Need Help!- Need controller for small DC brush motor

[tirebiter]

I am looking for a way to control a small 12 volt DC motor (1amp.) driving a model railroad
HO scale rotary dumper. This will rotate a train car for dumping a load of ore than
return to an upright position. The control must start at approximately 8 volts and
when a timed event occurs the voltage goes to 12 volts DC. Another timed event stops
the motion waits 1 second reverses the current and goes to a timed event and drops
to 8 volts DC and continues until tripping a limit switch. At this point the current
is reversed and the sequence is ready to be repeated. I would like to be able to
control both voltage settings and both timed events with trim pots. I do not need
any more than 2 amps capacity. I plan on offering this product for sale to the
public as soon as as I complete and test the dumper.

Can you help me?

You can see the dumper on Atlas HO model railroad forum.

Thanks in advance,
Steve Paul

[epineh]

Hi Steve, to get a better answer a few more details may help, but I will have a go...

I am interested in setting up a small train set for my young son so I have been quietly looking at model train set controllers lately while trying to keep going with current projects

First a couple of questions :

What does the controller consist of ? Will this be part of a DCC system ?

The power side is fairly simple, I would use something like this :

http://www.acroname.com/robotics/parts/R6-754410.html

This H Bridge chip (or one like it) would provide the means to spin the motor as required in both directions. You could also build you own discrete power stage using mosfet's or even transistors. Space constraints will be the major factor.

As for the control of this I would use a micro, something like a small PIC, I think the PICAXE controllers are good for this kind of thing, they use a simple basic instruction set. The other option would be to make your own discrete timing circuit's but IMHO the micro would be simpler to implement.

If this has to tie in with a DCC system then you will have to add whatever bus protocol the model train sets use, a micro would also help here. From what I have read they use some kind of serial comms superimposed over the 12V supply.

Good Luck !

Russell.

[epineh]

I just found your post on the train forum, nice work !

For some reason (maybe as it is late here) I thought you were mounting one of these per truck...now I have a better idea of what you are doing. I still think a micro would provide what you need. It is really a simple finite state process control, if this happens do this, then this, then wait for the process to start over.

What are the different voltages for ? I would think you would want to turn at a constant speed until upside down, hold then return at a constant speed.

Russell.

[tirebiter]

Thanks Russell,
Although I will be running my diorama with DCC, the dumper contol will be as simple as possible, being activated by a momentary switch (push or toggle). The full size dumper starts its rotation in a slow speed move until the hold down bars clamp the car to the track at about a 20 degree angle. At this point the dumper inreases speed until it reaches the dumping position at which it stops and hessitates for a second. The motion is reversed and continues until returning to the start possition. The 3 timed events and 2 voltage events need to be tuneable (pots). I made a video and sound recording of a power plant rotary dumper and would like to have a sound file that could be sinked to the cycle of the dumper. I was a coal miner underground repairman for many years and though I had to do electrical work, my main focus was mechanical.

I am looking for someone that can design this and I can allocate $$ or machine work in trade. I also have a CO2 lasar engraving machine.

Thanks,
Steve

[abasir]

Hi Steve,

I agree with Russel that a micro would do exactly what you want. I would use the LMD18200 H-bridge with 3A max current. Voltage control using PWM so you can vary the speed of the motor. I could make you a functional 'controller' (PCB design, micro codes, etc) but I'm half way across the world from you :-(

BTW, how fast you want this?
Can you get someone to assemble the board and program a PIC microcontroller?

Regards,
Basir

[tirebiter]

Hi Basir,
Thanks for the info. Your blogs are very informative. I have the ability to assemble a breadboard setup but am not use to working with microprocessors. The programming looks familiar since I studied several languages when I was retrained after the coal mine closed. I will check your website much more in depth in the next couple days. Is there a book or website that you could recommend for me to study up on this subject? I would like to have a functional setup within 2 months. If you can do this let me know how much this would cost. I could assemble the board myself and pay you for the programming help. I still have components to machine to complete this project so I am still several months away from completion.
Thanks,
Steve

[epineh]

I would love to help Steve, but unfortunately at the moment I am swimming in a sea of unfinished projects, it would take me 2 months to get a start on a design.

Basir would do a better job of the program anyway, I am more of a copy and paste kind of programmer

Cheers.

Russell.

[abasir]

Hi Steve,

Here's my plan... since I don't do such models, your feedback on my approach is critical... the programming part is easy :-)

I'll used a PIC microcontroller coupled with the LMD18200 chip thus allowing max current of 3A. Instead of voltage control, I'll use PWM to control the motor speed. The controller will have 3 pots and 1 switch.

1. Pot #1: Sets the duty cycle for the 'slow speed'.
2. Pot #2: Sets the number of seconds (T1) for the 'slow speed' mode (allowing 1 to 16s)
3. Pot #3: Sets the number of seconds (T2) for the 'fast speed' mode (allowing 1 to 16s)

When the switch is triggered, slow speed mode will be on for T1 seconds then PWM will go to 100% duty cycle (maximum speed) for T2 seconds. The controller then waits for ?? seconds then reverse the cycle.

Does the above sounds correct?

Any price target you are expecting? Digikey list the LMD18200 for USD14 each thus it will be the most expensive part of the controller.

You can pm me if you wish to take the discussion offline.

Regards,

p/s: I'll be away on assignment until next monday. I may not be monitoring this thread during the period...

[tirebiter]

Hi Basir,
This sounds great. I can start assembling components. Which microcontroller will you want to use? $$ are not critical. I have been looking at related websites and understand the duty cycle and speed controll. What kind of pots need to be purchased? I watched the video and the slow speed needs to be aproximately 6 seconds and the fast speed needs to be aproximately 16 seconds. The home position should be controlled with a limit switch so the tracks line up. I would like to control the high speed voltage but I could control this before it gets to the H-bridge. I ordered a Digikey catolog.

I also saw where you could control the limits by using an encoder fastened to the motor to locate postions by counting rotations. Is this a viable alternative or is it too expensive? It is very important to locate the home positon.

Thanks in advance,
Steve