PWM vs sine pulse throttle performance questions

[mmagliaro]

On my DC layout, I use a simple transistor throttle that has sine ripple riding on top of the
output to provide some pulse for low-speed smoothness.  For those of you who really go back a long way,
it is just the "High Capacity Throttle" circuit from the old Peter Thorne electronics book.
But in fact, it's about the same design as lots of basic transistor current amplifier circuits.

At low speed, the sine ripple makes up a significant portion of the output of the throttle.  As the speed goes up,
that ripple becomes less and less a percentage of the output.  That is, the sine amplitude is about 2v.  So at low speeds (say up to 6v), that is a significant ripple.  By the the time you are up to 8 or 10, a 2v ripple on top of that is
not really much "pulsing".

Some engines don't even start moving until the throttle is up to 4-5v.  So by the time they start moving, the really
nice pulse range is exhausted.

And now the question:
What about using a PWM circuit design instead?  I know about the general advantages of PWM for controlling
DC motors.  It also seems to me that PWM would also get rid of this quirky pulse drawback from
the old-school transistor design.  The pulses would be there, they widen as you turn up the throttle,
and the performance advantage would be there for any motor and would be immune to the particular
starting voltage of the engine.

Am I thinking right here?

[peteski]

The PWM-based throttles usually have fairly high frequency pulses which are averaged into a DC voltage at the motor.  At least in DCC decoders.  I guess if the pulses were low enough frequency then they would act like the pulses you want (around 20-100Hz). But then the motor will keep on buzzing.

If you really want a complete control of DC locos build yourself a TAT V throttle. I did that many years ago (following instructions in an article in Model Railroader).  You have full control of the pulses (frequency and amplitude) plus several other adjustments.

Here is mine (stuffed into an old MRC case). Notice the extra knobs and other features.

[mmagliaro]

Heh, somehow Peteski, I figured you'd be the one (and maybe the only one!)  to respond to this.

I had a Heathkit throttle for a long time, which is essentially a TAT (a IV, I think).  It had the adjustable
pulse width and frequency on it.

I don't want the throttle to get that complicated to use.  I remember tweaking those pulses for each engine
and while it works, it's more hassle than I want.

Essentially, that's one of the conveniences that a DCC decoder buys you.  You can tweak it to the engine.
It's like carrying around a customized PWM throttle inside each engine, ideally programmed to that engine's
characteristics.

I think what I really want is to have those sine pulses be strong "when the engine just starts to move".
That could be at 1 volt or ir could be at 4 or 5 volts.  But that might require some fancy footwork to make it that smart.

I'll tell you where I'm really going with this.   My coreless motor powered engines can typically start moving at under a volt.  But sometimes I have to use a 6v or 3v motor.  In that case, although I can over-driver them to higher voltages than that, I don't think I want to try driving a 3v motor all the way to 12v.  So, what to do?
Well, I could put a zener limiting circuit in there, or a regulator, to essentially "cap" the maximum so it won't fry
the motor.  But that takes up lots of room (that I usually don't have) and makes a lot of heat.
Alternately, I can just use an in-line resistor.  With the low currents these things draw, and the gearhead,
their current draw is usually quite stable over a wide range of speeds and loads, so the resistor does a fine job.

But that means that the 3v motor won't start moving until about 5-6 volts, and I have lost the advantages of my
pulse if I do that.  And yes, coreless or not, they definitely run better with a little ripple in that supply.

I suppose I could add a "starting voltage" control to the throttle (like that TAT or Heathkit also had),
and tie that same control to the pulse amplitude, so that if the starting voltage is 2, the pulse amplitude is around 2,
and if the starting voltage is more like 4-5, the pulse amplitude is 4-5.   The pulses would still descend as the
throttle is cranked up... hmmmm.... must ponder.

