There are several ways to motorize the trim wheel of the Airbus. The A320, unlike the Boeing 737, has autotrim built into the aircraft systems. Boeing pilots are required to trim the aircraft to an angle of attack of 1 G — this is done with the trim switch on the yoke.
On the A320 (also present on more automated Boeing aircraft) we have the luxury of a computer auto-triming the aircraft during flight. The only instance where manual trim has to be performed is prior to the aircraft takeoff (part of your checklist procedure).
To motorize a trim wheel on a simulator you will need to create a close loop system. This allows the software to feedback to a motor driving the shaft of the trimwheel, and to your simulator software.
There are two types of motors generally recommended to drive a trim wheel – DC, or Stepper. DC motors either run clockwise or anti-clockwise depending on the polarity of the power provided. Stepper motors have a built in encoder (feedback system) which allows you to keep track of the position of the motor.
I have enquired several cockpit builders which motor would be the best to use to motorize an A320 trim wheel. The advice has been split, with some suggesting a stepper motor, and others suggesting a DC motor and using a potentiometer to provide feedback. If you use a DC motor you will also have to attach a 360 degree pot (no limitations) to the trimwheel so position can be detected.
The interface cards commonly used for motorization of a trim wheel include the Opencockpit DC Motors card / Stepper Motors card or the Phidgets MotorControl card. Axel has written a nice flow chart on how an Adruino card could be used:
The software used to interface with the simulator is also critical — what use is the hardware if the software does not work correctly?
Robert has written software for his FDS A320 throttle and allows you to interface with Jeehell FMGS (using FSUIPC offsets) a DC motor and feedback 360 degree pot. You can see the trimwheel in action in this video. I have obtained his permission to post links to his software which you can download here, or on the resource page.
I am undecided which method to use at this stage. For simplicity I am considering implementing a DC motor with a feedback pot, as Robert’s software seems to work well with FMGS.
Update – Trim Wheel motorisation has now been implemented using a Pololu Jrk interface board and DC motor