Architecture model dilemma..

[ConnorP]

Hi,

I'm building an architectural model for one of my final year projects at uni, and i'm having trouble designing the operating circuit for it.

The model is of what is commonly known as 'The Sliding House' by drMM architects (give it a google and you'll see what I mean about the motor) and it is in 1:40 scale, which puts it at about 80cm unextended.

I'm a complete novice in electronics, although I'm having basic lessons from one of the pro Modelmakers at the company I'm doing work experience in, so I have a (very) basic grasp of it as of yet.

I'll explain what I'm trying to do first and hopefully one of you would be kind enough to teach me how such a system would operate if you could?

Ok, so I need to plug the model into the mains, just so you know where my power is coming from initially. (I know about fusing and transformers to convert so I'm ok up the the converted side of the transformer)

Ideally, due to aesthetics and ease of demonstration, I'd like to use a 3 position rocker switch (on-off-on) to control the direction of the motor. Unless there are any better suggestions of low visual impact switches that would suit better? Also, i'd like the switch to only have to be depressed for a second (not held down) but the model to move until it reaches either end of the rack.

I need to run a DC motor with low RPM to control a rack and pinion system, the rack being attached to the bottom surface of the outer part of the house, and the pinion being attached to the shaft of the motor to drive it.

I need to reverse the polarity of the motor when the switch is presses from either left or right to move the model left or right (sorry if this is obvious, just trying to include as much info as possible.

At both ends of the rack, I plan to install microswitches (have no idea what contact config) so that when the piece reaches the switch it stops the circuit and the model isn't working against itself, I believe this type of system is often found in model railways, although I want the piece to stop when it gets there.

Hopefully this is clear as of yet, sorry if I'm getting anyone confused! I actually saw an image of a circuit similar with a blue train and microswitches which looks kind of what i'm after, without the constant movement. That guy got loads of help so hopefully I'll get the same!

Ok, so here comes the hard part. The model has a 3rd position, the middle. It's important it stop at the right place as windows line up etc and it would ruin the effect if it wasn't precise. This is the part I'm really stuck on, I don't know if I need another micro switch in the middle (maybe a lever microswitch switch that gets depressed by a lump on the bottom of the moving piece that contacts it at the right position?), and even then I wouldn't have a clue how to wire it so it works from both directions.

So, positions A, B and C. In terms of switch operation I'd love it if there was a way to have a 3 position switch, the middle position being no movement, and each momentary touch of a direction to send the model the appropriate direction. So imagine we are in position A, one touch of the right hand side of the switch would see the model move slowly right into position B, and another touch seeing it move into position C. A touch on the left side of the switch would see it move back into B, and so on.

I know its long winded and fussy and everything, but I just have a vision of the movement in my head and I know with enough research and time (and grovelling) the solution is out there.

Thanks in advance for the bundles of expert advice i'm about to receive, it's appreciated

Cheers

 

[jpanhalt]

How big a motor do you need?

Have you considered a hobby servo (some are quite powerful) and a simple program to control its position. Positioning accuracy can be well within a degree. The program can be quite simple in either Assembly or C. I have done it in Assembly and use Microchip PIC controllers. No limit switches are needed, and you can have a switch select almost any number of pre-defined positions.

John

 

[ConnorP]

Well the outer part of the house is going to be made from a 2mm acrylic skeletal frame with 2mm acrylic (i know it's heavy but its all laser cut for accuracy) clad in lime wood so it will probably weigh somewhere around the 2kg+ range. I don't know exactly yet as I haven't started production.

There will be a shaft coming from the motor with two gears on it, mounted to lock into the racks on the bottom of each side of the outer sleeve, like this.

M-I----I-

(The motor being M, the gears being I and the shaft being -)

So I don't actually know how big I need the motor, but it needs to be slow and strong, any ideas on specifics?

I haven't no, is it a lot more difficult to make? Taking into account I have no first hand experience of them..

 

[jpanhalt]

Here's a link you may be interested in: https://www.servocity.com/

Click on servos and accessories. Two kg is nothing, if there is any semblance of a bearing. Big model aircraft today (excluding America's military drones) are 25 kg or so. Servo City is in the US. I have purchased from it and have gotten quite good service. I don't know whether it ships internationally. Hobby King does ship internationally. It is an honest dealer, but its shipping may not be the fastest.

You can do a lot with a servo and a simple "turn a port pin on, wait, turn port pin off, wait" Assembly program. Gears may not be needed. You can control servo speed in firmware, which is a little more complicated, but not by much.

John

 

[alec_t]

Doing this with a servo might be tricky, since 80cm of movement is needed. If I had to do this I'd probably use a geared hobby motor driving a threaded shaft, with nut-runners on the movable walls.

 

[MrAl]

Hi,

Another idea is a cart on grooved wheels on tracks, like a train car. The motor drives the wheels, everything controlled from within the 'house'.

There are a number of ways to do linear motion. There's a lot out there already like linear motion bearings, ground shafts, special threaded shafts, etc.

Limit switches are the worst choice for control because when they fail disaster follows.

 

[user_88]

Doing this with a servo might be tricky, since 80cm of movement is needed. If I had to do this I'd probably use a geared hobby motor driving a threaded shaft, with nut-runners on the movable walls.

There is a product called Acme Threaded Rod ... e.g.:
https://www.grainger.com/Grainger/T...5DPX7&ef_id=UIz1eQAAUXMVNX2P:20130920201928:s

This is designed for motion, when used with the corresponding Acme Nut. You can count the rotations of the rod, or use limit switches.

The most direct approach to achieve your goal would be to use a stepper motor ... With a stepper motor, you will be able to control the position of an object with some precision ... forward, reverse, or halt, using a micro-controller. The linear traverse capability for this material is within the design specifications which you have described.