to avoid any problem
of commutation noise. Star wiring
is therefore necessary.
In the current version of the Brushless
Controller for R/C Models, some of the
components are not required (place
has been reserved for future extension,
such as a regulation loop, for example).
Consequently, it is possible to omit the
following components:
- C19 on the power board (which is, in
any case, wired on the outside), this
additional capacitor may be needed for
more ‘heavy-duty’ power stages (as
mentioned above).
– Pull-up resistor R1 on the SCL line
from the unused EEPROM, (removed
from the diagram but present on the
30
layout) D2, R4, C4, K4, reserved for a
second additional path to adjust a
regulation loop, for example (not implemented
in this version of the software,
perhaps one day soon?).
Implementation and operation
The most appropriate spot for the
controller is in a ventilated part of the
scale model, if possible. This obviously
depends on available space. An
instruction manual, that may be downloaded
from our website, will help you
configure the controller. At the least,
you must have calibrated the throttle
for the initial tests. Modellers will recognize
traditional configurations for this
type of circuit, like active braking, softstart
to gentle start, for example, electrical
helicopter rotors, choice for PWM
operating frequency, configuration for
battery type. We have, however, opted
for a 2-point adjustable phase advance
(see instruction manual) allowing us to
have a linear variation (dependent on
the throttle position) between minimum
and maximum motor speed; it is
actually generally preferable to have
soft timing (10-15 degrees, for example)
and more aggressive with hard
timing (25-30 degrees).
If your motor is operating at a high
(electrical) frequency, please note that
it is better to have a high PWM frequency
(24 or even 48 kHz). For example,
for a motor rotating at 20,000 RPM
having eight pole pairs, we obtain a
commutation frequency of: 6 (commutations
per electrical cycle) * 8 (pole
pairs) * (20,000 RPM/60) = 16 kHz. In
order to guarantee a minimum number