Pitch Control Module
The last projects of the avionics portion is the pitch trim control module.
This ship has a simple MAC trim type elevator trim tab servo. It is
controlled by two buttons on the pilots stick marked up/down. I wanted the
ability to control the speed of the trim motor, as pilots report it is very
sensitive at cruise speeds. I also want to incorporate a altitude hold
function similar to the system sold as the Ez-Trim system by Cliff Cady.
Anyway... The requirement for the all these wants is assembled into a small
box called the PCM (Pitch Control Module). This box contains a
microprocessor (PIC 16F84A) , pressure transducer (Motorola MPXA-4115), dual
channel 12 bit A/D converter (LTC-1298) and an H-bridge MOSFET (LMD18200T)
motor driver.
The motor driver replaces two mechanical relays and allows PWM (pulse width
modulation) to control the speed of the trim motor to any thing I want. It
also incorporates electrodynamic braking to stop motor run-on.
In the cruise trim altitude hold mode, if the airplane drifts more that 16
feet off altitude, the system begins to blip the trim up or down as needed
to maintain level flight. While in this mode, the stick buttons can trim the
airplane up/down 8.1 feet per press to adjust for pilots whims. It works
only in smooth air, but works darn well.
Another freebie feature of the microprocessor based system is the runaway
trim protection. This limited protection will stop/brake the trim motor,
flash a lockout warning light, and inhibit further trim motion until reset.
I have the thing set to lockout if the trim moves for more than 2.5 seconds
continuously in either direction. As always, everything is built with
multi-pin connectors to allow EASY maintenance. The CPU chosen also allows
ICSP (in circuit serial programming) to ease firmware changes
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