Load sensing point motors

[AndyID]

Not sure where to put this. It applies just as much to DC as it Does to DCC, but here goes.

 

I'm pretty sure most of us have cars with electric windows. Why don't model railway point motors work the same way?

 

How does an electric car window know when the window is closed? Because it senses the motor current and shuts off the power when it exceeds a predetermined value that indicates the motor has stalled, or is close to stalling.

[cliff park]

In essence it is as you describe. And if it goes wrong it can cost several hundred pounds to put right, ask me how I know. They would not be feasable on solenoids anyway, and it is probably possible to adjust voltages on any other motors, including servos, to achieve a similar effect.

[Enterprisingwestern]

Model railway pointwork needs different amount of current to operate depending on the type of construction, manufacturer, how and where it is laid, etc, so current sensing what is already a very small load across a wide number of variables probably isn't practicable.

Servos do it by default, so that is probably as close as we can get.

 

Mike.

[Suzie]

Tortoise motors do this. There is just no need to cut the power at the stall point because the power involved (about ¹/₁₀th of a Watt) is so low.

[Crosland]

Not sure where to put this. It applies just as much to DC as it Does to DCC, but here goes.

 

I'm pretty sure most of us have cars with electric windows. Why don't model railway point motors work the same way?

 

How does an electric car window know when the window is closed? Because it senses the motor current and shuts off the power when it exceeds a predetermined value that indicates the motor has stalled, or is close to stalling.

 

It's not difficult but, given some of the solutions we see to point control, I suspect a lot of modellers would not pay the extra

 

Model railway pointwork needs different amount of current to operate depending on the type of construction, manufacturer, how and where it is laid, etc, so current sensing what is already a very small load across a wide number of variables probably isn't practicable.

Servos do it by default, so that is probably as close as we can get.

 

Servos will continue to draw (a lot of) current until they reach the commanded position. They turn off as a result of the in-built position feedback, not current sensing.

[Ian Morgan]

A Merg member is currently trying to develop a load sensing servo driver, so that it would not need any setting up (except speed, maybe). It would just move until the servo hit some sort of end stop, and would halt there.

[AndyID]

A Merg member is currently trying to develop a load sensing servo driver, so that it would not need any setting up (except speed, maybe). It would just move until the servo hit some sort of end stop, and would halt there.

 

I was thinking of a linear telescopic screw driven by a small motor or using the motor and gears in a cheap servo without any of the servo's electronics. The controller does not have to be expensive. As long as it's not necessary to throw more than one set of points at a time a single controller could drive a lot of points. The most expensive element might be directing the current to the target motor, but that should not be too expensive either.

[Suzie]

Driving a servo with it's built in electronics is easier (and cheaper) than driving a motor because you only need signal level power rather than motor level, and it is just as easy to detect the current drawn by a servo as it is a simple motor. If you just move one point at a time you can use a single global current detector for all the points.

 

It might just be a case that you calibrate the end points at power up and then move smoothly between them after that assuming that the points don't move.

[Junctionmad]

The cobalt analog IP point motor from DCCconcepts works like you describe , at the end of the throw , the power is cut to the drive motor ( unpon sensing stall )

 

Dave

[Junctionmad]

I was thinking of a linear telescopic screw driven by a small motor or using the motor and gears in a cheap servo without any of the servo's electronics. The controller does not have to be expensive. As long as it's not necessary to throw more than one set of points at a time a single controller could drive a lot of points. The most expensive element might be directing the current to the target motor, but that should not be too expensive either.

Given SG90 servos can be got for about a quid , I hardly see the point

[AndyID]

Driving a servo with it's built in electronics is easier (and cheaper) than driving a motor because you only need signal level power rather than motor level, and it is just as easy to detect the current drawn by a servo as it is a simple motor. If you just move one point at a time you can use a single global current detector for all the points.

 

It might just be a case that you calibrate the end points at power up and then move smoothly between them after that assuming that the points don't move.

 

Hi Suzie,

 

That's about it. Each point can have a designated "force" level based on the common return current, and assuming there is plenty of friction in the drive mechanisms, they should hold position. It's also quite practical for the controller to do a periodic "check up" to be sure that they are holding position.

 

My issue (as I'm sure you are already aware) is that the interface to typical servos is great on RC models, but it's absolute garbage in a model railway environment, but who am I to say? 

 

Andy

[Suzie]

Given SG90 servos can be got for about a quid , I hardly see the point

 

Or the even better SG92R that should give better indication of stall with more consistent current draw.

[newbryford]

 

Current sensing is great but:

All it will do is detect a stall, it doesn't prove the blades have moved fully as there could be something stuck between blade and stock rail causing a stall.

(And that applies to all stall type point motors).

 

Cheers,

Mick

[AndyID]

 

Current sensing is great but:

All it will do is detect a stall, it doesn't prove the blades have moved fully as there could be something stuck between blade and stock rail causing a stall.

(And that applies to all stall type point motors).

 

Cheers,

Mick

 

 

Hi Mick,

 

Doesn't that apply to all point motors? (Except the real ones obviously.)

 

Cheers,

 

Andy

[newbryford]

Hi Mick,

 

Doesn't that apply to all point motors? (Except the real ones obviously.)

