Servo Motors & Servo Controllers

[Grovenor]

1. The relatively un-sophisticated movement of the signal.

Which can be changed, as was covered in post #3 above. The standard firmware was intended for points.

Regards

Keith

Edited September 28, 2011 by Grovenor

[Steve Hewitt]

Which can be changed, as was covered in post #3 above. The standard firmware was intended for points.

Regards

Keith

 

Hi Keith,

 

Thanks for reminding me. In post #3, you said:

 

"MERG now has firmware available for the serv4 designed for signals. This provides for up to 3 individually adjustable bounce positions at each end of the travel. Each of the 4 servos can be adjusted separately. The original version was designed for points where bounce is not usually wanted."

 

Am I right in thinking that the new firmware cannot be used with the MERG Setter Box?

If so, is a PC connection needed for signal adjustment?

 

regards,

Steve.

[meil]

 

 

It is limited in two aspects:

1. The relatively un-sophisticated movement of the signal.

2. The requirement to have a Setting Box, to make adjustments. The cost and investment in time to build one of these must be considered when comparing costs.

 

Steve.

 

No to both.

Software is available for the Servo4 with all the bounce capability anyone would want.

A PC can be used to set-up the Servo4 making it much easier and controllable.

Edited September 28, 2011 by meil

[nick_bastable]

I have to agree the latest MERG bounce software / pic is a breeze to set up even managed to set up a demo 2mm version

 

Speaking as the most ham fisted 2mm modeller ever

[Grovenor]

Am I right in thinking that the new firmware cannot be used with the MERG Setter Box?

Yes and no You can use it with the setting box, but in that case you can't set the bounce, only the end positions and speed as with the basic firmware.

If so, is a PC connection needed for signal adjustment?

Yes.

 

Regards

Keith

[beeman]

I find many articles on using servos, and good for the price, but by the time one has purchased the control unit it starts to become expensive. Does not anyone use a simple geared motor with a crank on the o/p shaft, the length of the crank can be adjusted to give the correct 'length' of movement needed for the signal arm, a push button or switch to rotate the crank thro 180 , a further momentary push to lower. No expensive control units no limit switches.. A company selling electrics for robotics etc is selling small high ratio motors suitable. ok no good for the 'rivet counters' wanting bounce, but the bounce for me seeing vids of the results looks so artificial , a waste of time. keeping it simple is always more reliable. A more realistic bounce if you are insistant would be to operate by a suitably profiled cam. Beeman.

Edited October 9, 2011 by beeman

[lincolnshiremodeller]

I recently bought 4x Servos and 2x Servo Motor Controllers from Heathcote to power a set of 4x level crossing gates. I have wired the servos and controllers up as suggested, and have successfully programmed them all so that they open and close to the correct positions. I am powering them from a 16V AC supply.

Whilst this operation is as expected, I notice that when power is initially supplied to the servos, they all "kick" (i.e. they rotate randomly before returning to their start position - usually in the opposite direction to their programmed operation).

Can you please explain how this can be removed, would adding a capacitor across the supply remove this unwanted travel on power-up? I do notice as well that occasionally there is an unexpected random movement.

Have any members experienced this, and does anyone have a recommended method to overcome it?

Thanks

SJ

[Steve Hewitt]

snipped....

Whilst this operation is as expected, I notice that when power is initially supplied to the servos, they all "kick" (i.e. they rotate randomly before returning to their start position - usually in the opposite direction to their programmed operation).

Can you please explain how this can be removed, would adding a capacitor across the supply remove this unwanted travel on power-up? I do notice as well that occasionally there is an unexpected random movement.

Have any members experienced this, and does anyone have a recommended method to overcome it?

Thanks

SJ

 

 

Hi SJ,

 

I think the problem lies with the way the controller communicates with the servo.

It supplies two main items:

1. The 5v power needed to run the motor.

2. The control signal - a square wave form of defined length, between 1msec and 2msec.

The precise length between these limits defines the position to which the arm moves.

The rate at which the wavelength is changed controls the speed of movement.

 

I think the twitch is caused when the power is supplied to the servo before the control signal, causing the motor to try to move to some out of range location and at maximum speed.

I believe the software in the controller should should ensure on startup that the control signal is sent before the power supply is turned on.

