Servo stall motor for points

[AndyID]

Not sure this is the best place for this. It applies to the remote operation of points (turnouts) regardless of whether the layout is DC, DCC, clockwork, steam or battery but as there doesn't seem to be another forum for remote point control I'll put it here.

 

There are several point motors on the market that use "stall motors". Basically they use a DC motor to move the tiebar and when the tiebar can move no further the motor stalls. When it stalls it is still exerting a force to keep the tiebar in position. That's  bit different from many other types of point motor that only supply intermittent torque and either rely on friction or some sort of over-center spring to keep the point blades in position.

 

Generally it's not a good idea stall a DC motor. It's likely to overheat, but if the power supplied to the motor is limited to an amount that the motor can easily dissipate without overheating there isn't a problem.

 

SG-90 servos are very inexpensive. It's not at all difficult to access their motors directly and it's quite simple to modify them for continuous rotation. Why not use them as stall motors to control points? Based on my (limited) measurements it looks like a stalled SG-90 motor with 0.7 volts across its terminals (and passing a current of 0.17 Amps) will restart when the stall force is removed. In the stalled condition it is only having to dissipate 0.12 watts.

 

 

[JimFin]

2 hours ago, AndyID said:

SG-90 servos are very inexpensive. It's not at all difficult to access their motors directly and it's quite simple to modify them for continuous rotation. Why not use them as stall motors to control points? Based on my (limited) measurements it looks like a stalled SG-90 motor with 0.7 volts across its terminals (and passing a current of 0.17 Amps) will restart when the stall force is removed. In the stalled condition it is only having to dissipate 0.12 watts.

 

 

 

I don't understand why you would want to do this as the servos work perfectly well unmodified with an appropriate controller either DC ar DCC.

[Gordon A]

Reading between the lines I think Andy ID is looking to use the servos without an electronic interface. I have read of someone else doing this, I think on this forum.

The servos themselves are quite cheap, but the electronic interface could be considered a bit expensive. By modifying the servos and running them on a low voltage you could in theory replace the electronic interface with a standard switch.

 

Personally I stick with an electronic interface which gives control over the speed of throw and how many degrees of arc it operates over. On my current cameo layout I am using Megapoints to operate two points and when I build them a pair of gates.

 

Gordon A

[Nigelcliffe]

"Izzy" has done as Andy suggests on his small 2mm layouts.   I'm not sure if he's actually using them as stall motors, or just operating temporarily and then removing power when movement is complete (centre-sprung to off switches). 

 

I can see the reasons why, but to find out if it makes sense requires some real experiments.   At least with cheap SG90's it won't be an expensive experiment.

 

- Nigel

 

[Ian Morgan]

If you are proficient enough to consider modifying a servo, you could easily build a Merg Servo4 kit. At less than £7 and able to control up to 4 servos, the cost of the electronics can be affordable.

[ejstubbs]

2 hours ago, Gordon A said:

Reading between the lines I think Andy ID is looking to use the servos without an electronic interface. I have read of someone else doing this, I think on this forum.

 

IIRC someone posted a while back about using a servo motor with some kind of limit switch at the extremes of travel.

 

Here's a couple of example threads I found by Googling, though I don't think either of them is the one I was thinking of:

 

(I've just realised that the first one up there is probably what nigelcliffe was referring to.)

 

And here's an example of someone using "hacked" servos with just pushbutton switches to activate them i.e. no limit switches:

 

Edited March 10, 2020 by ejstubbs

[Izzy]

I first started using servos for point operation some years ago using Merg servo4 boards coupled with SG90’s. It all worked okay but the issues I had were that the SG90’s were quite noisy and coarse sounding in action, a bit like a cement mixer you could hear across a room, kept ‘chattering/fidgeting no matter what I did, and there seemed to be a lot of wires needed going back and forth to the boards, which had to be placed near the servos to keep signal interference down.

 

My next step was to replace the 9gm SG90’s with some JP 7.5gm digital servos. These have small 6x12 coreless motors and a very high gear reduction, 615-1 against the 320-1 of the SG90’s, ( which have 10x12 can’s). In operation these sounded nice and quite, sweet and refined you could describe it as. 
 

