The Adapter Plate.Adapting the MeccParts Stepper.
In the Motorvator FORUM section, I briefly introduced the MeccParts stepper motor. ( FORUM / Control Systems / Motorvator / The MeccParts Stepper ) Plan 1 shows the motor in diagram form.
The Adapter Plate.
Referring first to the design of the adapter plate, this is created in the overall form of a Metallus / Meccano 3 x 1.5 inch flat plate with revised internal perforation to accommodate the stepper motor and associated terminal block fixings. A fully dimensioned diagram appears as Plan 2.
I made the plate in 1.2mm thick aluminium, and used cheese head or pan head bolts for the attachment of the motor and terminal block. Countersunk fixings would have been better to provide a smooth surface on the shaft side, but rather thicker material would then have been appropriate. If you don't have facilities to make up a plate like this, you can give the diagram to a friendly model engineer to make it up for you.
I prefer a local terminal block that allows the stepper motor to be used as a self-contained unit in any project. The philosophy here is not just "Bolt-On" but "Bolt-On and Plug-In". I personally use Tyco HE14 connectors extensively to implement the "Plug-In" principle, but you need proprietary crimping tools for this that can be quite expensive. Using the 2.54mm (0.1 inch) pitch strip board shown, you could alternatively use similarly pitched screw connectors to obviate the need for special tools.
The orientation of the output connector is important. The arrangement shown means that output circuits leave the adapter assembly normal to the plane of the plate. An alternative is to employ right angle connectors so that output circuits leave parallel to the long axis of the plate. This might be convenient if the assembly is mounted on a moving structure where an output ribbon cable might be employed to accommodate the movement with respect to the static base. The use of two connectors in parallel makes it easy to install different terminal blocks with varying output connector orientation.
Note that the output shaft passes through a 6mm hole, and that the remaining end perforations are slotted. This assumes that the output shaft is supported elsewhere, and allows the whole unit to be moved with respect to this shaft. I favour this arrangement because it allows the controlled engagement of the first gear pair. The gearbox described below makes good use of this facility.
The Gearbox Photo Set 1 shows various views of the Metallus gearbox with stepper motor fitted, and Photo Set 2 a demonstration stand with or without the gearbox assembly attached. The bush wheel with radial marker (Photo Set 1(c)) is used to check that complete turns are being achieved without slipping steps.
Since the Meccparts stepper motor moves in 7.5 degree increments, it performs 48 steps for each complete revolution. It is quite common for commercial instrument gearboxes to provide ratios that lead to even hundred steps per revolution, so I have done the same.
In this instance, a ratio of 6:25 converts 48 revolutions to 200. This is conveniently achieved in two stages. The first stage uses the wide face Metallus 24 tooth gear with a 50-tooth partner. This pairing is closed up slightly using the slotting of the adapter plate mentioned above. The second stage is a standard 25/50 tooth pair spaced at the normal 1 inch. This arrangement can be achieved in a quite compact manner, as I hope Photo Set 1 demonstrates. (The gearbox is really only 1.5 inches deep; the side narrow girders are extended to 3 inches in order to accommodate the support stand.) Just add further 2:1 / 3:1 or 4:1 stages as required.
An alternative might be to create what I call a "One-Degree-Stepper" (360 steps per rev.) A ratio of 6:45 now converts 48 revolutions to 360 instead. This option uses the wide faced 16 tooth Metallus gear with a 60 tooth partner as the first stage, but now we use the slotting to slightly open up the shaft separation. Add a 2:1 stage as before, and there you have it!
In order to avoid excessive eccentricity, and possible large lateral forces on the (slightly overlong) stepper shaft, the first gear needs to be reasonably close to the stepper body. Failing that, you might want to consider additional shaft support.
The ability to establish different gearing arrangements at will is part of the convenience of a unitary construction system, but the result might not be as compact or as effective as a custom made commercial unit. Standard gear profiles and relatively low precision bearings will deliver somewhat lower accuracy.
Controller / Drivers and Programming.
The above arrangement can obviously be used with any stepper motor controller / driver equipment using associated programming techniques. Motorvator programming issues will be dealt with in a separate article in the appropriate area.
Please direct queries or comments to the FORUM / General Metallus Chat message section.
Bryn Jones (April 2010)