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Topic: Slow start for DC motor – Faller car system (Read 545 times) previous topic - next topic

vinceherman

Sep 21, 2018, 08:14 pm Last Edit: Sep 26, 2018, 10:55 pm by vinceherman
Edit:  This is starting to be less General Electronics and more Power and Motors.  I will request a move from the admins.

I am working on a portion of our club's model train layout where I will be automating the running of model cars.  I am using the Faller car system.
I AM using an Arduino to drive the traffic lights, read sensors and move servos to guide the cars but this specific question is more about general electronics.



The top circuit is the drive system for the model cars.  You turn them on and they go.  When the reed switch is near a magnet they stop.  When you remove the magnet they go.
This is all good.  The bard part is that when they go again, they instantly run at full speed ruining the realism of the model.  I had read that you might be able to reduce this jack rabbit start with a cap and resistor.
The bottom circuit represents my understanding of such an arrangement.
My hope is that when the reed closes (after removing the magnet) that the cap will go through a period of charging, and during this period, the motor will start slowly and eventually get up to full speed.  Even if that period is a half second it will be more realistic.
The batteries are 2 button cell NiCad batteries.  2.4v I think but it has been a while since I checked.
My questions:
Will the second circuit provide this behavior?
Do I run the risk of burning out the reed switch as the cap charges?
How do I compute the values of the cap and resistor?
Do I have the polarized cap the right way round? (not that having it right on the schematic means I will get it right when I start soldering)

hammy

#1
Sep 21, 2018, 08:42 pm Last Edit: Sep 21, 2018, 08:52 pm by hammy
I don't think that arrangement will work . Really it needs something to allow the voltage across the motor to ramp up -
Maybe if the capacitor was directly across the motor and the resistor in series with the power supply ? It might reduce top speed a bit tho.
Needs a bit more thinking !

JohnRob

hammy has the correct connection scheme, however I think the values you will need are not very practical.

I'm not sure the current of the motor but lest say its around 20 ma (pure guess).   For a 3 second ramp the capacitor would have to be in the order of 20,000 µF.   This was a very crude calculation but will be in the ballpark.

I'm not sure of an alternative.  Perhaps an ATTiny85 driving a small FET might fit. The would give you the most realistic startup because it would be a PWM signal which seems more realistic on models than voltage limiting.

Please do not PM me with thread based messages.  If your thoughts are worth responding,  the group should benefit from your insight.

outsider

Have you thought about a MOSFET in series with the motor and doing the R/C timing on the gate circuit?

vinceherman

I do have Attiny85s laying around.
Can you suggest a FET to purchase? 
The batteries are 2 cell NiCad 250mAh in series.  They run about 15 minutes on a charge.
Does 2.4v give me enough to run an Attiny85?

vinceherman

So, how simple could I make this with one of the ATTiny's?

If I have the ATTiny drive a FET to run the motor, on power up I can start at PWM 0, then ramp up to full PWM.
I could have the reed switch just be an input to the ATTiny.  If the reed switch is open, PWM 0.  When the reed switch goes closed, ramp up to full PWM.
Or I could have the reed switch remain inline with the power for the ATTiny and the FET.  When the reed switch is open, no power to the FET and the motor stops.  When the reed switch goes closed, the ATTiny will boot up and ramp up from PWM 0 to full PWM.
Any thoughts?
How simple would this be component wise?
A low voltage ATTiny
A logic level FET (please recommend an appropriate model)
Maybe a cap to decouple power since I will be running a motor on the same power supply.  I have many but any recommendations on a value to start with will help.
Is that enough?

Slumpert

Start with your power supply for the train... 
Would it be possible to adapt it so that a servo moves the power control dial?

Robin2

I had not seen this Thread before today. I have had a interest in the Faller car system for some time but I have not yet bought one.

@vinceherman, you have not said what size are the vehicles you are working with. Is there space inside them for an MCU?

I have built a small wireless controller with an nRF24 module, Attiny1634 and a ROHM BD621xxx h-bridge. The biggest part (by far) is the nRF24 and you would not need that. The Attiny 1634 and the BD621 are what I would describe as large surface mount components which are quite practical for hand soldering. IMHO the key thing for squeezing into a small space is NOT to use any PCB.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

septillion

I think accelerating gradual and stopping quickly does not look nice either and is indeed a bit harder without like a motor driver or more parts.