 

Steve.

[lincolnshiremodeller]

snipped

I believe the software in the controller should should ensure on startup that the control signal is sent before the power supply is turned on.

 

Steve.

 

Thanks Steve

Judging by the last part of your post I'm assuming this is not something you have experienced before? I've contacted Heathcote re this issue but have yet to receive a reply.

 

SJ

[Steve Hewitt]

 

Thanks Steve

Judging by the last part of your post I'm assuming this is not something you have experienced before? I've contacted Heathcote re this issue but have yet to receive a reply.

 

SJ

 

Hi SJ,

 

Yes, I've seen lots of "twitching" servos. The early MERG controller which I use on my demo. stand to run couple of Ground Signals, always pushes them up out of the ground on power up.

The earlier Embedded Controls units were similar, but they did seem to twitch "within range", so not putting undue stress on the model.

The final production of E.C. units didn't seem to twitch at all.

 

The G F Controls units I've recently been involved with are designed to avoid the problem.

 

Steve.

[Gordon H]

I think the problem lies with the way the controller communicates with the servo.

It supplies two main items:

1. The 5v power needed to run the motor.

2. The control signal - a square wave form of defined length, between 1msec and 2msec.

The precise length between these limits defines the position to which the arm moves.

The rate at which the wavelength is changed controls the speed of movement.

 

I think the twitch is caused when the power is supplied to the servo before the control signal, causing the motor to try to move to some out of range location and at maximum speed.

I believe the software in the controller should should ensure on startup that the control signal is sent before the power supply is turned on.

The control signal is not a square wave as such, but it is a logic level 'digital' pulse of defined length (as noted) sent periodically, typically 50 times a second (20ms intervals). Also, 'wavelength' is not really the correct term to use because that too has a specific meaning in electronic 'circles', particularly those involved with RF. 'Pulse Length' is more appropriate here.

Sorry to be pedantic about this, but correct use of terminology is important to avoid confusion and ensure everyone is talking about the same thing.

The 'servo twitch at start-up issue' is quite widely known and reported, and there are several aspects to it.

The essence is that the moment power is applied to the servo it doesn't really know what to do until a pulse is received, so the internal servo circuitry plays its own part in what to do in that situation. It might tell the servo to go to a default position, probably one or other endstops - or perhaps do nothing (which would be preferred).

It could also be that the current servo position does not corresepond with the length of the first received pulse - the servo may have moved between sessions or not have been restored to a default position before power was removed last time.

The circuit creating the control pulses may take a finite time to start producing valid pulses - this is part of the basic design of a Servo controller.

The cause of start-up twitching seems to be combinations of these effects, and a common 'one size fits all' solution for all circumstances may not be possible. The usual first attempt is to add a pull-up or pull-down resistor between the pulsed line and one or other supply lines. This at least sets the pulsed line to a known state while the controller sorts itself out. This is equivalent to Steve's final comment of 'sending the control signal before turning the power supply on'.

[Steve Hewitt]

Hi Gordon,

 

Thanks for your explanation, which I appreciate.

As I said earlier in this topic, I have very little knowledge of Electronics, and I've proved it yet again.

 

 

Steve.

[lincolnshiremodeller]

Since my last post I've done some further adjustments. In a similar vein to previous posts I have found that isolating the ground connection of the servo via an on/off switch so that it can be powered up after the supply and control signals limits the amount of movement on power up. Interestingly applying supply/gnd without the control signal sends the servo wild with constant random movement at top speed. Presumeably this is because the servo has no reference point with which to position itself.

 

I think as well my particular application of the servos amplifies the movement on power-up. I use each servo to open/close a level crossing gate, and due to the amount of movement required I use 1:1 gearing with the servo running at its slowest speed. This means that on power-up random movement at a high speed setting is much more noticeble and appears "violent". In an ideal world I would simply increase the speed and use further gearing, but this is not possible with the limited rotation of a servo. Perhaps for a semaphore signal the servo is moving at higher speed setting, so any random movement has less of a visual impact (though still has the ability to cause damage if pushing the signal mechanism the wrong way).

 

Anyhow we live and learn.....

