Megapoint Controllers

[Ian Fisher]

Did you get it all sorted out?

 

Hi Andy,

 

Its still being worked through....

 

Thanks for asking!

 

Ian

[vitalspark]

Interesting reading as this thread was started by one of our group ScRSG almost a year ago when we were considering the use of Megapoint controllers for the new 'Son of Alloa' project. 

 

Layout now well under way and panel almost there ..couple of pics showing the panel so far and the installation of the six boards.

 

Dave. 

 

 

 

 

 

 

 

[Vecchio]

Wiring very neat. As I can see you are not using the multipanel. And you have all multipoint controllers central, at the mimic board. Does the feedback for your mimic board come from the points itself (switches on servos)?

I like the space behind your mimic board. I crammed everything below an A4 board and now I am suffering from lack of space....

keep us updated!

Vecchio

[ScRSG]

Wiring very neat. As I can see you are not using the multipanel. And you have all multipoint controllers central, at the mimic board. Does the feedback for your mimic board come from the points itself (switches on servos)?

I like the space behind your mimic board. I crammed everything below an A4 board and now I am suffering from lack of space....

keep us updated!

Vecchio

Thanks for you comments, yes, the Megapoints Boards are placed inside the control panel. After a discussion with Dave Fenton ( a very helpful chap) we decided to do it this way for ease of adjustment and Dave reckons it should not be a problem for the distance from the panel to the servos, in our case up to 25ft. There is also the fact that instead of three wires going to each and every servo, a 5 Volt "bus" can be carried around the layout to supply each servo cutting down the wiring from the panel. Each servo then picks up the 5 volts from the bus and only then requires the signal feed, a single wire, from the board, cutting down on wiring.

 

The mimic panel, as you refer to it, is fed from the switches on the panel, DPDT switches are used, one side to send the servo signal and the other side to light the LED's on the panel.

 

As we are building a layout of a prototype location, this allowed us to build the panel in advance of the layout, knowing the exact track layout and location of signals etc. We hope this will make the wiring to the layout more straightforward, we will see!

Edited August 12, 2016 by ScRSG

[Colin Greenaway]

I am using a Megapoints controller with a DCC interface and the electronics, set up and use have been reliable so far BUT the constant ticking of the servos is very annoying. At a show it wouldn't be noticeable but in the relative quiet of the Railway Room it certainly is. I asked Dave and he said buzzing could be fixed but the ticking was common to all servos.

I gave up grovelling under the baseboard trying to set up micro-switches on blocks of wood to work reliably and not cause the servo to buzz. Invested in a hex frog juicer and couldn't be happier with the result; much easier to install and no adjustment needed; if a tad more expensive! :-)

Any one got any suggestions or ideas about removing servo tick? I guess they are slightly adjusting their position all the time, well every few 10s of seconds or so.

Colin in Lindum

[Vecchio]

Servo ticking is normal and for my opinion cannot be avoided. You have it also in RC models. Some servos are worse - some are better. The recommended micro servos (15g, hobbyking analogue servo, white) are rather quiet.

 

And of course - the more spring force you have (either from the point itself or from micro switches) the more chance you have for servo ticking. In fact the servos are adjusting position constantly.

 

By the way - can you tell me what you mean with a hex frog juicer? I am not familiar with this expression.

 

Vecchio

[Colin Greenaway]

Servo ticking is normal and for my opinion cannot be avoided. You have it also in RC models. Some servos are worse - some are better. The recommended micro servos (15g, hobbyking analogue servo, white) are rather quiet.

 

And of course - the more spring force you have (either from the point itself or from micro switches) the more chance you have for servo ticking. In fact the servos are adjusting position constantly.

 

By the way - can you tell me what you mean with a hex frog juicer? I am not familiar with this expression.

 

Vecchio

 

Those are the servos I am using!