[peteski]

Max,
pulses in DC throttles are truly beneficial with standard motors with iron rotor armatures.  The pulse vibrates the armature to minimize the cogging which those motors are infamous for. Plus the vibration overcomes some of the inertial in those rotors.  Corleless motors don't need pulses because they don't exhibit coggign (especially the higher-end ones with 5 or more windings).  The inertial is also almost non existent.   So, if I was in your position I would not worry about the coreless motor not getting its pulses to make it run smoothly it shouldn't need them. 

As far as PWM motor driving circuit goes, that alone is not what makes the DCC-decoder-driven motors run so well. Like I mentioned before, the PWM pulses are usually such a high frequency that the motor sees them as an averaged DC voltage (not as pulses).  It is the extra intelligence built into the decoders which makes the motors run so well at low speeds.  You can thank the BEMF feedback for that.  Using  BEMF the decoder senses the rotational speed of the armature and then adjusts how much power to send to the motor.  It is that feedback loop which makes the motor run at the speed corresponding to the speed step selected.  Another thing is that most decoder have a kick-start circuit which will send a kick-start voltage pulse to the motor to get it initially started before the BEMF takes over.

If you want this type of control from a DC throttle, you would have to design a PWM throttle with a BEMF feedback incorporated in it.  At least, that is how I see this.

[Ron McF]

Max,

for a year or so before converting my layout to DCC I used a PWM throttle from Ken Stapleton in Canada.  It worked very well, but not so much better than my home-made "Pacematic" throttles (built from an article by Thorne in the December 1977 MR) that I went out and bought more.

If you want to try PWM for next to no dollars, then you might take a look at the PWM motor controllers that are available on eBay for less than $5 delivered to your door.  I bought a couple of different types about a year ago and tested them out on a club layout using some old Tri-ang locomotives.  I was surprised at how well they worked.  I see that there is now a much wider variety available to choose from.

Regards,
Ron

[mmagliaro]

Peteski, I hear ya.  In theory, there should be no advantage to pulses with a coreless motor.  All I can tell you is what I observe in practice.  I can get a coreless-powered engine to creep visually slower with the pulses turned on.  I suspect that
with 60 Hz pulses, this has more to do with vibrating the mechanism than with vibrating the motor.  The motor can't "cog", but at super low speeds, the engine could reach a point in the gear or driver rotation
where there is a little extra friction, and kicking the motor 60 times per second is going to translate into hitting the gear train with a kick 60 times/sec also.  It's just like shaking the mechanism a little.

BEMF --- yes, that would be really cool to incorporate into a throttle.   

But remember, I really don't have any quibbles with engine performance that require anything that fancy, and BEMF
has to be tuned to each motor, so I'd rather avoid relying on it.

If I could get the pulse amplitude to be a significant part of the output just at the point where the motor starts to move,
it would be great. 

[peteski]

Peteski, I hear ya.  In theory, there should be no advantage to pulses with a coreless motor.  All I can tell you is what I observe in practice.  I can get a coreless-powered engine to creep visually slower with the pulses turned on.  I suspect that
with 60 Hz pulses, this has more to do with vibrating the mechanism than with vibrating the motor.  The motor can't "cog", but at super low speeds, the engine could reach a point in the gear or driver rotation
where there is a little extra friction, and kicking the motor 60 times per second is going to translate into hitting the gear train with a kick 60 times/sec also.  It's just like shaking the mechanism a little.

BEMF --- yes, that would be really cool to incorporate into a throttle.   

But remember, I really don't have any quibbles with engine performance that require anything that fancy, and BEMF
has to be tuned to each motor, so I'd rather avoid relying on it.

If I could get the pulse amplitude to be a significant part of the output just at the point where the motor starts to move,
it would be great.

Well then, that is what advanced DC throttles like TAT V would do for you.   

But you you want to take a PWM throttle for a spin than like Ron said, you can get a PWM motor controller circuit on the cheap on eBay. I recall VonRyan (Cody) getting one for his Nn3 layout.  He posted into about it on TRW (use the search function to get more info).  Or just go to eBay and search for PWM motor controller or throttle.

If you want to build one from scratch then there are ICs made just for that function. I have never built one but I'm sure there is plenty of info on how-to on the Internet.