Now, In operation these were all fed - the servo4 boards - from a separate power supply, an old SMS 2.5amp/12v brick from a redundant external DVD/RW drive - remember them? - because of the power drain multiple servos create at startup. This supply could just about serve the dozen or so servos I used without them ‘thrashing’ about at full travel, another issue I’d had to overcome so tie-bars & signals connected to them didn’t keep on getting wrecked when this happened.
 

All was well with the JP servos until my Gaugemaster/MRC DCC system was switched on when they would just go mad, constantly moving and twitching. The system didn’t even have to be plugged into the layout. Just energised within the same room was enough. 
 

At this stage I called it a day, and moved over to ‘hacked’ servos, using them with the electronics ripped out, so like high ratio motor/gearbox drives using ‘simple’ plain DC power. 
 

There are several ways they can be used like this, but it depends on what you want. To be used as fairly slow speed stall motors they really need to be fed no more than 1.5v to 3v max. The lower the better because the mechanical advantage they produce is such that given enough voltage they will not stop and can wreck/rip the gear train. The setup the link above is to is where they are fed 5-6v but the switches they are connected to stop excess movement as well as cutting/reversing power and switching the frog polarity. ( they need the full voltage to be able to move the switches). 
 

The 7.5gm JP’s will work on just 1.2v, the SG90’s on 1.5v, and powering them using single AA’s is useful, ( re- chargeable AA/AAA are of course just 1.25v v 1.5v of ordinary AA/AAA). Here momentary SPST’s (centre sprung) can be used to power them until the reach their travel end. Frog polarity needs to be by connected micro-switch. In this scenario they are allowed to traverse their full 87 degree normal travel and fine piano wire is used which can absorb the excess travel above what is needed. This wire retains it’s spring no matter how long it is held in tension. 
 

Sorry for the length of this post, but I hope the info is of use.

 

Izzy

Edited March 10, 2020 by Izzy
spelling, spelling!!!

[AndyID]

Thanks for the information Izzy.

 

My preference for hacked servos is similar to yours but I'd rather not turn this thread into a debate about the pros and cons of the many ways a servo can be used. I'm just interested to know if it's practical to use them as stall motors.

 

It looks like it could be practical but it might require a slightly fancy high current, low voltage PSU to power a lot of them.

 

Cheers!

Andy

[AndyID]

8 hours ago, Izzy said:

All was well with the JP servos until my Gaugemaster/MRC DCC system was switched on when they would just go mad, constantly moving and twitching. The system didn’t even have to be plugged into the layout. Just energised within the same room was enough. 

 

Anyone who is having a lot of trouble with twitchy servos might want to try this:

 

 

 

[Junctionmad]

8 hours ago, AndyID said:

 

Anyone who is having a lot of trouble with twitchy servos might want to try this:

 

 

I implemented a version of this idea and can attest to its usefulness 

 

Edited March 11, 2020 by Junctionmad

[AndyID]

I've tried supplying the servo's motor from various fixed voltages through resistors. That appears to be NBG. When the motor stalls its EMF drops to zero and because almost all the applied power is being dissipated in the resistor the motor does not restart when the load is removed.

 

I'll try applying pulse-power to see if that might work.

 

 

[Pandora]

On 10/03/2020 at 05:49, AndyID said:

Not sure this is the best place for this. It applies to the remote operation of points (turnouts) regardless of whether the layout is DC, DCC, clockwork, steam or battery but as there doesn't seem to be another forum for remote point control I'll put it here.

 

There are several point motors on the market that use "stall motors". Basically they use a DC motor to move the tiebar and when the tiebar can move no further the motor stalls. When it stalls it is still exerting a force to keep the tiebar in position. That's  bit different from many other types of point motor that only supply intermittent torque and either rely on friction or some sort of over-center spring to keep the point blades in position.

 

Generally it's not a good idea stall a DC motor. It's likely to overheat, but if the power supplied to the motor is limited to an amount that the motor can easily dissipate without overheating there isn't a problem.

 

SG-90 servos are very inexpensive. It's not at all difficult to access their motors directly and it's quite simple to modify them for continuous rotation. Why not use them as stall motors to control points? Based on my (limited) measurements it looks like a stalled SG-90 motor with 0.7 volts across its terminals (and passing a current of 0.17 Amps) will restart when the stall force is removed. In the stalled condition it is only having to dissipate 0.12 watts.