 

A hex frog juicer is a piece of electronic kit that detects a short circuit in the frog and switch blade of a turnout as it changes position and very very quickly reverses the polarity of the frog and blade. I am talking DCC here, they do not operate on DC. The changeover takes place within a few microseconds and is undetectable by the DCC system so no stuttering of the locos or resets occur.

See http://tamvalleydepot.com/products/dccfrogjuicers.html

An American product available from Digitrains amongst others.

Colin

[newbryford]

 

 

By the way - can you tell me what you mean with a hex frog juicer? I am not familiar with this expression.

 

Vecchio

 

A frog juicer is an electronic switch that does away with a micro switch. They are for DCC powered layouts only. It relies on the pickups bridging across to the frog and creating a momentary short circuit before it switches over the polarity.

 

There are those on here who are vociferously against this type of device as it relies on the short circuit to work. (I have no opinion either way, I haven't tried one)

 

Hex - as in a six-channel device. Single and dual channel units are available.

 

A quick link to the Tam Valley product

 

BTW, I used above board micro switches for my Megapoints fitted fiddle yard. I didn't need to hide them under the boards.

 

(I was originally going to use Peco point motors, hence the larger than required hole in the baseboard.)

 

 

 

Cheers,

Mick

[bgman]

 

By the way - can you tell me what you mean with a hex frog juicer? I am not familiar with this expression.

 

Vecchio

They are used to power the frog and provide the correct polarity when using DCC to switch the turnout road ensuring there isn't a problem with short circuits possibly caused when wheels touch the opposing rails.

 

http://www.tamvalleydepot.com/images/Hex_Frog_Juicer_Manual_v1_5_2p.pdf

 

 

I've used them very successfully with Peco Code 75 electro frog points which have been modified in the usual way.

 

Hope this helps.

ATB

[Vecchio]

Thanks to all of you  - got it now. In principle similar to the circuit changing the polarity in loops or track triangles. Never heard about it - but I had a few years of a model railway break. Probably a bit more expensive than the cheap Chinese micro switches I use?

 

20160625_203950[1].

 

 

By the way - analogue locomotives survive in my workshop only for a few weeks - after that they have a decoder... 

[Suzie]

Megapoints controller and DCC decoder + 2x hex frog juicers is £220. That makes motorising points quite expensive at £18.33 per point. I use the Signalist SC2 which works out at 11.75 per point (just a fraction more than the frog juicer alone!), has the frog switching built in, and is 4-way rather than 12-way which means the controller (and frog switching) can be more local to the points keeping the wiring shorter.

 

When you start adding things on to the Megapoints or don't need all 12-ways or use DCC it can cease to be the cost effective solution it at first appears.

[AndyID]

Any one got any suggestions or ideas about removing servo tick? I guess they are slightly adjusting their position all the time, well every few 10s of seconds or so.

Colin in Lindum

 

If they tick sometimes seconds apart, it's more likely a "twitch" caused by electrical interference. The interference produces an intermittent false position demand pulse that is cancelled out on the next cycle.

 

A servo that is trying to correct an error in its position but can't quite get there because of mechanical constraints will produce something more like a buzzing sound (typical drive circuits transmit a position demand pulse 50 times per second to produce a 50 Hertz buzz.).

 

To find out whether it's an electrical noise (interference) or mechanical constraint problem, disconnect the servo from its load (the point tie-bar) so there is is nothing to prevent the servo eliminating the position error. If it still "ticks" every few seconds, it's an interference problem.

[Suzie]

Modern servos with digital electronics are prone to ticking. They only move to a fixed number of defined positions. The servos sample the pulses and compare the pulse length with previous pulses, and ignore anything that is wildly different. If the applied pulse appears to be consistently different to the current position they will 'tick' as they move just one step to the adjacent position.

 

The work around for this is to use the fine adjust feature on your servo controller to ensure any pulses that are sent to the servo are in the middle of one of the defined positions. If you are right on the border between two defined positions then the servo will constantly tick between the two adjacent positions.

