<<<      Stepper motors for cockpit instruments

1. Automotive gauge stepper motors

No doubt, one of the best choices for replicating most of aircraft panel instruments is using small bipolar stepper motors X27.168 (or X25, VID29 and other similar ones) that are used in car and motorcycles dashboards nowdays. You can get these steppers for a price of $3..5 a piece on Ebay, AliExpress, etc.

To drive these very low-power motors (which draw about 20 milliamps per winding), there are specially designed controllers for them, such as VID6606, STI6606Z, AX1201728SG or STI6608 chips, that all provide very precise positioning - 12 steps per degree.

Note: The first 3 of these chips are identical and control four stepper motors, but the STI6608 controls 2 motors.

Any of these controllers can be found online for $1 ... $2 for the chip. Also, along with driver chips, you can buy very useful small boards (for $0.1 for a piece) that will allow you to make a breakout board yourself and save some time and money.

Using a small soldering iron you will make a good "breakout board" for 4 stepper motors just in a few minutes and spending $2 (chip + board + header strip).

All 4 Switec stepper motors should be connected directly to the controller outputs, without using additional diodes or resistors, as shown in the connection diagram for VID6606 below. Pay attention to the steppers pin numbering.

Instruments with limited rotation

Switec stepper motors with internal limiter have the shaft rotation of 315 degrees. For most instruments with an arrow rotation angle less than 315 degrees (like ASI, VSI, all engine instruments, etc.) you can use these stepper motors without any modification - no need to make zero-position sensor (though you still can).

When SimVimCockpit is started or reconnected, the program moves the motor shaft until it stops against the limiter and thus sets it into zero position.

Instruments with a full-circle rotation and zero-positioning

To make instruments with continuous rotation (like compass card, altitude indicator, directional gyro) you need a motor without internal limiter and it should have a sensor to define the zero-position (it can be a physical contact sensor, hall effect or optical sensor).

In these steppers you can easily remove the stop, just carefully open the pair of plastic half shells of the stepper's body (they snap together without glue), and cut off the plastic stop located on the gearwheel using a sharp blade (see the picture below). Then, you will need to add a sensor for the "zero" position, so that this motor might reset to its initial position on startup.

A good way is to use an optical (infra-red) sensor - a pair of infrared LED and IR photodiode, as shown in the picture, using the instrument needle as the "mirror" that deflects the light from the LED onto the sensor. You can glue a piece of tin foil on the back on the needle for better reflection, but for us just a white plastic arrow worked great, with diode connection shown on the diagram below.

The optical pair is placed behind the faceplate at a distance of about 3..5 mm and the faceplate has a small vertical gap in front of it (about 2 mm width) - this will exclude external light interference. For instruments that have no needle but have a rotating card just use a black matte surface on the back side of this card with a thin white line or strip of foil aligned with "zero" on the face side.

When SimVimCockpit is started or reconnected, the program moves the motor shaft until its needle/card cross the zero-position sensor to set the motor into initial position.

Calibration of instruments

Other stepper motors

Besides X27-like steppers, others stepper motors can be used with appropriate controllers/driver boards that have "STEP" and "DIR" inputs, for example A3967 chip ("EasyDriver" board) or DRV8825 and other.

NOTE: Most stepper motors can require quite much power to run. If your Arduino is powered using an external power source via the power jack (7-12V opposed to 5 volts from the USB source) and your stepper needs power source of 7..12V you can access it through the Vin pin on Arduino board. This allows you to power the stepper with the same power supply (but you need to be sure that this PS's maximum current will be enough for all connected stepper motors).

You will need to enter the number of steps per 360 degrees for your stepper when you select it in the configurator. The default is the number of microsteps for X25, X27 and similar steppers used with the controllers described above only!

28BYJ-48 stepper motor with gearbox

This is a stepper motor widely used in various hobby constructions, and you can use it with SimVimCockpit too. It's a unipolar motor but it can be easily converted into bipolar to work with controllers that drive bipolar stepper motors.

Information into this chapter will be added after all hardware tests with 28BYJ-48 stepper motor

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