To kind of accelerate and brake gradual you could add a cap parallel to the motor and a resistor in series. But I think you need a large-ish cap. You could do it with some extra components in a discrete way but it get's bit quick.

You could use a small micro to do it. Do note quite some people already do that with FCS and made the cars completely digital, DCC for FCS including small decoders for cars :D Including break lights, turn signals etc.
Use fricking code tags!!!!
I want x => I would like x, I need help => I would like help, Need fast => Go and pay someone to do the job...

NEW Library to make fading leds a piece of cake
https://github.com/septillion-git/FadeLed

vinceherman

#9
Sep 27, 2018, 02:11 pm Last Edit: Sep 27, 2018, 02:14 pm by vinceherman
@slumpert, I think you missed some of my message or I was not clear.  The Faller Car System uses battery powered cars that have limited functionality.  They have a magnet that follows a wire to steer the car.  They have a reed switch that stops the car when a magnetic field is near.  When the field is removed, the car runs again.  It starts running at full speed, creating a 'jackrabbit' start.  That is the property that I am trying to address with a slow start.

@Robin2, we have the HO sized system.  3 vehicles, a bus, a garbage truck and a straight truck.  All have enough room inside for a dead-bug ATTiny and a few components.  I can solder under my microscope and I can see using SMD but I imagine the proof of concept/prototype will be with the bigger through-hole type components.

I am not looking to control these remotely other than the stock features; wire following (with servos under the layout moving wires to turn the vehicle on to other wires) and stop control (with servos under the layout moving a magnet into position to activate the reed switch and stop the vehicle)

The one feature that I do want is a more realistic start.  I had started investigating this using passive electronics.  But it looks like an ATTiny and a signal level mosfet might be enough.

Oh, it would be way cool to have some of the features that Faller's digital system provides.  One is a close parallel to adaptive cruise control.

But for today, I am looking to use PWM to turn the jackrabbit start into a smooth, realistic-looking acceleration.

I have ATTiny's laying around.  Any recommendation on a fet?  Any ideas on other components?

I am going to order a few Attiny1634.  They look like fun!  The BD621 look like they start at 3v and go up.  I will be operating lower than that.


vinceherman

#10
Sep 27, 2018, 02:14 pm Last Edit: Sep 27, 2018, 02:14 pm by vinceherman
@septillion, you got in while I was typing my book.  :)
For now, I am just using the non-digital Faller cars.  No turn signals.  Just stop and go.
ATTiny.  Fet.  Anything else?  Any recommendation on a logic level FET?

septillion

As for the fet, any that can handle the current and is in linear region in normal operation. Simplest with a logic level mosfet / one with a low Vth.

A H-bridge looks like overkill to me. I use IRF7313 quite often (also in model trains). But it's also a bit large/overkill for this as it's a dual mosfet... Something like a 2SK3018 or AO3400A looks fine to me I think.

Don't forget to add a cap close to the uC and maybe an elco as well.
Use fricking code tags!!!!
I want x => I would like x, I need help => I would like help, Need fast => Go and pay someone to do the job...

NEW Library to make fading leds a piece of cake
https://github.com/septillion-git/FadeLed

Robin2

But it looks like an ATTiny and a signal level mosfet might be enough.
I had overlooked the fact that you do not need to reverse the vehicle so a MOSFET should be fine and they are available in small sizes.

But think what extra features you could apply with a little more program. A 3-point turn would look very cool and make the other Faller users green with envy.

...R
Two or three hours spent thinking and reading documentation solves most programming problems.

vinceherman

Step 1:  make the layout control piece work.  Reading IR sensors, controlling servos and managing the traffic lights.  Vehicles stop at intersections and the bus stop.
Step 2:  see if I can fix the jackrabbit start
Step 3:  have HO scale people board the bus at the bus stop and deboard at the train station.  And 3 point turns.  :-D

OK, if I could send data to the vehicles, turn signals would be a nice addition.  But if I were to properly model Ohio traffic, only half the vehicles would ever use signals.

septillion

But if I were to properly model Ohio traffic, only half the vehicles would ever use signals.
Half Ohio drives a BMW? :smiley-mr-green:
Use fricking code tags!!!!
I want x => I would like x, I need help => I would like help, Need fast => Go and pay someone to do the job...

NEW Library to make fading leds a piece of cake
https://github.com/septillion-git/FadeLed

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