 

SJ

[Steve Hewitt]

Since my last post I've done some further adjustments. In a similar vein to previous posts I have found that isolating the ground connection of the servo via an on/off switch so that it can be powered up after the supply and control signals limits the amount of movement on power up. Interestingly applying supply/gnd without the control signal sends the servo wild with constant random movement at top speed. Presumeably this is because the servo has no reference point with which to position itself.

 

I think as well my particular application of the servos amplifies the movement on power-up. I use each servo to open/close a level crossing gate, and due to the amount of movement required I use 1:1 gearing with the servo running at its slowest speed. This means that on power-up random movement at a high speed setting is much more noticeble and appears "violent". In an ideal world I would simply increase the speed and use further gearing, but this is not possible with the limited rotation of a servo. Perhaps for a semaphore signal the servo is moving at higher speed setting, so any random movement has less of a visual impact (though still has the ability to cause damage if pushing the signal mechanism the wrong way).

 

Anyhow we live and learn.....

 

SJ

 

Hi SJ,

 

Your approach to the "twitching" problem seems to be giving benefits. Well done!

 

With reference to the "gearing", I offer the following video of a typical application of a servo to drive a 4mm scale ground signal.

Any gearing is limited to keeping the "horn" as short as possible.

 

http://www.youtube.com/watch?v=LVqOpOYikoo

 

As you will see, the movement required between end positions is quite small, but most controllers can cope with the accuracy required without any problems.

 

With such a small movement, any "twitch" can be very noticable.

 

Steve.

[Steve Hewitt]

If you want to get your hands on some servos and controllers, drop by my demo stand at the Warley, NEC show next weekend.

 

I'll have working examples of the Embedded Controls, MERG, Switch Pilot and G F Controls devices available to try out.

 

I'll also have several working signals on display and be delighted to show you how they've been created, and do my best to help you with any signalling problems you might have.

 

Stand number is E44.

 

Steve.

Edited November 12, 2011 by SteveAtBax

[Steve Hewitt]

Two years since any postings on this topic...........

 

Once again I'm preparing my stuff for the Warley Show.

Rob and I will be on Stand E17 this year.

 

This afternoon I collected on of the new GF Controls 8-Way Servo Controllers.

I'll have it on my stand, connected up to the Liverpool Lime Street gantry which I built a few years ago.

 

This new controller uses the same basic technology of the GF Controls twin servo unit described previously.

 

It does have some new features however:

1. The switching logic for the control signal (On / Off) can be inverted if this is more to your liking.

i.e. where the twin unit is "Off or Open Circuit for Clear, you can use a slide switch to change the 8-way unit to "On or Closed Circuit" Clear.

 

2. The setting controls are now on a separate hand-held controller connected by a plug in "Curly Cable".

Only one such control unit is required for however many 8-way units you might have.

 

3. The 8-way unit displays several LEDs to show its status at any time.

 

The 8-way controller is built on one circuit board measuring some 130mm x 100mm.

 

The setter box:

 

The contents of a "Starter Set":

which also includes an A5 sized manual/guide of 10 pages.

 

Features to note:

• The eight servo connectors are at the top of the image above.
• On the right in Red are the dip-switches which enable "safe" mode, On-Off invertion, and Bounce control for each servo separately.
• Along the bottom edge is the 12v DC power connection and eight switch connections.
• Note these all share the same common return as the power supply.
• In the lower right hand corner is the telephone type socket for the Setter Box connection.
• To the left of this socket are four screw terminals into which an extension socket for the setter box can be connected.
• e.g if the 8-way unit was under the baseboard, the setter box socket could be extended to the front of the layout, or nearer the control panel etc.

I'll get some experience of using this new unit at the Warley Show this weekend so I can answer any questions about ease of use etc. after that.

If you get a chance, come to my stand E17 and have a go for yourself.

 

Steve.

 

edit for spelling.

Edited November 19, 2013 by SteveAtBax

[ndk]

Rather late to this thread, but very interesting. How was the 8-way unit received? Any news on cost?

[Steve Hewitt]

Rather late to this thread, but very interesting. How was the 8-way unit received? Any news on cost?

Hi,

 

The "eight-way unit" was received in the post.

 

The unit works very well, and has a couple of extra features not seen on the standard "two-way unit".