 

 

To lock,or   "parking brake" a DC motor and hold the armature in position just isolate the power to the motor then   put a shorting wire across the brushes, it will not move unless you apply a considerable  torque to the armature shaft

[Nile]

I find that unpowered servos need quite some force to move them, probably more than any point blade will exert. All of mine operate Peco points with the springs still fitted, so they are not relying on the servo holding the blades in place.

[Izzy]

4 hours ago, AndyID said:

I've tried supplying the servo's motor from various fixed voltages through resistors. That appears to be NBG. When the motor stalls its EMF drops to zero and because almost all the applied power is being dissipated in the resistor the motor does not restart when the load is removed.

 

I'll try applying pulse-power to see if that might work.

 

 

 

Yes, you will see in my original thread a post with the current draw at various voltages both free running and at stall. The stall rates are quite high. IIRC I also found that the lowest voltages I gave, 1.2v for the 7.5gm and 1.5v for the gm/SG90 were the lowest they would run on plain DC reliably. It was why I decided to use only momentary power via the sprung SPST's. I would presume some level of PWM/pulsed DC waveform might get movement at lower voltages, but whether this would be reliable enough to produce usable powered movement is the question I think. And of course what the resultant draw at stall would be. It will be interesting to see what you can come up with.

 

Izzy

 

 

[Andy Reichert]

On 09/03/2020 at 22:49, AndyID said:

Not sure this is the best place for this. It applies to the remote operation of points (turnouts) regardless of whether the layout is DC, DCC, clockwork, steam or battery but as there doesn't seem to be another forum for remote point control I'll put it here.

 

There are several point motors on the market that use "stall motors". Basically they use a DC motor to move the tiebar and when the tiebar can move no further the motor stalls. When it stalls it is still exerting a force to keep the tiebar in position. That's  bit different from many other types of point motor that only supply intermittent torque and either rely on friction or some sort of over-center spring to keep the point blades in position.

 

Generally it's not a good idea stall a DC motor. It's likely to overheat, but if the power supplied to the motor is limited to an amount that the motor can easily dissipate without overheating there isn't a problem.

 

SG-90 servos are very inexpensive. It's not at all difficult to access their motors directly and it's quite simple to modify them for continuous rotation. Why not use them as stall motors to control points? Based on my (limited) measurements it looks like a stalled SG-90 motor with 0.7 volts across its terminals (and passing a current of 0.17 Amps) will restart when the stall force is removed. In the stalled condition it is only having to dissipate 0.12 watts.

 

 

 

 

That's pretty much how the "MOLE" works. Very inexpensive and the kit contains all the extra mechanical parts, including the crossing power switch . Several thousand in stalled with zero problems.

 

Andy

 

C

[AndyID]

I was probably being a bit unfair to my servo's motor. It was setup for continuous rotation and I was stopping it to determine the current required to get it to restart.

 

It is now connected to a sturdy 5 volt supply (ex computer) through a resistor and a reversing switch. The resistor value is 35 ohms. The motor reverses every time I flip the switch and it does not produce an excessive amount of torque. The stalled motor current is 0.126 amps and the stalled motor voltage is 0.534 volts. That means the motor is only dissipating 67 milliwatts while it is stalled. The resistor has to dissipate a bit more but not an excessive amount, 562 milliwatts.

 

Of course these numbers are based on a sample of one servo but to me it looks like there's plenty of room for adjustment.

 

Edited March 13, 2020 by AndyID
typhoo

[AndyID]

It seems to work quite well with a stall current as low as 100mA (43 ohm resistor). I suppose I could arrange for the motor to drive a micro-switch to supply the point's frog but I'm not going to bother. It's much simpler to use a relay and a diode. Not expensive either. I can get two pole changeover relays for around $0.75.

[Andy Reichert]

I much prefer the stall technology approach over servos. All the duplicated electronics per mechanism, each of which needs individual accurate position set up is an major waste of computer and human processing time and expertise.