 

Applying some sound deadening to the servo might be another solution if you cannot adjust the servo pulse finely enough, or temperature changes are causing the ticking.

 

The ticking is the price you pay for having servos that don't move wildly when they get a rogue pulse. In the olden days of analogue servos they just readjusted fifty times a second whenever they got a pulse regardless of whether it was a big or small change and hysteresis prevented small movements and therefore the more accurate positioning you get with modern digital servos.

[t-b-g]

I haven't used Servos for operating points (yet) but I have installed plenty for signals. I did purchase a Megapoints board, tried it and thought that the signal arm twitching may represent some kind of problem and the good people at Megapoint were very happy to change it for another board, which I haven't tried out yet.

 

I have used lots of MERG Servo4 boards, which are very economical and ever so easy to make up from the kits. I think that they are around £6.50 to run 4 servos. Well worth the MERG membership subs just to get them! I do find that some of them seem to lose their memory slightly as you can go to a layout and find some signals not at a true horizontal position. They need to be re-set and stay OK for a while and then go slightly off again. If I need to do more, I will build in some more over-travel and physical stops on the signal to make the "on" and "off" positions right even if the servo changes slightly beyond the end travel.

 

The boards available from GF Controls are not cheap but they are superb. A couple of people I have built signals for have been willing to pay the extra for a dead easy to use "plug and play" board and I have probably fitted around a dozen of the dual servo board and have never had any problems. I have not seen any twitch or unwanted movement at all. They may not be ideal for points as they have a different speed of movement each way and are designed to make a signal go up slowly and return with a bit of help from gravity. The "bounce" feature is very nice too.

 

edited for spelling!

Edited October 8, 2016 by t-b-g

[Vecchio]

Just came back from Gaydon - where I met Dave Fenton (Mr. Megapoints...).

 

There is a new mega points frog/relay driver card - which can be used for frog polarity. If it is set up with the same address as the controller board each relay driver corresponds to the servo position of the controller and can therefore be used for frog polarity. With this we can get rid of mechanical switches which may not be helpful  for ticking noises as they add a constant spring force to the servo.

 

These boards can also be used stand alone for whatever switching tasks in case a multipanel or a DCC module is used on your layout. I really like the idea as before you had to use channels of your megapoints controller in case you need switching functions. For pure switching functions like light signals or just simply the light on the mainstreet of your village you do not need to be able to set operation speed or endpoints as you do for servos.

 

So I can free now several channels on my Donnersbachkogel modules (6 light signals on the station modules) and use this new board instead. Also the new board has a stronger on board voltage regulator - so the former external voltage regulator is no longer necessary.

 

So if somebody has the need of 12 switch functions which are not servo related this new board is now the cheaper option. The megapoints controller costs now 60£ - the new relay board costs 36.

 

And of course I purchased one as I want to test it...

Vecchio

[bgman]

The boards available from GF Controls are not cheap but they are superb. A couple of people I have built signals for have been willing to pay the extra for a dead easy to use "plug and play" board and I have probably fitted around a dozen of the dual servo board and have never had any problems. I have not seen any twitch or unwanted movement at all. They may not be ideal for points as they have a different speed of movement each way and are designed to make a signal go up slowly and return with a bit of help from gravity. The "bounce" feature is very nice too.

 

edited for spelling!

Having looked at their website and video the units seem very good, only problem there is no price list !

It would be good to have some idea of the unit price.

[t-b-g]

Having looked at their website and video the units seem very good, only problem there is no price list !

It would be good to have some idea of the unit price.

 

In the text on the website it mentions £25 each plus p & p.

[AndyID]

Modern servos with digital electronics are prone to ticking. They only move to a fixed number of defined positions. The servos sample the pulses and compare the pulse length with previous pulses, and ignore anything that is wildly different. If the applied pulse appears to be consistently different to the current position they will 'tick' as they move just one step to the adjacent position.