1. Each servo has its own LED to indicate its "ON" or "OFF" state.

2. The logic of "Switch Open" for signal Clear can be inverted to be "Switch Closed" for signal Clear is desired.

 

The actual software for the servo motion is I believe identical to the "two-way unit".

The operation of each servo is sequential, so each one has to complete its movement before the next one can start.

This means it is less suitable for use on "slotted" signals which require two servos to move simultaneously under certain conditions (also route Indicators if you use a separate servo for the route display, as I do).

 

The setting of the "eight-way unit" is done through a plug-in "setter Box".

The connection is a quite long "curly cable" telephone type connection.

 

The cost of the "eight-way unit" is £75.00.

Initially, however, you will need a "Starter Kit" which comprises one "eight-way unit" and a "Setter Box"  and Cable.

This comes at a price of £90.00, with the Setter Box being re-usable on multiple "eight-way units", so you only need the one.

Prices are "plus P&P".

 

I'm just completing a batch of signals and hope to make a short video of them using an "eight-way unit".

I'll post a link to it on my signals Topic if I'm successful (at making the video).

 

Steve.

[Michael Woolford]

Steve,

 

Do you happen to know how we would go about purchasing these? I've looked on the website but there doesn't seem to be a way to purchase through the website or a telephone number, only an email address for inquiries. Do you know if Geoff takes orders by email?

 

I would really like to use his servo boards for my signals when they are built (there are over 15 arms in the station).

Thanks in advance.

 

Michael

[Steve Hewitt]

Hi Michael,

 

I've just checked on the website, and there is a Tel. No and Postal Address on the Contacts page.

 

If you can't make progress, please PM me and I'll see if I can help.

 

Steve.

[ndk]

Thanks for the info, I'm still looking at options but the GF control circuits look very impressive.

 

 

I'm reminded by your comment of the Bonzo Dog Band who sang;

 

How did you find yourself this morning? well I just rolled back the sheets & there I was!

[Steve Hewitt]

Hi,

 

The "eight-way unit" was received in the post.

 

The unit works very well, and has a couple of extra features not seen on the standard "two-way unit".

1. Each servo has its own LED to indicate its "ON" or "OFF" state.

2. The logic of "Switch Open" for signal Clear can be inverted to be "Switch Closed" for signal Clear is desired.

 

The actual software for the servo motion is I believe identical to the "two-way unit".

The operation of each servo is sequential, so each one has to complete its movement before the next one can start.

This means it is less suitable for use on "slotted" signals which require two servos to move simultaneously under certain conditions (also route Indicators if you use a separate servo for the route display, as I do).

 

The setting of the "eight-way unit" is done through a plug-in "setter Box".

The connection is a quite long "curly cable" telephone type connection.

 

The cost of the "eight-way unit" is £75.00.

Initially, however, you will need a "Starter Kit" which comprises one "eight-way unit" and a "Setter Box"  and Cable.

This comes at a price of £90.00, with the Setter Box being re-usable on multiple "eight-way units", so you only need the one.

Prices are "plus P&P".

 

I'm just completing a batch of signals and hope to make a short video of them using an "eight-way unit".

I'll post a link to it on my signals Topic if I'm successful (at making the video).

 

Steve.

I suppose its better if I put the video link in here, as readers may not follow my Signals thread.

Here it is, just me setting up a batch of signals I've just completed:

 

Steve.

[bgman]

Hi there,

Very very interesting, may I ask, are the servos still operating at danger ? The reason I ask is there appears to be noise coming from them on the video ( which is extremely informative) once they return to said position.

Kind regards

Grahame

[Steve Hewitt]

Hello Grahame,

 

Yes, the servos are always under power.

 

I'm not sure what you can hear when the signals are "at danger".

I can hear the servos when they move, but I can't hear anything when they are steady.

 

Steve.

[Jon Fitness]

Many thanks for posting the very informative GF controls demo with your superb signals. I must confess to owning both a GF 2 way and 8 way board but I have yet to use them other than for testing purposes. I am hoping that the GF boards will be a bit more "refined" (is that the right word?) than the MERG boards as I do get the odd random strange noise from various servos in both the on and off positions with the MERG ones!

JF