 

FWIW, MOLE stall current is only 30 mA from a standard 12 v supply.   An also inexpensive (but "smart") PCB that provides multi-location push button operation is available, as is a "touch" switch upgrade for it. All of which makes for some very powerful control panel options, without doing a major micro programming exercise.

 

I can also vouch for the long term reliability. Many of the original Moles have been working faultlessly for around 10 years now. I don't know of one that has ever broken.

 

Andy

[JimFin]

2 hours ago, Andy Reichert said:

I much prefer the stall technology approach over servos. All the duplicated electronics per mechanism, each of which needs individual accurate position set up is an major waste of computer and human processing time and expertise.

Stall technology may be fine for points but not other applications  where servos can come into play, signals, crossing gates, shed doors and similar.

[Izzy]

Why? What’s the difference?

 

Izzy

[Crosland]

I suspect JimFIn is thinking of applications that require accurate positioning, but all the examples quoted can be implemented with a stall motor with mechanical end stops.

 

I can't think of any show-stopping differences.

 

Signal "bounce" would be a bit more complex with a stall motor. 

[cliff park]

OK, this is like, throwing an idea out there, may not even be new, may be nonsense. To save on the control electronics, how about if all the power supplies were common to several servos, but the control wire went through a one pole wafer switch. So you turn it to the servo you want to operate then twirl the knob or operate the switch to control the servo. Would obviously only work for basic end to end servos, no fancy signal bounce etc, but for points ? I have checked with several different servos, and they are really hard to move from the horn end, so I can't see them moving when they shouldn't. Failing the single pole idea, what about a three pole switch and switch the power leads as well ?

[Andy Reichert]

2 hours ago, JimFin said:

Stall technology may be fine for points but not other applications  where servos can come into play, signals, crossing gates, shed doors and similar.

 

Given the very specific topic, I was assuming the comparison to be for the point motor application only. In the case of a servo, the end points need to be set quite accurately, lest the the servo try to push the point blades past where the stock rails sit. In that case you will have a servo passing a high current continuously, and/or stock rails coming loose from their fixings.

 

Andy

[AndyID]

29 minutes ago, cliff park said:

OK, this is like, throwing an idea out there, may not even be new, may be nonsense. To save on the control electronics, how about if all the power supplies were common to several servos, but the control wire went through a one pole wafer switch. So you turn it to the servo you want to operate then twirl the knob or operate the switch to control the servo. Would obviously only work for basic end to end servos, no fancy signal bounce etc, but for points ? I have checked with several different servos, and they are really hard to move from the horn end, so I can't see them moving when they shouldn't. Failing the single pole idea, what about a three pole switch and switch the power leads as well ?

 

If you mean actual servos (with all their all innards still intact) you can drive all of them from two pulse generators that run continuously via single pole switches. One pulse is long and the other pulse is short. The snag is all the servos will rotate by the same angle and you'll have to adjust the throw at the points by some mechanical means. If you extend the intervals between the pulses the servos can be made to rotate quite slowly. There is no requirement to drive them every 20 milliseconds. (I am referring to cheap servos like the SG90).

 

The pulse generators can by coded in a simple micro-controller or wired up on veroboard from a couple of 555 timers if you prefer.

[newbryford]

On 10/03/2020 at 05:49, AndyID said:

 

 

There are several point motors on the market that use "stall motors". Basically they use a DC motor to move the tiebar and when the tiebar can move no further the motor stalls. When it stalls it is still exerting a force to keep the tiebar in position. That's  bit different from many other types of point motor that only supply intermittent torque and either rely on friction or some sort of over-center spring to keep the point blades in position.

 

Generally it's not a good idea stall a DC motor. It's likely to overheat, but if the power supplied to the motor is limited to an amount that the motor can easily dissipate without overheating there isn't a problem.

 

 

 

 

 

The Cobalt Analog IP and Digital IP motors switch down the current after a few seconds after stall - to about 5mA, (enough to power the electronics)  whereas the Cobalt Omega and Tortoise continually draw current at stall - typically 25mA.

IME, switching off power completely to any of the above does not cause any issue of the tiebar relaxing and allowing the blade to move away from the stock rail with the potential for derailment.

It is quite difficult to backdrive a Tortoise and as good as impossible to do it to a Cobalt without damaging the gears.