Ken Hough's Website

Project: A Stepper Motor Driven Barn Door Mount

My motor driven barn door mount finished as a rather complicated design, but it did eventually work well.

I began with the intention of using a curved bolt design which was to be driven by means of a stepper motor. An advantage of using a curved bolt design is that tracking accuracy should be maintained beyond small tracking angles. After beginnning the construction, I decided that the motor should be controlled via a computer, so I could have gone with a straight bolt design after all!

I wished to use this mount with digital SLR and 35mm film cameras coupled to a Sigma 100mm to 300mm F4 APO zoom lens. That's F4 at 300mm! A heavy lens! The total weight of this assembly is around 5lbs (ie >2 kg), so I needed the mount to be quite substantal.

Prototypes are rarely optimal, but others might find a few useful tips here. Hindsight can be very helpful!

Side view of the barn door, showing:

-- barn door panels hinged at right hand side
-- curved drive bolt
-- amera ball and socket mount fixed to top panel
-- gunsight to serve as a polar scope mounted on upper
-- electronics, etc (covering stepper motor) mounted below
    the lower panel
-- low level limit switch and stop fixed to edges of panels

The barn door panels were made out of 20mm thick melamine coated MDF -- 2 off, 350mm x 150mm (leftovers from fitting out a kitchen!).

The threaded drive rod was made from standard 5/16 inch Whitworth threaded rod (18 TPI), only because this was to hand. M8 metric thread could have been used. The radius of the curve in the drive rod (which must be equal to the distance between the hinge and the drive rod) was 300mm. As this drive and it's angular drive rate were to be controlled via a computer, this dimension and thread type are not critical. The controlling software could be set up to match.

Many simple barn door designs use a straight drive rod which is rotated within a fixed drive nut. This is not possible when using a curved drive rod. A curved drive rod must be constrained at one end by a gimbal mount so that the drive rod cannot rotate. The drive nut must be free to be driven to rotate round the drive rod.

Top view of the barn door, showing:

-- on the left hand side (ie hinge side), a mounting plate for ball and socket head. This was bent so that the ball and socket head was approximately horizontal when the barn door is set for local latitude.

-- mounting for the gunsight is situated near the middle of the top board. To the right hand side is a plate supporting a gimbal bearing that fixes the top end of the drive rod.

View between the barn door panels showing:

-- drive gears connecting the stepper motor shaft to the drive rod.

-- to the righ hand side can be seen the two brass hinges used to connect the barn door panels.

-- low level limit switch fixed to the side of the lower panel.

Closeup view of the drive gears.

These gear wheels are made of nylon or delrin (?) and were 'rescued' from old printers. The small upper gear section on the motor shaft is not used.

The centre of the drive rod gear was cut away to make room for a captive drive nut. The captive drive nut was made by soldering an ordinary threaded nut into a drilled mild steel plate. This was then bolted exactly centrally onto the drive gear.

View of underside of lower panel showing:

-- stepper motor drive circuitry. This is supported on metalwork which sits over the stepper motor.

-- connections for 12v power to the stepper motor and for connection of a computer (via ribbon cable) are shown.

'Exploded' view of drive assembly showing:

-- underside of gimbal mount at top of drive rod
-- drive rod gear wheel and captive drive nut
-- brass washer to reduce friction
-- plastic support bush/bearing with hight adjusting 'shim'

Parts associated with the drive system were obtained (collected?) from various old printers. The drive gears are approximately 70mm diameter for the drive rod and 50mm diameter for the stepper motor, so there is a small gearing advantage for the motor stepper motor.

Again, the plastic bush that was used for the lower bearing support was found in an old printer. This centre of this bush was drilled out to be a loose/working fit around the drive rod. The lower barn door panel was drilled through to retain the lower/narrower section of the plasic bush. The surrounding area was routered to sink the shoulder of the plastic bush a depth which resulted in the captive drive nut being approximately level with the upper surface of this panel. Final adjustment of height was done by including a 'shim' washer underneath the shoulder. A thin brass washer was fitted between the upper plastic bearing surface and the lower side of the captive nut. A little lubricating grease was applied to reduce friction in this bearing.

The stepper motor was fitted into the lower barn door panel (again by use of a router) so that the motor gear matched the height of the drive rod gear and so that the meshing of the gears could be adjusted.

Complete assembly of barn door with camera and lens

An equatorial wedge has been added. Refer to notes below.

The original tripod mounting plate on the underside of the bottom panel has been left in place. This had been drilled and tapped to 1/4 inch BSW at several points so as to provide balanced mounting points, depending on the weight of camera/lens used.

The total weight of the camera and large zoom lens was around 5lbs. The barn door added another 7lbs or so. The total weight of 12lbs (4.7kg) was more than the design maximum of 10lbs for the tripod head and resulted in some flexing as the barn door was driven outwards.

The flexing was eliminated removing the tripod head, fitting an equatorial wedge as shown in the image above, and fastening the wedge directly onto the tripod.

This did mean that initial adjustment of altitude and azimouth for polar alignment had to be done by adjusting the tripod, but the results were worthwhile.

Considering the substantial weight of barn door and camera/lens, a reasonably powerful motor was needed. The uni-polar stepper motor selected (again from an old printer) was marked as having winding resistances of 39 ohms. The motor itself was 55mm diameter. No other meaningful information was given. When driven from a 12 volt supply (and therefore drawing a current of approx 0.32 amps in single step mode), there was clearly sufficient torque available to drive the barn door.

Motor drive/control circuitry:
Using a unipolar stepper motor kept the drive circuitry fairly simple. In essence, transistors are used to switch each of the four phases/windings. The transistors must be protected from high voltage spikes that are produced when switching inductive loads such as motor windings. All of the necessary components can be provided in one DLL chip in the form of a ULN2803A (Maplin cat. no. QY79L). This chip includes 2.7k base resistors to allow for direct connection to TTL and 5v CMOS.

Switching was to be controlled via the parallel port of a laptop computer. To protect the computer from damage that might occur should there be a failure in the switching stage, a TIL193A quad opto isolator chip was included. (Maplin cat. no. YY63T)

In addition to the basic circuitry, the following were included:

-- powerline fuse
-- silicon diode in power line to protect from reverse polarity connection.
-- resistor/LED networks connected from power supply to each switching transistor -- helpful when developing
   software and tracing hardware faults.
-- connections for high level and low level switches -- only a low level switch was fitted on the finished barn door

Schematic diagram of circuit,
excluding power line and test LED components

Circuit assembled on strip board Board

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