 

Cheers,

 

Andy

 

I think that's sort of what I said.....

(Not knowing how you can detect stall current on a solenoid motor)

[Suzie]

Hi Suzie,

 

That's about it. Each point can have a designated "force" level based on the common return current, and assuming there is plenty of friction in the drive mechanisms, they should hold position. It's also quite practical for the controller to do a periodic "check up" to be sure that they are holding position.

 

My issue (as I'm sure you are already aware) is that the interface to typical servos is great on RC models, but it's absolute garbage in a model railway environment, but who am I to say? 

 

Andy

 

The high impedance input of the servos is less than ideal, but when driven from a low impedance source with appropriate and consistent pulse timings is perfectly fine and immune to the hostile environment under the baseboard.

 

The rules are:-

 

1. Servo signal line should not float when the controller is powered up but should be driven immediately to the correct polarity (prevents a wild swing at power on).

 

2. Servo signal line should be driven hard both ways and not left floating or just be pulled up or down with a resistor (prevents pickup of interference from adjacent wiring).

 

3. Servo pulses should be consistent within 500nS from one pulse to the next when stationary (prevents chatter when servo is supposed to be stationary).

 

4. Analogue servos should not be used (analogue servos will instantly respond to any interference but digital servos will filter it out, and digital servos will always move at full power until they reach the desired position overcoming any friction while analogue servos may stall on small movements and not actually reach the final position).

 

5. Keep servo wires as short as possible and well away from any potential sources of interference.

 

6. The power supply should be adequate allowing about an Amp (at 5V) for each servo that is moving and provide plenty of power at power up.

 

Ideally servo controllers should be within reach of the servo without requiring an extension lead, but I have found that up to 8' of extension to be fine if all of the above is taken in to account. Unfortunately not all commercial products follow the above guidelines and some are difficult to place/power because of trying to drive a lot of servos from a single controller rather than smaller more localised controllers. There is a lot of promotion of analogue servos like the SG90 as being suitable, when the often cheaper digital SG92R and similar are much better in the model railway environment. Railway modelling needs a high power servo, not a crap one that you can get away with in RC! Price is not always a good indicator of suitability.

Edited November 22, 2018 by Suzie

[AndyID]

I think that's sort of what I said.....

(Not knowing how you can detect stall current on a solenoid motor)

 

Hi Mick,

 

I'm missing your point. What's wrong with sensing the current for each turnout and turning off the supply when it peaks at some value? AFAIK that's exactly how window winders work.

 

Cheers,

 

Andy

[AndyID]

The high impedance input of the servos is less than ideal,

 

 

Hi Suzie,

 

Let's not fool ourselves. The interface is entirely inappropriate for use in a noisy environment like a model railway.

 

Cheers,

 

Andy

[newbryford]

Hi Mick,

 

I'm missing your point. What's wrong with sensing the current for each turnout and turning off the supply when it peaks at some value? AFAIK that's exactly how window winders work.

 

Cheers,

 

Andy

 

That's what I thought I said in the first place.

 

When you said all point motors, I thought you were implying types other than slow acting....

 

Going O/T

Part of the reason for window winders sensing current is so that it doesn't break fingers that shouldn't be there. It's not just for detecting end of travel. Some window winders have the ability to detect a stall current and also that the stall is not at the end of travel, thus working out that something is there that shouldn't be and the window automatically goes down.

[AndyID]

That's what I thought I said in the first place.

 

When you said all point motors, I thought you were implying types other than slow acting....

 

Going O/T

Part of the reason for window winders sensing current is so that it doesn't break fingers that shouldn't be there. It's not just for detecting end of travel. Some window winders have the ability to detect a stall current and also that the stall is not at the end of travel, thus working out that something is there that shouldn't be and the window automatically goes down.

 

Hi Mick,

 

Sorry, but you've completely lost me.

 

Andy

[AndyID]

The cobalt analog IP point motor from DCCconcepts works like you describe , at the end of the throw , the power is cut to the drive motor ( unpon sensing stall )

 

Dave

 

Hi Dave,

 

One inexpensive controller could drive a large number of very basic point mechanisms and save model railway enthusiasts quite a lot of money. Not only that, they would be completely immune from interference. What would be wrong with that?

 

Cheers,

 

Andy

Andy

[RAF96]

Piezo leg linear or rotary motors. Build enough of them and the price drops to pennies.

[newbryford]

Hi Mick,

 

Sorry, but you've completely lost me.

 

Andy

 

 I did say it was going

[Junctionmad]

I’ve used servos and setup correctly there is no interference or issues . There are miniature linear servos ( hobbyking ) where you could disable the electronics and just use stall currents

 

Again why bother , servos can operate well on a model railway if used correctly

[AndyID]

I’ve used servos and setup correctly there is no interference or issues . There are miniature linear servos ( hobbyking ) where you could disable the electronics and just use stall currents

 

Again why bother , servos can operate well on a model railway if used correctly

 

It looks like Hobbyking's micro linear servo is no longer with us.

 

According to what I've read and seen servos frequently do not operate well on a model railway, but if they work well for you that's OK.

 

Apart from susceptibility to interference there's also the slightly complicated business of setting up the correct throw distance, and for several reasons that might require adjustment over time. Pressure sensing would eliminate that.