 

The work around for this is to use the fine adjust feature on your servo controller to ensure any pulses that are sent to the servo are in the middle of one of the defined positions. If you are right on the border between two defined positions then the servo will constantly tick between the two adjacent positions.

 

Applying some sound deadening to the servo might be another solution if you cannot adjust the servo pulse finely enough, or temperature changes are causing the ticking.

 

The ticking is the price you pay for having servos that don't move wildly when they get a rogue pulse. In the olden days of analogue servos they just readjusted fifty times a second whenever they got a pulse regardless of whether it was a big or small change and hysteresis prevented small movements and therefore the more accurate positioning you get with modern digital servos.

 

I think Colin said he is using analog servos.

 

AFAIK, the digital servos still use a potentiometer to determine position. If so, they are still analog to a great extent. Are there some high-end versions with encoder wheels rather than pots?

[Vecchio]

A digital servo is not using a potentiometer to determine the position. They are more precise and respond faster - things we do not need when we want to switch a point or move a signal arm.  Digital servos are much worse if it comes to ticking. Stay with analog. They are also much cheaper.

Regards

Vecchio  

[AndyID]

A digital servo is not using a potentiometer to determine the position.

 

Not according to this guy.

 

"First off, there is no physical or main component difference between a digital servo or analog servo. The servo case, motor, gears, and even the feed back potentiometer all have the same functions and operations in both types."

[Vecchio]

Cannot open your link - but I have to correct myself. It has a potentiometer. And there is some information about the differences.

 

But this doesn't change anything of what I said at the end. For a model railway application a digital servo is not needed.

[AndyID]

Cannot open your link - but I have to correct myself. It has a potentiometer. And there is some information about the differences.

 

But this doesn't change anything of what I said at the end. For a model railway application a digital servo is not needed.

 

I agree. Analog is fine for model railways.

 

It's possible analog servos will disappear from the market if the digital electronics cost drops to the point where there is no real cost difference between the analog and digital versions.

[ScRSG]

I thought the main benefit of the digital servos was the fact that there is no twitch or kick on powering up, this to protect delicate uses like semaphore signals.

[AndyID]

I thought the main benefit of the digital servos was the fact that there is no twitch or kick on powering up, this to protect delicate uses like semaphore signals.

 

That's an interesting idea. I've only mucked around with analog servos, but as far as I know, analog and digital servos both basically "do what they are told" except that the digital versions are more responsive which is a good thing for RC applications.

 

I think the power-up excursions you mention might be more to do with the controller than the servos themselves. If the controller has some means to record the position of the servos when it was powered down, it should be able to restore the previous states on power-up to prevent any twitch/kicking.

[Suzie]

It is very hard to define the start-up conditions of a microprocessor based servo driver. Unless you arrange for the power to be applied to servo after the microprocessor is running there will be an undefined state on the servo control wire until the processor has completed boot and the firmware has started generating the pulses. These initial pulses may be appear very long or very short to the servo and cause it to twitch at power up. The only way I know to mitigate this is to make the controller with a nano-power processor that can be kept in sleep mode when power is removed so that it does not have to perform a reset sequence and can therefore generate pulses straight away - but this is an added complexity that can have knock on side effects.

 

Just using a digital servo gets around this 'twitch at startup' problem because the signal processing in the servo ensures that it ignores any initial out-of-range or wildly different sized pulses until a stream of similar length pulses start to be generated when the servo will start to respond only after the controller has settled. Any spikes induced in to the control wire will be ignored too. Digital servos have a lot of advantages.

 

Yes digital servos still have the potentiometer, but the output of this is digitised to make a series of steps, and the servo only moves when it perceives that the incoming pulse has changed sufficiently to move to the next step - and it does this with a distinct tick. I don't see the tick as being a big problem - careful adjustment does appear to minimise it.

 

Digital servos do appear to be just as cheap as their analogue equivalents now - it is hard to tell the difference in price between the standard analogue TowerPro SG90 and the digital with carbon fibre gears but otherwise equivalent TowerPro SG92R.